Experimental Verification on Simulating Shrinkage and Warpage of Thin Compression Moulded SMC Parts

Abstract

One of the important issues in designing plastic parts, especially those that are fibre reinforced, is to predict shrinkage and warpage of the moulded parts. Shrinkage and warpage result from material inhomogeneities caused by flow induced fibre orientation, curing, poor thermal mould lay-out, and processing conditions. Shrinkage and warpage are directly related to residual stresses which result from locally varying strain fields that occur during the curing or solidification stage of a manufacturing process. This paper presents research conducted in numerical simulation and experimental investigation of the shrinkage and warpage of compression moulded fibre reinforced composite parts. A coupled temperature and stress simulation program with a three-noded shell element formulation was developed to calculate the residual stress build-up during curing and solidification stages of a compression moulding process. The numerical model also included the spring forward effect present in curved parts. Correlation between experimentally measured and simulated shrinkage and warpage of the moulded composites is presented for various geometries and processing conditions. The simulated results have shown excellent agreement with the experimental data. The results indicated that fibre orientation distributions which vary with different charge locations have a significant effect on the final warpage. Shrinkage and warpage can be substantially reduced by using lower shrinkage polyester matrix or by increasing the fibre content within the charge. Unsymmetric curing caused by uneven mould temperatures leads to a thermal moment which could increase or possibly even reduce warpage of a finished part. Preheating or increasing the fibre content of the charge could also help reduce the final warpage. The spring forward effect was found to be significant in large-curved parts.