Fused Deposition Modeling of Composite Materials 0323988237, 9780323988230

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Woodhead Publishing Series in Composites Science and Engineering

Fused Deposition Modeling of Composite Materials Antonella Sola and Adrian Trinchi Commonwealth Scientific and Industrial Research Organisation (CSIRO), Manufacturing Business Unit, Clayton - Melbourne, VIC, Australia

Fused Deposition Modeling of Composite Materials

Woodhead Publishing is an imprint of Elsevier 50 Hampshire Street, 5th Floor, Cambridge, MA 02139, United States The Boulevard, Langford Lane, Kidlington, OX5 1GB, United Kingdom Copyright © 2023 Elsevier Ltd. All rights reserved. No part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Details on how to seek permission, further information about the Publisher’s permissions policies and our arrangements with organizations such as the Copyright Clearance Center and the Copyright Licensing Agency, can be found at our website: www.elsevier.com/permissions. This book and the individual contributions contained in it are protected under copyright by the Publisher (other than as may be noted herein). Notices Knowledge and best practice in this field are constantly changing. As new research and experience broaden our understanding, changes in research methods, professional practices, or medical treatment may become necessary. Practitioners and researchers must always rely on their own experience and knowledge in evaluating and using any information, methods, compounds, or experiments described herein. In using such information or methods they should be mindful of their own safety and the safety of others, including parties for whom they have a professional responsibility. To the fullest extent of the law, neither the Publisher nor the authors, contributors, or editors, assume any liability for any injury and/or damage to persons or property as a matter of products liability, negligence or otherwise, or from any use or operation of any methods, products, instructions, or ideas contained in the material herein. ISBN: 978-0-323-98823-0 For Information on all Woodhead Publishing publications visit our website at https://www.elsevier.com/books-and-journals Publisher: Matthew Deans Acquisitions Editor: Gwen Jones Editorial Project Manager: Veronica Santos Production Project Manager: Prasanna Kalyanaraman Cover Designer: Mark Rogers Typeset by Aptara, New Delhi, India

Contents

Preface

ix

1

Introduction to “Fused deposition modeling of composite materials” 1.1 Introduction: Why this book? 1.2 General outline of the present book References

1 1 1 4

2

Basic principles of fused deposition modeling 2.1 Introduction: Additive manufacturing and fused deposition modeling 2.2 Cost and quality considerations 2.3 How to print an object 2.4 Build-up mechanisms and governing parameters References

7 7 12 12 21 34

3

The need for fused deposition modeling of composite materials 3.1 Introduction: From monomaterials to composite feedstocks in FDM 3.2 Monomaterial filaments 3.3 Research trends in composite feedstock in FDM 3.4 Commercial composite filaments 3.5 Applications and case studies References

39 39 39 45 48 53 77

4

Production of composite filaments for fused deposition modeling 4.1 Introduction: Basic requirements of feedstock in FDM 4.2 Strategies for adding a filler 4.3 Key production steps 4.4 Additional issues References

89 89 90 93 100 102

5

Characterization and quality assurance in fused deposition modeling 5.1 Introduction: Properties and quality of filaments and printed parts 5.2 Materials characterization in FDM 5.3 Characterization issues with printed parts 5.4 Characterization of continuous fiber-reinforced parts 5.5 Quality assurance

109 109 109 116 120 121

vi

6

7

8

Contents

5.6 Quality assurance for the International Space Station References

122 123

Fused deposition modeling of polymer-matrix composites with discrete ceramic fillers 6.1 Introduction: Glass, ceramic, and carbonaceous fillers 6.2 Rationale for implementing discrete fillers 6.3 Mechanical reinforcement 6.4 Electrical conductivity 6.5 Thermal properties 6.6 Bioactivity and biological properties 6.7 Case studies and special applications References

129 129 129 130 136 144 146 158 163

Fused deposition modeling of polymer-matrix composites with metal fillers 7.1 Introduction: Metal fillers 7.2 Case studies and relevant applications References

175 175 176 186

Fused deposition modeling of polymer-matrix composites with natural fibers 8.1 Introduction: What is a “natural fiber”? 8.2 Structure and properties of natural fibers 8.3 Examples of FDM composite parts filled with natural fibers 8.4 Natural fibers: Pros and cons References

189 189 189 192 200 206

9

Fused deposition modeling of continuous fiber-reinforced composites and sandwich structures 211 9.1 Introduction: Rationale for adopting continuous fibers 211 9.2 “Dual extrusion” method 212 9.3 “In-nozzle impregnation” method 220 9.4 “Dual extrusion” vs “In-nozzle impregnation”: Critical considerations222 9.5 Other technological approaches to continuous fiber reinforcement 225 9.6 Multi-layered and sandwich structures 234 9.7 FDM with continuous reinforcements: A summary 238 References 241

10

Fused deposition modeling of fully inorganic parts: Shaping, debinding, and sintering (SDS) 10.1 Introduction: From a composite filament to a fully inorganic part 10.2 A three-step process: Shaping, debinding, and sintering 10.3 SDS and powder injection molding 10.4 Ceramic-based parts (fused deposition of ceramics, FDC)

249 249 250 260 262

Contents

11

12

vii

10.5 Metal-based parts (fused deposition of metals, FDMet) References

270 283

Open challenges and future opportunities in fused deposition modeling of composite materials 11.1 Introduction: Pros and cons of composite materials 11.2 Processing conditions optimization 11.3 Filler loading optimization 11.4 Environmental conditions 11.5 Porosity 11.6 Thermodilatometric compatibility 11.7 Improvement strategies 11.8 Sizing and surface modification of fillers 11.9 Isotropy vs anisotropy 11.10 Smart materials: Functionality beyond mechanical reinforcement 11.11 What is next? References

289 289 290 292 298 299 302 302 306 307 314 315 317

Fused deposition modeling of composite materials at a glance – supplementary tables 12.1 Introduction: A roadmap to FDM of composite materials 12.2 Supplementary table 1: State of the art 12.3 Supplementary table 2 References

329 329 330 392 404

Index

445

Preface

Fused deposition modeling (FDM, or fused filament fabrication, FFF) is generally recognized as the most popular additive manufacturing technique with applications that range from home-made figurines printed by hobbyists to advanced bespoke biomedical devices developed by surgeons. FDM already uses a wide palette of materials, from bio-based polymers through to high-performance thermoplastics. Nonetheless, neat polymers often fail to meet functional requirements. There is a clear and present need for polymer-matrix composites that can be 3D printed for satisfying the pressing and emerging needs in the “hot” and burgeoning industries of the 2020s and beyond. More and more often researchers receive requests for 3D printable advanced composites that can do “more” than conventional materials, with the main demand coming from industry sectors of space, biotechnology, defense, consumer goods, alternative energy, and mass transport systems. These areas are both intriguing and in demand for students, teachers, developers, and scientists everywhere. The main goal of our book is to provide the reader with an overarching description of the state of the art and future directions about the fabrication and usage of composite feedstocks in FDM for producing parts whose function and features exceed those attainable with standard neat polymers. After a brief, but clear and informative, introduction to the functioning mechanisms of FDM, our book extensively discusses the motives that drive research in composite materials for FDM with examples that demonstrate their progressive adoption in a variety of fields, from the design of advanced industrial tools to the implementation of multimaterial tagging features, from 3D pharming to 4D printing. As the foundation for any advanced 3D printed composite part is the feedstock filament, the subsequent chapters outline the basic requirements that filaments must meet to be processed by FDM. They also detail the fabrication methods that are currently used to achieve satisfactory composite filaments and the characterization techniques that are applied for checking the properties and suitability of composite filaments and printed parts. The focus then shifts to part production, where substantial sections examine the 3D printing of functional parts starting from composite feedstocks. Since polymer-matrix composite objects can be produced with different kinds of fillers, all of which can impart unique functionality from mechanical strength to thermal and electrical conductivity, to biocompatibility and antimicrobial effects, specific chapters are dedicated to ceramics, metals, natural fillers, and continuous fibers. Our book also accounts for the increasingly popular production of fully inorganic objects via the shaping, debinding and sintering method, where the filament is a hybrid organic-inorganic composite possessing an extremely high filler loading. For each class of materials, properties and relevant applications are described with a wealth of examples and case studies taken from the archival literature

x

Preface

and from commercial websites. Ultimately, based on the reviewed data, our book critically analyses the existing hurdles and the future areas of growth of FDM starting from composite filaments. Very detailed tables in the last chapter report technical information regarding filament fabrication and testing methods that has been gathered from hundreds of published papers and reports. Our book has been conceived to be informative and therefore the style is very clear and straightforward throughout the text. The introductory chapters have been especially written to take the reader on an educational journey, building their skills from the basic concepts right through to developing advanced composites for hightech industry sectors. Upon reviewing the literature, we have adopted a more critical approach and repeatedly stressed the open challenges in the field (What is the effect of a filler on the polymer’s processability? How can one-of-the-kind parts be qualified? How can we work out technological issues such as the voids associated to “U-turn” bends with continuous-fiber reinforced filaments? Is there a solution to the ubiquitous lack of bonding at the filler-matrix interface? …). Much attention has been dedicated to discussing the current limitations and expected advantages coming from the FDM of composite materials, as our aim is to engage the reader in a thoughtful discussion about the future of the field. Efficiently producing composite parts and inorganic objects by FDM is certainly an ambitious goal, but also an exciting opportunity, and we hope our book will give the reader sufficient insight and tools to embark on this challenge. Antonella Sola Adrian Trinchi 7 December 2021, Melbourne, Australia

Introduction to “Fused deposition modeling of composite materials” 1.1

1

Introduction: Why this book?

This book is dedicated to 3-dimensional (3D) printing composite materials and fully inorganic parts using the popular technique called “fused deposition modeling” (FDM), aka fused filament fabrication (FFF). FDM is by far the world’s most popular additive manufacturing (AM) method, but currently it is limited to printing basic polymers and only a small handful of composite materials. Many future industries, such as space, biomedicine, construction and defense are craving for the ability to 3D print composites and new functional materials with complex shapes and features, so that they can add all sorts of unique and customizable features to their parts. These features include biocompatibility, radiation shielding, high strength, rapid cooling, electrical and thermal conductivity and shape-memory, among others. Nowadays more and more research is emerging showing that this can soon become a reality. We first take the reader through the basics of what the FDM technique is all about and describe what are the advantages and new opportunities arising from 3D printing innovative materials, which include polymer-matrix composite parts and fully inorganic parts. We then review and discuss methods for making the different types of composite feedstock filaments that are needed to 3D print such parts by FDM. We talk about the challenges that should be considered in making the filaments and parts, and how to go about solving them, with many examples along the way. Finally, we end with some open challenges in the field and what critical steps and actions are needed by the research community in order to realize this ambitious goal in the coming years, hopefully giving the reader sufficient insight and tools to embark on this exciting challenge.

1.2

General outline of the present book

1.2.1 Time frame of the analyzed references At the time of writing this Preface (September 2021), a simple search on Scopus (Elsevier, https://www.scopus.com/search/form.uri?display=basic#basic) having “(FDM OR FFF) AND composite” as entries in title, abstract and keywords would return more than 1500 results, most of them published starting 2014. In fact, after relevant patents on FDM expired in 2009, an enormous open-source movement has appeared and the usage of composite feedstock in FDM has boomed. As shown in Fig. 1.1, the number of contributions being published every year is actually growing by an exponential trend, which clearly proves the strategic importance of this topic. Fused Deposition Modeling of Composite Materials. DOI: https://doi.org/10.1016/B978-0-323-98823-0.00007-X c 2023 Elsevier Ltd. All rights reserved. Copyright 

2

Fused Deposition Modeling of Composite Materials

Figure 1.1 Number of papers published per year having “(FDM OR FFF) AND composite” as search entries in title, abstract and keywords (data collected in Scopus on September 16, 2021; results are shown starting 2010 for the sake of clarity; 4 papers scheduled in 2022 have been included in the figure for 2021; data for 2021 is still provisional and expected to increase by the end of the year).

In the following chapters, the available literature and numerous websites have been reviewed with particular attention to recent developments. In fact, review papers on the ‘state of the art’ of FDM materials in general and about the usage of composite feedstocks for FDM date back to the ‘90s (for example, the papers by Balla et al. (2019), Bardot and Schulz (2020), Brenken et al. (2018), Dickson et al. (2017), El Moumen et al. (2019), Ferreira et al. (2019), Gao et al. (2020), Gonzalez-Gutierrez et al. (2018), Guerra et al. (2020), Hamzah et al. (2018), Kabir et al. (2020), Kehinde Aworinde et al. (2019), Kumar et al. (in press), Mazzanti et al. (2019), Mohan et al. (2017), Novakova-Marcincinova and Kuric (2012), Park and Fu (2021), Penumakala et al. (2020), Rahim et al. (2019), Rane and Strano (2019), Saroia et al. (2020), Shanmugam et al. (2021a, 2021b), Tambrallimath et al. (2021), Tümer and Erbil (2021), Valino et al. (2019), Valvez et al. (2020), Wang et al. (2017), Zhang et al. (2021), Zhao et al. (2019), Zindani and Kumar (2019); essential information about the content of these reviews is provided in Chapter 12, Supplementary table 1a). However, their focus has been more on single classes of materials (according to their intended end-use, for instance composites for electrical circuitry or materials for biomedical applications, or according to their specific composition, for example poly(lactic acid) (PLA)-matrix composites or fiber-reinforced materials) and their material performance, as opposed to producing fully functioning additively manufactured parts.

Introduction to “Fused deposition modeling of composite materials”

3

One of the main targets of this book is to provide the reader with a complete and updated description of the most recent trends, and therefore the time frame of interest has been focused on the last five years. The other target is to present how new materials could potentially impact AM. Consequently, numerous contributions from the past have also been selected and included on account of their key relevance to the progress of composites specific to FDM.

1.2.2 Nature of fillers considered herein The present book considers all kinds of composite filaments and printed parts that are loaded with inorganic phases, viz ceramics, glasses, carbonaceous fillers and metals. Moreover, on account of the progressive diffusion of filaments reinforced with biobased fillers, dedicated sections introduce natural fibers, wood flour and cellulose.

1.2.3 Specific perspective of the book The literature debates on composites in FDM under multiple viewpoints. These are carefully considered throughout the book, with topics including: the specific production of composite filaments and the optimization of the corresponding processing conditions, the microstructural peculiarities of composites fabricated by FDM, the effect of the filler on the thermo-mechanical performance or other functions of the polymer matrix, the interplay between filler arrangement and anisotropy, the advantages and disadvantages of composites obtained via FDM as compared to pure polymers and to composite materials produced by conventional techniques, the new challenges posed by additively manufactured composites in terms of characterization methods, quality check and standardization. This book emphasizes the material-related technological issues associated with the fabrication and printing of composite filaments. To this aim, it examines a wide range of fillers, which are categorized according to their nature (carbonaceous fillers, ceramics and glasses; metals; bio-based fillers) and their geometry (discrete fillers and continuous fibers). The book also describes the techniques that are currently available to produce, characterize and print composite feedstock filaments and discusses the consequences of the processing route on the final behavior of the printed parts. Both the achievement of functional polymer-matrix composites and the production of fully inorganic parts (fused deposition of ceramics and metals) are taken into consideration in the following chapters. Chapter 11 highlights the gaps, critical points and open questions that still exist in spite of the quick growth of composites in the field of FDM.

1.2.4 Supporting information The book has been conceived to be clear and understandable to a wide audience. However, in order to provide technicians and specialists with a quick tool to orient themselves, at the end of the book (i.e., Chapter 12) Supplementary tables 1a-1e are proposed that collect the essential data of all the cited contributions from the scientific

4

Fused Deposition Modeling of Composite Materials

Figure 1.2 Graphical outline of the book.

literature, including bibliographic information, nature of the constituent phases of the composite filament, adopted fabrication and processing routes, main goal of the research. Additionally, Supplementary tables 2a and 2b summarize the experimental approaches currently applied to measure the tensile and flexural properties of composite filaments and printed parts. Fig. 1.2 displays a graphical outline of the present book. It should be remarked that the book’s structure as presented in Fig. 1.2 also reflects the state of the art about FDM of composite materials and fully inorganic parts. Properties and relevant applications are described hereafter with a wealth of examples and case studies from the scientific literature and from commercial websites. Certain commercial equipment, instruments, or materials are also identified in this book to foster understanding. However, they are just an example of the emerging market trends.

References Balla, V.K., Kate, K.H., Satyavolu, J., Singh, P., Tadimeti, J.G.D., 2019. Additive manufacturing of natural fiber reinforced polymer composites: Processing and prospects. Compos. Part B-Eng. 174, 106956. http://doi.org/10.1016/j.compositesb.2019.106956. Bardot, M., Schulz, M.D., 2020. Biodegradable poly(lactic acid) nanocomposites for fused deposition modeling 3D printing. Nanomaterials 10, 2567. http://doi.org/10.3390/ nano10122567.

Introduction to “Fused deposition modeling of composite materials”

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Brenken, B., Barocio, E., Favaloro, A., Kunc, V., Pipes, B., 2018. Fused filament fabrication of fiber-reinforced polymers: a review. Addit. Manuf. 21, 1–16. http://doi.org/10.1016/ j.addma.2018.01.002. Dickson, A.N., Barry, J.N., McDonnell, K.A., Dowling, D.P., 2017. Fabrication of continuous carbon, glass and Kevlar fibre reinforced polymer composites using additive manufacturing. Addit. Manuf. 16, 146–152. http://doi.org/10.1016/j.addma.2017.06.004. El Moumen, A., Tarfaoui, M., Lafdi, K., 2019. Additive manufacturing of polymer composites: processing and modeling approaches. Compos. Part B-Eng. 171, 166–182. http://doi.org/ 10.1016/j.compositesb.2019.04.029. Ferreira, I., Machado, M., Alves, F., Marques, A.T., 2019. A review on fibre reinforced composite printing via FFF. Rapid Prototyp. J. 6, 972–988. http://doi.org/10.1108/RPJ-01-20190004. Gao, X., Yu, N., Li, J., 2020. Influence of printing parameters and filament quality on structure and properties of polymer composite components used in the fields of automotive. In: Friedrich, K., Walter, R., Soutis, C., Advani, S.G., Fiedler, B. (Eds.), Structure and Properties of Additive Manufactured Polymer Components. Woodhead Publishing Series in Composites Science and Engineering, Duxford, UK, Woodhead Publishing, pp. 303–330. http://doi.org/10.1016/B978-0-12-819535-2.00010-7. Gonzalez-Gutierrez, J., Cano, S., Schuschnigg, S., Kukla, C., Sapkota, J., Holzer, C., 2018. Additive manufacturing of metallic and ceramic components by the material extrusion of highlyfilled polymers: a review and future perspectives. Materials 11, 840. http://doi.org/10.3390/ ma11050840. Guerra, V., Wan, C., McNally, T., 2020. Fused deposition modelling (FDM) of composites of graphene nanoplatelets and polymers for high thermal conductivity: a mini-review. Funct. Compos. Mater. 1, 3. http://doi.org/10.1186/s42252-020-00005-x. Hamzah, H.H., Shafiee, S.A., Abdalla, A., Patel, B.A., 2018. 3D printable conductive materials for the fabrication of electrochemical sensors: a mini review. Electrochem. Commun. 96, 27–31. http://doi.org/10.1016/j.elecom.2018.09.006. Kabir, S.M.F., Mathur, K., Seyam, A.-F.M., 2020. A critical review on 3D printed continuous fiber-reinforced composites: History, mechanism, materials and properties. Compos. Struct. 232, 111476. http://doi.org/10.1016/j.compstruct.2019.111476. Kehinde Aworinde, A., Oluropo Adeosun, S., Adekunle Oyawale, F., Titilayo Akinlabi, E., Akinlabi, S.A., 2019. Parametric effects of fused deposition modelling on the mechanical properties of polylactide composites: a review. J. Phys.: Conf. Ser. 1378, 022060. http://doi. org/10.1088/1742-6596/1378/2/022060. Kumar, S., Singh, R., Singh, T.P., Batish, A., in press. Fused filament fabrication: a comprehensive review. J. Thermoplast. Compos. Mater. DOI: http://doi.org/10.1177/ 0892705720970629 Mazzanti, V., Malagutti, L., Mollica, F., 2019. FDM 3D printing of polymers containing natural fillers: a review of their mechanical properties. Polymers 11, 1094. http://doi.org/ 10.3390/polym11071094. Mohan, N., Senthil, P., Vinodh, S., Jayanth, N., 2017. A review on composite materials and process parameters optimisation for the fused deposition modelling process. Virtual Phys. Prototyp. 12, 47–59. http://doi.org/10.1080/17452759.2016.1274490. Novakova-Marcincinova, L., Kuric, I., 2012. Basic and advanced materials for fused deposition modeling rapid prototyping technology. Manuf. and Ind. Eng. 11, 24–27. Park, S., Fu, K.(K.), 2021. Polymer-based filament feedstock for additive manufacturing. Compos. Sci. Technol. 213, 108876. http://doi.org/10.1016/j.compscitech.2021. 108876.

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Penumakala, P.K., Santo, J., Thomas, A., 2020. A critical review on the fused deposition modeling of thermoplastic polymer composites. Compos. Part B-Eng. 201, 108336. http://doi.org/ 10.1016/j.compositesb.2020.108336. Rahim, T.N.A.T., Abdullah, A.M., Akil, H.M., 2019. Recent developments in fused deposition modeling-based 3D printing of polymers and their composites. Polym. Rev. 59, 589–624. http://doi.org/10.1080/15583724.2019.1597883. Rane, K., Strano, M., 2019. A comprehensive review of extrusion-based additive manufacturing processes for rapid production of metallic and ceramic parts. Adv. Manuf. 7, 155–173. http://doi.org/10.1007/s40436-019-00253-6. Saroia, J., Wang, Y., Wei, Q., Lei, M., Li, X., Guo, Y., Zhang, K., 2020. A review on 3D printed matrix polymer composites: its potential and future challenges. Int. J. Adv. Manuf. Technol. 106, 1695–1721. http://doi.org/10.1007/s00170-019-04534-z. Shanmugam, V., Das, O., Babu, K., Marimuthu, U., Veerasimman, A., Johnson, D.J., Neisiany, R.E., Hedenqvist, M.S., Ramakrishna, S., Berto, F., 2021a. Fatigue behaviour of FDM-3D printed polymers, polymeric composites and architected cellular materials. Int. J. Fatigue 143, 106007. http://doi.org/10.1016/j.ijfatigue.2020.106007. Shanmugam, V., Rajendran, D.J.J., Babu, K., Rajendran, S., Veerasimman, A., Marimuthu, U., Singh, S., Dash, O., Neisiany, R.E., Hedenqvist, M.S., Berto, F., Ramakrishna, S., 2021b. The mechanical testing and performance analysis of polymer-fibre composites prepared through the additive manufacturing. Polym. Test. 93, 106925. http://doi.org/10.1016/ j.polymertesting.2020.106925. Tambrallimath, V., Keshavamurthy, R., Patil, A., Adarsha, H., 2021. Mechanical and tribological characteristics of polymer composites developed by fused filament fabrication. In: Dave, H.K., Davim, J.P. (Eds.), Fused Deposition Modeling Based 3D Printing. Materials Forming, Machining and Tribology. Springer, Cham (Switzerland), pp. 151–166. http://doi.org/10.1007/978-3-030-68024-4_8. Tümer, E.H., Erbil, H.Y., 2021. Extrusion-based 3D printing applications of PLA composites: a review. Coatings 11, 390. http://doi.org/10.3390/coatings11040390. Valino, A.D., Dizon, J.R.C., Espera Jr, A.H., Chen, Q., Messman, J., Advincula, R.C., 2019. Advances in 3D printing of thermoplastic polymer composites and nanocomposites. Prog. Polym. Sci. 98, 101162. http://doi.org/10.1016/j.progpolymsci.2019.101162. Valvez, S., Santos, P., Parente, J.M., Silva, M.P., Reis, P.N.B., 2020. 3D printed continuous carbon fiber reinforced PLA composites: a short review. Procedia Struct. Integr. 25, 394– 399. http://doi.org/10.1016/j.prostr.2020.04.056. Wang, X., Jiang, M., Zhou, Z., Gou, J., Hui, D., 2017. 3D printing of polymer matrix composites: a review and prospective. Compos. Part B-Eng. 110, 442–458. http://doi.org/10.1016/ j.compositesb.2016.11.034. Zhang, H., Huang, T., Jiang, Q., He, L., Bismarck, A., Hu, Q., 2021. Recent progress of 3D printed continuous fiber reinforced polymer composites based on fused deposition modeling: a review. J. Mater. Sci. 56, 12999–13022. http://doi.org/10.1007/s10853-02106111-w. Zhao, H., Liu, X., Zhao, W., Wang, G., Liu, B., 2019. An overview of research on FDM 3D printing process of continuous fiber reinforced composites. J. Phys.: Conf. Ser. 1213, 052037. http://doi.org/10.1088/1742-6596/1213/5/052037. Zindani, D., Kumar, K., 2019. An insight into additive manufacturing of fiber reinforced polymer composite. Int. J. Lightweight Mater. Manuf. 2, 267–278. http://doi.org/10.1016/ j.ijlmm.2019.08.004.

Non-Print Items Abstract “Fused deposition modeling” (FDM), also known as “fused filament fabrication” (FFF), is presently the world’s most popular additive manufacturing (AM) method. However, commercial feedstock materials are limited to basic thermoplastics and a few composites. In the near future, industries such as aeronautics, biomedicine, construction and defense will need new advanced materials to enable parts with customizable properties. Nowadays research is progressing very fast, showing that this goal can soon become a reality. The main target of the present book is to provide the reader with an updated description of the most recent trends in this field. To this aim, the book outlines a wide range of fillers loaded into 3-dimensional (3D) printable filaments. These fillers are classified according to their nature (ceramics and glasses; metals; bio-based fillers) and their geometry (discrete fillers and continuous fibers). The book also describes the techniques currently available to produce, characterize and print composite feedstock filaments and discusses the consequences of the processing route on the final behavior and performance of the printed parts. The book accounts for both the obtainment of functional polymermatrix composites and the production of fully inorganic parts. This chapter provides an introductory roadmap to help the reader navigate across the wealth of information gathered in this book on the FDM of composite materials and fully inorganic parts. Keywords Fused deposition modeling; FDM; Fused filament fabrication; FFF; Additive manufacturing; Composite material; Scientific literature

Basic principles of fused deposition modeling 2.1

2

Introduction: Additive manufacturing and fused deposition modeling

Additive manufacturing (AM) is a new approach to fabrication, where components are manufactured by selectively adding material according to an iterative process, typically layer by layer, instead of removing material or shaping with a tool as usually happens in conventional (subtractive and formative) manufacturing methods, such as machining or computer numerical control (CNC) milling. This disruptive technological change was inspired by the evidence that nature itself creates parts by additive processes, such as organic structures or crystals that are grown atom by atom, molecule by molecule, layer by layer (Love et al., 2014). After 40 years of research and development, AM is progressing from a rapid prototyping solution to an industrial reality whose positive societal impact has been acknowledged in key areas such as the improvement of population health and quality of life through the production of bespoke healthcare products, the reduction of the environmental footprint for sustainable manufacturing, and the development of simplified and durable supply chain (Huang et al., 2013). As summarized in Table 2.1, “material extrusion” (ME) is the official designation of one of the seven AM technologies recognized by international standards (ISO / ASTM 52900, 2015). Basically, ME is an extrusion-based AM technique in which material is dispensed through a nozzle to build a 3-dimensional (3D) object. Although additional requirements should be taken into consideration as discussed in detail in Chapter 4, in principle, any material that can be pushed through a nozzle and can retain its shape afterwards may be used for ME. Build materials include plastics (polymers), concrete, clay, bioink, and even edibles like chocolate. ME is therefore an umbrella term that covers different processes that can be grouped in two macro-families depending on the physical state of the feedstock. If the extrusion takes place from a paste, the technique is known as liquid deposition modelling (LDM); otherwise, if the extrusion takes place from a molten filament, the technique in known as fused deposition modeling (FDM) (Zindani and Kumar, 2019). This method, originally patented by Stratasys Inc. (https://www.stratasysdirect.com/) (Crump, 1989), is also referred to as “fused filament fabrication” (FFF), a name introduced within the framework of the RepRap’s project (https://reprap.org/wiki/RepRap) to develop a self-replicating 3D printing machine. FDM is unanimously regarded as the most established and widespread AM technique for polymer-based feedstock. The chart in Fig. 2.1 compares the technological trends in 2019, 2020 and 2021 according to the outcomes of a survey published in the

Fused Deposition Modeling of Composite Materials. DOI: https://doi.org/10.1016/B978-0-323-98823-0.00001-9 c 2023 Elsevier Ltd. All rights reserved. Copyright 

Table 2.1 AM technologies according to the ISO / ASTM 52900 classification (ISO/ASTM 52900, 2015). 8

Vat photopolymerization

Material extrusion Sheet lamination

Powder bed fusion

Binder jetting

Material jetting

Selective Laser Sintering (SLS)

Binder jetting

Material jetting

Directed energy deposition

How it looks like

Popular Stereolithography technologies (SLA) Direct Light Processing (DLP) Continuous DLP (CDLP)

Fused Deposition Modeling (FDM) aka Fused Filament Fabrication (FFF)

Ultrasonic additive manufacturing (UAM) Laminated object manufacturing (LOM)

Selective Laser Melting (SLM) Direct Metal Laser Sintering (DMLS)

Laser Engineered Net Nano-particle jetting Shape (LENS) (NPJ) Drop-On-Demand (DOD)

How it works

Multi Jet Fusion (MJF) A photocurable resin A paste or a molten Sheets of material are A heat/energy source A print head sprays a fuses/bonds together binder onto selected material is dispensed bonded together hardens when areas of a thin selected areas of through a nozzle to layer by layer. exposed to a light powder layer to start powdered material beam that draws the draw the shape of In UAM, metal a chemical reaction. layer by layer. shape of each layer in each layer. After sheets are bonded deposition, the a resin-filled vat. MJF requires a fusing Post-processing is together by Post-curing is often material hardens ultrasonic waves and agent and infra-red required to increase through cooling required mechanical pressure (IR) energy source to the part strength, which may include: down or drying and In LOM, paper sheets fuse thermoplastic curing, sintering, retains its shape powders are bonded together infiltration, finishing by an adhesive coating and pressed

One or more printheads swipe across the build platform and shoot hundreds of micro-droplets of pre-heated photoreactive resin onto selected areas. The droplets are solidified either through cooling, ultra-violet (UV) light curing

The materials is deposited and simultaneously incorporated into a focused energy beam. Nozzle and energy source are connected to a multi-axis arm especially for repairing parts

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Fused Deposition Modeling of Composite Materials

Electron Beam Melting (EBM)

Cost Pros

Liquid resins

Pastes

Sheets

Powders

Powders (+ binder)

Liquid resins

Wires or powders

Photocurable polymer resins, mainly epoxy- or acrylate-based ones

Any material that can be pushed through a nozzle and is able to retain its shape. This includes: pure or composite thermoplastics, bioinks, concrete and clay mortars, edibles like chocolate

- UAM: metals like aluminum, copper, stainless steel, titanium alloys

- SLS: pure or composite thermoplastics, ceramics can be processed with additional steps

Powdered materials include ceramics, sand, metals

Liquid polymer resins, especially photocurable ones

Metals, including titanium alloys, Inconel, copper, aluminum, stainless steel

$ High accuracy Good surface finish Readily available materials

- LOM: A4 paper sheets

Waxes

More rarely: thermoplastics; ceramics (often Different printheads molten to a glassy state) can be fed with different materials to Powders can be changed create multi-material to create multi-material parts parts More rarely: metals

- SLM/DMLS: Pure or composite metals - EBM: Pure metals

$ Straightforward process

$ Straightforward process

Economical

Economical

Little post-processing Fast manufacturing

- MJF: Thermoplastics $$$ Good mechanical properties Wide range of materials

$$ - For metals:

$$ $$ Multi-material parts Multi-material parts

Cheaper than powder Multi-color parts bed fusion Fastest Lower thermal manufacturing stresses Extreme accuracy

Suitable for industrial production Faster manufacturing Good surface - For other materials: finishing

Suitable for Wide range of industrial production materials

Easy coloring

Large parts

Basic principles of fused deposition modeling

Processed materials

More affordable powders than PBF Efficient material usage Fast manufacturing Limited post-processing Good mechanical properties Suitable for applications in space

Cons

Post-curing

Low accuracy

Lengthy post-processing

Slow manufacturing Lengthy post-processing

- UAM:

Extremely expensive Expensive High complexity

Solvents required

- LOM:

Supports (often) required

Supports required

Low part strength

Poor surface finish

Very lengthy post-processing

Expensive

Suitable for existing part repair Expensive Less suitable to create new parts

9

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Fused Deposition Modeling of Composite Materials

Figure 2.1 Leading AM techniques in 2019, 2020 and 2021. Interviewees were asked: “Which 3D printing technologies do you use?” (Sculpteo, 2019, 2020, 2021). NOTE: the graph collects and elaborates the statistics originally published separately for each year. For the sake of clarity, some technologies have not been reported. For 2021, the figures for digital light processing (DLP) also include liquid crystal display (LCD).

yearly report by Sculpteo (a 3D printing service provider recently acquired by BASF, https://www.sculpteo.com/en/) (Sculpteo, 2019, 2020, 2021). FDM is widely recognized as the most popular AM technique on account of its affordability and simplicity (Gao et al., 2020). However, the data in Fig. 2.1 reveals another interesting trend. Whereas the greatest part of FDM users (more than 70%) operate their own printing machines internally, all other techniques are thriving through external printing services. This is due to the relatively low investment and maintenance costs of FDM. Besides its affordability, the main advantages of FDM that justify its extraordinary success and its prevalent usage as internal capability in many industries are listed in Table 2.2. FDM is a relatively inexpensive and simple technique. FDM printers range from desktop printers for private use to professional series for design studies, prototyping and even for industrial manufacturing. The technique is not considered harmful to the user, since it does not require solvents and high-temperature parts, although the temperature of the polymer within the liquefier exceeds the glass transition temperature, typically above 200°C. Filaments are clean (no dust) and simple to handle, although some polymers are hygroscopic and must be stored in sealed bags. The usage of material in the FDM printing process is relatively efficient, as compared to other AM methods (Rett et al., 2021). Song and Telenko (2016) collected and analyzed material scraps, removed supports and waste parts from failed jobs in a heavily utilized open

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Table 2.2 Main advantages of FDM that have supported the widespread uptake of the technique in industry and university. Advantages of FDM Less expensive than most AM techniques, especially for metals Low capital and operational expenditures Originally developed for thermoplastics, but suitable for composites Compatible with continuous fiber reinforcement Not solvent-based Not harmful to operators and environment Can use soluble support materials Can create sparse-fill parts Reliability Simplicity of use Fast forming time (as compared to other AM techniques) Automation Good dimensional accuracy and stability On-site (remote) production Geometric freedom (including feasibility of hollow parts) Part’s customization typical of AM Easy material change Limited waste of material Low maintenance costs Low working temperature Equipment compactness (portability, for desktop printers)

shop working with commercial FDM printers fed with ABS. Their analysis of the collected samples showed that just 34% of the ABS consumed in the workshop had to be disposed of. Waste material was mainly generated as a consequence of human or printer errors or a side effect of the printing process. It is worth noting that the volume of waste material would be expected to be smaller in a controlled environment like a research and development laboratory in academia or industry. As a term of comparison, Hann (2016) estimated that in laser-based powder bed fusion only 35% of the powder fed into the printer is actually fused in the finished part, which requires advanced powder recycling procedures to make the process economically competitive and environmentally sustainable. Also, the sparse infill pattern, which is a peculiarity of FDM, allows parts to be printed with a low density of the internal infill grid and thus to save material, whilst preserving good mechanical stability and perfect surface appearance. Another point of difference is that FDM is based on thermoplastic materials that can be processed repeatedly, and thus is amenable to recycling, as many of the waste materials can be collected and re-extruded into filaments. One of the main strengths of FDM is its extreme versatility in terms of feedstock materials. Although originally designed to process solely thermoplastic filaments, FDM is one of the few AM techniques that is actually capable of processing composite materials. In such cases the feedstock would consist of a polymer matrix combined with bio-based fillers or with inorganic phases, such as metals and ceramics, depending on the desired composition of the finished component.

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Fused Deposition Modeling of Composite Materials

2.2 Cost and quality considerations It is often claimed that one of the advantages of FDM is its affordability, since the feedstock materials and, most of all, the printing equipment employed in FDM are sensibly more economical than in other AM techniques. As a rule of thumb, this statement about the affordability of FDM is true, especially with respect to metal-based AM techniques. However, some additional considerations hold true: r

r

r

r

r

The cost of an FDM printer may greatly vary depending on its intended usage, spanning from 200-300 USD for a hobbyist model to over 30,000 USD for an industrial set-up; the cost may be even higher for professional fabrication lines for the production of metal parts, which may include the debinding station (if needed) and the sintering furnace in addition to the metal printer for around 100,000 USD (Kauppila, 2021); Very few companies declare the base price of their products or make it openly available online. This reluctance is partly due to commercial competitiveness reasons, but also takes into account the necessity of adjusting the price for customized solutions. Add-ons and optional functions, such as heating base platform, controlled printing chamber, or just a direct-Bowden drive adaptor, may significantly change the final cost over the baseline price; Starting a new research activity in FDM may require the production of experimental bespoke feedstock filaments. To this end, the investment cost should include not just the printer, but also one or more extruders, an appropriate storage system, and ancillary devices; Like any other manufacturing method, running a production line based on FDM brings about additional costs for energy supply, space usage, and personnel, which includes workforce with technical knowhow; As technology evolves very quickly, the average cost of FDM equipment is progressively becoming lower, but professional printers typically experience a rapid obsolescence.

As of September 2021, a comparison between polymer-based AM techniques, including FDM, stereolithography (SLA) and selective laser sintering (SLS) can be drawn according to the estimates published online (Formlabs, n.d.), as reported in Table 2.3. The table also lists some basic features, such as the nature and form of common feedstock materials, as well as the main pros and cons, of each technique, since several parameters, other than costs, should be accounted for in order to make an informed buying decision (Tully and Meloni, 2020).

2.3 How to print an object Whereas other ME techniques work with pellets (fed into a hopper) or with pastes (pushed through a syringe), the key feature of FDM is that the process uses a thermoplastic or wax filament of controlled size and properties as the feedstock to fabricate three-dimensional parts. At present, poly(lactic acid), aka polylactide (PLA), acrylonitrile butadiene styrene (ABS), polyamide (PA, “nylon”), polyethylene terephthalate (PET) and flexible plastics (thermoplastic elastomers) are the most commonly used feedstock materials for FDM. Engineering polymers with superior mechanical and thermal performance, such as polyetherimide (PEI) and various members of the poly-aryl ether ketone (PAEK) family including poly-ether ether ketone (PEEK) and

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Table 2.3 Direct comparison of the main features and cost reasons for FDM, SLA, and SLS (Formlabs, n.d.). SLA Thermoset resins

Feedstock Pros

FDM Thermoplastic polymers and composites Filament Fast building rate

Smooth surface finish

Cons

Large palette of materials Low accuracy

Materials

Low details

Equipment cost

Hobbyist printers: few hundreds USD Mid-range desktop printers: starting at 2,000 USD Industrial printers: starting at 15,000 USD

Material cost

50 to 150 USD/kg for standard and engineering filaments

Liquid High accuracy

Few options for composite feedstock Sensitivity to UV exposure Professional desktop printers: starting at 3,500 USD Large-format desktop printers: starting at 10,000 USD Large-scale industrial printers: starting at 80,000 USD 150 to 200 USD/l for standard and engineering resins

100 to 200 USD/kg for support materials Labor

Manual support removal (can be automated for soluble supports) Lengthy post-processing for high surface finish

Manual but simple support removal

SLS Thermoplastic polymers and composites Powder Strong functional parts No supports needed Low surface finish Limited material options Desktop printers: starting at 10,000 USD Industrial printers: starting at 100,000 USD

Around 100 USD/kg for nylon Unfused powder can be recycled, which lowers final material costs Cleaning to remove excess powder

Washing and post-curing (can be automated)

poly-ether ketone ketone (PEKK), are gradually emerging for demanding applications. However, these engineering polymers require specialized printing hardware and trained operators to achieve professional results, which still confines their usage to a market niche. Typically, the feedstock filament has a diameter of about 1.5-3 mm, with 1.75 mm and 2.85 mm being the standard commercial sizes. For desktop printers and small-scale systems, the feedstock is often a simple loose coil, but for industrial FDM plants the feedstock is preferentially coiled inside a cartridge (Turner et al., 2014).

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Fused Deposition Modeling of Composite Materials

Figure 2.2 Basic functioning mechanisms of the FDM process: the thermoplastic or wax filament is fed into the printhead, where it is heated and melted, and then deposited on the base platform layer-by-layer.

In principle, the build-up strategy is straightforward, since it is based on the simple heating of the filament to obtain a moderately viscous melt (Engineering product design, n.d.). As schematized in Fig. 2.2, the filament from a spool is moved through counter-rotating pinch rollers or wheels, and fed into the liquefier, where it is heated and melted. Then, the molten material flows through the extruding nozzle of the printhead and lands on the baseplate, where it builds up the desired 3D shape line-by-line, and further on layer-by-layer (Novakova-Marcincinova and Kuric, 2012). In “direct drive printers”, the stepper motor powering the feeding mechanism that pushes the filament into the liquefier is attached directly to the printhead. In “Bowden drive printers”, instead, the stepper motor is not co-located with the liquefier, rather it is placed at the rear of the printer. The difference is schematically shown in Fig. 2.3. As compared to direct drive printers, the printhead of Bowden drive systems is smaller, lighter and easier to move around, which allows for a larger build surface. Further, the low inertia improves the print quality, because the closely controlled motion of the nozzle avoids the ripples that are frequently observed as a consequence of vibrations (called “ringing” or “rippling” or “ghosting”). As a drawback, the filament has a longer way to go from the rear of the printer to the printhead and hence Bowden drive printers are not suitable for very flexible feedstock materials. For the same reason, Bowden drive printers typically process thick (2.85 mm) filaments, whereas thinner filaments (1.75 mm) are more appropriate for direct drive equipment. The majority of FDM printers identify each point within the build volume by Cartesian coordinates, where “x” and “y” locate the point on the horizontal plane, whereas “z” refers to the vertical axis along the build direction. Quite often, the

Basic principles of fused deposition modeling

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Figure 2.3 Whereas in direct drive systems the feeding mechanism is co-located with the liquefier, in Bowden drive systems it is placed at the rear of the printer.

Figure 2.4 Schematic illustration of different FDM printers, including Cartesian, Delta, and SCARA types.

printhead moves on a x-y gantry, which in turn moves along the z axis. However, as shown in Fig. 2.4, other variations are possible. For example, in Delta printers the printhead is hinged to three or more arms attached to vertical rails. The base platform is preferentially round, because the arms can hardly stretch out to reach receding corners. As a consequence, the base platform can only accommodate relatively small rectangular parts that are inscribed in the maximum allowed circular perimeter. The Delta hardware is thus well-suited to manage tall prints, but the build volume is relatively small. Selective compliance assembly robot arm (SCARA) printers mount the printhead on a robotic arm. Two motors control the movement of the robotic arm in the x-y plane, whereas another motor is responsible for the motion in the z direction.

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Fused Deposition Modeling of Composite Materials

SCARA printers do not usually have an attached base platform. In some printers (H-bot and CoreXY, for example), the printhead moves in the x-y plane, but the part’s growth along the z direction is accomplished by moving the base platform downwards, instead of moving the printhead upwards. Polar printers were first introduced in 2015. Their unique feature is that the build volume is mapped out according to a polar coordinate system, instead of a Cartesian one. The printhead is supported by a curved arm and swings on a spinning circular base platform. This hardware reduces the number of motors needed to move the printhead and the base platform, and therefore it is relatively inexpensive and quiet. However, the printing quality is low and, since the technology is still confined to a market niche, there is limited community support (O’Connell, 2020). The standard nozzle diameter is 0.4 mm, but different sizes are available, typically ranging from 0.15 to 1.0 mm. Working with a smaller nozzle diameter is preferable to achieve a higher printing resolution, since the thinner strands of printed material enable the reproduction of finer details. However, the flow of molten material becomes more difficult and this leads to the build-up of backpressure against the feeding mechanism and hence to an increased likelihood of filament buckling at the entrance of the liquefier, or of filament slippage through the feeding mechanism. Working with a smaller nozzle may be especially challenging with composite feedstocks, since oftentimes the addition of the filler increases the melt viscosity (Cano et al., 2020). Also, the presence of individual fillers or agglomerates whose size is comparable to the nozzle diameter may cause the printhead to clog (Zhang et al., 2020). The build volume of standard FDM printers does not exceed 20 × 20 × 20 cm3 (Roschli et al., 2019), but specialized printers are available with larger dimensions up to about 1 × 1 × 1 m3 (Daminabo et al., 2020). The Big Area Additive Manufacturing (BAAM) system, currently under development at the Oak Ridge National Laboratory, is a large-scale printer that works similarly to an FDM system (Duty et al., 2017; Roschli et al., 2019). Although it uses a single screw extruder to melt and deposit pelletized feedstock materials instead of processing thermoplastic filaments in a normal printhead, the BAAM printer is an interesting example of the undergoing research to extend the maximum printable size and throughput in FDM-related technologies. Its base platform can accommodate structures as large as 6 m in length, 2.4 m in width and 1.8 m in height, which is around ten times larger than standard FDM machines. The BAAM nozzle is also proportionally larger than conventional nozzles, with a standard diameter of 7.6 mm that prints an oval-shaped bead with a typical width of 8.4 mm and a thickness of 4 mm. The deposition rate can be as high as 50 kg/hour, more than 200 times faster than standard printers (Duty et al., 2017). The surface of the build platform plays a critical role in successfully completing a job. The first layer of the build-up sequence must adhere to this surface well enough so that the printed part does not prematurely detach in spite of the thermal stresses that develop upon printing, but not so well that the part cannot be removed after printing (Cano et al., 2019; Nabipour et al., 2020; Spoerk et al., 2018, 2020; Turner et al., 2014). Structurally complicated parts may require the extrusion of supports to hold overhanging geometries. In order to facilitate the removal of supports after printing, many

Basic principles of fused deposition modeling

17

Figure 2.5 Dual-nozzle printheads are able to deposit construction material and support material that are fed through two parallel lines. The same configuration allows the processing of a second construction material instead of the support material, so as to print multi-material and multi-color parts.

FDM machines mount two separate heated nozzles on the same printhead to allow for the simultaneous deposition of the construction material and of the dissolvable support material. Otherwise, dual-nozzle printers can be used to print in two different construction materials that will remain in the finished part, for example in two colors. An example of dual-nozzle equipment is schematically presented in Fig. 2.5. As discussed in Chapter 9, the dual nozzle (or, sometimes, dual printhead) layout is often adopted in commercial printers for continuous-fiber reinforced parts. This is the case, for instance, of Markforged Mark Two printers (Markforged Mark Two, n.d.) to produce continuous fiber-reinforced parts, since one nozzle is dedicated to the matrix filament and the other one to the fiber-reinforced filament. The standard configuration of Fiber printers from Desktop Metal includes 2 FDM printheads and a third separate head for the “micro automated fiber placement (μAFP)”. According to the specifications, the μAFP head has been designed with a closed-loop heat control

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Fused Deposition Modeling of Composite Materials

system to deposit continuous fiber reinforcement with low porosity (Desktop Metal Fiber, n.d.). However, other approaches are feasible. For example, the “in-nozzle impregnation method” requires simultaneous feeding of the neat polymer filament and fiber bundle in the liquefier and co-extruding them through the same nozzle (Kabir et al., 2020), which is similar to the layout of Anisoprint printers (Anisoprint solutions, n.d.). There is an increasing interest for printing multi-material objects (Dilberoglu et al., 2017), both for aesthetic purposes (to combine multiple colors in a single part) and for functional reasons (to enable different properties at different print locations, for example to emphasize the counter-intuitive mechanical response of metamaterials by the appropriate combination of flexible joints and stiff struts, which has already been demonstrated by PolyJet printing (Wang et al., 2015)). For this reason, research is underway to figure out new strategies for multi-material printing, which span from feedstock development (for example, programmable multi-material single filaments (Takahashi et al., 2020)) to advanced hardware solutions (for example, four-nozzle print head machines (Koslow, 2016)). FDM, like most AM techniques, produces 3D objects by the controlled deposition of a sequence of layers. As shown in Fig. 2.6, the appropriate choice of the layer thickness has important consequences on the surface finish, the mechanical behavior, and the final cost of a printed part. Due to the deposition of discrete layers instead of a continuum of material, curved shapes are approximated with stepped profiles (Brooks et al, 2011). This “staircase effect” (or “stair-stepping effect”) impairs the surface finish, but the aesthetical appearance of the printed object can be improved by reducing the layer thickness (Anitha et al., 2001). Although less obvious, the staircase effect also undermines the mechanical behavior, because the inter-layer indents may cause local stress concentration and hence premature failure. Again, the negative consequence of the staircase effect can be minimized by reducing the layer thickness. However, the layer thickness cannot be reduced arbitrarily, because the increased number of layers required to complete the object’s geometry substantially increases the printing time and hence the cost of the finished part. Moreover, the layer thickness is affected by technological constraints. The layer thickness in FDM typically lies in the 0.05 mm (with very small nozzles) to 0.4 mm range. This means that FDM allows to print with a layer thickness comparable to SLS, as the finest layer thickness in SLS is typically around 0.08 mm, but it cannot reach up to the finest layer height that can be achieved by SLA, which may be as low as 0.025 mm (Wickramasinghe et al., 2020). Alternatively, the staircase effect may be mitigated somewhat by re-orienting the part upon printing (Solomon et al., 2021), as shown in Fig. 2.7. However, for complex geometries, this approach may have contrasting results on different faces, as re-orienting is likely to improve the surface quality on some faces, and to simultaneously worsen the surface quality on other faces whose orientation becomes unfavorable. Also, supports must be completely redesigned and the build time may also change. Whereas the standard option is to keep the layer thickness constant for the whole part, adaptively computing the layer thickness, which is shown in Fig. 2.8, has the potential to provide good quality results while maintaining a reasonably short printing

Basic principles of fused deposition modeling

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Figure 2.6 Effect of the layer thickness. In this drawing, details colored in green represent advantages, and details colored in red represent disadvantages of having different layer thickness.

time. However, the implementation of adaptive slicing may be not straightforward, especially at the hobbyist level (Tyberg and Bøhn, 1999; Wasserfall et al., 2017). Since it is not possible to completely avoid the staircase effect, several post-printing treatments have been proposed in the literature to improve the surface roughness of FDM parts. Polishing with sand paper remains the easiest and cheapest method, readily available also to hobbyists. Traditional industrial techniques, such as CNC milling, capitalize on the extensive knowledge currently available in machining of polymers and offer a viable tool to improve the surfaces of the printed part that cope with functional requirements (Boschetto, et al. 2016). However, these approaches are often time consuming and costly when applied to complex geometries with fine details. Also, receding details and very complicated profiles may remain inaccessible. Other finishing methods leverage the peculiar nature of thermoplastic materials that can be melted upon heating or chemically dissolved by appropriate solvents. For example,

20

Fused Deposition Modeling of Composite Materials

Figure 2.7 Although the layer thickness remains the same, after reorienting the part the staircase effect is largely mitigated on surfaces A and A’. However, the surface finish worsens on surfaces B and B’, and supports are required to print overhangs. The build time is also likely to change after reorienting.

the stepped appearance of FDM parts can be smoothed by exposing the printed object to a flux of hot air that partly remelts the surface features (Adel et al., 2018), or by dissolving the undesired protrusions with a chemical treatment, either exposing the object to a solvent vapor (Jin et al., 2017; R. Singh et al., 2017) or directly soaking it in a solvent bath (Galantucci et al., 2009). Surface treatments may also serve to modify the surface wettability and to improve the inter-layer bonding (Li et al., 2021). However, at present it is still unclear how these finishing methods may be translated from neat polymer parts to composite ones, as few contributions specifically address the effect of surface finishing on composite parts produced by FDM (Chen and Zhang, 2019; Li et al., 2021).

Basic principles of fused deposition modeling

21

Figure 2.8 The implementation of adaptive slicing algorithms allows to locally reduce the layer thickness to mitigate the staircase effect while maintaining short build times.

2.4

Build-up mechanisms and governing parameters

As schematically demonstrated in Fig. 2.9, AM (and, thus, FDM) is to a large extent a “digital manufacturing technology” (Chen et al., 2019), since 3D printing the physical object is just the last step in a long workflow that starts from the creation of the virtual model, typically drawn via computer-aided design (CAD), or acquired by 3D tomography or by 3D scanning, or created by a combination of them. The CAD file must be converted into a readable format for the printing software, which is often the. STL format (STL being the abbreviation of standard tessellation language, or standard triangulate language, or stereolithography). Next, software packages are employed to slice the STL file, which means that the 3D model is broken down to 2D slices corresponding to the cross sections of the object to be printed. If support structures are needed, they must be included prior to slicing. Then, the 2D slices are converted to the G-code, which is a computer language that can be read by CNC-based systems, including those used for extrusion-based processes. The digital workflow comes to an end with the “physical” build-up of the object itself (Daminabo et al., 2020). For printing the same object, different toolpaths can be devised for different purposes, which can be grouped into three main classes: i) improving the quality and surface finish of the printed part, ii) saving material/time, and iii) achieving improved properties such as better mechanical, topological or functional properties

22

Fused Deposition Modeling of Composite Materials

Figure 2.9 Schematic representation of the AM workflow, including digital manufacturing steps and physical manufacturing operations. Printing may be accomplished through external service providers. These three aspects (digital manufacturing, physical manufacturing, and external services) are closely interconnected, as shown by red arrows.

(Jiang and Ma, 2020). Since the toolpath and G-code instructions govern the chronological sequence in which the material is deposited within each layer and then layerupon-layer, they have a deep effect on the development of thermal stresses (Daminabo et al., 2020). Additionally, Gkartzou et al. (2017) demonstrated that objects produced from the same CAD design and with the same user-defined process and toolpath parameters, but printed according to two different G-code files result in different fracture morphologies as a consequence of premature inter- and intra-laminar failures that are induced by over- or under-extrusion or by locally weak bonds. In terms of “physical manufacturing technology”, the quality and properties of the printed object will depend on the feedstock materials (both structural materials and support materials, as long as support materials do not survive to the final object,

Basic principles of fused deposition modeling

23

but affect the quality of the printing job), on the printing hardware (which is not limited to the printer itself, since the equipment may also include ancillary processing units such as post-processing facilities) and on the steady supply of energy while printing (Yampolskiy et al., 2017). As an additional level of complexity, it should be noted that many companies in industrial settings do not print internally and rely upon external service providers for this. Although this is more common with other AM technologies that require expensive equipment and dedicated staff like electron beam melting (EBM) or selective laser melting (SLM), outsourcing may be an option also for FDM. Generally speaking, the investment and labor costs for FDM are affordable, but outsourcing may be the most convenient option for companies that are still in the early stages of initiating 3D printing or that require 3D printing on an occasional basis (Berman, 2020). Using the terminology illustrated in Fig. 2.2, the filament upon printing is fed into the liquefier by the action of the counter rotating gears of the feeding system. The feed rate to the liquefier and, ultimately, the printing speed depend not only on the rotating speed of the gears, but also on their pinching efficiency on the filament, which in turn depends on the geometry of the gears, on the surface roughness of the filament, and on the diameter consistency of the filament. The liquefier is the core of the FDM process, since it melts the polymer to the right viscosity for printing. The filament at the entrance of the liquefier works like a piston (or a “ram”) and pushes the molten polymer through the liquefier and then through the printing nozzle. The extruded material lands on the build platform or on the previously deposited layers thus creating the part’s geometry. Despite of its apparent simplicity, the science that underlies the build-up mechanisms in FDM is indeed very complicated. The first hurdle to face is the stress state experienced by the filament. Due to the action of the feeding mechanism, the filament is pulled from its spool and pressed down into the liquefier (Turner et al., 2014). The filament is therefore in tension above the feeding gears and in compression below feeding gears. The pressure required for printing and hence the compressive stress state on the filament mainly depend on the melt viscosity, the nozzle geometry and the volumetric flow rate of the printing process (Das et al., 2021; Nienhaus et al., 2019; Turner et al., 2014). In turn, the melt viscosity is governed by multiple factors, including: the polymer chemistry, the potential presence of a filler, its volume fraction and its state of agglomeration, and the processing conditions, especially temperature and shear rate (Venkataram et al., 1999). If the compressive stress acting on the filament between the gears and the entrance of the liquefier exceeds a critical value, the filament can buckle as shown in Fig. 2.10. Buckling has been reported to be the most common failure mode for ceramic-filled composite feedstocks (Venkataram et al., 2000), but it is expected that metal-loaded composite filaments experience a similar scenario. The critical stress σ cr that governs buckling phenomena can be predicted according to the Euler buckling model, Eq. (2.1) (Venkataraman et al., 1999):

σcr =

π 2 Ed 2f 16L2f

(2.1)

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Fused Deposition Modeling of Composite Materials

Figure 2.10 Filament buckling between feeding gears and liquefier (details of feeding mechanism and printhead are not in scale).

where E is the compressive modulus of elasticity of the feedstock material, df is the diameter of the filament, and Lf is the length of the filament from the feeding mechanism to the entrance of the liquefier. Venkataraman et al. (1999) modified Eq. (2.1) and introduced a correction factor of 1.1 to account for geometric issues (with the barrel diameter of the liquefier being larger than the filament diameter). In order to verify the printability of ceramic-filled composite filaments, Venkataram et al. (2000) compared the critical stress value σ cr from modified Eq. (2.1) to the pressure drop experienced by the molten feedstock in a capillary rheometer that simulated the flow through the liquefier. The analytical model proposed by Venkataram et al. (2000) was validated by experimental evidence and proved that, for a nozzle diameter of 508 μm, buckling would not occur if the ratio between the modulus of elasticity of the feedstock material, E, and its apparent melt viscosity, η, exceeded a critical value between 3 × 105 and 3 × 105 s−1 (Venkataram et al., 2000). This provides a practical guideline for the development of new composite feedstock, since candidate materials whose E/η ratio falls below this threshold are unlikely to be successfully fed into the liquefier (Das et al., 2021). It is worth noting that, according to the model developed by Venkataram et al. (2000), the elastic modulus, “E” in Eq. (2.1), is “the elastic modulus (compressive modulus for FDC filaments)” (where FDC stands for “fused deposition of ceramics”,

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a technique that produces fully inorganic objects starting from FDM parts with high filler loadings, as explained in Chapter 10). The detail that the elastic modulus in Eq. (2.1) is actually the compressive modulus of the filament is often overlooked in the literature and the tensile modulus is used instead of the compressive one. Though particularly challenging, understanding the melt dynamics within the liquefier is essential to develop control algorithms that can effectively tune the flow rate and hence the size of the printed strand of material. A mathematical description of the melt dynamics within the liquefier can be found in dedicated contributions and the interested reader is referred to the review paper by Turner et al. (2014) for a survey of the analytical models and finite element (FE) simulations on this topic. However, it is worth mentioning that a key issue is the rheological behavior of the melt. Generally speaking, feedstocks for ME techniques have a shear thinning behavior and, in the first instance, their apparent melt viscosity, η, is assumed to depend on the shear rate γ˙ according to a power-low model, Eq. (2.2): η = K(γ˙ )n−1

(2.2)

where K and n are fit parameters (Turner et al., 2014). This is a very important property, because new composite feedstocks for FDM should have a shear-thinning behavior to be compatible with standard printing equipment (Park and Fu, 2021). In addition to the rheological properties of the melt, the heat transfer from the wall of the liquefier to the feedstock also plays a substantial role. Understanding the heat transfer within the liquefier is particularly problematic, because descriptive models should account for the intrinsic properties of the melt, especially its heat capacity (that depends on the temperature and changes significantly at the glass transition temperature and, for semi-crystalline polymers, at the melting temperature), and for the specific geometry of the liquefier. Since the addition of a filler inevitably modifies the heat capacity of the melt, the temperature profile of the walls of the liquefier must be adjusted accordingly. Strictly speaking, the flow through the nozzle can be described as a fully developed laminar flow of polymer through a capillary die with a generally circular cross section (Daminabo et al., 2020). More intuitively, the flow through the nozzle can be regarded as an extrusion process, and this is why FDM is classified as a “material extrusion” AM technique. This implies that in FDM the feedstock material must first be extruded in order to produce the filament, and this filament must then be extruded a subsequent time in order to print the part. However, for the sake of clarity, in the following chapters the term “extrusion” will be applied only to the production of the filament, whereas the flow through the nozzle will be termed “printing.” After leaving the nozzle, the material cools down via convective cooling from the environment. Concurrently, the material still receives heat from the melt in the liquefier via heat conduction. Although this may appear counterintuitive, the higher the thermal conductivity of the feedstock material, the slower the cooling down process due to the prevailing effect of heat conduction from the melt (Turner et al., 2014).

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Figure 2.11 Die swelling of the melt as a consequence of the recovery of elastic deformation. The die swelling phenomenon occurs at the exit of the print nozzle as well as at the exit of the extruder.

As it deposits on the build platform (or on previous layers), the material is referred to as “bead” or “raster” (also known as “road”, “strand”, “track” or “traxel”). As commonly observed in extrusion-based processes, the recovery of the elastic deformation causes the polymer melt to swell upon leaving the nozzle and this is responsible for an increase in the bead diameter as shown in Fig. 2.11. Quantitatively, the die swelling phenomenon is measured by the swelling ratio, S, which is defined as the ratio of the maximum diameter of the extrudate, D2 in Fig. 2.11, to the nominal opening of the orifice, D1 , according to Eq. (2.3): S=

D2 D1

(2.3)

As a consequence of die swelling, the increased diameter of the bead negatively affects the print resolution. Typical values of the swelling ratio for FDM materials upon printing range between 1.05 and 1.30 (Turner et al., 2014). However, the addition of fillers, especially inorganic ones, is expected to limit the elastic recovery of the melt, thus reducing the die swelling effect and improving the print quality (Dul et al., 2016; Yang et al., 2021). Although the bead is circular at the exit of the nozzle, it slightly flattens when deposited on the base platform (or on the previous layer), thus assuming an oblong cross section. As it spreads, the bead also cools down and therefore the melt viscosity increases until a solid state is reached. As a first approximation, the cross section of the newly deposited bead is often described as “elliptical”, although this is not geometrically exact because the bead is still soft when being deposited and therefore the bottom slightly flattens under the effect of gravity, while the top cools down to a

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Figure 2.12 Toolpath in the x-y plane with its basic terminology.

rounded edge (Wang et al., 2016). The degree of flattening reached by the solidified bead depends on several parameters, especially the viscosity of the polymer when it touches the base platform (or the previous layer), the time to cool down to solid state and the relative surface energies of the bead and the underlying surface. In addition to the properties of the feedstock material, also the interaction with the nozzle tip and the initial velocity profile of the bead may be relevant. For example, the FE simulations performed by Bellini (2002) suggest that keeping the tip of the printhead in contact with the bead (as opposed of keeping it above the bead) helps stabilize the bead and keep its surface flat and regular. Bead spreading brings about two important consequences. Firstly, the final bead width defines the print resolution and therefore a substantial spread will lead to a large bead width and ultimately to a poor print resolution. Secondly, the final bead width as compared to the bead-to-bead distance determines the contact area between neighboring beads and the size of inter-bead voids. In order to better understand this issue, which is paramount to control the mechanical strength and stiffness of the printed object, the layer-by-layer build-up of FDM parts must be analyzed in more detail, starting from the deposition of a single layer. As previously mentioned, in common (Cartesian) FDM printers, the printhead sits on a CNC gantry and moves across the x-y plane of the build environment thanks to the action of stepper motors. The planar route of the printhead, which is fully computercontrolled, is defined as “toolpath” and is responsible for the deposition of a single layer. This planar movement is combined with the z translation of the build stage (or of the printhead) to create the desired 3D object layer-by-layer. As shown in Fig. 2.12, the toolpath is typically comprised of a perimeter (aka contour) and of an interior. In order to obtain a smooth surface, the contour bead is printed around the perimeter of the part in a continuous path; very often, two or more contour beads are printed along

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Figure 2.13 The raster angle can be changed from layer to layer according to a predetermined stacking sequence in order to reduce the overall anisotropy in the x-y plane or to induce controlled anisotropic effects.

concentric lines to strengthen the exterior of the part and to improve the surface finish. The interior is filled with a raster of beads. Typically, the raster angle is changed from layer to layer according to a predetermined stacking sequence in order to reduce anisotropic effects or to induce a controlled anisotropic response in the printed object (Ahn et al., 2002), as exemplified in Fig. 2.13. Fig. 2.14 demonstrates that neighboring beads in the interior of a single layer can partly overlap (the distance between the centerlines of adjacent beads is smaller than the single bead width) or they can be completely separated (the distance between the centerlines of adjacent beads is larger than the single bead width), thus leaving air gaps in between. As compared to the “sparse-fill” strategy, the “highly packed” strategy maximizes the mechanical strength, but also increases the final weight of the part, the required printing time, and the likelihood that thermal deformation occurs after printing (Turner and Gold, 2015). In case the object does not have critical load-bearing functions, the sparse-fill printing mode is extremely advantageous because it reduces the part cost without affecting the exterior. Meanwhile, the presence of internal beads provides some mechanical support to the exterior, similar to bridging cables that add strength to the structure (Stratasys sparse fill, n.d.). This often corresponds to a “lightweight construction effect,” where the decrease in mechanical strength is less than linear to the decrease in infill degree and ultimately in density. For example,

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Figure 2.14 Different raster strategies: if the distance between the centrelines of adjacent beads is smaller than the bead width, the beads partly overlap (“highly packed” strategy); otherwise, the beads remain separated with a gap in between (“sparse-fill” strategy).

