Mechanical Properties and Applications of Additively Manufactured Thermoplastic Polyurethane Material A thesis submitted in fulfilment of the requirements for the degree of Master of Engineering Tao Xu Bachelor of Engineering, Tongji University School of Engineering College of Science, Technology, Engineering and Maths RMIT University August 2021 Declaration I certify that except where due acknowledgement has been made, the work is that of the author alone; the work has not been submitted previously, in whole or in part, to qualify for any other academic award; the content of the thesis is the result of work which has been carried out since the official commencement date of the approved research program; any editorial work, paid or unpaid, carried out by a third party is acknowledged; and, ethics procedures and guidelines have been followed. Tao Xu 23 August 2021 i Acknowledgements First of all, I would like to give my heartfelt thanks to my senior supervisor, Professor Yi Min (Mike) Xie, who provided me with the opportunity to study at RMIT and always helped me with my research. It has been a great privilege and joy to study under his guidance and supervision. I am particularly impressed by his sharp mind for detecting extremely detailed problems that exist in my research, which profoundly inspires me to be a rigorous and responsible researcher.
Professor Xie is not only my academic leader but also my life mentor. I am sure that I will forever beneĄt from his direction and treasure it all of my life. My sincere and hearty thanks and appreciation also go to my associate supervisor, Dr Xiaoshan (Susanna) Lin. After every dayŠs busy and exhausting work, she still devoted her considerate care and immense vigour in the supervision of my thesis writing, including the suggestions on wording, the help in forming the structure, and the efforts to the reĄnement of ideas in my thesis.
Without her keen insights and constant encouragement, the thesis would not have been Ąnished. I would like to thank all my current and former colleagues. I miss lunchtime with Yulin Xiong, Hu Xu, Wei Li, Anbang Chen, Wenzheng Xu, Yunzhen he, Qi Cai, Zicheng Zhuang, Minghao Bi and Zhonggao Chen. I wish to particularly thank Mr Dingwen Bao for giving me considerable care during my life in Melbourne.
I am grateful for the support and company from my friends Zhouhan Jia, Hange Yang, Shangchi Yao, Rong Peng, Chenxing Zuo, Qi Lu, Jiajun Fan, Bo Fan, Mengge Cai, Zhi Li, Yilun Chen, Jiezhuoma La, Xinrong Jiang, and Xiaohan Sun. I would like to express my ii appreciation to my former teacher Mr Ercüment Görgül, who introduced me to Professor Xie personally and offered me a lot of help and advice in my career. I am thankful to Nanjing Ameba Engineering Structure Optimization Research Insti- tute for providing technical support. I thank Mr Wei Shen for arranging the workspace and experimental equipment and Mr Donghui Wang for helping me operate machines and conduct manufacturing.
I thank my beloved girlfriend Yingqi Li, who has been with me and brought me count- less laughs and happiness. We met in spring, however, when I saw her, a poem by Shake- speare appeared in my mind: "Shall I compare thee to a summerŠs day?" I would like to express my appreciation for the funding for my research from the Aus- tralian Research Council (grant number: FL190100014). Finally, but most importantly, I would like to thank my parents, who offer me uncondi- tional support and encourage me to pursue my dreams. I will keep your love in the quietest place in my heart.
iii Contents 1 Introduction 1 1.1 Two Commonly Used Additive Manufacturing Technologies .2 Additively Manufactured TPU Material .3 Objectives and Scope .4 Layout of Thesis .1 Chemical Properties of TPU .3 Thermal Behaviour of TPU Material .4 Mechanical properties of TPU Material. 13 3 Tensile Tests of TPU Dumb-Bell Specimens Printed with Various Parameters 16 3.2 Materials and Testing Methods .3 Tensile Test Setup .4 Scanning Electron Microscope .5 Particle-Size Distribution .6 Melt Flow Rate .7 Differential Scanning Calorimetry .3 Results and Discussion .1 Effect of Build Orientation .2 Effect of Post-processing .3 Effect of Powder Mix Ratio. 35 4 A Novel Low-cost and Environmentally Friendly Method for Concrete Cast- ing Using Additively Manufactured TPU Moulds 37 4.1 Topologically Optimized 2D cantilever .2 3D Free-form Column .2 Pre-processing of the TPU Moulds .3 Casting and Demoulding .4 Manufacturing WorkĆow. 55 5 Conclusions 58 Bibliography 61 Appendix A Python Program for 2D Topology Optimization 72 A.1 Boundary Conditions and Stiffness Matrix .2 Finite Element Solver.
