In polymer processing weld-lines occur when a polymer melt is separated and then reunited. The internal structure of a weld-line region is determined by the rheological state of the reunited melt streams. An experimental investigation of the region of interest within the mold is almost impossible. Thus, computer simulation is necessary to examine weld-lines. This paper presents a detailed analysis of the flow behavior at a weld-line interface which was caused by a flow obstacle. The results 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 melt there is a weld-line over the full length of the part. Its formation mechanism was additionally described by experimental filling studies by the sandwich injection molding technique. In conclusion, the formation of the weld-line should be determined not only by the flow fronts, but also by the thermo-theological history of the reunited melt streams.