Development and optimization of selective laser sintered-composites and structures for functional applications
Date of Issue2018-01-18
School of Mechanical and Aerospace Engineering
Singapore Center of 3D Printing
Polymer nanocomposite technology is an emerging field in which nanoscale fillers are embedded into a polymer to produce materials with advanced functionalities. The properties imparted by carbon nanotubes (CNTs) to the polymer can be remarkably good. Thermoplastic polymers such as polyamide 12 (PA12) and thermoplastic polyurethane (TPU) incorporated with CNTs have the potential to create a whole new generation of composites with significantly improved performances through the selective laser sintering (SLS) system instead of the conventional manufacturing processes of nanocomposites. This PhD research aims to develop new polymeric nanocomposite powders, optimize the process parameters of SLS for printing these powders, and investigate the multi-functionality of the printed composite parts and their applications in energy absorption and dissipation. The development of composite powders is the primary step for the SLS process. A surfactant-facilitated latex technique was developed to prepare multi-walled CNTs (MWCNTs)-coated polymer powders, which possess the desirable microstructures and surface morphologies for the SLS process. This method of embedding nanoparticles into/onto polymeric powders was facile, green, low cost and scalable, providing a universal route for the rational design and engineering of configuration and morphology of composite powders. The thermal conductivity, thermal capacity, optical properties, rheological and viscoelastic properties, and flowability of the new powders were properly characterized for further evaluation. These powder properties affect the powder deposition, bed temperature control andsintering performance of the entire laser sintering process. A methodology of process optimization and powder evaluation was proposed, which applied a simplified theoretical model to calculate the energy required for polymer melting and decomposition and then predict the effective range of the input laser energy. This methodology can effectively narrow down the working range of sintering parameters and identify the set of optimal process parameters in the SLS process. Additionally, the established model can be generally applied to the prediction of the effective range of the input laser energy for semi-crystalline and amorphous polymers, such as PA12, TPU and their composites. To explore the end-use application of laser-sintered composite materials, their multi-functionality was investigated, and the process-structure-property relationship was studied to reveal the reinforcement mechanisms of nanofillers within the polymeric matrix. The laser-sintered MWCNTs-reinforced composites exhibited promising improvements in electrical conductivities and mechanical properties, as well as the slight enhancements in thermal conductivities. The sintering of the MWCNTs-coated polymeric powders could create the composites with segregated microstructures that built the three-dimensional (3D) CNTs-network and formed the continuous conductive pathways for electrons and phonons within polymer matrices. Meanwhile, after laser sintering, the MWCNTs were retained at the powder-boundaries and prohibited the movement of polymer chains upon mechanical forces, thus leading to the enhancements in the mechanical properties. With the improved mechanical toughness and strength, the CNT/PA12 composites were proposed to be used for the fabrication of the 3D cellular and auxetic lattices for the destructive energy absorption purpose. Another type of 3D soft auxetic lattices was fabricated by the laser-sintered TPU material, which could be highly flexible and recoverable upon cyclic compressive loading. Meanwhile, the energy absorption capability of 3D lattices could be engineered by tuning the structure designs and controlling the material formulations. Thus, the concept of the digitization and integration of materials and structures was implemented into the SLS system and the newly prepared polymeric nanocomposites was incorporated with complex 3D lattice structures. The multi-scale and multi-functional architectures could be designed and engineered to possess vibration resistance and impact shock absorption. This PhD dissertation developed a systematic and integrated methodology for material development, process optimization and structure design in the SLS process. This methodology provides an effective guidance for formulating materials and engineering their morphology to match the stringent requirements of the feeding material for the SLS process. The designed 3D auxetic or cellular lattices can be manufactured through the optimized system using the newly developed materials to achieve the desirable performance of energy absorption or dissipation.