An experimental study was carried out in this study to improve the surface grade of light-weight structural parts of compression molded SMC (sheet molding compounds). For this purpose, material property was characterized as a function of strain rate and temperature by simple compression test using grease oil between dies and SMC specimen. The characterized material property was verified by simulating the flat and cross-sectional T-shape molding based on thermo-viscoplastic finite element approach. The comparisons of loads between experiments and numerical simulation demonstrated the accuracy of the proposed material property.
Material flow and temperature variations during compression molding were experimentally observed in the present investigation. Simplified slab and one dimensional finite difference analyses were conducted in determining the load and temperature predictions, respectively. Depending on process variables such as molding temperatures, mold closing speeds, and mold geometries, the mechanical properties were measured in terms of tensile, 3-point bending, and IZOD impact tests. In addition, fiber orientations were examined through taking scanning electron microscopy and taking photographs of the burnt specimen.
In order to determine the surface quality of the molded parts, the surface roughness was measured at various locations. Then, the depth of the sink mark in the cross-sectional T-shape specimen was measured under various molding and geometric conditions. Based on the measured experimental data, non- dimensionalized empirical equation for predicting the sink mark depth was determined. The predicted values were compared well to experimental values for different molding cases.
From the present investigation, it was found out that the surface quality improves when low molding temperature, fast mold closing speed, and in-plane flow was applied.
Finally, in-mold coating set up was designed and manufactured to remove pin- holes or sink marks observed in the molded SMC parts. It was construed that the surface grade of light-weight structural parts of compression molded SMC could be improved by applying in-mold coating technique.