Laser Polishing of Vertically Oriented FDM-PLA Components: Influence of Laser Power and Polishing Speed on Surface Topography and Mechanical Response

Laser polishing represents a promising post-processing method for FDM-printed parts, enabling targeted manipulation of their surface topography and the associated functional properties. Despite extensive research in this field, no experimental investigation has yet addressed the configuration with vertical build orientation, which is critical in terms of interlayer stress transfer. Therefore, this study focuses on CO2 laser polishing of vertically printed PLA specimens at power levels of 10 and 12 W and polishing speeds of vf = 200, 400, and 600 mm·s−1, with a constant transverse displacement of 0.1 mm. The results showed that both laser power and polishing speed have a significant influence on the surface topography and mechanical properties of the samples. The lowest roughness values, Ra = 10.24 ± 0.14 µm at 10 W and Ra = 12.20 ± 0.43 µm at 12 W, were achieved at the highest polishing speed of vf = 600 mm·s−1; however, these still exceeded the roughness of the reference unpolished sample (Ra = 9.02 ± 0.21 µm). The increase in roughness was attributed to the formation of relief structures caused by insufficient overlap of laser passes, orthogonal to the printing-induced surface orientation. These reliefs further acted as local stress concentrators, resulting in a decrease in mechanical performance of up to 9% in comparison with the reference specimen. At 10 W, the surface profile was dominated by Rv > Rp, similar to the reference sample, whereas at 12 W, the opposite trend (Rp > Rv) was observed—attributed to more intense subsurface melting and valley filling by the melt pool. Therefore, the findings clearly confirm that the critical factor is the energy distribution and transverse displacement of the laser polishing paths, where improper parameter settings may visually improve the surface while structurally weakening it. These findings highlight the need for further research, focused on the precise optimization of the polishing trajectory and the energy distribution for vertically oriented samples.