Prediction of process-induced warpage in injection molded thermoplastics

The main cause of warpage in injection moldings is the imbalance of the thermal residual stresses that are caused by a non-uniform temperature distribution through the thickness of the moldings resulting from variation in cross sections, part geometries, and temperature difference between the mold surfaces. As the hot plastic melt is injected into the relatively cooler mold, a temperature gradient develops between the core of the molding and its surfaces, determining the magnitude of the residual stresses and warpage deflection. The relationship between the temperature difference of the two halves of the mold and warpage for a flat plate was measured and predicted by use of a finite element software package. The development of warpage in a 3D component (L-shaped bracket) was also measured, and the results were compared with computer predictions.