In this study, an injection molding machine with a low-fracture-fiber mechanism was designed with three stages: a plasticizing stage, an injection stage, and a packing stage. The fiber-fracture behavior is observed under the screw (plasticizing stage) of low-compression/shear ratio for the ultra-long fiber during the molding process. The molding material employed in this study was 25-mm-ultra-long-glass-fiber-reinforced polypropylene (PP/U-LGF). In addition, a thickness of 3 mm and a width of 12 mm spiral-flow-mold were constructed for studying the melt flow length and flow-length ratio through an experiment. The experimental results showed that the use of an injection molding machine with a three-stage mechanism decreased the fiber length when the screw speed was increased. On average, each fiber was shortened by 50% (>15 mm on average) from its original length of 25 mm. Longer glass fibers were more resistant to melt filling, and as the fiber length was reduced, the mixing between the melt and glass fibers was improved. Thus, the melt fluidity and fiber ratios were increased. In addition, the mixing/flow direction of the melt had an impact on the dispersion and arrangement of glass fibers, thus the tensile strength of PP/U-LGF increased.
