In-situ ultrasonic measurement of molten polymers during injection molding

Injection molding is regarded as the most significant process to manufacture high-quality polymer products. However, the molten polymer undergoes drastic variations of temperature in the cavity during injection molding, which strongly affects the dimensional accuracy, weight consistency and crystallinity of the molded products. Therefore, it is of great significance to achieve online temperature measurement of molten polymers (melt). An ultrasonic method to in-situ measure the melt temperature during injection molding is proposed for the first time. An ultrasonic velocity equation and PVT (Pressure-Volume-Temperature) equation for polymers are used to construct a correlation model of the temperature, ultrasonic velocity and pressure of molten polymers. The results of the proposed ultrasonic method are much closer to the results of an infrared fiber optic temperature sensor compared to a thermocouple. Moreover, experiments were carried out to measure the melt temperatures of LDPE (low density polyethylene) at different injection speeds and melt temperatures where measurement errors are less than 7.5 %. The melt temperature of PVC (polyvinyl chloride) was also measured at different injection speeds and the measurement errors are less than 13.5 %. The results of the proposed method have a good consistency in the shape and trend with those from an infrared sensor throughout the process. The proposed method possesses advantages, such as low-cost, easy installation, informative output, real-time operation and high precision, which shows a great potential for applications in the polymer industry.