Fafenrot et al. (2017) observed that samples printed with 60% infill degree achieve much more than 60% of the maximum flexural strength corresponding to 100% infill degree. However, for the same infill degree, the final strength is affected by the specific infill pattern and orientation. Many infill patterns are feasible and the infill degree may vary from 100% (fully solid part) to 0% (fully hollow part), which is presented in Fig. 2.15. The best infill strategy should be identified according to the mechanical properties (stiffness and strength) of the feedstock material and according to the final application of the printed part. Even in case the targeted part is fully dense, voids are likely to be present at the bead junctions. In the simple configuration of keeping the raster angle constant from layer to layer, these interbead voids result in empty channels whose cross section (shape and size) depends on the degree of flattening as well as on the deposition strategy, as exemplified in Fig. 2.16 for rectangular and skewed deposition patterns. It is also clear that, for a constant value of distance between neighboring beads, the degree of overlap and hence the contact area between neighboring beads depend on the bead flattening, as illustrated in Fig. 2.17. The contact area between adjacent beads plays a fundamental role in the consolidation of FDM parts. In fact, at the microscale, the consolidation of an FDM-printed polymer can be described as a sintering process according to a modified FrenkelEshelby model (Turner et al., 2014), which is illustrated in Fig. 2.18. The formation of bonds between neighboring beads occurs through surface wetting (“contact”), necking, viscous flow and molecular diffusion (“reptation”) of polymer

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Figure 2.15 Examples of different infill degrees with the same infill pattern (top) and different infill patterns with similar infill degree (bottom).

chains across the bead-bead interface. The interfacial bond formation, also known as “healing”, is influenced by several parameters, though especially by the melt viscosity (Bellehumeur et al., 2004; Blok et al., 2018; Das et al., 2021; Sun et al., 2008; Turner et al., 2014). Polymer sintering is a temperature-driven process and requires the polymer to be above its glass transition temperature (melting temperature for semi-crystalline polymers) (Das et al., 2021). Of course, at the exit of the nozzle tip the temperature of the melt is well above the glass transition temperature. However, the temperature of the build environment is typically much lower than the glass transition temperature.

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Figure 2.16 Formation of inter-bead voids in solid parts.

Figure 2.17 For solid parts, if the distance between neighboring beads is constant, the degree of overlap depends on the final bead width that in turn depends on the bead flattening.

Figure 2.18 Frenkel-Eshelby sintering process from initial contact to polymer chain randomization between neighboring beads via molecular diffusion of polymer chains (viscous flow sintering). For the sake of clarity, the figure illustrates the role of molecular diffusion and randomization, but does not account for the surface tension of the polymer melt that governs the bonding process on the base platform (Das et al., 2021; Turner et al., 2014).

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The bead still receives heat from the melt in the liquefier, but it also loses heat to the material below (base platform or previous layers) via conduction and to the air in the build chamber via convection. In a typical FDM set-up, this implies that the printed material cools down to below its glass transition temperature in about 2–3 sec, which usually impedes complete coalescence and sintering (Das et al., 2021). Conceivably, the thermal properties of the polymer, especially its thermal conductivity and heat capacity, are key to enabling effective bonding (Turner et al., 2014). Also, printing within a closed build chamber and, most of all, mounting a heated build platform may be very useful to control the bead temperature. For dual-nozzle printers, these hardware-related solutions to prolonging the available time for bond formation become critical in facilitating heat transfer-based inter-molecular diffusion between two different polymers that may have sensibly diverse thermal properties (Yin et al., 2018). Post-printing thermal annealing may be effective to promote additional bond formation and thus to increase the structural strength of the printed part. As a rule, annealing is generally conducted at temperatures about 20–40°C lower than the temperatures at which the polymer begins to flow, but there is not a standard rule for the duration (Das et al., 2021). A very fine tuning of time and temperature is required to avoid overheating and subsequent damage of the printed part’s geometry, especially collapse of tiny details and very thin structures. Besides the temperature, also pigments and additives in the polymer matrix have been proven to play a fundamental role on the molecular mobility and hence on the formation and growth of necks (Wittbrodt and Pearce, 2015). Even more so, the presence of a filler will impact the sintering process, interfering with the polymer chain mobility and changing the thermal properties of the neat polymer (Das et al., 2021). There is no general model to predict the consequence of a filler on the sintering process, because competing mechanisms may be active. For example, rigid fillers may limit the polymer chain mobility, but they may also increase the heat transfer efficiency between neighboring beads (Blok et al., 2018). In this regard, carbon nanotubes (CNTs) on the surface of neighboring beads have been demonstrated to effectively induce inter-bead welding when exposed to radio frequency (RF) waves (microwaves). Both single-wall CNTs and multi-wall CNTs (MWCNTs) are indeed excellent susceptors and, when targeted by RF excitation, they provide local heating and improve the polymer chain diffusion through the beadbead interface. As opposed to massive heating of the whole printed part, the locally induced radio frequency (LIRF) welding technology does not cause warping and morphology disruption. However, the targeted heating is very efficient in promoting polymer sintering and thus increasing the fracture strength (Sweeney et al., 2017). On account of the different effect that fillers may have, the sintering behavior of composite feedstock must be assessed on a case-by-case basis. It is intuitive that the more efficient the sintering process, the stronger the inter-bead and inter-layer bonding, and hence the better the mechanical performance of the printed part. Fig. 2.19 provides a graphical outline of the parameters that govern the build-up mechanisms in FDM. Understandably, various steps are mutually related and some variables affect multiple steps.

Basic principles of fused deposition modeling

Figure 2.19 Parameters affecting the key steps of FDM build-up mechanisms.

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Solomon, I.J., Sevvel, P., Gunasekaran, J., 2021. A review on the various processing parameters in FDM. Mater. Today 37, 509–514. http://doi.org/10.1016/j.matpr.2020.05.484. Song, R., Telenko, C., 2016. Material waste of commercial FDM printers under realistic conditions. In: Solid Freeform Fabrication 2016: Proceedings of the 267th Annual International Solid Freeform Fabrication Symposium – An Additive Manufacturing Conference, 810 August, 2016, Austin (Texas, U.S.A.). The University of Texas at Austin, pp. 1217– 1229. Spoerk, M., Gonzalez-Gutierrez, J., Sapkota, J., Schuschnigg, S., Holzer, C., 2018. Effect of the printing bed temperature on the adhesion of parts produced by fused filament fabrication. Plast. Rubber Compos. 47, 17–24. http://doi.org/10.1080/14658011.2017.1399531. Spoerk, M., Holzer, C., Gonzalez-Gutierrez, J., 2020. Material extrusion-based additive manufacturing of polypropylene: A review on how to improve dimensional inaccuracy and warpage. J. Appl. Polym. Sci. 2020, 48545. http://doi.org/10.1002/APP.48545. Stratasys sparse fill, n.d.. Sparse fill vs. solid FDM parts. https://www.stratasysdirect.com/ technologies/fused-deposition-modeling/sparse-fill-vs-solid-FDM-parts (accessed September 1, 2021). Sun, Q., Rizvi, G.M., Bellehumeur, C.T., Gu, P., 2008. Effect of processing conditions on the bonding quality of FDM polymer filaments. Rapid Prototyp. J. 14, 72–80. http://doi. org/10.1108/13552540810862028. Sweeney, C.B., Lackey, B.A., Pospisil, M.J., Achee, T.C., Hicks, V.K., Moran, A.G., Teipel, B.R., Saed, M.A., Green, M.J., 2017. Welding of 3D-printed carbon nanotube– polymer composites by locally induced microwave heating. Sci. Adv. 3, e1700262. http:// doi.org/10.1126/sciadv.1700262. Takahashi, H., Punpongsanon, P., Kim, J., 2020. Programmable filament: Printed filaments for multi-material 3D printing. In: UIST ’20: Proceedings of the 33rd Annual ACM Symposium on User Interface Software and Technology, October 2020, New York (NY, U.S.A.). Association for Computing Machinery, pp. 1209–1221. http://doi.org/ 10.1145/3379337.3415863. Tully, J.J., Meloni, G.N., 2020. A scientist’s guide to buying a 3D printer: How to choose the right printer for your laboratory. Anal. Chem. 92, 14853–14860. http://doi.org/ 10.1021/acs.analchem.0c03299. Turner, B.N., Gold, S.A., 2015. A review of melt extrusion additive manufacturing processes: II. Materials, dimensional accuracy, and surface roughness. Rapid Prototyp. J. 21, 250–261. http://doi.org/10.1108/RPJ-02-2013-0017. Turner, B.N., Strong, R., Gold, S.A., 2014. A review of melt extrusion additive manufacturing processes: I. Process design and modeling. Rapid Prototyp. J. 20, 192–204. http:// doi.org/10.1108/RPJ-01-2013-0012. Tyberg, J., Bøhn, J.H., 1999. FDM systems and local adaptive slicing. Mater. Des. 20, 77–82. http://doi.org/10.1016/S0261-3069(99)00012-6. Venkataraman, N., Rangarajan, S., Matthewson, M.J., Safari, A., Danforth, S.C., Yardimci, A., 1999. Mechanical and rheological properties of feedstock material for fused deposition of ceramics and metals (FDC and FDMet) and their relationship to process performance. In: Bourell, D.L., Beaman, J.J., Crawford, R.H., Marcus, H.L., Barlow, J.W. (Eds.), Solid Freeform Fabrication Proceedings, Austin (TX, U.S.A.). University of Texas at Austin, pp. 351–360. http://doi.org/10.26153/tsw/827. Venkataraman, N., Rangarajan, S., Matthewson, M.J., Harper, B., Safari, A., Danforth, S.C., Wu, G., Langrana, N., Guceri, S., Yardimci, A., 2000. Feedstock material property – process relationships in fused deposition of ceramics (FDC). Rapid Prototyp J. 6, 244–252. http://doi.org/10.1108/13552540010373344.

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Wang, K., Chang, Y.-H., Chen, Y.W., Zhang, C., Wang, B., 2015. Designable dualmaterial auxetic metamaterials using three-dimensional printing. Mater. Des. 67, 159–164. http://doi.org/10.1016/j.matdes.2014.11.033. Wang, J., Xie, H., Weng, Z., Senthil, T., Wu, L., 2016. A novel approach to improve mechanical properties of parts fabricated by fused deposition modeling. Mater. Des. 105, 152–159. http://doi.org/10.1016/j.matdes.2016.05.078. Wasserfall, F., Hendrich, N., Zhang, J., 2017. Adaptive slicing for the FDM process revisited. In: In: 2017 13th IEEE Conference on Automation Science and Engineering (CASE), 20-23 August 2017, Xi’an (China), pp. 49–54. http://doi.org/10.1109/COASE.2017.8256074. Wickramasinghe, S., Do, T., Tran, P., 2020. FDM-based 3D printing of polymer and associated composite: a review on mechanical properties, defects and treatments. Polymers 12, 1529. http://doi.org/10.3390/polym12071529. Wittbrodt, B., Pearce, J.M., 2015. The effects of PLA color on material properties of 3-D printed components. Addit. Manuf. 8, 110–116. http://doi.org/10.1016/j.addma.2015.09.006. Yampolskiy, M., King, W., Pope, G., Belikovetsky, S., Elovici, Y., 2017. Evaluation of additive and subtractive manufacturing from the security perspective. In: Rice, M., Shenoi, S. (Eds.). Critical Infrastructure Protection XI. ICCIP 2017. IFIP Advances in Information and Communication Technology, 512. Springer, Cham (Switzerland) http://doi. org/10.1007/978-3-319-70395-4_2. Yang, D., Zhang, H., Wu, J., McCarthy, E.D., 2021. Fibre flow and void formation in 3D printing of short-fibre reinforced thermoplastic composites: an experimental benchmark exercise. Addit. Manuf. 37, 101686. http://doi.org/10.1016/j.addma.2020.101686. Yin, J., Lu, C., Fu, J., Huang, Y., Zheng, Y., 2018. Interfacial bonding during multi-material fused deposition modeling (FDM) process due to inter-molecular diffusion. Mater. Des. 150, 104–112. http://doi.org/10.1016/j.matdes.2018.04.029. Zhang, P., Wang, Z., Li, J., Li, X., Cheng, L., 2020. From materials to devices using fused deposition modeling: a state-of-art review. Nanotechnol. Rev. 9, 1594–1609. http://doi.org/ 10.1515/ntrev-2020-0101. Zindani, D., Kumar, K., 2019. An insight into additive manufacturing of fiber reinforced polymer composite. Int. J. Lightweight Mater. Manuf. 2, 267–278. http://doi.org/10.1016/ j.ijlmm.2019.08.004.

Non-Print Items Abstract Fused deposition modeling (FDM), also known as fused filament fabrication (FFF), is a widespread additive manufacturing (AM) technology belonging to the family of “material extrusion” (ME) methods. The conventional feedstock in FDM is a thermoplastic (or wax) filament of standard diameter of either 1.75 mm or 2.85 mm. However, a key advantage of FDM is the ability to process thermoplastic-matrix composites, which enables the production of complicated objects with bespoke functionality and even the fabrication of fully inorganic parts. In principle, printing an object by FDM is relatively simple. The filament is fed into the printer and pushed into the liquefier by counter-rotating gears. After melting, the feedstock flows through the print nozzle and lends onto the base platform according to a computer-controlled path. The process is repeated layer upon layer, until the desired three-dimensional geometry is completed. Dual-nozzle printers allow multi-material printing. However, in spite of its apparent easiness, the part’s buildup is actually a complex process governed by numerous, and often inter-related, variables. After introducing the basics of FDM, this chapter details how to print an object and what parameters should be considered to reach the desired performance. Keywords Fused deposition modeling; FDM; Fused filament fabrication; FFF; Additive manufacturing; Build-up mechanism; Polymer sintering

The need for fused deposition modeling of composite materials 3.1

3

Introduction: From mono-materials to composite feedstocks in FDM

Although originally intended to process wax and thermoplastic feedstocks, fused deposition modeling (FDM), aka fused filament fabrication (FFF), can be readily extended to process thermoplastic-matrix composite materials. Incorporating an appropriate filler endows the polymer matrix with enhanced mechanical properties or with functional features, such as thermal or electrical conductivity (Park and Fu, 2021). Also, composite filaments with a very high filler loading (typically exceeding 45 vol%) enable the fabrication of fully organic parts after printing, removing the polymer binder and sintering (Gonzalez-Gutierrez et al., 2018). The shift from neat polymer feedstocks to composite filaments is anticipated to overcome the functional limitations of commodity plastics and thus to speed up the progress of FDM from “rapid prototyping” to “industrial manufacturing”. After a critical assessment of the key properties of popular mono-material filaments, this chapter will survey the emerging trends in FDM of composite materials taking into account both the scientific literature and the new products available in the marketplace. Numerous case studies and practical applications will demonstrate how composite materials are already changing the state of the art in FDM.

3.2

Mono-material filaments

The greatest part of commercial filaments for FDM is presently mono-material. Table 3.1 compares some basic properties of ordinary FDM materials, including poly(lactic acid) (aka polylactide, PLA), acrylonitrile butadiene styrene (ABS), polyamide (PA, “nylon”), polyethylene terephthalate (PET) and its glycol-modified version (PETG), and flexible plastics (thermoplastic elastomers, TPEs; thermoplastic polyurethane, TPU). The general structure of these polymers is shown in Fig. 3.1. Table 3.1 and the information provided hereafter have been completed taking into consideration several sources available on-line (3d matter, n.d.; Simplify3d, n.d.; Treatstock, n.d.) and in the archival literature (Calignano et al., 2017; Park and Fu, 2021; Peterson, 2019). Nonetheless, any comparison should be considered with caution, because materials properties (e.g., mechanical performance, density, maximum service temperature) and technological parameters (e.g., ease of print, printing temperature, recommended post-processing) may vary with the specific composition, grade and purity of the polymer. Also, a certain amount of variability exists as a result of the extrusion conditions applied to produce the filament. Fused Deposition Modeling of Composite Materials. DOI: https://doi.org/10.1016/B978-0-323-98823-0.00004-4 c 2023 Elsevier Ltd. All rights reserved. Copyright 

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Table 3.1 Main properties of common thermoplastic materials used to produce FDM filaments. General properties Extrudability Printability

Colors

ABS Commodity thermoplastic Excellent Easy; May warp

PET/PETG General purpose thermoplastic Very good Easy

PA (Nylon) Very durable thermoplastic Very good Difficult; May produce odor

TPE/TPU Rubber-like material

Produces odor

Good Very difficult

Odorless 190–210

210–250

250–260

235–260

210–225

Good Easy to post-process

Medium Easy to post-process

Medium Easy to post-process

Can be smoothed with sandpaper

Can be smoothed with sandpaper

Can be smoothed with sandpaper

Medium Moderately easy to post-process

Low Difficult to post-process

Can be baked to improve strength

Can be finished with acetone vapors

Available in many colors

Can be machined Available in many colors

Easy to paint with acrylics

Easy to paint with acrylics

Good silky finish Can be smoothed with sandpaper

Easy to paint with acrylics

Easy to paint/dye

Can be semi-transparent

(continued on next page)

Fused Deposition Modeling of Composite Materials

Printing Temperature (°C) Visual quality Post-processing/ finishing

PLA Bio-based thermoplastic Good Very easy; Low shrinkage

PLA Bio-degradable, not durable Mechanical properties High stiffness (rigid) Brittleness Chemical durability

Low impact resistance

ABS Durable

PET/PETG Durable, food-grade

High mechanical properties

Good abrasion resistance

Good toughness Good abrasion resistance

PA (Nylon) Durable, but hygroscopic High strength

TPE/TPU Good resistance to oils and grease Very flexible

Very high impact resistance

High mechanical properties

High wear resistance

Very high impact resistance

Thin parts may be flexible

98

Moderate heat resistance 73

80–95

Good wear/abrasion resistance Moderate heat resistance 60–74

1.04 $

1.23 $$

1.06–1.14 $$

1.19–1.23 $$

Heat resistance

Low heat resistance

High heat resistance

Maximum service temperature (°C) Density (g/cm3 ) Cost

52 1.24 $

High heat resistance

The need for fused deposition modeling of composite materials

Table 3.1 Main properties of common thermoplastic materials used to produce FDM filaments—cont’d

41

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Fused Deposition Modeling of Composite Materials

Figure 3.1 General structure of (A) PLA, (B) ABS, (C) PET, (D) PA, and (E) TPU.

PLA (Fig. 3.1A) is the reference material for FDM as it is extremely easy to print. Good-quality parts can be also obtained with hobbyist-like printers, since PLA has a relatively low printing temperature and, thanks to its thermal properties, does not strictly require a heated platform (Park and Fu, 2021). Also, PLA is inexpensive and readily available in a variety of colors and variants. As a plus, PLA is one of the most environmentally friendly filaments on the market. Since PLA derives from crops such as corn and sugarcane, it is renewable and biodegradable (Tümer and Erbil, 2021), and, unlike other commonly used plastics in 3-dimensional (3D) printing, it does not emit any foul odors when heated up during printing. Also, with respect to ABS and other common thermoplastic filaments for FDM, the emission of particles and other by-products when printing is lower, because the printing temperature of PLA is lower (Manoj et al., 2021). As such, the usage of PLA may be regarded as potentially less harmful for human health, especially when printing is conducted in an enclosed space or even at home (Cowley et al., 2019). The mechanical properties are relatively good, with the high elastic modulus being one of the main advantages of PLA (parts printed in the x-y plane typically have a Young’s modulus around 3 GPa). However, PLA is very brittle and scarcely durable (Liu et al., 2019). In fact, PLA can be bio-degraded by simple hydrolysis of the ester bond, even without enzymes. This represents a shortcoming for long-term usage, but it is also a key advantage in biomedical applications where bio-resorption is needed, since the degraded hydrolysate is nontoxic and is easily metabolized by the human body (G. Li et al., 2020). Also, the biodegradation rate can be modulated to fit the natural healing rate of the original tissue (DeStefano et al., 2020). The printability and non-toxicity of PLA have proven essential to fabricate face masks and other personal protection equipment (PPE), as well as biomedical devices during the pandemic (Vaˇnková et al., 2020). PLA, with a density around 1.24 g/cm3 , is much denser than other polymers commonly used in FDM, including ABS (whose density is around 1.04 g/cm3 ) and polyolefins (around or

The need for fused deposition modeling of composite materials

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even less than 1 g/cm3 ). This may be a disadvantage whenever light-weight structures are sought after as the main goal of printing, but this is also a key advantage for recycling, since PLA can be easily sorted out from ordinary thermoplastic waste based on its density and then processed separately (Ahmed et al., 2020; Niaounakis, 2019). ABS (Fig. 3.1B) is an “umbrella” name that includes many blends and copolymers of acrylonitrile-, butadiene-, and styrene-containing polymers (Peterson, 2019). ABS was one of the first plastics to be used with industrial 3D printers and its long-lasting success depends on its favorable balance between good mechanical properties and low cost. Though relatively easy to print, ABS is more demanding as compared to PLA, because it requires a higher printing temperature to account for its higher glass transition temperature, and a heated build platform, to minimize the thermal stresses that may develop because of the tendency to shrink upon cooling (Park and Fu, 2021). However, ABS offers very good toughness and impact resistance (for instance, the extreme durability of ABS is the main reason why classic LEGO bricks are made from this material (Lego, n.d.)). The maximum working temperature is almost two times as much as that of PLA. PET (Fig. 3.1C) is a commodity thermoplastic very popular for the production (by injection and blow molding) of plastic bottles. Due to its good thermal properties, PET cools down very efficiently with negligible warpage. It is a semi-rigid material with good impact resistance, but the relatively low hardness makes it sensitive to scratch and wear. PETG is the glycol-modified version of PET, where the addition of glycol makes PETG even less brittle and more pliable than PET, but also more sensitive to moisture uptake (Park and Fu, 2021). Although, strictly speaking, PET and PETG have different properties, many materials companies and 3D printers use the names interchangeably. PA (Fig. 3.1D) has very good mechanical properties, excellent wear resistance and the best impact strength among all non-flexible thermoplastic filaments (Park and Fu, 2021). PA requires printing temperatures around 250°C (Calignano et al., 2017), which makes it unsuitable for hobbyist 3D printers. Nowadays, some brands offer chemicallymodified PA that can be printed at much lower temperatures, around 220°C. However, the easier processability usually comes at the expenses of the mechanical properties. Printing with PA may be a difficult task due to the poor inter-layer bonding. Also, PA is extremely hygroscopic and the correct storage of filaments is key to preventing the absorption of moisture that may be responsible for several print and quality issues. Flexible plastic filaments are made of TPEs, which are blends of hard plastic and rubber in various ratios. There are several types of TPEs, however TPU (Fig. 3.1E) is currently the most popular elastomer for the extrusion of FDM filaments. Very often, the terms TPE, TPU, flexible filaments and some popular trade names such as NinjaFlex (NinjaTek, n.d.) are used interchangeably in the market, which may cause some confusion. It is important to remark that the degree of elasticity largely depends on the specific type of TPE and on its chemical formulation. Oftentimes, the term “flexible” is also used to mean different properties, since some “flexible” filaments can be compliant to bending loads, like a car tire, whereas other filaments can be stretchable to axial loads, like a rubber band. Fig. 3.2 (which is based on the data from (3d matter, n.d.)) provides a graphical summary of the main characteristics of common thermoplastic materials in FDM.

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Fused Deposition Modeling of Composite Materials

Figure 3.2 Graphical comparison of the main characteristics of common thermoplastic materials in FDM, including PLA, ABS, PET, PA, and TPU (data elaborated from (3d matter, n.d.)).

High-performance mono-material filaments, such as poly-ether ether ketone (PEEK) and poly-ether ketone ketone (PEKK), offer enhanced mechanical and thermal properties, whilst others possess inherent flame retardancy, such as polyetherimide (PEI), or improved ultraviolet (UV) and weather resistance for outdoor usage, such as acrylonitrile styrene acrylate (ASA). However, their implementation is non-trivial, not only because they are more expensive than ordinary FDM materials, but they also require customized printing hardware (Valino et al., 2019). In fact, since the liquefier of most commercially available FDM printers has a maximum operating temperature of around 300°C, high melting point materials cannot be processed using standard equipment (Mohan et al., 2017). For instance, PEEK would be ideal for printing dental implants, owing to its low moisture absorption, or cartilage substitutes, since the mechanical properties, especially tensile stiffness and strength, are comparable to those of natural tissues, but it requires dedicated printers with a nozzle temperature between 360°C to 400°C and a heated bed at 120°C (Haleem and Javaid, 2019; G. Liu et al., 2020). PEKK is also a semicrystalline polymer, whose melting temperature (around 385°C) is slightly higher than that of PEEK (around 343°C). However, the crystallization rate of PEKK is around three orders of magnitude lower than that of PEEK, which allows PEKK to be processed like an amorphous polymer (Park and Fu, 2021). Polyolefins, such as polyethylene (PE) and polypropylene (PP), are rarely seen in FDM, as they pose substantial challenges in terms of adhesion to the base platform, geometry retention and warpage upon printing. As a consequence, few polyolefin filaments are available in the marketplace and, oftentimes, commercial filaments are heavily modified for the ease of printing (Nabipour, 2020; Spoerk et al., 2020).

The need for fused deposition modeling of composite materials

45

Figure 3.3 Popularity of different AM materials in 2019 and in 2020. Interviewees were asked: “Which 3D printing materials do you use?” (Sculpteo, 2019, 2020). NOTE: the graph collects and elaborates the statistics originally published separately for each year.

3.3

Research trends in composite feedstock in FDM

As shown by the results of the survey conducted annually by Sculpteo (2019, 2020), and elaborated in Fig. 3.3, thermoplastic polymers (“plastics”) are nowadays the most popular materials used in AM. This predominance of thermoplastics largely depends on the fact they are the typical feedstock materials for FDM, as well as for other widespread 3D printing technologies, including selective laser sintering (SLS) and multi-jet fusion (MJF). On the other hand, according to recent statistics presented in Fig. 3.4 (based on the outcomes of the survey conducted by Sculpteo (2019, 2020)) AM is still being used primarily in a prototyping context or for very small runs. As illustrated in Fig. 3.5, the statistics published by HUBS (2021) suggest that 64% of professionals are still choosing AM for small production volumes up to 10 parts, whereas just 2% of those surveyed are scaling up AM for larger batches above 1000 parts (HUBS, 2021). The development of new materials with advanced functionality is expected to play a fundamental role to promote the further uptake of AM in industrial production (Dilberoglu et al., 2017). This urgent need for new materials particularly applies to FDM. In fact, despite the advantages of standard thermoplastic filaments, including low density, good processability, availability and inexpensiveness, AM components made from pure polymers are mainly considered as conceptual templates and educational models. Such polymer parts are generally produced by FDM for design

46

Fused Deposition Modeling of Composite Materials

Figure 3.4 Main applications of AM in 2019 and in 2020. Interviewees were asked: “What is the purpose of your 3D prints?” (Sculpteo, 2019, 2020). NOTE: the graph collects and elaborates the statistics originally published separately for each year.

Figure 3.5 Typical production volumes in AM reported in 2020–2021: 64% of users still prefer AM for small production volumes up to 10 parts. Data elaborated from (HUBS, 2021).

The need for fused deposition modeling of composite materials

47

verification, functional testing, and rapid prototyping, but they are rarely employed for structural purposes due to the lack of stiffness and strength in load-bearing applications (Rahim et al., 2019; Saroia et al., 2020; Valino et al., 2019; Zhao et al., 2019). Also, PLA, ABS and other common thermoplastics for FDM have functional limitations, such as non-conductivity (electrical and thermal), that impair their advancement in industrial production (Tümer and Erbil, 2021). The research is now open to identify new materials that are able to break these barriers. Polymer blends may represent an option (Torrado et al., 2015), but the combination of two polymers is not always effective to meet the service requirements for demanding applications in aerospace, automotive, building/construction, oil, gas and medical industries (Siqueiros and Roberson, 2017; Valino, et al. 2019). For this reason, composite filaments are slowly but steadily displacing neat polymer filaments. The addition of particles, fibers, and nanoreinforcements has the disruptive potential to improve the mechanical properties of thermoplastics, to extend their functionality and, at the same time, to stabilize their shape and geometry upon printing (Caminero et al., 2018; Hwang et al., 2015a, 2015b; F. Li et al., 2020; Love et al., 2014; Weng et al., 2016; Word et al., 2021). In this regard, a critical analysis of the available literature reveals two different trends: the first being the development of functional polymer-matrix composites, where a filler can be added to improve the performance of the polymer matrix; and the second being the formulation of composite filaments directed to produce fully inorganic components via a three-step (shaping/printing + debinding + sintering) process, where the polymer matrix is a sacrificial binder. It is worth noting that the latter approach starts from a composite feedstock, but the polymer phase is removed to achieve a fully inorganic part and the final product is not a composite anymore (unless it consists of multiple inorganic phases). As an additional option, the composite feedstock may contain a soluble filler (space holder) that, after printing, can be easily removed to create a controlled porosity (Shalchy et al., 2020). In this case, a composite filament can result in a neat polymer scaffold.

3.3.1 Polymer-matrix functional composites In polymer-matrix functional composites, the polymer matrix itself is targeted as the desired functional material in the component being produced and appropriate fillers are introduced in order to enhance the thermo-mechanical properties of the neat polymer or to provide it with new features, such as thermal or electrical conductivity or bioactivity (Castles et al., 2016; Masood and Song, 2004, 2005; Park and Fu, 2021; Tümer and Erbil, 2021; Wang et al., 2021). For example, in polymer-ceramic composites, hydroxyapatite (HAp) is widely used for biomedical applications, whereas carbonaceous fillers are the preferred option for electrical conductivity. According to a recent review (Mohan et al., 2017), if polymermetal composites are considered instead, iron and aluminum powders are often chosen to reinforce ABS due to the ferromagnetic properties and high wear resistance of the former, and due to the high conductivity and self-lubricating properties of the latter.

48

Fused Deposition Modeling of Composite Materials

In parallel, the widespread use of software packages that work with “open parameters”, thus enabling the optimization of the printing parameters for new materials, and the progressive reduction of the cost of FDM printers have prompted experimental research to test these new composite materials and to optimize the corresponding processing conditions (Dudek, 2013; Melenka et al., 2016; Wittbrodt and Pearce, 2015).

3.3.2 FDM of fully inorganic parts from composite feedstock The other major pathway forward in FDM is the production of inorganic components. This approach utilizes a composite filament with a high inorganic filler loading, generally exceeding 45 vol%. After printing, the polymer matrix, which is typically a two-part mixture, can be removed by solvent, thermal, and/or catalytic methods; subsequently, the inorganic phase (or the inorganic phases, in case inorganic composites are targeted after sintering) is sintered into a fully solid part. In this case, the polymer merely acts as a temporary binder to allow for 3D printing a metal or ceramic part having a complicated shape that otherwise could not be produced by conventional methods. Although the expression “fused deposition of metals” (FDMet) had initially been introduced to describe the production of metal parts via FDM, the term “fused deposition of ceramics” (FDC) was often extended also to metal components (Gonzalez-Gutierrez et al., 2018). However, the new label “fused deposition modeling and sintering” (FDMS) is emerging to account for metal-based systems (B. Liu et al. 2020). The process to manufacture inorganic objects via material extrusion-based AM methods as a whole is also called “shaping, debinding, and sintering” (SDS) (GonzalezGutierrez et al., 2018). Comprehensive reviews of the SDS method have been recently published by Gonzalez-Gutierrez et al. (2018) and by Rane and Strano (2019), who recalled the challenges connected to this technology, which needs extremely high filler loadings in the composite feedstock, requires complicated formulations of the polymer binder to induce a controlled debinding, and often causes relevant dimensional changes upon sintering the printed part. The fabrication of inorganic components was the subject of widespread investigations in the early stages of FDM development, especially at the end of the 1990s. Nowadays, the technique is in high demand again and the number of “metal filaments” on the market is rapidly growing, as witnessed for example by the recent launch of the 316L Ultrafuse filament as a key part of the strategic development of the “Forward AM” brand of the BASF Group (https://forward-am.com/). Other filaments with high inorganic filler loadings are commercially available, usually containing stainless steel (either 316L or 17-4PH) or ceramic (mullite, fused silica, titania, yttria stabilized zirconia, Si3 N4 ) particles.

3.4 Commercial composite filaments In order to demonstrate the advantages of polymer-matrix composites over neat polymers, Figs. 3.6–3.9 map the properties of FDM parts printed from some commercial polymer and polymer-matrix composite filaments. Care should be taken when reading

The need for fused deposition modeling of composite materials

49

Figure 3.6 Correlation between density and tensile modulus of parts printed from commercial filaments. ABS, acrylonitrile butadiene styrene; ASA, acrylonitrile styrene acrylate; Composites, composite materials; CPE, co-polyesters; HIPS: high impact polystyrene; PA: polyamides; PC: polycarbonate, and polycarbonate-based polymer blends; PEI, polyetherimide; PEKK/PEEK: polyether ketone ketone and polyether ether ketone; PLA, poly-lactic acid; PP, polypropylene; PPS/PSU/PPSU, polyphenylene sulfide, polysulfone, and polyphenylsufone; PVDF, poly(vinylidene) fluoride; Supports, support materials; TPU, thermoplastic polyurethanes and thermoplastic elastomers. NOTE: tensile samples were printed and tested under different conditions as explained in the text.

these charts, since there is not yet an international standard to define a consistent and repeatable way to determine the mechanical properties of FDM parts. As such, the values reported in the technical data sheets from different suppliers often refer to parts that have been printed under different parameters (temperature of the nozzle, temperature of the build platform, raster angle, air gap, layer thickness, etc.) and tested under different conditions. Further, as a rough approximation, in drawing these graphs no distinction has been made between samples printed “flat” or “on edge” on the base platform. In other words, parts that have been printed parallel to the growth direction have been discarded, but the potential anisotropy in the x-y plane (parallel to the base platform) has been neglected. This is consistent with the general recommendation that printing in the upright position should be avoided, especially if tensile strength is key, whereas minimal variation is observed in tensile performance between flat and on-edge orientations (Gordelier et al., 2019). Moreover, it should be underlined that, due to their peculiarities, continuous-fiber reinforced materials have not been included. Taking into consideration these simplifying hypotheses and limitations, Figs. 3.6–3.9 are not meant

50

Fused Deposition Modeling of Composite Materials

Figure 3.7 Correlation between tensile strength and tensile modulus of parts printed from commercial filaments (for full list of abbreviations: see caption of Fig. 3.6). NOTE: tensile samples were printed and tested under different conditions as explained in the text.

to provide a ranking between commercial FDM materials, rather, they are intended to visualize some general trends through the properties of about 100 materials that are currently available in the market (the full list of materials is reported in Table 3.2). The first element that emerges from Figs. 3.6–3.9 is that much attention is being paid to the tensile properties of FDM parts. In fact, basically all data sheets report the tensile properties of the printed parts at least in one direction. For this reason, all charts in Figs. 3.6–3.9 use the “tensile modulus” as the main coordinate. Whereas the tensile modulus of most polymers is below 3 GPa, advanced polymers such as PEEK and PEKK are slightly stiffer and can have values up to about 4 GPa. However, reinforcing fibers are required to further increase the tensile modulus up to about 10 GPa. Interestingly, as shown in Fig. 3.6, the augmented stiffness implies just a moderate increase in density. Although the density of many composites tends to be slightly higher than that of lightweight polymers such as PP, PA and some thermoplastic elastomers, it is still comparable to that of PLA and most technical polymers. The density of some composites can even be lower than that of some specialized polymers such as poly(vinylidene) fluoride (PVDF). The relatively low density of most commercial composites is due to the low volume fraction of the filler, typically lower than 30%, and to the prevailing usage of chopped carbon fibers (whose density is around 1.8 g/cm3 ) instead of glass fibers (2.6 g/cm3 ) or other mineral fillers. However, fillers may have a contrasting effect on the tensile resistance. In fact, the comparison between Fig. 3.7

Table 3.2 List of commercial filaments considered to draw Figs. 3.6–3.9.

Stratasys

Ultimaker

Ultrafuse

Filament name Prusament PETG; Prusament PLA

Link to technical data sheets https://help.prusa3d.com/en/category/ material-guide_220

Technical data sheets accessed on June 23, 2020 ABS-M30; ABS-ESD7; ABSi; ABS-M30i; ABSplus - P430; Antero 800NA; Antero https://www.stratasys.com/materials/ 840CN03; ASA; Diran 410MF07; PPSF/PPSU; FDM Nylon 12; FDM Nylon 12CF; FDM search Nylon 6; PC; PC-ABS; PC-ISO; PLA; FDM TPU 92A; ULTEM 1010; ULTEM 9085 Technical data sheets accessed on Natural; ULTEM 9085 Black June 23, 2020 Ultimaker CPE; Ultimaker TPU 95A; Ultimaker ABS; Ultimaker Nylon; Ultimaker PC tr.; https://ultimaker.com/materials Ultimaker PC b/w; Ultimaker PLA Technical data sheets accessed on June 23, 2020 Ultrafuse ABS Fusion+; Ultrafuse ABS; Ultrafuse ASA; Ultrafuse BVOH; Ultrafuse HiPS; https://www.ultrafusefff.com/materialUltrafuse PA; Ultrafuse PAHT CF15; Ultrafuse PET CF 15; Ultrafuse PET; Ultrafuse PLA data/ PRO1; Ultrafuse PLA; Ultrafuse PP GF30; Ultrafuse PP; Ultrafuse rPET; Ultrafuse TPU Technical data sheets accessed on 85A; Ultrafuse Z PCTG June 25, 2020

Markforged Nylon White; Onyx; Onyx FR

https://markforged.com/materials Technical data sheets accessed on June 30, 2020 https://www.3dxtech.com/tech-datasheets-safety-data-sheets/

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3DXTECH 3DXMAX ABS; ABScent Transparent ABS; Firewire Flame retardant ABS; 3DXSTAT ESD-ABS; ECOMAX PLA; 3DXSTAT ESD-PLA; 3DXFLEX TPE made using PEBAX elastomer; 3DXSTAT ESD-TPC (90A); 3DXMAX HIPS; 3DXMAX ASA; ThermaX High Technical data sheets accessed on Temp PETG; 3DXMAX PETG; 3DXPro Low-Gloss PETG; 3DXSTAT ESD-PETG; iOn October 18, 2020 Nylon 6 Copolymer; 3DXSTAT ESD-PC; 3DXMAX Polycarbonate; 3DXMAX PC/ABS; 3DXMAX PC/ASA; FluorX PVDF; 3DXSTAT ESD-PVDF; ThermaX PPS; 3DXSTAT ESD-PPS; ThermaX PSU; ThermaX PPSU; ThermaX PEI (Made using ULTEM 1010); ThermaX PEI (Made using ULTEM 9085); 3DXSTAT ESD-Ultem; ThermaX PEEK; ThermaX PEKK; 3DXSTAT ESD-PEKK; AmideX PA6-GF30 Glass Fiber Nylon; GlassX Glass-Fiber PETG; CarbonX Carbon Fiber ABS; CarbonX CF-PLA; CarbonX Carbon Fiber PETG; CarbonX Carbon Fiber Nylon (Gen3); CarbonX Carbon Fiber Polycarbonate; CarbonX Carbon Fiber Ultem PEI; CarbonX Carbon Fiber PEEK; CarbonX Carbon Fiber PEKK (aerospace); CarbonX Carbon Fiber PEKK (industrial)

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Producer Prusa

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Figure 3.8 Correlation between elongation at break and tensile modulus of parts printed from commercial filaments (for full list of abbreviations: see caption of Fig. 3.6). NOTE: tensile samples were printed and tested under different conditions as explained in the text.

and 3.8 shows that the addition of inorganic fillers generally increases the tensile strength (defined here as the maximum tensile stress) at the expense of the elongation at break. On the other hand, this behavior is predictable, because inorganic fillers generally act as rigid inclusions that increase the stiffness and strength, but reduce the ductility as compared to the neat polymer matrix (Gibson, 2012). The available data about other mechanical properties is more fragmentary, and Fig. 3.9 specifically collects some values of impact strength as measured according to ASTM D256 (latest version of the standard: (ASTM D256, 2018). In this case, the performance of composite materials is scattered on a wide range of impact strength values. However, the impact response is mainly governed by the matrix and the optimization of the impact strength is largely dependent on the adoption of the appropriate polymer matrix, such as TPE and TPU. Overall, Figs. 3.6–3.9 are interesting to highlight some relevant features about the mechanical behavior of commercial FDM materials, especially composite ones. However, these graphs let emerge only one facet of a bigger picture, since composite materials have the potential to address a diversity of functional requirements, other than just improving the mechanical response. As shown by the literature, composite materials can be printed by FDM to obtain parts with complicated geometries, and at the same time with new properties that cannot be achieved by common polymers, such as thermal or electrical conductivity or bioactivity.

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Figure 3.9 Correlation between impact strength and tensile modulus of parts printed from commercial filaments (for full list of abbreviations: see caption of Fig. 3.6). NOTE: samples were printed and tested under different conditions as explained in the text.

3.5

Applications and case studies

FDM of composite materials is experiencing a very fast growth in numerous industries. If AM is generally recognized as the key enabling technology of the Industry 4.0 revolution, the shift from mono-material feedstocks to composite feedstocks and the development from mono-material to multi-material printing are driving innovation in future industries such as biomedicine, space and defense, where new functionality is required in addition to customization (Dilberoglu et al., 2017). The websites of manufacturers of FDM printers and filaments report a wealth of information and case studies, where FDM of composite materials has been implemented at various readiness levels to solve industrial needs and supply chain bottlenecks. Some examples are summarized in the following paragraphs, but many more can be found on-line with a simple search. The cases reported below have been chosen to provide the reader with a big picture of FDM’s potential when applied to composite materials and therefore they cover numerous businesses, from the fabrication of advanced light-weight structures, to the design of biomedical scaffolds with embedded functionality, to the creation of toys. A paragraph outlines the formulation of personalized pharmaceutical products. Whereas most active principles are chemical compounds rather than fillers, drug-loaded filaments for 3D pharming present substantial analogies with composite feedstocks and the technical difficulties observed upon extruding and printing are also very similar. The

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last two examples proposed in this chapter, namely an industrial lifting tool developed and currently run by Wärtsilä, and the replica of a legacy carburetor produced by Tecron, have been chosen from the 2020 Additive Trends Report recently published by Markforged (Markforged, 2020) and from Markforged’s library comprising more than 100 case studies (Markforged applications, n.d.). These applications have been selected to showcase the impact that FDM of polymer-matrix composite materials (lifting tool) and of inorganic parts (carburetor) is already having on today’s manufacturing. However, these are just two examples out of the many that are currently being posted by Markforged (Markforged applications, n.d.), Stratasys (Stratasys case studies, n.d.), Anisoprint (Anisoprint cases, n.d.), Desktop Metal (Desktop Metal case studies, n.d.), and many other FDM providers.

3.5.1 Colorful filaments for new toys and toy rescue One of the main advantages of FDM is that filaments are available in a whole range of different colors. Surprisingly, the literature is mainly interested in mechanical performance and high-tech properties such as conductivity, whereas the color of feedstock materials is often neglected, likely because it is just considered an ornamental feature. However, this approach to color is misleading, because the capability of producing colored objects is actually crucial to foster the uptake of AM for everyday-life objects. In this regard, the history of colors in the automotive field may be exemplary. Henry Ford’s statement that “Any customer can have a car painted any color that he wants so long as it is black” (or the abbreviated citation “Any color so long as it is black”) has become proverbial, but the truth is that the first Ford models, identified by the letters A to T, actually came in many different colors. For instance, the original Model A 19031904 and, later one, the Model R were available only in red, the Model F was rich deep green with yellow running gear, and the Model K was royal blue (OpLaunch, 2015). The Model T itself was made available in 4 to 5 different colors between 1908 and 1914. All of them were dark shades, but none of them was black. Then, from 1914 to 1926, all Model T cars were indeed sold in black, but this was due to a purely economic reason. Ford wanted to produce as many cars as possible in the least amount of time in order to satisfy the ever-increasing demand for cars on the American market and black paint at the time was the most affordable and the fastest to dry. Since then, the selection of car colors has expanded to such an extent that nowadays it covers almost any shade of the spectrum (still with a preference for darker shades or for neutral colors like silver, though). The attention of car industry to color is not surprising, because the color of their possessions is recognized as one of the most important ways that people have to express themselves, their individuality and their personal taste (Veer, n.d.). Even more so, color is key in toys, as proved by the strategic importance of the colors’ palette in LEGO bricks. At the beginning of the LEGO history, bricks were already available in 7 different colors (white, grey, black, blue, red, yellow and, though rarely, green), which grew to 16 by the early 1980s.The palette was progressively extended until the late 1990s and then substantially revised around 2005, when the tints were changed from

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a yellowish-grey dominant to a bluish-grey dominant (the so-called “bley”). Starting 2016, the official LEGO colors’ palette includes 39 different solid colors, 3 metallic colors, 1 glowing color, and 14 translucent colors. In spite of the impressive portfolio of LEGO bricks, devotees and professionals are asking for further improvement, especially new colors and more consistency (Alphin, n.d.). In FDM, the importance of color options is highlighted by the availability of filaments in almost any color. The development of new feedstock with fancy features such as color change or luminescence, the possibility of coembedding multiple functionalities such as antistatic and antibacterial, and the multi-material printing capability are some of the advantages of FDM, in addition to its affordability, that have attracted the attention of the toy industry. FDM printers are the preferred choice for hobbyists that want to create their own figurines and gadgets (Petersen et al., 2017). However, 3D printing toys may be a real business, not just an amusement. The growing interest of the toy industry towards AM and, in particular, towards FDM is prompted by three main scenarios that are partly interconnected. First of all, AM and especially FDM are ideal solutions to cut down the lead time from design to prototype. For example, the adoption of FDM to create prototypes, demonstration models and even functional components has shortened the time required to develop a drone from one year or more to just around 8 months (Stratasys for toy company, n.d.). Secondly, AM enables the production of personalized toys, and customization substantially increases the added value of toy products. This was one of the core businesses of the Horizon 2020 iBUS project, which applied the “Design your play” concept to invent and fabricate customized toy articles (CORDIS, 2021). Thirdly, with the implementation of FDM short products series become feasible for public places with special needs, such as kindergartens, or with limited access, such as hospitals. In particular, toys are extremely important in hospitals, not just as a source of enjoyment for children, but also as therapeutic aids. However, in order to avoid the potential spread of infections, toys should be new and washable. Also, they should be non-toxic and durable, so that they do not break easily (León-Cabezas et al., 2017). León-Cabezas et al. (2017) proved that additives can be mixed with PLA to obtain printable filaments with thermo/photo-chromic change effects, luminescence can be imparted to PLA and TPU, and analogously ABS can be modified with nano-sized zinc oxide suspensions or powders to induce antimicrobial response. Depending on their specific composition, additives may cause side-effects such as the altered melt flow index (MFI, which is representative of the flowability of the material at high temperature) and the reduced flexibility of the functionalized filaments as compared to the neat polymer counterparts. However, all filaments could be printed and the toy demonstrators passed the tests according to EN71 Safety Toy Standard, Part 1, physical and mechanical safety (EN 71-1, 2014), and Part 3, chemical safety, migration of certain elements (EN 71-3, 2013). Printed prototypes included a PLA jig-saw puzzle whose color changed in the sun light, a teether toy that became luminescent in the dark and a toy horse with antimicrobial properties against Staphylococcus Aureus and Escherichia Coli. As remarked by León-Cabezas et al. (2017), in principle comixing more than one additive at the same time would enable the obtainment of multifunctional filaments, that combine, for example, color change or luminescence with

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antimicrobial properties, thus paving the way to the fabrication of personalized toys that are at once nice, enjoyable and safe. In addition to fabricating customized toys, new businesses in the toy industry are drawing on the suitability of AM to produce spare parts for repairing old toys. Quite often, spare parts for toys do not exist at all, or they are not produced individually. Implementing an AM-based approach allows single parts to be printed on-demand, when they are needed and where they are needed, and this is expected to revolutionize the whole supply chain (Sculpteo on 3D printing toys, n.d.). Dagoma, a French company specializing in 3D printing services (https://www.dagoma3d.com/en_US/), has launched the “Toy rescue” initiative that aims at reducing the volume of waste toys (https://toy-rescue.com/). The inspiring idea of the project is: “Take care of your toys, Take care of the planet”. According to Dagoma experts, oftentimes toys are thrown away not because they are completely destroyed, but just because they have a single broken or missing part. The first step of the initiative was therefore to identify the most frequently broken or missing parts on toys over the last 40 years. Subsequently, Dagoma experts created an inventory of stl files to print such parts and made them freely available on-line. Different categories of spare parts account for characters, dolls, board games, vehicles, and “other stuff” (game consoles, for instance). Particular attention has been paid to plastic toys, because the defective parts can be easily printed by FDM with common colored feedstock materials such as ABS and PLA. Some important businesses in the toy industry are already offering print-on-demand or repair services by AM. For example, the toy store chain Toys’R’Us has agreed a partnership with PieceMaker Technologies to install 3D printers in some stores that might be accessed to create customized toys, spare parts or even wearables (Sevenson, 2014). In spite of its increasing success, the widespread adoption of AM to fabricate or to repair toys is expected to pose substantial issues in terms of intellectual properties. Printing a spare part of a toy means printing a part of an existing object that is likely protected, as the company manufacturing the toy may hold the rights for it. Whereas 3D scanning and printing a protected object without authorization may constitute counterfeited reproduction, making a new design file is not considered reproduction, since it is done independently by the 3D software user. Nonetheless, 3D printing a protected object or a part of a protected object must always comply with intellectual property regulations, to avoid unauthorized copies, or denatured objects that deviate from the intended original form, shape and function (Sculpteo on 3D printing toys, n.d.). Printing colored filaments may also bring about technical difficulties, because pigments and other additives are known to change the thermal behavior of the neat polymer matrix, to modify its crystallization and ultimately to affect the mechanical properties of the printed parts (Tanikella et al., 2017). As previously mentioned, LeónCabezas et al. (2017) observed that the MFI of PLA doubled with the addition of 5 wt% of thermo- and photo-chromic additives to induce color change, and it almost trebled with 10 wt% of luminescent masterbatch. The effect of the luminescent masterbatch on the MFI was so pronounced due to its ethylene-vinyl acetate (EVA) matrix, which is known to deeply modify the original behavior of PLA (Singla et al., 2017). The

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additives also increased the Young’s modulus and reduced the tensile strength of the composite filaments. As for commercial filaments, Wittbrodt and Pearce (2015) compared the properties of parts obtained with an open-source printer starting from commercial PLA filaments in different colors. As remarked by Wittbrodt and Pearce (2015), the exact composition of commercial filaments is usually unknown and therefore it is very difficult to identify the reasons behind the color effect. However, one of the main findings of the research conducted by Wittbrodt and Pearce (2015) was that dyes change the crystallinity of neat PLA, thus changing all its properties. Also, Wittbrodt and Pearce (2015) suggested that, since the thermal behavior of PLA is affected by the presence of dyes, each PLA colored filament should have a different printing temperature to obtain optimal tensile properties. Similarly, Spina (2019) compared a transparent PLA filament, Crystal Clear, and a black PLA filament, Onyx Black. Although the molecular weight distribution was basically the same for both materials, the different degree of crystallinity led to different thermal and rheological properties, as already observed by Wittbrodt and Pearce (2015). After printing, samples made with Crystal Clear featured higher elastic modulus and higher tensile strength than their Onyx Black counterparts (Spina, 2019). The presence of pigments and dyes is also known to change the tribological properties of FDM parts, as proved by Hanon et al. (2019) who registered a higher coefficient of friction and a more severe average wear for grey PLA parts than for white counterparts. Besides the mechanical properties, the geometric accuracy and surface roughness of PLA parts are also influenced by the color of the feedstock filaments. Valerga et al. (2018) argued that the printing quality, expressed in terms of dimensional accuracy and surface finish, is usually superior for light colors, such as transparent (natural) and grey, than for dark colors, such as pink and green, because the substantial presence of additives needed to obtain deep colors hinders the molecular chain mobility and hence the ability of the polymer to receive the intended shape (Valerga et al., 2018). However, Cicala et al. (2018) reached contrasting results. They considered three PLA filaments in different colors and observed that all of them had similar rheological behavior under high shear rate, which represents the condition of the molten polymer when flowing through the nozzle. However, under low shear rate to simulate the condition of the polymer after flowing through the nozzle, the low viscosity of the green PLA caused dripping phenomena and resulted in very poor printing quality. Conversely, the black and the white PLA filaments in the molten state were more “structured”, probably due to the presence of additives in the form of solid particulate, which helped to retain the intended shape while building up the part and reduced the printing defects (Cicala et al., 2018). Whereas most of the published research is focused on PLA, also other polymers are similarly affected by the presence of additives used to change their color. For instance, the effect of dyes on ABS was confirmed by the research by Alves Guimarães et al. (2020). When printed under the same parameters, the tensile strength of white ABS samples (almost 19 MPa) largely surpassed that of blue counterparts (around 12 MPa), with the difference being an impressive 57.5%. The green samples (around 13.5 MPa) performed slightly better than the blue ones, but still much worse than the white ones (Alves Guimarães et al., 2020).

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The examples reported so far clearly show that the role of pigments, dyes and coloring agents in FDM is still open for discussion, as contrasting results are reported in the literature, likely because of the variability in materials and processes used in commercial products coming from different manufacturers (Kim et al., 2020). Another issue is that, very often, manufacturers provide the same technical data sheet for all filaments nominally made from the same polymer, although they actually come in different colors (Pandzic et al., 2019). However, some producers acknowledge the color effect and provide separate technical specifications for different colors. As seen in Table 3.2, this is the case, for example, for ULTEM 9085 filaments by Stratasys, whose properties are individually categorized for “natural resin” and for “black resin” (Stratasys ULTEM 9085, n.d.). As a concluding remark about colored feedstocks, it should be mentioned that additives, including pigments and dyes, are expected to worsen the environmental footprint of FDM. There is a growing consensus that ultra-fine particles (UFPs) and volatile and semi-volatile substances (volatile organic compounds, VOCs) are released upon printing due to thermal degradation of the thermoplastic filament. Recent studies suggest that the feedstock color has a minor (though still perceptible) effect on the emissions (Davis et al., 2016, 2019; Sigloch et al., 2020; Sittichompoo et al., 2020; Stefaniak et al., 2017), but new additives may have unpredictable effects on the thermal stability of the feedstock material, thus actually affecting the polymer degradation (Alberts et al., 2021; Davies et al., 2019). Also, during recycling, parts printed in different colors should be separated as currently done for glass, in order to avoid color mix-ups and unwanted interactions between additives (Fries and Durna, 2018).

3.5.2 Tagging features At present, the fabrication of prototypes and functional models is still the main application of AM. However, the role of AM to produce bespoke components for hightech areas in aerospace, automotive, defense and biomedicine has grown steadily over the last decade and nowadays the usage of AM is evolving from “rapid prototyping” to “industrial production.” As AM progresses from a prototyping tool to a production technology, the need has emerged to develop new tagging strategies to certify the printed objects, in order to authenticate and identify them. Although “authenticate” and “identify” are often assumed as synonyms, according to the outcomes of a briefing held by the United States Pharmacopeia–National Formulary (USP–NF) the two terms are not interchangeable, as long as the “integrity” of a product implies “Ensuring the identity and authenticity of a material or product by a set of procedures” (USP-NF, n.d). In the first instance, “authentication” means that an item is certified in order to verify its origin from a specified brand/company, whereas “identification” means that an item is certified to track it down individually. Certifying an additively manufactured part, either for authentication or for identification, may serve numerous purposes. Among them, anti-counterfeiting is particularly urgent. Due to the freedom in geometry that is typical of AM, the shape of an object can be deftly reproduced from stolen design data or from a 3D scan of the prototype. However, the performance and the reliability of the printed part depend on multiple

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variables, including feedstock quality and printing parameters, and these are not obvious to the final user (Eisenbarth et al., 2020). As a consequence, counterfeited parts can easily pass a visual inspection, but they are likely to miss the service requirements or even to undergo premature failure when in use if they are printed from low-quality materials or with non-optimized processing parameters (Chen et al., 2019a). As such, counterfeited parts not only impair the quality of the associated devices they are installed in, but may also pose threats to human lives (Wei et al., 2018). Further, counterfeiting is known to be a major cause of financial loss and reputational damage to the original producers. For example, according to the statistics published by Red Points (Red Points, n.d.), a firm working in brand protection, the counterfeiting of sports footwear is worth 14 billion USD per year, which corresponds to approximately 10% of the genuine market value. Interestingly, 35% of all counterfeited purchases were made not knowing the item was fake (Red Points, n.d.) and this clearly demonstrates the importance of enforcing appropriate authentication strategies to verify and make evident the genuine origin of a product. Also, authentication tags are often used on a company’s branding strategy in order to advertise and to create the myth of “distinctiveness”. Especially for high-end parts and for luxury products, it is important that a brand finds a way to certify that a given item actually belongs to their exclusive limited editions and, possibly, to make it manifest (Flank et al., 2017). Very often, the overt exhibition of authenticity is a primary need for the end-user, who experiences the item’s authenticity as a reason for self-gratification and for social promotion. Although authentication of a part may be sufficient for merchandising and for avoiding counterfeiting, unique identification is necessary to comply with traceability requirements and to manage the logistics, especially to track the flow of items down the supply chain of large-volume productions (Binder et al., 2019). The enforcement of an effective certification strategy requires legal measures to cover the technical and intellectual properties, preventive measures to discourage the final customer from buying an imitation instead of the original item, communication measures to help the client make an informed purchase, and technical measures to allow for authentication or identification of the physical object (Jahnke et al., 2013). In the field of AM, quality assurance requires the complete traceability of the component downstream from conceptualization to final usage. This is obviously a cross-disciplinary task, since the complete workflow of AM parts includes several digital and physical fabrication steps (Chen et al., 2019a). The certification of AM parts is thus challenging, with legal, digital and technological issues. However, materials engineering and technology offer powerful tools to provide the physical object with a distinctive feature for certification. Broadly speaking, AM parts can be tagged by two different methods: r r

Parts can be fitted with a sensor, namely a mechatronic component that can be detected by or can interact with a reader (Binder et al., 2019); Parts can be printed with a “structural” tagging feature, namely with a physical mark that is integral to the structure of the part itself; this can be, for instance, a logo printed in plain sight on the part’s surface, a bar code or a quick response (QR) code, a chemical fingerprint, or a random distribution of “spots” such as pores or impurities or optical markers (e.g., dyes, quantum dots, etc.).

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As for structural tagging features, which is where composite materials and multimaterial printing become extremely useful, various strategies are feasible. First of all, marks can be either clearly visible on the part’s surface, or printed in such a way that they are invisible to the naked eye. Generally speaking, visible marks are more practical if the check has to be done by the end-user, because they do not require dedicated equipment to be seen. However, invisible features requiring a specific detection technology are more difficult to counterfeit and to tamper (Jahnke et al., 2013). The same object may receive multiple marks, both visible and invisible, for additional security. Another basic distinction occurs between deterministic and non-deterministic structural tagging features. A deterministic mark is conceived to communicate a meaningful message. Decoding the message of a deterministic mark can be very easy, as it may happen with branding logos that are designed to be obvious, or very difficult, as it may happen with symbols that require a key to be understood. As opposed to deterministic marks, non-deterministic marks are random in nature. As such, they do not convey any logical information, but they are extremely secure, as they cannot be reproduced even if all the variables are known (Eisenbarth et al., 2020). Nondeterministic marks are common in nature, with human fingerprints, the sequence of nucleotides in deoxyribonucleic acid (DNA) and the structure of the iris being just a few examples of distinctive features that make individuals unique (with some exceptions: for example, twins share the same genetics and hence the same DNA, and this is why multi-modal biometrics increase the recognition accuracy of individuals by analyzing more than one biometric trait in combination) (UNICEF, 2019). In terms of AM, for instance, pores can be regarded as the equivalent of a fingerprint, since the location, size and shape of pores are caused by stochastic fluctuations in the manufacturing process, and therefore they are unique to each part and cannot be reproduced even if the same printing parameters are applied identically (GE Additive, 2021; Sola and Nouri, 2019). As an alternative to pores, random spots can be intentionally caused in the printed part if discrete particles or other dispersed fillers are mixed with the feedstock material. In both cases, regardless of the nature (either pores or particles) of the random spots, the object itself is the tag, which is an example of physical cryptography (Ivanova et al., 2014). The implementation of multi-material printing and the adoption of composite feedstock materials are extremely advantageous to equip AM parts with structural tagging features. For this reason, much effort in the literature has been devoted to demonstrate the printability of physical marks by FDM, which is one of the most versatile printing methods in terms of feedstock availability and management. Chen et al. (2019a) opted for a deterministic approach and investigated the printability of QR codes by direct metal laser sintering (DMLS), by mono- and bi-material InkJet printing, and by FDM. The mark could be effectively printed by FDM with an ABS filament being processed in combination with a removable support material. However, the smallest printable size of the (square) QR code was 42 mm, as opposed to 4.54 mm and 3.8 mm for InkJet printing, and to 7.7 mm for DMLS. Chen et al. (2017) introduced anti-counterfeiting features working at the computeraided design (CAD) file level. The basic idea was to integrate tricky details in the

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design file. If printed according to standard processing parameters, such details would cause defective parts. If coupled with specific printing parameters, such details would be harmless to the finished part. Due to the presence of such tricky features, the standard CAD file used for printing would be useless by itself, since the appropriate printing conditions to achieve a defect-free part would be required as a separate piece of information. As such, the strategy detailed by Chen et al. (2017) would mainly address the security of the digital object (model). However, Chen et al. (2019b) subsequently combined the two previous methods to make the anti-counterfeiting strategy more effective and to concurrently account for the physical object and for its digital model. To this aim, Chen et al. (2019b) designed two inter-penetrating QR codes, one reading as “counterfeit” and the other one reading as “genuine.” Whereas the faulty QR code could be easily printed and reproduced with standard processing parameters, the authentic QR code required special processing parameters that should be stored separately from the CAD file for improved security. Further, the two QR codes could be broken down in several segments and each segment printed on a different layer to make counterfeiting more difficult and to minimize the effect of the marks on the mechanical properties of the part. In order to generate more intricate patterns, some segments were shared by both QR codes. Chen et al. (2019b) proved the feasibility of this anticounterfeiting technology by printing interpenetrating QR codes by FDM and by InkJet 3D printing. In the FDM prototypes, the marks were built with the support material embedded in ABS and could be easily read by micro-computed tomography (micro-CT). Kennedy et al. (2017) conceived the tagging feature, specifically a QR code, as the link between the physical object and its digital twin on a blockchain platform. The QR code redirected to metadata on the blockchain that contained the signature information of the object and additional details regarding design parameters, G-code and printing set-up. Kennedy et al. (2017) conducted their feasibility study on a dualnozzle FDM printer. The object was printed with standard PLA, which was chosen owing to its popularity in FDM and to its relevance across multiple industries. For printing the QR code, instead, Kennedy et al. (2017) compounded a lanthanide-aspartic acid nanoscale coordination polymer/PLA composite filament that could be detected through visible fluorescence emission under UV light. The emission from the printed QR code was read and quantified in terms of color with a common smartphone camera, and then connected to the blockchain entry. In this preliminary contribution, the mark was printed in plain sight, layering the white QR code on a black PLA background. However, the lanthanide-aspartic acid nanoparticles are visible under UV light regardless of the color of the polymer matrix in visible light and therefore, in case of need, they can be made invisible. Maia et al. (2019) developed a new tagging scheme, called LayerCode, that is inspired by the resemblance between the parallel “black and white” bars of a bar code and the parallel layers of a 3D printed object. As exemplified in Fig. 3.10, the foundational concept of LayerCode is that the object itself is a bar code, provided that it is built up by two distinguishable layer types. This can be accomplished by printing in two different colors, for example by multi-material FDM. If combined with the usage of composite feedstock, the multi-material approach also enables the production of monochromatic objects, where the same base polymer, with and without hidden

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Figure 3.10 LayerCode, the tagging strategy proposed by Maia et al. (2019), is based on the resemblance between parallel layers, which are the building blocks of additively manufactured parts, and parallel bars, which are the building blocks of bar codes (the image is merely representative of the functioning mechanism of LayerCode and does not show real bar codes).

additives or fillers (for example, near-infrared dyes), is employed to print both “black” and “white” bars of the code. Otherwise, for single-material printers, the distinction between “black” and “white” bars of the code can be rendered through variable layer heights, for example producing “black” bars with thin printing layers and “white” bars with thick printing layers (Maia et al., 2019).

