DETERMINING OPTIMUM VENT SIZES FOR INJECTION MOLDS

摘要

A mold was designed and built to study the optimum size of the vents needed in injection molding. Results demonstrated that vent sizes primarily were dictated by the part design immediately behind the vent. The localized shear rate of the polymer is determined by the flow cross section of the part in the vent area. The higher the shear rate, the lower is the viscosity of the polymer, thus limiting the vent depths in the mold. The experimentation proved that for a given polymer material larger vent sizes can be accepted in the mold with no flash when the shear rate was low. For the same mold and under identical processing conditions, when the localized shear rates were high, the polymer flashed and the vent depths had to be reduced. The injection phase of the molding cycle is the high shear phase and the pack and hold phases are the low shear phases. This leads us to believe that the vent depths for the mold in the injection phase area can be significantly different from the vent depths at the end of fill where the pack and hold phases come into play.