Optimization of clamping force for low-viscosity polymer injection molding

Abstract

Due to its low cost and high efficiency, injection molding is used for the mass production of many plastic products nowadays. However, when processing low-viscosity plastic materials, i.e., materials with an excellent fluidity, an inappropriate setting of the clamping force often results in a poor appearance and dimensional accuracy of the final product. Thus, operators usually take the upper limit of the clamping force as a default in setting up the machine in an attempt to improve the quality of the molded parts. However, such an approach shortens the machine and mold life, increases the energy consumption, and leads to poor air venting. Consequently, more scientific methods for determining the clamping force setting are required. To meet this demand, the present study proposes a clamping force search methodology for determining the optimal clamping force setting of a hydraulic cylinder clamping injection molding machine in the processing of low-viscosity plastics such as thermoplastic polyurethane (TPU) and polypropylene (PP). Based on the characteristic extracted from the sensing tie-bar elongation profile under different clamping force settings, a regression analysis on these data points is implemented to seek for an optimal clamping force. The experimental results show that for an injection molding machine with a hydraulic cylinder clamping mechanism, the effect of the mold temperature on the clamping force is sufficiently small to be ignored, which has an impact on the toggle type clamping unit. Furthermore, compared to traditional methods, the optimal clamping force obtained using the method proposed in the present study results in a significant improvement in the yield rate. Overall, the results confirm that for low-viscosity polymer resins, the optimal clamping force determined using the proposed method results in a higher and more consistent quality of the molded parts than that achieved using the proper clamping force setting for ordinary-viscosity resins.