83 v List of Figures 1.1 Schematic of selective laser sintering.2 Schematic of fused deposition modelling.3 TPU materials for additive manufacturing: (a) TPU powder for SLS (im- age credit: SINTERIT); (b) TPU granules for FDM (image credit: Kingoda) and (c) TPU Ąlament for FDM (image credit: SUNLU).1 Two-step synthesis of polyurethanes: (a) Synthesis of urethane prepoly- mer via reacting two diisocyanates with polyols. (b) Synthesis of polyurethane via reacting the prepolymer with chain extenders.2 Particle shapes attainable by different powder generation processes (Schmid and Wegener [2016]).3 Typical DSC-Thermogram with nature of Śsintering windowŠ as LS pro- cess temperature (Schmid and Wegener [2016]).4 SEM image of an SLS part surface (Launhardt et al.1 Shape of the dumb-bell test specimens.2 3D printed dumb-bell specimens.3 Three build orientations.4 Comparison of specimen surfaces: (a) specimen with post-processing and (b) specimen without post-processing. 22 vi LIST OF FIGURES 3.5 Tensile test setup.6 Illustration of sieve analysis.7 Stress-strain curves of specimens printed in three different directions.8 Average and standard deviation of the results: (a) Maximum tensile strength and (b) strain at break.9 Moduli of specimens printed in different orientations.10 Illustration of selective laser sintering (SLS) processing.11 Comparison of the average tensile strength. ŤYŤ indicates specimens with post-processing, and ŤNŤ means specimens without post-processing.12 Comparison of the average strain at break.
ŤYŤ indicates specimens with post-processing, and ŤNŤ means specimens without post-processing.13 Comparison of moduli of specimens at different strains. ŞYŤ indicates specimens with post-processing, and ŤNŤ means specimens without post- processing.14 Comparison of (a) maximum tensile strength and (b) strain at break.15 Microstructure of new TPU powder under different magniĄcations: (a) 500×; (b) 1000× and (c) 2000×.16 Microstructure of reused TPU powder under different magniĄcations: (a) 500×; (b) 1000× and (c) 2000×.17 Particle size distribution results for the new powder and the reused powder.18 Differential scanning calorimetry (DSC) traces of the new powder and the reused powder: (a) heating traces and (b) cooling traces.1 Boundary conditions of the cantilever.2 Topology optimization result.3 Smoothed shape of the topology optimization result.4 Extruded solid model.5 TPU Mould Design .6 Block design and cutting directions of the mould.8 Geometrical articulations of NerviŠs column.9 Mould design for the 3D column: white parts are the pieces of the mould, and the black part is NerviŠs column. 46 vii LIST OF FIGURES 4.10 Six sections of the mould and the corresponding bases.11 Mould printing with FDM technique.12 Additively manufactured TPU mould of the 2D cantilever.13 Additively manufactured TPU mould of 3D column (top part).14 Water-based putty on the surface of the mould.15 Machine oil (red) on the water-based putty.16 Pre-processing of the mould for 3D column (top part): (a) Cutting the mould into four pieces; (b) Appling TeĆon release paper on the internal surfaces; (c) Assembling the four pieces on a board.19 Demoulding process of the 2D cantilever.20 After demoulding of the 2D cantilever: (a) TPU mould after demoulding; (b) 2D concrete cantilever; (c) Surface of the 2D concrete cantilever.21 Six sections of NerviŠs column.22 Assembled NerviŠs column: (a) Assembled column; (b) Surface of the column. 57 viii List of Tables 3.1 Dimensions of the dumb-bell specimens.2 Processing parameters used in this study.3 Eight sets of specimens printed with different processing parameters.1 Comparison of water-based putty and oil-based putty.
