Effects of Process Conditions on Shrinkage and Warpage: Experiments and Simulations

Abstract

In the injection mold process, a pressure gradient exists from the polymer entrance to the last-fill location. At different planar locations of a part, when the polymer melt cools down to the transition temperature and freezes (changes from liquid to solid) at different pressures, shrinkage at the various locations will be different. If cooling channels are not arranged properly, the mold wall temperatures on the cavity and core sides can be different. This unbalanced cooling can also cause the melt at the upper and lower halves of the cavity to shrink differently, because they freeze at different times and different pressures. These two types of non-uniform shrinkage will cause parts to warp. Reducing shrinkage and warpage is one of the top priorities for improving the quality of injection molded parts. In addition to part design and material properties, process conditions are the most important determinants of part quality. In this paper, the relationship between process conditions and in-cavity residual stress will be studied. In-cavity residual stress is the driving force that causes parts to deform after they are taken out of the mold. The effects of process conditions on injection-molded part quality (in terms of shrinkage and warpage) will be discussed. Different packing pressure levels, together with unbalanced cooling from mold wall temperatures, will be examined. Deformation of injection molded parts will be measured. Comparisons between experimental and numerical simulation results will be reported.