Design and development of glass-reinforced polymeric composites using multi jet fusion process

• Main Author: Tey, Wei Shian
• Other Authors: Zhou Kun
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2024
• Subjects: Engineering::Materials::Non-metallic materials
• Online Access: https://hdl.handle.net/10356/173286
• Institution: Nanyang Technological University

The Hewlett-Packard (HP) Multi Jet Fusion (MJF) process is a recently emerged polymer powder bed fusion (PBF) additive manufacturing (AM) process that utilizes a combination of powder bed and inkjet printing to fabricate functional parts with voxel resolution. While the MJF technology offer several advantages over other PBF processes such as Selective Laser Sintering (SLS), MJF applications are limited to its small material palette of engineering polymers, which do not cater to applications where high performance is required. As research of MJF is still in its infancy, literature on the MJF materials and MJF printing mechanisms are scarce and do not provide sufficient depth to aid in the development of new MJF materials. Therefore, this Ph.D. research aims to provide an in-depth analysis of the existing MJF materials, provide some insight on the MJF printing mechanisms, and develop new high-performance composites using existing MJF materials as the polymer matrix. A thorough understanding of the physical, chemical, and mechanical properties of the MJF materials is essential for the selection of a suitable polymer matrix for MJF polymeric composite development. Three commercial MJF materials, namely polyamide 12 (PA12), polyamide 11 (PA11), and thermoplastic polyurethane (TPU) were characterized to determine the most suitable material candidate to serve as the polymer matrix for composite development based on their printability and print quality. With a powder morphology consisting of near-spherical particles with smooth surfaces and the highest sintering window (32°C) among all three material candidates, PA12 possessed the best powder flowability and the largest printing temperature range. Although the mechanical strength of PA12 (49.6 MPa) was slightly lower than that of PA11 (50.9 MPa), PA12 was deemed to be a more suitable candidate for composite development due to a higher sintering window which allows for less precise control over the printing temperature. HP PA12 GB powder is a commercially available MJF glass bead-reinforced PA12 (GB/PA12) composite powder that possesses enhanced tensile modulus at the cost of a drastic reduction in tensile strength. To alleviate the significant reduction in tensile strength, alternative compositions of GB/PA12 composites were designed and fabricated using an MJF testbed printer (Dalmata printer). An alternative high-temperature annealing method, which can be used to replace the existing post-print annealing process in commercial MJF printers, was performed on the GB/PA12 specimens to further improve their mechanical performance. Printing optimization of the GB/PA12 specimens revealed that the powder bed temperature must be set above the onset crystallization temperature of the GB/PA12 powder (160°C) to avoid printing defects such as the “staircase” effect or layer shifting. In comparison to the commercial PA12 GB parts, the fabricated 10% GB/PA12 specimens achieved a 92% increase in tensile strength while maintaining a similar tensile modulus of 3.37 GPa. The fabricated GB/PA12 specimens also achieved a 19% increase in tensile strength and a 113% increase in tensile modulus when compared to MJF-printed PA12 parts. Through the use of the Dalmata printer, several fiber-reinforced PA12 composites with anisotropic directional properties have been developed. To reduce the effect of anisotropy, a novel composition of high-strength glass bead/glass fiber/polyamide 12 (GB/GF/PA12) hybrid composite was developed and fabricated using the Dalmata printer. Compared to the previously fabricated glass fiber-reinforced PA12 (GF/PA12), the degree of anisotropy in observed in the 5GB/20GF/PA12 specimens was notably reduced as it exhibited a 14.1% and 10.7% increase in tensile strength when printed along the X (powder recoating direction) and Z (layer building direction) orientations, respectively. Overall, the GB/GF/PA12 composite specimens achieved a 45% increase in tensile strength and 253% increase in tensile modulus when compared to commercial PA12 parts. The GB/GF/PA12 fabricated in this thesis attained the highest tensile strength and modulus among all reported glass-reinforced PA12 composites printed using PBF processes. The compatibility of the GB/GF/PA12 powder with commercial MJF printing was later showcased on a commercial MJF printer. The GB/GF/PA12 specimens fabricated using the MJF 5200 printer possessed similar mechanical properties to their Dalmata-printed counterparts, demonstrating that the GB/GF/PA12 is a viable high-strength polymer composite candidate for commercial MJF printing. This Ph.D. thesis provides an in-depth analysis of the existing MJF materials, the development and optimization of a new composition of GB/PA12 composites with enhanced mechanical performance using an alternative high-temperature annealing method, the development of a novel composition of high-strength GB/GF/PA12 glass hybrid composites using the Dalmata printer, and a demonstration print of the GB/GF/PA12 composite powder using a commercial MJF printer to showcase its compatibility with the MJF print mode. This thesis serves as an effective guideline for the printing optimization of MJF-printed PA12 composites and can be used as a foundation for future composite development in MJF.