3.5.3 Scaffolds for biomedical applications In the biomedical field, AM techniques in general, and FDM in particular, are gaining momentum for their ability to produce unique and complicated geometries, which is crucial to create customized implants and highly porous scaffolds with a controlled architecture. Orthoses, often called braces, are artificial external devices that support the limbs or spine, prevent or assist relative movement and modify the structural and functional characteristics of human neuromuscular and musculoskeletal systems. Prostheses are instead devices that replace a missing part of the body after birth defects, trauma or disease. Although off-the-shelf orthoses and prostheses are widespread owing to their affordability, customized implants can fit the patient’s anatomy and physiology and perform better than standard ones. As discussed in the review by Chen et al. (2016), surveys conducted across Vietnam and Iraq war veterans and patients holding longterm prostheses have pointed out that the appropriate fit of orthoses and prostheses is the most important factor for the recipient’s satisfaction, having a deep impact on their quality of life. In this regard, bespoke implants outperform out-of-the-shelf ones because they respect the patient’s anatomy. However, conventional methods

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for manufacturing customized biomedical devices rely on labor-intensive and timeconsuming methods. AM drastically cuts down the labor, cost and time required for customization, thus providing an ideal pathway forward for the uptake of personalized biomedical devices (Chen et al., 2016; Jin et al., 2015). Whereas orthoses and prostheses are intended to sustain or replace affected limbs or organs in the human body, scaffolds are highly porous structures designed to support and promote the spontaneous healing process of natural tissues, especially bone. To this aim, the scaffold architecture should meet some basic principles in order to allow for cell movement, re-vascularization and removal of physiological by-products (Filippi et al., 2020; Koons et al., 2020): r r r

Pores must be open to their environment and fully interconnected Pores should be at least 300 μm in diameter, with pore-to-pore openings at least 100 μm in diameter Pores should have a moderately rough surface to facilitate the anchorage of cells

Additional requirements are addressed to the scaffolding material, which should be biocompatible, bioactive and, for orthopedic applications, osteoinductive and osteoconductive. Next-generation biomaterials for bone implants are engineered to trigger specific cellular responses at the molecular level. Moreover, depending on the specific kind of application, the scaffolding material may be stable for long periods or bioresorbable. In case of bioresorbable implants, the bio-degradation rate of the scaffold should correspond to the healing rate of natural tissues. Understandably, the degraded by-products must be non-cytotoxic (Schroeder and Mosheiff, 2011). For bone tissue repair, the stiffness of the scaffold should equal the stiffness of natural bone to avoid the stress shielding effect, which is the progressive resorption of bone as a consequence of the altered stress distribution with respect to normal physiological conditions. The presence of a controlled porosity also governs the density of the scaffold and hence the weight of the implant, which, again, should match the properties of natural bone (Sola et al., 2016). Many techniques have been developed to fabricate polymer-based scaffolds, such as controlled foaming and solvent-casting particulate-leaching. However, these conventional approaches allow the architecture to be controlled in terms of average pore size and average pore distribution. The shift to AM makes it possible to design and build the exact porous structure that is optimal for a given application and, if required, to mimic the anatomy of the patient (Trachtenberg et al., 2014). Although originally conceived for use as bone implants, scaffolds have found other relevant biomedical applications, for example the treatment of liver and peripheral nerve tissue (Mozafari et al. eds., 2019a, 2019b). The development of scaffolds for different tissue engineering applications is fostered by the advancement of 3D printing technologies that combine new materials and cells (Trachtenberg et al., 2014). In addition to supporting natural healing processes, scaffolds can serve as threedimensional models for in-vitro testing of new drugs and therapies (Sola et al., 2019). In this regard, the adoption of AM is key to ensuring the consistency of the experimental framework and hence the statistical soundness of the acquired data (Abar et al., 2021). As already mentioned, the architecture of scaffolds produced with “conventional”

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Figure 3.11 The architecture of scaffolds produced by conventional methods can only be governed in terms of “average properties” and therefore scaffolds with the same mean porosity may actually have different architectures. As opposed to conventional fabrication methods, AM techniques enable the production of scaffolds with controlled and repeatable architecture.

methods can only be controlled in terms of “average” properties. Conversely, the architecture of scaffolds produced with AM techniques can be carefully designed and consistently reproduced point by point (Zhang et al., 2019). Fig. 3.11 shows this difference. During the last two decades, FDM has proved to be a reliable technique to print accurate and highly repeatable porous architectures (Abar et al., 2021; Górecka et al., 2020; Grémare et al., 2018; Hutmacher et al., 2001; Ravi, 2020; Tang et al., 2020; Zein et al., 2002). Combined manufacturing technologies, such as FDM in conjunction with selective polymer removal or with foaming, or again with particulate leaching, have been proposed to create hierarchical porous structures (Choi et al., 2020; Sanz-Horta et al., 2020; Shalchy et al., 2020; Song et al., 2018) and post-printing treatments have been designed to control the surface roughness and modify the hydrophobicity (KosikKozioł et al., 2019, 2020). It is interesting to note that, in case of particulate leaching combined with FDM, the initial feedstock is a polymer-matrix composite loaded with salt particles, typically sodium chloride (NaCl) or other water-soluble salts, but the finished part is made of pure polymer, since the inorganic filler is just a temporary space holder to control the final multi-level porosity. In bone tissue engineering, polymer scaffolds are often combined with inorganic fillers in order to mimic the natural structure of bone tissue, which is a nanocomposite comprising organic proteins (mainly collagen type I), inorganic minerals (mainly calcium phosphates and particularly carbonated HAp) and several cell types (Koons et al., 2020). Whereas the mechanical properties of neat polymer scaffolds

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would be too weak for load-bearing applications, organic-inorganic hybrid composites can closely reproduce the mechanical behavior of bone (Koons et al., 2020; Zhang et al., 2019). Many polymers including PLA generate an acidic environment when they (bio-)degrade in the human body with adverse consequences on cell viability. The presence of inorganic phases such as HAp mitigates the burst in pH and improves cell survival and proliferation (Hajiali et al., 2018; Hassan et al., 2019; Wang et al. 2021). Some fillers, such as iron oxide, have been reported to respond to external static magnetic fields to promote bone growth (Yun et al., 2016). Further, fillers can impart additional functionalities to the porous polymer matrix, such as shape memory effect or photoluminescence, which contribute to improved clinical handling or postimplantation tracking. The incorporation of functional fillers enables therefore unprecedented bio-mimicking and healing solutions that would otherwise be unattainable with neat polymers (Koons et al., 2020). The development of new composites is thus key to remediating the lack of diversity in printable biomaterials that is currently recognized as one of the main limitations when FDM and other AM techniques are applied in the biomedical field (Javaid and Haleem, 2020).

3.5.4 3D pharming A new exciting scenario is opening with the adoption of FDM for printing functional medications and drug delivery systems for pharmaceutical applications. Alongside inkjet 3D printing, FDM is the AM technique currently experiencing the fastest advancement towards the production of personalized tablets (Acosta-Vélez and Wu, 2016). Owing to its unique versatility, FDM can be easily extended to fabricate systems for oral delivery having virtually any desired dosage form and release profile (Awad et al., 2018). According to the conventional paradigm of therapeutics, pharmaceutical companies are used to manufacture tablets that contain standard dosage amounts. However, these universally-designed tablets benefit only a small fraction of the patients that take them, because the real effectiveness of the treatment depends not just on the properties of the tables, such as nature and concentration of the active principles, dosage form, specific surface area, etc., but also on the genetic makeup and the gene expression of a person, as well as on several other factors coming from environment, diet, lifestyle, and microbiome (Acosta-Vélez and Wu, 2016). Further, mass-oriented treatments are inappropriate for critical patients, such as geriatric patients, whose therapies need to be continuously adjusted to cope with fast modifications of the physiological and metabolic conditions, and pediatric patients, whose dosage units need to be scaled down based on body mass (Goole and Amighi, 2016). As opposed to standardized medicine, the emerging “personalized medicine” aims at providing each patient with a specific treatment, which is tailored to their pathophysiology and life conditions. With a focus on solid oral dosage forms, this therapeutic approach needs a radical change in manufacturing, since it requires new technologies to fabricate bespoke tablets on-demand, with customized drug ingredients and dosage. Whereas conventional tableting machines are still based on punches and dies, thus lacking flexibility, new and more customizable platforms are needed to enable the

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design, production and dispensing of personalized medicines (Curti et al., 2020). The new production methods should be compatible with a wide range of active substances, be versatile and consistent in controlling the relative amounts of each single drug, and be economically viable. Moreover, the new fabrication procedures should necessitate a minimal manufacturing time, in order to enable the immediate production of the required tablets for a timely treatment of pathological conditions (Acosta-Vélez and Wu, 2016). 3D pharming is the application of AM technologies to directly fabricate customized pharmaceutical tablets and drug delivery systems to meet the specific medical needs of the individual patient (Mathew et al., 2020). AM methods, and especially FDM, offer strategic features to make personalized therapeutics become a reality (Acosta-Vélez and Wu, 2016): r r r r r

They can incorporate multiple active substances and ingredients in a single tablet; They are suitable to precisely control the dose of each component; They allow printing of tablets with designed structures to modulate the release kinetics; They impose modest requirements in terms of space and technological equipment; They can be digitally controlled by healthcare staff with reasonable operational training.

The first demonstration of the suitability of AM to fabricate pills dates back to 1996 (Wu et al., 1996), but it was only in 2015 that the Food and Drug Administration (FDA) Agency in the U.S.A. first granted the approval of a 3D printed tablet, Spritam, manufactured by Aprecia Pharmaceuticals (now: Aprecia – The 3DP Pharmaceutical Company). Spritam is a fast disintegrating/dissolving tablet based on levetiracetam that dissolves in 15 sec in saliva or in 10 mL of water for the treatment of epileptic seizures (Aprecia, 2016; Pandey et al., 2020). The acceptance of Spritam by regulatory agencies has demonstrated the efficacy of orodispersible 3D printed drug delivery systems in the treatment of epileptic seizures, however van Tienderen et al. (2018) has recently discussed the potential of FDM and other polymer-based AM techniques to shift from orally-delivered therapeutics to more effective implant-based systems. Implantable antiepileptic drugs are expected to play a key role in the treatment of patients unresponsive to oral administration, which are currently estimated to be 30% of all epileptic patients (Kwan and Brodie, 2000). In order to understand the relevance of the problem, it is important to remark that epilepsy affects around 1% of the worldwide population and that epileptic seizures may have dramatic consequences culminating in loss of awareness, injury, psychological and social disability, and ultimately mortality (van Tienderen et al., 2018). In principle, several AM techniques could be suitable for preparing medications. Nonetheless, inkjet 3D printing (mainly binder jetting and drop-on-demand, DOD) and FDM are currently prevailing in 3D pharming due to their ability to build multimaterial parts and compositionally graded structures, which is hardly feasible by other AM techniques (Acosta-Vélez and Wu, 2016). FDM is anticipated to lead the progress in 3D pharming, as it has some key advantages over inkjet printing, especially its lower cost, its versatility to easily integrate different polymers/materials is a single part, and its capability to print hollow and porous structures with good mechanical strength (Acosta-Vélez and Wu, 2016; Pandey et al., 2020). Further, unlike other AM

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techniques such as inkjet-based systems and pressure assisted microsyringe (PAM) printing, FDM, if conducted from a hot-melt compounded filament, does not require to use solvents in order to print. This is a substantial benefit, because the pharmaceutical stability guidelines for industries provided by the International Conference on Harmonisation (ICH) dictate stringent recommendations about the presence of impurities and especially residual solvents (Goole and Amighi, 2016). However, 3D pharming with FDM faces several challenges, especially in terms of printing materials. Few pharmaceutical-grade thermoplastic materials have the appropriate rheological behavior to be processed by FDM (Alhijjaj et al., 2016; Azad et al., 2020; Melocchi et al., 2016) and those few that are suitable for printing often have poor mechanical properties (Singhvi et al., 2018). Also, the exposure to high temperature required to melt the polymer matrix, albeit only for short time, may cause the degradation of the drugs incorporated in the feedstock (Kollamaram et al., 2018; Okwuosa et al., 2016). The first examples of using FDM for 3D pharming are relatively recent. In 2014, Goyanes et al. (2014) printed fluorescein-loaded polyvinyl alcohol (PVA) filaments. In order to avoid the long permanence of fluorescein at high temperature during filament extrusion, fluorescein was loaded by swelling the PVA filaments in a fluoresceinethanol solution. As a result, the amount of fluorescein incorporated into the filaments was relatively low, about 0.29 wt%. Also, fluorescein was mainly concentrated at the surface of the filaments due to the slow diffusion into the polymer matrix. The loaded filaments could be printed in tablets with different infill degrees. Interestingly, dissolution tests carried out in a bicarbonate buffer at pH 6.8 proved that the drug release rate of fluorescein could be controlled by changing the infill degree of the tablets, with lower infill degrees resulting in faster release rates (Goyanes et al., 2014). The release rate has also been proven to depend on the surface area-to-volume ratio of the tablet, with higher ratios inducing faster release rates (Goyanes et al., 2015). Whereas the maximum drug loading attainable by passive diffusion into the filament is generally lower than 2%, hot-melt compounding can be very effective to increase the drug loading of active molecules that are not thermally labile (Awad et al., 2018; Zhang et al., 2017). Also, if the extrusion conditions are properly set, direct compounding into the molten polymer produces a more uniform dispersion (Zhang et al., 2017). For example, Goyanes et al. (2016) evenly incorporated up to 8.2 wt% of paracetamol and up to 9.5 wt% of caffeine as model drugs in PVA filaments by hot-melt compounding. Another advantage of hot-melt compounding is that the same recipe of polymer matrix and excipients can be translated to incorporate different active principles. Since the filament composition must not be redesigned completely for different drugs, the development of new feedstocks can be streamlined (Awad et al., 2018). Research is now on-going to leverage the unique technological features of FDM to print multi-material and compositionally graded tablets, in order to combine different therapeutics in a single pill (Awad et al., 2018; Melocchi et al., 2020). The approach also integrates nanotechnology and AM, in order to maximize the effectiveness of drugs thanks to the increased bioavailability and specific targeting (Singhvi et al., 2018). On account of the geometric freedom that is particular to AM, the “engineering approach” aims to control the properties of a tablet through the design of its geometry,

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and not just through its composition. For example, the drug release profile can be tuned through the infill density or, generally speaking, through the introduction of channels or other features, such as by adding gaps to the tablet design, called “gaplets”, to increase the surface area-to-volume ratio (Araújo et al., 2019; Awad et al., 2018; Curti et al., 2020; Melocchi et al., 2020). The engineering approach seems to be particularly promising to speed up the status of 3D pharming from “lab” to “fab.” In fact, it is envisaged that the pharmaceutical industry can fabricate drug-loaded filaments on a large scale as an intermediate product with the necessary quality and safety guarantees. Later on, local pharmacies can transform the filaments into personalized medicines according to specific prescriptions by a simple change in the tablet shape, size, and architecture (Araújo et al., 2019). Further, 3D pharming is expected to play a key role in telemedicine, where diagnosis and prescriptions are set to become fully virtual (Araújo et al., 2019). As the printing technologies and materials for 3D pharming are growing at a remarkably fast pace, regulatory issues are also emerging to ensure the safety of the patient and the real efficacy of the treatment (Chen et al., 2020). At present, no regulatory pathway for personalized 3D printed oral dosage forms has been established (Araújo et al., 2019). The FDA approval of Spritam represented a milestone, however it should be underlined that, strictly speaking, Spritam cannot be considered as a personalized product, since it is only available in four dosages (Curti et al., 2020). In principle, printing a tablet with a bespoke concentration of drug is technically simple, however various adaptations to standard FDM printers are required in view of pharmaceutical production, in order to avoid any contamination and meet Good Manufacturing Practices (Araújo et al., 2019; Awad et al., 2018). Also, the integrity and dosage of the drug(s) should be monitored on-time, and this requires the development of appropriate in-situ quality assurance systems (Awad et al., 2018). Further, the effect of customized treatments should be carefully predicated before administering, and regularly checked upon dispensing them. Whereas off-the-shelf medicines are statistically trialed and verified before approval and commercialization, the same approach does not apply to bespoke tablets that are one-of-a-kind products and regulation agencies are now called to set new standards for 3D pharming (Acosta-Vélez and Wu, 2016; Goole and Amighi, 2016) and the new regulations should be accepted internationally in order to minimize the disparities between different countries (Araújo et al., 2019).

3.5.5 4D printing The expression “4D printing” has been recently introduced to define an emerging technology that involves printing objects whose size, shape or function can change with time as a result of some external stimuli. More precisely, the name was first employed in 2013, when Skylar Tibbits, Director of the Self-Assembly Lab at the Massachusetts Institute of Technology (MIT), presented a multi-material 3D printed prototype that gradually changed its shape from a linear rope to the “MIT” monogram when immersed in warm water. This paved the way for a radical shift in AM. According to the definition provided by Tibbits (2014), 4D printing “entails multi-material prints with the capability to transform over time, or a customized material system that can

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Figure 3.12 As a consequence of differential water uptake, the bilayer assembly experiences a controlled bending that can be exploited to design complex structures with shape change or size change. In FDM, the differential water uptake can be achieved either by coupling different materials (multi-material hygromorphs) or by printing the same material under different orientations due to anisotropic effects (anisotropy-driven hygromorphs).

change from one shape to another, directly off the print bed”. As the name itself suggests, the disruptive idea of 4D printing is that “time” is added to 3D printing as the fourth dimension in which printed parts actuate their function (Kuang et al., 2019; Momeni et al., 2017). 4D printing takes its inspiration from the behavior of natural structures such as pinecones, whose scales bend and open to release the seeds when exposed to moisture (hygroscopic actuation). Basically, the scales of a pinecone are composed of two layers that experience different tissue swelling in the presence of water. Since the two layers are firmly connected to each other by a compositionally graded interphase, they are mutually bound to each other and therefore the differential swelling of the bilayer system causes the assembly to bend (Le Duigou et al., 2016). The hygroscopic actuation of pinecones has suggested the design of the so-called “hygromorph biocomposites”, which are natural fiber-reinforced polymer composites whose morphing ability is activated by moisture uptake, as schematically exemplified in Fig. 3.12. However, a very wide range of adaptive materials are being developed besides “hygromorph biocomposites”. For instance, many shape memory polymers (SMPs) experience a change in shape or size as a consequence of heating (Lendlein and Gould, 2019), whereas electro-active polymers (EAPs) exhibit a shape

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change when stimulated by an electric field (Bar-Cohen and Anderson, 2019). Depending on their nature and formulation, stimuli-responsive materials can react to different kinds of external factors, including humidity, temperature, ultraviolet light, electric and magnetic fields, and they can change different features of their functionality, including shape change, property change, self-assembly, or self-repair. Although stimuli-responsive materials have long been investigated because of their controlled transformation, the additional advantage of 3D printing them (which is the essence of 4D printing) is that actuated structures (rather than materials) can be fabricated with an unprecedented freedom and accuracy in geometry. In particular, FDM is very well-suited to produce stimuli-responsive structures (Aberoumand et al., 2021), which can be accomplished by means of two different strategies. Stimuliactivated structures can be designed by alternating layers made of different materials. This multi-material approach has been proposed, for example, by Li et al. (2019), who constructed a bio-hygromorph structure by coupling a fish swim bladder-based hydrogel as the hydrophilic material, and a wood flour-filled PLA scaffold printed by FDM as the rigid layer. As an alternative approach, the 4D ability may be based on the anisotropic response of layers that have been printed with the same material, but under different raster angles. For example, Le Duigou et al. (2016) proposed a 4D structure that was purely based on the different printing orientation of a woodfilled biocomposite bilayered structure. Opting for the anisotropy-driven morphing approach poses additional design constraints over the multi-material strategy, because the structural change can only be controlled through the stacking sequence of the printed layers. However, anisotropy-driven 4D printing offers key advantages in terms of environmental footprint and of recyclability at the end of life, because there is no need to break apart and sort parts or details printed with different feedstock materials. Further, working with the same feedstock across the whole part is expected to reduce potential delamination issues between heterogeneous materials. As remarked by Le Duigou et al. (2016), unlike static composites, which are designed to remain unchanged and to deliver a uniform performance over time, stimuliresponsive composites are dynamic materials that provide time-dependent self-shaping actuation as a new functionality. As an additional advancement, new multi-responsive materials are able to react and change under multiple environmental stimuli, which opens the way for the development of biomedical implants and, broadly speaking, of new structures capable of interacting with the surrounding environment (G. Liu et al., 2020). 4D printing is expected to attract increasing attention in the future, because, as stated by Castro et al. (2017), it has the potential to change the current paradigm “from ‘what can we do with these materials/technologies?’ to ‘how can we move the concept/ technology forward to achieve what we need?”’ over time.

3.5.6 Manufacturing of composites Interestingly, AM may present some similarity to conventional methods for manufacturing composite materials and objects. A meaningful term of comparison may be hand lay-up, which is schematically presented in Fig. 3.13. In this

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Figure 3.13 Hand lay-up is a conventional technique for manufacturing composite materials where pre-impregnated laminas are stacked in a mold and the part is built up in a layer-wise manner, which may remind of AM.

procedure, pre-impregnated laminas (aka pre-pregs: usually, unidirectional or woven fibers partially impregnated with thermoset resin) are manually layered onto a mold having the shape of the desired object. Each lamina is usually cut to the proper size and shape starting from rolls or tapes, typically in the 0.125 to 0.300 mm thickness range. Multiple layers are stacked to achieve the required strength for the part (that obviously depends on the intrinsic properties of the prepregs and on the final total thickness of the stack, not to mention the quality of the inter-layer bonds). The relative orientation between subsequent layers can be

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changed to obtain a (nearly) isotropic behavior or to induce specific anisotropic effects. Resin may be added to improve the impregnation and hence the inter-layer adhesion. The operator may also use a roller to apply a certain pressure in order to further improve the impregnation and remove potential bubbles. After completing the layering sequence, the stack is processed to make the resin cure. Whereas the reaction can be completed at room temperature, it is more common that a vacuum bag is applied and an autoclave cycle is performed in order to exploit the combined effect of temperature and vacuum in order to achieve the best viscosity of the resin for perfect impregnation, to remove any surviving air bubbles and off-gases developed as a consequence of curing, and to complete the hardening process that is necessary to retain the shape of the laminate (from the mold negative). In principle, the hand lay-up method enables the production of parts with optimal properties, which are ideal for marine and aerospace structures. However, the actual quality of the finished part greatly depends on the skill of the operator, especially for complicated geometries with recessing details. In this regard, hand lay-up is actually best suited for relatively simple shapes (Elkington et al., 2015; Jamir et al., 2018; Meola et al., 2017). Many process variations exist, including more automated ones that limit the effect of the operator’s manual ability on the finished part. Nonetheless, it is worth noting that the core of the process is always the same, as subsequent layers are stacked on top of each other so as to build up the wanted part, as commonly observed in many AM methods. In spite of this apparent similarity, the two processes, hand lay-up and AM, are substantially different. If FDM is considered, a first difference is the kind of polymers that are processed, since FDM works with thermoplastics, whereas hand lay-up works with thermosets. As compared to thermoplastics, generally speaking thermosets feature superior mechanical properties, especially in terms of stiffness, but are less tough and less impact resistant. Also, thermosets require curing treatments and pose serious concerns in terms of recyclability. Another key difference is the kind of objects that can be obtained. Whereas FDM enables the fabrication of almost any geometry, including complicated 3D objects, hand lay-up results in shells, whose thickness is in the millimeter range and whose geometry is restricted to the shape of the mold. However, hand laid-up parts can be much larger than typical FDM components, unless the FDM printhead is installed on a robotic arm. This also implies a difference in productivity. An FDM printer is well suited to produce unique parts or to run smallscale productions, and it can complete hundreds of prints (provided that appropriate maintenance is performed). Before the surface is worn out, a hand lay-up mold enables the production of a few hundreds of identical parts. The volume production for a specific part is higher in hand lay-up than it is in FDM, but the geometry is strictly constrained. Also, an FDM printer is capable of processing a variety of feedstock materials, whilst hand lay-up is less versatile. In fact, many kinds of pre-pregs are commercially available, but materials are rarely changed on the production line because this would bring about lengthy and expensive revisions to the processing parameters. In terms of investment costs, it is difficult to draw meaningful comparisons in general terms, since the cost of an FDM printer may vary from few hundreds of (American) dollars up to 30,000-50,000 USD for industrial equipment. Likewise, molds for hand

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Table 3.3 Main similarities and differences between FDM and hand lay-up. Feature Parts’ build-up

Feedstock materials

Obtained parts Parts’ geometry

Parts’ size

Production volumes Expected lifetime of equipment Investment costs

Labor

FDM Layer-wise: Molten material deposited layer-upon-layer Thermoplastics and thermoplastic-matrix composites Three-dimensional objects Almost any geometry

Relatively small, typically smaller than 20 cm x 20 cm, for standard printers; meter-scale for robotic arms Unique parts or small-scale runs Hundreds of jobs (under appropriate maintenance) Great variability between different printers, may be high (30,000-50,000 USD) for industrial equipment Largely automated

Hand lay-up Layer-wise: Pre-pregs deposited layer-upon-layer Thermoset-matrix laminas

Shells Identical parts that reproduce the same mold’s shape Variable, up to several meters

Dozens of parts Dozens of parts before the mold’s surface becomes worn Great variability depending on mold’s shape and size, may increase with oven or autoclave for curing Affected by ability and experience of the operator

lay-up are relatively inexpensive as compared to other conventional manufacturing techniques, but the actual cost of the mold depends on its geometric complexity and on its size (up to several square meters). Also, hand lay-up often needs post-processing to fully cure the resin, which requires additional equipment like a large-chamber oven (order of magnitude: 30,000 USD, but depends on the size) or an autoclave (order of magnitude: 300,000-400,000 USD, but again depends on the size) (Bauswein et al., 2017). Since the quality of hand laid-up parts largely depends on the skill and experience of the operator, specialized labor is another relevant cost. The main features of the two processes are summarized in Table 3.3. In the near future, composite parts produced by FDM and by other AM techniques are expected to overtake and replace conventional composite parts, since the integration of FDM printing technology with advanced multi-axis motion platforms and the increasing availability of continuous fiber-reinforced feedstock filaments offer extreme design freedom and, at the same time, close control on fiber alignment. Meanwhile, AM is already playing a key role in “conventional” manufacturing of composites, for example enabling the rapid production of molds. In conventional manufacturing of

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Figure 3.14 FDM and, generally speaking, AM are revolutionizing conventional methods for manufacturing composite materials by enabling the time- and cost-saving production of mold tools, sacrificial mold tools for hollow components, and ancillary tools.

composites, molds are mostly made of metals, although non-metallic materials may be also used, especially fiber-reinforced polymers. However, the production of molds is typically very complicated and lengthy, thus increasing final costs and lead times. Since single parts can be affordably obtained by FDM, the uptake of FDM for fabricating molds reduces machining costs, material waste and lead time. Also, molds printed by FDM can receive new optimized geometries and can be produced with lightweight (low density) materials instead of metals, thus eliminating heavy-lift procedures and simplifying the logistics. As an additional advantage, FDM can process washable supports that are the ideal solution for mold tooling when hollow parts are required. In fact, unlike conventional wash-out materials, FDM support materials are resistant to handling and dimensionally stable. Further, FDM is becoming more and more popular for the production of specialized ancillary tools for manufacturing composites. Composite parts produced by hand layup and other infusion methods are “near-net shape”, which means they must receive additional post-processing operations after demolding in order to achieve the prescribed quality standards. All secondary operations, such as trimming, machining, drilling, polishing, painting, require specific tools, and FDM can be successfully applied to produce the whole tooling range. The three-way impact of FDM on conventional manufacturing of composites is summarized in Fig. 3.14. However, the successful usage of FDM for mold tooling depends on meeting some basic principles (Stratasys ebook, n.d.): r

Most thermoset-matrix composites must be heat-cured at temperatures around 180°C. The mold must be made of a material that is thermally stable up above the curing temperature. Some FDM-compatible high-temperature thermoplastics such as PEI and polyethersulfone (PES) have a glass transition temperature in excess of 200°C, are heat-resistant and stable;

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r

r

r

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The coefficient of thermal expansion of the material used for the mold should be as close as possible to that of the composite part to be produced. Whereas neat thermoplastics typically have high coefficients of thermal expansion, the addition of small amounts of fillers may be enough to reduce the thermal expansion. In this regard, changing from neat thermoplastics to thermoplastic-matrix composites is key in functional mold tooling; If the composite parts to be produced must be autoclaved or must receive other highpressure/high temperature treatments, the design of the FDM mold tools must be adapted to withstand the extreme processing conditions; FDM mold tools need surface finishing. FDM parts unavoidably present internal pores as a result of inter-bead voids (Turner and Gold, 2015; Turner et al., 2014), and furthermore, the surface is not perfectly smooth as a consequence of the stair-case effect, as previously explained in Chapter 2. Depending on the nature, geometry and size of the composite parts to be produced, different strategies are feasible to make the surface of FDM mold tools perfectly sealed and smooth. For relatively simple parts, the surface of FDM mold tools may simply receive an adhesive-backed release film, otherwise they can be hand-sanded and then sealed with a high-temperature resin, most commonly epoxy. Molds for large parts may need more complicated finishing operations, including CNC machining. If this is the case, FDM mold tools should be printed in a near-net shape and slightly oversized to compensate for material removal upon finishing; FDM mold tools should be devised according to the “design for additive manufacturing” criteria to exploit all the advantages of AM (Leary, 2020). However, it is also very important to consider the intended usage, in order to optimize the design (including materials selection) as a function of the composite parts to be produced; In terms of the production volume, if very few composite parts are expected, the mold can be developed in such a way to minimize the cost. Instead, if the composite part is required for an urgent maintenance or repair, minimizing the lead time is priority. For large volume productions, a thoughtful trade-off should be reached between investment costs, mold quality and durability. Even if high-performance thermoplastics are employed and even if fillers are added to tune mechanical resistance and thermal properties, FDM mold tools inevitably have a shorter life-time than metal molds (Stratasys ebook, n.d.).

The increasing success of FDM and other AM methods is not anticipated to completely replace conventional methods for manufacturing composite materials, but offers new opportunities to design composite parts that are functionally oriented to their end-usage. This dictates the shift from “design for manufacturing” to “design for performance”.

3.5.7 Industry case study: Wärtsilä lifting tool Back in 2020, Markforged reported more than 100 industrial applications for FDM parts (Markforged, 2020). Markforged analysts estimated that more than 47% of the industries using AM within their survey were in manufacturing. Most of them 3D printed composite materials or neat metals by FDM in order to obtain complicated architectures with increased structural performance. A relevant example is provided by the lifting tool developed by Wärtsilä Corporation (https://www.wartsila.com/), a Finnish company that operates in power sources, engines and other equipment in the marine and energy markets. In order to build new

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engines and service broken ones both indoor and outdoor, Wärtsilä staff often requires custom-made lifting tools to move pistons, cylinders and other immensely heavy components. These lifting tools are often manufactured on demand, which makes them very expensive and time-consuming to fabricate with conventional methods. Also, the lifting tools themselves are usually made of solid steel to withstand the extreme loadbearing requirements and are therefore very cumbersome to transport and to put in place. The breakthrough at Wärtsilä was to change from conventional manufacturing to FDM. With this new AM-oriented approach, lifting tools can be printed remotely where they are needed and all the logistics can be simplified, with a substantial reduction in shipment time and warehouse capacity. The design flexibility that is typical of 3D printing makes it possible to cut down lead time and investment costs for producing bespoke lifting tools. As an additional advantage, solid steel can be replaced with lightweight (low density) materials. In this regard, neat polymers do not achieve the required mechanical strength, but continuous carbon fiber-reinforced polymers do. The specialists at Wärtsilä decided therefore to verify the viability of the AMbased approach and opted for FDM of continuous fiber-reinforced composites. To transition from conventional fabrication to 3D printing, the design of the lifting tool was completely revolutionized and included splitting the structure into several smaller parts in order to optimize the reinforcing effect of carbon fibers and, at the same time, in order to meet the size limits of the FDM printer (the volume chamber of the X7 printer used for this job nominally measures 330 mm x 270 mm x 200 mm (Markforged X7, n.d.)). The total weight of the tool was reduced by 75%, but the loading capacity was not compromised. The FDM tool was able to lift up to 960 kg without deformation, yet the nominal work load was set to 240 kg, because a safety factor of 4 is the standard in manufacturing applications. 3D printing the lifting tool instead of machining it also led to a substantial saving of around 100,000 € (corresponding to about 110,000 USD) on tooling over the first eight months (Markforged on Wärtsilä case study, n.d.). It is important to remark that the 3D printed Wärtsilä lifting tool is more than a demonstrator or a prototype. Wärtsilä partnered with Bureau Veritas (https://group.bureauveritas.com/), an international certification agency headquartered in Paris, to outline a process to certify the new 3D printed tool. After numerous checks and tests, Wärtsilä tool was thus the first 3D printed lifting tool worldwide to receive the Type Approval and CE-Certification (Markforged on Wärtsilä certification, n.d.).

3.5.8 Industry case study: Tecron replica carburetor Tecron (https://tecron.cz/en/contact/) is a Czech company located in Prague that specializes in the production of legacy parts for vintage race cars. The company holds a reputation for the high quality of their components, which have attracted the attention of numerous automotive companies across Europe. Tecron’s parts are fabricated in a conventional way, that combines modern instruments and, wherever possible, original instruments from the past, typically from the 1970s to the 1990s. However, Tecron’s business is obviously based on high-added value, large-variety and low-volume productions, which closely correspond to the philosophy of AM. For this reason, Tecron has recently adopted high-resolution scanning and metal 3D printing

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to fix or replace legacy parts whose production has been discontinued. For example, ´ Skoda Motor, one of their most important clients, contacted Tecron to manufacture a race car carburetor that was unavailable in the marketplace. The original carburetor was a three-part assembly made by die-casting. However, the sole supplier had interrupted the production and the die itself had gone lost over the years. Tecron was able to reverse-engineer the missing carburetor and, with some design optimization, 3D print it by FDM (SDS) in 17-4 PH stainless steel. Some redundant features of the original carburetor were not reproduced and all surfaces were made available for machining in order to have very smooth mating faces for bolting the carburetor to the manifold. The uptake of FDM allowed Tecron to produce a working carburetor faster than it would be possible with conventional manufacturing methods, and more affordably than it would be possible with other metal-based AM techniques. Also, the extreme precision and quality enabled them to make the part to specification (Keane, 2020; Markforged on Tecron case study, n.d.).

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Non-Print Items Abstract Most commercial filaments for fused deposition modeling (FDM), aka fused filament fabrication (FFF), still consist in neat thermoplastics, with poly(lactic acid) (PLA), acrylonitrile butadiene styrene (ABS), polyamide (PA, “nylon”), polyethylene terephthalate (PET) and its glycol-modified version (PETG), and flexible thermoplastic elastomers (TPEs) being the most popular options. However, it is also possible to process thermoplastic-matrix composite filaments with FDM. Adding a functional filler provides the polymer matrix with new properties that can be tailored to match the assigned service requirements yielding functional active materials. Composite filaments with high filler loadings can be printed, debound and sintered into fully organic parts. After a detailed analysis of conventional mono-material filaments, this chapter summarizes the emerging trends in FDM of composite feedstocks. Ashby-like diagrams map out the mechanical properties of around 100 commercial filaments and help visualize the functional advantages and limitations of composite materials over mono-material feedstocks. Selected applications and case studies illustrate how composite materials with embedded functionality are already revolutionizing 3-dimentional (3D) printing by FDM in numerous high-tech industries, such as composites manufacturing, biomedicine and 3D pharming, as well as in everyday life. Keywords Fused deposition modeling; FDM; Fused filament fabrication; FFF; Composite material; Shaping debinding and sintering; Color; Authentication; Scaffold; 3D pharming; 4D printing

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Introduction: Basic requirements of feedstock in FDM

Das et al. (2021) recently discussed the thought-provoking question: “What makes a polymer printable?” According to the results of the research conducted by Duty et al. (2018), Das et al. (2021) have concluded that a (thermoplastic) polymer in fused deposition modeling (FDM, or fused filament fabrication, FFF) must satisfy four basic requirements in order to be printable: 1. It must be able to flow out of the nozzle, 2. It must be able to hold its shape after leaving the nozzle, 3. It must be able to bridge a gap of a specific length once landed on the previous layers, while retaining the ability to support subsequent layers, 4. It must remain geometrically stable as it cools to room temperature.

Also, the thermo-mechanical properties of the feedstock material must be compatible with the functioning mechanisms of the printhead. Upon printing, the filament works similarly to a piston when it enters the liquefier, as it pushes the molten material ahead out of the nozzle. In order to accomplish this function, the filament for successful FDM must satisfy some basic requirements in terms of melt viscosity, strength, flexibility and modulus of elasticity/stiffness. As explained in Chapter 2, if the stiffness of the filament is too low or the melt viscosity is too high, buckling is likely to occur at the entrance of the liquefier. If the melt viscosity is very high, then either the stiffness of the filament must also be very high to avoid buckling, or the operating temperature must be increased to lower the melt viscosity, though this may cause overheating and polymer degradation (Das et al., 2021; Turner et al., 2014; Venkataram et al., 1999, 2000). Moreover, the temperature dependence of viscosity is also relevant, since the filament must preserve its stiffness up to the entrance of the liquefier, in spite of the increasing temperature encountered through the printhead (McNulty et al., 1998). The addition of a dispersed phase to obtain a composite material has complicated consequences on the properties of the neat polymer and therefore on the characteristics of the filament, especially if high filler loadings are involved (Gonzalez-Gutierrez et al., 2018). For example, some fillers such as talc (Gao et al., 2019) and montmorillonite (Coppola et al., 2017) are known to act as nucleating agents for poly(lactic acid) (PLA), favoring its crystallization. Also, depending on the nature of the fillers, some may increase the melt viscosity, whereas others may decrease it. For example, most ceramic and metal particles increase the viscosity of molten Fused Deposition Modeling of Composite Materials. DOI: https://doi.org/10.1016/B978-0-323-98823-0.00006-8 c 2023 Elsevier Ltd. All rights reserved. Copyright 

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thermoplastics (like adding flour to a dough) (Cano et al., 2020). Conversely, short carbon fibers have been reported to decrease the apparent viscosity of molten PLA due to the repulsive interactions and due to fiber alignment upon processing (Gao et al., 2019; Olesik et al., 2019). Dispersants can be necessary to adjust the viscosity and stabilize the filler dispersion (Masood and Song, 2004, 2005; McNulty et al., 1998; Nikzad et al., 2011). In cases where the composite filament is targeted towards the fabrication of inorganic components and debinding and sintering steps are required (the process, generally named “shaping, debinding, and sintering”, SDS, is presented in Chapter 10), the polymer matrix should meet additional specific requisites. The debinding process should be compatible with the filler not to damage it. Ideally, the solid residue after debinding and thermal removal should be nil. Some constraints also apply to the filler, as the particle size should not exceed 10% of the targeted structural detail (Nötzel et al., 2018). It is worth noting that the solid loading should be maximized in order to mitigate sintering-induced shrinkage phenomena. This is expected to have a deep impact on the rheological properties of the molten filament. However, the better the filler is dispersed through the polymer matrix, the lower is the melt viscosity of the dispersion and hence the higher the usable solid fraction (Gonzalez-Gutierrez et al., 2018; McNulty et al., 1998; Nötzel et al., 2018). Often plasticizers and surfactants can be added to the polymer to help the filament spool and improve handling. In addition to the abovementioned requirements, the fundamental prerequisite for FDM to be feasible is that the material must be extrudable, as the feedstock material must be extruded into a filament, first, and then extruded again through the print nozzle upon printing (Das et al., 2021). Producing high-quality filaments is pivotal to obtain high-quality parts. Filaments with agglomerates, high porosity or uneven microstructure have poor mechanical properties with respect to dense and homogeneous filaments. This may compromise the filament production and spooling, the feeding process into the printer, and ultimately the successful achievement of functional parts (Cano et al., 2020). Whereas the production of neat polymer filaments for FDM is a well-established procedure, adding a filler brings about new technical challenges and appropriate manufacturing strategies should be put into place to produce high-quality composite filaments, which is the first step towards the obtainment of high-quality printed parts.

4.2 Strategies for adding a filler Presently, all filaments for FDM are produced by extrusion, either with an extruder or sometimes with a capillary rheometer, which is only feasible at the lab scale. However, there is not a universal rule for adding a filler, as the best way of incorporating a filler must be identified on a case-by-case basis taking into account the filler nature and the desired filler distribution across the filament. In this regard, Fig. 4.1 provides a schematic diagram of the most common methods to prepare composite filaments for FDM.

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Figure 4.1 Different incorporation strategies should be applied as a function of the filler type and distribution.

4.2.1 Filler geometry The first variable to consider when choosing the right filament production method is the nature of the filler and especially its geometry, since different conditions apply to “continuous” fillers (continuous fibers) as opposed to “discrete” fillers (particles, short fibers, nano-platelets and nano-tubes, etc.). Continuous fibers indeed require specific compounding strategies not to compromise their length and integrity. Although some alternative approaches have been proposed in the literature, continuous fibers can be printed either by the “dual nozzle” method, which implies separately feeding the neat polymer filament and the polymer-pre-impregnated continuous fiber-reinforced filament into two different print nozzles, or by the “in-nozzle impregnation” method, which implies feeding the polymer matrix filament and the fiber bundle into the same liquefier, where the fibers become impregnated with the molten polymer and co-printed through the same nozzle (Heidari-Rarani et al., 2019; Kabir et al., 2020; Zhang et al., 2021). Most commonly, discrete fillers are instead pre-mixed in the polymer matrix. There are three main approaches to produce a well-dispersed composite, namely hot-melt compounding, solvent mixing and in-situ polymerization. Hot-melt compounding and solvent mixing are based on the mechanical action and physical interactions between the filler and the polymer matrix (Silva et al., 2021). To this aim, the thermoplastic matrix must be in a liquid state, which can be accomplished either by heating and melting it, in hot-melt compounding, or by dissolving it in a solvent, in solvent mixing (Chong et al., 2022). In situ polymerization follows a chemical approach,

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which requires preparing a stable dispersion/suspension of the filler in the liquid monomer, followed by polymerization (Silva et al., 2021). In the literature, hot-melt compounding and solvent mixing largely prevail on in-situ polymerization for the preparation of composite feedstock for FDM, as they are more economically viable and scalable for large production (Chong et al., 2022). Then, the composite feedstock must be extruded to obtain a filament with the appropriate diameter, usually 1.75 mm or 2.85 mm. Then, the filament can be spooled and stored until printing.

4.2.2 Filler distribution Another relevant parameter to consider when choosing the appropriate incorporation strategy is the desired filler distribution in the filament. For continuous fiber-reinforced parts, the “dual nozzle” method allows the volume fraction and distribution of the fibers to be governed by means of the part’s design. In fact, the polymer-to-fiber proportion in the reinforced filament is a fixed parameter and therefore the volume fraction and distribution of the fibers in the finished part depend on how much of each filament (neat polymer filament/reinforced filament) is printed and where it is distributed (Chacón et al., 2019). As opposed to the “dual nozzle” method, the “in-nozzle impregnation” method allows for a superior compositional freedom, since in theory it is possible to continuously adjust the polymer-to-fiber ratio by changing the feeding rates of the polymer filament and of the fiber bundle into the liquefier (Goh et al., 2018). Nonetheless, as a drawback, finely tuning the fiber loading may be cumbersome in practice, because the polymer-to-fiber proportion is governed indirectly through the feeding rates into the liquefier (Matsuzaki et al., 2016). Moreover, if the “in-nozzle impregnation” method is applied on a printer equipped with a single nozzle, fibers will be present in the whole printed part, unless the job is interrupted to stop feeding the fiber bundle and skip from “composite” printing to “neat polymer” printing and vice versa. Quite often, if discrete fillers are being used, the targeted composite material should have a uniform filler dispersion and therefore much attention is usually paid to the mixing step in order to ensure an even distribution of the filler into the polymer matrix. However, this is not always the case, as sometimes special distribution profiles can be more effective than a uniform distribution. Multi-polymer core-shell filaments have already been demonstrated to improve printing quality and mechanical properties (Park and Fu, 2021). For example, multi-polymer core-shell filaments have been developed and printed by FDM to combine the stiffness of a PC/ABS core with the toughness of a PE outer shell (Peng et al., 2019). As for composite filaments, it has been proven that a preferential distribution of carbonaceous fillers on the surface of the filament can lead to the establishment of localized conductive paths in the printed part and hence to the achievement of the percolation threshold for electrical conductivity with a lower amount of filler than is commonly observed with a homogeneous distribution (Shi et al., 2019). In other words, in some advanced applications the filler can be selectively loaded only where it is needed to accomplish its function and, if this is the case, special methods are designed to distribute the filler accordingly.

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Even if fillers are evenly distributed with the aim of producing a homogenous composite system, the so-called “wall slip phenomenon” is frequently observed in composite filaments and printed beads. When the composite material flows through the die of the extruder or through the nozzle of the printer, the rigid filler is unable to pack the wall of the orifice as efficiently as the molten polymer, and this leads to the formation of a slip layer of neat polymer near the wall boundary. In principle, provided that the reliability and consistency of the printed parts are not compromised, the wall slip phenomenon may be tolerable, as long as the presence of the polymer skin may facilitate the inter-bead and inter-layer bonding mechanisms (Bhagia et al., 2020). However, this may impair the obtainement of a unifrom distribution of the filler across the printed part.

4.2.3 Strategies for alternative fillers and additives It should be mentioned that “non-conventional” functional fillers may require dedicated compounding strategies. For example, many drugs and medicines are thermally labile and therefore they may be incompatible with hot-melt compounding (Kollamaram et al., 2018; Okwuosa et al., 2016). At the same time, drugs should not be altered or contaminated with potentially toxic solvents (Araújo et al., 2019). For this reason, pre-extruded PVA filaments and other pharmaceutical-grade thermoplastic filaments can be soaked and swollen with an ethanol-based solution of the drug, where ethanol can be easily removed by evaporation (Goyanes et al., 2014).

4.3

Key production steps

The following paragraphs will be dedicated to the production of composite filaments having discrete fillers, with a focus on the pre-mixing strategies and the extrusion process. Specific information about the fabrication of numerous composite filaments can be found in Chapter 12, Supplementary table 1.b (for polymer-based parts) and Supplementary table 1.d (for fully inorganic parts). As previously mentioned, continuous fibers require dedicated technologies for compounding and printing. For this reason, details about the production of continuous fiber-reinforced feedstocks and printed components are discussed separately in Chapter 9, and further summarized in Chapter 12, Supplementary table 1.c.

4.3.1 Constituent blending and mixing Many polymers are sensitive to humidity and other environmental variables, including temperature and light irradiation. Polymer pellets must be stored in a safe condition until ready for processing, and a pre-heating step to remove any moisture is often recommended. Similarly, as explained in Chapter 8, natural fillers are prone to water uptake and subsequent degradation, and require special attention during storage and additional drying before compounding (Nouryon, 2020).

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Figure 4.2 When preparing a composite filament for FDM, polymer and filler can be preliminary blended to improve homogeneity (“blending”), but the final goal is to achieve a thorough mixing of the filler into the polymer matrix (“mixing”).

The first step to produce a composite filament is to put the constituent phases, namely polymer matrix and filler (or fillers), together. In this respect, it should be mentioned that the terms “mixing” and “blending” are often used interchangeably in everyday language. Nonetheless, in science and in many industrial applications, especially in food manufacturing, there is a subtle difference, since “mixing” generally implies that many different substances, including both dry and wet ingredients, are put together to form a new product. As opposed to “mixing,” “blending” entails combining only dry components. (Dry) powder blending usually aims at creating a fine powder with the appropriate ratio of ingredients for further processing. In some instances, a small amount of liquid may be added to produce granules, but if the majority of the ingredients are dry, the process is still classified as blending (Dure Foods, 2019). When producing a composite filament for FDM, polymer and filler can be preliminary blended to improve the homogeneity of the filler distribution, however it is essential to achieve a thorough mixing of the filler into the polymer matrix. The difference is exemplified in Fig. 4.2. As previously mentioned, two main approaches are feasible to realize accurate mixing, namely the “hot-melt compounding method” and the “solvent mixing method.” In the hot-melt compounding method, the polymer matrix is heated until molten and the filler is dispersed into the polymer melt. In the solvent mixing method, the filler (or a liquid dispersion of the filler) is combined with a solution of the polymer matrix in an appropriate solvent that does not affect the filler. As a key advantage, the hotmelt compounding method is clean, safe and economical, because it does not require adding, handling or removal of any solvents, and therefore it is particularly suitable for industrial scale-up. On the other hand, the solvent mixing method usually facilitates a very even distribution of the constituent phases (Chong et al., 2022). In particular, solvent mixing assisted by (ultra)sonication can be very effective to break down filler agglomerates, which are clusters of individual fillers held together by inter-particle

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attraction forces (Nichols et al., 2002). Breaking down agglomerates is key to take full advantage of the reinforcing or functionalizing action of the filler, to minimize the risk of clogging the nozzle upon printing and to reduce local stress concentrations in the printed part.

4.3.1.1

Hot-melt compounding

The easiest way to accomplish mixing by means of the hot-melt compounding method is to directly feed polymer pellets and filler together into the extruder. Although theoretically very straightforward, unfortunately most of the times this method fails to produce a homogeneous filler distribution. In order to improve the dispersion, preliminarily the polymer pellets can be milled into a fine powder, whose particle size distribution is proportional to the particle size distribution of the filler (Masood and Song, 2004; 2005). For example, nylon pellets were ground to 500-800 μm in order to achieve a uniform distribution with iron particles ranging between 20 μm up to 100 μm (Masood and Song, 2005). Before compounding, the polymer and filler may be blended in single batches using a shaker, a roll mill, or a plastic bag. The mechanical action for blending contributes to breaking down potential filler aggregates and this obviously improves the dispersion. Alternatively, a more energetic blending action can be exerted with a planetary centrifuge. Otherwise, polymer and filler can be combined in an internal mixer, whose function is melting the polymer and distributing the filler through the polymer melt. To some extent, the functions of the internal mixer overlap with those of the extruder. However, the primary target of the internal mixer is to homogenize the mix and to remove bubbles and other defects, and not yet to produce the filament. The composite lumps that result from the internal mixer must be crushed and pelletized prior to being fed into the extruder for further processing. Since achieving a fine distribution can be particularly challenging for low filler loadings, repeated extrusion can be a viable strategy, whereby the filament is extruded first, chopped down and fed again into the extruder. Otherwise, especially if several different filler loadings should be considered for comparison purposes, a masterbatch can be produced with the highest filler loading. Then, the masterbatch can be chopped down and fed into the extruder with additional neat polymer to dilute the filler loading and obtain the targeted (lower) filler concentrations (Brounstein et al., 2021). Working with a masterbatch may be useful for safety reasons to minimize handling of loose nanoscale fillers (Berretta et al., 2017).

4.3.1.2

Solvent mixing

The key advantage of solvent mixing is that the presence of a liquid medium significantly improves the uniformity of the filler distribution (Siemann, 2005). Since the filler is dispersed in a liquid medium, sonication and even ultra-sonication can be used to break down filler agglomerates and improve the dispersion. Above a certain ultrasonic intensity, cavitation occurs in a low-viscosity fluid and, once created, the cavitation

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bubbles collapse and exert an extremely high strain rate in the surrounding fluid, which effectively separates agglomerated fillers. However, a drawback of sonication is that the delivered energy may be sufficient to damage the filler, especially if the filler is characterized by a high aspect ratio (for instance, this may occur for carbon nanotubes and other strongly elongated particulates). A trade off must be reached between the energy that is required for separating the individual particles and the energy that may cause the filler to fracture (Huang and Terentjev, 2012). Also, with the solvent mixing approach, the filler is readily dispersed and thus bound in a liquid (which may be the same polymer solvent, or another liquid vehicle compatible with the polymer solvent), which is extremely advantageous when handling nano-sized fillers that can be hazardous if inhaled or manipulated in the dry state. The primary requirement of the solvent mixing technique is to identify a solvent for the polymer that does not interact (dissolve or react with) the filler. Moreover, after completing the mixing step, the ideal solvent should be completely removed, preferentially in a short time. Eliminating any residual traces of solvent is crucial to avoid defects or technical issues during the subsequent steps of extruding and printing. However, thoroughly removing the solvent by natural evaporation is often a very lengthy process, if not even unfeasible, and therefore the combined action of heating and vacuum degassing is often necessary to tackle this issue.

4.3.2 The extrusion process The extrusion step is the core of the filament production process (Park and Fu, 2021). Very briefly, polymer and filler, either pre-mixed or in their original condition, are fed into the extruder, which is responsible for melting the polymer, redistributing the filler, and forming the filament at the exit of the spinneret. After leaving the spinneret tip, the filament is often stretched and cooled on a conveyor belt. Otherwise, it can be cooled down with a fan or even quenched in a water bath, sometimes added with ice. Cooling plays a pivotal role as it contrasts the die swelling effect, stabilizes the diameter of the filament, and for semi-crystalline polymer matrixes, governs the crystallization. Ultimately, the filament is spooled by means of a winding system that exerts a calibrated drawing tension to adjust the diameter of the filament. Extruders for FDM filaments can be single screw extruders or twin-screw extruders (Park and Fu, 2021). Multiple screw extruders also exist in the marketplace (Patil et al., 2016), but they are not practical for the production of filaments for FDM. Catalysts’ extruders, instead, are mainly used in the chemical industry. Although originally designed to process water-based pastes rather than polymer-bound systems, they can be adapted to the production of filaments with high filler loadings for SDS (Devyatkov et al., 2015).

4.3.2.1

Single screw extruders

Single screw extruders are very popular, because they are more affordable and easier to set-up and maintain than twin-screw extruders (Patil et al., 2016). Basically, the feedstock enters from the hopper into the feed throat and moves toward the exit due to the rotary motion of the screw. The polymer gradually changes into a melt as a

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Figure 4.3 Schematic representation of the cross section of the barrel of a twin-screw extruder either with corotating screws (“corotating”) or with counter-rotating screws (“counter-rotating”).

consequence of the combined action of the mechanical shear from the screw and of the heat from the barrel (Shrivastava, 2018). An important limitation of single screw extruders is that extrusion speed and temperature are not independent variables. In fact, in single screw extruders the material is mainly pushed forward by frictional forces and increasing the rotational speed increases the frictional heat, which may degrade thermally sensitive feedstocks. An additional drawback is that single screw extruders develop a very high pressure within the barrel, and this may compress the filler particles and cause them to agglomerate (Park and Fu, 2021). However, single screw extruders ensure a very high and consistent throughput even for heavily loaded composite systems (Patil et al., 2016). For this reason, they are often preferred as the last step in the production of filaments for SDS.

4.3.2.2

Twin-screw extruders

As compared to single screw extruders, twin-screw extruders have a more complicated structure, with a pair of parallel screws sitting inside a barrel with an 8-shaped cross section. Twin-screw extruders are comprised of several subsequent sections for different functions, such as feeding solids and liquids, for mixing, and for vacuum degassing. Each section can work at a different temperature in order to create the best temperature profile to melt the polymer and mix the filler without causing any thermal degradation. In twin-screw extruders heating comes from external sources and the temperature within each section can be regulated independently of the rotational speed (Patil et al., 2016). The two screws can be corotating or counter-rotating, as explained in Fig. 4.3. Corotating systems offer a higher self-cleaning efficiency and a closer control on the residence time. In counter-rotating systems, all the material is forced in the central part of the barrel where the two screws meet. Since the screws are intermeshed, not all material will be able to pass through, but the fraction of material that does pass experiences a high degree of shear that is conducive to a very fine and homogeneous distribution (Shrivastava, 2018). Although the detailed description of the two systems is beyond the scope of this book (very in-depth descriptions can be found in the literature regarding polymer processing technologies, for instance (Osswald, 2017)), the main point to remark is that twin-screw extruders are more complicated and expensive than single screw extruders, but have a superior mixing capability with a lower energy input (Patil et al., 2016).

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Another relevant point to consider is that extruders also differ in many other parameters, such as diameter, length, speed range, power limits and screw design. All these variables obviously impact the productivity of the extruder, which is often expressed in terms of extruded mass per unit time or, in the field of FDM, in terms of extruded length (of filament) per unit time. In many cases, the choice of the extruder should also take into consideration the specification of the die and the downstream equipment (Griff, 2018).

4.3.2.3

Improvement strategies

The quality of the as-produced filament may be impaired by several kinds of defects, especially shape fluctuations, diameter inconsistency, uneven filler distribution and presence of voids. A very common way to improve the characteristics of the filament, especially to remove air pockets and to improve the filler distribution, consists in cutting the filament and extruding it again repeatedly, until the desired properties are achieved (Díaz-García et al., 2020; Waheed et al., 2019). However, this increases the extrusion time and cost. Moreover, the prolonged exposure to high temperature and the multiple heating-cooling cycles may degrade the polymer and become detrimental to thermally labile molecules. Generally speaking, the extrusion step is particularly challenging for composite materials reinforced with natural fibers, because the extrusion temperatures that are required to melt the polymer are likely to damage the natural filler (Ahmed et al, 2020). Other experimental approaches are emerging to produce composite filaments with minimal laboratory equipment. For example, Díaz-García et al. (2020) have demonstrated the feasibility of composite magnetic filaments with uniform and repeatable filler distribution by encapsulating the maraging steel powders in custom-made polymer capsules that are fed in a simple single screw extruder. The composite capsules can easily reach the melting area of the extruder and favor a very fine and homogeneous distribution of the metal filler. In this way, consistent composite filaments can be obtained without running expensive equipment and without re-processing the feedstock material several times (Díaz-García et al., 2020).

4.3.3 Filament spooling After leaving the extruder, the filament can be stretched and simultaneously cooled down on a conveyor belt. However, the diameter and circularity of the filament largely depend on the appropriate adhesion of the filament to the belt. Otherwise, the filament is often cooled down or even quenched in ice-cold water. As previously mentioned, this is necessary to mitigate the die swelling effect (namely, as explained in Chapter 2, the recovery of the elastic component of the deformation received by the feedstock material upon flowing through the spinneret (Turner et al., 2014)), that otherwise would increase the diameter of the filament to exceed the targeted value, and to stabilize the circularity of the cross section (Stoof and Pickering, 2017; Stoof et al., 2017). Also, PLA and other polymers that are commonly used in FDM are semi-crystalline. Since the sudden drop in temperature at the exit of the die hinders the crystallization phenomena, the polymer

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is likely to remain in an amorphous state, which improves its printability (Mazzanti et al., 2019). After cooling down, the filament is progressively wound on a spool, which can be either integral to the extrusion line, as it often happens in desktop extruders for FDM filaments, or designed and added to the extrusion line on purpose. The speed of the spooling system should match the extrusion speed and should be chosen in such a way that the correct tension is induced on the filament to stabilize its diameter. The identification of the most appropriate winding speed is indeed pivotal to make the diameter consistent. After spooling, the filament should be protected from physical damage and kept in a controlled atmosphere to avoid moisture uptake, especially in case of hygroscopic polymer matrixes such as polyamides and PLA, and of natural reinforcements.

4.3.4 Filament diameter control and monitoring The first parameter that governs the quality of a filament is its diameter tolerance. In the scientific literature, for small-scale production as proof-of-concept, large variations are accepted, up to 19% (Ponsar et al., 2020). However, for 1.75 filaments, the gold standard across the industry for filament tolerance is +/-0.05 mm. In practice, a filament advertised as 1.75 mm will likely have an average measurement between 1.66 mm and 1.84 mm (Toor, 2017). The second parameter is roundness. Unlike diameter tolerance, scientific literature on the specific effect of roundness fluctuations is still limited. However, based on technical data, divergence from perfect roundness may be tolerated up to 5% of ovality (Toor, 2017). Fluctuations in the filament diameter or deviations from the nominal value may have adverse consequences upon printing, because current FDM printers do not compensate variations (Ponsar et al., 2020). If the diameter is too thin, the filament is likely to break under the load of the feeding mechanism, or cannot be pinched correctly. Also, a gap between filament and wall of the liquefier may occur as a consequence of the filament being too thin. Due to the reduced heat transfer, melting and flow of molten material through the printhead may be uneven or get interrupted, thus bringing about voids and faults in the printed part (Gkartzou et al., 2017). On the contrary, if the filament is too thick, the excess material is likely to cause clogging (Turner et al., 2014). Even if the job can be completed, fluctuations in the filament diameter lead to a very poor surface finish (precision3Dfilament, 2014). Adjusting the filament diameter is particularly challenging, because it depends on the interplay between numerous variables, such as properties of the melt, extrusion parameters (including screw speed, powder feed rate, barrel filling degree, temperature profile), cooling rate, and drawing tension. The easiest and most economical way to check the filament diameter is measuring it manually with a caliper. However, the operation should be repeated every few centimeters, in order to assess the uniformity of the diameter along the entire length of the filament, and should be performed with great attention, because the diameter should be tightly controlled (Toor, 2017). The main limitation, however, is that the measurement with a caliper is often conducted off-line

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Figure 4.4 The molecules of the sizing often have a functional group that links to the matrix and a functional group that links to the filler, thus bridging the matrix-filler interface.

and therefore it does not allow for real-time retrofitting. Working with a manual caliper is also prone to human error. Since the accurateness of the diameter is so critical for printing, many extruders for FDM filaments are equipped with an automated system for the in-line detection of the filament diameter. Typically, the measurement unit, for example a real-time laser-based module, is placed between the cooling stage and the winder. In automated systems, the in-line measurement determines the diameter of the filament as the filament is being extruded and this triggers the closed-loop adjustment of the extrusion and/or spooling parameters, especially the spooling speed, to achieve the targeted diameter. As a drawback, in-line measurement can be affected by vibrations and movements of the filament, thus requiring off-line verification (Ponsar et al., 2020).

4.4 Additional issues One of the main difficulties in processing composite feedstocks for FDM is the attainment of a strong bond at the matrix-filler interface, which is a prerequisite for the efficient transfer of loads from the matrix to the reinforcement in working conditions. The interaction may be very poor in hybrid composites as a consequence of the dissimilar nature of the organic matrix and the inorganic filler (Olonisakin et al., in press). Sizing and other surface treatments are frequently applied in conventional polymer-matrix composites in order to enhance the chemical affinity and similar approaches can be applied to produce composite filaments for FDM. Very often, the basic idea of a sizing is to introduce selected molecules having a functional group that links to the matrix and a functional group that links to the filler, thus creating a bridge at the interface, as exemplified in Fig. 4.4. Although organic in nature, also natural fibers frequently require a surface treatment to improve the interface bonding, because natural fillers are typically hydrophilic, whereas several polymer matrixes are hydrophobic (Olonisakin et al., in press). Natural fibers may receive additional treatments to adjust their cellulose content, structure and

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original moisture content (Azwa et al., 2013; Bledzki and Gassan, 1999; Faruk et al., 2012; Sanjay et al., 2019). Interestingly, the same surface treatment may target several issues at the same time. For example, silanes are very efficient coupling agents because they make the filler surface more hydrophobic, which improves the dispersibility, and, if their chemical structure is properly chosen, they also create a chemical bond between matrix and filler as shown in Fig. 4.4. Further, silane molecules can react with each other and build up a multi-layer structure on the filler surface, thus leading to a very tight siloxane network. As an additional advantage, it has been demonstrated that a selected sizing may protect the polymer matrix from possible filler-catalyzed thermal degradation effects. For instance, this is the case of PLA-zinc oxide (ZnO) composites. ZnO is known to catalyze both the transesterification reactions of PLA, which reduce the molecular weight of PLA, and the “unzipping” depolymerization of PLA, which ultimately results in the selective formation of lactide. However, this degradation effect can be mitigated by treating the ZnO particles with proper silanes, such as triethoxy caprylylsilane (Murariu et al., 2011, 2021; Pantani et al., 2013). When left at 200°C for 30 minutes, a high-grade PLA loaded with 3 wt% of ZnO nanorods experienced a weight loss of 31.7% with untreated particles, and of only 7.7% with triethoxy caprylylsilane-treated particles (Murariu et al., 2011). Nonetheless, surface treatments are rarely seen in the literature about FDM, likely because they imply additional costs and longer processing times, which is contrary to the philosophy of “rapid fabrication” that is a driving force of FDM and, generally speaking, of AM. Also, surface treatments commonly require solvents and other chemicals, thus compromising the “green” nature of hot-melt compounding methods. As an additional drawback, sizing agents may have unpredictable effects upon printing, which asks for accurate preliminary investigations. Sometimes plasticizers and surfactants are added to adjust the rheological behavior of the melt and to improve the filler dispersion, respectively. These modifiers are extremely important when the filler loading is very high. Maurel et al. (2019), for example, noticed that increasing the active material loading as much as possible is vital to obtain appreciable electrochemical performances that are required for printing electrodes, but this is detrimental to the integrity of the filament that becomes too brittle to be fed into the printer. To overcome this pitfall, Maurel et al. (2019) incorporated poly(ethylene glycol) dimethyl ether (average molecular weight of 500) in PLA as plasticizer. Various experiments were conducted to produce the composite filaments to print the negative electrode, the positive electrode and the separator to build up a lithium battery in a single job. The results lead to the conclusion that the total amount of fillers (sum of active material and conductive additives) must not exceed about 30 vol% of neat (un-plasticized) PLA matrix, otherwise the filament is not printable. However, the total amount of fillers can be increased up to 50 vol% when a plasticizer is introduced. Even more so, the role of plasticizers and surfactants is of utmost importance for the development of feedstock filaments for SDS, where the volume fraction of inorganic particles largely exceeds 45 vol% (McNulty et al., 1998, 1999). However, the presence of plasticizers and surfactants also has some relevant downsides. First of all, plasticizers and surfactants modify the behavior of the melt during printing. They are likely to

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promote filament buckling and to change the build-up process of the printed part. Moreover, plasticizers and surfactants are known to alter the conduction of heat and electricity, with adverse consequences on the establishment of conductive paths. Some additives cannot be incorporated in a stable way. For example, poly(ethylene glycol) dimethyl ether with a relatively high molecular weight (2000) and acetyl tributyl citrate gradually exude from PLA. Other additives, such as the aforementioned poly(ethylene glycol) dimethyl ether 500 and propylene carbonate, are volatile, and this reduces the shelf life of the filament (Maurel et al., 2018). Further, the ongoing evaporation may generate pores in the filament and ultimately in the printed part, with potential harm to the structural integrity (Maurel et al., 2019). Analogously, the potential effects of plasticizers and surfactants on biocompatibility and bioactivity become critical for biomedical applications. Some additives can have primarily an aesthetic function, which is the case of pigments to provide the material with special colors, to create a satin- or metal-like finish, or to correct the natural hue of the polymer (ABS, for example, is basically white, but with undesirable yellowish undertones) (Asiaban and Taghinejad, 2010). The homogeneous distribution of these additives plays a key role to ensure the aesthetic quality of the filament and, ultimately, of the printed part. It is also important to remark that “aesthetic” additives are known to modify the rheology and crystallization of the polymer matrix, with direct consequences on the printing conditions and even on the mechanical performance of the printed part (Asiaban and Taghinejad, 2010; Wittbrodt and Pearce, 2015). Some issues may arise with specific fillers. For example, the greatest part of inorganic fillers, including glass and carbon fibers, as well as many ceramic particles, are harder than copper, steel, and other metals that are commonly used to fabricate the components of blenders, mixers and extruders. The abrasive action of these fillers unavoidably undermines the strict geometric tolerances that are necessary for the correct functioning of the extruder’s components, such as the screws and barrels required for the accurate control of the filament diameter at the die exit. Some fillers, such as nanodiamond particles, are so abrasive that they need custom-made components in titanium or other wear-resistant materials to withstand deterioration issues (Waheed et al., 2019). On the other hand, the interaction with the internal surfaces of the extruder is known to damage brittle fillers, especially glass and carbon fibers, whose average length can be severely reduced during extrusion and printing operations (de Toro et al., 2020). Since the fiber length is one of the most important parameters that govern the matrixto-filler load transfer (Gibson, 2012), the fragmentation of fibers during extrusion can impair the efficiency of the reinforcing mechanisms in the printed part.

