Dimensional consistency is a critical attribute of molded products, yet part dimensions are frequently only estimated from cavity pressure or part weight measurements. An in-mold shrinkage sensor is designed and validated. The design includes a deflectable diaphragm instrumented with strain gages connected in a full bridge circuit. In operation, the diaphragm is deflected due to the pressure of the melt in the mold cavity. Molded part shrinkage is then measured as the polymer melt solidifies, shrinks, and retracts from the mold wall. A design of experiments is conducted to validate the performance of the sensor as a function of packing pressures, cooling time, melt temperature, and coolant temperature. The results indicate the sensor outperforms both cavity pressure transducers and regression models, and is able to measure the shrinkage to an absolute accuracy of 0.01 mm for a 2.5 mm thick part.

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