Extrudate-Level Analysis of Thermoplastic Properties in Material Extrusion Additive Manufacturing

Polymer-based material extrusion additive manufacturing techniques, including Fused Filament Fabrication (FFF) and Fused Granular Fabrication (FGF), are rapidly expanding across multiple industries. These processes build parts layer by layer by depositing thermoplastic extrudates through a heated nozzle. However, their 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 (PLA) 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 and an ~ 85% and ~ 60% loss in toughness for extrudates and printed samples, respectively. Digital Image Correlation (DIC) revealed more uniform strain distribution and delayed failure in low-rate samples, whereas high-rate samples exhibited early strain localization. Differential Scanning Calorimetry (DSC) and structured light profilometry demonstrated that higher extrusion rates increased crystallinity and surface roughness in both extrudate and printed samples. 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 direct consequences for printed part performance, highlighting that process optimization should account for extrudate-level behaviour in FFF and FGF.