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Todoroki, A., Hirano, Y., 2016. Three-dimensional printing of continuous-fiber composites by in-nozzle impregnation. Sci. Rep. 6, 23058. http://doi.org/10.1038/srep23058. Maurel, A., Courty, M., Fleutot, B., Tortajada, H., Prashantha, K., Armand, M., Grugeon, S., Panier, S., Dupont, L., 2018. Highly loaded graphite-polylactic acid composite-based filaments for lithium-ion battery three-dimensional printing. Chem. Mater. 30, 7484–7493. http://doi.org/10.1021/acs.chemmater.8b02062. Maurel, A., Grugeon, S., Fleutot, B., Courty, M., Prashantha, K., Tortajada, H., Armand, M., Panier, S., Dupont, L., 2019. Three-dimensional printing of a LiFePO4 /graphite battery cell via fused deposition modeling. Sci. Rep. 9, 18031. http://doi.org/10.1038/ s41598-019-54518-y. Mazzanti, V., Malagutti, L., Mollica, F., 2019. FDM 3D printing of polymers containing natural fillers: a review of their mechanical properties. Polymers 11, 1094. http://doi.org/ 10.3390/polym11071094. McNulty, T.F., Cornejo, I., Mohammadi, F., Danforth, S.C., Safari, A., 1998. Development of a binder formulation for fused deposition of ceramics. Rapid Prototyp. J. 4, 144–150. http://doi.org/10.1108/13552549810239012. McNulty, T.F., Shanefield, D.J., Danforth, S.C., Safari, A., 1999. Dispersion of lead zirconate titanate for fused deposition of ceramics. J. Am. Ceram. Soc. 82, 1757–1760. http://doi.org/10.1111/j.1151-2916.1999.tb01996.x. Murariu, M., Doumbia, A., Bonnaud, L., Dechief, A.-L., Paint, Y., Ferreira, M., Campagne, C., Devaux, E., Dubois, P., 2011. High-performance polylactide/ZnO nanocomposites designed for films and fibers with special end-use properties. Biomacromolecules 12, 1762–1771. http://doi.org/10.1021/bm2001445. Murariu, M., Benali, S., Paint, Y., Dechief, A.-L., Murariu, O., Raquez, J.-M., Duboi, P., 2021. Adding value in production of multifunctional polylactide (PLA)-ZnO nanocomposite films through alternative manufacturing methods. Molecules 26, 2043. http://doi.org/10.3390/ molecules26072043. Nichols, G., Byard, S., Bloxham, M.J., Botterill, J., Dawson, N.J., Dennis, A., Diart, V., North, N.C., Sherwood, J.D., 2002. A review of the terms agglomerate and aggregate with a recommendation for nomenclature used in powder and particle characterization. J. Pharm. Sci. 91, 2103–2109. http://doi.org/10.1002/jps.10191. Nikzad, M., Masood, S.H., Sbarski, I., 2011. Thermo-mechanical properties of a highly filled polymeric composites for fused deposition modeling. Mater. Des. 32, 3448–3456. http://doi.org/10.1016/j.matdes.2011.01.056. Nötzel, D., Eickhoff, R., Hanemann, T., 2018. Fused filament fabrication of small ceramic components. Materials 11, 1463. http://doi.org/10.3390/ma11081463. Nouryon, 2020. Making composites from natural fibers. Nouryon, published September 2020. https://www.nouryon.com/news-and-events/features-overview/composites-from-naturalfibers/(accessed September 1, 2021). Okwuosa, T.C., Stefaniak, D., Arafat, B., Isreb, A., Wan, K.-W., Albed Alhnan, M., 2016. A lower temperature FDM 3D printing for the manufacture of patient­specific immediate release tablets. Pharm. Res. 33, 2704–2712. http://doi.org/10.1007/s11095-016-1995-0. Olonisakin, K., fan, M., Xin-Xiang, Z., Ran, L., Lin, W.S., Zhang, W., Wenbin, Y., in press. Key improvements in interfacial adhesion and dispersion of fibers/fillers in polymer matrix composites; focus on PLA matrix composites. Compos. Interfaces. DOI: http://doi.org/10.1080/09276440.2021.1878441. Olesik, P., Godzierz, M., Kozioł, M., 2019. Preliminary characterization of novel LDPEbased wear-resistant composite suitable for FDM 3D printing. Materials 12, 2520. http://doi.org/10.3390/ma12162520.

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Osswald, T.M., 2017. Understanding polymer processing: Processes and governing equations, 2nd ed., Carl Hanser Verlag, Munich, Germany http://doi.org/10.3139/9781569906484. Pantani, R., Gorrasi, G., Vigliotta, G., Murariu, M., Dubois, P., 2013. PLA-ZnO nanocomposite films: Water vapor barrier properties and specific end-use characteristics. Eur. Polym. J. 49, 3471–3482. http://doi.org/10.1016/j.eurpolymj.2013.08.005. Park, S., Fu, K.(K.), 2021. Polymer-based filament feedstock for additive manufacturing. Compos. Sci. Technol. 213, 108876. http://doi.org/10.1016/j.compscitech.2021.108876. Patil, H., Tiwari, R.V., Repka, M.A., 2016. Hot-melt extrusion: from theory to application in pharmaceutical formulation. AAPS PharmSciTech. 17, 20–42. http://doi.org/10.1208/ s12249-015-0360-7. Peng, F., Jiang, H., Woods, A., Joo, P., Amis, E.J., Zacharia, N.S., Vogt, B.D., 2019. 3D printing with core-shell filaments containing high or low density polyethylene shells. ACS Appl. Polym. Mater. 1, 275–285. http://doi.org/10.1021/acsapm.8b00186. Ponsar, H., Wiedey, R., Quodbach, J., 2020. Hot-melt extrusion process fluctuations and their impact on critical quality attributes of filaments and 3D-printed dosage forms. Pharmaceutics 12, 511. http://doi.org/10.3390/pharmaceutics12060511. precision3Dfilament, 2014. Filament tolerances. precision3Dfilament, Published July 29, 2014. https://precision3dfilament.com/blogs/news/15202543-filament-tolerances (accessed September 1, 2021). Sanjay, M.R., Siengchin, S., Parameswaranpillai, J., Jawaid, M., Pruncu, C.I., Khan, A., 2019. A comprehensive review of techniques for natural fibers as reinforcement in composites: Preparation, processing and characterization. Carbohydr. Polym. 207, 108–121. http://doi.org/10.1016/j.carbpol.2018.11.083. Shi, S., Chen, Y., Jing, J., Yang, L., 2019. Preparation and 3D-printing of highly conductive polylactic acid/carbon nanotube nanocomposites via local enrichment strategy. RSC Adv. 9, 29980. http://doi.org/10.1039/c9ra05684j. Shrivastava, A., 2018. Introduction to Plastics Engineering. Elsevier Inc., Amsterdam (The Netherland) http://doi.org/10.1016/C2014-0-03688-X. Siemann, U., 2005. Solvent cast technology – a versatile tool for thin film production. Progr. Colloid. Polym. Sci. 130, 1–14. http://doi.org/10.1007/b107336. Silva, M., Pinho, I.S., Covas, J.A., Alves, N.M., Paiva, M.C., 2021. 3D printing of graphenebased polymeric nanocomposites for biomedical applications. Funct. Compos. Mater. 2, 8. http://doi.org/10.1186/s42252-021-00020-6. Stoof, D., Pickering, K., 2017. 3D printing of natural fibre reinforced recycled polypropylene. Processing and Fabrication of Advanced Materials-XXV. The University of Auckland, Auckland (New Zealand), pp. 668–691. Stoof, D., Pickering, K., Zhang, Y., 2017. Fused deposition modelling of natural fibre/polylactic acid composites. J. Compos. Sci. 1, 8. http://doi.org/10.3390/jcs1010008. Toor, R., 2017. Is Recycled 3D Printer Filament Bad Quality? Filamentive, published October 30, 2017. https://www.filamentive.com/is-recycled-3d-printer-filament-bad-quality/#:∼:text= The%20first%20quality%20parameter%20is,1.66mm%20and%201.84mm (accessed September 1, 2021). Turner, B.N., Strong, R., Gold, S.A., 2014. A review of melt extrusion additive manufacturing processes: I. Process design and modeling. Rapid Prototyp. J. 20, 192–204. http://doi. org/10.1108/RPJ-01-2013-0012. Venkataraman, N., Rangarajan, S., Matthewson, M.J., Safari, A., Danforth, S.C., Yardimci, A., 1999. Mechanical and rheological properties of feedstock material for fused deposition of ceramics and metals (FDC and FDMet) and their relationship to process performance. In: Bourell, D.L., Beaman, J.J., Crawford, R.H., Marcus, H.L., Barlow, J.W. (Eds.), Solid

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Non-Print Items Abstract The key pre-requisite for a material to be printable by fused deposition modeling (FDM, aka fused filament fabrication, FFF) is that it must be easily processed into a filament with a round cross-section having a consistent diameter of 1.75 or 2.85 mm, which are the standard sizes in the marketplace. Whereas extruding neat polymer filaments is a well-established procedure, composite materials bring about new technical challenges because the presence of the filler modifies the thermal and rheological properties of the thermoplastic matrix. Specific applications may require a selective filler distribution, however, oftentimes the filler should be uniformly dispersed, which is usually accomplished by solvent mixing or by hot-melt compounding. Potential filler agglomerates should be carefully eliminated. The composite material must then be extruded into a filament with closely controlled geometry. Surface modification of the filler is often necessary to strengthen the interface bonding with the thermoplastic matrix, whilst surfactants and plasticizers may improve the workability, especially for high filler loadings. This chapter explores the effect that a filler may have on the extrudability of thermoplasticbased materials, compares different mixing strategies and introduces the functioning mechanisms of single screw and twin-screw extruders. Suggestions are provided to tackle common issues such as filler agglomeration, poor distribution or weak interaction with the polymer matrix. Keywords Fused deposition modeling; FDM; Fused filament fabrication; FFF; Composite material; Extrusion; Solvent mixing; Hot-melt compounding; Filler

Characterization and quality assurance in fused deposition modeling 5.1

5

Introduction: Properties and quality of filaments and printed parts

Numerous characterization methods are frequently applied in fused deposition modeling (FDM, aka fused filament fabrication, FFF) to examine the properties of the filament, before printing, and the quality of the final object, after printing. The goal of the following sections is to outline the kind of information that can be obtained about composite filaments and printed parts, whereas a detailed explanation of the functioning principles of the characterization techniques can be found elsewhere in specialized textbooks (Roylance, 2001; Zhang et al., 2008). To conclude, current challenges in quality assurance are discussed with a special attention for composite parts.

5.2

Materials characterization in FDM

ASTM F3049 (2014) describes the techniques for characterizing metal powder feedstocks used in powder-based additive manufacturing (AM) processes. Although mainly addressed to metal powders for binder jetting, directed energy deposition, and powder bed fusion, this standard provides general guidelines that may be extended to the filler particles to be used in FDM of composite materials. Depending on the particle size, sieving tests (applicable above 45 μm), light scattering and scanning electron microscopy (SEM) or transmission electron microscopy (TEM) image analysis can be applied to measure the particle size distribution. Although there are no standard methods to classify powder shapes, powder morphology can be investigated by light scattering and by image analysis. X-ray fluorescence and optical emission spectroscopy can be used for chemical analysis, whereas tap and skeletal density can be measured by a helium or nitrogen pycnometer (ASTM F3049, 2014; Rane and Strano, 2019; Vock et al., 2019). As for the filaments, oftentimes a visual inspection may be sufficient to spot out macroscopic features such as the presence of open pores or surface anomalies such as the sharkskin effect. However, observing the filament’s cross section by optical microscopy (OM) (Grubb, 2012) or by SEM (Inkson, 2016), the latter typically coupled with a local chemical analysis, is recommended to check the filler distribution and orientation, the formation of aggregates, the presence of internal voids of various types (for example, internal bubbles and voids at the matrix-filler interface), and potential Fused Deposition Modeling of Composite Materials. DOI: https://doi.org/10.1016/B978-0-323-98823-0.00011-1 c 2023 Elsevier Ltd. All rights reserved. Copyright 

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damages that may be caused both to the polymer matrix and to the filler by the combined action of heat and shear stress through the extruder. The analysis can be focused either on the cut and polished section of the filament, or on the fracture surface of a sample broken under tensile loading or other testing conditions. The analysis of the cut and polished cross section is more time consuming, because it implies the metallographic preparation of the sample, but gives accurate information about the microstructure of the filament. In particular, the orientation of the cutting plane to the filament axis can be controlled closely, which is important to prepare normal sections for the assessment of the volume fraction, distribution and preferential orientation (if applicable) of the filler and for the determination of the void content. The visual inspection of the polished cross section offers additional information about the geometric features of the filament, such as shape accuracy and consistency (Domm et al., 2021). Fracture surfaces are instead very meaningful to identify the “weakest link” of the microstructure that initiates failure. Although very rare, the recourse to TEM or to other high-magnification systems may be deemed necessary to visualize fillers in the nano-scale (Inkson, 2016). The density of the filament can be measured by means of the Archimedes’ principle. However, the density value obtained with this technique depends on two competitive variables, namely the filler content and the porosity content of the filament, and therefore should be complemented with other characterization methods such as image analysis of the cross section or fiber content determination via thermolysis or solvolysis. Mercury porosimetry provides useful information about the porosity, the skeletal and apparent density, the pore size distribution and the specific surface area of a sample. However, since pores are measured according to the intrusion of mercury into pores under increasing values of applied pressure, mercury porosimetry cannot be used to analyze closed pores (Giesche, 2006). Similarly, gas (helium) pycnometry measures the skeletal volume of a sample, which is the volume inaccessible to the gas. In this way, the volume of closed pores is compounded with the solid volume of the material. An additional limitation of gas pycnometry is that helium has some degree of permeability through low density solids, especially cellulosic materials and polymers, which may cause uncertainty in the measurement of the solid volume (Haugen and Bertoldi, 2017). X-ray computed tomography (CT) is a very accurate and reliable way to determine the three-dimensional architecture of composites, including size, shape and distribution of voids, as well as the distribution and orientation of filler particles (Chisena et al., 2020; Garcea et al., 2018; Little et al., 2012). Also, time-lapse X-ray CT enables to monitor time-dependent phenomena, such as processing-induced or in-service degradation (Garcea et al., 2018). Chisena et al. (2020) applied the micro-CT technique to short carbon fiber-reinforced filaments and printed parts with a minimum allowable feature size of 3.3 μm3 for the former and of 4.5 μm3 for the latter. The micro-CT conducted on the filament samples returned a filler loading of 36.2 vol%, which was very close to the nominal value (35 wt%), and demonstrated a preferential orientation of the fibers parallel to the extrusion direction. The pore volume fraction was as low as 1.6 vol%, with the pores being radially arranged within the filament. Whereas the largest pores were detected near the center of the filament, the highest pore density occurred at the periphery of the filament’s cross section. Chisena et al. (2020) proposed

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various possible mechanisms to justify this preferential pore distribution. During the extrusion process, large bubbles can remain entrapped in the filament core, whereas small pores can develop on the outer rim of the filament as a consequence of the cooling down step after extruding. Another reason can be the uneven thermal shrinkage, with the periphery of the filament cooling down and thus contracting more quickly than the core. In the printed parts, the porosity significantly increased to 9.8 vol%. Examination of the cross sections parallel to the growth direction (z axis) revealed that the porosity was almost absent below the interface between subsequent layers, but it was very high just above the interface. Chisena et al. (2020) argued that this preferential distribution of pores was intrinsically caused by the build-up process, with air bubbles being able to escape from the top of the newly deposited material, but irreversibly entrapped at the bottom. Additional reasons for the layer-wise distribution of pores could be the thermal gradient, stress distribution and differential contraction between bottom and top of the bead. The extrusion-induced orientation of the short carbon fibers survived to a great extent in the printed part, but was locally disrupted at the intersection between beads deposited under different printhead travel trajectories (Chisena et al., 2020). As demonstrated by the microstructural characterization reported by Chisena et al. (2020), micro-CT is a powerful tool to explore the volume fraction and distribution of pores and fillers. However, X-ray CT may be only viable to map relatively small volumes of materials. According to the detailed discussion provided by Garcea et al. (2018), the sample should be no larger than 1000–2000 times the smallest feature size of interest. For example, Chisena et al. (2020) had to cut a 3 mm long fragment from the filament and to excise a cube specimen with 3 mm edge length from the printed part for micro-CT scanning. In this regard, the maximum size of the measurable sample can compromise the “non-destructive” nature of micro-CT. Also, appropriate strategies have been developed to tackle other potential challenges of micro-CT, as images of polymer-matrix composites can suffer from poor phase contrast between matrix, filler and pores or cracks, and from long acquisition times (Garcea et al., 2018). In principle, ultrasound-based methods can be useful for the identification of internal pores and, unlike SEM and other microscopy techniques, they are not destructive and can be easily performed in-situ (Jin et al., 2020). Ultrasonic testing can be conducted off-line on filaments and printed parts to determine the presence of defects. Additionally, ultrasonic testing is one of the most powerful techniques for in-situ monitoring of AM processes (Honarvar and Varvani-Farahani, 2020). However, depending on the technique in use, it has been reported that pores smaller than 0.5 mm may be difficult to detect due to the low penetration depth and low resolution associated with the sound wavelength (Chisena et al., 2020; Zeltman et al., 2016). The crystallographic composition of the raw materials (filler and pristine polymer) and of the extruded filament is assessed by X-ray diffraction (XRD). This is important to verify that the original crystalline nature of the filler has not been altered by the extrusion process, and to assess the extent of the crystallization phenomena potentially occurred in semi-crystalline polymer matrixes, such as PLA and PCL, that may be influenced by the presence of the filler (Epp, 2016). The degree of crystallinity and the nature of the crystalline phases eventually present in the composite material affect the mechanical performance, as well as the thermal behavior of the filament upon printing.

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Ideally, a new composite filament should be printable on any FDM equipment working with the appropriate diameter in order to obtain rapid market uptake and be commercially successful. To this aim, the first requirement to check is that the rheological behavior of the composite material corresponds to the rheological behavior of standard polymers for FDM. Common terms of comparison are indeed neat acrylonitrilebutadiene-styrene (ABS) and neat poly(lactic acid) (PLA) filaments. The rheological behavior of the composite material at different temperatures can be thoroughly examined with a rheometer (Osswald and Rudolph, 2013). Different technologies are available to measure the rheological properties of polymers and composites on-line and off-line. Capillary rheometers are usually preferred for measuring melt properties at higher shear rates than rotational rheometers, and allow the flow behavior under typical processing conditions to be determined. The functioning principle of a capillary rheometer is indeed similar to the FDM system, in that a solid piston pushes a molten pool of polymer through a narrow orifice (Das et al., 2021). Otherwise, a first insight in the rheological behavior can be attained by measuring the melt flow index (MFI). This technique is very common in industry, because a single-point measurement is taken under standard testing conditions that are specific to each material. According to ASTM D1238 (2020), polymer pellets (or granules of a pre-compounded composite) are heated in the barrel of the melt flow indexer until molten. Then, the melt is extruded through a short cylindrical die using a piston actuated by a weight. The filament is cut after 10 min. The weight (in grams) of the polymer extruded during the 10-minute test is the MFI of the polymer. Similarly, ISO 1133-1 (2011) defines the MFI as the mass of molten polymer that flows through a standard die (2.095 mm x 8 mm, same as per ASTM D1238 (2020)) at a given temperature and under a given standard weight applied to the piston that pushes down the material. However, ISO 1133-1 (2011) describes two different procedures. According to Procedure A, the melt mass-flow rate (MFR) is expressed in g/10 min and is determined by the manual measurement of the mass of each material segment that is extruded in 10 minutes. In Procedure B, instead, the melt volume rate (MVR) is automatically measured as the piston displacement. The MFI can be inferred from the MVR provided that the density of the melt is known or separately measured at the same temperature (ISO 1133-1, 2011). When testing the printability of a new feedstock, measuring the MFI gives a simple estimate of the “flowability” of the material, which can be compared to standard filaments. Creep recovery test and relaxation-stress test are two rheological tests that provide information about the viscoelastic properties. In a creep recovery test, the sample is subjected to a constant stress and the strain is measured as a function of time. At the end of a fixed time interval, the stress is removed and the recoverable strain of the sample is measured as a function of the recovery time. A relaxation-stress test is intuitively the counterpart of a creep recovery test, since a fixed strain is applied to a sample that is initially stress-free and then the stress required to maintain the strain is monitored as it decays over time. Although viscoelastic measurements are conventionally conducted on samples in the solid state by differential thermal analysis (DMA, which is also called DMTA for dynamic mechanical thermal analysis) as described in the following paragraphs, they can also be performed on a rheometer and can be employed, for example, to investigate the die swelling effect (Das et al., 2021).

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Differential thermal analysis (DTA) and differential scanning calorimetry (DSC) give information about the structural transformations, such as phase transitions, order-disorder transitions, glass transitions, and about the occurrence of chemical reactions, as the temperature changes. In DTA, the instrument measures the difference in temperature between the sample and the inert reference while the sample and the reference are subjected to a controlled heating/cooling program. In DSC, the instrument measures the difference in energy input that is required to keep the sample and the inert reference at the same temperature while the sample and the reference are subjected to a controlled temperature program (Schick et al., 2012). Very often, thermal tests on polymers include multiple heating and colling steps, because the first heating ramp is necessary to erase the thermal history of the polymer. The usage of DSC is widespread for determining the potential glass transition temperature(s), crystallization temperature(s) and melting temperature(s) of polymers, polymer blends and polymer-matrix composites, as well as the reaction temperature(s) if any, and the energy involved in each reaction, which can also be manipulated to calculate the degree of crystallization or, for thermoset polymers, the degree of curing. The presence of fillers is expected to change the thermal behavior of the polymer, sometimes in an unpredictable way, because fillers simultaneously affect various properties such as the molecular chain mobility and the thermal conductivity of the polymer. The thermal characterization of a composite feedstock is therefore essential to determine its behavior upon heating and cooling, which is conducive to the appropriate choice of the printing temperature and, whenever available, of the chamber or platform temperature. Thermogravimetric analysis (TGA) measures the changes in weight occurring in a material sample as a function of temperature and time. Conducting a TGA is the most straightforward method to verify the filler loading that is actually present in the filament after extrusion, which is often lower than the target value as a consequence of filler agglomeration and pore formation. However, this method (thermolysis) should be purposely adjusted to deal with organic fillers that would thermally decompose in the same range of temperature as the polymer matrix (Nabinejad et al., 2015). As an alternative, the matrix can be removed by selective chemical methods (solvolysis). Otherwise, the filler loading can be estimated via image analysis of the filament cross section (Domm et al., 2021). Gel permeation chromatography (GPC), a type of size-exclusion chromatography (SEC), is widely utilized to determine the molecular weight distribution (MWD) of polymers (Moore, 1964). The test is often repeated at different stages from virgin polymer to printed parted in order to record the potential damage occurred to the polymer chains through each processing step. The mechanical properties of experimental filaments are frequently determined by means of tensile and bending tests. The combination of “longitudinal resistance” and “transverse resistance” is sometimes referred to as the “mechanical resilience” of the filament and it is a key factor regarding the printability (Korte and Quodbach, 2018; Zhang et al., 2017). As for the longitudinal properties, much attention in the literature is being paid to the tensile behavior (modulus of elasticity; tensile strength) of the filament. However, the results of tensile tests on filaments should be considered with caution, due to the difficulty of clamping the sample (Domm et al., 2021). Further,

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as discussed in Chapter 2, the risk of buckling at the liquefier entrance depends on the compressive properties of the filament, rather than on its tensile properties, and therefore compressive tests should be carried out in addition to tensile tests (Venkataraman et al., 1999, 2000). Another point of concern is the absence of international standards for testing the mechanical properties of filaments. Janek et al. (2020) have recently pointed out that mechanical tests on fibers and other stringlike materials are preferentially conducted on long samples, because these provide a better statistical representation. However, when a new material is being developed for FDM, few meters of experimental filament are usually available for testing and therefore working with shorter samples would be preferable to have a statistical population. Unfortunately, the choice of the length of the sample may affect the results of mechanical testing. According to the results reported by Janek et al. (2020) for two different polyvinyl alcohol (PVA) filaments loaded with 50 wt% of hydroxyapatite (HAp) and for a commercial PLA filament loaded with about 27 wt% of gypsum, the tensile strength did not depend on the sample length, but the compressive strength drastically decreased when the nominal distance of the gripping heads was increased from 16 mm, to 36 mm and then up to 76 mm. With the filament diameter being constant, the increasing gripping distance resulted in a higher slenderness ratio and hence in a stronger tendency to Euler buckling (Janek et al., 2020), which suggests the need for the definition of standard test procedures in order to obtain comparable experimental results. Filaments should not break under transverse loads, for example between the gears of the feeding mechanism. Bending tests are often conducted to estimate the transverse strength of a filament, because the load in the gear mechanism is applied normal to the filament axis (Korte and Quodbach, 2018; Zhang et al., 2017). Also, bending tests are useful to evaluate the ability of the filament to be bent without breaking, which is informative about its “spoolability” (Wu et al., 2017). Combining longitudinal and transverse tests is important to gain a comprehensive understanding of the mechanical properties of a filament, especially if the material behavior is anisotropic (Korte and Quodbach, 2018; Zhang et al., 2017). Since no international standards exist to regulate these types of tensile/compressive and bending tests on filaments, mechanical tests in the literature are not performed in a uniform way and therefore the obtained results should be compared with caution. Although DMA is more common for printed parts than for filaments, this kind of measurement can be conducted in addition to or in place of static tests to attain a deeper understanding of the thermo-mechanical behavior of the filament. The mechanical performance of polymeric materials is indeed very sensitive to temperature, and DMA can be resolutive to reveal to combined effect of temperature and mechanical load. In its easiest version, DMA implies to apply an oscillating “load”, which can be a fixed stress or strain, within the elasticity range of the polymer (or composite) and to measure the response of the material as the temperature increases (Chartoff et al., 2009). The primary information given by this procedure is the dependence of the storage modulus on temperature. The storage modulus, which is related to the stiffness (elastic response) of the material, drops across a range of temperature that is representative of the glass transition temperature of the polymer. Additionally, the loss modulus, which is related

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to the dissipative (plastic) response of the material, can be quantified as a function of temperature. The so-called “tan δ” is defined as the ratio between the storage modulus and the loss modulus. If performed on a composite feedstock, the analysis can demonstrate the effect of the filler on the storage modulus and on the glass transition temperature of the neat matrix. As to the glass transition temperature, it should be underlined that its nominal value can be determined by different techniques, especially DMA and DSC (also, by DTA). However, if the glass transition temperature can be seen with both methods, it is very likely that the value obtained by DMA is different from that obtained by DSC, with a discrepancy of up to 25°C (Fernandes et al., 2021). There are various reasons for this discrepancy. First of all, the glass transition occurs on a temperature range and therefore the identification of a single “representative” value is always arbitrary, although inspired by some general rules. Then, the two techniques are based on different physical phenomena (thermally-induced structural relaxation for DMA, heat flow for DSC). Further, the two methods also apply to different kinds of samples (centimeter-sized specimens having a defined geometry for DMA, fragments weighing a few milligrams for DSC), which may cause some delays in heat transfer in larger samples. Specific tests can be planned in view of specific applications. For example, dedicated measurements can be conducted to quantify the thermal and/or electrical conductivity of the filament, if the targeted FDM part is intended to be conductive. Analogously, bioactivity tests are mandatory for any material directed to biomedical applications. Many tests are repeated on the filament, first, and on the corresponding printed parts, later on. This is essential to evaluate the effect of printing on the properties of the finished part. In fact, the behavior of the final component depends both on the feedstock material and on the printing parameters. For example, it is commonly observed in the body of literature that the tensile strength of the filament is higher than that of the corresponding FDM parts. This is due to the occurrence of printing-induced defects, such as inter-bead and inter-layer faults deriving from a poor sintering process, or to the thermal degradation within the liquefier. Analogously, as previously mentioned, several GPC analyses may be scheduled to track the MWD of the polymer (or polymer matrix) at different processing stages. Due to their geometry, filaments are not suitable for all characterization techniques. Standard XRD equipment, for instance, preferably works on relatively large and flat samples. If this is the case, custom samples may be prepared by applying the same processing conditions, but extruding different geometries, such as sheets or rods. However, this may change some features of the samples, like the polymer chain orientation, thus affecting the result of the characterization (Chan and Lee, 1989). To conclude, it is worth noting that, apart from checking the value and consistency of the filament diameter which can be accomplished on the extrusion line directly, most of the analyses mentioned above are performed ex-situ (namely post-mortem), since a fragment must be cut off from the filament to complete the test. Moreover, as previously mentioned, the lack of international standards to define repeatable measurement procedures limits the relevance of the experimental results, which should be understood as merely comparative values. As a consequence, the characterization of composite filaments for FDM, as it is performed now in the literature, is more suitable

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Figure 5.1 Exemplary tensile (stress-strain) curves for brittle and ductile materials.

for an exploratory assessment of the effect of a filler on the filament’s properties than for a systematic quality check in industrial production, where potential defects should be detected and corrected timely to limit the waste of material, time and energy.

5.3 Characterization issues with printed parts Alongside materials development, metrology is currently recognized as one of the two key enabling technologies of AM to fully achieve its effectiveness (Tofail et al., 2018). International standards to normalize the characterization methods for parts produced by FDM and, broadly speaking, by AM are still under development. Since the reasons for variability are numerous when testing AM parts (Dal Maso and Cosmi, 2018), the lack of standard guidelines for testing may cause fluctuations in the reported data and thus hamper the ability of stakeholders to compare materials and machines (Forster, 2015). This is particularly obvious with tensile and other mechanical tests. Exemplary tensile (stress-strain) curves for ductile and brittle materials are shown in Fig. 5.1. For example, Goh et al. (2018) detected a significant discrepancy between their experimental values and the manufacturer’s data for the tensile and flexural modulus of continuous fiber-reinforced parts processed from the Markforged Mark One technology (Markforged on the Mark Two launch, n.d.). Both sets of values were correct, with the difference being attributable to the non-standardized methods applied to measure the strain. For example, on account of the anisotropic behavior that is frequently observed in FDM parts and, generally speaking, in many AM components, just the simple orientation of a tensile sample on the build platform may significantly impact its performance (Ahn et al., 2002). Following the nomenclature in Fig. 5.2, the literature

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Figure 5.2 Schematic representation of a tensile sample (dogbone-like; dimensions not corresponding to international standards) printed flat on the build platform, on an edge, or upright in a vertical position parallel to the growth direction.

presents contrasting results about the effect of printing flat on the build platform as opposed to printing on an edge. However, printing in the vertical direction, that is, parallel to the growth direction, is universally considered as the worst condition (Das et al., 2021; Gordelier et al., 2019; Rahim et al., 2019). The anisotropy of FDM parts built under different orientations is known to affect not just the tensile behavior, but also the fatigue performance, with the growth direction often being the most problematic one due to the stratification of inter-layer surfaces (Shanmugam et al., 2021a; 2021b). The absence of specific regulations is primarily due to the extraordinarily fast growth of AM techniques, which are now becoming mainstream, with a global industry accelerating beyond aerospace and biomedical niches. Although very active in the field, international organizations for standardization have so far not been able to keep pace with the technological advancement. However, as depicted in Fig. 5.3, the slow implementation of characterization standards also derives from some intrinsic features of AM. In fact, AM is a cost-effective solution for the high-mix-low-volume fabrication of customized products (Kwok et al., 2017). Since one-off parts are designed and printed to fit specific applications, it is not possible to define general functional requirements. Concurrently, as the production volumes are typically very low, 3D printed parts are expensive and hence conventional destructive characterization techniques are not economically viable. Moreover, significant differences may exist between the properties of testing specimens, whose geometry is typically very simple and linear, and the real performance of manufactured end-parts, whose geometry is often very complicated (Popescu et al., 2018). With AM techniques, composites are created at once with the part’s production and the effects of materials properties and geometry may become blurred. As the FDM community waits for dedicated guidelines for AM parts, standards for polymers, for conventional composites and for laminates are often used instead

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Figure 5.3 Main reasons for the delayed publication of international standards concerning the characterization of additively manufactured parts.

(Dizon et al., 2018; Ferreira et al., 2017; Gkartzou et al., 2017; Justo et al., 2018; Ning et al., 2015; Shanmugam et al., 2021a, 2021b; Word et al., 2021). However, ordinary characterization techniques should be applied with caution, because not all of them are able to account for the peculiarities of AM components. For example, Goh et al. (2018) demonstrated that micro-CT, with a minimum detection threshold of 16.4 μm, underestimated the porosity of continuous carbon fiber-reinforced parts produced with the Markforged Mark One technology to 2.7 vol%, vs 10 vol% as determined from the density calculation method. Moreover, in the same contribution, Goh et al. (2018) were not able to ascertain the possible formation of micro-cracks in the carbon fibers as a result of the laying down process, since micro-cracks could not be observed below the detection threshold of the laser scanning confocal microscope in use, nor of the microCT scanner. This demonstrates the need for new techniques and standardized methods to univocally assess the properties and performance of AM parts (and of FDM parts among them), which is a prerequisite for the growth of AM in industrial settings. Back in 2015, Forster (2015) worked out a detailed analysis of the applicability of existing standards to mechanical testing of polymer materials and parts in AM. In order to provide an update with a specific focus on FDM of composite materials, two tables are included in Chapter 12 to give an insight into the protocols that are currently applied in the scientific literature regarding tensile properties (Chapter 12, Supplementary table 2a) and flexural (bending) properties (Chapter 12, Supplementary table 2b) of composite filaments and printed parts. These tables do not consider lattices, because they require specific techniques to account for their porous architecture, do not consider fully inorganic parts, because they are not polymermatrix composite anymore, and do not consider FDM parts printed from commercially available colored filaments, because in the literature they are usually treated

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as mono-material systems. Sometimes, printed coupons receive post-processing operations, such as cutting and sewing, or finishing treatments to meet the requirements of the applied standard. For example, continuous fiber reinforced parts can be printed and then cut to remove the turns of the fibers at the part’s periphery and thus to obtain neat unidirectional samples (Blok et al., 2018). Mechanical tests are often conducted according to “customized” protocols (no international standards are considered), and “customized” approaches are indeed necessary when assessing the properties of irregular geometries, such as curved parts (Zhang et al., 2020). However, many authors prefer to conform to international standards originally developed for “conventional” (not AM) materials. According to the data in Supplementary table 2a (Chapter 12) and in Supplementary table 2b (Chapter 12), the most commonly applied standards in the literature to determine the tensile properties of composite parts produced by FDM are thus: r

r r

ASTM D638, “Standard test method for tensile properties of plastics” (ASTM D638, 2014); according to ASTM D5592 (2018), ASTM D638 can be extended to the development of engineering design properties for load-bearing plastic components (Forster, 2015), ASTM D3039 / D3039M, “Standard test method for tensile properties of polymer matrix composite materials” (ASTM D3039 / D3039M, 2017), and ISO 527, “Plastics — Determination of tensile properties — Part 1: General principles” (ISO 527-1, 2019),

Whereas the prevailing standards for flexural tests are: r r r

ASTM D790, “Standard test methods for flexural properties of unreinforced and reinforced plastics and electrical insulating materials” (ASTM D790, 2017) ISO 14125, “Fiber-reinforced plastic composites — Determination of flexural properties” (ISO 14125, 1998), and ISO 178, “Plastics — Determination of flexural properties” (ISO 178, 2019),

The same testing conditions may be applied both to the original filament and to the printed part, in order to obtain comparable data. However, the geometry is completely different, with filaments being strand-like sanples, as opposed to printed samples that are typically (dumbbell-shaped) dogbones or rectangualr coupons, Apart from few exceptions such as the contribution by Bi et al. (2020), which deals with continuous fiber-reinforced filaments and printed parts and applies ISO 3597 (2003) to evaluate the flexural properties of the composite filaments, bending tests on filaments are prevalently carried out according to customized protocols. These may be intended to evaluate the flexibility and handleability of the filaments in addition to its mechanical properties (Wu et al., 2017). A recurrent issue with tensile and flexural tests on printed parts is the prescribed size of the samples, that must be often scaled down to meet the limits of common build platforms or to save feedstock material and printing time (Dal Maso and Cosmi, 2018; Gkartzou et al., 2017; Ning et al., 2017a, 2017b; U¸sun and Gümrük, 2021). This point would likely deserve further attention in future studies. Whereas some researchers did not observe any size effect due to scaling down the samples (Ning et al., 2017a, 2017b), Drummer et al. (2012) investigated the mechanical behavior of PLA-tricalcium phosphate composite parts and reported that larger samples achieved higher stiffness.

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Similarly, larger parts had higher tensile strength and elongation at break. Drummer et al. (2012) attributed the superior tensile performance of larger parts to two synergistic factors. Firstly, large and small parts were printed with the same bead diameter. As a consequence, larger samples required a higher number of beads to complete each layer, which led to a higher stiffness. Secondly, in larger samples the effect of potential weak points on the part’s surface was less impactful due to the larger cross section (Drummer et al., 2012). Tensile properties are sometimes deduced from toughness tests such as ASTM D5045 (2014), which is for example the approach followed by Papon and Haque (2019). Modified bending tests are often performed to measure the interlaminar shear strength (ILSS) of printed parts. The most popular standards to this aim are ISO 14130 (Fiber-reinforced plastic composites — Determination of apparent interlaminar shear strength by short-beam method (ISO 14130, 1997)), as proposed, for example, by Caminero et al. (2018) and by Domm et al. (2017), and ASTM D 2344 / D2344M (Standard test method for short-beam strength of polymer matrix composite materials and their laminates (ASTM D2344 / D2344M, 2016)), as described by Berretta et al. (2017), Fernandes et al. (2021), by Mosleh et al. (2021) and by O’Connor and Dowling (2019). However, also customized bending tests are occasionally performed to estimate the ILSS (Azarov et al., 2019)

5.4 Characterization of continuous fiber-reinforced parts A concluding remark should be addressed to the difficulty of testing the mechanical properties of continuous-fiber reinforced parts printed by FDM. AM techniques make it possible to print customized geometries and tabs, but the preparation of the specimen geometry is crucial to obtain reliable and consistent results. In the absence of a specific international standard, tensile properties are often measured according to ASTM D638, “Standard test method for tensile properties of plastics” (latest release of the standard: (ASTM D638, 2014)), and according to ASTM D3039/3039M, “Standard test method for tensile properties of polymer matrix composite materials” (latest release of the standard: (ASTM D3039/D3039M, 2017)). However, these test methods are addressed to polymers and composite materials produced with conventional methods and therefore they do not take into account the specificity of AM parts (Ahn et al., 2002). Croccolo et al. (2013) revised the geometry of ASTM D638 dumbbell-shaped samples (latest release of the standard: (ASTM D638, 2014)) to reduce the stress concentration at the fillet and thus to avoid premature failure of FDM plastic parts. Subsequently, Pyl et al. (2018) focused on continuous carbon fiber-reinforced parts printed with the so-called “dual nozzle” method (which implies printing a neat polymer filament and a continuous-fiber reinforced one through two separate nozzles (HeidariRarani et al., 2019; Kabir et al., 2020; Zhang et al., 2021)) and compared the tensile behavior of several types of specimen geometries, including ASTM D638 dumbbellshaped samples (latest version of the standard: (ASTM D638, 2014)), the variant proposed by Croccolo et al. (2013), and constant rectangular specimens with longitudinal fiber orientation according to ASTM D3039/D3039M (latest version of the standard:

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(ASTM D3039/D3039M, 2017)). The effect of glued tabs and printed end tabs (with and without fiber reinforcement) was also investigated. In order to avoid premature failure due to a bad fiber arrangement, the start/stop point of the reinforced filament in each layer was located in the end tabs and changed from layer to layer in order to spread as much as possible the potential locations for damage initiation. Under tensile load, flat rectangular specimens performed better than standard and modified dumbbell specimens. This is predictable, since additional start/stop points and additional neat polymer matrix are required to print the specimen’s shoulder, which is a well-known failure-prone region of dumbbell shaped-specimens because the cross-sectional area steeply changes from the large end tabs to the narrow gauge section. Non-bonded paper tabs or a light abrasive between the grip and a rectangular specimen were proved to be an efficient solution to obtain repeatable results (Pyl et al., 2018). Analogously, in the absence of a dedicated standard test method for the determination of the shear properties of continuous fiber-reinforced parts, the ILSS is often determined with bending tests on a short beam configuration, according to customized protocols (Azarov et al., 2019) or according to existing standards such as ASTM D2344/D2344M, “Standard test method for short-beam strength of polymer matrix composite materials and their laminates” (ASTM D2344/D2344M, 2016), or ISO 14130, “Fibre-reinforced plastic composites — Determination of apparent interlaminar shear strength by short-beam method” (ISO 14130, 1997), as described above.

5.5

Quality assurance

According to the stages of the product life cycle, quality control activities in conventional manufacturing can be classified into incoming quality control, in-process quality control and outgoing quality control, also known as quality assurance. This classification in AM should be extended to account for the preliminary product design and process planning (Wu and Toly Chen, 2018). As a matter of fact, the performance of 3-dimensional (3D) and 4-dimensional (4D) printed objects (where 4D printing refers to 3D printing of stimuli-responsive materials having the capability to transform over time (Tibbits, 2014)) depends as much on the materials properties as on the part’s design, on the printing history and, in case they are required, on the post-printing and surface finishing treatments (Kim et al., 2018). Consequently, testing the behavior of the printed part as a whole, in addition to verifying the material- and process-related variables, is essential for quality control in AM. Whereas some examples of engineering process control applied to FDM have been proposed in the literature (a brief summary was published by Huang et al. (2015)), conventional statistical approaches to quality control can be hardly applied to bespoke and one-of-a-kind parts. In fact, since FDM is rarely run in mass production, the available data for a specific kind of product is insufficient for a statistical process control (Wu and Toly Chen, 2018). Hollister et al. (2015) designed a quality control procedure where sacrificial parts are produced on purpose to preliminary conduct destructive tests and verify the appropriateness of the printing conditions to meet specific functional requirements. The flowchart proposed by Hollister et al. (2015) also includes additional non-destructive tests (for example, by micro-CT) to ensure the final

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part actually meets the design inputs. This two-stage verification method, originally conceived for modular scaffolds printed by selective laser sintering (SLS), has the potential to be extended to other AM techniques, including FDM, and may actually provide the necessary framework for quality control in critical and load-bearing applications, such as in the biomedical and aeronautic industries. However, it still requires iterative testing, which increases lead time and materials’ waste (Kim et al., 2018). In order to enable a statistical approach to quality control that tackles these issues, it has been suggested that data extracted from a specific FDM process could be normalized based on the product design or based on the printing path of each layer in order to generalize the information and make it suitable for a statistical process control, but additional research would be required to validate the feasibility of this approach (Huang et al., 2015). Quality control of AM products is very challenging also because the attributes (primarily aesthetics, conformance to specifications and performance) largely vary with the materials in use, with the processing conditions, and with the object produced. For example, Wu and Toly Chen (2018) argued that warping may be a serious issue for parts with slender and elongated shapes, but not for parts featuring vertical structures with uniform cross-sectional area. Whereas the American Society for Testing and Materials International (ASTM International) has issued some guidelines for fabricating safe and high-quality devices by using PBF methods involving laser and electron-beam sources (ASTM on additive manufacturing, n.d.), the absence of dedicated international standards and the rapid progress currently happening in materials and technologies are additional issues that impact quality control of FDM parts. In other words, as stated by Wu and Toly Chen (2018), there are multiple options, but not an absolute rule for choosing among them.

5.6 Quality assurance for the International Space Station As recently reported by O’Hara IV et al. (2018), the challenges in quality assurance of AM parts became particularly obvious when an FDM printer was applied to manufacture the first AM device to be used by the crew as part of the nominal operations onboard the International Space Station (ISS). In 2015–2016, the opportunity arose to print onboard the plastic adapter that connects the oxygen generator system and the hotwire anemometer that are part of the airflow system of the ISS. The conceptual design of the adapter took several months to account for the critical dimensions and tolerances of the adapter, which are essential to achieve a proper fit between the adapter and the other pieces of equipment onboard. Also, supports and some structural features of the adapter were redesigned to strike a balance between amount of feedstock material required to print and probability of having a successful print. Another concern was that different printers may have different levels of accuracy. After some preliminary tests directed to demonstrate the printability of the adaptor, it was decided that the prototypal parts to verify the validity of the design would be printed using exactly the ground equivalent of the onboard printer. Even so, some doubts remained about the correspondence between ground-printed parts and in-flight-printed parts as a consequence of the microgravity

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environment. With the design optimization being almost completed, it was necessary to activate the procedure to review the safety of the adaptor’s design and usage, as every single piece of hardware that goes to (or is printed onboard) the ISS must receive a safety hazards assessment. This started a very complicated discussion between the technical office, the safety office and ultimately the crew office, as the in-flight-printed parts were not accessible to quality assurance specialists for in-person inspection before usage. In order to overcome this hurdle, a safety review demonstration was planned on November 12, 2015. The demonstration was attended by representatives of various NASA offices and also served as a procedure verification. The Safety Review Panel approval was issued the day after, but the final safety approval for printing and using the adapter was provided almost one month later on December 12, 2015. As an additional precaution, it was agreed that the crew would inspect the in-flight-printed part for burs or sharp edges before mounting it. The extrusion-based Additive Manufacturing Facility (AMF) is now a permanent manufacturing facility on the ISS, which allows spare parts to be produced on demand and delivers groundbreaking experiments for understanding the viability of 3D printing to be conducted on long-term missions (NASA AMF, n.d.).

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Non-Print Items Abstract Numerous characterization methods are available to investigate the filament properties before printing and the quality of the final object after printing by fused deposition modeling (FDM, or fused filament fabrication, FFF). When composite feedstocks are being processed, repeating the same tests before and after printing may be useful to understand the effect of adding a filler on the feedstock’s printability. However, the absence of dedicated international standards jeopardizes the meaningfulness of any comparison among published data in different studies, as the properties of additively manufactured parts largely depend on the specific printing conditions, such as part’s orientation and location of start/stop points. This becomes critical in quality assurance, since the available data for a specific kind of product is not sufficient for a statistical process control. As discussed in detail in this chapter, the advancement of metrology and the development of new approaches to quality control are thus deemed as key enabling factors for the widespread adoption of FDM and, generally speaking, of additive manufacturing (AM) in industry. Keywords Fused deposition modeling; FDM; Fused filament fabrication; FFF; Characterization method; Quality assurance; Composite material; Continuous fiber

Fused deposition modeling of polymer-matrix composites with discrete ceramic fillers 6.1

6

Introduction: Glass, ceramic, and carbonaceous fillers

Glass, ceramic and carbonaceous fillers used for functional polymer-matrix composites in fused deposition modeling (FDM, or fused filament fabrication, FFF) can be roughly divided into two main categories that comprise (1) “discrete” fillers (nanoparticles, particles and short fibers) and (2) continuous fibers. Discrete fillers may be added for different aims, which range from the mechanical improvement of the polymer matrix to the introduction of new functionalities such as bioactivity and thermal or electrical conductivity. Continuous fibers, conversely, are mainly addressed to reinforce the thermoplastic matrix, increasing its stiffness or its load-bearing capacity for structural applications, in an analogous way that steel is added to concrete to increase its strength. According to this general classification, the following sections will summarize numerous examples taken from the literature about the production of composite parts with discrete fillers. Due to their peculiarities, continuous-fiber reinforced parts are dealt with in a separate chapter (Chapter 9). Glass and ceramic filaments for obtaining fully inorganic parts by the shaping-debinding-sintering (SDS) method, aka fused deposition of ceramics (FDC), are discussed in Chapter 10. A critical assessment of the opportunities and challenges that emerge when FDM parts are loaded with discrete fillers, including glass, ceramic and carbonaceous ones, is presented in Chapter 11. Details about the preparation of composite filaments are reported in Chapter 12.

6.2

Rationale for implementing discrete fillers

Discrete fillers represent a wide group of particles and short fibers. Based on their size, they can be nanoscale materials or microscale materials. Also, based on their geometry, they can be zero-, one-, two- or three-dimensional materials, such as nanocrystals, nanotubes, nanoplatelets, and nanoparticles, respectively. These materials may have natural origins (for instance, silicates and other minerals) or synthetic origins (for instance, carbon fibers and nanotubes, graphene, shredded glass, etc.). As previously mentioned, the addition of discrete fillers may have different goals. Discrete fillers may improve the mechanical properties of the polymer matrix, especially in terms of stiffness under tensile and flexural loading conditions. Although their reinforcing effect is inferior to continuous fibers, incorporating discrete fillers is Fused Deposition Modeling of Composite Materials. DOI: https://doi.org/10.1016/B978-0-323-98823-0.00012-3 c 2023 Elsevier Ltd. All rights reserved. Copyright 

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easier and more economical, and therefore represents an interesting trade-off between performance, practicality, and cost. Besides mechanical reinforcement, the addition of discrete fillers is often targeted to provide the neat polymer with new functionalities, the most common ones in the literature being thermal and electrical conductivity and bioactivity. The general outline of the following sections, which are entirely dedicated to glass, ceramic and carbonaceous discrete fillers, mirrors this functional classification with four sections focused on mechanical reinforcement (section 6.3), electrical conductivity (section 6.4), thermal properties (section 6.5), and bioactivity (section 6.6). However, it should be noted that discrete fillers also play a key role in numerous applications that do not fall in these four categories. Some examples are discussed in section 6.7. Also, it is worth noting that any functional classification is arbitrary, in that the addition of fillers always exerts multiple effects (Park and Fu, 2021). For example, carbon nanotubes (CNTs) can impart multiple functionalities to the polymer matrix, which include not only mechanical reinforcement, but also electrical conductivity and thermal conductivity (Dorigato et al., 2017). Analogously, hydroxyapatite (HAp) and other calcium phosphates are often chosen in biomedical applications due to their bioactivity and bone-bonding properties, but they also contribute to increasing the inherently low stiffness of the polymer matrix to values closer to that of natural bone tissue (Russias et al., 2006).

6.3 Mechanical reinforcement The addition of fillers always modifies the mechanical behavior of the polymer matrix. The following paragraphs provide some examples of fillers that are purposedly introduced to improve the properties of the neat polymer, especially to increase its stiffness.

6.3.1 Glass particles Olesik et al. (2019) demonstrated the feasibility and printability of low-density polyethylene (LDPE) filaments reinforced with glass particles (containing polyvinyl butyral) from shredded car windscreen waste. The modulus of elasticity increased from 193 MPa for neat LDPE to 219 MPa for the composite with 30 vol% of shredded glass powder. The wear resistance of LDPE increased by 50% due to the presence of the glass particles that promoted the formation of a sliding film on the sample’s surface. The introduction of glass particles slightly increased the friction coefficient as compared to the neat polymer, but also stabilized the friction coefficient reducing its fluctuations as a function of the sliding distance. The wear behavior was anisotropic, since the response of the composite depended on the relative orientation between friction direction and printed path direction, likely due to differences in the removal of wear products from the friction area for different path directions (Olesik et al., 2019).

6.3.2 Alumina particles Singh et al. (2016) focused their research on the effect of the particle size distribution of alumina powder on the tensile properties of nylon-matrix filaments. They compared

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single particle size (SPS) systems having single particle size of 100 μm, dual particle size (DPS) systems having two particle sizes of 100 μm and 120 μm in equal proportion by weight, and triple particle size (TPS) systems having three particle sizes of 100 μm, 120 μm and 150 μm in equal proportion by weight. Although SPS filaments achieved the highest elongation at break, DPS filaments proved to be the best for all other tensile properties, including tensile strength, yield strength and modulus of elasticity. In fact, DPS composites took their advantage from the uniform mixing and from the efficient spatial distribution enabled by the two different particle sizes. The coexistence of two different particle sizes in DPS composites led to improved particle packing and distribution over SPS systems. Meanwhile, the particle size in DPS materials was smaller than in TPS counterparts, which mitigated potential drawbacks (for example, local stress concentration) coming from large inclusions (Singh et al., 2016). Boparai et al. (2016a) investigated the printability of nylon-matrix composites reinforced with aluminum and alumina powders. No plasticizers were added on account of the self-lubricating properties of aluminum. Since acrylonitrile-butadiene-styrene (ABS) is a standard feedstock for FDM, it was considered as a benchmark. The first requirement for the nylon-aluminum/alumina composite system was thus to meet the melt flow index (MFI, which is a measure of the flowability of the material in the molten state) of ABS. To this aim, the solid weight fraction in the composite was limited to 40 wt%. Then, the response surface methodology (RSM) was applied for the design and analysis of experiments, in order to optimize the aluminum-to-alumina ratio and, at the same, the single screw extruder parameters including mean barrel temperature and die temperature. The appropriate choice of composition (30 wt% aluminum, 10 wt% alumina) and processing conditions provided the filament with adequate values of tensile strength and of diameter consistency to allow for successful FDM. Boparai et al. (2015, 2016b) proved the superior tribological properties of the composite printed parts that derived not only from the extreme hardness and thermal stability of alumina particles, but also from the self-lubricating properties of aluminum particles. With respect to standard parts produced from ABS filaments, their composite counterparts exhibited a lower friction force due to the self-lubricating properties of aluminum. Moreover, aluminum particles contributed to dissipating the heat generated at the sliding interface. Meanwhile, during sliding the alumina particles acted similarly to ball bearings and prevented material loss. However, the content of alumina could not be increased arbitrarily. In fact, the aluminum-alumina reinforcements totaled to 40 wt%, but the weight fraction of alumina ranged between 10 to 14 wt% to adjust the MFI of the composite and to limit abrasion of the screw and barrel of the extruder, and of the print nozzle upon processing. Whereas abrasion and surface delamination were the prevalent wear mechanisms for ABS, adhesion and, to a minor extent, abrasion prevailed for the composite materials (Boparai et al., 2015). Unfortunately, the potential dependence of the wear mechanisms on the printing orientation was not investigated.

6.3.3 Mineral fillers In an introductory contribution, Sang et al. (2019b) successfully printed poly(lactic) acid (PLA)/(silanized) basalt fiber composites. However, the mechanical performance did not correlate positively with the fiber content, as both the tensile strength and the

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impact strength decreased when the fiber fraction increased from 10 wt% to 20 wt%. For this reason, in a subsequent contribution addressed to printing composite honeycomb structures for civil engineering applications, Sang et al. (2019a) chose to add 15 wt% of (silanized) basalt fibers as the maximum filler loading to modify a PLApolycaprolactone (PCL) blend matrix. The presence of PCL reduced the elastic modulus of the composite, but improved the deformation at break, as well as the fibermatrix interfacial bonding and the interlayer adhesion of the printed parts. Accordingly, whereas PLA/basalt fiber honeycombs showed a brittle behavior under compressive load, the deformation process of PLA-PCL/basalt fiber honeycombs was more ductile, with the highest energy absorption occurring for 30 wt% PCL (Sang et al., 2019a). In a recent contribution, Zhou et al. (2020) considered the addition of montmorillonite, which is an aluminum-rich phyllosilicate of the smectite group very common in nature, as an eco-friendly nanofiller to enhance the mechanical behavior of FDM components. Zhou et al. (2020) also introduced a compatibilizer (ethylene-methyl acrylate copolymer) in order to improve the interface adhesion to the matrix. It was demonstrated that the montmorillonite nanoparticles were effective to increase the tensile strength of the ABS-PC blend matrix. Although the details of the experiment were omitted, Zhou et al. (2020) observed that the presence of the filler significantly reduced the post-processing shrinkage of the printed parts, which resulted in a smaller porosity with respect to the pure ABS-PC matrix. Whereas the addition of a rigid filler was beneficial to reduce the shrinkage, the compatibilizer had instead a controversial effect: on the one hand, it improved the interfacial strength between ABS and PC, which increased the strain at break of the composites; on the other hand, it promoted the molecular motion of the polymer matrix, which favored post-deposition shrinkage phenomena and pore formation. According to Zhou et al. (2020), an interesting outcome of the tensile tests was that the necking of the FDM samples could spread throughout the total gauge length and the strain could exceed 100% when the compatibilizer and the nanoparticles were added to the ABS-PC matrix, which was attributed to the achievement of a balance between bonding properties and ductility. Similarly, the advantageous effect of montmorillonite on the tensile and bending properties of ABS was described by Weng et al. (2016). The filler-matrix interaction was improved by a surface modification with stoichiometric benzyldimethylhexadecylammonium chloride. The addition of increasing fractions of organically modified montmorillonite progressively increased the tensile and bending strength of the FDM composite parts. However, the resistance of the printed samples remained lower as compared to injection molded counterparts due to the week inter-bead and inter-layer bonding. The coefficient of thermal expansion decreased with increasing content of montmorillonite, thus reducing deformation and warping of the composite objects (Weng et al., 2016). Montmorillonite was also proposed to reinforce PLA (Coppola et al., 2017). The addition of 4 wt% organically modified (modifier: methyl, tallow, bis-2-hydroxyethyl, quaternary ammonium salt, MT2EtOH) montmorillonite (Cloisite 30B) increased the degree of crystallinity of PLA and caused the growth of two different crystalline forms (α and α’). The intercalated/exfoliated morphology of the reinforcement improved

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the tensile modulus of elasticity by 15% as compared to neat PLA, but reduced the deformation at break (ductility). The presence of the rigid filler that hindered the polymer deformation and changed the crystallization behavior of the PLA matrix itself increased the resistance to shrinkage and hence the printing accuracy (Coppola et al., 2017). Nanoclays, including montmorillonite, may be suitable for biomedical applications owing to their antimicrobial properties, adsorption ability of toxins and sterilizing effect. However, once incorporated into the polymer matrix, nanoclays may act as rigid fillers and reduce the mechanical strength, thus undermining their reliability and safe usage. Optimizing the tensile strength of PLA-montmorillonite FDM parts for biomedical applications was the main goal of the research conducted by Mihankhah et al. (in press). Since the tensile strength is governed by numerous parameters, a Taguchi experimental design was developed in order to investigate the effect of filler loading (2-4 wt%), printing temperature (190-210-230°C) and raster angle (0-45-90°) simultaneously, so as to minimize the experimental effort. Although all entry variables were influential, the printing temperature was the most important parameter to control the tensile strength, since a higher printing temperature induced a stronger inter-bead bonding. The experimental design identified therefore the combination of printing temperature of 230°C, montmorillonite loading of 4 wt%, and raster angle of 0° as the optimum condition, leading to a tensile strength of 38.98 MPa (Mihankhah et al., in press). Esposito Corcione et al. (2018a) contributed to revive the local economy and imaginative craftmanship of Lecce stone (LS) in the Salento area (Italy) by recycling the powder waste from the LS processing and using it as reinforcement in PLAmatrix parts. The as-received stone waste contained up to 88 wt% of water, which was incompatible with its intended usage in FDM. However, some basic steps such as drying, manual grinding and sieving were sufficient to reduce the water content and the powder size. Composite filaments loaded with up to 60 wt% of LS powder were produced by melt extrusion and successfully tested on a standard FDM printer. In order to reduce the environmental footprint of reinforced filaments, Ahmed et al. (2020) combined recycled PLA from waste FDM materials and local silica sand from the United Arab Emirates. Assorted leftover PLA was collected from the prototyping laboratory of the university, sorted based on the color, then shredded and milled. Silica sand was obtained from local resources and finely ground to a mean particle size of 3.10 μm. With the sand being a natural raw material, the composition was not pure silica, but a mixture of silicates (around 47 wt%), carbonates (26 wt%) and quartz (up to 14 wt%). Recycled polymer and sand were hot-melt compounded in a twin-screw extruder and then compression molded into thin sheets. Although the sand did not receive any surface treatment to improve its dispersion and affinity to the PLA matrix, tensile tests on compression-molded dogbones proved that Young’s modulus, tensile strength, deformation at break, ductility and toughness consistently increased with increasing amounts of sand up to 10 wt%, but then dropped at 15 wt%. The adverse effect of higher filler loadings above 10 wt% was ascribed to agglomeration phenomena, which led to the formation of large inorganic inclusions acting as preferential initiation points for cracks. In spite of the promising results obtained from the mechanical characterization,

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the printability of the recycled PLA-local sand composites was not demonstrated and left for future investigation (Ahmed et al., 2020).

6.3.4 Carbonaceous fillers Gao et al. (2019) compared the properties of various filaments and FDM parts obtained by mixing either 2 wt% of talk (Mg3 Si4 O10 (OH)2 ) or 5 wt% of low-aspect-ratio carbon fibers into a PLA matrix. Interestingly, based on differential scanning calorimetry (DSC) results, whereas the presence of carbon fibers did not change the very slow crystallization behavior of the polymer matrix, the addition of talc, which is known to be a strong nucleating agent for PLA (Li and Huneault, 2007; Yu et al., 2012) induced a sharp crystallization of the composite filament at 110°C. Vice versa, whereas the presence of talc did not alter the viscosity of the molten PLA matrix, introducing 5 wt% of carbon fibers was enough to reduce the melt viscosity under shear flow due to the upsurge of repulsive interactions between carbon fibers and due to the progressive alignment of the carbon fibers. Generally speaking, the tensile properties of the FDM composite parts were similar to those of their respective counterparts produced by injection molding, at least in terms of modulus of elasticity. However, as often stated in the literature, the tensile strength of the FDM parts was sensibly lower. In spite of the nearly complete absence of inter-bead voids, the mechanical performance of the carbon fiber-reinforced composites was worse than expected due to the poor wetting of the carbon fibers and the consequent development of intra-bead voids. On the contrary, strong interface adhesion was achieved between talc particles and PLA due to hydrogen bonding. In terms of printability, no warpage, distortion, or delamination were observed for any parts, but the surface quality varied with different feedstock materials and sensibly decreased with decreasing melt viscosity (Gao et al., 2019). Ning et al. (2015) measured the tensile and bending properties of ABS-matrix composites reinforced with short carbon fibers with several fiber weight fractions up to 15 wt%. As a general trend, adding carbon fibers increased the tensile strength and modulus of elasticity of the neat polymer matrix, but decreased toughness, yield strength, and ductility. The specimens with 5 wt% of reinforcement had the highest tensile strength, whereas those with 7.5 wt% of carbon fibers had the highest modulus. Bending strength, modulus and toughness were the best with 5 wt% of carbon fibers. However, the mechanical properties did not follow a linear trend and reached minimum values at about 10 wt% of fiber loading as a consequence of porosity. In fact, pores were generated from defects in the composite filaments and from physical gaps at the layer interfaces in the printed parts. The porosity of the composite parts initially decreased as the fiber loading increased from 0 wt% to 3 wt%, but then increased to its largest mean value (9.04%) at 10 wt% of carbon fibers and then reduced again to 3.27% when the carbon fiber content increased further to 15 wt% (Ning et al., 2015). Papon and Haque (2019) explored the fracture toughness of short carbon fiberreinforced PLA composites. All composites showed superior fracture resistance as compared to the pure polymer matrix. Whereas the fiber weight fraction clearly affected the mechanical behavior, with the highest improvement corresponding to 5 wt% of

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reinforcement, the print orientation proved to be almost uninfluential. The implementation of an experimental square-shaped nozzle resulted into a relevant reduction of the inter-bead voids between adjacent beads, which contributed to boost the fracture toughness of the composite parts (Papon and Haque, 2019). de Toro et al. (2020) directly compared the properties of polyamide 6 (PA6)-short carbon fiber composite parts produced either by FDM or by injection molding. The standard brass nozzle of the commercial FDM equipment used in their study was replaced with a wear-resistant one in order to account for the abrasive action of the filler. The authors measured the average fiber length before and after processing and proved that, regardless of the manufacturing method, the fiber length was significantly reduced, with negative consequences on the load transfer mechanisms. Injection molded parts are usually reported to perform better than FDM printed parts under tensile loads. This behavior was confirmed by the experimental results of de Toro et al. (2020), but it is worth noting that the difference was not as large as expected, probably as a result of the relatively high filler content (20 wt%) of the commercial filaments used to print the tensile specimens. The difference in mechanical properties was even less under compressive load. Interestingly, de Toro et al. (2020) observed a higher compressive stiffness for FDM parts than for injection molded ones. On the other hand, it has been often reported that 3-dimensional (3D) printed materials are generally stronger in compression than in tension (Kehinde Aworinde et al., 2019). Conceivably, the mechanical performance was worse when the infill density decreased from 100% to 60%, with different results for different infill strategies (de Toro et al., 2020). Lin et al. (2019) conducted tribological tests on polyether ether ketone (PEEK)short carbon fiber composites (namely, composite coatings printed on a pure PEEK substrate), with and without feeding silica nanoparticles on the sliding interface during testing, and detected that both the friction coefficient and the wear resistance were lower when sliding took place perpendicular to the carbon fiber direction as compared to parallel, with the carbon fibers being almost perfectly aligned with the printing direction. The introduction of silica nanoparticles on the interface lowered the friction coefficient because of the rolling effect and because of the reduced adhesion between the friction pair. However, the wear resistance was not improved, since the presence of silica nanoparticles alleviated the effect of wear on the carbon fibers, but also caused significant abrasion of the PEEK matrix (Lin et al., 2019). CNTs have been proposed in the literature to reinforce high-performance hightemperature polymers such as PEEK in order to take advantage of their nano-size, which leads to a large specific surface area capable of interacting with the polymer matrix, and of their thermal stability, which is characterized by an onset degradation temperature in excess of 500°C. However, Rinaldi et al. (2021) demonstrated that the addition of CNTs drastically reduces the processability window of PEEK, due to the heterogenous nucleating effect of CNTs that shifts the crystallization temperature upon cooling towards higher values (from 290°C for neat PEEK filaments to 307°C for filaments with 10 wt% of CNTs). The premature crystallization hinders the polymer chain mobility and thus impairs the healing phenomena that are responsible for the development of strong inter-bead and inter-layer bonds (Turner et al., 2014). Also, according to the results presented by Rinaldi et al. (2021), CNTs decrease the MFI of

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neat PEEK and hence limit the spreadability of the polymer and, ultimately, the interlayer contact area. Although the thermal conductivity of PEEK was almost doubled by the addition of CNTs, the thermal conductivity of the printed composites was lower than the theoretical value due to extensive phonon scattering at the interfaces and pores. The reduced processability of PEEK-matrix composites impacted the mechanical performance of the printed parts. Whereas the Young’s modulus under tensile load increased from about 3.0 GPa to about 3.8 GPa with 10 wt% of CNTs, the composite parts ultimately experienced a general decrease of the tensile strength and a general embrittlement compared to neat PEEK as a consequence of the weak inter-layer bonding (Rinaldi et al., 2021). Similarly, Berretta et al. (2017) observed that the ultimate tensile stress largely diminished when 5 wt% of CNTs was added to PEEK. The tensile behavior was repeatedly tested on the composite filaments, on single FDM deposited layers and on FDM dogbones, and this led Berretta et al. (2017) to hypothesize that every step of processing resulted in structures of lower properties. The under-performance of the printed dogbones was attributed to several interacting phenomena, mainly CNT agglomeration and development of pores upon printing. Short beam shear stress (SBSS) tests were conducted to estimate the inter-laminar shear strength, which clearly dropped with the addition of 5 wt% of CNTs due to the modified thermal behavior and rheology of PEEK (Berretta et al., 2017). Instead of dispersing the filler evenly throughout the printed part, Mei et al. (2019) proposed a layered reinforcement pattern, where a suspension of SiC nanowires was periodically brushed between the printed PLA layers. The addition of up to 3 SiC layers increased the compressive modulus of elasticity, but the compressive yield strength decreased correspondingly (Mei et al., 2019).

