Particle size of wood fillers used in FDM 3D printing filaments is a topic not commonly discussed in the literature. Research on traditional wood–polymer composites (WPCs) suggests that bigger particles improve the composite’s tensile properties. Is that the case at the 3D printing scale? Five variants of composites were prepared using recycled PLA and sawdust, differentiated by filler particle size (<0.2 mm, 0.2 mm–0.4 mm, 0.4 mm–0.6 mm, 0.6 mm–0.8 mm, 0.8 mm–1 mm). Current draw during extrusion, as well as tensile strength and tensile modulus, were tested. Test results of tensile strength, ranging from 9.21 MPa to 14.28 MPa, and tensile modulus, ranging from 802 MPa to 1014 MPa, have shown no clear correlation between wood particle size and tensile properties of the composites at the 3D printing scale. A clear increase in forces needed to extrude composites containing larger particles of wood was discovered, as well as the inability to extrude composites filled with the biggest tested particle size. To further explore this topic, SEM/EDS imaging of the tested composites was performed. Based on the test results, wood particle sizes ranging from one-fifth to one-half of the nozzle size are recommended for use as fillers in wood–PLA composites intended for 3D printing.

This study explores the possibilities of utilisation of coniferous bark as a filler in wood–polymer composites (WPCs), its impact on properties such as the modulus of rupture (MOR), modulus of elasticity (MOE), thickness swelling (TS) and water absorption (WA) after 2 h and 24 h of immersion in water and the significance of this impact compared to other factors. Six variants of bark–polylactic acid (PLA) WPCs were manufactured, differentiated by their filler content and filler particle size. As a comparison, analogous composites filled with coniferous sawdust were also manufactured. Bark-filled composites were characterised by lower TS and WA after both 2 h and 24 h of immersion, as well as lower water contact angles and surface free energy. The bark filler decreased the composites’ MORs and MOEs, while greater differences were noticed for variants filled with small particles. The type of filler was the second most important factor contributing to variance in this study, with the filler content being the most important one.

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.