Mixing Element Structure in Direct Fiber Feeding Injection Molding: Correlating Microstructure and Fiber Morphology With Length Retention and Mechanical Properties in GF / PP Composites

ABSTRACT This study systematically evaluated screw mixing elements for direct fiber feeding injection molding (DFFIM), addressing the critical yet unresolved trade‐off between fiber length retention and fiber dispersion in glass fiber/polypropylene (GF/PP) composites. A comparative analysis between DFFIM and the conventional “two‐step” method confirmed that DFFIM retained significantly longer fibers, with 30.79% of fibers exceeding 2 mm in length compared with 12.4% for the “two‐step” method, but exhibited poorer dispersion. To address this limitation, four screw mixing elements, standard screw element (SWE), cylindrical pin element (CPE), pineapple pin element (PPE), and SAXTON element (SNE), were designed and evaluated. PPE, characterized by a low shear and long residence time profile, achieved the best overall balance between fiber length retention and dispersion, resulting in the highest tensile and bending strengths (92.27 and 128.74 MPa, respectively). In contrast, the high‐shear CPE prioritized dispersion uniformity and produced the maximum impact strength (84.91 kJ/m 2 ). This study elucidates the fiber length–dispersion trade‐off governed by shear stress and residence time and provides clear selection guidelines based on target mechanical properties, with PPE recommended for strength‐critical applications and CPE for optimal impact resistance.