Yao Xu

• Fused filament fabrication (FFF) is an additive manufacturing technique with signif-icant potential for producing customized and functional polymer-based components. Using polypropylene (PP) as a representative system, the morphology, thermal be-havior, and mechanical performance of pure and microfibrillar-filled semi-crystalline polymers fabricated via FFF were explored, emphasizing the influence of tempera-ture control, material properties, and processing parameters on final part perfor-mance. Results demonstrate that contact temperature is the key parameter control-ling interfacial diffusion and bonding strength, a trend consistently validated across various semi-crystalline polymer systems. A contact temperature model was devel-oped and validated, enabling process condition optimization and achieving interfacial properties comparable to those of injection-molded parts. To exploit the anisotropic properties of FFF using microfibrillar composites (MFC), an unexpected phenome-non of in situ fibrillation was observed, wherein PET droplets were fully transformed into aligned microfibers during printing under suitable thermal and flow conditions. These fibers enhance tensile properties and promote crystallization. These findings establish a comprehensive framework and practical strategy for optimizing the FFF process of semi-crystalline polymers and MFC, enabling simplified processing, en-hanced performance, and geometry-aware parameter control for advanced light-weight structural applications.