Scale-Dependent GD&T Conformity and Surface Roughness in PLA Parts Manufactured by Material Extrusion: A Metrological Assessment

Dimensional accuracy and surface roughness in material extrusion (MEX) have been widely studied; however, the effect of part scale on the simultaneous fulfilment of geometrical tolerances and surface finish remains less clearly established. This study evaluates the scale-dependent quality of PLA parts manufactured by material extrusion (MEX) using a full 3 × 3 × 3 factorial design, combining three scales (1×, 0.75×, and 0.5×), three commercial PLA filaments, and three printing speeds (40, 60, and 80 mm/s). A dedicated test artefact was inspected by a coordinate measuring machine to quantify GD&T-related deviations, including flatness, perpendicularity, parallelism, angularity, circularity, cylindricity, and coaxiality. Surface roughness was characterized by 2D profilometry using Ra, Rz, and Rq on representative horizontal and vertical surfaces. Results showed that part scale was the dominant factor affecting geometrical conformity, especially for orientation- and location-related tolerances such as perpendicularity, parallelism, and coaxiality, which deteriorated markedly as specimen size decreased. Filament type also influenced several geometrical responses, whereas printing speed showed no significant main effect on geometrical tolerances within the evaluated range. Surface roughness exhibited clear anisotropy, with vertical surfaces showing higher values and stronger statistical dependence on process factors. Unlike studies focused only on dimensional accuracy or surface roughness, this work provides an integrated GD&T-based and surface-texture assessment of scale-dependent quality loss in PLA parts manufactured by MEX. These findings may help designers and manufacturers define inspection criteria, select suitable commercial PLA filaments, and identify critical geometrical features when scaled MEX parts are intended for functional applications.