FLOW DISTURBANCE IN POLYMER MELT BEHIND AN OBSTACLE

In polymer processing weld-lines occur when a polymer melt is separate d and then reunited. The internal structure of a weld-line region is d etermined by the rheological state of the reunited melt streams. An ex perimental investigation of the region of interest within the mold is almost impossible. Thus, computer simulation is necessary to examine w eld-lines. This paper presents a detailed analysis of the flow behavio r at a weld-line interface which was caused by a flow obstacle. The re sults show a gradual increase of the velocity within a short distance behind the obstacle which coincides with a short weld-line determined by the advancement of the flow fronts obtained from conventional mold filling studies. However, based on the residence time history of the m elt there is a weld-line over the full length of the part. Its formati on mechanism was additionally described by experimental filling studie s by the sandwich injection molding technique. In conclusion, the form ation of the weld-line should be determined not only by the flow front s, but also by the thermo-theological history of the reunited melt str eams.