6.4 Electrical conductivity Polymer-matrix composites containing electrically conductive fillers such as carbon allotropes or metals are increasingly adopted to print customized parts for electrochemical research (Hamzah et al., 2018), advanced robotics (Xia et al., 2016) and a wide range of electro- and bio-mechanical sensors (Schouten et al., 2021). The ability to 3D print highly conductive traces is crucial for achieving reliable structural electronics (Baker et al., 2021). Conductive filaments are gaining in popularity in the marketplace for small do-it-yourself (DIY) projects in electronics (for instance: (Multi3D, n.d.; ProtoPasta, n.d.; Amolen, n.d.), but new conductive filaments are continuously introduced in the marketplace). Most commercial filaments are suitable for low-voltage devices (typically, up to 60 V at 100 mA, according to data recently reported on-line (O’Connell, 2021)). However, not all materials have the same performance. A critical parameter in this regard is the resistivity, which measures how much a material resists the flow of current. Basically, resistivity is the inverse of conductivity and therefore a material with a low resistivity will be able to conduct electricity very well. Some representative values of resistivity for reference materials, composites and conductive filaments are listed in Table 6.1. Although this point is sometimes overlooked especially by websites that aim to convey very simplified information to a non-specialistic

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Table 6.1 Resistivity of some reference materials, composites and filaments for FDM. Material Resistivity (·m) Reference materials Graphite, single crystal, in plane 1 × 10−9 Graphene nanoplatelets 4 × 10−6 MWCNTs 1 × 10−6 Graphite (1.47-1.90) × 10−5 Silver (293 K) 1.587 × 10−8 Copper (293 K) 1.678 × 10−8 Gold (293 K) 2.214 × 10−8 Aluminum (293 K) 2.650 × 10−8 Mercury (298 K) 96.1 × 10−8 Gallium (273 K) 13.6 × 10−8 ABS 1 × 1010 PLA 1 × 1014 Composites used in the literature for FDM filaments ABS + 3.8 wt% graphene 1.6 × 104 PLA + 10 wt% CNTs 9.03 × 10−3 PLA + 10 wt% CNTs + 5 wt% Cu 8.67 × 10−3 PLA + 10 wt% CNTs + 5 wt% ZnO 7.58 × 10−3 Commercial conductive filaments Electrifi conductive filament 6 × 10−5 Conductive PLA Protopasta (4.8-7.2) × 10−5 Conductive Black PLA Amolen 1.42 × 10−2 Experimental conductive filaments in the literature PLA + 12 wt% CNTs 6.7 × 10−3 PLA + 6 wt% graphene 4.2 × 107 PLA + 6 wt% MWCNTs 3.0 × 10−2 PLA + 3 wt% graphene + 6 wt% MWCNTs 1.4 × 10−2

Reference (Baker et al., 2021) (Spinelli et al., 2020]) (Spinelli et al., 2020]) (Baker et al., 2021) (Baker et al., 2021) (Baker et al., 2021) (Baker et al., 2021) (Baker et al., 2021) (Baker et al., 2021) (Baker et al., 2021) (Dorigato et al., 2017) (Mora et al., 2020) (Wei et al., 2015) (Junpha et al., 2020) (Junpha et al., 2020) (Junpha et al., 2020) (Multi3D, n.d.) (ProtoPasta, n.d.) (Amolen, n.d.) (Mousavi et al., 2020) (Spinelli et al., 2020) (Spinelli et al., 2020) (Spinelli et al., 2020)

readership, it is important to stress that material’s resistivity, ρ (expressed in  m or, more commonly, in  cm), and object’s resistance, R (expressed in ), are not the same thing, as the resistance will also depend on the device’s geometry through its cross-sectional area, A, and its length, l, according to the (Eq. 6.1): l (6.1) R=ρ A Whereas few polymers are intrinsically conductive, electrical conduction is possible in polymer-matrix composites with conductive fillers either due to direct inter-particle contact or due to electrons being able to jump from one filler particle to the next one through a thin polymer layer (which is the so-called “tunneling effect”). In either case, the filler particles generate a continuous conductive network, also known as “percolated network”. The formation of a percolated network is possible only if the filler loading exceeds a critical concentration, which is the “percolation threshold” (Mora et al., 2020). As schematically shown in Fig. 6.1, the conductivity of the composite material is a function of the filler loading. At first, the conductivity is still governed by the matrix and remains very low for increasing filler loadings, because the conductive filler creates

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Figure 6.1 Electrical conductivity of a polymer matrix composite as a function of the volume fraction of the conductive filler.

conducting islands within the insulting polymer matrix. The electrical conductivity starts to increase very rapidly as the conducting islands become interconnected, thus generating a continuous path for the electric charge to flow, which is the “percolation threshold.” After a sudden increase, the electrical conductivity flattens at a certain value that is the maximum conductivity of the composite. The resistivity of a filament obviously depends on the nature, properties and volume fractions of the constituent phases. Another important parameter to consider is the spatial arrangement of the filler. Whereas filler aggregation is detrimental to electrical conductivity, it has been proven that segregated structures, wherein fillers are constrained to certain locations within the matrix, are very advantageous to lower the percolation threshold as compared to composites having uniform filler distribution (Mora et al., 2020). In segregated structures, some areas contain a higher filler concentration than the average content inside the polymer. The reduced inter-particle distance in these areas promotes the flow of electrons and thus reduces the percolation threshold (Mora et al., 2020). However, in order for electrical conduction to take place, the segregated areas must form a continuous phase, which can be the “sea” in a seaisland morphology, or one of the continuous phases or the interface of a co-continuous morphology (Mi et al., in press). The resistivity of 3D printed devices may also depend on some printing-related specificities, such as processing-induced defects and anisotropic behavior. For example, the resistivity of FDM parts is typically influenced by the inter-bead and interlayer bonding, with defective interfaces hindering electrical conduction (Dijkshoorn et al., 2021). This is often responsible for a higher resistivity of printed objects over the original filament (Dorigato et al., 2017). On the other hand, elongated fillers are likely to receive a preferential orientation upon extruding and printing, which produces

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Figure 6.2 As discussed by Mousavi et al. (2020), when the overlap is close to zero (“kissing bond”), a strain applied normal to the raster direction separates the adjacent beads. The reduced contact area significantly increases the electrical resistance of the printed part.

an anisotropic electrical behavior and facilitates the flow of electrical charge along the alignment direction (Spinelli et al., 2019). Also, as demonstrated by the multi-directional sensors developed by Mousavi et al. (2020), the electrical response of a printed part as a whole largely depends on its architecture, especially on the infill pattern and degree. Mousavi et al. (2020) observed that, for an infill degree corresponding to the establishment of a “kissing bond” between parallel neighboring beads (nearly zero overlap), an increasing strain applied normal to the raster direction leads to a progressive separation between adjacent tracks. The reduced contact area that is available for the flow of electrons significantly increases the electric resistance of the printed part and this allows the sensor’s sensitivity to be controlled through its design. The effect of strain on the inter-bead contact area and, ultimately, on the electrical resistance is schematically shown in Fig. 6.2. Alongside producing stand-alone devices, it has been proven that PLA-matrix conductive composites can be successfully printed onto PLA fabrics, thus paving the way for the development of wearable sensors and devices (Sanatgar et al., 2017). Generally speaking, direct printing onto textiles is an enabling technology for the production of personalized wearable items and even orthopedic devices with customized shape and size (Ahrendt and Romero Karam, 2020). Yang et al. (2019) thoroughly investigated the printability of PLA-CNT composites having different filler ratios and also determined the mechanical properties and the electrical conductivity of the corresponding FDM parts. Preliminary thermal tests on the composites demonstrated that, in general, the addition of CNTs was disadvantageous to the printability of PLA, because the glass transition temperature, the melting temperature and the tendency for cold crystallization of PLA progressively increased, whereas the MFI dropped with increasing filler content. Coherently with these outcomes, Yang et al. (2019) reported that it was not possible to print reliable parts for filler contents exceeding 6 wt%. As a combined result of the very homogenous distribution of the filler and of the extensive graft polymerization that occurred at

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the PLA-CNT interface, the adhesion bonding and the load transfer mechanism were very efficient, with substantial benefits for the modulus of elasticity, for the tensile strength and for the corresponding bending properties of PLA that increased with increasing filler content. The electrical resistivity of neat PLA progressively decreased from 1 × 1012 to 1 × 102 /sq when the filler content increased to 8 wt%. In fact, the increased concentration of CNTs promoted the establishment of permeable conductive paths and the distribution of electrical charges on the surface of the composite. For a given filler content, the study by Yang et al. (2019) also proved that the printing parameters have a relevant effect on the electrical conductivity. In particular, with the aim of maximizing the conductivity, a lower printing speed and a higher liquefier temperature should be applied in order to favor an even distribution of the CNTs; to the contrary, a greater layer thickness should be selected to limit the presence of inter-layer voids (Yang et al., 2019). As previously mentioned, the addition of electrically conductive fillers such as CNTs also affects other properties, such as thermal conductivity and mechanical characteristics (tensile stiffness, strength and elongation at break). This complicated interplay in ABS-CNT composites was explored, for instance, by Dorigato et al. (2017). Although the MFI measurement showed a decreased processability of the material above 4 wt% of CNTs, composite parts with 6 wt% of CNTs could be successfully printed under different orientations and then characterized to determine tensile properties, electrical properties and thermal conductivity. The addition of CNTs increased the Young’s modulus, but drastically reduced the elongation at break of the printed parts. The embrittlement was particularly evident for parts printed upright (namely, with the loading direction parallel to growth direction), due to the tensile load being applied normal to the inter-layer interfaces. Whereas the presence of 6 wt% of CNTs was sufficient to endow the ABS matrix with electrical conductivity, the behavior of the printed parts was anisotropic, with the lowest resistivity value measured on samples printed upright. Also, the electrical resistivity was slightly dependent on the applied voltage for the samples printed flat on the base platform. This suggests that the electrical conductivity of the 3D printed parts was undermined by the weak bonding that impaired the charge flow between neighboring beads. The thermal conductivity of the printed parts with 6 wt% of CNTs was slightly higher than that of the neat ABS counterparts, but significantly lower (-21%) than that of compression molded composite parts with the same filler loading. This discrepancy in thermal conductivity was attributed to an unfavorable orientation of the filler in the FDM samples (Dorigato et al., 2017). K. Kim et al. (2017) leveraged the dependence of electrical conductivity on the spatial distribution of multi-walled carbon nanotubes (MWCNTs) to print a multi-axial force sensor. The sensor consisted of two components, namely a structural component and a sensing component, which were printed simultaneously with different functional filaments. The structural component was fabricated with a commercial thermoplastic poly-urethane (TPU) filament, whereas the sensing component was produced with an experimental TPU-based filament functionalized with 4 wt% of MWCNTs. TPU was chosen because of its flexibility, which would be useful for wearable electronics, and it was used both for the structural component and for the matrix of the sensing

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component in order to impart a good adhesion between them. The functioning principle of the sensing component proposed by K. Kim et al. (2017) is based on the number of conduction paths through the MWCNT network changing when a strain is applied to a TPU-MWCNT composite (or, generally speaking, to a CNT-functionalized conductive composite). This increases the composite’s resistance (piezoresistivity). The structural component was made of three orthogonal beams forming a cubic cross and each beam received a sensing layer on its surface. The thickness of the sensing layer was a trade-off between the printing accuracy, which could not be lower than 0.2 mm for the TPU-MWCNT composite feedstock, and the need of having a sensing layer as thin as possible to capture the maximum strain on the upper or lower surface of the curved beam under bending. The whole sensor prototype could be successfully printed in a single job with a dual-nozzle printer. However, the resistivity of the sensing layers was affected by the orientation upon printing. In particular, the sensing layer parallel to the growth direction had a resistivity higher than expected due to the numerous interfaces and occasional cross-contamination from neat TPU. Similarly, the mechanical response of structural beams printed under different orientations was anisotropic. A small hysteresis was also observed during cyclic loading due to the viscoelastic behavior of TPU. Nonetheless, the multi-axial sensor was integrated in a haptic device that could perceive the pressure exerted by fingers on the beams’ tips (K. Kim et al., 2017). Various papers in the literature describe the introduction of graphene nanoplatelets (GPNs), which are representative of prevalently bi-dimensional nanofillers as opposed to mono-dimensional nanotubes, to obtain conductive printable filaments. The incorporation of graphene was successfully accomplished both by solution mixing (Wei et al., 2015) and by solvent-free hot-melt compounding (Dul et al., 2016). PLA-graphene composite filaments were printed into electrochemical energy storage architectures, which were effectively employed as a potential graphene-based lithium-ion anode and a solid-state graphene supercapacitor. Additionally, the printed PLA-graphene electrodes demonstrated the ability to electrochemically produce hydrogen, via the hydrogen evolution reaction, as an alternative to standard platinum-based electrodes (Foster et al., 2017). However, as proven by Camargo et al. (2019), the presence of graphene deeply influences the mechanical behavior of PLA-graphene composites, whose tensile and flexural strength can be optimized through the appropriate choice of printing parameters such as infill strategy and layer thickness (Camargo et al., 2019). Gnanasekaran et al. (2017) directly compared the effectiveness of CNTs and thermally expanded graphite (graphene) to obtain electrically conductive polybutylene terephthalate (PBT)-matrix composites. Both the filaments and the FDM parts containing graphite were rough and brittle as compared to those functionalized with CNTs. In fact, heating upon printing caused the evaporation of residual moisture in the thermally expanded graphite and the consequent development of bubbles was responsible for surface roughness and brittleness. The different geometry of the nanofillers (prevalently mono-dimensional vs prevalently bi-dimensional) and the presence of pores and potential aggregates in the graphite-based material resulted into a remarkable difference in the percolation threshold to obtain conductive filaments, which was estimated at about 0.49 wt% for CNTs (corresponding to 0.31 vol%) and at 5.2 wt% (3.3 vol%)

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for graphite. Probably as a consequence of the preferential orientation of the filler, the electrical conductivity of CNT-loaded composites at the percolation threshold was strongly anisotropic. A joint Italo-Bulgarian research group conducted an extensive investigation dedicated to the electrical and mechanical properties of PLA-matrix nanocomposites functionalized with MWCNTs and with GNPs (Batakliev et al., 2019; Ivanov et al., 2019; Spinelli et al., 2019, 2020). The structure of the composite filaments strongly depended on the filler type and composition, shifting from an aggregated structure for 6 wt% GNPs, to a homogeneous network for 3 wt% GNPs + 3 wt% MWCNTs, and to a segregated structure for 6 wt% MWCNTs (Spinelli et al., 2020). DSC tests conducted on the composite filaments, on the FDM parts and, as a term of comparison, on the hot-pressed nanocomposites revealed that the 3D printing process induced the highest degree of crystallinity in the PLA matrix, probably because of the slow cooling rate associated with the layer-wise build-up. Both fillers served as nucleating agents, with the nucleation effect of MWCNTs being slightly stronger than that of GNPs (Spinelli et al., 2020). Coherently with the percolation theory and with the observations of other authors (Kalsoom et al., 2020), as soon as the filler concentration increased, the electrical resistivity of the composite materials decreased (Spinelli et al., 2020). The coexistence of GNPs and MWCNTs exerted a synergistic effect on the distribution of the nanophase promoting the establishment of a continuous network for electrical conduction. For the composite systems comprising a single nanofiller, the percolation threshold was generally lower for MWCNTs than for GNPs, as a consequence of their different morphology. The preferential orientation induced by the extrusion step promoted the development of a directional percolation pattern that facilitated the achievement of the percolation threshold, but also induced anisotropic mechanical properties (Spinelli et al., 2020). Based on the assumption that one of the main advantages of nanofillers is the relatively low percolation threshold to conductivity, Shi et al. (2019) argued that it is possible to further reduce the filler loading by means of a local enrichment strategy that leads to a selective distribution of the conductive filler where it is strictly needed to create the conductive path. Preliminary computational simulations demonstrated that the flow of molten PLA through the liquefier channel was steady and this implied that the filler particles, if properly distributed on the surface of the PLA filament used as feedstock, would basically remain at their original state around the polymer core during the whole printing process. In order to put into practice this local enrichment strategy, CNTs were dispersed in a PCL solution in dichloromethane and the PLA filament was drawn through the suspension to obtain an even PCL-CNT coating. After printing, the locally enriched composite achieved an electrical conductivity that was 8 orders of magnitude higher as compared to a composite having the same composition and produced with the conventional hot-melt compounding strategy and thus having a uniform filler distribution (Shi et al., 2019). Many conductive composites for FDM contain nanofillers such as the aforementioned CNTs and GNPs. The key advantage is that CNTs and GNPs largely benefit from their nanometer size and mono- or bi-dimensional geometry (Clancy et al., 2020), which brings about a very high specific surface up to 750 m2 /g and hence a more than

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linear relationship between filler loading and effect on the polymer matrix (Rothon, 2016). However, these nanofillers are extremely difficult to disperse homogenously, as they tend to agglomerate as the filler loading increases (Haslam and Raeymaekers, 2013). Also, as a substantial drawback, these fillers are reportedly very expensive. Kwok et al. (2017) proposed carbon black as a low-cost alternative for 3D printed circuits and sensors. Polypropylene (PP) was chosen as an affordable alternative to PLA, which is currently the most commonly used thermoplastic matrix in conductive filaments, especially commercial ones (O’Connell, 2021). The filaments produced by Kwok et al. (2017) achieved very low resistivity values (below 10−2  m) for a filler loading exceeding 30 wt%. In this regard, it is worth noting that the percolation threshold was determined in the literature to fall between 3 and 6 wt% for PLAGNP composites, and to be even lower, between 1.5 and 3 wt%, for PLA-MWCNT composites (Spinelli et al., 2019), which means about one order of magnitude lower than carbon black. This is consistent with the general observation that the larger is the aspect ratio of the filler, the lower is the percolation threshold, although the achievement of the percolation threshold is known to depend on several parameters other than the aspect ratio, such as the dispersion level and the presence of agglomerates, the fabrication process, the nature of the matrix, the matrix-filler interaction and the interfacial effects (Spinelli et al., 2019). Besides investigating the feasibility of inexpensive electrical devices via FDM, Kwok et al. (2017) also remarked the importance of defining standard test methods to verify the long-term usability of the printed components under different environmental conditions. The electrical resistance of the carbon black-loaded composites investigated by Kwok et al. (2017) remained substantially unchanged under ultraviolet (UV) radiation up to 1 month and under applied voltage of 12 V in alternating current up to 1 week. The performance of the conductive composites was also favored by the superior thermal stability of the PP matrix as compared to other polymers commonly used in FDM, such as PLA. Bi- and three-dimensional circuits, as well as thermal sensors and wearable flex sensors, could be printed as proof of concepts. However, the non-polarity of PP limited the printability of the composites directly onto polar substrates such as ABS (Kwok et al., 2017). Leigh et al. (2012) experimented with the usage of PCL, which they preferred to other thermoplastic materials on account of its low melting point (around 60°C) and glass transition temperature (about -60°C) that helped to lower the processing temperature. Adding 15 wt% of carbon black was sufficient to reach the percolation threshold and amenable to print without modifying the standard FDM equipment. The PCL-carbon black composite filament, named “carbomorph”, was successfully printed into flex sensors, capacitive sensors and smart vessels responsive to the amount of liquid inside them. Xiang et al. (2019) highlighted the importance of the matrix-filler interface on the mechanical and physical properties of composites, including the electrical conductivity of conductive polymer-matrix composites. In order to improve the sensing performance of a highly flexible strain sensor, Xiang et al. (2019) noncovalently modified the surface of MWCNTs with 1-pyrenecarboxylic acid to facilitate their dispersion and strengthen the interaction with a flexible TPU matrix. The tensile properties of the FDM parts, with and without surface modification of the nanofillers, were superior to those of the cast

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counterparts as a result of the disentanglement and partial alignment of the MWCNTs due to the shear stresses acting on the material in the single screw extruder and in the print nozzle. For the printed parts, the modulus of elasticity, the tensile strength and the elongation at break increased after surface modification. However, the elongation at break dropped when the filler content was increased from 1.5 to 3 wt%, probably as a consequence of partial filler agglomeration in spite of the surface modification. The percolation threshold of the unmodified printed composites was calculated to be 1.98 wt%, whereas the percolation threshold of the modified printed composites was reduced to 0.95 wt%. All the printed sensors were anisotropic due to the alignment of the nanofillers. For a nanofiller content of 3 wt%, the sensitivity of the modified printed composite was higher than that of the unmodified printed composite over the whole strain range under examination. Also, it was reported that a lower concentration of modified nanofillers in the printed sensor contributed to a higher sensitivity, with the 1.5 wt% system having the highest sensitivity over the whole strain range. The printed sensors proved to be very stable under cyclic strain up to 1000 cycles. Prototypal devices were successfully applied to monitor human motions, including finger and joint movements, respiratory frequency and speech recognition (Xiang et al., 2019).

6.5 Thermal properties The addition of fillers is often intended to improve the thermal behavior of the polymer matrix. One of the main requirements is to provide the polymer matrix with a thermal conductive pathway, which is essential, for example, to dissipate heat in electronic devices and thus to prolong their service life (Guerra et al., 2020). Heat conduction in solids may occur by two main mechanisms, namely the flow of charge carriers (e.g., electrons and holes) and the propagation of phonons, which are the energy quanta associated to atomic lattice vibrations. Whereas heat conduction in metals largely depends on electronic-based mechanisms, the thermal conductivity of insulators and semiconductors is governed by the contribution from phonons. This is also the case for common thermoplastic materials that typically have a very low thermal conductivity (around 0.1–0.5W/(m·K)) as a consequence of phonon scattering, structural defects and (especially in semi-crystalline materials) grain boundaries. Heat conductive fillers may thus be introduced to increase the thermal conductivity of neat polymers. Many fillers, such as carbonaceous-based fillers (graphene, graphite, CNTs) and metal particles, are amenable to simultaneously imbue the polymer matrix with thermal and electrical conductivity. However, some heat conductive fillers, such as aluminum oxide (alumina) and boron nitride, are electrically insulating, which makes them the ideal option for those applications that require high thermal conductivity and electrical insulating properties. Oftentimes, the development of a continuous filler network is the key to enable high thermal conductivity in composite materials. However, this may pose technical difficulties, as high filler loadings are generally required to create a continuous conductive network, and research is ongoing to figure out new strategies for obtaining high thermal conductivity with lower filler loadings, for example by means of a careful design of the filler spatial arrangement (Chen et al., 2016).

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Different fillers can be selected not just to provide the polymer matrix with thermal conductivity, but also to reduce its thermal deformation and to increase its degradation temperature (Das et al., 2021). Li et al. (2020) proved by finite element (FE) simulation and experimental validation that the addition of 20 wt% carbon powder significantly reduced the thermal deformation of polyamide 66 (PA66) during printing. The thermallyinduced warpage was lowered by 49% and the maximum displacement by 11.4% (Li et al., 2020). The effect exerted by the addition of fillers on the thermal stability of the polymer matrix was also pointed out by Vinyas et al. (2019a, 2019b). Through a comparison of pure PLA and commercial PLA-based composite filaments with various fillers, it was proven that PLA attained the highest mechanical strength. Among the composite materials, the system PLA + 10% of carbon fibers achieved the highest tensile strength (Vinyas et al., 2019a, 2019b). However, all the composites under investigation had better thermal characteristics than pure PLA. In particular, the presence of 30% of nylon glass fibers improved the thermal stability and increased the degradation temperature of PLA. Vinyas et al. (2019a) therefore reached the conclusion that neat PLA should be considered the best option for applications requiring high mechanical strength, whereas the addition of nylon glass fibers should be considered whenever thermal stability is a priority. The PLA + glycol-modified polyethylene terephthalate (PET-G) blend, which reached the highest hardness values (Vinyas et al., 2019b), was identified instead as the best compromise between mechanical resistance and thermal stability (Vinyas et al., 2019a). Love et al. (2014) proved that the addition of 13% of chopped carbon fibers reduced the coefficient of thermal expansion by one order of magnitude and doubled the thermal conductivity as compared to neat ABS when the composite samples were tested parallel to the direction of deposition, whereas the coefficient of thermal expansion and conductivity values remained comparable to neat ABS in the transverse direction. The reduced coefficient of thermal expansion and the increased thermal conductivity significantly mitigated the thermal gradients within the printed parts and dramatically improved the geometric accuracy. In order to benchmark the printing quality of carbon fiber-reinforced ABS, Love et al. (2014) performed a test using the NIST reference geometry (Moylan et al., 2012). The same shape was printed with carbon fiber-reinforced ABS without heating the chamber, with neat ABS without heating the chamber, and with neat ABS whilst heating the chamber. All printed samples were smaller in size than the reference CAD model, but the carbon fiber-reinforced parts caused the least reduction in size. The printing precision was comparable for all three samples. The composite part had comparable dimensional accuracy to the ABS part printed in the heating chamber and significantly better accuracy than the ABS part printed without heating chamber (Love et al., 2014). Similarly, Ranganathan et al. (2019) observed that both the thermal conductivity and the heat distortion temperature of PA6 increased steadily by incorporating increasing amounts of glass spheres and of crushed glass fibers. Being the filler weight fraction the same, glass spheres were more effective than glass fibers due to a very even and pore-free distribution of the filler.

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6.6 Bioactivity and biological properties A wide range of fillers have been proposed to improve the mechanical properties of polymer-based biomedical devices, to control the bio-degradation rate and mechanisms, and to bring in new functionality, such as electrical conductivity for neuroregenerative treatments. The following paragraphs summarize the main outcomes of the on-going research in the field of FDM of composite materials for biomedical applications based on the filler’s nature. As will be seen later, materials like calcium phosphates are often deployed as they have striking similarities, both chemically and structurally, to bone. Other fillers such as ceramic oxides including titania and zinc oxide can offer mechanical reinforcement and at the same time antimicrobial properties, and, from a biomedical standpoint, carbonaceous fillers can provide a host of functional features to parts whilst maintaining bio-compatibility. The wealth of examples provided hereafter clearly demonstrate that the fabrication of composite parts for biomedical applications by FDM has recently experienced a tremendous and exciting advancement, however, it is important to remark that some points still remain critical. First of all, the incorporation of bioactive ceramic particles has proven to be effective for increasing the affinity and attachment of cells to polymers, especially to PLA. However, in doing so, most of the particles remain embedded within the polymer matrix and therefore are not exposed to cells directly, as only the surface of the material is available for directly interacting with the cells. Although some particles emerge as a consequence of the polymer matrix degradation, the process is often too slow to be practical. For this reason, new techniques are sought after to coat the surface of polymer parts with HAp and other bioactive ceramics (Dascalu et al., 2020). This is still the object of open debate, because appropriate coating methods are required to account for the geometric complexity of AM parts (Mondal et al., 2020). Moreover, El Moumen et al. (2019) recently remarked that, in spite of substantial progress achieved in the recent past, most of the available literature about 3D printing of composite materials for tissue engineering is still dedicated to bone tissues, whereas scarce attention has been devoted to other applications, such as cardiac tissue. Similarly, there is still a substantial lack of understanding about the printability of lowstiffness high-strength materials and lattices for the replacement of weight-bearing soft tissues like hyaline cartilage or spinal disks (Abar et al., 2021). This is a substantial limitation, since different tissue substitutes require different specifications in terms of morphology, porosity and mechanical behavior, which impedes the automatic extension of the results achieved for bone tissues to other tissues. Innovative research is needed to bridge this gap in knowledge.

6.6.1 Hydroxyapatite Hydroxyapatite, HAp, Ca10 (PO4 )6 (OH)2 is a calcium phosphate commonly used in the biomedical industry. The development of HAp-reinforced composites is a theme of major interest because HAp, besides reinforcing the polymer matrix and improving its thermal stability (Haq et al., 2019), is a bioactive material with demonstrated

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osteo-conductive properties (Hassan et al., 2019; Kattimani et al., 2016; Trombetta et al., 2017). According to the estimations of Russias et al. (2006), at least 70 wt% of HAp should be added to PLA to match the stiffness of natural cortical bone. Although promising attempts have been made to increase the HAp loading up to 50 wt% (Esposito Corcione et al., 2018b, 2019; Wang et al., 2021), the normal concentration of HAp in FDM filaments is usually lower than 15 wt% in order to preserve the printability (D. Wu et al., 2020). Also, the addition of HAp particles may have adverse effects on the geometric accuracy of FDM parts, especially if fine geometries are involved such as highly porous scaffolds for bone tissue engineering and bone gap healing. The effect of fillers such as HAp particles on the printability of composite materials in FDM is still under debate. This issue was recently investigated by Gendviliene et al. (2020), who demonstrated that PLA filaments containing up to 10 wt% of HAp can be successfully produced by hot-melt compounding and extrusion, and then used as feedstock to produce highly porous scaffolds (48% porosity). Interestingly, Gendviliene et al. (2020) also observed that the printing accuracy of the composite filament was equal or even better than that of pure PLA. Hypothetically, the improved printing accuracy may be attributed to the presence of the rigid filler that, as often reported in the literature, hinders the polymer chain movement in the melt and thus mitigates the die swelling effect at the exit of the nozzle (Blok et al., 2018; Das et al., 2021; Yang et al., 2021). Moreover, it is worth noting that, according to the results achieved by Gendviliene et al. (2020), the printing accuracy was mainly affected by the specific FDM printer in use, rather than by the addition of the inorganic filler. Wang et al. (2021) compared the printability of several PLA filaments functionalized with up to 50% of nanosized HAp particles. Whereas the scaffolds containing up to 40% of filler could be successfully printed with completed structures and clear pores, obvious fractures and defects appeared when the filler loading was increased further up to 50%. However, it was not possible to detect any statistically relevant difference in porosity between the different groups of scaffolds, which led to the conclusion that the proportion of nanosized HAp in the composite filaments did not affect the porosity (Wang et al., 2021). Sahmani et al. (2020) analyzed the behavior of PLA-HAp scaffolds printed by FDM with different architectures and porosity values after immersion in a simulated body fluid (SBF). As predicted, the growth rate of re-precipitated HAp in SBF was different for different scaffolds. However, since the samples investigated by Sahmani et al. (2020) differed by geometry and by porosity amount at the same time, clarifying the effect of the single variables based on the published results is not straightforward, whereas a focused investigation of each parameter would be recommended to control the biological reactivity of scaffolds produced by AM through an appropriate design of the shape, size and amount of pores. Xu et al. (2014) produced composite filaments with PCL and HAp at a mass ratio of 7:3 and printed them to replicate natural goat femurs by means of computed tomography (CT)-guided FDM. SEM images acquired on the cross section of the PCLHAp artificial bones proved that the structural features of cancellous bone had been reproduced faithfully, with interconnected pores having an average diameter of about 765 μm and struts having an average thickness of about 280 μm. The high printing

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accuracy was probably facilitated by the nanometric size of the HAp particles, that measured 40-150 nm in length and 20-30 nm in diameter (Xu et al., 2014). D. Wu et al. (2020) tested the printability by FDM of real trabecular structures derived from 3D images of a femoral head (synchrotron radiation micro-CT) with PLA-based composite filaments containing up to 15 wt% of HAp particles preliminary sieved to below 75 μm. Basic patterns could be reproduced successfully, but real trabecular structures could be printed only if scaled-up by a factor of 2-4. Moreover, the printing accuracy diminished with increasing fractions of HAp. Vice versa, the incorporation of increasing fractions of HAp had beneficial effects on the mechanical properties of the printed parts. In fact, the presence of HAp improved the modulus of elasticity and the screw pull-out load of the printed trabecular structures, whose properties were closer to human bone than those of commercially available synthetic substitutes (D. Wu et al., 2020). Besides mimicking the natural composition of bone (Koons et al., 2020), Senatov et al. (2016b) proved that HAp particles may also act as the (rigid) fixed phase to enhance the intrinsic shape memory properties of PLA. PLA-matrix composites containing 15 wt% of HAp microparticles achieved the maximum recovery stress of 3.0 MPa at 70°C, which was higher than that of neat PLA by a factor of 1.7 (Senatov et al., 2016b). Synchrotron X-ray techniques confirmed that the fixing ratio of the PLAHAp composite was 65%, and the recovery ratio was almost 100% (Sui et al., 2019), which closely matched the shape memory recovery ratio of 98% observed at the macroscopic level (Senatov et al., 2016b). PLA-HAp composite scaffolds printed by FDM could withstand up to three compression-heating (i.e., recovery)-compression cycles without delamination and with a final shape recovery of about 96%. Nonetheless, as pointed out by Senatov et al. (2016b), the composite scaffolds undoubtedly have the potential to be implanted as self-fitting devices for bone replacement, but further studies are required to lower the shape memory activation temperature to below the body temperature. Under low-cycle compression fatigue tests, the PLA-HAp composite scaffolds exhibited hysteresis, which implied the formation of defects and the accumulation of plastic deformation. Understandably, the rate at which defects formed increased with increasing peak stress. However, the introduction of HAp particles increased the crack resistance of the composite scaffolds, especially in the initial stages of cyclic loading, and substantially inhibited the growth of cracks (Senatov et al., 2016a). As suggested by the introductory investigations by Niaza et al. (2016), an effective technological advancement to FDM scaffolds for bone reconstruction may derive from the adoption of nano-sized HAp fillers, such as nanoparticles and nanorods, instead of conventional micron-sized ones. In fact, if properly distributed, nanofillers promote the formation of an ultrafine structure and the greater surface interaction between nanofillers and polymer matrix induces a ‘hardening effect’, being the stiffness of the nano-composite sensibly higher than that of the corresponding microcomposite. Moreover, in a technological perspective, very minute fillers are less likely to clog the extruder and printing nozzle (Niaza et al., 2016). In order to improve the dispersion of nano-HAp without using solvents, Esposito Corcione et al. (2017) pre-mixed HAp nanoparticles and PLA pellets in a rotomolding machine. Thanks to the appropriate setting of temperature, time and rotational speed,

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the polymer pellets were uniformly coated by a thin HAp layer. Filaments were meltextruded from the coated pellets with HAp loadings up to 15 wt%. Although the nanoparticles had the tendency to agglomerate, the rotomolding step effectively led to a good dispersion of the filler in the composite filaments. The HAp nanoparticles did not experience any phase transformation upon processing and also the degree of crystallinity of the PLA matrix remained unchanged in spite of the incorporation of HAp. This was an important achievement, since not just the FDM processability, but also the bioactivity rate strongly depends on the crystalline properties of both HAp and PLA. The compressive modulus of the neat polymer significantly increased when adding 15% of HAp, while the yield stress and the corresponding strain did not change significantly (Esposito Corcione et al., 2017). Whereas the HAp content in composite scaffolds printed by FDM rarely exceeds 15 wt%, Zhang et al. (2021) successfully obtained poly(L-polylactic acid) (PLLA)HAp scaffolds with up to 50% of nanosized HAp by an optimized strategy that relied on solvent mixing, in order to improve the particles’ dispersion, and on surface treatment with dodecyl trimethoxy silane, in order to enhance the matrix-filler bonding. Since the addition of HAp lowered the melting point of neat PLLA and reduced its crystallinity, the printing temperature had to be reduced in order to ensure sufficient printing accuracy. Whereas the elastic modulus of the scaffolds under a compressive load was almost unaffected by the addition of HAp, the compressive strength decreased from 45 MPa for neat PLLA to 15 MPa for composites with 50% of HAp. Nonetheless, the compressive strength still remained higher than that of neat HAp ceramic scaffolds with the same porosity, which, according to the literature data discussed by Zhang et al. (2021), has been reported to be about 4.5 MPa. Also, after soaking in a phosphatebuffered saline solution, the compressive strength of pure PLLA scaffolds decreased more rapidly than that of the composite scaffolds. The presence of a very high fraction of filler significantly improved the hydrophilicity of the composites and mitigated the acidity of the PLLA degradation products, which led to a superior capability of osteoregeneration in vivo (Zhang et al., 2021). Comparable results were reported by Yu et al. (2017) for poly-(ester urea) (PEU) scaffolds filled with HAp nanorods. Solvent mixing in hexafluoroisopropanol led to a very homogeneous filler distribution up to 40 wt% and there was no detectable loss of HAp nanorods during processing and cell culture. The compressive modulus of the composite scaffolds with 75% of porosity ranged between 65 and 85 MPa regardless of the filler loading. The lack of change in modulus was likely due to the balance between the stiffening effect of HAp on PEU and the weakening effect of the unoptimized matrix-to-filler interface. When tested at physiological temperature (37°C), the compressive modulus slightly decreased to 49-65 MPa due to the softening of the polymer matrix as the temperature approached the glass transition temperature (54°C). After 1 week in phosphate buffered solution at physiological temperature, the compressive modulus was further reduced to 38-54 MPa due to the plasticization of PEU in water and due to the weakening of the matrix-to-filler interface. However, the compressive stiffness remained very close to natural cancellous bone (50–100 MPa). The addition of HAp nanorods significantly promoted the proliferation and differentiation of MC3T3-E1 cells, especially in samples with a high filler loading. The scaffolds

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Figure 6.3 Core-shell filaments comprising a polymer internal core and a bioceramic external shell of micron-sized HAp particles. The amount of powder adhering to the filament’s surface increases with the applied pressure, the heating temperature and the holding time (exemplifying sketch of the procedure followed by Dascalu et al. (2020)).

with 30 wt% of HAp showed the highest cell number, indicating these scaffolds are suitable to promote MC3T3-E1 cell proliferation likely due to increased roughness or the fact that HAp may contribute to the integrin-mediated osteoblast adhesion (Yu et al., 2017). In order to eliminate any complications related to the even dispersion of HAp particles across the filament, Dascalu et al. (2020) adopted a different approach by producing core-shell filaments comprised of an external layer of micron-sized HAp particles coated on an internal polymer core, as schematically shown in Fig. 6.3. Two different types of commercial filaments, either PLA or TPU, were laid down on a bed of HAp particles and completely covered with additional HAp particles. Then, an external pressure ranging between 0.5 10−4 MPa to 5 10−4 MPa was applied to make the particles adhere to the filaments’ surface. In order to irreversibly glue the HAp particles to the polymer substrates, the temperature was increased from 195°C to 215°C with a holding time from 30 minutes to 120 minutes. As the final step, all filaments were sonicated so that any excess powder would drop off. The characterization of the core-shell filaments proved an increasing gain in weight with increasing pressure, time and temperature. However, the combined action of pressure and heating led to geometric deformation of the filaments, especially the PLA ones, whose cross section largely deviated from the original circularity. The optimized parameters (temperature of 205°C, exposure time of 60 minutes and pressure of 0.5 10−4 MPa for PLA; temperature of 215°C, exposure time of 60 minutes and pressure of 2.5 10−4 MPa for TPU) led to the best trade-off between HAp adhesion and preserved geometry of the filaments (Dascalu et al., 2020). The proposed core-shell structures completely avoid the problem of targeting a perfect distribution of the filler. As an additional

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advantage, HAp particles are preferentially located on the surface, where the reactions with the human body are expected to primarily occur after implantation. However, based on the available data, it is not clear whether the HAp external layer may have any consequences on the filament’s diameter and hence on its compatibility with standard printers. Also, the inter-bead adhesion, as well as the inter-layer adhesion in the printed parts are likely affected by the presence of a ceramic shell and this point needs further investigation, especially if load-bearing applications are sought. Singh et al. (2019) and Ranjan et al. (2020) put the addition of chitosan forward to increase the tissue in-growth rate of PLA-HAp composites. In a multi-step analysis, 91 wt% PLA-8 wt% HAp-1 wt% chitosan was identified as the best composition and the extrusion conditions and printing parameters were adjusted accordingly in order to optimize tensile and flexural strength (Ranjan et al., 2020). Tian et al. (2020) used ε-poly-L-lysine (EPL), a natural homo-polyamide made of 25-35 L-lysine with antimicrobial activity, to coat PCL-HAp composite scaffolds printed via FDM. The incorporation of HAp particles made the surface of PCL slightly rough and heterogeneous. The static contact angle of PCL (about 98°) decreased to about 76° after adding HAp and dropped to about 38° after coating with EPL, which demonstrated a substantial improvement in hydrophilicity of the coated scaffolds. According to in vitro release studies, more than 80% of the EPL was released within the first 24 hours, but then a slower release occurred during the following 3 days. Although MC3T3-E1 cells could not spread completely on the PCL scaffolds, excellent cell attachment, and interaction could be observed on the composite scaffolds, especially after EPL coating, as a result of the increased hydrophilicity and surface roughness. Moreover, EPL-coated PCL-HAp composite scaffolds exhibited an excellent broadspectrum antibacterial activity against S. aureus, E. coli and S. mutans in vitro, and the antibacterial activity was retained for an extended period (Tian et al., 2020).

6.6.2 Tricalcium phosphate and other phosphates Besides HAp and its derivatives such as carbonated-HAp (Oladapo et al., 2020), tricalcium phosphate (TCP), Ca3 (PO4 )2 , and other calcium phosphates are frequently considered to control the bioactivity of polymer-matrix composites (El Moumen et al., 2019; Koons et al., 2020). In fact, calcium phosphate ceramics are widely researched for bone tissue engineering and drug delivery applications on account of their biocompatibility in vitro, their excellent bone regenerative activity in vivo and their effectiveness for the localized delivery of drug molecules or cells (Trombetta et al., 2017). As underlined by Walejewska et al. (2020), the addition of hydrophilic components such as calcium phosphates accelerates the degradation kinetics by improving the wettability of the composite. Also, calcium phosphate-based ceramics mitigate the drop in pH that often accompanies the bio-degradation of polyesters such as PLA and that may be responsible for acidification-induced inflammatory response in vivo (Hassan et al., 2019; Yu et al., 2017). Already in 2003, Kalita et al. (2003) demonstrated that FDM can be applied to fabricate a variety of scaffolds with different architectures from a PP-TCP composite filament. Interestingly, wax and vegetable oil were introduced as viscosity regulator and

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as plasticizer, respectively. Although required to impart plasticity and processability during mixing, these processing aids actually undermined the tensile properties of the composite material as compared to neat PP. The presence of the ceramic phase partially counterbalanced the loss in modulus of elasticity, but not the drop in tensile strength, probably as a consequence of the defective PP-TCP interface. The compressive behavior of the composite scaffolds was comparable to that of cortical bone, being the compressive stiffness and strength inversely related to porosity (Kalita et al., 2003). Drummer et al. (2012) could extrude PLA filaments loaded with 2.5 wt% and with 5.0 wt% of TCP. However, since no plasticizers were added to improve the handleability, the filaments with the highest filler loading were too brittle for printing. The DSC analysis revealed that the glass transition temperature slightly decreased when moving from the original granules to the extruded filament, to the extruded filament with 2.5 wt% of TCP, and finally to the printed part. Nonetheless, the change was not significant enough to be considered a cue of the molecular degradation of PLA upon processing, Instead, the presence of impurities and the addition of TCP had a nucleating effect that promoted the crystallization of PLA above the glass transition temperature. As a result, whereas the original PLA pellets had an amorphous behavior, the PLA in the printed parts came to have a semi-crystalline behavior. However, the specific volume change between molten and solid material was minimal. The relatively small change in specific volume between molten and solid states is usually considered to be advantageous to limit warpage phenomena that may occur due to temperature gradients while printing. Tensile test samples built at different temperatures experienced different degrees of crystallisation, with a direct correlation existing between crystallinity and tensile properties, especially stiffness. Another relevant parameter was the size of the samples, as larger samples had a different thermal history as compared to smaller ones. The larger samples required a larger number of beads and layers to be built, which caused repeated heating and thus led to a higher stiffness. Also, the influence of weak points on the surface was mitigated due to the higher cross section of the larger specimens. Consequently, larger specimens had higher tensile strength and elongation at break with respect to smaller samples. However, all printed parts had a stiffness appropriate for a potential future medical use (Drummer et al., 2012). Since β-TCP and, even more, its polymorph α-TCP are less stable than HAp, they degrade more rapidly and promote resorption phenomena activated by osteoclasts. However, TCPs may resorb too fast as compared to natural bone regrowth. Biphasic calcium phosphates (BCPs) are a mixture of HAp and β-TCP whose degradation rate can be tuned to match the bone healing rate by changing the relative amounts of the constituent phases. The biological response of BCPs is primarily governed by the HAp/TCP ratio, but it is also affected by their physical properties, with BCPs in the nanometric scale being responsible for a significant improvement in osteoblast adhesion and proliferation, collagen production and calcium deposition. Whereas conventional mixing methods to obtain BCPs with a controlled compositional ratio often require multiple steps and at least a post-processing calcination at high temperature, the solution combustion technique has been proposed to obtain the targeted HAp/TCP ratio in a single step. Nevado et al. (2020) synthesized BCP powders with around 70 wt% of HAp by solution combustion and observed that, after treating at 800°C for 2 hours, the

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crystalline phases basically remained the same, the amount of HAp slightly changed from around 72.5 wt% to around 68.6 wt% and some α-TCP was transformed into βTCP. However, the thermal treatment decreased the surface area and pore size of the BCP nanoparticles and helped to completely remove any residue of organic matter from the solution combustion synthesis. The BCP powders could be easily dispersed through the PLA matrix in a ratio of 15 wt% by ball milling and hot-melt compounding in a single screw extruder. However, the BCP agglomerates, which consisted of granular particles smaller than 100 nm and of platelets around 300 nm in diameter, survived into the filaments. Pores were also detected, especially in the filament containing the as-synthesized (not heat-treated) powder. Both filaments, with original and calcined BCP powders, were successfully fed in a low-cost FDM printer to produce scaffolds with complicated geometry. The printed parts presented a hierarchical porosity, with macro-scale pores deriving from the CAD design and microscale pores generated by the filament and the printing conditions. The composite filaments appeared to be nontoxic for several cell lines, including Detroit cells, Saos-2, and U937 macrophages, and did not affect the proliferation of these cells (Nevado et al., 2020).

6.6.3 Bioactive glasses A viable alternative to HAp and, generally speaking, to calcium phosphates are bioactive glasses. These include a wide group of surface-reactive glasses that are able to bond to mineralized bone tissue in physiological environment through the release of biologically-active ions. The first bioactive glass composition, the so-called Bioglass 45S5 (composition in wt%: 45% SiO2 , 24.5% CaO, 24.5 % Na2 O, 6 wt% P2 O5 ) was formulated by Prof. L. Hench in 1969 and, due to its ability to bond to living bone and to stimulate osteogenesis, started a revolution in healthcare and set the basis for modern biomaterial-driven regenerative medicine (Baino et al., 2018). Bioactive glasses have been successfully incorporated into a number of different matrices yielding a variety of composites. The printability of PLA-Bioglass 45S5 filaments was already proved by Martel Estrada et al. in 2017 (Martel Estrada et al., 2017). Composite scaffolds were produced by FDM and soaked in a SBF to ascertain their potential bonding to bone through the reprecipitation of carbonated HAp (Martel Estrada et al., 2017). Subsequently, Distler et al. (2020) investigated the effect of the glass weight fraction on the processability of PLA-Bioglass 45S5 filaments and on the mechanical and biological behavior of the composite scaffolds produced with them. Although filaments could be successfully extruded with filler loadings as high as 10 wt%, the consistency of the cross-sectional diameter became problematic for filler loadings above 2.5 wt% and the tensile properties of the filaments started to decline for glass contents above 1 wt%, likely due to the weak interphase bonding between PLA and glass particles. Although strut merging and loss in definition were observed in the printed scaffolds, the structures printed from the composite filaments basically achieved the same geometric accuracy and printing resolution as the neat PLA. The failure mode under compressive load changed from buckling for 1 wt% of glass to brittle fracture for 10 wt% of glass. Accordingly, the compressive strength dropped from about 18 MPa

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for neat PLA scaffolds to about 12 MPa for scaffolds with 1 wt% of glass, down to 1 MPa for scaffolds with 10 wt% of glass. All the disks printed with the composite filaments were able to induce the precipitation of carbonated HAp in SBF and induced increased osteogenic differentiation of adipose-derived human stem cells in vitro as compared to neat PLA (Distler et al., 2020).

6.6.4 Calcium carbonate The addition of calcium carbonate, CaCO3 , to PLA was evaluated for orthotics, splint and cast manufacturing and medical modelling. Although superior to neat PLA, the thermal stability of the PLA-calcium carbonate composite filaments negatively correlated with the filler loading in the 20-50 wt% range. This is an important issue, since the thermal stability determines the printing conditions and the feasibility of thermal sterilization (Maróti et al., 2019). The further addition of silver (Ag) nanoparticles into the PLA-calcium carbonate composite filaments not only provided the printed PLAmatrix composites with anti-bacterial properties, but also improved the thermal stability of PLA (Maróti et al., 2020). Given the relative dearth of published experiments on the FDM of polymer-calcium carbonate composite filaments, there is certainly scope for further investigation.

6.6.5 Titania Titania (titanium dioxide, TiO2 ) is a multi-functional oxide that is largely explored in the literature to provide polymer-matrix composites with a variety of different features. Titania is often added to polymers in order to produce functional composites on account of its photocatalytic and antibacterial properties (Al-Hydary and Al-Rubiae, 2019; Sangiorgi et al., 2019; Skorski et al., 2016; Viskadourakis et al., 2018). Since it is generally reported as the most effective semiconductor for the oxidative degradation of organic compounds, and since it is very resistant to corrosion, titania can be successfully adopted for the controlled degradation of drug residues in water, as proved by Sevastaki et al. (2020) for the treatment of paracetamol contaminants. In order to mimic the mechanical properties of cancellous bone, Nájera et al. (2018) added titania to a blend of PLA and PCL. The presence of relatively small and medium titania particles (diameters not defined) up to 1 wt% increased both the ultimate tensile strength and the deformation at break of the polymer blend matrix and MC3T3-E1 pre-osteoblast cells could proliferate on all printed composites, even if the proliferation rate was lower as compared to the neat polymer blend. The good cell adhesion after 14 days post-seeding proved that the composite could promote cell growth without any cytotoxic effect (Nájera et al., 2018). Although Nájera et al. (2018) observed an improvement in the tensile strength, the effect of titania on the mechanical properties of polymer-matrix composites is still under debate, especially because titania particles are likely to agglomerate. This undermines the interaction between filler and matrix, with adverse consequences on the tensile strength and the flexural strength of the composite as remarked by Sudeepan et al. (2016). Torrado Perez et al. (2014) analyzed the fracture surface of tensile

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samples printed with an ABS filament loaded with 5 wt% of titania nanoparticles and clearly noticed diffused agglomeration phenomena. Unlike other fillers and polymers used in the same investigation by Torrado Perez et al. (2014) such as jute fibers and thermoplastic elastomer, titania particles, which are inherently stiff, reduced the freedom of the ABS macromolecules to slide, thus inducing a brittle failure. The critical threshold above which titania particles are likely to agglomerate depends on both the polymer matrix composition and the titania particle size distribution. The role played by the fabrication route of the composite is also decisive (Singh et al., 2018). Although agglomeration has occasionally been detected when surpassing 30 wt% (Soundararajan et al., 2019), the critical filler loading is often around 1-1.5 wt% (Asiaban and Taghinejad, 2010; Skorski et al., 2016). Interestingly, whereas the greatest part of the literature reports a decrease in the ultimate tensile strength as a consequence of agglomeration, Skorski et al. (2016) described the opposite trend. They argued that the presence of titania nanoparticles in their printed composites had a negative effect on the strength of the ABS matrix. As a consequence of agglomeration, some titania nanoparticles were sequestered within the aggregates and their reduced interaction with the polymer matrix improved the tensile resistance of the composite material (Skorski et al., 2016). Titania obviously modifies the mechanical strength and wear properties (S. Kumar et al., in press). Besides this, the addition of titania also modifies the physical properties of the polymer matrix. For example, Tsukuda et al. (1997) reported that the heat capacity of ABS-titania composites decreased linearly with the filler loading, but the experimental values were higher than the additivity in the whole temperature range from 20°C to 150°C. Except in a low temperature range up to about 80°C, the thermal conductivity of ABS increased by adding titania nanoparticles in good agreement with the theoretical values calculated by the Maxwell and Bruggeman equations (Tsukuda et al., 1997). Santos et al. (2014) demonstrated the efficiency of sub-micron titania particles, used alone or in combination with carbon black, to reduce the photo-oxidative degradation of neat ABS. Also, titania has been introduced as a white pigment to modify the optical properties of ABS. In fact, ABS typically looks yellowish in color and slightly translucent due to blue wavelength absorption and partial light transmission. Asiaban and Taghinejad (2010) proved that the addition of 6 wt% of titania substantially increased the whitening index of ABS from 5.3 to about 60, whereas the yellowness index decreased from 32.1 to about 16. Also, the contrast ratio, which is an index of opacity, increased from 75.4% to 100% for a filler loading as low as 1.5 wt%. The strong opacifying effect of titania particles mainly derives from their substantial scattering power (Asiaban and Taghinejad, 2010).

6.6.6 Zinc oxide Zinc oxide, ZnO, like titania, is a thermally stable ceramic with well-known antimicrobial properties. Its effectiveness in several biological applications depends on the ability to induce the production of reactive oxygen species (ROS), to release Zn2+ ions and to induce cell apoptosis (Chong et al., 2022). Also, zinc oxide is one of the favorite antimicrobial agents in popular industrial products like rubber,

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paints and coatings, cosmetics and food packaging due to its inexpensiveness (Jiang et al., 2018). Brounstein et al. (2021) compared the effectiveness of zinc oxide and titania to provide PLA filaments with antimicrobial properties. Whereas the greatest part of the literature is focused on nanoparticles, Brounstein et al. (2021) considered instead ceramic powders in the micron-size range, in order to avoid the potential risk of toxicity that part of the literature has attributed to nano-sized particles (Kononenko et al., 2017). Since the addition of ceramic fillers up to 30 wt% was expected to make the filaments too stiff and brittle for 3D printing them, poly-(ethylene glycol) (PEG) was included as a plasticizer. PEG offers several advantages, since it is biocompatible and biodegradable like PLA, and is miscible with PLA in quantities up to 20 wt% without causing phase separation. In order to achieve a very homogeneous distribution, all composite filaments were produced by solvent mixing in chloroform and subsequent extrusion. Although the filaments with titania were more thermally stable than those with zinc oxide, all composites were stable up to 200°C, which is indicative of their suitability for FDM. The mechanical properties of the filaments could be finely tuned over a wide range of values by changing the relative amounts of ceramic particles and plasticizer, and the grade (average molecular weight) of the plasticizer. For example, the Young’s modulus spanned from about 700 MPa for PLA filaments modified with 10 wt% PEG 1k to about 2700 MPa for PLA filaments reinforced with 30 wt% of zinc oxide microparticles (no added PEG). When incubated in soil, the neat PLA filaments experienced a substantial alteration, with the growth of extensive colonial and filamentous patterns on the filament surface. However, the presence of ceramic fillers, especially zinc oxide, significantly reduced pitting and degradation phenomena caused by microbial activity. PEG proved to be useless for increasing the antimicrobial resistance of PLA; however, it did not impair the antimicrobial effect of the ceramic fillers. On account of this neutral antimicrobial role, PEG might be considered as a functional ingredient that helps adjust the physical and mechanical properties of composite filaments, whereas the biological modifications are imparted by other fillers (Brounstein et al., 2021). R. Kumar et al. (in press) focused on the effect of zinc oxide nano-powder on the shape memory behavior of PLA filaments for FDM. To this aim, several composite filaments were extruded according to a design of experiment based upon a Taguchi L9 orthogonal array, which analyzed filler loading, applied extrusion load and torque as the input parameters. The filler loading had the strongest effect on the microstructure and on the mechanical behavior of the filaments. The presence of 1% of zinc oxide led to porous filaments, with weak mechanical properties and low shore hardness. Increasing the filler loading to 2% reduced the porosity and improved both the mechanical properties and the surface hardness. The shape memory effect was tested using a water bath as the triggering stimulus. The samples were soaked for 5 hours either in water at room temperature (25°C) or in water at 40°C to mimic the human body. The shape recovery at room temperature was maximum for composites containing 1% of zinc oxide, but composites with 2% of zinc oxide performed best at 40°C, with a percentage recovery close to 95%. These results suggest that the shape memory effect can be tuned through the filler loading, which affects both the maximum shape recovery and the triggering temperature. In particular, for biosensing purposes, mixing PLA and

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zinc oxide at 2% is expected to optimize the shape memory effect exactly at body temperature (R. Kumar et al., in press). León-Cabezas et al. (2017) mixed ABS with either a nano-zinc oxide water suspension or with a nano-zinc oxide powder in order to print toys with antibacterial properties to limit the spread of infections in hospitals. The addition of 5 wt% of nano-zinc oxide was not enough to reach the minimum antimicrobial activity and it was necessary to increase the amount of suspension up to 15 wt% to stop the bacterial growth. However, the nano-zinc oxide powder was more efficient, with a good antimicrobial response at 5 wt% of loading. The different efficiency was due to the better dispersion of the nano-zinc oxide powder, which resulted in a higher active surface (León-Cabezas et al., 2017). The antibacterial activity is actually governed by the surface that is available for interacting with bacteria cells (Abudula et al., 2020). In addition to its antimicrobial properties, zinc oxide also features a strong photocatalytic activity under UV radiation, with potential applications for self-cleaning materials (Virovska et al., 2016) and pollutant removal systems (Sevastaki et al., 2021). Creating efficient de-contamination systems poses substantial technical challenges because working with loose particles, especially nanosized ones, allows for the largest active surface possible, but separating them from the purified liquid medium is extremely difficult. A smart solution consists in immobilizing the photocatalytic nanoparticles on a scaffold having an engineered structure to maximize the surface exposed to the contaminated liquid. In this regard, the contribution of FDM is extremely favorable, because customized polymer-based skeletons can be easily printed and then coated with the photocatalytic nanoparticles. Son et al. (2018), for instance, combined ABS-zinc oxide composite scaffolds and zinc oxide hierarchical coatings. For the backbone structures, ABS and zinc oxide nanoparticles were compounded by solvent mixing in acetone and the composite material thereof was extruded to obtain a filament for FDM. After printing, zinc oxide nanorods and nanoflower secondary hierarchical structures were directly synthesized onto the surface of the composite scaffolds. The photocatalytic activity was verified by the efficient degradation of methylene blue (Son et al., 2018). The effect of zinc oxide on the printability and final properties of common thermoplastics is still under debate, with contrasting results depending on the particle size and on the printing parameters. Vidakis et al. (2020) systematically compared the mechanical properties of ABS-zinc oxide composite parts produced with either micron-sized or nano-sized particles. Contrary to the common expectation that nanoscale fillers would provide better mechanical properties due to the size effect, the microcomposites achieved higher mechanical (tensile and flexural) strength than the nano-composites having the same filler loading. As observed by Vidakis et al. (2020), this may be due to the strong tendency of nanoparticles to agglomerate, with the agglomerates behaving like large-scale clusters responsible for local stress concentration and hence for premature failure. However, as an additional point open for discussion, Vidakis et al. (2020) hypothesized that zinc oxide particles may have a pro-degradant effect on the ABS matrix upon processing. If so, micron-sized particles having a smaller active surface may limit the degradation process and thus contribute to

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preserve the mechanical reliability of the matrix. Whereas the overall mechanical performance of the composite parts likely depends on reaching a trade-off between these competing mechanisms, the printing parameters are also very important. In this regard Aw et al. (2018) explored the effect of infill density and printing pattern to improve the tensile, dynamic mechanical, and thermoelectric properties of conductive ABS (CABS)-zinc oxide composites, where CABS is a mixture of ABS and carbon black residue. Increasing the infill density from 50% to 100% improved the stiffness (Young’s modulus and dynamic storage) and tensile strength, but reduced the elongation at break, as well as the loss modulus and damping factor. Also, the higher infill density led to a slight increase in thermal conductivity and to a substantial improvement in electrical conductivity. The line pattern worked better than the rectilinear one, especially in terms of tensile properties and electrical conductivity (Aw et al., 2018). Unfortunately, the paper by Aw et al. (2018) does not specify the size of the filler and therefore it is unclear whether the zinc oxide filler in use was nano- or micron-sized.

6.6.7 Carbon-based fillers Carbonaceous fillers, such as graphene, CNTs, fullerenes, carbon dots and carbon nanofibers, have been also considered to improve the mechanical performance of parts and to embed new functionality while preserving the biocompatibility of the polymer matrix. The importance of carbon-based nanomaterials in the biomedical field stems from their unique combination of electrical, thermal, optical and mechanical properties, with electrical conductivity being conducive to electrical stimulation and enhanced cell growth in neural tissues (Hassan et al., 2019). In order to develop new biomaterials with a robust mechanical response, Chen et al. (2017) reported a significant increase of the stiffness under compression of a TPU-PLA blend as a consequence of graphene oxide incorporation. Although the recorded values were anisotropic, the stiffness steadily increased as the filler loading increased. However, the modulus of elasticity under tension reached a maximum at only 0.5 wt% of graphene oxide loading, coherently with the hypothesis that a percolation threshold for tensile modulus in the TPU-PLA matrix exists below 2 wt %. The deformation at break constantly decreased since graphene oxide acted as rigid filler in the elastic matrix. Although all printed composites supported cell growth and proliferation (specifically, NIH3T3 cells), the highest cell density was achieved by the samples with 0.5 wt% graphene oxide, which outperformed even the neat TPUPLA blend likely due to the regular arrangement of graphene oxide nanoplatelets in a flat position (Chen et al., 2017).

