Process‐Induced Anisotropy and Fracture Behavior of Pellet‐ 3D Printed PLA and PLA /Flax Biocomposites

ABSTRACT Fused granular fabrication (FGF) enables cost‐effective, large‐scale polymer additive manufacturing; however, the mechanical behavior and fracture response of pellet extruded parts, particularly for natural‐fiber composites, remain insufficiently characterized. This study evaluates the tensile response and fracture mechanisms of neat polylactic acid (PLA) and flax fiber‐reinforced PLA (PLA/Flax) produced by FGF. Two hundred specimens are tested to quantify the influence of raster orientation and layer height on process‐induced anisotropy. Tensile strength, elastic modulus, and density are measured, utilizing density‐normalized metrics to reduce scatter, while fracture surfaces are analyzed by 3D optical profilometry. FGF‐processed PLA reaches tensile strengths up to 59.5 MPa for a 0° raster orientation, whereas PLA/Flax achieves up to 45.82 MPa. Both materials present reduced elastic stiffness, with moduli approximately 40%–50% lower than common reference values. Reducing the layer height generally improves the mechanical performance and reduces the orientation sensitivity but increases the printing time and production cost. Profilometry confirms that anisotropy is governed by bead orientation and bonding quality. A reduction of approximately 46% in the average fracture surface roughness of representative PLA/Flax composite specimens was observed as layer height decreased, correlating with an increase in density‐normalized tensile strength. These results provide practical guidance for selecting raster strategies to balance mechanical performance with manufacturing efficiency in pellet‐based printing.