Analysis of mold filling associated with unsteady flow in injection molding process

Surface defects in injection molded parts due to the unsteady flow are related to the dimension of gate, operational conditions and rheological properties of polymer. In this study we have examined surface defects in injection molding for PC, PBT and PC/ABS alloy with several injection speeds. We have used various cavity shapes that are tensile, flexural and impact test specimens with various gate and cavity thicknesses. Through this study we have observed that the formation of surface defect associated with jetting during filling stage in injection molding is strongly related to not only die swell but retardation of die swell. Large die swell eliminates jetting however the large retardation of die swell stimulates jetting. Reducing the thickness ratio of cavity to gate can reduce or eliminate jetting and surface defects. It also enlarges process window that can produce steady flow of polymer melt in injection molding.