Investigation of the in-flow effect on weld lines in injection molding of glass fiber reinforced polypropylene

In the injection molding process, weld lines can occur when two flow fronts rejoin due to either multi-gated molds or obstacles in the mold cavity. The weakness of plastic at weld lines provides serious difficulties for the design and long term durability of injection molded parts. Various methods to reduce the strength loss of weld lines include optimization of material composition, mold design and process conditions. To this purpose, this paper experimentally explores the influence of in-flow on the strength of weld lines for a commercial polypropylene compound reinforced with glass fibers. In-flow is defined as the flow within the mold cavity, below the solidified layer, that continues after the local region of the mold cavity is filled. In particular, the comparison of the weld line strength between specimens manufactured with and without in-flow was carried out and related to the reinforcement distribution in the welding zone.