Determination of process parameters based on cavity pressure characteristics to enhance quality uniformity in injection molding

Abstract Injection molding is widely used for the mass production of plastic components. However, an appropriate choice of the process parameters is essential to ensure the precision and uniformity of the molded parts. The flow length-to-wall thickness (L/t) ratio for most molded components lies in the range of 100∼200. For higher values of the L/t ratio, the flow resistance increases dramatically at the end of the molded part and the injection pressure propagates less easily through the mold cavity. Consequently, the geometric dimensions of the molded parts in the near-gate region are frequently different from those in the far-from-gate region as well as resulting in lower uniformity. In practice, the variation in the geometric dimensions is significantly dependent on the filling-to-holding (V/P) switchover point, injection speed, and holding condition. Accordingly, the present study commences by proposing a simple least-squares regression-based approach for determining the appropriate V/P switchover point based on the quality requirement for the geometric dimensions of the molded component. In addition, a multi-stage holding condition is used to improve the geometric uniformity of the molded component by minimizing the residual pressure difference between the near-gate and far-from-gate regions of the mold at the end of the packing stage. Finally, a deceleration strategy is considered for the injection screw at the end of the filling stage to further reduce the quality variation. The experimental results show that for molded components with a large L/t ratio, the V/P switchover point should be determined based on the quality requirement for the part in the far-from-gate region in order to avoid insufficient quality compensation in the early holding stage. Furthermore, compared with the single-stage holding condition, the multi-stage holding condition reduces the residual pressure difference between the near-gate and far-from-gate regions of the cavity by around 74.5% (from 48.7 to 12.5 MPa). As a result, the difference in width between the near-gate region of the molded component and the far-from-gate region is reduced by about 66.7% (0.09 to 0.03 mm), while the difference in thickness is reduced by approximately 32.1% (0.028 to 0.019 mm). The deceleration strategy for the injection screw has no noticeable effect on the quality uniformity of the molded part provided that the V/P switchover point and multi-stage holding pressure are appropriately designed.