50 ix Abstract Thermoplastic polyurethane (TPU) is a polymer material that has high ductil- ity, good biocompatibility and excellent abrasion resistance. These properties open a pathway to manufacturing functional TPU parts for applications in var- ious Ąelds, such as aerospace engineering, medical devices and sports equip- ment. The mechanical properties of additively manufactured TPU material can be affected by various parameters, such as the build orientation, the mix ratio of new and reused printing powder, and whether the printed part is post- processed. The settings of printer parameters, i., printing speed, printing path, and processing temperature, have been the focuses of previous studies.
However, the inĆuences of other factors have not been systematically investi- gated, which would limit the application of TPU material. In this project, the mechanical properties of additively manufactured TPU material affected by three different processing parameters are Ąrstly investigated, including build orientation, mix ratios of the new and reused powder and post-processing. Then, additively manufactured TPU moulds are applied to cast complex con- crete structures. A series of tensile tests are conducted on TPU dumb-bell specimens.
It is found that the mix ratio of the new and reused powder is the most critical factor in the mechanical properties of the printed TPU parts. Compared to reused powder, new powder has plumper particle and properer thermal be- haviour that is more suitable for the additive manufacturing process. Besides, TPU parts printed in Ćat and on-edge orientations show better tensile strength and deformability than those printed in upright orientation. In addition, post- processing is found to enhance the deformability of TPU parts by more than 40%.
Once the mechanical properties of printed TPU are characterised, a novel, low cost, and environmentally friendly method for concrete casting using printed TPU moulds is proposed in this thesis. Additively manufactured TPU mould is developed to cast complex concrete structures. A planar topologically opti- mized cantilever and a free-form 3D column are cast using the proposed TPU mould. Good surface quality is obtained for the cast concrete elements, and the moulds can be reused many times after cleaning.
The Ąndings of this thesis will provide clear guidelines for the selection of processing parameters for additively manufactured TPU parts and the method of casting complicated concrete structures with additively manufactured TPU mould.1 Overview Additive manufacturing (AM) is a technology that melts or sinters or extrudes speciĄc materials to form components layer by layer. AM has many advantages over traditional manufacturing methods. AM does not require traditional cutting tools and various pro- cessing procedures (Bikas et al. It can quickly and accurately manufacture parts of complex geometries so as to realize the Şfree manufacturingŤ of parts (Agarwala et al.
[1995]; Kruth et al. [2005]; Lamikiz et al. [2007]; Calignano et al. AM has been increasingly employed to manufacture functional parts in various areas, including aerospace (Barroqueiro et al.
[2019]), medical engineering (Berry et al. [1997]; Liu et al. [2019]) and sports (Mărieş et al. Three types of materials can be used in additive manufacturing: polymers, ceramics and metals.
Thermoplastic polyurethane (TPU) is an additively manufacturable polymer 1 1.1 Overview material that has high ductility, good hydrolysis resistance, excellent biocompatibility and excellent abrasion resistance (Lu et al. [2003]; Li et al. [2008]; Ma and Yang [2008]; Ma and Yang [2008]; Aurilia et al. [2011]; Feng and Ye [2011]; Mi et al.
[2013]; Lee et al. It can be used in structures that require high ductility, such as energy-absorbing structures (Bates et al. [2016]) and wearable devices (Scarpello et al. [2012]; Li et al.1 Two Commonly Used Additive Manufacturing Technologies There are many different AM technologies.
The differences among them lie in the mate- rial that can be used and how the parts are printed. Two commonly used AM technologies are adopted in this thesis, i. selective laser sintering (SLS) and fused deposition mod- elling (FDM). SLS technique could be used to manufacture TPU parts that require high dimensional accuracy, while large components can be printed at a low cost with the FDM technique.
Selective Laser Sintering SLS uses a laser as its energy source and powder as the primary processing material.1 shows the schematic of SLS.