Impact of polymer molecular weight blends on the powder bed fusion process and the properties of polypropylene printed parts

Abstract

Designing and controlling the molecular characteristics of polymeric feedstocks is crucial for creating robust structures via the powder bed fusion (PBF) process. To explore the impact of a powder's molecular weight on printed part structure and properties, thermally induced phase separation was employed to produce spherical, appropriately sized polypropylene (PP) powders formed from individual unimodal molecular weights and molecular weight blends. More precisely, these powders are composed of 12 000 Daltons PP (12k), 250 000 Daltons PP (250k), or 340 000 Daltons PP (340k), as well as their blends (50/50 wt% of 12k/250k, 12k/340k, 250k/340k, and 33/33/33 wt% of 12k/250k/340k). Analysis of the printed parts from these powders shows that the blended molecular weight (M_w) samples exhibit lower void space and higher crystallinity than the unimodal M_w counterparts. More importantly, dynamic mechanical analysis of the printed parts shows a substantial increase in storage modulus for blended molecular weight samples compared to unimodal M_w counterparts. This significant enhancement in the mechanical property of the blended molecular weight samples is due to improved coalescence dynamics driven by the powders’ decreased melt viscosity. Improved coalescence reduces the void space in the printed parts, thereby improving mechanical performance. These results, therefore, provide a molecular-level understanding of the mechanism by which low M_w additives improve PBF processability, presenting avenues to augment the macroscopic properties of the printed parts. Additionally, the powder design approach presented in this work is cost-effective and offers a simple strategy to enhance the final part properties across various materials in additive manufacturing.