Cure Characterization of Thick SMC Parts Using Dielectric and Finite Difference Analysis

Curing characteristics of thick composite parts have long been known to be a function of thickness and location within the parts. While cure state as a function of thickness has often been modeled, actual experimental verification has been difficult. In this work, disposable and permanently mounted dielectric sensors were used to characterize the cure in polyester sheet molding compound at various locations through the thickness of the part in a simulated molding environment. Using established tech niques, the dielectric and temperature information were combined to yield local cure state information for each sensor. Parts under five millimeters thick were found to cure rather uniformly, while parts greater than this had increasing degrees of nonuniformity in cure behavior through the thickness. Cure time differences of up to three minutes, between center and edge, were observed in parts 20 millimeters thick. These observed cure state data are compared to finite difference model predictions. The model simulations show that in thick parts, a cure "wave" develops, starting at the SMC/mold interface and proceeds toward the center of the part. The "wave" increases in velocity as it moves toward the center of the part. The overall effect is that a relatively uniform cure rate is observed at the SMC/mold interface while in the middle of the SMC part, a long induction period occurs, followed by an extremely fast cure. The model predictions, which are confirmed by the sensor cure data, may be used to predict the behavior and molding cycle time required for new parts during the design process.