Flow-Enhanced Crystallisation in Fused Filament Fabrication*

The most common 3D printing method is known as fused filament fabrication (FFF). This process involves melting a thermoplastic, followed by layer-by-layer extrusion, cooling and re-solidification. The main concern with FFF is the strength at the welds between printed filaments; bulk strength is never achieved in these regions and the reason is currently unclear. Advancing FFF relies on a molecular understanding of how thermoplastics behave during the printing process. We employ a non-isothermal molecularly-aware model for FFF processing of a semi-crystalline polymer melt to show how typical FFF conditions can stretch the polymer molecules prior to cooling. Enhanced nucleation due to residual polymer stretch leads to accelerated crystallisation times at the surface of a deposited filament, whilst the bulk of the filament is governed by slower quiescent kinetics. Consequently we find a cross-sectional variation in the crystal morphology of single filament, with smaller spherulites forming in an outer skin layer. Furthermore, our model of a multi-filament walls reveals significant variations in crystal morphology from filament to filament.