Melt Processability and Post-Processing Treatment of High Temperature Semi-Crystalline Thermoplastics for Extrusion Deposition Additive Manufacturing

Composites of high performance semi-crystalline thermoplastics are emerging as viable candidates for autoclave tooling applications, which typically demand good mechanical and thermal performance, as well as chemical resistance. With the growing demand for tooling, there is a need for versatile customizable tools that can be inexpensively manufactured with low lead times. Extrusion Deposition Additive Manufacturing (EDAM) technique can be extremely beneficial for the manufacture of low-cost composite tools with high degrees of customizability and short lead times. However, EDAM processes for such high temperature polymers can pose several challenges for extrusion and deposition due to their inherent properties as well as their thermal and oxidative responses. In addition, to use these printed parts in an application, the desired mechanical property requirements have to be realized. The use of semi-crystalline polymers in AM not only adds to the already existing complexity of mechanical anisotropy of the printed parts, but also makes the final part properties dependent on the crystallization in the deposited beads. For out of the oven EDAM processes, crystallization after extrusion and deposition takes place non-isothermally, further complicating crystallization mechanisms. This work encompasses strategies to address challenges encountered both, during deposition, as well as post-deposition, of commercial grades of carbon fiber reinforced poly (ether ketone ketone) (PEKK) and polyphenylene sulfide (PPS). The first section of this work involves determining the melt processability by evaluating the thermal and rheological characteristics of these materials under different processing conditions that can be encountered during EDAM processing. Establishing the link between rheological properties and printing can guide the development of such new materials as well as printer design. The second section focuses on post-processing isothermal annealing as a strategy to enhance crystallinity, thereby improving the mechanical properties of printed carbon fiber reinforced PPS parts. In addition, a detailed analysis of the possibility of crosslinking in PPS, its effect on crystallinity and the mechanism dominating the changes in mechanical properties has also been discussed. Overall, this work provides a broad framework for developing high performance semi-crystalline thermoplastics for high temperature tooling applications employing 3D printed tools.