Non-standard raster angles in FFF-printed PLA: Process–mesostructure–property relationships governing tensile and fracture behavior

Fused filament fabrication (FFF) enables polymer components with complex geometries, but mechanical performance is often limited by orientation-induced anisotropy. This study examines how non-standard raster angles, combined with build orientation, affect the tensile and fracture behaviour of FFF-printed polylactic acid (PLA), with direct comparison to conventional raster configurations. ASTM D638 Type IV specimens were printed in three build orientations (Flat, On-Edge, and Upright) using seven raster angle pairs ranging from [0°/ −90°] to [90°/ 0°]. Tensile testing showed that build orientation governs load transfer and ductility, with Flat and On-Edge specimens consistently outperforming Upright builds due to improved filament alignment and reduced dependence on weak interlayer interfaces. The Flat configuration with the non-standard [15°/ −75°] raster achieved the highest median tensile strength of 77.6 ± 2.05 MPa and a strain at failure of 0.7311 ± 0.0941, representing improvements of 58.4% in strength and 62.8% in ductility relative to the weakest Upright specimens. Fractographic analysis revealed a clear transition in failure mechanisms. Upright specimens failed by brittle interlayer separation and exhibited low fracture surface roughness (Sa = 27–86 μm). In contrast, Flat specimens printed with oblique rasters showed highly developed fracture surfaces, with the [15°/ −75°] configuration reaching the highest roughness values (Sa ≈ 296 μm, Ra ≈ 224 μm), indicating tortuous crack paths, extensive filament deformation, and high energy absorption. On-Edge specimens displayed intermediate roughness and mixed-mode fracture behaviour, depending on raster angle. The strong correlation between fracture surface roughness and tensile strain at break provides quantitative evidence linking microstructural damage evolution to macroscopic mechanical performance. These results establish a process–mesostructure–property relationship and demonstrate that non-standard raster angles can effectively mitigate anisotropy and improve the apparent tensile and fracture performance of FFF-printed PLA under quasi-static loading conditions.