Improving the Surface Quality of FDM 3D-Printed Wood–PLA Composites Through CNC Milling

Additive manufacturing enables the production of complex components, but the layered deposition process can result in pronounced surface irregularities, particularly when wood particles are incorporated into the polymer matrix. This study investigates the mechanical performance and surface characteristics of wood–PLA materials and evaluates CNC milling as a post-processing method. Three wood–PLA composites, PLA, and ASA, were examined. Tensile and flexural properties as well as density were determined for the investigated materials. Cutting forces were measured during milling, while surface roughness was evaluated before and after milling for specimens produced in different orientations. The wood–PLA composites exhibited lower mechanical properties than neat PLA, with tensile strength reduced by 24–43% and flexural strength by 36–57%. Internal voids and imperfect fusion between deposited strands were identified as important factors affecting the measured performance. Their initial roughness was higher than that of the neat polymers, indicating a substantial contribution from material heterogeneity. Milling substantially reduced roughness for all wood–PLA composites and required cutting forces up to 48% lower than the neat polymers under the investigated conditions. CNC milling therefore constitutes an effective approach for improving the surface quality of these materials, although machining parameters should be selected with their layered structure in mind.