CURE CHARACTERIZATION OF THICK SMC PARTS USING DIELECTRIC AND FINITE-DIFFERENCE ANALYSIS

Curing characteristics of thick composite parts have long been known t o be a function of thickness and location within the parts. While cure state as a function of thickness has often been modeled, actual exper imental verification has been difficult. In this work, disposable and permanently mounted dielectric sensors were used to characterize the c ure in polyester sheet molding compound at various locations through t he thickness of the part in a simulated molding environment. Using est ablished techniques, the dielectric and temperature information were c ombined to yield local cure state information for each sensor. Parts u nder five millimeters thick were found to cure rather uniformly while parts greater than this had increasing degrees of nonuniformity in cur e behavior through the thickness. Cure, time differences of up to thre e minutes, between center and edge, were observed in parts 20 millimet ers thick. These observed cure state data are compared to finite diffe rence model predictions. The model simulations show that in thick part s, a cure ''wave'' develops, starting at the SMC/mold interface and pr oceeds toward the center of the part. The ''wave'' increases in veloci ty as it moves toward the center of the part. The overall effect is th at a relatively uniform cure rate is observed at the SMC/mold interfac e 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 proce ss.