Surface Integrity and Mechanical Performance of Vertically Printed FDM-PLA After Laser Polishing: Coupled Effects of Scan Orientation and Hatch Distance

Vertically printed fused deposition modelling (FDM) components exhibit pronounced surface irregularities on their sidewalls at the interfaces between deposited layers, while limited interlayer bonding can impair their tensile performance. Although laser polishing has shown potential to improve the surface quality of vertically printed FDM components, the combined influence of laser scan orientation relative to the printed layers and hatch distance on the surface integrity and mechanical response of PLA remains insufficiently understood. To address this gap, a novel methodology for determining hatch distance (H) is introduced, with H defined as 25%, 50%, 75%, and 100% of the experimentally measured groove width (GW). To maximize structural and surface performance, three scanning strategies, parallel (0°), perpendicular (90°), and oblique (45°) to the layer interfaces, were evaluated using ANOVA, regression models, and SEM analysis. The results demonstrate a fundamental synergy between scan orientation and melt-flow dynamics. At H = 25%, perpendicular scanning produced the largest Sa reduction (59%), whereas parallel scanning yielded the lowest Sdr (3.14%, compared with 20.74% for the as-printed raw surface). At the same H, the Sdr was 4.45% for perpendicular and 8.45% for oblique scanning. SEM analysis indicates that while all strategies eliminated the inherent layered texture, the oblique strategy (45°) additionally led to more thorough valley filling through melt flow from two adjacent peaks. Specific surface porosity was identified at modified interfaces, providing indications of gas entrapment and bubble expansion within the heat-affected zone (HAZ). Statistical height distribution (positive Ssk and Sku < 3) demonstrates that HAZ-driven surface rounding occurs across all orientations. The greatest improvement in tensile performance was observed with oblique scanning (45°) at H = 100%, where peak force increased by 11.2% relative to that of the raw material. Fractography indicates that this improvement originates from a preserved ductile skin-layer that delays delamination, shifting the failure mode toward localized internal fibre breakage. This work establishes scanning orientation as the primary determinant of topography, while hatch distance dictates homogenization intensity, offering a systematic framework for optimising the laser polishing of polymers.