Process control of surface quality and part microstructure in selective laser sintering involving highly degraded polyamide 12 materials

Abstract

Polyamide 12 is one of the most extensively used semi-crystalline polymer materials to date in selective laser sintering (SLS) additive manufacturing, or SLS 3D printing. In this powder-based direct digital manufacturing process, a substantial amount of expensive materials remains un-sintered, recyclable, and reusable. Recently, understanding the mechanisms of degradation and the reusability of reclaimed polyamide 12 powders has attracted increasing industrial and research interests. However, using reclaimed polyamide 12 powder in SLS results in problems with part surface quality such as undesirable part surface finish with poor textures and numerous un-sintered particles. Limited research is available on the improvement of part surface quality. In particular, results barely exist on improving or modifying the surface quality of parts using extremely aged powders – powders that are held close to the heat-affected zones (HAZs) and suffer from severe degradations during the sintering process. To improve the surface quality and to build interrelations between process parameters and surface quality, we propose a novel approach for SLS with (extremely) aged polyamide 12 powders. By combining material preparation, powder and part characterizations, and SLS with customized post-heating, we obtain parts with improved surface quality (e.g., reduced roughness and porosities, and eliminated un-sintered particles). Particularly, parts 3D-printed using the 30%-30%–40% new-aged-extremely-aged powder mixtures exhibit the smoothest and flattest surface with no unmolten particles and nearly zero porosity.