Understanding effect of flow-induced crystallization in Fused Filament Fabrication (FFF)-printing of HDPE

Polyethylenes are amongst the most widely produced polymers globally, offering low cost and wide availability, yet their adoption in the FFF-printing industry remains limited due to severe deformation and poor part quality. Along with high crystallinity and fast-crystallization kinetics, the changes observed in crystal morphology of these polymers during processing strongly influence this behavior. Even though the effect of flow has been evaluated in processes like injection molding, its effect in printing remains largely unexplored. Thus, this work focuses on understanding the impact of flow in FFF-printed HDPE and its effect on printability, part quality and strength. Our approach focuses on understanding the effect of shear and temperature on crystal morphology changes from spherulite to shish-kebab using rheological characterization and SAXS-XRD. These insights are extended to printing conditions to identify onset of flow-induced crystallization and the effect of crystal morphology changes on mechanical performance and dimensional stability. We also investigate how elongational flow during printing influences crystal morphology evolution and compare it with other HDPE processing routes like molding and fiber drawing. This work provides a deeper understanding of HDPE behavior during FFF printing and offers insights into leveraging flow-induced crystallization to enhance mechanical strength and dimensional stability, enabling broader adoption of polyethylenes in 3D printing.