A major factor that weakens the weld line in injection moldings is the V‐notch structure. Though the existence of a V‐notch is well known, its depth variation with molding conditions has not been detailed. The aim of this paper is to clarify the V‐notch structure and its effect on the strength of general purpose polystyrene injection moldings. A dog bone type tensile specimen with a weld line was molded under several molding conditions. The surface of the weld line was partially eliminated by cutting with a milling machine to seven levels of cut depth (Dc). As a result, the weld strength increased with Dc to about 50%. The relationship between the weld strength and Dc made it possible to determine the V‐notch depth, which vas defined as the “depth of the weld line.” From these results, a hypothesis is proposed that the V‐notch has a structure with a fine groove on the surface and a poorly bonded inner layer. This study considered the relationships among the weld strength, the depth of the weld line, and molding conditions.

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