A Novel and Simple Method to Improve Thermal Imbalance and Sink Mark of Gate Region in Injection Molding

Among the appearance defects of injection molded products, sink mark near the gate accounts for a large proportion, especially for the precision products that require a high appearance quality. The traditional method to relieve gate sink mark is to increase packing pressure and packing time. Even so, the problem cannot be completely solved. In current study, a three-dimensional numerical simulation method was employed to analyze the formation mechanism of gate sink mark. The analyses point out that high shear in the filling stage, melt refilling in packing stage and high mold temperature lead to the imbalance of shear heat generation and cooling between the gate and other regions, and finally result in the relatively high temperature and the sink mark defect in the gate region. Based on these results, a novel and simple method for improving gate sink mark by adding beryllium copper insert in the runner was proposed and implemented. Further simulation shows that the method is effective in eliminating the gate “hot spot” and improving the temperature uniformity of the entire product. The experiment results also prove that the heat sink mark in the gate area can be effectively avoided by using the mold with insert for both amorphous ABS and semi-crystalline PP products. The proposed method can not only solve the sink mark problem effectively, but also has the advantage of being simple and easy to achieve because the slide type side gate design is adopted.