When thermosetting plastic injection molding (also called extrusion molding) or thermoplastic injection molding, if the part has an insert or the mold cavity is equipped with a core for molding holes, the molten plastic surrounds these inserts or When the core flow is filled, a weld line (or weld seam) will be generated at the confluence of the material flow on the workpiece. If the forming mold of an article has more than two feed gates, weld lines will also be produced at the confluence of the two streams at the time of molding. The mechanical strength of the weld line is much lower than the rest of the piece. When the parts are subject to vibration, shock or environmental temperature changes, the parts can easily crack from the weld line. In some cases, even when stored under normal storage conditions, cracks may form from the weld line. Especially for thermosetting plastics, when the insert or core is far from the gate, or the size of the insert or core is large, the flow of the molten plastic around the insert or the core is long, and the curing reaction has been performed Deep, the material's fluidity and fusion are poor. In this case, the strength at the weld line of the article is lower and it is easier to crack from the weld line.