6.7 Case studies and special applications The 3D printing of parts for high frequency applications can present a number of challenges, particularly where the dielectric properties must be precisely designed to achieve the desired part performance. Czarny et al. (2018) developed a new printable composite material based on cyclic olefin copolymer (COC) loaded with titania at 36 vol%. The appropriate choice of the constituent phases and the high

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filler loading resulted in a very high permittivity, εr , up to 9.2, a low loss of 1.10−3 at 9.3 GHz and good thermal stability (glass transition temperature of 130°C). The dielectric composite filament was successfully printed into complex geometries prefatory to diffraction prisms in the radio-frequency range (Czarny et al., 2018). In a 2016 report by Castles et al. (2016), they were able to 3D print ABS-barium titanate (BaTiO3 ) composite parts with tailored and relatively high (as compared to conventional unloaded polymers) real permittivity at microwave frequencies. Interestingly, the dielectric properties of the printed parts were analogous to those of the corresponding unprinted materials, which confirmed the reliability of FDM to process materials with high dielectric properties. The real part of the permittivity and the loss tangent increased with the BaTiO3 fraction, but, in the absence of any plasticizer or compatibilizer, the maximum filler fraction viable for printing was 70 wt% (corresponding to 29 vol%), being filaments with higher loadings too brittle to be handled. In order to improve the handleability and printability of ABS-BaTiO3 composite filaments, Wu et al. (2017) added 1 wt% of octyl gallate (C15 H22 O5 ) as surfactant and 5 wt% of dibutyl phthalate (C18 H22 O4 ) as plasticizer. As already proposed by Castles et al. (2016), a solvent mixing procedure in acetone was applied to improve the homogeneity of the filler dispersion. The composite filament, which contained up to 32 vol% of BaTiO3 microparticles, was easily printed into a range of optical devices with a relative dielectric permittivity of about 11 when measured at 15 GHz. In order to compound BaTiO3 microparticles in ABS, Khatri et al. (2018) replaced the solvent mixing procedure with a kneading step with the addition of a maximum of 1.1 wt% of stearic acid as surfactant. The kneaded feedstock was then fed in a single screw extruder to produce 1.75 mm filaments. Several filler amounts were considered, but composites containing more than 35 vol% of BaTiO3 were very challenging to extrude and basically impossible to print due to their rheology (stick-slip behavior). The ultimate tensile strength as well as the flexural strength of the printed parts decreased with the addition of filler, because the BaTiO3 particles reduced the molecular mobility and hence the bulk ductility of the polymer matrix. The effect was more than linear for high filler loadings (typically, above 10 vol%), because the particles tended to agglomerate and to facilitate the entrapment of air pockets. However, the presence of BaTiO3 had positive outcomes on the relative permittivity, whose value increased with increasing filler loadings over the whole range of frequencies under investigation, from 250 Hz to 200 kHz. However, for high filler loadings, the effectiveness of BaTiO3 was partly impaired by the presence of pores (Khatri et al., 2018). Piezoelectric pressure sensors were successfully printed by dispersing BaTiO3 nano-particles in a poly(vinylidene) fluoride (PVDF) matrix and applying an innovative in-situ electric poling system during the FDM process (H. Kim et al., 2017; Kim et al., 2018a). PVDF is a semi-crystalline polymer consisting of long molecular chains based on the –[CF2 -CH2 ]– repetitive unit. PVDF molecules have a large dipole moment, especially in the β polymorph. In order to induce the transformation from the natural α phase to the more efficient β phase, PVDF is commonly stretched by a factor of 4:1 and then exposed to a high electric field to align the dipole molecular structures. This approach can be coupled with the FDM process, since the polymer chains receive a preferential orientation upon flowing through the print nozzle; then, a high electric field is applied between the nozzle tip and the build platform to complete the in-situ

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poling process. In order to enhance the molecular orientation and poling of PVDF, H. Kim et al. (2017) and Kim et al. (2018a) modified the standard in-situ poling set-up to increase the maximum capacity of the applied electric filed. This allowed multilayered structures with a β phase fraction as high as 55.91% at 15 wt% of BaTiO3 to be printed. The output current of the pressure sensors printed with the PVDFBaTiO3 composite filament progressively increased with the filler loading. However, as a side effect, the tensile strength and the bending strength progressively decreased due to aggregation phenomena. The highest fatigue strength was achieved at 3 wt% of BaTiO3 , but decreased for higher filler loadings (H. Kim et al., 2017). Kim et al. (2018b) studied the effect of adding CNTs to PVDF-BaTiO3 printable materials for the production of highly efficient energy storage devices. Although both CNTs and BaTiO3 particles have a strong tendency to agglomerate when used singularly, when combined together CNTs and BaTiO3 particles mutually interact and remain separated in the polymer matrix, which significantly increases the dielectric properties of ternary composites over binary ones. Further, the dielectric constant of the composite increases as the filler loading approaches the percolation threshold. Accordingly, the highest dielectric constant (118) at 1 kHz was obtained with 1.7 wt% of CNTs and 45 wt% of BaTiO3 particles (Kim et al., 2018b). Qian et al. (2018) combined Li0.44 Zn0.2 Fe2.36 O4 (LZFO) particles and PLA to print efficient microwave absorbers with good mechanical properties. However, it was observed that the composites filled with 20 wt% of LZFO had lower mechanical properties than those with 5 wt% or 10 wt% of filler, due to partial clogging of the nozzle upon printing and microstructural defects. Interestingly, increasing the filler content above 5 wt% impaired the thermal stability of neat PLA, probably because increasing the filler loading led to poor compatibility between filler and matrix, thus impeding an effective heat transfer. The enhanced microwave absorption properties of the printed composites as compared to neat PLA counterparts were attributed to the multi-level microstructure, with the matrix-particle interfaces acting as discontinuities, and to the improved impedance matching between magnetic loss by LZFO and dielectric loss by PLA (Qian et al., 2018). The development of materials capable of providing electromagnetic shielding is of great interest for a broad range of applications, such as the protection of medical equipment, data servers, and aerospace devices, and the ability to produce such materials with complex geometries can have significant benefit. Making steps forward in this area has been the work of H. Wu et al. (2020), who compared the absorbing efficiency of electromagnetic waves of different composites printed by FDM as a function of the nature and concentration of the absorbing agent. Different absorbing mechanisms were detected for different absorbing agents. Whereas the PLA-graphene absorber relied on the absorption of electromagnetic waves through the conductive and sheet-like structure of graphene, the PLA-nano-Fe3 O4 absorber absorbed electromagnetic waves through magnetic loss and surface absorbing protrusions. The PLA-graphene/nanoFe3 O4 mixed system synergistically combined the advantages of both graphene and nano-Fe3 O4 absorbers. Schmitz et al. (2018) focused instead on the electromagnetic interference shielding ability of ABS-matrix parts. MWCNTs and carbon black were preferred to other

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conductive fillers owing to their low density. After performing preliminary rheological tests, the filler loading was fixed to 3 wt% to avoid clogging the extruder and especially the printer. The electrical conductivity of the composite filaments was effectively increased in comparison to neat ABS, with the highest value being measured for the composite with MWCNTs, followed by the hybrid systems and, ultimately, by the carbon black system. These results were predictable, since MWCNTs have a higher aspect ratio than carbon black, which facilitates the establishment of a continuous conductive network, and they are intrinsically more conductive, as also observed by Spinelli et al. (2019). However, the electrical conductivity of the printed parts was approximately 5-6 orders of magnitude lower than the respective filament. This substantial drop was due to the interfaces existing between neighboring beads and between subsequent layers, as confirmed by the dependence of the electrical conductivity on the printing orientation. Accordingly, the shielding effectiveness was found to be a function of both the filler type and the printing direction of the specimens. As already seen for the electrical conductivity, the shielding effectiveness was the highest for MWCNTs, but the performance of the hybrid composites was almost as good, in spite of the significantly lower content of expensive MWCNTs. As for the printing direction, the key variable in order to maximize the shielding effectiveness was to minimize the density of structural interfaces. In spite of their good performance, none of the composite parts was able to achieve the minimum attenuation threshold of 20 dB that is required to ensure that the electronic equipment does not generate, or is not perturbed by, electromagnetic interference. A higher filler loading is likely necessary to reach the safety threshold, but this is expected to pose a new technological challenge due to the increased viscosity of the molten composite (Schmitz et al., 2018). Conductive polymers and polymer-matrix composites have been investigated for sensing solvents and humidity. Sathies et al. (2019) used a commercial PLA-carbon black composite filament to fabricate solvent sensors by FDM. The research was targeted to determine the effect of the infill density on the sensing efficiency and the consequences of solvent dipping in terms of reusability of the device. The sensitivity was negatively affected by increasing the thickness of the printed sensor, but positively affected by reducing the infill density (unless a complete fusion of the printed structure occurred). From the reusability study that included up to 5 cycles of solvent dipping, Sathies et al. (2019) observed that the sensitivity of the sensors progressively improved. Vice versa, as predictable, both the tensile strength and the impact strength of the sensors diminished after dipping into the solvent, especially in dichloromethane, which is a good solvent for PLA. Despite this, the mechanical properties only deteriorated in a very gradual fashion with exposure time, without experiencing a sudden break down. Kalsoom et al. (2020) demonstrated the utility of compounding ABS with borondoped diamond (BDD) microparticles, acting as the electrode, and with LiCl microparticles, acting as the humidity sensing material. Based on previous optimization studies (Waheed et al., 2019), the constituent phases, including 60 wt% BDD and 2 wt% LiCl, were pre-mixed through a wet route, dried, pelletized and repeatedly extruded until complete elimination of air pockets (Kalsoom et al., 2020). The weight fraction of BDD microparticles was a trade-off between two opposite needs, since increasing the amount of conductive fillers proved to amplify the connectivity, but at the same time

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higher filler loadings resulted in brittle and unstable filaments. Also, introducing more than 2 wt% of LiCl induced rapid and excessive humidity absorption, with consequent jamming issues during 3D printing. In order to withstand the abrasive nature of BDD microparticles, the commercial FDM printer was modified with a custom-made titanium alloy gear, printed on-demand by selective laser melting, and with a larger nozzle than the standard one to avoid clogging upon printing. The FDM-printed sensors, which were manufactured by a one-step process unlike conventional sensors, demonstrated very good humidity sensing properties as a result of the synergistic action of the two fillers. In fact, for humidity levels exceeding 11%, the LiCl crystals hydrated into Li+ and Cl− ions that formed a strong network with each other, and also connected with the conductive BDD microparticles. This resulted into a strong continuous network of conductive fillers. As the humidity increased, the conductivity also increased due to an increase in conductive ion concentration. The sensors also possessed short response and recovery times, and good stability on the long term (Kalsoom et al., 2020). Junpha et al. (2020) modified the electrochemical behavior of CNT-based conductive filaments with the addition of copper or zinc oxide fillers in order to fabricate an electronic tongue. The base filament contained 10 wt% of CNTs as the conductive filler in a PLA matrix modified with 10 wt% of PCL and with 10 wt% of styrene-butadienestyrene (SBS), where PCL and SBS were added to improve the filler dispersion. Two multi-component filaments were obtained from the base composition by adding either copper or zinc oxide by 5 wt%, with 5 wt% being the highest filler loading that allowed extruding and printing. Prismatic electrodes were printed from the three different filaments and tested to hydrogen peroxide (H2 O2 ), to nicotinamide adenine dinucleotide and to potassium hexacyanoferrate (K4 Fe(CN)6 ), which were chosen on account of their important role in the production of adenosine triphosphate within living cells. Copper and zinc oxide, working as electroactive additives, modified the original electrochemical properties of the PLA-CNT composite system and the cyclic voltametric data was stable and distinct enough to differentiate the different biological compounds in various concentrations (Junpha et al., 2020). The development of FDM materials and components for analytical applications presents significant challenges, especially in terms of precision and accuracy for the measurement they are undertaking, not to mention the potentially complex environments in which they operate. In a very recent example, Vanˇecˇ ková et al. (2020) were the first to apply FDM to the fabrication of electrodes for the spectroelectrochemical measurement of ultraviolet/visible (UV/VIS) absorption starting from PLA-CNT composite filaments. The optimized activation procedure resulted into the lowest intrinsic kinetic barrier height of 72 ± 2 mV ever reported for 3D printed electrodes. Vanˇecˇ ková et al. (2020) detected an increase of the faradaic response as a consequence of FDM printing and concluded that the printing procedure likely reduced the intrinsic kinetic barrier height for the electron transfer and, at the same time, increased the density of electrochemically active sites at the surface of the composite. The proposed activation procedure, which exploited the anodic oxidation of the electrode/electrolyte interface, was effective at the nanoscale and most likely increased the exposure of CNTs.

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In addition to implantable biomaterials and scaffolds for tissue engineering, the medical industry is one fraught with numerous applications and with that a significant number of technical challenges, not to mention the regulatory requirements. FDM parts must be capable of performing at the high standards required, with precision and reliability, particularly where radiotherapies are being delivered to patients. Hashimoto et al. (2019) developed new filaments specifically designed to print dosimeters with complex geometries for advanced radiation therapy. They added a fixed amount (7.5 wt%) of radio-photoluminescence glass powder to different polymer matrixes, including PLA, ABS and PCL. The PCL matrix proved to be superior in terms of radiation sensitivity and of flexibility. Both ring-shaped and ear-shaped dosimeters could be successfully printed (Hashimoto et al., 2019). Interestingly, Dong et al. (2015) exploited the optical properties of titania not to adjust the color of the polymer matrix, but to finely tune its light-scattering properties in order to print tissue-simulating phantoms to calibrate and validate bio-imaging equipment. The feedstock materials included transparent gel wax as the matrix, titania powder as the scattering phase, and graphite powder as the absorption phase. Waxtitania and wax-graphite composites were prepared ex-situ by melting the gel wax, adding the filler at a designated concentration, degassing in a vacuum chamber, and cooling down. Then, using an experimental FDM printer, the neat gel wax, the titaniafilled composite and the graphite-filled composite were pre-heated, separately fed through independent feeding lines, mixed in the printhead, and deposited layer-bylayer to create the required shape. If needed, magnetic resonance images could be processed as model geometries. The continuous adjustment of the volume fractions of the constituent phases allowed the structural and optical heterogeneities of natural tissue to be simulated, as proved by printing the sections of a human brain model. The optical properties of natural tissues could be reproduced faithfully and a rat head phantom was also printed with embedded vasculature to demonstrate the potential to mimic the physiologic architecture of complex living systems (Dong et al., 2015).

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Non-Print Items Abstract Thermoplastics, such as poly(lactic acid) (PLA), acrylonitrile-butadiene-styrene (ABS), and thermoplastic polyurethane (TPU), are the ideal feedstock materials for 3dimensional (3D) printing by fused deposition modeling (FDM, aka fused filament fabrication, FFF). This is because they are amenable to be extruded into regular filaments and are very easy to print. However, thermoplastic materials are unsuitable for load bearing applications due to their relatively poor mechanical properties. Also, they lack functional features such as bioactivity, thermal and electrical conductivity. Glass, ceramic and carbonaceous fillers can be introduced in order to provide the thermoplastic matrix with the required characteristics. After discussing the main advantages of discrete fillers over continuous reinforcements, this chapter reviews the opportunities and challenges that arise when FDM is translated to composite feedstock materials that are functionalized with glass, ceramic and carbonaceous fillers. Coherently with the available literature, great attention is paid to structural reinforcement, thermal and electrical conduction, and bioactivity. However, numerous examples from other fields are also presented in order to demonstrate the extraordinary versatility of these fillers. Keywords Fused deposition modeling; FDM; Fused filament fabrication; FFF; Composite material; Glass; Ceramic; Carbonaceous filler; Additive manufacturing

Fused deposition modeling of polymer-matrix composites with metal fillers 7.1

7

Introduction: Metal fillers

Metals are commonly employed as discrete fillers, including nano- and micro-particles and rods, in order to improve the mechanical performance, thermal and electrical properties, and even the bioactivity of polymer-matrix composites for fused deposition modeling (FDM, aka fused filament fabrication, FFF) (Wanasinghe et al., 2020; Zare and Shabani, 2016). Discrete metal fillers are very versatile. However, the adoption of metal fillers in lightweight structures may be impaired by their relatively high density. Most properties of composite materials are dictated by the volume fractions of their constituent phases, rather than by their weight fractions (Gibson, 2012). As compared to glass, ceramic and carbonaceous fillers, most metals have higher density, which increases the part’s weight for the same volume fraction of filler. Further, as to the best of the Authors’ knowledge, the usage of continuous metal fibers has not been implemented in FDM yet. An exception may be the fiber encapsulation additive manufacturing (FEAM) method described by Saari et al. (2015), as explained in Chapter 9. However, according to the FEAM method, the metal fiber/wire is not actually printed; rather, it is guided close to the nozzle tip by an external pipe and then encapsulated in the thermoplastic matrix after the extrudate has left the nozzle. The gap in the literature regarding the usage of continuous metal fibers in FDM is probably due to the technological issues that arise in critical processing operations upon printing, such as fiber impregnating, folding and cutting, as a consequence of the stiffness and surface properties of metals. Moreover, the extreme reactivity of most metals with oxygen requires working under protective atmosphere, thus imposing additional costs and processing constraints. A completely different approach, defined as Hybrid Fused Deposition Modelling (HFDM), has been proposed to embed continuous metal meshes in the printed part (Butt and Shirvani, 2018; Butt et al., 2020). Although the final product can be regarded as a composite, since metal meshes are inserted between polymer layers, the HFDM process does not actually require to compound the metal additive into the polymer matrix, but just to alternate strata with different compositions upon printing. In this regard, the HFDM is similar to the procedure described by Mori et al. (2014) to reinforce FDM parts with continuous carbon fibers and therefore this method is described in detail in Chapter 9, which is dedicated to continuous fibers and sandwich structures.

Fused Deposition Modeling of Composite Materials. DOI: https://doi.org/10.1016/B978-0-323-98823-0.00008-1 c 2023 Elsevier Ltd. All rights reserved. Copyright 

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Figure 7.1 Some examples of the numerous applications of FDM applied to composite materials with metal fillers.

Polymer-metal composite filaments with a high filler loading are the feedstock material for producing fully metal parts by means of the shaping-debinding-sintering (SDS) process, aka fused deposition of metals (FDMet), which is described in detail in Chapter 10. The following sections provide a wealth of examples to illustrate the successful development of polymer-metal composite parts by FDM in areas as diverse as rapid mold tooling and biomedical implants, reactive (energetic) materials and friction welding, as shown in Fig. 7.1. A critical assessment of the advantages and disadvantages of processing composite parts by FDM, including metal-filled ones, is provided in Chapter 11. Details about the preparation of composite filaments are reported in Chapter 12.

7.2 Case studies and relevant applications Some general remarks about the main advantages and disadvantages of adding discrete fillers are provided in Chapter 6. The goal of the following paragraphs is to exemplify how the specific properties of discrete metal fillers can be leveraged in order to embed new functionality in the printed part.

7.2.1 Direct rapid tooling The development of metal-filled functional composites by FDM can be traced back to early dates. Almost 20 years ago, researchers at Swinburne University of Technology started to investigate both iron- and copper-polymer composites processed by FDM for direct rapid tooling of injection molding dies and inserts. Table 7.1 summarizes the various powders used by Masood and Song (2004, 2005) and by Nikzad et al. (2011) through their research on metal particle composites for direct rapid tooling.

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Table 7.1 Characteristics of the polymer matrix and metal fillers processed in the research at Swinburne University of Technology (Masood and Song, 2004, 2005; Nikzad et al., 2011). Polymers in use: acrylonitrile-butadiene-styrene (ABS), and polyamide (PA). Reference Masood and Song (2004)

Matrix Polymer PA (P301 Stratasys Inc.)

Masood and Song (2005)

PA (P301 Stratasys Inc.)

500–800 μm

Nikzad et al. (2011)

ABS (P400 Stratasys Inc.)

450–500 μm

Size 200–500 μm

Particles Metal Size Iron Coarse: 50–80 μm (Sigma-Aldrich) Fine: PLA+talc > PLA+carbon fibers. As a result, according to the sintering model that describes the fusing mechanisms of FDM parts, the addition of talc and, even more, the addition of carbon fibers favored the consolidation process through the improved coalescence between adjacent beads. Furthermore, the increasingly homogenous microstructure implied an increasingly isotropic behavior. This is consistent with the results published by Torrado et al. (2015) regarding the effect of various fillers and polymer blends on the anisotropic behavior of ABS parts. It was observed that some additives may provide the polymer matrix with a higher propensity to flow, which finally results in a more homogenous microstructure. Since the presence of adjacent beads and successive layers is thus obscured, the mechanical behavior (specifically, the failure mode under tensile load) depends more on the bulk material properties and less on the printing-induced artefacts. However, it is worth noting that for most of the systems described by Torrado et al. (2015) the increased isotropy of the printed parts came at the expense of the tensile resistance, since the addition of additives such as styrene ethylene butadiene styrene (SEBS) and ultra-high molecular weight polyethylene (UHMWPE) made the modified material weaker than neat ABS, both in the in-plane direction as well as in the growth direction.

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The anisotropic behavior of composite parts reinforced with continuous fibers is also very complicated. Starting from a highly anisotropic feedstock, composite parts reinforced with continuous fibers may have a variable degree of anisotropy depending on the arrangement of the fibers within the single layer (for example, “concentric mode” vs “isotropic mode” as explained in Chapter 9 for the Markforged technology; alternating areas made of neat polymer and of composite material; potential presence of gaps deriving from sharp turns of the composite beads for angular geometries) and on the stacking sequence of different raster angles across subsequent layers. However, it has been anticipated that, if fibers can be freely aligned and printed with a curved trajectory, a variable stiffness composite structure (VSCS) can be achieved, where the load is transferred along the longitudinal direction of the fibers (Malakhov et al., 2020). Accordingly, new design methodologies are being formulated to determine the optimized curved fiber trajectory that ensures structural reliability with substantial weight saving (Suzuki et al., 2020). Besides the mechanical properties, the electrical and thermal properties of conductive composites processed by FDM have also been reported to be direction-dependent. As discussed in detail in Chapter 6, the electrical conduction within each layer depends not only on the conductivity of the feedstock material, but also on the deposition path, the meandering connections and the interfaces between neighboring beads (Mousavi et al., 2020); in the growth direction, interfaces between subsequent layers often present pores and defects due to imperfect bonding, which cause contact resistance (Dijkshoorn et al., 2021). A clear example is provided by the PCL-carbon black (“carbomorph”) composite parts described by Leigh et al. (2012), since their electrical resistance changed by 25% when tested parallel or perpendicular to the growth direction due to the insulating effect of the residual inter-layer voids. Analogously, Gnanasekaran et al. (2017) reported that the electrical conductivity of carbon nanotube-loaded composites at the percolation threshold was strongly direction-dependent, with an anisotropy factor (defined as the ratio between the conductivity measured in line with the printed layers and the conductivity measured normal to the printed layers) close to 7. This implied the conductivity in the longitudinal direction was 7 times higher than in the transverse direction. In order to achieve a uniform connectivity across the FDM deposition lines in spite of the strong anisotropic behavior, it was recommended therefore to have concentrations of conductive nanofillers far above the percolation threshold. Likewise, the thermal conductivity of PEEK loaded with 5 wt% of continuous carbon fibers was almost three times higher in the fiber direction than in the perpendicular direction when tested between room temperature and 300°C (Stepashkin et al., 2018). The anisotropic behavior of FDM composite parts is commonly regarded as a drawback, because the response of the composite is optimized along a single direction (Torrado et al., 2015). For example, Mulholland et al. (2018b) analyzed the effect of the FDM-induced preferential orientation of short copper fibers on the thermal conductivity of PA6-matrix composites for the production of air-cooled heat exchangers. The strong orientation, close to 70%, was reflected in the thermal conductivity, where the conductivity along the flow direction, parallel to the prevalent fiber direction, was about seven times larger than along the build direction. This posed a substantial challenge, since the observed anisotropy in heat conduction was in contrast with the geometric

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requirements of the heat exchanger, thus calling for a synergic optimization of material properties and part’s design (Mulholland et al., 2018a, 2018b). Nonetheless, special applications may require a direction-dependent performance and anisotropy may be sought after deliberately with aligned or spatially organized fillers (Niendorf and Raeymaekers, 2021). For example, thermal management materials in electronic devices should possess a very high through-plane thermal conductivity, although a good in-plane thermal conductivity is also useful to avoid local heat concentration. Geng et al. (2019) purposely exploited the preferential orientation of planar flake-like hexagonal boron nitride (BN) to enhance the through-plane thermal conductivity of planar devices printed from PA6 modified with polyolefin elastomer grafted maleic anhydride (POE-g-MAH). Although the through-plane thermal conductivity also depended on the preferential alignment of the matrix macromolecules, as well as on the formation of porosities and structural voids between adjacent beads, Geng et al. (2019) proved the effectiveness of controlling the thermal properties of electronic devices through the preferential orientation of elongated fillers. Zhuang et al. (2017) fabricated PLA-graphene composite parts with a desktop printer that could melt and print two different filaments through the same nozzle. A neat PLA filament and a PLA-graphene composite filament were thus “passively mixed” in the nozzle and the relative amount of the two materials was adjusted through the corresponding extrusion ratio through the nozzle. The authors exploited the layered structure of FDM parts and the variable extrusion ratio to provide the printed parts with a controlled anisotropy and a controlled compositional/functional gradient. The selected distribution of graphene corresponded to a local change in electrical conductivity and hence to an anisotropic temperature field under the applied electric field that was expected to serve for some special chemical reactions, temperaturecontrolled surface wettability and phase change processes (Zhuang et al., 2017). Similarly, Isakov et al. (2016) leveraged the multi-material printing capability of FDM to fabricate dielectric resonator structures consisting of thin coupons (where “thin” implies the coupon’s thickness was smaller than the wavelength of the incident microwave radiation, in the range from 12 to 18 GHz, i.e., around 1.6 to 2.5 cm) made of alternate layers of relatively low (polymer only) and high (composite system) dielectric constant materials. Either neat ABS or PP were used to print the low permittivity regions. For the high permittivity regions, bespoke filaments were produced to incorporate microparticles (diameter below 3 μm) of different perovskite oxides. Nearly metamaterial-like properties (namely, near-zero or negative permittivity) could be achieved by the appropriate choice of the constituent materials (PP for the low permittivity areas, BaTiO3 -filled polymer for the high permittivity ones) and their spatial arrangement (Isakov et al., 2016). An interesting example of intentionally induced anisotropy in the field of 4dimensional (4D) printing is provided by hygromorphs, whose shape changes with environmental humidity as a result of the controlled combination of an anisotropic structure with a hydrophilic material. 3D printing has proved to be the ideal technique to create different scaffolds as the deformation-controlling structure (Li et al., 2019). Yu et al. (2020) outlined a detailed workflow to design thermally-activated bending units consisting of bi-layered structures. In fact, each bending unit consists of two

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blocks, i.e. the top actuator block and the bottom constraint block. The actuator block is made of neat PLA and printed in the longitudinal direction, whereas the constraint block is made of carbon fiber-reinforced PLA and is printed in the transverse direction. If the system is heated above the glass transition temperature of PLA, the release of printing-induced residual stresses causes the shrinkage of PLA along the printing path direction, but the actuator block shrinks more than the constraint block and creates a difference in shrinkage that results in a controlled bending behavior. The appropriate combination of multiple bi-layered structures was exploited to create complex parts, such as lamp structures, bottle-holders and shoe supports (Yu et al., 2020).

11.10

Advanced materials: Functionality beyond mechanical reinforcement

In 2013, Michael Hayes, a technical lead engineer with Boeing, identified and discussed some of the main hurdles retarding the industrial success of AM, including (Tibbits, 2014): r r r

The need for larger build chambers to enable the production of increased scale parts for realworld applications The need for structural materials that can withstand severe service conditions The need for multi-functional and smart/responsive materials

Materials science and technology in AM have witnessed a substantial advancement since then. Nonetheless, the lack of adequate materials is still perceived as one of the most relevant limiting factors that AM should overcome to become viable for serial production (Proff and Staffen, 2019). One reason for the limited availability of materials is the proprietary nature of many 3D printing technologies, which forces AM customers to buy their materials from their printer manufacturer and to process them according to closed (often blind) parameters. Also, as previously discussed in Chapter 5, the certification of AM materials is often complicated and time consuming. Gauging AM materials with traditional materials is challenging, since the properties of AM parts also depend on the geometry and on the printing history, and not just on the feedstock. However, the situation is gradually changing, with some companies, like Ultimaker and HP, offering open-platform systems and cooperating with external materials suppliers (AMFG, 2019). The way forward is the development of new materials. Of course, structural parts fabricated by AM must meet the pre-existing, widely accepted and often standardized requirements of industrial production. However, it is not just a matter of structural strength or reliability. Modern manufacturing asks for materials with new functionalities that feature advanced properties other than mechanical performance – or that feature advanced properties in addition to mechanical performance. As seen previously, metal particles and carbonaceous fillers may impart electrical and thermal conductivity. Selected materials, such as bioactive glasses, hydroxyapatite and other ceramics, improve the performance of plastic materials for biomedical

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applications. However, there is more than this and the possibility of mixing and compounding to create new composite filaments provides engineers and developers with the right materials to make their ideas come to life. The possibilities are endless, and the same material can target multiple final applications. For example, thermoplastics can be compounded with thermochromic additives that modify their color as a consequence of a change in temperature. The transformation is reversible and the material turns back to its original color when the temperature reverses. Filaments with thermochromic effect can be printed into fancy toys just for fun (León-Cabezas et al., 2017), but they can also be used to fabricate functional parts, such as water pipe connectors that change color with the temperature of the water flowing inside (Ahroni et al., 2020). Photochromic additives provide the polymer matrix with photo-responsive effect. They change their physicochemical properties and, most notably, their color as a reaction to light irradiation at appropriate wavelengths. The change is proportional to light intensity. The usefulness of photochromic filaments has been demonstrated to produce bracelets whose color changes with UV light. These wearable indicators are very helpful to protect sensitive individuals, such as very pale phototypes or children, from excessive exposure to UV light (Ahroni et al., 2020). Also, the appropriate combination of different fillers may provide the polymer matrix with multiple functionalities. Going back to toys, in principle thermochromic additives can be combined with antibacterial agents in order to obtain fancy toys that also meet the safety requirements to be handled in hospitals (León-Cabezas et al., 2017). Simultaneously embedding multiple functionalities is not any easy task, since fillers can mutually interact or have an adverse effect. For example, most antibacterial particles, such as zinc oxide or titanium oxide (titania), modify the original color and opacity of the polymer (Asiaban and Taghinejad, 2010) and this is expected to interfere with the colors induced by thermochromic additives. In this regard, the optimization of each filler loading plays a key role in minimizing the side-effects on the other fillers. Different structural properties, such as stiffness, strength, toughness and impact resistance, as well as structural and non-structural functions, such as thermal and electrical conductivity, energy harvesting and self-healing features, can be incorporated in the same material and translated into the printed structure (Martins et al., 2021). In spite of the difficulties, the coexistence of multiple functionalities provides exciting opportunities to build next-generation technical devices, such as custom biomedical implants with controlled degradation rates (Daminabo et al., 2020), and sensors and actuators for soft robotics that combine printability, flexibility, and touch-sensing capability all at the same time (Mousavi et al., 2020).

11.11

What is next?

According to recent statistics gathered and published by Sculpteo (2019, 2020, 2021), when interviewed about their future usage of AM, most technologists foresee they will explore new materials for new applications. The results of the survey for the past three years are summarized in Fig. 11.3.

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Figure 11.3 Forecast development areas of AM in 2019, 2020 and 2021. Interviewees were asked: “How do you expect your use of 3D printing to evolve over the next year?” (Sculpteo, 2019, 2020, 2021). NOTE: The graph collects and elaborates the statistics originally published separately for each year.

It is interesting to note that the aim of investigating more uses and applications for AM (that corresponds to about 68% of the preferences) calls for the introduction of new materials, rather than for the initiation of new technologies. Understandably, this trend is caused by the investment costs, since the expenditure for purchasing a new AM platform (which includes not just the printer, but also various satellite equipment for post-processing, finishing, etc.) is several orders of magnitude greater than the expenditure required to buy a new feedstock material that can be fed into the printing equipment already available in-house. Also, starting a new 3D printing capability requires dedicated staff, often with an extremely high level of technical qualification. This point is crucial, because it demonstrates the need for developing and commercializing new materials for the existing AM technologies (Blanco, 2020; Word et al., 2021). Generally speaking, there is a certain specificity, since each AM method is appropriate for some materials, but not for others, and vice versa (Loke et al., 2019). For example, electron beam melting (EBM) works only on metal powders, whereas stereolithography (SLA) requires photocurable resins, and multi jet fusion (MJF) currently supports a few polyamide (PA)-based powders (mainly PA-11 and PA-12). However, many techniques are potentially capable of processing a wide range of materials, and this paves the way for the creation of new feedstocks with customized properties (Singh et al., 2017; Tofail et al., 2018). In this respect, FDM offers a key advantage, because it is an extremely versatile technique that applies perfectly well to a variety of thermoplastic polymers and composite materials. In addition to composite filaments where the filler is uniformly dispersed, multi-material filaments can be produced where disparate materials are

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arranged in elaborate microstructures, sheltered by a protective layer and combined with an external adhesion promoter, in order to print ready-to-use devices with complicated geometries for optoelectronics, sensing and energy storage (Loke et al., 2019). As an additional advantage of FDM, even inorganic parts can be fabricated via the SDS approach, including metal, ceramic, and metal-ceramic composite systems (GonzalezGutierrez et al., 2018; Rane and Strano, 2019). In terms of feedstock materials, the flexibility of FDM surpasses all other AM technologies. However, the palette of commercial filaments for FDM is still surprisingly limited. If neat polymers and polymer-matrix functional composites are considered, filaments are normally produced from a handful of thermoplastics, usually ABS, PLA, PA, polyethylene terephthalate (PET) and its glycol-modified version (PETG), and flexible plastics (thermoplastic elastomers, TPEs). Advanced polymers such as PEI, PEEK, and PEKK are commercially available, but they are less common, typically very expensive and difficult to print. Commercial composite filaments are prevalently reinforced with inorganic particles (mineral fillers, salts), chopped glass fibers or chopped carbon fibers, with a filler loading not exceeding 30 vol%. Some “environmentally friendly” filaments are modified with natural fibers and wood flour, but they still are a rarity. As largely discussed in Chapter 9, at present just a handful of commercial filaments are reinforced with continuous fibers and they require a dedicated printer. As to SDS, which is presented in Chapter 10, commercial filaments either for metal 3D printers or for standard FDM printers include some examples of steel (stainless steel, tool steel, high tensile steel), Inconel 625, and copper, with Ti-6Al-4V being under development. Few examples of filaments for the production of ceramic parts have been recently put on sale, but the data about them is still sporadic. Apparently, no filaments are yet available in the marketplace for the fabrication of metal-ceramic composites. The applicability of FDM can be substantially broadened through the targeted development of new materials (Roberson et al., 2015; Word et al., 2021). The appropriate combination of polymer matrix and functional filler can enable the fabrication of components with customized properties to meet the ever-increasing needs of highgrowth industries and high-tech applications. With SDS, the cost and time-to-market of fully inorganic parts can be cut with respect to all other AM techniques. Future industries call for a substantial revision of manufacturing. This is a challenge, but this is also an opportunity that FDM is ready to take on.

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Non-Print Items Abstract Fused deposition modeling (FDM, aka fused filament fabrication, FFF) is a versatile additive manufacturing (AM) technique capable of processing a wide variety of thermoplastic materials and thermoplastic-matrix composites. Another advantage of FDM is that fully inorganic parts can be manufactured starting from composite filaments by means of the “shaping, debinding and sintering” (SDS) approach, which entails printing the composite feedstock, removing the polymer matrix acting as sacrificial binder, and sintering the part to consolidate the inorganic object. As critically reviewed in this chapter, incorporating a filler often causes technical challenges, as the printability of the neat polymer is deeply affected. Also, fillers may have complicated consequences on the porosity and the anisotropic behavior of the printed parts. However, the shift from neat thermoplastics to composites is the turnkey to endow FDM products with new embedded functionality and thus to contribute to the substantial revision of manufacturing that is necessary for the successful progress of future industries in the scenario of the Industry 4.0 revolution. Keywords Fused deposition modeling; FDM; Fused filament fabrication; FFF; Shaping, debinding and sintering; Composite material; Additive manufacturing

Fused deposition modeling of composite materials at a glance – supplementary tables 12.1

12

Introduction: A roadmap to FDM of composite materials

Fused deposition modeling (FDM), aka fused filament fabrication (FFF), can be easily extended to print a variety of thermoplastic-matrix composite materials either to achieve composite parts with new functionality or to fabricate inorganic components. The following Supplementary tables draw a roadmap to this field of research. In more detail, Supplementary table 1 provides an overarching description of the state of the art and includes: r

r

r r

r

Supplementary table 1a: review papers on FDM, additive manufacturing (AM) and characterization methods, as well as foundational papers that have initiated key research topics such as 4D printing; Supplementary table 1b: research papers on the FDM of neat polymers and composites, with detailed information about matrix, filler, filament manufacturing and basic goal of the research; Supplementary table 1c: research papers on the FDM of continuous fiber-reinforced parts and sandwich structures; Supplementary table 1d: research papers on shaping, debinding and sintering (SDS), aka fused deposition of ceramics (FDC) and fused deposition of metals (FDMet); great attention is paid to the binder’s composition, which is crucial to produce a printable filament with an extremely high filler loading; Supplementary table 1e: research papers that, though not specifically dedicated to FDM, are relevant to the understanding and future advancement of this field.

Supplementary table 2 focuses instead on mechanical testing methods applied to composite filaments and printed parts produced by FDM. In summary: r r

Supplementary table 2a: tensile testing of composite filaments and printed parts; Supplementary table 2b: flexural testing of composite filaments and printed parts; interlaminar shear strength testing of printed composite parts.

Supplementary table 2 does not include 3D printed composite scaffolds, as specific experimental approaches should be applied to lattices porous materials. Common abbreviations are listed before the list of references.

Fused Deposition Modeling of Composite Materials. DOI: https://doi.org/10.1016/B978-0-323-98823-0.00003-2 c 2023 Elsevier Ltd. All rights reserved. Copyright 

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2 Supplementary table 1: State of the art 12.2.1 Supplementary table 1a – review papers Supplementary table 1a Review papers and foundational papers on new research topics. Reference Topic Review papers and contributions on FDM, SDS, and ME Aberoumand et al., 2021 Review on 4D printing by FDM Ahmed et al., 2020b Review on natural fiber composites in FDM Bardot and Schulz, 2020 Review on FDM of PLA-matrix nanocomposites Bellini, 2002 PhD thesis on FDC (SDS) Brenken et al., 2018 Review on fiber composites in FDM Daminabo et al., 2020 Review on materials and technologies for FDM Das et al., 2021 Review on polymer rheology in ME Dickson et al., 2020 Review on fiber composites in FDM Ferreira et al., 2019 Review on fiber composites in FDM Fries and Durna, 2018 Guidelines on filaments’ recycling Gao et al., 2020 Book chapter on FDM of composite parts for automotive applications Gonzalez-Gutierrez et al., Review on SDS 2018 Gordelier et al., 2019 Review on tensile strength of FDM parts Guerra et al., 2020 Review on FDM of polymer-graphene nanocomposites Huang et al., 2015 Review on quality control in FDM Kabir et al., 2020 Review on AM (FDM) of continuous-fiber reinforced parts Kehinde Aworinde et al., 2019 Review on mechanical properties of PLA parts by FDM S. Kumar et al., in press a Review on FDM materials Liu et al., 2019a Review on PLA in FDM Manoj et al., 2021 Review on particulate emission of PLA vs ABS in FDM Mazzanti et al., 2019 Review on FDM of natural fiber composites Mohan et al., 2017 Review on FDM composites Novakova-Marcincinova and Review on FDM Kuric, 2012 Nurhudan et al., 2021 Review on FDM of metal parts (FDMet - SDS) Park and Fu, 2021 Review on composite feedstock and printing by FDM Penumakala et al., 2020 Review on FDM composites Peterson, 2019 Review on ABS in FDM Popescu et al., 2018 Review on printing parameters-mechanical properties relationship in FDM Rahim et al., 2019 Review on FDM of polymers and composites Rane and Strano, 2019 Review on SDS Rett et al., 2021 Review on renewable materials in FDM Schouten et al., 2021 Review on electro- and biomechanical sensors by material extrusion (continued on next page)

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Supplementary table 1a Review papers and foundational papers on new research topics— cont’d Reference Shanmugam et al., 2021a

Topic Review on fatigue behavior of FDM parts, composites and lattices Shanmugam et al., 2021b Review on mechanical and thermal properties of fiber-reinforced parts by FDM Solomon et al., 2021 Review on processing parameters in FDM Spoerk et al., 2020 Review on FDM of PP (reducing warpage) Tambrallimath et al., 2021 Review on FDM composites Tümer and Erbil, 2021 Review on FDM of PLA-matrix composites Turner and Gold, 2015 Review on basic science of FDM Turner et al., 2014 Review on basic science of FDM Valvez et al., 2020 Review on FDM of PLA-continuous carbon fiber composites Wickramasinghe et al., 2020 Review on FDM composites, especially about defects Word et al., 2021 Review on research at the Polymer Extrusion Lab, The University of Texas at El Paso P. Zhang et al., 2020 Review on FDM from materials to devices H. Zhang et al., 2021 Review on FDM of continuous-fiber reinforced parts Zhao et al., 2019 Review on FDM composites Review papers and contributions on AM Abudula et al., 2020 Review on antibacterial AM with metal/metal oxide fillers Acosta-Vélez and Wu, 2016 Review on 3D pharming Araújo et al., 2019 Review on 3D pharming Atwood et al., 1998 Foundational paper on LENS technique Awad et al., 2018 Review on 3D pharming Azad et al., 2020 Review on 3D pharming Baker et al., 2021 Review on conductive materials for 3D printing of structural electronics Berman, 2020 Review on social effects (especially financial ones) of AM Blanco, 2020 Review on AM of composites Buj-Corral et al., 2020 Review on metal AM in the biomedical field Calignano et al., 2017 Review on AM technologies Castro et al., 2017 Review on 4D printing R.K. Chen et al., 2016 Review on AM of custom orthoses and prostheses Z. Chen et al., 2019 Review on AM of ceramics Chen et al., 2020 Review on 3D pharming Chong et al., 2022 Review on AM of PLA-zinc oxide (ZnO) (nano)composites Curti et al., 2020 Review on 3D pharming Dass and Moridi, 2019 Review on DED Dilberoglu et al., 2017 Review on role of AM in Industry 4.0 Ding et al., 2015 Review on metal wire additive manufacturing Dizon et al., 2018 Review on mechanical characterization and properties of AM polymer parts (continued on next page)

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Supplementary table 1a Review papers and foundational papers on new research topics— cont’d Reference El Moumen et al., 2019 Fidan et al., 2019 Forster, 2015 German, 2019 Gibson et al., 2015 González-Henríquez et al., 2019 Goole and Amighi, 2016 Haleem and Javaid, 2019 Hamzah et al., 2018 Hassan et al., 2019 Honarvar and Varvani-Farahani, 2020 Huang et al., 2013 Jahnke et al., 2013 Javaid and Haleem, 2020 Jiang and Ma, 2020 Jin et al., 2015 Kim et al., 2018a King et al., 2015 Kuang et al., 2019 Le Duigou et al., 2020 Leary, 2020 G. Liu et al., 2020 Martins et al., 2021 Mathew et al., 2020 Melocchi et al., 2020 Momeni et al., 2017 Niendorf and Raeymaekers, 2021 Pandey et al., 2020 Sanchez-Rexach et al., 2020 Saroia et al., 2020 Silva et al., 2021 Sing and Yeong, 2020 S. Singh et al., 2017 Singhvi et al., 2018 Sola and Nouri, 2019 Tibbits, 2014

Topic Review on AM of composites Review on fiber composites in AM Review on applicability of international standards to polymer materials and parts in AM Review on metal sintering after 3D printing by binder jetting and FDMet-SDS Review on DED Review on antimicrobial polymers and composites for AM Review on 3D pharming Review on AM of PEEK in dentistry Review on AM of electrochemical sensors Review on AM of polymer nanocomposite for biomedical applications Review on ultrasonic testing applied to AM Review on societal impact of AM Review on anti-counterfeiting strategies for AM parts Review on AM of organs and tissues Review on toolpath planning strategies in AM Review on AM of custom orthoses and prostheses Review on quality control in AM Review on metal PBF Review on 4D printing Review on 3D and 4D printing of natural fiber composites Book on design for additive manufacturing Review on 2D, 3D and 4D printing of bioimplants Review on AM of multi-functional materials Review-Editorial on 3D pharming Review on 3D pharming (hollow systems for oral delivery) Review on 4D printing Review on 3D printing of composites with aligned filler Review on 3D pharming Review on renewable polymers in AM Review on AM of composites Review on AM of polymer-graphene composites for biomedical applications Review on metal PBF Review on new materials (especially bio-) in AM Review on 3D pharming Review on porosity and defects in PBF Foundational paper on 4D printing (continued on next page)

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Supplementary table 1a Review papers and foundational papers on new research topics— cont’d Reference Tofail et al., 2018 Travitzky et al., 2014 Trombetta et al., 2017 Tully and Meloni, 2020 Valino et al., 2019 Vock et al., 2019 Wang et al., 2017 Wrobel et al., 2013

Topic Review on new materials and metrology needs in AM Review on AM of ceramics Review on 3D printing of calcium phosphates Review on AM printers (guide to buy a 3D printer) Review on AM of composites Review on powders for PBF Review on AM of composites Review on group’s research in radiation shielding and thermal insulation systems by 3D printing Wu and Toly Chen, 2018 Review on quality control in AM Yampolskiy et al., 2017 Book chapter on security issues in AM vs subtractive manufacturing L. Zhang et al., 2019 Review on AM of scaffolds for bone repair Zindani and Kumar, 2019 Review on AM of composites Zocca et al., 2015 Review on AM of ceramics Review papers and contributions on characterization techniques Chartoff et al., 2009 Book chapter on DMA Epp, 2016 Book chapter on XRD Garcea et al., 2018 Review on X-ray computed tomography of polymer composites Giesche, 2006 Review on mercury porosimetry Grubb, 2012 Book chapter on optical microscopy Haugen and Bertoldi, 2017 Review on characterisation methods for porous materials (morphology) Inkson, 2016 Book chapter on electron microscopy Moore, 1964 Foundational paper on GPC for molecular weight distribution of polymers Osswald and Rudolph, 2013 Book chapter on rheometry Roylance, 2001 Book on mechanical properties and measurement Schick et al., 2012 Book chapter on DSC and DTA Zhang et al., 2008 Book on materials characterization Review papers and contributions on natural fibers and composites Azwa et al., 2013 Review on natural fibers and composites Balla et al., 2019 Review on natural fibers and composites Bledzki and Gassan, 1999 Review on natural fiber composites Célino et al., 2014 Review on natural (plant) fibers – hygroscopic behavior Faruk et al., 2012 Review on natural fibers and composites George and Sabapathi, 2015 Review on cellulose nanocrystals Neubauer, 2010 Book chapter on natural fibers and composites Peças et al., 2018 Review on natural fibers and composites Sanjay et al., 2019 Review on natural fibers and composites Satyanarayana et al., 2009 Review on natural fibers and composites Shah, 2014 Review on natural fiber and composites Summerscales et al., 2011 Review on natural fiber and composites (continued on next page)

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Supplementary table 1a Review papers and foundational papers on new research topics— cont’d Reference Topic Review papers and contributions on biomaterials and biomedical applications Bellucci et al., 2010 Review on devitrification of bioactive glasses DeStefano et al., 2020 Review on PLA in modern medicine Filippi et al., 2020 Review on polymer scaffolds Gudkov et al., 2021 Review on ZnO nanoparticles for antibacterial activity Hajiali et al., 2018 Review on PCL-matrix composites for biomedical applications Jiang et al., 2018 Review on ZnO nanoparticles in biomedical applications Kattimani et al., 2016 Review on hydroxyapatite Koons et al., 2020 Review on materials design for bone tissue engineering Kwan and Brodie, 2000 Analysis of early treatment of drug-resistant epilepsy Mozafari et al. eds., 2019a Book on scaffolds Mozafari et al. eds., 2019b Book on scaffolds Schroeder and Mosheiff, 2011 Review on tissue engineering for bone repair Sola et al., 2016 Review on functionally graded materials for biomedical applications Trachtenberg et al., 2014 Book chapter on polymer scaffold fabrication van Tienderen et al., 2018 Review on treatment of epilepsy Zare and Shabani, 2016 Review on polymer-matrix metal-nanofiller composites for biomedical applications Review papers and contributions on materials science and technology (miscellaneous) Baino et al., 2018 Book on polymers and polymers engineering Banerjee and Joens, 2019 Book chapter on debinding and sintering in MIM Bar-Cohen and Anderson, Review on electro-active polymers 2019 Bauswein et al., 2017 Cost comparison of conventional composite manufacturing techniques Brinson and Brinson, 2008 Book on polymers and polymers engineering H. Chen et al., 2016 Review on polymer-matrix composites with thermal conductivity Clancy et al., 2020 Review on metal mimics Devyatkov et al., 2015 Review on catalysts’ extrusion German, 2014 Book on sintering Gibson, 2012 Book on composites science Huang and Terentjev, 2012 Review on agglomerate breakdown under sonication Jamir et al., 2018 Book chapter on hand lay-up and conventional composite manufacturing Jamróz et al., 2019 Review on nanofillers in biopolymer-based films Jesson and Watts, 2012 Review on interface and interphase in polymer-matrix composites and their effect on mechanical properties Lendlein and Gould, 2019 Review on shape memory polymers G. Li et al., 2020 Review on PLA Liu and Chen, 2014 Book on porous materials (continued on next page)

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Supplementary table 1a Review papers and foundational papers on new research topics— cont’d Reference Meola et al., 2017 Niaounakis, 2019 Nichols et al., 2002 Olonisakin et al., in press Osswald, 2017 Patil et al., 2016 Petersen et al., 2017 Rothon, 2016 Shrivastava, 2018 Siemann, 2005 Ter Maat et al., 1992 Wanasinghe et al., 2020

Topic Book chapter on hand lay-up and conventional composite manufacturing Review on recycling of biopolymers Review on terms "aggregate" vs. "agglomerate" Review on surface treatment of fillers in PLA-matrix composites Book on polymer processing Review on extrusion in pharmaceutical industry Analysis of economic implications of DIY toys and gadgets Review on nanofillers Book on polymer processing Review on solvent casting technique Foundational paper on the development of new binder for catalytic debinding (Catamold method, BASF) Review on composites for electromagnetic interference (EMI) shielding

12.2.2 Supplementary table 1b – research papers on FDM Supplementary table 1b Research papers on FDM of neat polymers and composite materials. Contribution Abar et al., 2021

Matrix Polycarbonate urethane

Filler //

Adel et al., 2018

PLA

//

Ahmad et al., 2020

ABS

5 wt% oil palm fibers

Ahn et al., 2002

ABS, different colors

//

Ahrendt and Romero Karam, 2020

PETG

20% short carbon fibers

Filament’s production Filament extrusion; drying; printing; collagen coating

Main research topic Replacement of weight-bearing soft tissues: effect of pore size Commercial Surface polishing of FDM parts by hot air jet Pre-mixing; Development of extrusion with single new composite screw extruder system with oil palm fibers Commercial Effect of printing parameters on anisotropy of ABS parts by FDM and design guidelines Commercial 3D printing on textile for customized orthotics (case study for an anterior cruciate ligament rupture)

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Supplementary table 1b Research papers on FDM of neat polymers and composite materials— cont’d Contribution Matrix Filler Ahroni et al., 2020 CCT BLUE, N.A. thermochromic polymer

Filament’s production Commercial

Main research topic 4D printing - color change and smart devices with commercial filaments

CCU RED, photochromic polymer PLA, semi-transparent Alberts et al., 2021

Alhijjaj et al., 2016

PLA, conductive ABS

Tungsten (in ABS) Commercial

PLA

Copper (in PLA)

Eudragit + Polysorbate 10 wt% felodipine + Polyethylene glycol + Polyethylene Oxide Soluplus + Polysorbate + Polyethylene glycol + Polyethylene Oxide

Alves Guimarães et al., 2020

PVA + Polysorbate ABS

Anitha et al., 2001 //

Aw et al., 2018

ABS Conductive ABS (commercial filament with about 28 wt% carbon black)

Badouard et al., 2019

PLLA PLLA-PBS 50 wt% PBAT

Batakliev et al., 2019

PLA (powdered)

Various colors

Blending and pestling in mortar; extrusion with twin-screw extruder

Particulate emissions with and without metal additives 3d pharming: effect of polymer matrix blend

Commercial

Effect of printing parameters and ABS color on tensile strength of parts produced with open-source printer // // Effect of printing parameters on surface roughness of FDM parts 14 wt% ZnO Solvent mixing in Effect of printing acetone under parameters on magnetic stirring; conductive mechanical grinding ABS-ZnO as with mortar and thermoelectric pestle to materials de-agglomerate the wet powder; drying at room conditions. 10-20-30 wt% flax Hot melt Development of fibers compounding and new composite system with flax 10 wt% flax shives extrusion with twin-screw extruder; fibers pelletization; extrusion with single screw extruder 1.5-3-6-9 Hot-melt Preliminary MWCNTs compounding and investigation on extrusion with potential composite 1.5-3-6-9 GNPs extruder feedstock (graphene nanoplatelets)

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Supplementary table 1b Research papers on FDM of neat polymers and composite materials— cont’d Contribution Bayraktar et al., 2019

Matrix PLA

Bellehumeur et al., 2004

ABS

Belter and Dollar, ABS 2014 Berretta et al., 2017

PEEK

Bhagia et al., 2020 PLA

Filament’s Main research production topic Synthesis of silver Antibacterial nanowires by the activity due to solution-based silver nanowires polyol method, which provides a PVP surface layer; solvent mixing in acetone with pre-dried PLA; solvent removal under vacuum; filament extrusion with (micro) twin-screw extruder // // Models of necking in polymer sintering (healing) // Commercial Strength improvement by fill composting 1-5 wt% MWCNTs Masterbatching Effect of CNTs on from commercial PEEK for FDM masterbatch (10 wt%): manual pre-mixing; hot-melt compounding in twin-screw extruder; drying at 150°C before testing or printing 20 wt% poplar Knife milling of Comparison wood flour PLA pellets to between wood flour 15 wt% fibrillated 2 mm; processing of and fibrillated poplar wood to flour cellulose, and effect cellulose from OR preparation of of genetic poplar tree fibrillated cellulose; variability of pre-mixing with a vegetable fillers spatula at 21°C; moisture removal in vacuum oven at 80°C for 8 hours; hot-melt compounding in single screw extruder; pelletization; storage under vacuum; filament extrusion with single screw extruder

Filler 2.5-5-10 wt% silver nanowires (actual: 1-2-4 wt%)

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Supplementary table 1b Research papers on FDM of neat polymers and composite materials— cont’d Contribution Boparai et al., 2015

Boparai et al., 2016a

Matrix Nylon

Nylon

Filament’s production Vacuum heating; tumbler mixing; filament extrusion 28 wt% aluminum with single screw particles + 12 wt% extruder Al2 O3 particles

Main research topic Improvement of tribological behavior of Nylon 6

Filler 26 wt% aluminum particles + 14 wt% Al2 O3 particles

30 wt% aluminum particles + 10 wt% Al2 O3 particles 26 wt% aluminum Vacuum heating; particles + 14 wt% tumbler mixing; filament extrusion Al2 O3 particles 28 wt% aluminum with single screw particles + 12 wt% extruder

DOE to optimize filament fabrication

Al2 O3 particles

Boparai et al., 2016b

Nylon

Boschetto et al., 2016

ABS

30 wt% aluminum particles + 10 wt% Al2 O3 particles 30 wt% aluminum Tumbler mixing; particles + 10 wt% filament extrusion alumina particles with single screw extruder // Commercial

Braconnier et al., 2020

ABS

//

Commercial

Brooks et al., 2011

//

//

//

Brounstein et al., 2021

PLA + PEG, various

10-20-30 wt% ZnO Solvent mixing in microparticles chloroform (1:5 10-20-30 wt% TiO2 w/w); addition of fillers (and microparticles chloroform to tune viscosity) in planetary mixer; casting on Teflon plate; drying in air; cutting to 1 cm squares; drying under vacuum; filament extrusion; gravity-driven dropping in water

Improvement of tribological behavior of Nylon 6 Surface polishing of FDM parts by CNC machining Materials informatics applied to FDM to analyze the effect of printers and printing parameters Concept design for the variable diameter nozzle Tunable antimicrobial PLA filaments with titania, ZnO, PEG

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Supplementary table 1b Research papers on FDM of neat polymers and composite materials— cont’d Contribution Camargo et al., 2019

Casavola et al., 2016 Castles et al., 2016

Chacón et al., 2017

Chávez et al., in press

Chen and Zhang, 2019 F. Chen et al., 2017

Q. Chen et al., 2017

F. Chen et al., 2019a

F. Chen et al., 2019b

Main research topic Characterization of mechanical behavior of composite parts ABS // // Classical Laminate Theory applied to PLA FDM parts Solvent mixing in ABS Up to 70 wt% Production of acetone; evaporation dielectric BaTiO3 microparticles under periodical composites mixing; complete evaporation; granulation; drying; filament extrusion with extruder PLA // Commercial Effect of printing parameters on tensile and bending properties under different part orientations PLA 5% (for PLA) or Silanization of metal In-situ wire 5-10% (for ABS glass particles; drawing of ABS and for ABS blend) hot-melt low-melting point Blend of ABS with low-melting point compounding and filler styrene ethylene SnBi filament extrusion butylene styrene (SEBS) with desktop single grafted with maleic screw extruder anhydride PLA 6.95 wt% chopped Commercial Effect of laser aluminum fibers polishing on FDM composite parts ABS // Commercial Embedding of QR codes at the CAD Washable support level for tagging material (acrylic Matrix PLA

copolymer) TPU + PLA, 7:3

Filler Graphene

ABS

//

Solvent mixing; precipitation in alcohol; drying; filament extrusion with mini-extruder Commercial

Washable support material (acrylic copolymer) ABS

//

Commercial

Washable support material

0.5-2-5 wt% home-made graphene oxide

Filament’s production Commercial

Composites for biomedical applications

Comparison of QR codes printed with different techniques for tagging Obfuscation of QR codes printed with different techniques for tagging

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Supplementary table 1b Research papers on FDM of neat polymers and composite materials— cont’d Contribution Chisena et al., 2020

Matrix 10/12 Nylon

Filler 35 wt% short carbon fibers

Choi et al., 2020

PLA AND

//

+ 0.5-1-1.5-2 wt% epoxide chain extender + 5 wt% chemical foaming agent Cicala et al., 2018 PLA

Various colors

Comminal et al., 2019 Coppola et al., 2017

PLA

//

PLA

4 wt% organically modified montmorillonite

Cowley et al., 2019

PLA (red dyed)

//

Croccolo et al., 2013

ABS (M30)

//

Czarny et al., 2018

Cyclic olefin copolymer 36 vol% TiO2

Dal Maso and Cosmi, 2018

PLA

Bronze powder in PLA (+ bronze powder) PLA Thermoplastic copolyester elastomer

Filament’s production Commercial

Main research topic Micro-computed tomography applied to short-carbon fiber reinforced filaments and FDM parts Blending; filament Hierarchical porous extrusion with structure through twin-screw extruder foamable filament

Commercial

Research article on the correlation between rheology and printing quality for PLA in different colors Commercial Toolpath planning at corners Pre-mixing; vacuum Development of drying; new composite compounding in system with layered twin-screw extruder; nano-silicates pelletization; filament extrusion with single screw extruder Commercial FDM under variable gravity conditions (parabolic flight) Commercial Redesign of dumbbell-shaped specimens for tensile tests Spray-drying and Radio-frequency pre-sintering of and microwave TiO2 ; mixing; applications filament extrusion with extruder Commercial Tensile properties of FDM parts printed from different feedstock and under different orientation

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Supplementary table 1b Research papers on FDM of neat polymers and composite materials— cont’d Contribution Dascalu et al., 2020

Matrix PLA or TPU

Daver et al., 2016 Linear low-density polyethylene AND + 5 wt% maleic anhydride grafted polyethylene (compatibilizer)

Davis et al., 2016 ABS, different colors

Filler HAp in-house produced micros-sized particles: (for filament coating)

30 wt% de-vulcanized rubber particles AND 3 wt% MWCNTs

//

PLA, different colors Nylon Davis et al., 2019 ABS, different colors PLA, different colors

Additives (metal particles, CNTs)

Filament’s Main research production topic Laying filament on Feasibility study HAp powder bed; for core-shell covering with HAp bioceramic powder; pressing filaments and heating for various time intervals; sonication to remove excess powder Filament extrusion Effect of with twin-screw de-vulcanized extruder rubber particles from recycled tires, of compatibilizer, and of MWCNTs on parts either compression molded or printed by FDM Commercial Emissions upon printing Commercial

Emissions upon printing

Commercial

Comparison between FDM and polymer injection molding of short fiber-reinforced parts Feasibility of extrusion from composite capsules

Nylon PVA de Toro et al., 2020

High impact polystyrene PA6 20 wt% short carbon fibers

Díaz-García et al., PLA 2020

5-8-12 vol% (47.5 wt%) maraging steel particles

Dijkshoorn et al., // 2021

//

3D printing of PLA pre-capsules; filling with steel particles; drying; sealing with PLA 3D printed film with acetone; cutting; extrusion with single screw extruder // Analytical and computational simulation of anisotropy in electrical conduction

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Fused Deposition Modeling of Composite Materials

Supplementary table 1b Research papers on FDM of neat polymers and composite materials— cont’d Contribution Matrix Distler et al., 2020 PLA

Dong et al., 2015

Gel wax

Dorigato et al., 2017

ABS

Drummer et al., 2012

PLA

Dudek, 2013

ABS

Dul et al., 2016

ABS

Filament’s production Glass powder sieving below 80 μm and anti-static ioniser treatment; mixing in rotationally mixing; filament extrusion with desktop filament extruder Absorption Melting the gel wax, material: graphite mixing with the powder filler(s) at a Scattering material: designated concentration, TiO2 powder degassing in a vacuum chamber for 15 min, and cooling down 6 wt% CNTs Hot-melt (1-2-4-6-8-15 wt% compounding from commercial CNTs for masterbatch with 15 compression wt% CNTs: molding) pre-drying of ABS pellets and masterbatch; hot-melt compounding with neat ABS in internal mixer; compression molding OR grinding and filament extrusion with twin-screw extruder 2.5-5 wt% TCP Hot-melt (but 5 wt% NOT compounding in printable) mixer; filament extrusion with micro-extruder 1:1 HAp particles Hot-melt compounding and filament extrusion with single screw extruder 4 wt% graphene Hot meltnanoplatelets compounding in internal mixer; filament extrusion with twin-screw extruder Filler 1-2.5-5-10 wt% Bioglass 45S5

Main research topic In-depth investigation of PLA-bioglass 45S5 composites for biomedical usage

Tissue-simulating phantoms to calibrate and validate bio-imaging

Effect of different CNT loading under different printing directions on mechanical and thermo-electric properties

Suitability of PLA-TCP composites for FDM Feasibility study of new composite

Development of composite system (graphene)

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Supplementary table 1b Research papers on FDM of neat polymers and composite materials— cont’d Contribution Matrix Esposito Corcione PLA et al., 2017

Esposito Corcione PLA et al., 2018a

Esposito Corcione PLA et al., 2018b

Esposito Corcione PLA et al., 2019

Fafenrot et al., 2017

PLA

Filament’s production Rotomolding of nano-HAp onto PLA pellets; hot-melt compounding and filament extrusion with twin-screw extruder; water cooling 50-60 wt% Lecce Waste drying, Stone powder from manual grinding, industrial waste sieving; hot melt compounding and filament extrusion with twin-screw extruder 5-15-30-50 wt% Hot-melt HAp microparticles compounding and filament extrusion with twin-screw extruder; water cooling 50 wt% Hot-melt mesoporous compounding and micron-sized filament extrusion spherical HAp with twin-screw extruder; water cooling 78 wt% (= 36 Commercial vol%) bronze particles Filler 5-15 wt% home-made nano-HAp

46 wt% (= 12 vol%) iron particles FerrándezMontero et al., 2020

PLA

5-10-30-50 wt% Mg microparticles (10 wt% in filaments), surface-treated with dispersants: 0.2 wt% polyethylenimine (with respect to Mg) 0.2 wt% cetyltrimethylammonium bromide (with respect to Mg)

Colloidal route: pre-treatment of Mg particles in water and tetramethylammonium hydroxide to adjust the pH; addition of dispersants; centrifuging; dissolution of PLA in THF; addition of Mg particles; vacuum drying; filament extrusion with single screw extruder

Main research topic Biomedical applications (bone tissue)

Re-using of Lecce stone waste

Biomedical applications (bone tissue)

Scaffolds for biomedical applications (bone tissue)

Assessment of mechanical behavior of parts produced from commercial metal-reinforced composite filaments Investigation of PLA-Mg interface

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Fused Deposition Modeling of Composite Materials

Supplementary table 1b Research papers on FDM of neat polymers and composite materials— cont’d Contribution Ferreira et al., 2017

Matrix PLA

Filler 15 wt% chopped short carbon fibers

Fleck et al., 2017

PVDF

20 wt% aluminum (4.5 μm spherical particles)

Flowers et al., 2017

Polyester

Copper particles

Foster et al., 2017 PLA

10 wt% graphene

Galantucci et al., 2009

ABS

//

Gao et al., 2019

PLA

2 wt% talc

Garg and Singh, 2016

Nylon (PA) 6

5 wt% low-aspect-ratio carbon fibers 40-50-60 iron particles

Gavali et al., 2018 PLA

Carbon fiber powder

Gendviliene et al., PLA 2020

9:1 (10.43 wt%) HAp particles

Geng et al., 2019

10-20-30 vol% hexagonal BN

PA6 with POE-g-MAH (as warpage modifier)

Filament’s production Commercial

Main research topic Characterization of mechanical behavior of composite parts Solvent mixing in a FDM of energetic co-solvent of materials (safety acetone and DMF: issues and part dissolution of PVDF consistency) in vortex mixer; addition of aluminum particles and mixing in a digital sonifier; drying on metal tray; pelletization; filament extrusion with extruder operated remotely Commercial Printability of multi-material electronic components Commercial 3D printable electrochemical energy storage architectures Commercial Surface polishing of FDM parts by soaking in solvent Commercial Characterization of mechanical behavior of composite parts //

Pre-mixing; extrusion repeated twice with extruder Pre-mixing; extrusion with single screw extruder Pre-mixing; extrusion repeated twice with twin-screw extruder

Preliminary investigation on potential composite feedstock (iron particles) Composites

Geometric assessment of FDM scaffolds Enhancement of through-plane thermal conductivity

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Supplementary table 1b Research papers on FDM of neat polymers and composite materials— cont’d Contribution Gkartzou et al., 2017

Matrix PLA

Gnanasekaran et al., 2017

PBT

Goyanes et al., 2014

PVA (commercial filament)

Goyanes et al., 2015

PVA (commercial filament)

Goyanes et al., 2016

PVA (commercial filament)

Gray IV et al., 1998

PP

Filament’s production Vacuum-drying at 50°C for 24 hours; for bulk (baseline) (5-10-15-20 wt% purified kraft pine samples: hot-melt compounding in lignin for twin-screw compression internal mixer molded samples) around 180-190°C; compression molding at 120°C; for FDM samples: filament extrusion with single screw extruder Multi-walled Solvent mixing in carbon nanotubes isopropanol; solvent evaporation under Thermally expanded graphite stirring; complete drying; filament extrusion with mini-extruder Fluorescein Filament swelling in ethanolic solution of fluorescein 5 wt% paracetamol Filament cutting; mixing with paracetamol in shaker-mixer for 10 minutes; filament extrusion with single screw extruder 5-10 wt% Filament cutting, (nominal) milling and sieving paracetamol below 1 mm; mixing with drug in mortar 5-10 wt% (nominal) caffeine and pestle; mixing with paracetamol in shaker-mixer for 10 minutes; filament extrusion with single screw extruder 20-40 wt% TLCP Special dual extrusion to change TLCP from droplets to fibrils: pre-processing of PP and TLCP in separate extruders, injection of TLCP in the PP extruder, extrusion; chopping; second extrusion Filler 5 wt% purified kraft pine lignin

