In 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. Whereas the complete elimination of weld lines is not always successful, their adverse effects on part appearance and mechanical performance can be minimized. To reduce the strength loss of weld lines, various approaches are available, although the improvement is sometimes rather limited. The methods include optimization of material composition, mold design, and process conditions. To this purpose, the paper experimentally explores the influence of in-flow on the strength of weld lines for two polypropylene compounds, respectively reinforced with glass fibers and talc. 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. To provide a better understanding of the phenomenon, structural finite element software was used to predict the weld line strength as a function of the reinforcement distribution and alignment and the presence of voids and internal defects.