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Glass fiber-reinforced polypropylene composites (GF/PP) were fabricated in this study by direct fiber feeding injection molding technology (DFFIM) equipped with specially designed plasticizing units. By analyzing the fiber length distribution, scanning electron microscopy micrographs, and the mechanical properties of GF/PP composites, the effect of three process parameters, namely the number of fiber bundles, screw rotational speed, and injection pressure, on the properties of GF/PP composites fabricated by DFFIM were investigated. The results show that the mechanical properties of GF/PP composites gradually increase as the fiber content increases and the screw rotational speed decreases; however, when the fiber content exceeds a certain range, the enhancement effect of mechanical property is reduced due to the increase of fiber breakage and fiber aggregation. Appropriately increasing the injection pressure can increase the fiber length and make the fiber and PP more tightly bonded, and the mechanical properties of GF/PP composites gradually improve, but the fiber breakage phenomenon is more severe under excessive injection pressure, the mechanical property enhancement effect is reduced or even the mechanical property is decreased due to the decrease of the fiber length. KEYWORDS fiber length, fiber-reinforced, GF/PP composites, injection molding, mechanical properties, 1 | INTRODUCTION Fiber-reinforced thermoplastics (FRTP) are made from thermoplastic resin as the matrix and various fibers as the reinforcing material with the advantages of high strength, low density, corrosion [...]