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.

注射成型被认为是制造高质量聚合物产品的最重要过程。然而，熔融聚合物在注射成型期间在型腔中经历急剧的温度变化，这极大地影响了成型产品的尺寸精度，重量一致性和结晶度。因此，在线测量熔融聚合物（熔体）的温度具有重要意义。首次提出了一种超声波方法在注射成型过程中原位测量熔体温度。使用聚合物的超声速度方程和PVT（压力-体积-温度）方程构建熔融聚合物的温度，超声速度和压力的相关模型。与热电偶相比，所提出的超声方法的结果更接近于红外光纤温度传感器的结果。而且，进行了实验以测量在不同的注射速度和熔融温度下的LDPE（低密度聚乙烯）的熔融温度，其中测量误差小于7.5％。还测量了不同注射速度下的PVC（聚氯乙烯）的熔融温度，测量误差小于13.5％。在整个过程中，所提出的方法的结果在形状和趋势上与红外传感器的结果具有良好的一致性。该方法具有成本低，安装简便，输出信息量大，实时性强，精度高等优点，在聚合物工业中具有广阔的应用前景。