Polymer Testing (May 2022)
Optimization process parameters and adaptive quality monitoring injection molding process for materials with different viscosity
Abstract
Injection molding is a cyclic process of producing plastics products by injecting molten plastic materials. There is much complexity in the process because it is a highly non-linear process; many factors affect the product quality after long-run fabrication processes. In this study, product weight is defined as product quality, and this research aims to minimize the variation in the product weight. In this study, a standard process parameter setup procedure is proposed in order to set the process parameters in a close to optimized value based on the information collected by the sensors so the adaptive process control system can work well. Three polypropylene materials with low to high MI (melting index) values were used to verify the adaptive control system. The study with the information collected through two sensors installed in the machine—the injection nozzle pressure sensor and tie-bar strain gauge. Based on a series of results in the preliminary experiments, the appropriate V/P switchover point and clamping force could be judged from the peak pressure and clamping force peak for materials with different viscosities, respectively. The viscosity index and peak pressure strongly correlated with the weight of the injection molded parts toward the materials of different viscosity. The viscosity index, peak pressure, and clamping force peak are defined in this research as the quality monitoring characteristics used to guide the adjustment and increase the stability of the process in the presented control system. The experiment reveals that a combination of nozzle pressure curve-based adaptive process control incorporated with clamping force peak monitoring can decrease the weight variation of the injection molded parts. With this system, the coefficient of variation of product weight (Cv) of different viscosity materials from low to high can be decreased to 0.031%, 0.020%, and 0.024%, respectively.