Main research topic Smart reusage of lignin in FDM

Development of composite system for electrically conductive structures

3D pharming: feasibility of FDM 3D pharming: effect of tablet shape

3D pharming: effect of hot melt compounding on different drugs

Development of composite system reinforced with thermotropic liquid crystalline polymers

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Supplementary table 1b Research papers on FDM of neat polymers and composite materials— cont’d Contribution Gregor-Svetec et al., 2020

Matrix HDPE

Grémare et al., 2018

PLA

Guessasma et al., PLA 2019

Hanon et al., 2019 PLA, various colors ABS

Haq et al., 2019

PCL

Hashimoto et al., 2019

PLA ABS PCL

Huang et al., 2019 ABS

Hutmacher et al., 2001

PCL

Hwang et al., 2015a

ABS

Main research topic Development of new composite system with cardboard // Commercial Effect of pore size on mechanical properties and cell interaction of scaffolds Hemp filament: 10 Commercial Development of vol% short hemp new composite fibers system with hemp fibers // Commercial Effect of materials and color of PLA commercial filaments on tribological properties of printed parts 2-3-4 wt% Pre-mixing; Preliminary montmorillonite extrusion with single investigation on screw extruder; potential composite 10 wt% HAp + water cooling feedstock with 2-3-4 wt% montmorillonite montmorillonite 7.5 wt% radiopho- Hot-melt Fabrication of toluminescence compounding and radiophotoluminesglass powder filament extrusion cence with extruder dosimeters 1-2-3-5 wt% Drying; pre-mixing; Development of cellulose hot-melt new composite nanocrystals/silica compounding in system with nanohybrids (silane twin-screw extruder; cellulose coupling) OR second extrusion nanocrystals-silica with twin-screw nanohybrids cellulose extruder nanocrystals OR Filler 20-50-75 wt% cardboard dust

Filament’s production Pre-mixing; filament extrusion with single screw extruder

blend of cellulose nanocrystals and silica nanoparticles // Drying; filament with fiber-spinning machine 10 to 50 wt% Production of copper particles composite pellets; 10 to 50 wt% iron filament extrusion with extruder particles

FDM of polymer scaffolds Development of new composite system with iron and copper particles

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Supplementary table 1b Research papers on FDM of neat polymers and composite materials— cont’d Contribution Hwang et al., 2015b

Isa et al., 2015

Matrix ABS

ABS

Filament’s production Production of composite pellets; filament extrusion with extruder

Main research topic Development of new composite system with iron and copper particles

Hot-melt compounding in mixer; pelletization; injection molding

Preliminary investigation (MFI) on potential composite feedstock with copper

Solvent mixing in acetone; drying; (ABS +) 30 vol% milling in high-speed grinder; Ba0.64 Sr0.36 TiO3 filament extrusion (PP +) 27 vol% with single screw CaTiO3 extruder 1.5-3-6 wt% GNPs Melt extrusion; hot pressing 1.5-3-6 wt% MWCNTs

Multi-material printing to produce dielectric resonator structures (metamaterial-like properties)

Filler 10 to 50 wt% copper particles 10 to 50 wt% iron particles 10-20-30-40 vol% copper particles AND + Up to 8 vol% binder

Isakov et al., 2016 ABS PP

Ivanov et al., 2019 PLA

+ Up to 2 vol% surfactant (ABS +) 27 vol% BaTiO3

1.5 wt% GNPs + 1.5 wt% MWCNTs

Preliminary investigation on potential composite feedstock with graphene and/or MWCNTs

1.5 wt% GNPs + 4.5 wt% MWCNTs 3 wt% GNPs + 3 wt% MWCNTs 4.5 wt% GNPs + 1.5 wt% MWCNTs //

∗

Jabbari and Abrinia, 2018

Sn-15Pb

//

Jin et al., 2017

PLA

//

Commercial

Jin et al., 2020

ABS

//

Commercial

10 wt% CNTs

Solvent mixing in chloroform; drying at 70°C for 6 hours; filament extrusion

Pb-40Sn

Junpha et al., 2020 PLA + 10 wt% PCL + 10 wt% SBS

10 wt% CNTs + 5 wt% copper 10 wt% CNTs + 5 wt% ZnO

Metal printing

ME of a neat metal filament (thixo-extruder) Surface polishing of FDM parts by solvent vapor treatment Ultrasound characterization of FDM parts Electrodes with different electrochemical charactistics for electronic tongue

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Fused Deposition Modeling of Composite Materials

Supplementary table 1b Research papers on FDM of neat polymers and composite materials— cont’d Contribution Matrix Kalita et al., 2003 PP

Filler 20.5 vol% TCP (not stoichiometric) AND + 13.4 vol% hard paraffin (viscosity regulator)

Kalsoom et al., 2020

ABS

Kariz et al., 2018

PLA

Kennedy et al., 2017

PLA

Khatri et al., 2018 ABS AND + up to 1.1 wt% stearic acid (surfactant) H. Kim et al., 2017

PVDF

+ 14.4 vol% vegetable oil PLASTICIZER 60 wt% boron-doped diamond microparticles + 2 wt% LiCl 10-20-30-40-50 wt% (beech) wood flour

Filament’s production Hot-melt compounding in mixer; crushing; extrusion with single-screw extruder

Main research topic Scaffolds for biomedical applications (bone tissue)

Solvent mixing in acetone; drying; pelletization; extrusion with extruder Milling and sieving the wood flour below 0.237 mm; pre-drying of filler and polymer pellets; compounding; extrusion with single-screw extruder 89:11 Synthesis of lanthanide-aspartic Ln3+ -(D)-Asp NC powder; solvent acid nanoscale mixing in CHCl3 for coordination PLA and ethyl polymers (Ln = acetate for Eu, Tb) Ln3+ -(D)-Asp NCs; bath sonication; casting on Teflon; heating to partly remove solvents; drying in vacuum furnace at 70°C overnight; extrusion with custommachined plunger extruder 10-20-30-35-40-45- Kneading; Drying; Filament extrusion 50 vol% BaTiO3 microparticles with single screw extruder Solvent mixing in 3 up to 15 wt% DMF and ultraBaTiO3 powder sonication for 20 min; casting and heating to evaporate the solvent; pelletization; filament extrusion with desktop extruder

Fabrication of humidity sensors

Effect of different wood flour loading

Composite filament with chemical fingerprint for printing QR codes and connection to blockchain

Printable composites with tuneable dielectric properties Piezoelectric composites for (pressure) sensors

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Supplementary table 1b Research papers on FDM of neat polymers and composite materials— cont’d Contribution K. Kim et al., 2017

Matrix TPU

Kim et al., 2018b PVDF

Kim et al., 2018c

PVDF

Kim et al., 2020

PLA, various colors Nylon 645

Kollamaram et al., Kollidon VA64 + 2018 excipients and/or

Korte and Quodbach, 2018

Kollidon 12PF + excipinets Ammonio methacrylate copolymer, A AND

Filament’s Main research production topic Shear mixing in Multiaxial force torque rheometer; sensor by pelletization; multi-material 3D filament extrusion printing with extruder Solvent mixing in 9 wt% BaTiO3 Piezoelectric powder DMF and composites for ultrasonication for sensors 20 min; casting and heating to evaporate the solvent; pelletization; filament extrusion with desktop extruder 6-12-15-30-45-60- Solvent mixing in Ternary dielectric DMF and 75 wt% BaTiO3 composites for powder ultrasonication for energy storage 12 wt% BaTiO3 + 20 min; casting and devices 0.1-0.4-0.7-1-1.3- heating to evaporate the solvent; 1.7 wt% CNTs pelletization; 12-30-45-60 wt% filament extrusion BaTiO3 + 1 wt% with desktop CNTs extruder 12-30-45-60 wt% BaTiO3 + 1.7 wt% CNTs // Commercial Effect of color of PLA commercial filaments and printed parts with different printing parameters 3 wt% ramipril Mixing in mortar 3d pharming: effect and pestle; filament of low-temperature with single screw processing extruder

Filler 1-2-4 wt% MWCNTs (only 4 wt% conductive)

30 wt% anhydrous Sieving to break theophylline agglomerates; blending in turbula + 0.4 wt% anhydrous mixer; hot-melt colloidal silica compounding and (flowability) filament in + 3.5-7 wt% stearic acid twin-screw extruder; (lipophilic) plasticizer cooling and OR stretching on + 5-10 wt% conveyor belt polyethylene glycol 4000 (hydrophilic) plasticizer Kumar et al., 2018 ABS 40 wt% iron Filament extrusion powder with twin-screw PA6 extruder

Correlation between extrusion parameters and printing results in 3D pharming

Friction welding on FDM parts

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Fused Deposition Modeling of Composite Materials

Supplementary table 1b Research papers on FDM of neat polymers and composite materials— cont’d Contribution Matrix R. Kumar et al., in PLA press

Filler 1-2% ZnO nanoparticles

S. Kumar et al., in PLA press b PA6 (+ TiO2 )

30 wt% TiO2 (in PA6)

Kwok et al., 2017 PP

Up to 32.5wt% carbon black

Laureto et al., 2017

PLA

copper particles

Lazarus et al., 2019

ABS

Le Duigou et al., 2016

PLA + PHA

Le Guen et al., 2019

PLA

Lederle et al., 2016

ABS

Lee et al., 2019

PLA

Filament’s production Synthesis of ZnO nanoparticles; mechanical blending of PLA pellets and ZnO nanoparticles with linseed oil for improved dispersion; filament with twin-screw extruder Ready-to-use granules; filament with twin-screw extruder

Hot-melt compounding and extrusion with single-screw extruder Commercial

Main research topic Properties and shape memory effect of filaments with different filler loading and processing parameters

Flexural and wear properties of multi-material samples (layers printed from different filaments) Fabrication of circuits and sensors

Thermal conductivity of FDM parts from stainless steel commercial magnetic iron filaments with metal fillers Copper particles (in Commercial Selective polyester) electroplating for multi-material electronic components 15.2 ± 0.9 wt% Commercial 4D printing of recycled wood moisture-actuated fibers hygromorphic parts 10 wt% rice husk Pre-mixing in plastic Effect of different biomasses on 10 wt% Pinus wood bag; hot-melt compounding in extrusion and flour twin-screw extruder; printability of PLA filament with twin-screw extruder // Commercial Nitrogen atmosphere to prevent polymer oxidation upon printing 5-10-15-20 vol% Ti Mixing for 30 min in Scaffolds for particles a shear mixer at biomedical 180°C; filament applications extrusion with single-screw extruder bronze particles

Polyester

Nylon copolymer

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Supplementary table 1b Research papers on FDM of neat polymers and composite materials— cont’d Filament’s production Solvent mixing in DCM; solvent evaporation; manual filament forming: repeatedly warming to 80°C and rolling between two glass plates Hot-melt compounding and extrusion; diameter control unit

Main research topic Fabrication of electronic sensors

5 wt% nano-ZnO powder //

Commercial

Strengthening effect of ultrasonic vibration Strengthening effect of ultrasonic vibration 4D printing: Fabrication of bio-hygromorph Modelling and validation of thermal deformation Effect of IR laser irradiation on surface texture and properties of FDM parts Improvement of tribological behavior of PEEK

Contribution Matrix Leigh et al., 2012 PCL

Filler 15 wt% carbon black

León-Cabezas et al., 2017

PLA, various mixes +

PLA TPU ABS

5 wt% thermochromic additive

Printability of functionalized filaments for personalized toys

5 wt% photochromic additive 5 wt% luminescent masterbatch in EVA 5 wt% MWCNTs (as received; surface treated) TPU + 5 wt% luminescent masterbatch in EVA ABS, various mixes + 5–15 wt% nanoZnO suspension

Li et al., 2018a

ABS PLA

Li et al., 2018b

ABS

//

Commercial

Li et al., 2019

PLA

5 wt% wood flour

Commercial

F. Li et al., 2020

PA66

20 wt% carbon powder

Li et al., 2021

PEEK

10 wt% carbon fibers

Hot-melt compounding and extrusion with twin screw extruder Commercial

Lin et al., 2019

PEEK

30 wt% carbon fibers

Commercial

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Fused Deposition Modeling of Composite Materials

Supplementary table 1b Research papers on FDM of neat polymers and composite materials— cont’d Contribution Liu et al., 2019b

Matrix PLA

Filler 3:2 wood powder

Filament’s production Commercial

Main research topic Characterization of mechanical behavior of composite parts with different fillers

3:2 ceramic powder 3:2 copper powder 3:2 aluminum powder

Loke et al., 2019

Multi-layer (PC, internal; cyclic olefin copolymer, external)

Love et al., 2014

ABS

Maia et al., 2019

Various polymers

Malakhov et al., 2020

//

Maróti et al., 2019 PLA

Maróti et al., 2020 PLA

Martel Estrada et al., 2017

PLA

Masood and Song, Nylon 2004

3:2 carbon fiber powder Various Co-extrusion of components (wires, core-shell structures particles)

Feasibility of one-step printing of optoelectronic devices by 3D printing of structured composite filaments 13% chopped Hot-melt Effect of carbon carbon fibers compounding in fibers on mixer; filament with mechanical plunger-type batch properties and extrusion unit printing accuracy Near-infrared dyes // LayerCode: tagging for SLA resin strategy based on resemblance between printed layers and bars in bar codes // // Modeling and optimization of curved fiber trajectory in AM of continuous fiberreinforced parts 20-50 wt% CaCO3 Commercial (patent Preliminary applied), investigation on Twin-screw extruder new potential composite feedstock Silver nanoparticles Commercial Development of new composite system with silver nanoparticles Sol-gel Bioglass PLA milling; Sol-gel synthesis of 45S5 mechanical mixing Bioglass 45S5 and with glass; filament printability of extrusion with composites extruder 30-40 vol% coarse Pre-mixing; FDM for rapid iron particles AND extrusion with tooling single-screw + surfactant, extruder plasticizer 40 vol% fine iron particles AND + surfactant, plasticizer

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Supplementary table 1b Research papers on FDM of neat polymers and composite materials— cont’d Contribution Matrix Masood and Song, Nylon 2005

Filament’s Filler production 30 vol% coarse iron Pre-mixing; particles AND extrusion with single-screw + surfactant, extruder plasticizer

Main research topic FDM for rapid tooling

30-40 vol% fine iron particles AND

Maurel et al., 2018

PLA + AND plasticizer, up to 100:60 wt/wt: - poly(ethylene glycol) dimethyl ether 500 - poly(ethylene glycol) dimethyl ether 2000

+ surfactant, plasticizer" 61.7-62.1-62.5 wt% Solvent casting in graphite DCM; production of 0.6-1.2 conductive films by doctor filler (either carbon blade casting and solvent evaporation; black or carbon filament extrusion nanofibers) with extruder

Development of composote feedstock to print negative electrode for lithium batteries

- propylene carbonate Maurel et al., 2019

Mei et al., 2019

- acetyl tributyl citrate PLA + poly(ethylene glycol) dimethyl ether 500 (plasticizer), 100:40 wt/wt

PLA

Negative electrode: Solvent casting in FDM of lithium 49 wt% graphite + DCM; production of battery cell in a 5 wt% carbon black films by doctor single job Positive electrode: blade casting and 47-49-50-52 wt% solvent evaporation; LiFePO4 + 8-4-2-0 filament extrusion wt% carbon black with extruder Separator: 13-1822 wt% amorphous SiO2 5 wt% short carbon Commercial fibers

Reinforcement via layers brushing

2 wt% graphene

Melocchi et al., 2016

Various thermoplastics

Brushed layers of SiC nanowires //

(pharmaceutical grades)

Mihankhah et al., PLA in press

2-4 wt% montmorillonite

Pre-heating at 40°C where necessary; mixing in mortar and pestle; filament with twin-screw extruder; manual pushing through 1.80 mm die Masterbatch at 10 wt%: predrying of PLA granules; masterbatching in twin-screw extruder; drying; filament extrusion with single screw extruder

3d pharming: effect of low-temperature processing

Taguchi model to maximize tensile strength

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Fused Deposition Modeling of Composite Materials

Supplementary table 1b Research papers on FDM of neat polymers and composite materials— cont’d Contribution Milosevic et al., 2017

Matrix Recycled PP

Filler 10-20-30 wt% hemp fibers AND + 2 wt% MAPP as compatibilizer 10-20-30 wt% harakeke fibers AND + 2 wt% MAPP as compatibilizer //

Filament’s production Drying; hot-melt compounding in twin-screw extruder; granulation; extrusion with larger twin-screw extruder

∗

Mireles et al., 2012

Bi58Sn42 (eutectic)

//

Mondal et al., 2020

PLA

//

Montalvo N. and Hidalgo, 2015

PE/PP/PLA/ABS

Montalvo Navarrete et al., 2018

PLA/PP

Mora et al., 2020

PLA

10-20-30 vol% Manual mixing of sugar cane bagasse polymer pellets and filler powder; hot pressing; pelletization; filament extrusion with twin-screw extruder 10-20-30 wt% Hot pressing; wood flour pelletization; filament extrusion with twin-screw extruder 0.25-0.5-1-2-4-6 Vacuum pre-drying wt% CNTs (60°C for PLA and HDPE; 100°C for CNTs); manual pre-mixing with acetone; filament extrusion with twin-screw extruder

Sn60Bi40 (non-eutectic)

HDPE

Filament extrusion; printing; HAp coating applied post-processing

Mousavi et al., 2020

PLA

12 wt% CNTs

Solvent mixing in DCM; drying; extrusion with twin-screw extruder

Moylan et al., 2012

//

//

//

Mulholland et al., PA 2018a

14.1 wt% (12 Commercial vol%) short carbon fibers

Main research topic Development of composite systems (recycled PP; natural fibers)

Metal printing

FDM of a neat metal filament (low melting) with re-designed liquefier Feasibility of nano-HAp coating on scaffolds by post-processing treatment Feasibility of printing composite filaments reinforced with natural fibers commonly available in Colombia Effect on wood flour on PLA and PP to reduce the cost of AM feedstock/composites Micromechanicsbased modeling and experimental validation of electrical conduction in segregated and agglomerated structures Sensors with tunable sensitivity through "kissing bond" for soft robotics Test geometry for quantifying printing accuracy Fabrication of heat exchangers

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Supplementary table 1b Research papers on FDM of neat polymers and composite materials— cont’d Contribution Matrix Mulholland et al., PA6 2018b

Nabipour et al., 2020

PE AND + Additives (paraffin wax and stearic acid)

Nájera et al., 2018 PLA 4043D and PCL

Nevado et al., 2020

PLA

Filament’s Main research Filler production topic 20-25 vol % copper Hot-melt Fabrication of heat fibers compounding and exchangers) extrusion with copper spheres twin-screw extruder (only for compression molding) 25-50-75 wt% copper particles

Preliminary dry-mixing at 80 rpm and 140°C for 40 minutes; filament extrusion with twin-screw extruder, cooling in water bath and drawing equipment 0.25/0.50/0.75/1 % Hot-melt TiO2 particles with compounding the different sizes polymers and adding (50 nm, 150 nm the filler; and 300nm) pelletization; filament extrusion 15 wt% BCP from Ball milling; solution filament extrusion combustion with single screw extruder

Niaza et al., 2016 PLA

15 wt% HAp nanoparticles

Nienhaus et al., 2019

PLA, black color

//

Nikzad et al., 2011

P400 ABS (powdered)

5-10-20-30-40 vol% fine copper particles AND + surfactant, plasticizer

FDM of PE with the addition of copper particles

Potential biomaterials for bone tissue repair

Properties of filaments with BCP powders, as synthesized and calcined Hot-melt Scaffolds for compounding and biomedical extrusion with screw applications (bone extruder tissue) Commercial Effect of nozzle geometry (conical angle; capillary length) on back pressure and printability (of PLA) Hot-melt FDM for rapid compounding and tooling extrusion with screw extruder

5-10-20-30-40 vol% coarse copper particles AND + surfactant, plasticizer 5-10-20-30-40 vol% fine iron particles AND + surfactant, plasticizer

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Fused Deposition Modeling of Composite Materials

Supplementary table 1b Research papers on FDM of neat polymers and composite materials— cont’d Contribution Ning et al., 2015

Matrix ABS

Ning et al., 2017a ABS

Ning et al., 2017b ABS

Filler 3-5-7.5-10-15 wt% Panex 30 short carbon fiber

Filament’s production Pre-mixing; extrusion repeated twice with extruder

3-5-7.5-10-15 wt% Panex 35 short carbon fiber 5 wt% chopped Commercial carbon fibers 5 wt% chopped carbon fibers

Effect of printing parameters on tensile properties Comparison between carbon fibers and graphite as reinforcement Effect of low-pressure printing

Commercial

5 wt% graphite O’Connor and Dowling, 2018 O’Hara IV et al., 2018 Okwuosa et al., 2016

ABS

PA6 (transparent blend) // // Polyvinylpyrrolidone (50 wt%) AND

+ Theophylline/dipyridamole, drug (10 wt%) PLA

Olesik et al., 2019 LDPE

Pandzic et al., 2019

PLA, various colors

Papon and Haque, PLA 2019

Peng et al., 2019

Commercial

PLA

+ Triethyl citrate, plasticizer (12.5 wt%)

Oladapo et al., 2020

//

//

First FDM part on the ISS 3d pharming: feasibility of FDM at low temperature

27.5 wt% talc filler Hot-melt compounding and extrusion in mini-extruder

0-5-10 wt% carbonated HAp

15-30 vol% Glass particles (+ polyvinyl butyral) from car glass waste //

Main research topic Development of new composite system with carbon fibers

Hot-melt compounding in roller mixer; filament extrusion with extruder Pre-mixing; extrusion repeated twice with extruder

Effect of carbonated HAp

Commercial

Effect of color of PLA commercial filaments and printed parts Customized square-shaped nozzle

Improvement of tribological behavior of LDPE

3-5-7-10 wt% short Pre-mixing; carbon fibers extrusion x 3 with single-screw extruder PC/ABS copolymer // Pre-drying in FDM printing from blend as core vacuum oven at multi-polymer 110°C; core-shell core-shell filaments HDPE or LDPE as shell filament extrusion with two single screw extruders; quenching in room-temperature water bath; winding onto a take-up wheel

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357

Supplementary table 1b Research papers on FDM of neat polymers and composite materials— cont’d Filament’s production Blending in turbula mixer; hot-melt compounding and + 10 wt% triethyl citrate filament with (plasticizer) twin-screw extruder; + 0.4 wt% fumed silica cooling; conveyor (powder flow) belts with winder Qian et al., 2018 PLA 5-10-20 wt% Solvent mixing in home-made THF; solvent Li0.44 Zn0.2 Fe2.36 O4 evaporation; (LZFO) particles pelletization; filament extrusion with single-screw extruder Ranganathan et PA6 10-20-30 wt% glass Pre-heating/drying; al., 2019 beads hot-melt compounding and 10-20-30 wt% crushed glass fibers extrusion with extruder Ranjan et al., 2020 PLA 8 wt% HAp AND Hot-melt + 1 wt% chitosan compounding and extrusion with extruder Ravi, 2020 PLA // Commercial Contribution Matrix Ponsar et al., 2020 82.93 wt% ethyl cellulose AND

Filler 16.67 wt% hypromellose

Ravi et al., 2016

ABS

//

Rinaldi et al., 2021

PEEK

3-5-10 wt% MWCNTs

Roberson et al., 2015

ASB and/or PC

Commercial

Main research topic Correlation between extrusion parameters and printing results

Fabrication of microwaves absorbers

Assessment of thermal properties

Biomedical applications (bone tissue) Effect of infill density on real porosity of scaffolds Effect of local remelting by near-IR laser treatment Effect of CNTs on PEEK for FDM (processability window)

Masterbatching from commercial masterbatch (10 wt%): predrying at 150°C for 24 hours and storage in vacuum bag before extruding; filament extrusion with single screw extruder; pelletization; repeated extrusion Various (TiO2 , ZnO Hot-melt Utility of nanorods, compounding and customized MayaCrom extrusion in materials in FDM palygorskite) twin-screw extruder

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Fused Deposition Modeling of Composite Materials

Supplementary table 1b Research papers on FDM of neat polymers and composite materials— cont’d Contribution Matrix Russo et al., 2019 XFN FRS 6002A/B/Green-lite_14415

Filler N.A.

Elan-tech EPO 0 2L/ HexForce G092 Sahmani et al., 2020

PLA

Sakunphokesup et ABS al., 2019

30 wt% HAp particles

1 wt% graphene

Filament’s production Impregnation; pelletization; extrusion with single-screw extruder Pre-mixing; milling; filament extrusion with extruder Different strategies: - Solvent mixing in DMF; precipitation in ethanol; drying; mixing with additional ABS; extrusion with extruder

Main research topic High performance composite materials

Scaffolds for biomedical applications (bone tissue) Evaluation of different mixing techniques in filament production

- Solvent mixing in DMF; solvent evaporation; mixing with additional ABS; extrusion with extruder

Sanatgar et al., 2017

PLA

Sang et al., 2019a PLA + PCL, various

Sang et al., 2019b PLA

- Pre-mixing; extrusion with extruder 0.5 up to 5 wt% Hot-melt MWCNTs compounding of masterbatch at 1 up to 7 wt% 10 wt% in carbon black twin-screw extruder; pelletization; dilution with PLA and filament extrusion; water cooling 15 wt% silanized Hot-melt basalt fibers compounding in twin-screw extruder; extrusion with single screw extruder 5-10-20 wt% 1-2-3 Drying; Hot-melt mm-long silanized compounding in basalt fibers internal mixer with 5-10-20 wt% 1-2-3 two counter-rotating roller blades; mm-long carbon crushing; extrusion fibers with single-screw extruder

Direct FDM on textiles

Development of new composite system with basalt fibers for honeycombs Development of new composite system with basalt fibers

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Supplementary table 1b Research papers on FDM of neat polymers and composite materials— cont’d Contribution Sangiorgi et al., 2019

Matrix Filler PLA (D-isomer content: 15/30 wt% TiO2 nanopowder, 4.25%) anatase:rutile ratio of 80:20 AND + surface functionalization with branched PEI

Sanz-Horta et al., PLA + PCL, various 2020

Sathies et al., PLA 2019 Sau’de et al., 2013 ABS

Schmitz et al., 2018

ABS

Senatov et al., 2016a

PLA

Senatov et al., 2016b

PLA

Filament’s production - Surface functionalization of TiO2 powder: TiO2 dispersion in deionized water with 6 vol% solid content; addition of 6 wt% (referred to powder) of PEI at pH 8; ball-milling for 90 min; centrifuging

Main research topic Feasibility of photocatalytic filters)

- Solvent mixing in THF; ultrasonication; solvent evaporation in a rotary evaporator, drying at 80°C in kiln; grinding; filament extrusion with home-made single-screw extruder // Commercial Hierarchical porous filaments structure through supercritical CO2 and breath figures Around 20 wt% Commercial Fabrication of carbon black solvent sensors 22-23-24-25-26 Hot-melt Preliminary vol% copper compounding in investigation of particles AND mixer; pelletization; new potential injection molding composite + 15-19 vol% for samples feedstock with surfactant copper 3 wt% MWCNTs Drying; pre-mixing Effect of filler type and printing 3 wt% carbon black in internal mixer; pelletization; drying; orientation on 3 wt% (50:50 filament extrusion electromagnetic MWCNTs + carbon with twin-screw interference black) extruder; drying shielding effectiveness 15 wt% HAp Drying; hot-melt Scaffolds for micro-particles compounding and biomedical extrusion with screw applications (bone extruder tissue) 15 wt% HAp Drying; hot-melt PLA treated as micro-particles compounding and shape-memory extrusion with screw polymer extruder

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Fused Deposition Modeling of Composite Materials

Supplementary table 1b Research papers on FDM of neat polymers and composite materials— cont’d Contribution Sevastaki et al., 2020

Matrix PS, recycled

Filler 20-40 wt% TiO2 nanoparticles

Sevastaki et al., 2021

PLA

//

Shalchy et al., 2020

PLA and PVA (PVOH) (various)

NaCl (various)

Shemelya et al., 2015

PC

1-3-5 wt% (0.1-0.2-0.3 vol%) tungsten microparticles (silane-treated)

Shi et al., 2019

PLA

CNTs on the filament surface

Sigloch et al., 2020

ABS, different colors

Additives (metal particles, CNTs)

PLA, different colors Singh et al., 2016 Nylon

R. Singh et al., 2017

ABS

Singh et al., 2018 PA6, recycled

Filament’s production Crushing of PS waste objects to about 0.2 mm particles; solvent mixing in toluene; precipitation in ethanol; drying at 60°C for 24 hours; cutting into mm-size pieces; filament extrusion with extruder Commercial; printing of PLA substrate; ZnO coating Hot-melt compounding in mixing chamber; filament extrusion with single screw extruder Masterbatching by hot-pressing and manually kneading; gramulation; manually mixing with neat PC; filament extrusion with twin-screw extruder Local Enrichment Strategy: filament drawing through a suspension of CNTs in a PCL solution in DCM Commercial

40-50 wt % Composite batches alumina particles in in extruder three 100/120/150 (micron) grades // Commercial

30 wt% TiO2

Filament extrusion with twin-screw extruder

Main research topic Photocatalysis of drug contaminants (paracetamol)

Photocatalysis of drug contaminants (paracetamol) Hierarchical porous structure through particulate leaching and through PVA selective removal X-ray radiation-shielding applications in space

Effect of local enrichment on conductivity threshold)

Emissions upon printing Effect of particle size distribution of Al2 O3 reinforcement Surface polishing of FDM parts by solvent vapor treatment Feasibility of recycling PA6; investigation of extrusion parameters

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Supplementary table 1b Research papers on FDM of neat polymers and composite materials— cont’d Main research topic Preliminary investigation of potential composite 8 wt% HAp + 2 feedstock for wt% chitosan biomedical applications 5 wt% SiC + 5 wt% Hot-melt Investigation of Al2 O3 compounding by extrusion twin-screw extruder; conditions for 10 wt% SiC filament extrusion composite + 10 wt% Al2 O3 with single screw feedstock from 15 wt% SiC extruder waste HDPE + 15 wt% Al2 O3 5 wt% of (PEKK at N.A. Biomedical 94 wt% + HAp at 4 applications wt% + chitosan at 2 wt%) 5 wt% banana Filament extrusion Effect of banana fibers with twin-screw fibers on recycled extruder thermoplastics for FDM 40 wt% aluminum N.A. Friction welding on powder FDM parts

Contribution J. Singh et al., 2019

Matrix PLA

N. Singh et al., 2019

HDPE, recycled waste

R. Singh et al., 2019a

PLA

R. Singh et al., 2019b

ABS

R. Singh et al., 2021

ABS

Siqueiros and Roberson, 2017

2.5-5-10-20% (for PLA) or 5-10-20% 50:50 wt/wt blend of ABS with SEBS grafted (for ABS blend) low-melting point with maleic anhydride phosphate glass particles ABS, different colors //

Sittichompoo et al., 2020 Skorski et al., 2016

Somireddy and Czekanski, 2020

PA6

PA6 PLA

ABS, different colors

ABS

Filler 8 wt% HAp + 1 wt% chitosan

Filament’s production Pre-mixing; filament extrusion with twin-screw extruder

Salinization of in-house made glass particles; hot-melt compounding with desktop extruder

In-situ wire drawing of low-melting point filler

Commercial

Emissions upon printing Expanding the chemical capabilities of 3D printed structures from commodity plastics with adding a filler

Solvent mixing in acetone; casting in Teflon-lined aluminum pans; evaporating the solvent at 80°C; cutting the films; extrusion of rods with twin-screw extruder; pelletization; filament extrusion with desktop 3D extruder Short carbon fibers Commercial

1-5-10 wt% TiO2 nanopowder (80-90%)

Applicability of classical laminate theory to FDM parts

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Fused Deposition Modeling of Composite Materials

Supplementary table 1b Research papers on FDM of neat polymers and composite materials— cont’d Contribution Son et al., 2018

Matrix ABS

Song and Telenko, ABS 2016

Song et al., 2018

PLA-PVA (PVOH) 50-50 blend

Sood et al., 2010

ABS oil 0-2%, tallow 0-2%, wax 0-2%

Soundararajan et al., 2019

PA6 (Heraeus Instruments)

Spina, 2019

PLA, various colors

Spinelli et al., 2019

PLA

Spinelli et al., 2020

PLA

Filler 50 wt% ZnO nanoparticles

//

Filament’s production Pre-annealing of ZnO at 500°C for 2 hours; solvent mixing in acetone; filament extrusion with extruder; after printing, surface synthesis of hierarchical ZnO nanostructures //

Main research topic Photocatalysis of contaminants with tunable architectures

Estimation of polymer waste in FDM from open shop // Blending; mixing Hierarchical porous with twin-screw structure through extruder; CO2 seeding and foaming and pelletization; filament with torque through PVA rheometer and single selective removal screw extruder // // Statistical analysis of mechanical properties of FDM parts vs processing parameters 10-20-30 wt% TiO2 Pre-drying; filament Production and (Heraeus extrusion with characterization of Instruments) 3Devo-NEXT composites Filament Extruder // Commercial Effect of color of PLA commercial filaments Up to 12 wt% Hot-melt Fabrication of MWCNTs compounding in conductive parts for twin-screw extruder; electromagnetic Up to 12 wt% extrusion with single applications GNPs screw extruder Up to 12 wt% MWCNTs + GNPs 6 wt% MWCNT Hot-melt Fabrication of compounding in conductive parts 6 wt% GNPs twin-screw extruder 1.5 wt% MWCNTs repeated two times; + 4.5 wt% GNPs extrusion with single 3 wt% MWCNTs + screw extruder 3 wt% GNPs (bi-filler composites 4.5 wt% MWCNTs from mono-filler + 1.5 wt% GNPs composites)

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Supplementary table 1b Research papers on FDM of neat polymers and composite materials— cont’d Filler //

Filament’s production Commercial

ABS, different colors

//

Commercial

PLA, different colors PP (pre-consumer/postconsumer)

10-20-30 wt% hemp fibers

Specific pre-treatments of recycled PP, natural fibers, recycled gypsum; mixing in plastic bag; forced-fed extrusion with twin-screw extruder; filament with second extrusion with larger twin-screw extruder Specific pre-treatments of natural fibers; pre-mixing in intensive mixer at 185°C; granulation; filament with twin-screw extruder Hot-melt compounding and extrusion with screw extruder //

Contribution Matrix Spoerk et al., 2018 ABS

Main research topic Evaluation of adhesion as a function of base platform temperature Emissions upon printing

PLA

Stefaniak et al., 2017 Stoof and Pickering, 2017

10-20-30 wt% harakeke fibers 10-20-30-40-50 wt% recycled gypsum

Stoof et al., 2017

PLA

10-20-30 wt% hemp fibers 10-20-30 wt% harakeke fibers

Sui et al., 2019

PLA

15 wt% HAp nano-particles

Sun et al., 2008

ABS

//

Suzuki et al., 2020 //

//

Sweeney et al., 2017

MWCNT coating

PLA

Viability of post-consumer recycled PP as matrix

Feasibility study of new composites

Synchrotron characterization techniques

Bending tests on special FDM samples in order to determine the inter-bead adhesion // Optimization of curved fiber trajectory in FDM of continuous fiber-reinforced parts Either bath coating Demonstration of of PLA filament in locally-induced MWCNT ink; radio-frequency (LIRF) welding of OR melt co-extrusion of CNT neighboring beads masterbatch over a neat polymer core

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Fused Deposition Modeling of Composite Materials

Supplementary table 1b Research papers on FDM of neat polymers and composite materials— cont’d Filament’s production Multi-material filaments by 3d printing

Contribution Takahashi et al., 2020

Matrix PLA and/or TPU (various colors)

Filler //

Tang et al., 2020

PLA (theoretical)

//

//

Tanikella et al., 2017

Various commercial filaments, rigid/flexible in different colors

//

Commercial

Tao et al., 2017

PLA

5 wt% wood flour

Tao et al., 2019

PLA

Wood flour

Pre-heating/drying; hot-melt compounding and extrusion with single-screw extruder Commercial

Tekinalp et al., 2014

ABS

Tian et al., 2020

PCL

Torrado et al., 2015

ABS

10-20-30-40 wt% Hot-melt 3.2 mm long carbon compounding in fibers (epoxy-sized) mixer; extrusion with extruder 30 wt% HAp Hot-melt particles compounding and extrusion 5 wt% jute fibers Hot-melt from rope compounding and 2 wt% MayaCrom extrusion with twin-screw extruder Blue (polymer-

Main research topic Research article on programmable multi-material filaments Modeling of the relationship between porosity and mechanical properties in PLA scaffolds Effect of color on tensile strength of commercial filaments Preliminary investigation of new potential composite feedstock with wood flour Fabrication of cellular structures from wood-flour composite filaments Development of new composite system with carbon fibers Biomedical applications Effect of fillers/blends on anisotropy

organonanoclay composite) 5 wt% TiO2 2 wt% ZnO nanorods 5 wt% SrTiO3 microparticles 5 wt% Al2 O3 microparticles

Torrado Perez et al., 2014

ABS (Cyclolac, GE ABS resin)

Various polymer blends with SEBS and UHMWPE 5 wt% jute fibers 5 wt% TiO2 nanoparticles 5 wt% TPE (polymeric blend)

Filament extrusion with twin-screw extruder

Effect of fillers/blends on tensile properties and fracture surface of ABS-matrix parts

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Supplementary table 1b Research papers on FDM of neat polymers and composite materials— cont’d Contribution Matrix Tyberg and Bøhn, ABS 1999 Tymrak et al., Various 2014

Filler //

Valentini et al., 2019

PHBH

0.5-1-3 wt% fibrillated nano-cellulose (solution mixing)

Valerga et al., 2018

PLA, various colors

//

Vanˇecˇ ková et al., 2020

PLA

Carbon nanotubes

Vaˇnková et al., 2020

PLA

//

Vidakis et al., 2020

ABS

0.5-2.5-5-10-20 wt% ZnO nano-particles

//

0.5-2.5-5-10-20 wt% ZnO micro-particles Vinyas et al., 2019a

PLA

10% carbon fibers

Filament’s production Commercial

Main research topic Adaptive slicing in FDM Commercial Mechanical behavior of FDM parts printed in realistic environmental conditions with open-source printers Masterbatches by Development of solvent mixing in new composite chloroform; drying; system with crushing; addition of nanocellulose PHBH granules and filament extrusion with single-screw extruder Commercial Effect of color on PLA filaments and printed parts Commercial) Fabrication of electrodes for UV/VIS spectroelectrochemistry Commercial Cleaning of personal protection equipment made of PLA Physical pre-mixing Effect of particle in mechanical size (nano/micro) homogenizer; on drying; filament thermo-mechanical extrusion with single properties of screw extruder ABS-ZnO composites Commercial

30% Nylon Glass Fibers (not on sale) PET-G polymer blend (not on sale)

Vinyas et al., 2019b

PLA

10% carbon fibers 30% Nylon Glass Fibers (not on sale) PET-G polymer blend (not on sale)

Commercial

Characterization of mechanical behavior of composite parts from commercial filaments Characterization of mechanical behavior of composite parts from commercial filaments

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Fused Deposition Modeling of Composite Materials

Supplementary table 1b Research papers on FDM of neat polymers and composite materials— cont’d Contribution Viskadourakis et al., 2018

Matrix PS

Filler 20 wt% ZnO nanoparticles 20 wt% TiO2 nanoparticles

Waheed et al., 2019

ABS

Wang et al., 2016 Polywax (based on PE wax)

37.5-60 wt% high-pressure high-temperature micro-diamonds

2-5-8-11 wt% Expancel 930DU120 thermally expandable microspheres

Wang et al., 2021 PLA

10-20-30-40-50 nanosized HAp

Wasserfall et al., 2017

//

//

Filament’s production Solvent mixing in toluene; ethanol addition to precipitate the PS-filler composite; drying filament extrusion with home-made extruder Solvent mixing in acetone; manual pelletization of the dried slurry; extrusion with single-screw extruder repeated six times Pre-mixing of masterbatch at 20 wt%; hot-melt compounding in twin-screw extruder; addition of Polywax and extrusion with single-screw extruder; Post-processing: thermal treatment required to activate volume expansion Separation of HAp nanoparticles from water-based suspension; dispersion in acetone; dissolution of PLA in dichloromethane; solvent mixing; solvent removal; break down of the composite bulk in a crusher; filament extrusion with extruder //

Main research topic Investigation of the effect of architecture on photocatalytic properties of FDM parts

Improvement of thermal performance for heat sinks

Expandable microspheres

Effect of different HAp loading on printability and on mechanical behavior and bioactivity of PLA scaffolds

Adaptive slicing in FDM

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Supplementary table 1b Research papers on FDM of neat polymers and composite materials— cont’d Contribution Wei et al., 2015

Matrix ABS PLA

Weng et al., 2016 ABS

Wittbrodt and Pearce, 2015

PLA, different colors

Wu et al., 2017

ABS

Wu et al., 2018

ABS

D. Wu et al., 2020 PLA

Filament’s production Oxidization of graphite flakes to GO, solvent mixing in Nmethylpyrolidone; reduction of GO with hydrazine hydrate; fractional precipitation in water; centrifuge; vacuum drying; extrusion with single-screw extruder 1-3-5 wt% Pre-mixing in organically homogenizer; modified hot-melt montmorillonite compounding in (sizing with twin-screw extruder; benzyldimethylhex- pelletization; drying; adecylammonium extrusion through chloride) single-screw extruder // Commercial Filler 0.4-0.8-1.6-2.3-3.85.6-7.4 wt% reduced GO

30 vol% BaTiO3 micro-particles AND

Main research topic Development of new composite system with graphene

Development of new composite system with montmorillonite

Effect of color of PLA commercial filaments High-dieletric (permittivity) constant composites

Solvent-mixing in acetone; stirring; addition of surfactant and + 1 wt% octyl gallate as surfactant plasticizer; spreading to film and + 5 wt% dibutyl solvent evaporation; phthalate as crushing; filament plasticizer extrusion with desktop single-screw extruder // Commercial Strengthening effect of ultrasonic vibration 5-10-15 wt% HAp Solvent mixing in Scaffolds for micro-particles dichloromethane; biomedical cutting of the dried applications slurry; filament extrusion with single-screw extruder

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Fused Deposition Modeling of Composite Materials

Supplementary table 1b Research papers on FDM of neat polymers and composite materials— cont’d Contribution Matrix H. Wu et al., 2020 PLA

Filler 5 wt% graphene + 20 wt% nano-Fe3 O4

Filament’s production //

Main research topic Fabrication of microwaves absorbers

Solvent mixing in dimethylformamide; cutting of the dried slurry; filament extrusion with single-screw extruder Hot-melt compounding and filament extrusion with twin-screw extruder

Fabrication of strain sensors

10-20-30 wt% nano-Fe3 O4

Xiang et al., 2019 TPU

Xiao et al., 2019

PLA

5-7-9 wt% graphene 1.5-5 wt% MWCNTs modified with 1-pyrenecarboxylic acid (4:1)

10 phr to 40 phr hemp hurd AND + 13 wt% Poly-(butylene adipate-coterephthalate) (toughening agent)

+ ethylene-methyl acrylate-glycidyl methacrylate terpolymer (sizing) HAp in-house Hot melt produced compounding and nanoparticles filament extrusion with twin-screw extruder 2-4-6-8 wt% CNTs Pre-mixing in blender; drying; hot-melt compounding in twin-screw extruder; water cooling; granulation; extrusion with single-screw extruder 13.34 vol% short Commercial carbon fibers

Xu et al., 2014

PCL

Yang et al., 2019

PLA

Yang et al., 2021

PA (nylon)

Yao et al., 2020

PLA

//

Commercial

Yin et al., 2018

ABS

//

Commercial

TPU (multi-material printing)

Development of new composite system with hemp hurd

Printability of computedtomography acquired bone structures Development of new composite system with CNTs

Evolution of porosity and of fiber orientation and length upon printing Effect of printing orientation on tensile strength Interfacial bonding in multi-material FDM

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Supplementary table 1b Research papers on FDM of neat polymers and composite materials— cont’d Contribution Yu et al., 2017

Matrix PEU

Filler 0-10-20-30-40 wt% lab-made HAp nanorods

Yu et al., 2020

PLA

Chopped carbon fibers

Zein et al., 2002

PCL

//

J. Zhang et al., 2017

PLA + additives (various)

30 wt% acetaminophen

W. Zhang et al., 2017

ABS

8 wt% CNTs

Zhang et al., 2018 ABS

15 wt% short carbon fibers 8 wt% CNTs

X. Zhang et al., 2019

PLA

B. Zhang et al., 2021

PLLA

15 wt% short carbon fibers 6.95 wt% aluminum micro-fibers 0-30-50 % HAp nanoparticles

Filament’s production Synthesis of PEU; synthesis of HAp; solvent mixing in hexafluoroisopropanol; lyophilization; filament extrusion with capillary rheometer Commercial

Main research topic PEU-HAp FDM scaffolds for bone tissue engineering

Commercial

Effect of fillers on interfacial bonding strength

Commercial

Effect of raster angle on mechanical properties Optimization of PLLA-HAp scaffolds with high filler loading by solvent mixing and silanization

Fabrication of thermally actuated parts // Scaffolds for biomedical applications (bone tissue) Tumble mixing; Correlation sieving to break between extrusion aggregates; hot-melt parameters and compounding and printing results filament extrusion with twin-screw extruder; conveyor belt Commercial Analysis of thermal stresses and deformation

Removal of water from HAp suspension with centrifuge; dispersion in acetone; addition of dodecyl trimethoxy silane; solvent mixing in DCM; dispersion and sonication; solvent evaporation at room temperature; crushing; filament extrusion with extruder Reference filaments prepared by hot-melt compounding

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Fused Deposition Modeling of Composite Materials

Supplementary table 1b Research papers on FDM of neat polymers and composite materials— cont’d Contribution Matrix Zhong et al., 2001 Modified ABS

Filament’s Filler production 10.2-13.2-18.0 wt% Dilution with ABS short glass fiber of 30 wt% commercial masterbatch in twin-screw extruder; pelletization; filament extrusion with single-screw extruder 2 wt% nano-sodium ABS+PC blended in montmorillonite high-speed mixer; (filler) AND hot-melt compounding and + 5 wt% filament extrusion ethylene-methyl acrylate copolymer with twin-screw (as compatibilizer) extruder

Main research topic Development of new composite system with short glass fibers

In-nozzle passive mixing of two filaments – FGMs Comfort and antibacterial properties of prosthetic finger

Zhou et al., 2020

ABS+PC 1:1 blend

Zhuang et al., 2017

PLA

11 wt% graphene (graphite)

Commercial

Zuniga, 2018

PLA

1% copper-based antibacterial nanoparticles

Commercial

Analysis of bonding strength in composite parts

12.2.3 Supplementary table 1c – research papers on continuous fiber-reinforced parts

Supplementary table 1c Research papers on continuous fiber-reinforced parts and sandwiched structures produced by FDM. Filament production/ manufacturing Main research topic

Contribution Matrix Continuous fiber-reinforced parts Adumitroaie et al., PLA 2019

Filler

Akhoundi et al., 2019

49.3 vol% continuous Modified in-nozzle E-glass fiber yarns impregnation

PLA

Anisoprint: continuous carbon fibers (thermoset pre-impregnated)

In-nozzle impregnation of pre-impregnated composite fibers from commercial filaments

Basic concept of Anisoprint bi-matrix composites from in-nozzle impregnation of continuous fibers pre-impregnated in thermoset resin Demonstration of the "simultaneous in-melt feeding of continuous fibers"

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Supplementary table 1c Research papers on continuous fiber-reinforced parts and sandwiched structures produced by FDM—cont’d Contribution Al Abadi et al., 2018

Matrix Nylon MarkForged

Alspach et al., 2019 N.A.

Araya-Calvo et al., Nylon MarkForged 2018 Azarov et al., 2017 Various thermoplastics

Azarov et al., 2019 PLA

Azarov et al., 2020a N.A.

Filament production/ manufacturing Main research topic Commercial Elastic properties of continuous fiber-reinforced parts, experimental vs. MarkForged: modelled continuous glass fibers in nylon

Filler MarkForged: continuous carbon fibers in nylon

MarkForged: continuous kevlar fibers in nylon MarkForged: continuous carbon fibers in nylon MarkForged: continuous carbon fibers in nylon Anisoprint: continuous composite fibers (thermoset pre-impregnated)

Anisoprint: continuous carbon fibers (thermoset pre-impregnated)

Bettini et al., 2017

PLA

8.6 vol% (9.5 wt%) continuous aramid fibers

Bi et al., 2020

PA6

Continuous carbon fiber, T700-12K

Nylon

In-nozzle impregnation of pre-impregnated composite fibers from commercial filaments

In-nozzle impregnation of pre-impregnated composite fibers from commercial filaments Anisoprint: continuous In-nozzle carbon fibers impregnation of (thermoset pre-impregnated pre-impregnated) composite fibers from commercial filaments

N.A.

Nylon MarkForged

Commercial

Anisoprint: continuous carbon fibers (thermoset pre-impregnated)

Azarov et al., 2020b

Blok et al., 2018

Commercial

Continuous carbon fiber, T700-1K MarkForged: continuous carbon fibers in nylon

In-nozzle impregnation of pre-impregnated composite fibers from commercial filaments In-nozzle impregnation

Pre-impregnation

Commercial

Printing the hull of Soft-bubble, tactile sensor for robots DOE to optimize compression and flexural properties Basic concept of Anisoprint bi-matrix composites from in-nozzle impregnation of continuous fibers pre-impregnated in thermoset resin Proof of concept of Anisoprint technology: frame of a small size unmanned aerial vehicle Technological development: design of large-scale printer with a robotic arm for aerospace and aviation load-bearing parts Topological optimization of anisotropic structures

Alternative approach to FDM of composite parts reinforced with continuous fibers Feasibility study for continuous-fiber reinforced filaments Comparison between short and continuous carbon fibers reinforcement

Nylforce: 6 wt% Commercial chopped carbon fibers

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Fused Deposition Modeling of Composite Materials

Supplementary table 1c Research papers on continuous fiber-reinforced parts and sandwiched structures produced by FDM—cont’d Contribution Caminero et al., 2018a

Caminero et al., 2018b

Matrix Nylon MarkForged

Nylon MarkForged

Filament production/ manufacturing Main research topic Commercial Characterization of mechanical behavior (impact resistance) of composite parts with MarkForged: continuous fibers continuous glass fibers in nylon

Filler MarkForged: continuous carbon fibers in nylon

MarkForged: continuous kevlar fibers in Nylon MarkForged: continuous carbon fibers in nylon

Commercial

Characterization of mechanical behavior (interlaminar shear strength) of composite parts with continuous fibers

Commercial

Characterization of mechanical behavior of composite parts with continuous fibers

MarkForged: continuous glass fibers in nylon

Chacón et al., 2019 Nylon MarkForged

MarkForged: continuous kevlar fibers in nylon MarkForged: continuous carbon fibers in nylon MarkForged: continuous glass fibers in nylon

Cheng et al., 2021

PLA

Cox et al., 2017

BendLay (MyThings4U) ABS family PEI

de Backer et al., 2018

Dickson et al., 2017 Nylon MarkForged

MarkForged: continuous kevlar fibers in nylon Continuous twisted ramie fiber yarn Continuous copper wire

Effect of printing parameters on in-nozzle impregnated ramie yarns Encapsulation in Fiber encapsulation extrudate after printing additive manufacturing

Continuous fibers

Pre-impregnation

MarkForged: continuous carbon fibers in nylon

Commercial

MarkForged: continuous glass fibers in nylon

In-nozzle impregnation

Feasibility of multi-axis and multi material printing with industrial robotic arm for continuous fiber-reinforcement Characterization of mechanical behavior of composite parts with continuous fibers

MarkForged: continuous kevlar fibers in nylon

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373

Supplementary table 1c Research papers on continuous fiber-reinforced parts and sandwiched structures produced by FDM—cont’d Contribution Matrix Domm et al., 2017 PP

Domm et al., 2021 PP

Fernandes et al., 2021

Onyx MarkForged

Gardner et al., 2016 ULTEM

Goh et al., 2018

Nylon MarkForged

He et al., 2020

Nylon MarkForged

Hedayati et al., 2020

PCL

Heidari-Rarani et al., 2019

PLA

Hou et al., 2018

PLA

Hu et al., 2018

PLA

Filament production/ manufacturing Main research topic Pre-impregnation Feasibility of Fibre Integrated Fused Deposition Modeling (direct printing of pre-impregnated filament without additional in-nozzle impregnation) Continuous glass fibers Commercial Standardized quality analysis process for continuous fiber-reinforced strands (pre-impregnated filaments) MarkForged: Commercial Characterization of continuous carbon thermo-mechanical fibers in nylon behavior of composite parts with continuous carbon fibers 70/90 wt% highly Pre-impregnated: Dual-nozzle printing of densified continuous Continuous ULTEM and carbon nanotube yarn, impregnation in a ULTEM-impregnated ULTEM-impregnated solution of ULTEM in carbon nanotube yarn dimethylacetamide MarkForged: Commercial Characterization of continuous carbon mechanical behavior fibers in nylon (and fracture mode) of composite parts with MarkForged: continuous fibers continuous glass fibers in nylon MarkForged: Commercial Characterization of continuous carbon mechanical behavior of fibers in nylon composite parts with continuous fibers 22 vol% continuous Modified in-nozzle Demonstration of the PGA yarn impregnation "simultaneous in-melt feeding of continuous fibers" 28.2 vol% continuous In-nozzle Alternative approach to carbon fibers impregnation FDM of composite parts reinforced with continuous fibers Up to 10l% continuous In-nozzle Production and kevlar fibers impregnation characterization of FDM lightweight structures Continuous carbon Prepreg filament: Alternative approach to fibers pre-impregnation of FDM of composite parts continuous fibers reinforced with through single-screw continuous fibers extruder and coaxial extrusion molds Filler 30 vol% continuous glass fibers

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Fused Deposition Modeling of Composite Materials

Supplementary table 1c Research papers on continuous fiber-reinforced parts and sandwiched structures produced by FDM—cont’d Contribution Imeri et al., 2018

Matrix Nylon MarkForged

Filler MarkForged: continuous carbon fibers in nylon MarkForged: continuous glass fibers in nylon

Justo et al., 2018

Kousiatza et al., 2019

Nylon MarkForged

Nylon MarkForged

Le Duigou et al., 2019

PLA

Li et al., 2016

PLA

Luan et al., 2019

PLA

Matsuzaki et al., 2016

PLA

Matsuzaki et al., 2018

ABS

Melenka et al., 2016

Nylon MarkForged

MarkForged: continuous kevlar fibers in nylon MarkForged: continuous carbon fibers in nylon

Filament production/ manufacturing Main research topic Commercial Effect of isotropic vs concentric mode on tensile-tensile fatigue properties

Commercial

MarkForged: continuous glass fibers in nylon MarkForged: Commercial continuous carbon fibers in nylon MarkForged: continuous glass fibers in nylon 30.4 vol% (34.5 wt%) Pre-fabrication: continuous flax fibers Extrusion coating (coating fabrication) 34 vol% continuous carbon fibers after sizing in PLA-methylene dichloride solution Continuous carbon fiber tows

In-nozzle impregnation

Combined "Dual extrusion" "In-nozzle impregnation" methods 6.6 vol% continuous In-nozzle carbon fiber tow impregnation 6.1 vol% continuous In-nozzle twisted jute fiber yarn impregnation 1K (33.5 vol%), 4K Pre-impregnation (29.4 vol%), 8K (21.2 starting from 1K vol%) continuous PAN bundle, then carbon fiber bundles subsequent bundling of 4 1K filaments or 8 1K filaments MarkForged: Commercial continuous kevlar fibers in nylon

Characterization of mechanical behavior of composite parts with continuous fibers

In situ monitoring of thermal-induced strain

Alternative approach to FDM of composite parts reinforced with continuous (flax) fibers Alternative approach to FDM of composite parts reinforced with continuous fibers Damage and load self-sensing composite structure Demonstrative of FDM of composite parts reinforced with continuous fibers Assessment of printed curvature radius vs nominal curvature radius

Characterization and modelling of mechanical behavior of composite parts with continuous fibers

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375

Supplementary table 1c Research papers on continuous fiber-reinforced parts and sandwiched structures produced by FDM—cont’d Contribution Matrix Mosleh et al. 2021 ABS

Filler 11.4 wt% continuous carbon fibers

O’Connor and Dowling, 2019

MarkForged: continuous carbon fibers in nylon

Nylon MarkForged

Filament production/ manufacturing Prepreg filament: pre-impregnation of continuous fibers in ABS solution in acetone and Commercial

Main research topic Alternative approach to FDM of composite parts reinforced with continuous fibers; definition of process window Effect of low-pressure printing

MarkForged: continuous glass fibers in nylon

Pertuz et al., 2020

Nylon MarkForged

MarkForged: continuous kevlar fibers in nylon MarkForged: continuous carbon fibers in nylon

Commercial

Characterization of mechanical behavior and fatigue strength of composite parts with continuous fibers

Commercial

Analysis of the effect of the number of layers on open hole tensile strength

Commercial

Analysis of the effect of the number of layers on impact strength

MarkForged: continuous glass fibers in nylon

Prajapati et al., 2021a

Onyx MarkForged

Prajapati et al., 2021b

Onyx MarkForged

Pyl et al., 2018

Nylon MarkForged

Saari et al., 2015

BendLay (MyThings4U) ABS family Onyx, MarkForged

Sanei et al., 2019

Shen et al., 2019

Shape memory polymer

Silva et al., 2019

PA12

MarkForged: continuous kevlar fibers in nylon MarkForged: continuous high strength high temperature (HSHT) fiberglass MarkForged: continuous high strength high temperature (HSHT) fiberglass MarkForged: continuous carbon fibers in nylon Continuous metal wire

MarkForged: continuous carbon fibers in nylon Up to 7.26 vol% continuous carbon fibers PA12 fiber - long carbon fiber commingled yars

Commercial

Analysis of the effect of specimen geometry and tabs on tensile behavior Encapsulation in Fibre encapsulation extrudate after printing additive manufacturing Commercial

Tensile tests for anisotropic properties

N.A.

