Cure Characteristics of Sheet Molding Compound in Panels with Integrated Ribs

Abstract

The cure characteristics of sheet molding compound (SMC) panels with integrated ribs were studied using a ribbed mold instrumented with 38 thermocouples, 15 pressure transducers, and 4 linear position transducers (LPTs). The experiments showed that the polymerization of the material opposite the ribs takes place later than the material in the adjacent areas. This delayed reaction was identified by a time lag in the occurrence of the peak exotherm for the material opposite the ribs. The significance of this observa tion is that because the polymerization of the material is accompanied by some degree of volume change, this time lag leads to a differential volume change in the neighboring regions which in turn leads to a higher degree of rib readout. In order to better understand the nature of this time lag, the effects of variation in the amounts of inhibitor and initiator, the preheating of the material, and the variation in the types of resin were investigated. It was observed that, for a given resin chemistry, reducing the amount of initiator or increas ing the amount of inhibitor in the formulation decreases the time lag. The preheating of the material, on the other hand, was found to increase the time lag. This is in contrast to the general belief that preheating can be beneficial to surface appearance. As to the varia tion in the chemistry of the resin, the data showed that the time lag for a given resin can not be predicted solely based on its peak exotherm time. Therefore, it was concluded that whenever a new type of resin is used, the time lag has to be measured directly.