Preparation of Thermoplastic Polyurethane Parts Reinforced with in Situ Polylactic Acid Microfibers during Fused Deposition Modeling: The Influences of Deposition-Induced Effects

3D printing technologies have exhibited their superiorities in the polymer processing field. In this work, thermoplastic polyurethane (TPU)-based soft parts reinforced with in situ polylactic acid (PLA) microfibers were successfully fabricated using a customized fused deposition modeling (FDM) printer. The non-printability of composite filaments was theoretically proved. The morphology of PLA microfibers was carefully characterized using scanning electron microscopy, and microfibers with a large aspect ratio can be observed in extruded strands and printed parts. The thermal and mechanical properties of printed samples were studied and compared with a compression molding sample. It was found that the crystallinity of PLA in compression molding and FDM samples increased from 17.72% to above 24.92%. The tensile strength of FDM samples was about 90% higher than that of the compression molding sample, while elongation at break reduced by half. The results can be attributed to the deposition-induced effects, and the formation mechanism of PLA microfibers was proposed. These results provide a new method to industrially manufacture TPU parts with in situ microfiber-reinforced structures, making it possible for preparing complex parts with advanced properties.