Shape memory effect in continuous-fiber reinforced parts Development of printing equipment for commingled yarns

Commercial

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Fused Deposition Modeling of Composite Materials

Supplementary table 1c Research papers on continuous fiber-reinforced parts and sandwiched structures produced by FDM—cont’d Contribution Stepashkin et al., 2018

Matrix PEEK

Tian et al., 2016a

ABS

Filament production/ Filler manufacturing 20 wt% short carbon Single step hot-melt fibers (for comparison) compounding and extrusion with screw extruder from composite pellets diluted with PEEK 5 wt% continuous Modified in-nozzle carbon fiber yarns impregnation 10 wt% continuous In-nozzle carbon fibers impregnation

Tian et al., 2016b

PLA

Up to 27 wt% continuous carbon fibers

In-nozzle impregnation

Ueda et al., 2020

Nylon MarkForged

MarkForged: continuous carbon fibers in nylon Continuous carbon fibers

Commercial

van der Klift et al., Nylon MarkForged 2016

MarkForged: continuous carbon fibers in nylon

Commercial

Vaneker, 2017

PP

60 wt% E-glass commingled yarn

Yang et al., 2017

ABS

10 wt% continuous carbon fibers

Commercial: commingled yarns + pultrusion to reshape to round filament In-nozzle impregnation

Yasunaga et al., 2018

PLA

Continuous carbon fibers

In-nozzle impregnation

U¸sun and Gümrük, PLA 2021

Dedicated pre-impregnation line

Main research topic Development of new composite system with carbon fibers from modified in-nozzle impregnation method

Alternative approach to FDM of composite parts reinforced with continuous fibers (Demonstration of 3D printing in space environment; demonstration of path planning on curved surface) Alternative approach to FDM of composite parts reinforced with continuous fibers Effect of compaction upon printing Alternative approach to FDM of composite parts reinforced with continuous fibers Characterization of mechanical behavior of composite parts with continuous fibers Alternative approach to FDM of composite parts reinforced with continuous fibers Alternative approach to FDM of composite parts reinforced with continuous fibers from modified in-nozzle impregnation method Alternative approach to FDM of composite parts reinforced with continuous fibers from modified in-nozzle impregnation method

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377

Supplementary table 1c Research papers on continuous fiber-reinforced parts and sandwiched structures produced by FDM—cont’d Contribution Yu et al., 2019

Matrix Onyx, MarkForged

H. Zhang et al., 2020

PLA

J. Zhang et al., PLA 2020 Sandwich and multilayered structures Butt and Shirvani, ABS (COMM.) 2018 Butt et al., 2020

PLA

Galatas et al., 2018 ABS

Mori et al., 2014

ABS

Nakagawa et al., 2017

ABS

Sugiyama et al., 2018

//

Filament production/ manufacturing Main research topic Commercial Characterization of mechanical behavior of composite parts with continuous fibers Continuous flax fibers Pre-extrusion of PLA Printing of curved fiber pellets from single trajectories with screw extruder to continuous flax fibers “composite mold” (= hy barrel); drawing of continuous fibers through molten polymer in composite mold; spooling. Continuous carbon Pre-impregnation Effect of compaction fibers, 10.3% upon printing

Filler MarkForged: continuous carbon fibers in nylon

Copper sheet

Copper sheet Copper particles (in commercial filament) Carbon fiber fabric impregnated with epoxy resin

Multi-layered structures: Stacking upon printing Multi-layered structures: Stacking upon printing

Hybrid Fused Deposition Modelling (HFDM) with ABS Hybrid Fused Deposition Modelling (HFDM) with PLA

Sandwich structures: FDM of ABS core with different infill degrees; sanding; cutting the fabric to match the polymer core; impregnation in epoxy and gluing to polymer core; curing under applied load 1.4-1.6 vol% Multi-layered continuous carbon structures: fibers fibers sandwiched after (layered after printing printing and thermally bonded) Continuous carbon Multi-layered fibers structures: fibers (layered after printing sandwiched after and thermally bonded) printing

Sandwich structures with FDM-printed core: mechanical behavior and proof of concept (unmanned aerial vehicle)

MarkForged: continuous carbon fibers in Nylon

Proof of concept of one-step sandwich fabrication

Sandwich structures: core and skin printed in one piece

Reinforcement via fibers’ layering: mechanical behavior

Reinforcement via fibers’ layering: mechanical behavior

378

Fused Deposition Modeling of Composite Materials

12.2.4 Supplementary table 1d – research papers on shaping, debinding and sintering

Supplementary table 1d Research papers on shaping, debinding and sintering, SDS (production of fully inorganic parts). Reference Abe et al., 2021

Abel et al., 2019

Matrix Binder (multicomponent system comprising POM, PP and paraffin wax) Binder (multicomponent system, comprising thermoplastic elastomer compound, functionalized polyolefin and 5 vol% stearic acid as surfactant)

Filler 60 vol% 17-4PH stainless steel, gas atomized powder (average grain size: 10 μm) 47 vol% tetragonal zirconia, ZrO2 (d50 = 0.5 μm) OR 47 vol% 17-4PH stainless steel

Filament production Filament extrusion with capillary rheometer (piston-type extruder) - Pre-treatment: For zirconia: pre-drying to avoid agglomeration For 17-4PH: attrition milling and planetary ball milling to adjust the grain size and match the sintering behavior of zirconia

Main research topic Effect of print orientation and aging treatment on tensile properties of 17-4PH parts by FDMet (SDS) General procedure for multi-material SDS with alternating layers of zirconia and 17-4PH stainless steel:

- For producing each composite filament:

Arnesano et al., 2020

Pre-compounding in a roller rotors mixer; pelletization or granulation; hot-melt compounding in twin-screw extruder or shear roll extruder; pelletization or granulation; repeated extrusion and pelletization/granulation, if necessary; filament extrusion with single screw extruder Binder (multi50-57 vol% alumina, Single-step hot-melt compounding and component system, Al2 O3 , powder comprising LDPE, (AES-23, Sumitomo extrusion with chemicals, Japan) twin-screw extruder PLA, paraffinic wax and PEgMA)

Feasibility of fully ceramic parts via FDC (SDS) + Glass infiltration for dental restoration

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379

Supplementary table 1d Research papers on shaping, debinding and sintering, SDS (production of fully inorganic parts)—cont’d Reference Cano et al., 2019

Matrix Binder (multicomponent system), various formulations with:

Filler 47 vol% (85.3 wt%) tetragonal zirconia, ZrO2 , stabilized with 3 mol% yttria, (spray-dried - 35 vol% backbone: acrylic granules, average acid-grafted HDPE particle size 90 nm) - 10 vol% surfactant: stearic acid

Cano et al., 2020

Damon et al., 2019

- 55 vol% soluble fraction: amorphous polyolefin (neat; mixed with paraffin wax; mixed with paraffinic extender oil); OR styreneethylene/butylenestyrene copolymer (neat; mixed with paraffin wax; mixed with paraffinic extender oil) Binder (multicomponent system, comprising thermoplastic elastomer compound and polyolefin grafted with a polar component)

Polyacetal binder system

47 vol% (85.3 wt%) tetragonal zirconia, ZrO2 , stabilized with 3 mol% yttria, (spray-dried granules, average particle size 90 nm)

> 80 wt% AISI 316LX particles

Filament production Compounding of binder and pre-dried zirconia by hot-melt compounding in twin-screw internal mixer; granulation by cutting mill; filament extrusion with capillary rheometer, cooling down by natural convection of PTFE conveyor belt (filaments consisting of binder solely: extrusion with single screw extruder)

Binder preparation: hot-melt compounding in twin-screw extruder, water cooling, pelletization, drying;

Main research topic Binder formulation for the production of zirconia by FDC (SDS)

Effect of infill orientation on porosity and flexural properties of zirconia processed by FDC (SDS)

Filament preparation by compounding zirconia powder and binder pellets in twin-screw extruder, air cooling, pelletization; Filament extrusion with single screw extruder, cooling down by natural convection of PTFE conveyor belt Commercial (Ultrafuse 316LX, BASF AG)

Effect of orientation on porosity and mechanical properties of 316LX FDMet (SDS) parts

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Fused Deposition Modeling of Composite Materials

Supplementary table 1d Research papers on shaping, debinding and sintering, SDS (production of fully inorganic parts)—cont’d Reference Ebrahimi and Ju, 2018

Matrix PLA

Egorov et al., 2021

Organosilicon polymers AND + Additives (various)

Esslinger et al., 2021

PLA AND + 0-18-27 vol% PEG

Filament Filler production 40 vol% copper Commercial (The (particle size = 32 ± Virtual Foundry) 30 μm) 40 to 80 wt% Al2 O3 Solvent mixing in (micro/nano: 50/50) toluene with OR mechanical stirrer; addition of solid 40 to 80 wt% SiC (micro/nano: 50/50) loading with mechanical disperser; OR drying overnight; 40 to 80 wt% BN vacuum-drying (micro/nano: 50/50) OR 40 to 80 wt% ZrO2 (micro/nano: 50/50) 55 vol% β-tricalcium phosphate, TCP (d50 = 6.1 μm, d90 = 14.1 μm)

Godec et al., 2020 Binder (multi55 vol% 17-4PH component system, steel, gas atomized comprising powder (d10 = 4.2 μm, d50 = thermoplastic 12.3 μm, d90 = elastomer and grafted polyolefin) 28.2 μm) Gong et al., 2019 Binder

GonzalezGutierrez et al., 2017

SS 316LX particles

Binder (multi53-55-60 vol% component system, strontium ferrite comprising (SrFe12 O19 ) thermoplastic elastomer AND grafted polyolefin)

Gorjan et al., 2019 Various mixtures of:

Hot-melt compounding in twin-screw extruder; filament with single screw extruder Hot melt compounding and extrusion with twin-screw extruder; pelletization; filament extrusion with single-screw extruder Commercial (Ultrafuse 316LX, BASF AG) Hot-melt compounding in kneader with counter-rotating rollers; granulation by cutting mill; filament extrusion with capillary rheometer on PTFE conveyor belt Hot-melt compounding with high shear mixer; filament extrusion with piston extruder

Up to 50 vol% micron-sized γ -Al2 O3 (d50 = polymethylsiloxane 5.3 μm) OR resin (SiO2 source) Up to 50 vol% micron-sized - EVA γ -Al2 O3 (d50 = 14.8 μm) Hot-melt Gorjan et al., 2020 Various mixtures 50 vol% Al2 O3 (median particle size compounding with of: 0.5 μm) high shear mixer; - EVA filament extrusion - stearic acid with piston extruder

Main research topic Feasibility of FDMet (SDS) from standard filaments, thermal conductivity Suitability of organosilicon polymers as binders for SDS of ceramics

Feasibility study of β-tricalcium phosphate parts via FDC (SDS); role of PEG DOE to optimize the tensile strength of green parts for FDMet (SDS) based on extruder temperature, flow rate multiplier and layer thickness Fully metal parts via FDMet (SDS) Effect of filler loading on filaments for FDMet (SDS)

In-situ reaction to obtain mullite upon SDS

Effect of stearic acid in EVA-based binder for SDS of alumina

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381

Supplementary table 1d Research papers on shaping, debinding and sintering, SDS (production of fully inorganic parts)—cont’d Reference Iyer et al., 2008

Matrix Binder (multi-component organic system

Filler 55 vol% Si3 N4 powder (median particle size 0.5 μm) AND

Filament production Main research topic Batch compounding; Demonstrative of FDC extrusion with (SDS) feasibility single-screw extruder

commercial investment casting + oleyl alcohol (coating on particles wax) as surfactant) Jafari et al., 2000 Various Various //

Janek et al., 2020 PVA

Kukla et al., 2017a

Kukla et al., 2017b

50 wt% HAp, d50 ≤ 35 μm OR

50 wt% HAp, d50 ≤ 16 μm PLA 27 wt% gypsum Binder (multi55 vol% SS 316L component system, particles (average comprising particle size 5.5 μm) thermoplastic OR elastomer and 55 vol% SS 316L grafted polyolefin) particles (average

Solvent mixing in water; granulation; filament with single screw extruder

Commercial Hot-melt compounding in kneader with counter-rotating rollers; granulation by cutting mill; particle size 8.6 μm) filament extrusion with capillary rheometer on PTFE conveyor belt Hot-melt Binder (multi55 vol% SS 316L component system, (d50 = 6.05 μm) OR compounding in comprising 55 vol% Ti-6Al-4V capillary rheometer thermoplastic (d50 = 14.97 μm) OR for small quantities, in single screw elastomer and grafted polyolefin 55 vol% NdFeB (d50 extruder for large = 28.29 μm) OR quantities and dispersant/ 55 vol% SrFe12 O19 compatibilizing (d50 = 1.35 μm) OR agents)

Kukla et al., 2019 Binder (multicomponent system), different compositions - thermoplastic elastomer compound and polyolefin grafted with a polar component OR

50 vol% ziconia, ZrO2 (d50 = 0.6 μm) 50 vol% tetragonal zirconia, ZrO2 , stabilized with 3 mol% yttria (d50 = 0.6 μm)

Compounding of binder and pre-dried zirconia by hot-melt compounding in internal mixer; granulation by cutting mill

Proof of concept of multi-materials FDC (SDS) Tensile and compressive testing conditions on filaments for FDC (SDS) Effect of average grain size of filler on filaments for FDMet (SDS)

Feasibility study of printing and debinding for fully inorganic parts via SDS

Preliminary investigation on chemical debinding of zirconia processed by FDC (SDS)

- thermoplastic elastomer compound, polyolefin grafted with a polar component and stearic acid

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Fused Deposition Modeling of Composite Materials

Supplementary table 1d Research papers on shaping, debinding and sintering, SDS (production of fully inorganic parts)—cont’d Reference Kurose et al., 2020

Matrix Binder (multicomponent system, comprising POM and paraffin wax)

B. Liu et al., 2020 Binder (multicomponent system, comprising of POM and additives such as PP, dioctyl phthalate, dibutyl phthalate and ZnO) Marsh et al., 2021 Binder (multicomponent system, comprising elastomer, polyolefin and 5-8 vol% fatty acids as surfactants) Mashekov et al., Binder 2021 (POM-based)

McNulty et al., 1998

Binder (multicomponent system, comprising polyolefin base binder + tackifier + wax + plasticiser)

McNulty et al., 1999

Binder for FDC (SDS)

Nötzel et al., 2018 Binder (multicomponent system, comprising LDPE, paraffin, stearic acid as surfactant, release agents) Pekin et al., 1998a Binder (multicomponent system, comprising EVA copolymers; microcrystalline wax)

Filler 60 vol% 316L stainless steel (average particle size: 10 μm) 88 wt% SS 316L particles, 30–50 μm

46.4 to 48.9 vol% silver-modified 58S bioactive glass (particle size between 20 to 38 μm)

316L stainless steel

Filament production Filament extrusion with capillary rheometer (piston-type extruder) Commercial

Sol-gel production of doped glass particles; drying at 40°C for 1 h; filament with twin-screw extruder; immediate spooling

Starting from MIM feedstock (Catamold 316L) in granules; filament extrusion with extruder Hot shear mixing in 55 vol% (93 wt%) PZT (surface area = Haake System 9000 torque rheometer in 2.58 m2 /g AND + stearic acid as conjunction with a surfactant OR HAp single-screw extruder (surface area = 3.40 m2 /g) AND + stearic acid as surfactant 55 vol% PZT Hot shear mixing in (surface area = 2.58 Haake System 9000 m2 /g) AND + torque rheometer surfactant (various options) 10 to 60 vol% (32–86 Hot-melt compounding in wt %) mixer-kneader; sub-micron-sized Al2 O3 particles (d50 extrusion with = 0.1 μm) single-screw extruder) //. 60 wt% alumina, Al2 O3

Main research topic Effect of print orientation and layer thickness on tensile properties of FDMet (SDS) parts Fully metal parts via FDMet (SDS)

Feasibility study of silver-doped bioactive glass scaffolds by FDC (SDS)

Suitability of MIM feedstock for FDMet (SDS)

Feasibility study of fully ceramic parts via FDC (SDS)

Optimization of surfactant (chemical composition and concentration) via TGA for FDC (SDS) FDC (SDS) Fully ceramic parts, binder removal (solvent + thermal)

Thermal removal of a binder for FDC (SDS):

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383

Supplementary table 1d Research papers on shaping, debinding and sintering, SDS (production of fully inorganic parts)—cont’d Reference Matrix Pekin et al., 1998b Binder (multi-component system, comprising EVA copolymers; microcrystalline wax) Pistor, 2001 Binder (multicomponent system, comprising EVA, EEA, heavy mineral oil, methoxypolyethylene glycol, Al2 O3 ) Binder (multi-component system, comprising various polymers, wax) Prathumwan and PLA Subannajui, 2020

Schumacher and Moritzer, 2021

Binder (POM-based)

Singh et al., 2020 Binder (multi-component system)

P. Singh et al., 2021

Binder

Suwanpreecha et al., 2021

Binder

Thompson et al., 2019

Binder (multi-component system, comprising thermoplastic elastomer compound and grafted polyolefin)

Filler 60% Alcoa A-17 grade alumina

Filament production //

51 vol% Si3 N4 // (average particle size < 1 μm)

50 vol% PZT // (average particle size ∼1 μm)

70 wt% aluminum Blending; hot-melt (average particle size compounding and = 10 μm) filament extrusion with extruder Stainless steel 316L Commercial particles (Ultrafuse 316L, BASF) 59 vol% Ti-6Al-4V Hot-melt powder (D50 : 13 μm) compounding in OR 59 vol% torque rheometer Ti-6Al-4V powder with prep-mixer; (D50 : 30 μm) filament extrusion with capillary rheometer 59 vol% Ti-6Al-4V Hot melt powder (D50 : 13 μm) compounding in OR 59 vol% torque rheometer Ti-6Al-4V powder with prep-mixer; (D50 : 30 μm) filament extrusion with capillary rheometer Commercial 63 vol% 17-4PH stainless steel powder (d50 : 3.97 μm) 55 vol% 316L stainless steel (spherical particles, average particle size: 17.7 μm)

Hot-melt compounding in twin-screw extruder; filament extrusion with single screw extruder

Main research topic Formulation of a binder for FDC (SDS)

Special features for FDC (SDS):evaluation of the effect of vacuum pre-treatment of composite feedstock via modulated DSCevaluation of anisotropic coefficient of thermal expansion on green parts

Alumina/aluminum core/shell composites by partial oxidation upon sintering in SDS Fully metal parts via FDMet (SDS) vs. SLM counterparts Predictive model of printability based on shear strength and pressure drop

Systematic analysis of processing parameters and powder size on density (and tensile properties) of Ti-6Al-4V parts by FDMet (SDS) Effect of print orientation on tensile properties of FDMet (SDS) parts Tuning of process parameters to reduce porosity of 316L stainless steel parts produced by FDMet (SDS):

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Fused Deposition Modeling of Composite Materials

Supplementary table 1d Research papers on shaping, debinding and sintering, SDS (production of fully inorganic parts)—cont’d Reference Venkataraman et al., 1999

Matrix ECG9 binder

Venkataraman et al., 2000

Various binders (RU9, ECG9 and ECG2)

Y. Zhang et al., 2020

Binder (multi-component polyolefinthermoplastic system)

Filament production Mixing of PZT powder with 3 wt% solution of stearic acid in toluene for 4 hours; vacuum filtering and drying for 12 hours; hot-melt compounding at 140°C in torque rheometer; cooling; granulation; filament extrusion with single-screw extruder 52.6 vol% PZT, Hot-melt AND + 3 wt% stearic compounding; acid (surfactant) OR granulation; 55 vol% Si3 N4 AND extrusion with + 3 wt% oleyl single-screw extruder alcohol (surfactant) OR 55 vol% graphite OR 55 vol% graphite AND + 3 wt% stearic acid (surfactant) OR 58 vol% 17-4PH (spherical particles) AND + 1 wt% stearic acid (surfactant) OR 58 vol% 17-4PH (irregular particles) AND + 1 wt% stearic acid (surfactant) Powder 55-59 vol% pre-treatment: Ti-6Al-4V powder (d10 ∼ 1.81 μm, d50 rinsing by acetone, ∼ 2.66 μm, d90 ∼ 2-propanol, and 6.84 μm) deionized water; 24-h dehydration in a conventional oven at 40°C Filler 52.6 vol% ead zirconate titanate, PZT (median particle size = 1.2 μm) AND + 3 wt% (vs powder) stearic acid (surfactant)

Hot-melt compounding in mini-extruder; pelletization; filament extrusion with single-screw extruder at 170-200°C (2.85 mm)

Main research topic Analysis of criteria to avoid filament buckling, esp. in SDS Compressive modulus-to-apparent viscosity ratio to avoid buckling

Analysis of criteria to avoid filament buckling, esp. in SDS

FDMet (SDS) of Ti-6Al-4V parts

Fused deposition modeling of composite materials at a glance – supplementary tables

385

12.2.5 Supplementary table 1e – other relevant research papers

Supplementary table 1e Other research papers relevant to FDM of composite materials and fully inorganic parts. Composite’s Matrix Filler production Recycled PLA from 0-5-10-15-20 wt% Sorting, shredding and FDM local silica sand milling of PLA leftover; grinding of silica sand; hot-melt compounding in twin-screw extruder; hot pressing Al-Hydary and ABS 1-5-10-20-35 wt% Solvent mixing in acetone by dissolving Al-Rubiae, 2019 anatase (TiO2 ) nanoparticles ABS in acetone under magnetic stirring at 50°C and dispersing TiO2 in acetone (under sonication at room temperature for 2 h; mixing the two systems under magnetic stirring at room temperature overnight; aging; water induced precipitation; drying the paste at 80°C for 24 h; crushing with mortar; hot pressing Asiaban and 3:2 wt/wt blend of 0.2-0.5-1.5-3-6 wt% Pre-mixing SAN and Taghinejad, 2010 styrene-acrylonitrile TiO2 with alumina g-ABS; adding TiO2 ; strand extrusion with copolymer (SAN) surface functionalization twin-screw extruder and styreneacrylonitrile0.2-0.5-1.5-3 wt% (550 rpm; temperature profile from 180°C to (grafted)-butadiene TiO with 2 (g-ABS) organophosphates 220°C); pelletization; compression molding or surface injection molding functionalization Contribution Ahmed et al., 2020a

Binder et al., 2019 //

//

//

Chan and Lee, 1989

PP

//

Sheet extrusion

Cifuentes et al., 2017

PLLA

0.5-1-3-5-7 wt% (= 0.36-0.72-2.173.64-5.13 vol%) magnesium microparticles

Extrusion with twin-screw (minilab) extruder; melting and pressing

Main research topic Research article on the effect of silica sand on recycled PLA from FDM filaments

Determining the TiO2 wt% to balance mechanical strength and anti-bacterial properties in samples obtained by hot pressing

Research article on the effect of TiO2 pigments on optical aspects of ABS (white index, opacity, yellowness) and side effects on mechanical properties of compression- or injection molded parts Research article on tagging features for metal parts by AM (chips) Effect of extrusion parameters and sheet thickness on molecular chain orientation Effect of magnesium microparticles on structural integrity of PLLA matrix and on microstructural and mechanical properties of pressed samples

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Fused Deposition Modeling of Composite Materials

Supplementary table 1e Other research papers relevant to FDM of composite materials and fully inorganic parts—cont’d Contribution Duty et al., 2017

Matrix ABS

Duty et al., 2018

Various

Eckel et al., 2016 // Eisenbarth et al., 2020

//

Elkington et al., 2015 Fan et al., 2008

//

Fan et al., 2009

Multi-component binder

Filler 13 wt% short carbon fibers

Composite’s production Commercial pellets; printing by BAAM

Main research topic Research article on BAAM; comparison with injection-molded counterparts Various Commercial materials; Development of a various polymer-based viscoelastic model to predict printability by processing techniques including BAAM; FDM; material extrusion techniques direct writing // // Ceramic parts from SLA polymer parts // // Research article on tagging features for metal parts by AM // // Research article on hand operations in hand lay-up 93 wt% (61.8 vol%) Hot-melt compounding Role of different carbonyl iron (kneading); PIM swelling inhibitors in powder PIM

(polyethylene 40 wt%, paraffin wax 55 wt%, stearic acid 5 wt%) Multi-component 93 wt% (61.8 vol%) Hot-melt compounding carbonyl iron (kneading); PIM binder powder (polyethylene

40 wt%, paraffin wax 55 wt%, stearic acid 5 wt%) Flank et al., 2017 // //

79 vol% martensitic Giberti et al., 2016 Binder stainless steel AISI (water-soluble polyethylene glycol) 630 Binder 84.2 vol% yttria-stabilized (water-soluble) zirconia, ZrO2 Górecka et al., PCL // 2020

Hann, 2016

//

//

Haslam and Raeymaekers, 2013

Resin

CNTs

//

Commercial feedstock for PIM; printing with new equipment from pellets

Determination of minimum amount of binder removal to create connected pores in the first stage of debinding in PIM Research article on tagging features for metal parts by AM Development of new equipment to print pellets for PIM for subsequent debinding and sintering

Effect of diameter on properties of (single) printed beads and ultimately on properties of scaffolds produced by Precision Extruding Deposition // Research article on powder reuse in laser-based powder bed fusion Mixing under sonication Quantification of CNT dispersion by means of new indexes Solution casting of films; printing with BioScaffolder (Precision Extruding Deposition)

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387

Supplementary table 1e Other research papers relevant to FDM of composite materials and fully inorganic parts—cont’d Contribution Hollister et al., 2015

Matrix PCL powder

Composite’s production Printing by SLS

Main research topic Research article on quality control of biomedical devices (tracheobronchial splint; spinal cage) based on modular scaffolds Ivanova et al., // // // Research article on 2014 tagging features for PolyJet parts Jayabal and // // // Research article on Sivanarutchelvan, natural fiber-reinforced 2009 composites Kononenko et al., // // // Research article on effect 2017 of particle size on ZnO genotoxicity PCL // Printing with in-house Effectiveness of Kosik-Kozioł et al., 2019 bio-extruder acetone-based surface treatment to control scaffolds for bone tissue engineering Kosik-Kozioł et PCL // Printing with in-house Surface-modified al., 2020 bio-extruder scaffolds in multi-material systems for cartilage repair Kurgan, 2014 AISI 316L stainless // Pressing the powder Effect of porosity on steel with lubricant; sintering tensile properties of AISI under controlled 316 stainless steel atmosphere medical implants Li and Huneault, PLA AND Nucleating agents: Hot-melt compounding Investigation on PLA 2007 and extrusion with nucleation/crystallization + plasticizers - talc twin-screw extruder for phenomena - sodium stearate masterbatch (20% of - calcium lactate filler); second extrusion with additional PLA to dilute; water cooling; pelletization; injection molding Li et al., 2003 Multi-component 55-58-64 vol% Pre-mixing of binder on Condition for safe binder (high-density mixture of carbonyl an impeller mixer; thermal debinding polyethyiron pre-mixing of powder (if critical thickness lene/paraffin wax (98 wt%) and nickel required); binder-filler (powder injection 70:30) (2 wt%) powders mixing on molding) counter-rotating roller 55-58-64 vol% 316L stainless steel mixer; granulation on plastic extruder; PIM (gas atomized) Little et al., 2012 Resin

Filler //

Continuous carbon Commercial prepregs; fibers lamination

Void content determination by different methods in fiber-reinforced composite materials (laminates)

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388

Fused Deposition Modeling of Composite Materials

Supplementary table 1e Other research papers relevant to FDM of composite materials and fully inorganic parts—cont’d Contribution Liu and Shao, 2021

Matrix //

Filler //

Composite’s production //

Main research topic Analytical modeling of dielectric constant of polymer matrix-BaTiO3 nanocomposites taking into account interphase effects Little et al., 2012 Resin Continuous carbon Commercial prepregs; Void content fibers lamination; oven curing determination by under vacuum different methods in fiber-reinforced composite materials (laminates) Research paper on effect Masterbatch of CNTs Mi et al., in press PA6 and PP Various amounts of phase morphology and (10 phr) in PP by (up to 8 phr) of CNTs (segregated to hot-melt compounding orientation on electrical conductivity and in twin-screw PA6 phase) mechanical properties of micro-compounder; shredding; diluting with segregated structures (“Filler-transfer-induced PP and PA6; dispersion” method for compression molding improved dispersion of OR injection molding CNTs) OR intermittent injection molding Up to 3 wt% ZnO Drying; pre-mixing in Murariu et al., PLA AND Effect of silane on 2011 turbo-mixer; PLA-ZnO composites + 0.3 wt% Ultranox nanorods, with/without melt-compounding in for films and fibers 626A (thermal triethoxy twin-screw extruder; stabilizer) caprylylsilane granulation; films with surface treatment twin-screw micro-compounder or fibers by melt-spinning OR

Murariu et al., 2021

PLA AND

Masterbatches:

+ 0.3 wt% Ultranox 10-20-30-40 wt% 626A (thermal ZnO nanorods, stabilizer) with/without triethoxy caprylylsilane surface treatment Final composites: 1-2-3 wt% ZnO nanorods, with/without triethoxy caprylylsilane surface treatment

Drying; meltcompounding in bench-scale kneader; plates by compression molding Films from masterbatches: Drying; melt-compounding in twin-screw extruder; granulation; PLA dilution and films with single-screw extruder

Masterbatch-based processing to reduce thermal degradation of PLA

Films with conventional processing: Drying; pre-mixing in turbo-mixer; melt-compounding in twin-screw extruder; granulation; films with single-screw extruder

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Supplementary table 1e Other research papers relevant to FDM of composite materials and fully inorganic parts—cont’d Contribution Nabinejad et al., 2015

Matrix //

Filler //

Pantani et al., 2013 PLA AND

0.5-1-2-3 wt% ZnO + 0.3 wt% Ultranox nanorods, with/without 626A (thermal triethoxy stabilizer) caprylylsilane surface treatment

Park et al., 2009

Binder (multi-component system): - backbone: PP AND PE

51% solid loading of hydride-dehydride (HDH) Ti powder

57% solid loading - paraffin wax AND of spheroidized hydride-dehydride - stearic acid Ti powder

Roschli et al., 2019 //

Composite’s production //

Drying; pre-mixing in turbo-mixer; melt-compounding in twin-screw extruder; granulation; films with twin-screw micro-compounder Hot-melt compounding in a twin-shaft, co-rotating mixer; granulation in a rotary feedstock granulator; PIM

Main research topic Research article about TGA on natural fiber composites Effect of silane on PLA-ZnO films for packaging

Research article about rheological properties and thermal debinding in PIM of titanium

67% solid loading for gas atomized Ti powder // //

Design guidelines for BAAM Russias et al., 2006 // // // Bioactivity of composites obtained from hot pressing route Sa et al., 2018 Binder (for Biphasic calcium Suspension; printing by Ceramic scaffolds from suspension) phosphate + 10 wt% material extrusion from material extrusion of ZrO2 suspension ceramic-polymeric slurry - TiO2 masterbatch: Evaluation of the effect Santos et al., 2014 ABS (high Impact) Rutile (TiO2 ) of TiO2 , carbon black nanoparticles with Mixing TiO and a 2 and additives on the alumina surface low-viscosity matrix photo-stability of ABS functionalization (acrylonitrile-styrene, and + Furnace SAN) in internal mixer on extruded tapes carbon black (16 at high pressure nm) and + Light - Extrusion of tapes: stabilizers, antioxidants and a combination of them

Seiler and Kindersberger, 2014

//

//

Singla et al., 2017 //

//

For ABS-TiO2 samples: pre-drying ABS at 80°C for 1 h; hot-melt compounding with TiO2 masterbatch; tape extrusion with twin-screw extruder For ABS-TiO2 -carbon black samples: addition of carbon black and dispersant tape extrusion with twin-screw extruder // Effect of interphase on electrical properties of silicon-matrix composites // Research article on PLA-EVA blends

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Fused Deposition Modeling of Composite Materials

Supplementary table 1e Other research papers relevant to FDM of composite materials and fully inorganic parts—cont’d Contribution Sola et al., 2019

Matrix PMMA OR PU

Sudeepan et al., 2016

ABS pellets

Sun et al., 2017

//

Tsukuda et al., 1997

ABS

Composite’s production Solvent mixing in THF for PMMA or in DMF for PU; casting; solvent evaporation; salt removal with water 5/10/15/20 wt% Pre-drying at 60°C for 6 TiO2 microparticles h, manual pre-mixing in (mean particle size: plastic bag; rod extrusion with 1.5-1.9 μm) single-screw extruder; cooling in water; pelletization; Compression molding // // Filler NaHCO3 OR NaCl (various granulometries)

E-glass spheres (2 to 30 μm, surface with glycide silane coupling reagent)

- Systems with dispersant: double extrusion with single-screw extruder

AND/OR

- Systems without

Main research topic Research article on scaffolds for bone marrow niche by solution casting-particle leaching method Mechanical and tribological characterization of composites produced by compression molding and optimization by DOE Research article on carbon-fiber epoxy-matrix composites for aeronautics Evaluation of the effect of fillers on heat capacity and thermal conductivity of ABS on molded or hot-pressed samples

dispersant: double TiO2 rutile extrusion with nanospheres twon-screw extruder; (surface with alumina, silica, and - Compression molding zinc oxide) with or hot pressing N,Nethylene- bis (steramide) as dispersing agent (20 wt% on TiO2 weight) OR

Virovska et al., 2016

PLLA

TiO2 rutile nanospheres //

Electrospinning of PLA+ electrospraying of ZnO OR ZnO-expanded graphite OR ZnO-fullerene coatings Walejewska et al., PLGA (L-lactide + 20-40 wt% Solvent mixing in 2020 glycolide) PLCL tricalcium methylene chloride; (L-lactide + PCL) phosphate casting; drying; cutting; printing with BioScaffolder (Precision Extruding Deposition) Wambua et al., PP (very high MFI) Kenaf fibers Preparation of PP films; 2003 stacking of PP films with Coir fibers fiber/mat layers; Sisal fibers pre-heating; hot Hemp random mat pressing; cooling under pressure Jute random mat

Self-cleaning ability of PLA-ZnO/expanded graphite and PLA-ZnO/ fullerene eletrospun/ electrosprayed structures Properties of biodegradable scaffolds with structural shift produced by Precision Extruding Deposition Basic research on natural fiber- and glass fiber-reinforced composites

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391

Supplementary table 1e Other research papers relevant to FDM of composite materials and fully inorganic parts—cont’d Contribution Matrix Wang et al., 2015 //

Filler //

Composite’s production //

Wei et al., 2018

//

//

//

Wu et al., 1996

//

//

//

Xia et al., 2016

Kraton D1161 P

23-25-27-29-30-3335 vol% silver-coated nickel particles 50 vol% zirconia, ZrO2 , stabilized with 3 mol% yttria (average grain size: 0.12 μm OR 0.15 μm) HAp and TCP

//

Crystalbond 555 (Beeswax) Xie et al., 2020

Multi-component binder (various compositions)

Yang et al., 2006

Binder (for suspension)

Yu et al., 2012

PLA

Yun et al., 2016

PCL

Zeltmann et al., 2016

//

Zhao et al., 2021

Polycarbosilane + // PP (1:0.01-1:0.051:0.10 wt/wt)

Hot-melt compounding in double roller; PIM

Suspension; printing by material extrusion from suspension Up to 30 wt% Pre-mixing by hand nominal (24.3 wt% shaking in bag; real) talc and + 0.3 single-step hot-melt wt% (on PLA wt) compounding and 3-aminopropylextrusion with triethoxysilane twin-screw extruder; (coupling agent) pelletization; drying; injection molding 5-10 wt% (in Solvent mixing in solution) lab-made chloroform; drop-wise magnetic addition of the nanoparticles polymer-magnetic nanoparticle suspension to a plastic mold filled with NaCl particles; freeze-drying + leaching in water // //

Solvent mixing in xylene; drying; grounding, milling and sieving; material extrusion from syringe

Main research topic Research article on multi-material meta-structures by PolyJet Research article on tagging features for metal parts by AM Feasibility of 3D pharming by inkjet 3d printing Fabrication of electrical junctions by TEAM

Condition for safe thermal debinding and definition of critical thickness in PIM

Ceramic scaffolds from material extrusion of ceramic-polymeric slurry Investigation on PLA nucleation/crystallization phenomena

Research article on composite scaffolds with magnetic iron oxide particles to promote bone growth

Research article on security challenges in 3D printing (exemplified with PolyJet technique) Material extrusion SDS from pre-ceramic polymer mixed with similar thermoplastic (feedstock in powder form)

392

12.3

Fused Deposition Modeling of Composite Materials

Supplementary table 2

12.3.1 Supplementary table 2a – tensile tests

Supplementary table 2a Tensile tests on composite filaments and printed parts. Reference Composite material Method Tensile tests on filaments Abel et al., 2019 Binder + 47 vol% yttria-stabilized ZrO2 Customized Batakliev et al., 2019 PLA + up to 9 wt% graphene nanoplatelets Customized Berretta et al., 2017 Bi et al., 2020 Boparai et al., 2016a Brounstein et al., 2021

PLA + up to 9 wt% MWCNT PEEK + up to 5 wt% MWCNTs PA6 + continuous carbon fibers Nylon + up to 40 wt% (aluminum + Al2 O3 ) particles PLA + up to 30 wt% ZnO particles

Customized ASTM D3039 ASTM D638 Customized

PLA + up to 30 wt% TiO2 particles Cano et al., 2019 Cano et al., 2020 Distler et al., 2020 Domm et al., 2021 Dul et al., 2016 Gonzalez-Gutierrez et al., 2017 Gregor-Svetec et al., 2020 Guessasma et al., 2019 Janek et al., 2020

(PLA + up to 10 wt% PEG) Binder, various formulations + 47 vol% yttria-stabilized ZrO2 Binder + 47 vol% yttria-stabilized ZrO2 PLA + up to 10 wt% Bioglass 45S5 PP + 60 wt% continuous glass fibers

Customized Customized DIN 53455 ISO 527

ABS + up to 8 wt% graphene nanoplatelets Binder + up to 60 vol% strontium ferrite (SrFe12 O19 ) HDPE + up to 75 wt% cardboard dust PLA + 10% hemp fibers (COMM.) PVA + 50 wt% hydroxyapatite, d50 ≤ 35 μm

ISO 17129 ISO 527 Customized ASTM D2256 Customized Customized

PVA + 50 wt% hydroxyapatite, d50 ≤ 16 μm Kariz et al., 2018 Korte and Quodbach, 2018 Kukla et al., 2017a

PLA + 27 wt% gypsum (COMM.) PLA + up to 50 wt% (beech) wood flour Ammonio-methacrylate copolymer A + 30 wt% theophylline + plasticizer + silica Binder + 55 vol% SS 316L, average size: 5.5 μm

Customized Customized Customized

Binder + 55 vol% SS 316L, average size: 8.6 μm (continued on next page)

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393

Supplementary table 2a Tensile tests on composite filaments and printed parts—cont’d Reference Composite material Method Kukla et al., 2017b Binders and additives + 55 vol% SS 316L Customized Binders and additives + 55 vol% Ti-6Al-4V Binders and additives + 55 vol% NdFeB Binders and additives + 55 vol% SrFe12 O19 R. Kumar et al., in press Le Duigou et al., 2016 Masood and Song, 2004 Mousavi et al., 2020 Ponsar et al., 2020 Ranjan et al., 2020 Schmitz et al., 2018

Binders and additives + 50 vol% ZrO2 PLA + up to 2% ZnO nanoparticles PLA and polyhydroxyalkanoates + 15.2 wt% recycled wood fibers (COMM.) Nylon and additives + up to 40 vol% (76 wt%) iron PLA + 12 wt% CNTs Ethyl cellulose + 16.67 wt% hypromellose + plasticiser + 0.4 wt% silica PLA + up to 20 wt% (hydroxyapatite + chitosan) ABS + 3 wt% MWCNTs

Customized Customized Customized Customized Customized ASTM D638 ISO 527

ABS + 3 wt% carbon black

Singh et al., 2018 N. Singh et al., 2019 Singh et al., 2020 Spinelli et al., 2020 Stoof and Pickering, 2017

ABS + 3 wt% (50:50 MWCNTs + carbon black) PA6 + 30 wt% TiO2 Waste HDPE + up to 30 wt% (SiC + Al2 O3 , 1:1) Binder + 59 vol% (87 wt%) Ti-6Al-4V (either 13 μm or 30 μm) PLA + up to 6 wt% (MWCNTs and graphene nanoplatelets) PP (recycled) + up to 50 wt% hemp fibers

Customized Customized Customized Customized Customized

PP (recycled) + up to 50 wt% harakeke fibers

U¸sun and Gümrük, 2021 Valentini et al., 2019 Waheed et al., 2019 H. Zhang et al., 2020 Tensile tests on printed samples Adumitroaie et al., 2019 Ahmad et al., 2020 Akhoundi et al., 2019

PP (recycled) + up to 50 wt% gypsum powder PLA + continuous carbon fibers Polyhydroxyalkanoates + up to 3 wt% fibrillated nanocellulose ABS + up to 60 wt% micro-diamond PLA + continuous flax fibers

Anisoprint 25-27 vol% (composite) continuous carbon fibers in PLA ABS + 5 wt% oil palm fibers PLA + up to 49.3 vol% continuous E glass fibers

ASTM D3039 Customized Customized ASTM D4018

Customized ASTM D638 ASTM D638 (continued on next page)

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Fused Deposition Modeling of Composite Materials

Supplementary table 2a Tensile tests on composite filaments and printed parts—cont’d Reference Composite material Method Al Abadi et al., 2018 MarkForged continuous carbon fibers ASTM D3039 MarkForged continuous glass fibers Aw et al., 2018 Azarov et al., 2017 Azarov et al., 2019 Badouard et al., 2019 Berretta et al., 2017 Berretta et al., 2017 Bettini et al., 2017 Bhagia et al., 2020

MarkForged continuous Kevlar fibers ABS or conductive ABS (with 28 wt% carbon black) + 14 wt% ZnO Anisoprint 35 vol% (composite) continuous carbon fibers in PA Anisoprint 20 vol% (composite) continuous carbon fibers in PLA PLLA or PLLA/PBS or PBAT + 10 wt% flax shives or up to 30 wt% flax fibers PEEK + up to 5 wt% MWCNTs, single-layer PEEK + up to 5 wt% MWCNTs PLA + 8.6 vol% (9.5 wt%) continuous aramid fibers PLA + 20 wt% poplar wood flour

Butt and Shirvani, 2018

PLA + 15 wt% fibrillated cellulose from poplar tree MarkForged continuous carbon fibers (cut samples) Nylon + 6 wt% chopped carbon fibers (COMM.) ABS (neat polymer, COMM.)

Butt et al., 2020

Neat ABS + copper mesh(es) by Hybrid FDM (HFDM) PLA (neat polymer, COMM.)

Blok et al., 2018 Blok et al., 2018

ASTM D638 Customized Customized ISO 527 ASTM D 3379-75 ISO 527 Customized ASTM D638

ASTM D3039 ASTM D638 ISO 527

ISO 527

PLA + copper particles (COMM.)

Camargo et al., 2019 Chacón et al., 2019

Neat PLA + copper mesh(es) by Hybrid FDM (HFDM) PLA + graphene (COMM.) MarkForged continuous carbon fibers

ASTM D638 ASTM D3039

MarkForged continuous glass fibers Chávez et al., in press

MarkForged continuous Kevlar fibers PLA + 5 wt% of in-situ wire drawn SnBi particles

ASTM D638

ABS + up to 10 wt% of in-situ wire drawn SnBi particles

Chen and Zhang, 2019 Q. Chen et al., 2017 Cheng et al., 2021 Coppola et al., 2017

ABS blend + up to 10 wt% of in-situ wire drawn SnBi particles PLA + 6.95 wt% aluminum (COMM.) TPU + PLA blend + up to 5 wt% graphene oxide PLA + continuous twisted ramie yarns PLA + 4 wt% nanoclay

ASTM D638 Customized Customized ASTM D638 (continued on next page)

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395

Supplementary table 2a Tensile tests on composite filaments and printed parts—cont’d Reference Composite material Method Dal Maso and Cosmi, 2018 PLA + bronze powder (COMM.) ASTM D1822 Daver et al., 2016 Linear LDPE + 30 wt% de-vulcanised ASTM D638 rubber particles + 5 wt% maleic anhydride grafted PE + up to 5 wt% MWCNTs de Backer et al., 2018 PEI + continuous fibers Customized de Toro et al., 2020 PA6 + 20 wt% short carbon fibers ISO 527 (COMM.) Dickson et al., 2017 MarkForged continuous carbon fibers ASTM D3039 MarkForged continuous glass fibers Domm et al., 2017 Dorigato et al., 2017 Drummer et al., 2012 Dul et al., 2016 Fafenrot et al., 2017

MarkForged continuous Kevlar fibers PP + 30 vol% continuous glass fibers ABS + 6 wt% CNTs PLA + 2.5 wt% tricalcium phosphate ABS + up to 8 wt% graphene nanoplatelets PLA + 78 wt% bronze (COMM.)

Gao et al., 2019

PLA + 46 wt% iron (COMM.) Onyx MarkForged continuous carbon fibers PLA + 15 wt% chopped carbon fibers Sandwich structure with ABS printed core + carbon fiber-reinforced epoxy skins PLA + 2 wt% talc (COMM.)

Gardner et al., 2016

PLA + 5 wt% short carbon fibers (COMM.) ULTEM + continuous CNT yarn

Gkartzou et al., 2017

PLA + 5 wt% purified kraft pine lignin

Godec et al., 2020

Binder + 55 vol% 17-4PH steel, gas atomized powder MarkForged continuous carbon fibers

Fernandes et al., 2021 Fernandes et al., 2021 Ferreira et al., 2017 Galatas et al., 2018

Goh et al., 2018 Gray IV et al., 1998 Guessasma et al., 2019 He et al., 2020 Hedayati et al., 2020 Heidari-Rarani et al., 2019 Huang et al., 2019 Hwang et al., 2015a

MarkForged continuous glass fibers PP + thermotropic liquid crystalline polymers PLA + 10% hemp fibers (COMM.) MarkForged continuous carbon fibers PCL + 22 vol% continuous poly glycolic acid yarn PLA + continuous carbon fibers ABS + up to 5 wt% cellulose nanocrystals/silica nanohybrids ABS + up to 30 wt% copper

ISO 527 ISO 527 ISO 527 ISO 527 ISO 527 ASTM D638 D3039/D3039M ASTM D638 ASTM D638 ISO 527

ASTM D638 ASTM D1708 ASTM D638 (scaled by 0.6) Customized ASTM D3039 ASTM D638 Customized ASTM D3039 Customized ASTM D3039 GB/T 10402006 ASTM D638

ABS + up to 50 wt% iron (continued on next page)

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Fused Deposition Modeling of Composite Materials

Supplementary table 2a Tensile tests on composite filaments and printed parts—cont’d Reference Composite material Method Hwang et al., 2015b ABS + up to 30 wt% copper ASTM D638 Justo et al., 2018

ABS + up to 50 wt% iron MarkForged continuous carbon fibers

ASTM D3039

Love et al., 2014 Matsuzaki et al., 2016

MarkForged continuous glass fibers ABS + up to 35 vol% BaTiO3 PVDF + up to 15 wt% BaTiO3 PLA and polyhydroxyalkanoates + 15.2 wt% recycled wood fibers (COMM.) PLA + continuous flax yarns PLA + up to 20 vol% titanium PLA + continuous carbon fibers PLA + various (wood, ceramic, copper, aluminum and carbon fibers) (COMM.) ABS + 13% chopped carbon fibers PLA + continuous carbon fibers

ASTM D638 Customized

Melenka et al., 2016 Mihankhah et al., in press Milosevic et al., 2017

PLA + continuous twisted jute fibers MarkForged continuous Kevalr fibers PLA + up to 4 wt% montmorillonite PP and MAPP + up to 30 wt% hemp fibers

ASTM D638 DIN 53504-S3A ASTM D638

PP and MAPP + up to 30 wt% harakeke fibers PLA + up to 30 wt% wood flour

ASTM D638

Khatri et al., 2018 H. Kim et al., 2017 Le Duigou et al., 2016 Le Duigou et al., 2019 Lee et al., 2019 Li et al., 2016 Liu et al., 2019b

Montalvo Navarrete et al., 2018 Mori et al., 2014 Mosleh et al., 2021 Nájera et al., 2018 Nakagawa et al., 2017 Ning et al, 2015 Ning et al., 2017a Ning et al., 2017b

Papon and Haque, 2019 Pertuz et al., 2020

ASTM D638 Customized Customized ISO 527 ASTM D638 Customized Customized

PP + up to 30 wt% wood flour ABS + sandwiched continuous carbon fibers ABS + continuous carbon fibers (11.4 wt%) PLA and PCL + up to 1 wt% TiO2 ABS + sandwiched continuous carbon fibers ABS + up to 15 wt% short carbon fibers ABS + 5 wt% short carbon fibers

Customized ASTM D3039 ASTM D638 Customized

ABS + 5 wt% short carbon fibers (COMM.)

ASTM D638 ASTM D638 (scaled by 0.5) ASTM D638 (scaled by 0.5)

ABS + 5 wt% graphite (COMM.) PLA + up to 10 wt% milled carbon fibers MarkForged continuous carbon fibers

ASTM D5045 ASTM D638

MarkForged continuous glass fibers Pyl et al., 2018

MarkForged continuous Kevlar fibers MarkForged continuous carbon fibers

ASTM D638 ASTM D3039

Qian et al., 2018

PLA + 20 wt% Li0.44 Zn0.2 Fe2.36 O4 (LZFO)

Customized ASTM D638

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397

Supplementary table 2a Tensile tests on composite filaments and printed parts—cont’d Reference Composite material Method Ranjan et al., 2020 PLA + up to 20 wt% (hydroxyapatite + ASTM D638 chitosan) Rinaldi et al., 2021 PEEK + up to 10 wt% MWCNTs ASTM D638 Roberson et al., 2015 ABS and/or PC + various fillers (TiO2 ; ASTM D638 ZnO nanorods, MayaCrom blue) Sakunphokesup et al., 2019 ABS + 10 wt% graphene ASTM D638 Sanei et al., 2019 MarkForged continuous carbon fibers (X7 ASTM D3039 printer) Sang et al., 2019a PLA and PCL + 15 wt% basalt fibers ISO 527 Sang et al., 2019b PLA + up to 20 wt% basalt fibers ISO 527 Sathies et al., 2019 Schmitz et al., 2018

PLA + up to 20 wt% carbon fibers PLA + carbon black (COMM.) ABS + 3 wt% MWCNTs

ASTM D638 ISO 527

ABS + 3 wt% carbon black

Silva et al., 2019 Singh et al., 2016 R. Singh et al., 2019b

ABS + 3 wt% (50:50 MWCNTs + carbon black) PC + up to 5 wt% (0.3 vol%) tungsten micro-particles Shape memory polymer + continuous carbon fibers PA12 + 30 vol% continuous glass fibers PA6 + up to 50 wt% Al2 O3 Recycled ABS + 5 wt% banana fibers

Siqueiros and Roberson, 2017

Recycled PA6 + 5 wt% banana fibers PLA + up to 15 wt% of in-situ wire drawn phosphate glass particles

ASTM D638

ABS-PLA blend + up to 20 wt% of in-situ wire drawn phosphate glass particles ABS + up to 10 wt% TiO2 PA6 + up to 30 wt% TiO2 PLA + up to 6 wt% (MWCNTs and graphene nanoplatelets) PP (recycled) + up to 50 wt% hemp fibers

ASTM D638 ASTM D638 Customized

Shemelya et al., 2015 Shen et al., 2019

Skorski et al., 2016 Soundararajan et al., 2019 Spinelli et al., 2020 Stoof and Pickering, 2017

ASTM D638 ASTM D3039 Customized ASTM D638 ASTM D638

Customized

PP (recycled) + up to 50 wt% harakeke fibers

Stoof et al., 2017

Sugiyama et al., 2018

PP (recycled) + up to 50 wt% gypsum powder PP (recycled) + up to 30 wt% hemp fibers PP (recycled) + up to 30 wt% harakeke fibers MarkForged continuous carbon fibers (custom printer)

Customized

JIS K 7165 (continued on next page)

398

Fused Deposition Modeling of Composite Materials

Supplementary table 2a Tensile tests on composite filaments and printed parts—cont’d Reference Composite material Method Tao et al., 2017 PLA + 5 wt% wood flour ASTM D638 Tekinalp et al., 2014 ABS + up to 40 wt% chopped carbon fibers ASTM D638 Torrado et al., 2015 ABS + up to 5 wt% various additives/fillers ASTM D638 Torrado Perez et al., 2014

(ABS + up to 30 wt% other polymers) ABS + 5 wt% jute fibers

ASTM D638

ABS + 5 wt% TiO2 Ueda et al., 2020 U¸sun and Gümrük, 2021 Valentini et al., 2019 van der Klift et al., 2016 Vidakis et al., 2020

Vinyas et al., 2019a Vinyas et al., 2019b Wang et al., 2016 Weng et al., 2016 Xiang et al., 2019 Xiao et al., 2019 Yang et al., 2017 Yang et al., 2019 Yasunaga et al., 2018 Yu et al., 2019 Yu et al., 2020 Zhang et al., 2018 X. Zhang et al., 2019 J. Zhang et al., 2020 Zhong et al., 2001

(ABS + 5 wt% TPE) MarkForged continuous carbon fibers, compaction roller PLA + continuous carbon fibers Polyhydroxyalkanoates + up to 3 wt% fibrillated nanocellulose MarkForged continuous carbon fibers ABS + up to 20 wt% nano-ZnO (< 50 nm) ABS + up to 20 wt% micro-ZnO (< 5 μm) Various polymers and PLA-matrix composites (COMM.) Various PLA-matrix composites and blends (COMM.) Polywax + 20 wt% expandable glass micro-spheres ABS + up to 5 wt% montmorillonite (mod.) TPU + up to 5 wt% CNTs, with pyrenecarboxylic acid PLA and PBAT with EGMA blend + up to 40 phr hemp hurd ABS + continuous carbon fibers (around 10 wt%) PLA + up to 8 wt% CNTs PLA + continuous carbon fibers (yarn) MarkForged continuous carbon fibers + Onyx PLA + carbon fibers (COMM.) ABS + 7 wt% carbon nanotubes ABS + 15 wt% short carbon fibers PLA + 6.95 wt% aluminum micro-fibers (COMM.) PLA + 10.3% continuous carbon fibers, pressure roller ABS, modified, + up to 18 wt% short glass fibers

ISO 527 ASTM D3039 (scaled by 0.5) ISO 527 Customized ASTM D638

ASTM D638 ASTM D638 ISO 527 ASTM D638 ISO 37 ASTM D638 ISO 527 (smaller) ISO 527 Customized Customized Customized ASTM D3039 ASTM D638 GB/T 1040.1-2006 Customized

Fused deposition modeling of composite materials at a glance – supplementary tables

399

12.3.2 Supplementary table 2b – bending tests

Supplementary table 2b Bending tests on composite filaments and printed parts; interlaminar shear strength tests. Reference Bending tests on filaments Bi et al., 2020 Gregor-Svetec et al., 2020 Korte and Quodbach, 2018 Ponsar et al., 2020

Wu et al., 2017 J. Zhang et al., 2017 Bending tests on printed samples Ahmad et al., 2020 Ahrendt and Romero Karam, 2020 Araya-Calvo et al., 2018 Blok et al., 2018 Blok et al., 2018 Butt et al., 2020

Composite material

Method

PA6 + continuous carbon fibers HDPE + up to 75 wt% cardboard dust Ammonio-methacrylate copolymer A + 30 wt% theophylline + plasticizer + silica Ethyl cellulose + 16.67 wt% hypromellose + plasticizer + 0.4 wt% silica ABS and surfactant + up to 33.2 vol% BaTiO3 PLA + 30 wt% acetaminophen + additives (various)

ISO 3597 Customized Customized

ABS + 5 wt% oil palm fibers PETG + 20% carbon short fibers (COMM.) MarkForged continuous carbon fibers MarkForged continuous carbon fibers Nylon + 6 wt% chopped carbon fibers (COMM.) PLA (neat polymer, COMM.)

Customized

Customized Customized

ASTM D790 ISO 14125 ASTM D790 ASTM D7264 ASTM D7264 ISO 178

PLA + copper particles (COMM.)

Camargo et al., 2019 Chacón et al., 2019

Neat PLA + copper mesh(es) by Hybrid FDM (HFDM) PLA + graphene (COMM.) MarkForged continuous carbon fibers

ASTM D790 ASTM D790

MarkForged continuous glass fibers Daver et al., 2016

MarkForged continuous Kevlar fibers Linear low-density polyethylene + 30 wt% devulcanized rubber particles

ASTM D790

de Toro et al., 2020

+ 5 wt% maleic anhydride grafted polyethylene + up to 5 wt% MWCNTs PA6 + 20 wt% short carbon fibers (COMM.) MarkForged continuous carbon fibers

ISO 178

Dickson et al., 2017

ASTM D790

MarkForged continuous glass fibers MarkForged continuous Kevlar fibers (continued on next page)

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Supplementary table 2b Bending tests on composite filaments and printed parts; interlaminar shear strength tests—cont’d Reference Domm et al., 2017 Fafenrot et al., 2017

Composite material PP + 30 vol% continuous glass fibers PLA + 78 wt% bronze (COMM.)

Method ISO 14125 ISO 178

Goh et al., 2018

PLA + 46 wt% iron (COMM.) MarkForged continuous carbon fibers

ASTM D790

He et al., 2020 Heidari-Rarani et al., 2019 Hu et al., 2018 Huang et al., 2019 Khatri et al., 2018 S. Kumar et al., in press b Le Guen et al., 2019 Li et al., 2016 Liu et al., 2019b Montalvo Navarrete et al., 2018 Mosleh et al., 2021 Nabipour et al., 2020 Nakagawa et al., 2017 Ning et al., 2015 Ranjan et al., 2020 Sang et al., 2019b Silva et al., 2019 R. Singh et al., 2019a Somireddy and Czekanski, 2020 Tian et al., 2016a Tian et al., 2016b Ueda et al., 2020 U¸sun and Gümrük, 2021

MarkForged continuous glass fibers MarkForged continuous carbon fibers PLA + continuous carbon fibers PLA + continuous carbon fibers ABS + up to 5 wt% cellulose nanocrystals/silica nanohybrids ABS + up to 35 vol% BaTiO3 PA6 + (30 wt%) TiO2 PLA + 10 wt% rice husks PLA + 10 wt% Pinus wood flour PLA + continuous carbon fibers PLA + various (wood, ceramic, copper, aluminium, and carbon fibers) (COMM.) PLA + up to 30 wt% wood flour PP + up to 30 wt% wood flour ABS + continuous carbon fibers (11.4 wt%) PE + up to 75 wt% copper particles (< 20 μm) ABS + sandwiched continuous carbon fibers ABS + up to 15 wt% short carbon fibers PLA + up to 20 wt% (hydroxyapatite + chitosan) PLA + up to 20 wt% basalt fibers PLA + up to 20 wt% carbon fibers PA12 + 30 vol% continuous glass fibers PLA + 5 wt% (PEKK + hydroxyapatite + chitosan) ABS + short carbon fibers (COMM.) ABS + continuous carbon fibers (around 10 wt%) PLA + continuous carbon fibers (around 27%) MarkForged continuous carbon fibers, compaction roller PLA + continuous carbon fibers

Customized ASTM D790 GB/T 1449 GB/T 9341 ASTM D7264M ASTM D790 Customized Customized Customized ASTM D790 ASTM D790 ASTM D790 Customized ASTM D790 ASTM D790 ASTM D790 ASTM D790 ASTM D790 ASTM D7264 Customized ISO 14125 ISO 14125 ISO 14125 (continued on next page)

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Supplementary table 2b Bending tests on composite filaments and printed parts; interlaminar shear strength tests—cont’d Reference Vaneker, 2017 Vidakis et al., 2020

Composite material PP + E-glass commingled yarn (around 60 wt%, COMM.) ABS + up to 20 wt% nano-ZnO (< 50 nm)

ABS + up to 20 wt% micro-ZnO (< 5 μm) Weng et al., 2016 ABS + up to 5 wt% montmorillonite (mod.) Xiao et al., 2019 PLA and PBAT with EGMA blend + up to 40 phr hemp hurd Yang et al., 2017 ABS + continuous carbon fibers (around 10 wt%) Yang et al., 2019 PLA + up to 8 wt% CNTs Yu et al., 2019 MarkForged continuous carbon fibers + Onyx Yu et al., 2020 PLA + carbon fibers (COMM.) H. Zhang et al., 2020 PLA + continuous flax fibers, flat parts H. Zhang et al., 2020 PLA + continuous flax fibers, curved parts J. Zhang et al., 2020 PLA + 10.3% continuous carbon fibers, pressure roller Inter-laminar shear strength (ILSS) tests on printed samples Azarov et al., 2019 Anisoprint 20 vol% (composite) continuous carbon fibers in PLA Berretta et al., 2017 PEEK + up to 5 wt% MWCNTs Caminero et al., 2018b

MarkForged continuous carbon fibers

Method ISO 14125 ASTM D790

ASTM D790 ASTM D790 ISO 14125 ISO 178 ASTM D6272 Customized ISO 14125 Customized GB/T 1449: 2005 Customized ASTM D2344/ D2344M ISO 14130

MarkForged continuous glass fibers Domm et al., 2017 Fernandes et al., 2021

MarkForged continuous Kevlar fibers PP + 30 vol% continuous glass fibers Onyx (PA + short carbon fibers)

Fernandes et al., 2021

MarkForged continuous carbon fibers

He et al., 2020 Mosleh et al., 2021

MarkForged continuous carbon fibers ABS + continuous carbon fibers (11.4 wt%) MarkForged continuous carbon fibers

O’Connor and Dowling, 2019 Yang et al., 2017

MarkForged continuous glass fibers MarkForged continuous Kevlar fibers ABS + continuous carbon fibers (around 10 wt%)

ISO 14130 ASTM D2344/ D2344M ASTM D2344/ D2344M ASTM D5528 ASTM D2344/ D2344M ASTM D2344/ D2344M ISO 14130

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Fused Deposition Modeling of Composite Materials

Common abbreviations 3D 4D ABS AM ASA ASTM BAAM CAD CNC CNT CT DCM DED DIY DMA DMF DOE DSC DTA EGMA EMI EVA FDC FDM FDMet FE FGM GO GNP GPC HAp HDPE HSHT ILSS LDPE LENS LIRF ME MFI MFR MJF MWCNT MVR PBF PA PA6 PAN PAEK

Three-dimensional Four-dimensional Acrylonitrile butadiene styrene Additive manufacturing Acrylonitrile styrene acrylate American society for testing and materials Big area additive manufacturing Computer aided design Computer numerical control Carbon nanotube Computed tomography Dichloromethane Direct energy deposition/Directed energy deposition Do it yourself Dynamic mechanical analysis Dimethylformamide Design of experiment Differential scanning calorimetry Differential thermal analysis Ethylene-methyl acrylate-glycidyl methacrylate terpolymer Electromagnetic interference Ethylene-vinyl acetate Fused deposition of ceramics Fused deposition modeling Fused deposition of metals Finite element Functionally graded material Graphene oxide Graphene nanoplatelets Gel permeation chromatography Hydroxyapatite High-density poly(ethylene) High strength high temperature Interlaminar shear strength Low-density poly(ethylene) Laser-engineered net shaping Locally induced radio frequency Material extrusion Melt flow index Mass flow rate Multi-jet Fusion Multi-wall carbon nanotube Mel volume rate Powder Bed Fusion Polyamide Polyamide, also known as Nylon 6 Polyacrylonitrile Poly-aryl ether ketone

Fused deposition modeling of composite materials at a glance – supplementary tables

PBAT PBS PC PCL PEEK PEG PEgMA PEI PEKK PES PET PETG PGA PHA PIM PLA PLLA PMMA POE-g-MAH POM PP PPE PS PU PVDF PZT QR RF ROS SBF SCARA SDS SEBS SEM SLA SLS SMP TCP TEAM TEM TGA THF TLCP TPE TPU UFP UHMWPE UV XRD

Poly-(butylene adipate-co-terephthalate) Poly-(butylene-succinate) Polycarbonate Polycaprolactone Polyether ether ketone Poly-(ethylene glycol) Polyethylene grafted maleic anhydride Polyetherimide Poly-ether ketone ketone Polyethersulfone Polyethylene Terephthalate Glycol-modified polyethylene terephthalate Poly-(glycolic acid) Poly-(hydroxyalkanoate) Powder injection molding Poly-(lactic acid) Poly-(L-lactic acid) Polymethyl methacrylate Polyolefin elastomer grafted maleic anhydride Polyoxymethylene Polypropylene Personal protection equipment Polystyrene Polyurethane Poly-(vinylidene) fluoride Lead zirconate titanate Quick response (code) Radio frequency Reactive oxygen species Simulated body fluid Selective compliance assembly robot arm Shaping debinding and sintering Styrene ethylene butylene styrene Scanning electron microscope/microscopy Stereo Lithographic Apparatus Selective Laser Sintering Shape memory polymer Tricalcium phosphate Thermoplastic elastomer additive manufacturing Transmission electron microscope/microscopy Thermo-gravimetric analysis Tetrahydrofuran Thermotropic liquid crystalline polymers Thermoplastic elastomer Thermoplastic polyurethane Ultra-fine particles Ultra-high molecular weight polyethylene Ultraviolet X-ray diffraction

403

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Non-Print Items Abstract The fused deposition modeling (FDM, aka fused filament fabrication, FFF) of composite materials and fully inorganic parts is gaining momentum as an effective approach for merging the geometric freedom that is typical of additive manufacturing (AM) with the versatility in functionality that is one of the main advantages of composite materials. Driven by the Industry 4.0 revolution, the development of new composite filaments for FDM represents a fast-growing field of research with hundreds of scientific papers published every year. This chapter includes several supplementary tables in order to provide the reader with an agile description of the state of the art. Supplementary tables 1a to 1e collect details about review papers, experimental research in the FDM of neat polymers and composite materials, development of continuous fiber-reinforced parts, advancement of shaping, debinding and sintering (SDS) for the obtainment of fully inorganic parts, and other relevant research activities. Supplementary Tables 2a and 2b focus instead on the characterization methods applied in the literature to estimate the mechanical properties (tensile and flexural) of composite parts produced by FDM. These supplementary tables are intended to be a handy reference where the reader can find practical information about various kinds of composites, how to process them into a filament and how to characterize the FDM parts printed with them. Keywords Fused deposition modeling; FDM; Fused filament fabrication; FFF; Additive manufacturing; Shaping, Debinding and sintering; Composite material; Mechanical testing; Characterization methods

Index

Page numbers followed by “f” and “t” indicate, figures and tables respectively. A Acrylonitrile butadiene styrene (ABS), 12, 131 filament, 155 matrix composites, 134, 178 Active material loading, 101 Additive manufacturing (AM) approach, 7 technologies, 8 Agglomeration phenomena, 133 Alternative fillers and additives, 93 Alumina particles, 130 Anisotropy, 307 Antimicrobial properties, 157 Archimedes’ principle, 110 Automated systems, 100, 99 B Bending tests, 114 Bi-dimensional geometry, 142 Big Area Additive Manufacturing (BAAM) system, 16 Bioactive glasses, 153 material, 146 Bioactivity biological properties, 146 Biomedical applications, 180 Biphasic calcium phosphates (BCPs), 152 C Capillary rheometers, 112 Carbonaceous fillers, 134, 144, 158 Carbon nanotubes, 135 Characterization techniques, 109 Composite filament, 112 material, 89

Computer numerical control (CNC), 7, 27 Conductive polymers, 161 Continuous fibers, 91 Co-rotating systems, 97 Creep recovery test, 112 Crystallization phenomena, 98, 111 Custom-made polymer capsules, 98 D Debinding process, 90 Differential mechanical analysis (DMA), 114 Differential scanning calorimetry (DSC), 113 Differential thermal analysis (DTA), 113 Direct rapid tooling, 176 Discrete fillers, 91, 129 Dual extrusion method, 212, 228 Dual extrusion technology, 213 Dual-nozzle method, 91, 92 printers, 16 printheads, 17f Dynamic mechanical analysis (DMA) tests, 178 E Electrical conduction, 137 Electrical conductivity, 136 Electrically conductive fillers, 140 Electrically conductive materials, 182 Electrifi, 183 Engineering polymers, 12 Experimental filaments mechanical properties, 113 Extruder, 102 Extrusion induced orientation, 110 parameters, 99 process, 96

446

F Fabrication methods, 238 FEAM process, 233f Feedstock material, 90 Fiber-reinforced filaments, 110 Filaments, 90, 114 buckling, 24f diameter, 99 monitoring, 99 spooling, 98 Fill compositing method, 304 Filler geometry, 91 loading optimization, 292 loadings, 89 matrix interaction, 132 Finite element (FE) simulations, 25, 145 Fluctuations, 99 Frenkel-Eshelby sintering process, 31f Friction welding, 185 Fused deposition modeling, 1, 289, 329 advantages of, 11t, 12 cost and quality considerations, 12 features, 13t illustration, 15f Fused filament fabrication (FFF), 1, 2, 289, 329 G Gas pycnometry, 110 Gel permeation chromatography (GPC), 113 Graphene-based lithium-ion anode, 141 Graphene nanoplatelets (GPNs), 141 H HAp adhesion, 150 Healing phenomena, 135 Heat conduction, 144 High-magnification instruments, 109 High-quality composite filaments, 90 Hotmelt compounding method, 91, 94, 95 Hydroxyapatite, 146 Hygroscopic polymer matrixes, 99

Index

International Space Station, 122 Isotropy, 307 L Liquid deposition modelling (LDM), 7 “Longitudinal resistance,”, 113 Low-density polyethylene (LDPE) filaments, 130 Low-voltage devices, 136 M Magnetic filaments, 98 Magnetic Iron filaments, 179 Magnetic resonance images, 163 Markforged Mark One system, 214 Markforged technology, 214 Masterbatch, 95 “Material extrusion” (ME), 7, 12 Matrix-filler interface, 100f, 100 Mechanical reinforcement, 130 Melt flow index (MFI), 296 Melt mass-flow rate (MFR), 112 Melt volume rate (MVR), 112 Mercury porosimetry, 110 Metal fillers, 175 Metal-oxide particles, 180 Mineral fillers, 131 Montmorillonite, 132, 133 nanoparticles, 132 Multi-directional sensors, 139 Multi-layered and sandwich structures, 234 Multiple screw extruders, 96 Multi-polymer core-shell filaments, 92 Multi-walled carbon nanotubes (MWCNTs), 140 N Nanocellulose, 199 Nanoclays, 133 Nanofillers, 142 polymer matrix, 148 Natural fibers, 100, 189, 190, 192, 200 reinforced composites, 204f structure and properties, 189

I “In-nozzle impregnation” method, 220, 222 In-process strengthening methods, 303

O Orthopedic devices, 139

Index

P “Percolation threshold,”, 137 Pharmaceutical-grade thermoplastic filaments, 93 Piezoelectric pressure sensors, 159 PLA-graphene absorber, 160 PLA matrix, 101 composite filaments, 193 Plasticizers, 101, 131 PLA-zinc oxide, 101 Polyether ether ketone (PEEK)-short carbon fiber composites, 135 Polymers, 93, 96 based biomedical devices, 146 matrixes, 96, 100, 113, 133, 136, 137, 138f degradation, 146 metal composite filaments, 176 pellets, 93 sintering, 30 Polypropylene, 142 Post-printing thermal annealing, 32 Printing-related specificities, 138 Printing techniques, 299 Processing-induced defects, 138 Q Quality assurance, 121 R Raster angle, 28f strategies, 29f Reactive materials, 184 Reactive oxygen species (ROS), 155 Reference materials, 137t Relaxation-stress test, 112 Response surface methodology (RSM), 131 Rheological tests, 112 S Selective Compliance Assembly Robot Arm (SCARA), 14 Selective laser sintering (SLS), 12 Short beam shear stress (SBSS) tests, 135 Short natural fibers, 194 Single screw extruders, 96 Sintering-induced shrinkage phenomena, 90 Size-exclusion chromatography (SEC), 113

447

Smart materials, 314 Smart solution, 157 Solvent mixing, 95 approach, 96 method, 94 technique, 96 Storage modulus, 114 Surface finishing, 185 Surface treatments, 19, 101 Surfactants, 101 Swelling effect, 98 phenomenon, 26f, 26 Synchrotron X-ray techniques, 148 T Temperature-driven process, 30 Tensile tests, 113, 152 Thermal conductivity, 140 Thermal percolation threshold, 135 Thermal tests, 139 Thermodilatometric compatibility, 302 Thermogravimetric analysis (TGA), 113 Thermoplastic poly-urethane (TPU) filament, 140 Time-dependent phenomena, 110 Titania, 154, 155 Transesterification reactions, 101 Transmission electron microscope (TEM), 109 Tricalcium phosphate, 151 Triple particle size (TPS) systems, 131 Twin-screw extruders, 97f, 97 “Twisting model,”, 224f Two-stage verification method, 121 U Ultrasound-based methods, 111 V Variable stiffness composite structure (VSCS), 312 Vegetable fibers, 202 W Wax-graphite composites, 163 Wood flour, 196

448

Index

X

Z

X-ray computed tomography, 110 X-ray diffraction (XRD), 111 X-ray radiation shielding capability, 182 X-ray shielding and aerospace industry, 181

Zinc oxide, 155 based nanoparticles, 132 nanoparticles, 157

Y Young’s modulus, 133, 140