Monitoring, prediction and control of injection molding quality based on tie-bar elongation

Abstract

Quality consistency is essential in the injection molding process for maximizing the yield rate and minimizing the production cost, and this issue is more concerned proceeding with recycled material. Effective methods are therefore required for performing the real-time monitoring and control of the injection molded part quality and tuning the injection molding process parameters. Accordingly, the present study proposes a real-time quality monitoring and control system based on the clamping force increment characteristic, as determined by the measured tie-bar elongation. Notably, the proposed method is applicable to both virgin raw materials and recycled plastic materials. The effectiveness of the proposed approach is evaluated for three different processing parameters, namely the barrel temperature, the back pressure and the V/P switchover point. It is shown that the three processing parameters all have a significant effect on the molded part weight and thickness. Moreover, for each parameter, the part weight and thickness are strongly correlated. The clamping force increment is also highly correlated with the molded part weight. It is hence inferred that the clamping force increment provides an appropriate quality index for monitoring the injection molding quality. Furthermore, through the integration with a V/P switchover point calibration model, the clamping force increment index can be used to control the molded part weight to within a small tolerance on a shot-by-shot basis. When performing the injection molding process with virgin raw material, the tensile strength of the molded part increases with an increasing barrel temperature due to an increasing part weight if the proposed quality monitoring and control system is not applied. However, when the proposed system is employed, the tensile strength reduces slightly at a higher barrel temperature. When using recycled material in the injection molding process, the tensile strength remains approximately constant as the barrel temperature increases, irrespective of whether or not the proposed quality control system is applied. The difference in tendencies of the two materials can be attributed to a difference in their viscosities (i.e., molecular weights).