In-Situ Shrinkage Sensor for Injection Molding

Dimensional consistency is a critical attribute for injection molded part quality and is highly dependent on the polymer morphology, the thermal expansion, and various processing parameters. The dimensional shrinkage can be estimated by knowing the pressure-volume-temperature behavior of the polymer but with limited accuracy. There are various process monitoring systems available in the market; none of which has the capability of directly monitoring and controlling the real time shrinkage and part dimensions online. With a view to measuring in-mold shrinkage, a button cell type in-mold shrinkage sensor was developed, validated, and compared against the traditional shrinkage prediction and estimation methods. The shrinkage sensor consists of an elastic diaphragm instrumented with strain gages connected in a full bridge circuit. The sensor is placed beneath the movable pin that is protruded into the mold cavity and remains in contact with the sensor diaphragm. The sensor diaphragm is deflected due to the melt pressure acting on the pin into the mold cavity and is retracted back toward its original position as the melt solidifies and shrinks away from the mold cavity wall. The sensor signals acquired during each molding cycle were analyzed to validate the sensor performance in a design of experiments as a function of packing pressure, melt temperature, cooling time, and coolant temperature. The regression results indicate that the shrinkage sensor outperforms cavity pressure transducers and other methods of predicting the in-mold shrinkage. For polypropylene, the shrinkage sensor is able to measure the shrinkage to an average accuracy of 0.01 mm for a molded part with a nominal thickness of 2.5 mm. The coefficient of correlation, $R2$⁠, between the sensor’s final positions to the final part thickness was 0.921 for the in-mold shrinkage sensor. Other dimension prediction methods had lower correlation coefficients.