Crystallization-coalescence relationships in laser powder bed fusion: Moving beyond the “sintering window”

Abstract

Laser-based polymer powder bed fusion (PBF-LB/P) additive manufacturing (AM) creates objects through layerwise repetition of selective consolidation of polymer powder particles. The specific molecular- and meso-scale mechanisms responsible for consolidation are important to understand to rapidly identify potential new materials for PBF-LB and correctly attribute observed failures, defects, and deviancy to either feedstock issues or process issues for quality assurance. Such understanding must draw from both material science principles and a deep comprehension of how automated hardware interacts with the feedstock during the manufacturing process. The so-called “Sintering Window” or “PBF Processing Window” is a prevalent tool claimed by many to adequately and rapidly summarize these key relationships between feedstock properties and the manufacturing process. This tool has been common parlance in PBF-LB/P research since the early days of commercialized PBF-LB/P (a.k.a., Selective Laser Sintering, SLS) in the mid 1990’s. The author argues in the present work that lack of progression beyond the rudimentary Sintering Window is hampering advancement of this AM modality as it elevates secondary factors (e.g., crystallization) above primary factors (e.g., coalescence) and does so in a manner disconnected from the real manufacturing environment. The present work outlines four issues with the overuse of the Sintering Window in fundamental research and provides alternative methodologies for reconciling the present body of fundamental polymer science with the present understanding of PBF-LB process physics. Namely, an increased emphasis on coalescing flow behavior that is ultimately arrested by crystallization at the point of physical gelation is recommended for investigating potential suitability of the typical semicrystalline polymer for PBF-LB. Six varieties of nylon-12 and one commercially available polypropylene material are used as exemplars.