Process optimization for selective laser sintering of polypropylene

• Main Author: Wu, Kunlin
• Other Authors: Zhou Kun
• Format: Thesis-Master by Coursework
• Language: English
• Published: Nanyang Technological University 2021
• Subjects: Engineering::Mechanical engineering
• Online Access: https://hdl.handle.net/10356/149967
• Institution: Nanyang Technological University

Additive manufacturing (AM) has been evolving from a technique of rapid prototyping to an indispensable rapid manufacturing method for producing end-use parts. Among the various subclass of additive manufacturing processes, powder bed fusion (PBF) holds great potential for small batch production, due to its advantages of elimination of the supporting structures, good and consistent mechanical properties compared with conventional manufacturing processes, etc. For polymer material, selective laser sintering (SLS) is a typical powder bed fusion process, which is now well-established. This research aims to evaluate the properties of polypropylene (PP), including the powder particle distribution, mechanical properties, thermal properties of the printed parts. The process parameter optimization was conducted in order to find out the optimal experiment parameters. The effects of different building orientations on the tensile, flexural, impact and shrinkage properties and failure mechanism of the specimens produced by selective laser sintering process are also examined in order to investigate the anisotropy of the specimens printed by selective laser sintering process. The result shows that the polypropylene powder used in this research has a near-spherical shape, smooth particle surfaces, fairish particle size and wide sintering window, which ensure the great flowability of the powder, thus leading to excellent printing performance. Meanwhile, 16 W is the optimal value for laser power, which can be used in the orientation optimization. The result reveals that specimens printed in X and Y orientation exhibited higher tensile strength, tensile strain and tensile modulus than the parts built in Z orientation. The result of flexural tests shows that specimens printed in Z orientation had worse performance in flexural properties, compared to specimens built in X, Y orientation. Impact tests also showed similar results. The SLS-printed specimens show a fracture mechanism where crack initiates from unmelted powder particles, propagation through craze and form tearing edges. From the fatigue fractography, the area with unmelt powder particles acted as the stress concentrator, thus leading to the crack.