Long-fiber polymers offer the advantage of a lower production cost because specific tool designs are required for conventional injection molding equipment to produce long-fiber polymer parts. The use of long fibers allows relatively high fiber aspect ratios to be obtained, thereby enhancing composite stiffness, strength, creep endurance, and fatigue endurance. However, the multigate design of the injection-molded part can result in weldline formation during the molding process, which reduces the structural strength of the molded part. Therefore, in this study, the surface quality, fiber compatibility, and structural strength of long-glass-fiber-reinforced polypropylene (PP/LGF) injection-molded samples were compared in the use versus nonuse of a mold-cavity overflow-well area and the mold-face infrared heating method. The experimental results indicate that the mold-cavity overflow-well area more greatly improved the surface roughness of the PP/LGF molded samples. Moreover, the infrared heating of the mold-face decreased the weldline depth of the samples. Optical-microscopy images and mold-cavity pressure distributions indicated that the weldline tensile strength and the interface compatibility between fibers and melts at the weldline region during the molding stage were higher in the use than in the nonuse of the mold-cavity overflow-well and mold-face infrared heating method.

This study used the cavity-surface infrared rapid-heating method and tensile specimens (ASTM-D638) to test the fiber compatibility and tensile characteristics of long glass fiber reinforced PA66 (LGF-PA66) composites. The weldline molding experiment on the injected specimen was conducted using a double-gate. In addition, the integration of infrared heating and cavity pressure measurement were developed to achieve cavity-surface heating and observe the effect of with/without cavity-surf ace heating on weldline tensile strength variation and correlated pressure distribution near the weldline region. On the other hand, a vent -well was designed in the center position of the specimen (vent-well depths are 0.9 mm, 1.8 mm, and 3.6 mm), in order to discuss the effect of the two-melts on fiber and melt compatibility in the compatible process. The experimental results show that vent-well provides better weldline tensile strength; moreover, infrared heating cavity -surface also provides the better weldline tensile strength of LGF-PA66 composites molded parts. In addition, from SEM images and cavity pressure distributions, considering in presence or absence of vent-well design and cavity-surface heating, it provides better weldline tensile strength and interfaces compatibility between fiber and resin at weldline region during molding stage.