**About the Editors**

**Swee Leong Sing** is a Presidential Postdoctoral Fellow at the Singapore Centre for 3D Printing (SC3DP) and School of Mechanical and Aerospace Engineering (MAE), Nanyang Technological University (NTU), Singapore. He has been active in the 3D printing field for more than 8 years. He obtained his BEng (Hons) in Aerospace Engineering and PhD in Mechanical Engineering with a topic in additive manufacturing (AM) in 2012 and 2016, respectively. Swee Leong's research focuses on using advanced manufacturing techniques as enablers for materials development and to create strategic values for the industries. He is also active in inter-disciplinary research and translational work. His research has been awarded the Best PhD Thesis Award by MAE, NTU, Singapore as well as the Springer Theses Award from Springer Nature, Germany in 2017. Swee Leong has worked on numerous 3D printing projects with government agencies, universities, research institutes and industrial collaborators. Swee Leong has filed five patents pertaining to 3D printing processes and materials. He has published 1 book, 4 book chapters and more than 50 peer reviewed journal and conference articles. Swee Leong currently has a h-index of 25 with more than 3000 citations (Web of Science).

**Wai Yee Yeong** Active in 3D printing research since 2004, Associate Professor Wai Yee Yeong has created multiple frontiers in 3D printing, taking the leads in 3D bioprinting, electronics printing and metal printing research. Prof Yeong is currently serving as Associate Chair (Students) at School of Mechanical and Aerospace Engineering (MAE), Nanyang Technological University (NTU), Singapore. She has been awarded with research grant in the order of >USD 10 million and her works have been featured on media such as CNA, The Straits Times and other channels. Her portfolio includes serving as Programme Director (Aerospace and Defence) at Singapore Centre for 3D Printing (SC3DP), Program Director (3D Printing) at NTU-HP Digital Manufacturing Lab and Co-director of NTU Institute for Health Technologies. Her research and innovative leadership are well-recognized internationally, serving as Technical Chair for international conferences and Associate Editor for the top tier journal, Virtual and Physical Prototyping. She is the inaugral winner for the TCT-Women in 3D Printing Innovator Award in 2019 for her overall achievement in 3D printing. She has also been named as a finalist for Lush Prize (Science Category) in 2018 for her work in bioprinting of tissue models to replace animal testing. She has filed more than eight patents and know-hows, creating new processes, 3D printable metal alloy and hydrogel. At scholarly front, she has co-authored 3 textbooks, 7 book chapters, and over 150 technical papers with more than 5000 citations and h-index of 38.

### *Editorial* **Process–Structure–Properties in Polymer Additive Manufacturing**

**Swee Leong Sing \* and Wai Yee Yeong**

Singapore Centre for 3D Printing, School of Mechanical & Aerospace Engineering, Nanyang Technological University, 50 Nanyang Ave, Singapore 639798, Singapore; wyyeong@ntu.edu.sg **\*** Correspondence: sing0011@e.ntu.edu.sg or slsing@ntu.edu.sg

Additive manufacturing (AM) methods have grown and evolved rapidly in recent years. AM for polymers is an exciting field and has great potential in transformative and translational research in many fields, such as biomedical, aerospace, and even electronics. Current methods for polymer AM include material extrusion, material jetting, vat polymerisation, and powder bed fusion [1,2].

With the promise of more applications, detailed understanding of AM, from the processability of the feedstock to the relationship between the process-structure-properties of AM parts, have become more critical. More research work is needed in material development to widen the choice of materials for polymer additive manufacturing [3]. Modelling and simulations of the process will allow the prediction of microstructures and mechanical properties of the fabricated parts while complementing the understanding of the physical phenomena that occurs during the AM processes.

In this Special Issue, state-of-the-art review and current research, which focus on the process–structure–properties relationships in polymer additive manufacturing, are collated. In their review article, Dickson et al. looked into using fused filament fabrication, a type of material extrusion AM, for the fabrication of parts with fibre-reinforced thermoplastic composites [4]. For original research, Bere et al. looked into the manufacturing of complex parts with fibre-reinforced polymers using moulds fabricated by material extrusion AM [5]. Chen et al. looked into the crystallisation and thermal behaviours of poly(ethylene terephthalate)/biphenols complexes through melt post-polycondensation [6]. Abdolmaleki and Agarwala looked into using polyvinylidene difluoride added with barium titanate (PVDF–BaTiO3) for printed electronics [7]. Nagarajan et al. explored a methodology for the manufacture of magnetic composites using an in-house developed material jetting AM machine [8]. Sahay et al. demonstrated the use of a parallel plate for electrospinning-based AM in the fabrication of helicoidally arranged polyacrylonitrile fibre-reinforced polyvinyl alcohol polymer thin films [9]. Platek et al. analysed the deformation process of regular cell structures, manufactured using fused filament fabrication under quasi-static loading conditions [10]. Udroiu and Braga explored a new methodology for process capabilities analysis for material jetting AM [11]. Luis et al. studied the fabrication of silicone meniscus implants using a novel heat cured material extrusion technique [12]. Stoia et al. studied the mode I fracture toughness of polyamide and alumide samples fabricated using selective laser sintering [13]. Lastly, Andreaczyk et al. have developed a novel method for the experimental validation of numerically optimised turbomachinery components that are fabricated using AM [14].

**Acknowledgments:** This research is supported by the National Research Foundation, Prime Minister's Office, Singapore under its Medium-Sized Centre funding scheme.

**Conflicts of Interest:** The authors declare no conflict of interest.

**Citation:** Sing, S.L.; Yeong, W.Y. Process–Structure–Properties in Polymer Additive Manufacturing. *Polymers* **2021**, *13*, 1098. https:// doi.org/10.3390/polym13071098

Received: 1 March 2021 Accepted: 29 March 2021 Published: 30 March 2021

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