Injection molding is a common polymer processing technique due to its versatile and economical capability to produce parts having complex geometries and precise tolerances. Shrinkage is an intrinsic polymer property which has a significant effect over the final aesthetic and dimensional quality of the part. The shrinkage mainly depends on the polymer morphology, the coefficient of thermal expansion, and various processing parameters. The shrinkage can be estimated by knowing the pressure-volume-temperature behavior and processing history of the polymer but such estimates are limited in accuracy.This research focuses on the design and development of a novel elastic sensor having the capability to monitor the in-mold shrinkage. The sensor design is derived from button-type load cells which can be placed underneath a pin such as an ejector pin(s) within the ejector assembly. The deflection of the sensor diaphragm under the melt pressure during the filling stage can provide information related to the cavity pressure and the changes in the dynamic process behavior. Moreover, the return of the diaphragm during the packing and cooling stages monitors the shrinkage of the solidifying melt within the cavity. The shrinkage sensor developed in this study has a high correlation coefficient as a function of final part thickness, on the order of 0.95, and can characterize the behavior of in-mold shrinkage in real time. The shrinkage information acquired during the cycle can be used to monitor the process parameters and to achieve in-situ robust quality control through a closed loop control system. The sensor can also be used to provide conditions for more accurate prediction of the post molding shrinkage and the annealing conditions from the part dimensions within the mold.
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