Modelling flow-enhanced crystallisation during fused filament fabrication of semi-crystalline polymer melts

Abstract

Achieving better control in fused filament fabrication (FFF) relies on a molecular understanding of how thermoplastic printing materials behave during the printing process. For semi-crystalline polymers, the ultimate crystal morphology and how it develops during cooling is crucial to determining part properties. Here crystallisation kinetics are added to a previously-developed model, which contains a molecularly-aware constitutive equation to describe polymer stretch and orientation during typical non-isothermal FFF flow, and conditions under which flow-enhanced nucleation occurs due to residual stretch are revealed. Flow-enhanced nucleation leads to accelerated crystallisation times at the surface of a deposited filament, whilst the bulk of the filament is governed by slower quiescent kinetics. The predicted time to 10% crystallinity, t10, is in quantitative agreement with in-situ Raman spectroscopy measurements of polycaprolactone (PCL). The model highlights important features not captured by a single measurement of t10. In particular, the crystal morphology varies cross-sectionally, with smaller spherulites forming in an outer skin layer, explaining features observed in full transient crystallisation measurements. Finally, exploitation of flow-enhanced crystallisation is proposed as a mechanism to increase weld strength at the interface between deposited filaments.