Estimation of sink mark depth in compression molded SMC parts with substructures

Abstract

Sink mark, a surface defect along the centerline over a substructure is observed when compression molding of SMC (Sheet Molding Compounds) applies in fabrication of a thin plate with substructures. The magnitude of sink mark formed depends on process parameters used in manufacturing process. The surface quality of the external panels in automotive industry is so critical that this kind of defect should be eliminated during manufacturing stages. Thus, the effect of process parameters on sink mark depth formed in the compression molded thin plate with substructures was experimentally investigated in the present study. In order to estimate the effect of the process parameters such as molding temperature, mold closing speed, depth of the rib, corner radius of the rib, and final molded part thickness of flat portion on the magnitude of sink mark depth under the given experimental conditions, the non-dimensionalized molding parameters were determined based on physical relevances among various parameters. Then, the non-dimensionalized depth of sink mark was determined through dimensional analysis based on the experimental data. It was further tested by comparing the estimated values of sink mark depth with the experimental data. According to this comparison, reasonably accurate prediction was obtained. Compared to the previous study in the literature, the sink mark depths were quantitatively determined according to the various processing parameters and the non-dimensional equation to predict the sink mark level was derived in the present investigation. In addition, the orientation and distribution of fiberglass which directly affect the formation and magnitude of the depth of sink mark were investigated by taking the photographs of the cross-sectional area of the compression molded specimen by SEM (Scanning Electron Microscope). The non-dimensionalized equation for prediction of sink mark depth proposed in the present investigation will be useful in estimating the magnitude of sink mark depth in compression molded SMC plates with substructures and the experimental observations were useful in understanding the formation of sink mark during compression molding.