Dr. Day, CURE CHARACTERIZATION OF THICK SMC PARTS USING DIELECTRIC AND FINITE-DIFFERENCE ANALYSIS, Journal of reinforced plastics and composites, 13(10), 1994, pp. 918-926
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.