Extrudate-level analysis of extrusion rate effects in fused filament fabrication

Fused filament fabrication (FFF) is rapidly expanding across multiple industries. This process builds parts layer by layer by depositing thermoplastic extrudates through a heated nozzle. However, its wider adoption for high-performance applications remains limited due to challenges in predicting mechanical properties and the generally reduced performance of printed parts. These challenges are typically attributed to insufficient interlayer bonding and meso-scale effects related to extrudate geometry. However, the influence of process conditions on the intrinsic properties of the extrudates, which are the fundamental building units of printed parts, has received limited attention. In this study, polylactic acid was extruded at different rates and examined at both extrudate and printed levels. Tensile testing showed that increasing the extrusion rate did not affect ultimate tensile strength or stiffness but caused substantial reduction in ductility, with an 85% loss in toughness for single extrudates and a corresponding 60% loss for printed samples produced from those extrudates. Digital image correlation revealed more uniform strain distribution and delayed failure in low-rate samples, whereas high-rate samples exhibited early strain localization. Differential scanning calorimetry and structured light profilometry demonstrated that higher extrusion rates increased crystallinity in both extrudate and printed samples, and increased surface roughness in extrudates only. Notably, the effect on these properties is non-proportional, with substantial changes occurring only beyond a threshold. These results demonstrate that extrusion rate markedly influences extrudate properties, with effects that carry over to printed part performance, highlighting that process optimization should account for extrudate-level behaviour in FFF.