Plane-strain compression molding analysis of sheet molding compounds in flat and cross-sectional T-shape molds

Abstract

Sheet molding compounds (SMC) made of unsaturated polyester resin and other additives, reinforced with randomly distributed chopped fiberglass strands, have emerged recently as a substitute for steel automotive outer body panels because of their low weight, high stiffness, and non-corrosiveness per unit mass. During the mold filling and curing stages of the compression molding of SMC, the process is non-isothermal since the molding temperature is usually about 150 degrees C. Thus, an understanding of heat transfer and flow characteristics during the fabrication of SMC under various molding conditions is of importance. In the present study, an experimental sep-up was designed and used for plane-strain molding experiments of class-B and A SMC in flat and cross-sectional T-shape molds in order to investigate the effect of molding parameters such as the mold geometry, the thickness of initial charge dimension, three different mold closing speeds of 15, 45, and 50 mm min(-1), and two different molding temperatures of 130 and 150 degrees C on the flow characteristics. Experiments with different colored SMC layers were used in capturing the flow patterns at various compression stages. A one-dimensional finite-difference solution in the thickness direction was obtained and compared to the measure temperature data obtained from molding experiments at two locations in the SMC charge. For determination of the machine capacity for practical use, approximate solutions were obtained by applying the slab method under various constant shear frictional conditions. The calculated pressure distributions at the center of the top mold, and the load requirements for flat and cross-sectional T-shape moldings obtained from slab analyses, were compared with the measure data. The predicted values of temperature, pressure, and load were found to compare reasonably well with the measured data. The experimental observations obtained from the present investigation will be useful for mold design and process control of compression molding of SMC in practice.