PREDICTING SHRINKAGE AND WARPAGE OF THIN COMPRESSION MOLDED FIBER-REINFORCED THERMOSET PARTS

One of the most challenging tasks in designing plastic parts, especial ly those that are fiber reinforced, is to predict shrinkage and warpag e of the molded parts. Shrinkage and warpage result from material inho mogeneities caused by flow induced fiber orientation, curing, poor the rmal mold lay-out, and processing conditions. Shrinkage and warpage ar e directly related to residual stresses which result from locally vary ing strain fields that occur during the curing or solidification stage of a manufacturing process. This paper presents research conducted in modeling, analysis, and process simulation of the thermomechanical be havior of compression molded fiber reinforced composite parts. A coupl ed temperature and stress simulation program with a three-noded shell element formulation was developed to calculate the residual stress bui ld-up during curing and solidification stages of a compression molding process. Results from the simulation revealed that fiber orientation distributions which vary with different charge locations have a signif icant effect on the final warpage. Shrinkage and warpage can be substa ntially reduced by using lower shrinkage polyester matrix or by increa sing the fiber content within the charge. Unsymmetric curing caused by uneven mold temperatures leads to a thermal moment which could increa se- or possibly even reduce warpage of a finished part.