How to Control Weld Line Position through Gate Optimization

Weld lines form when two or more molten plastic flow fronts converge inside the mold cavity. As the starting point of melt entering the cavity, the gate determines the flow track, convergence position, fusion temperature and fusion quality of the material flow. Reasonable gate optimization is the most direct and efficient means to control weld lines, which can reduce the workload of repeated trial mold modification in the early stage of mold development. The core idea of gate optimization is not to completely eliminate weld lines, but to transfer them to non-critical areas or improve the fusion quality at the convergence position.

Adjust gate location to shift weld lines to non-critical zones

The gate should be arranged to guide the melt flow, so that the material convergence point falls on hidden assembly positions, thick wall areas or secondary surfaces, avoiding main appearance surfaces, sealing surfaces and stress-bearing parts such as threads. For frame and ring products with single-point gating, the weld line will appear on the opposite side of the gate. If this position is the appearance surface, the gate can be moved to side buckles, ribs and other sheltered structures. For products with through holes and bosses, the gate is set upstream of the hole position, so that the melt converges after bypassing the hole structure, preventing weld lines on the matching surface around the hole.

Reduce gate quantity to minimize flow convergence points

Every additional gate will add a new material convergence point. Single-point gating only produces one weld line, which is preferred for appearance parts when the flow length ratio meets the material limit. For large products with long flow distance that have to adopt multiple gates, it is necessary to ensure that the melt arrives at the convergence position at the same time. If one flow front cools in advance, the low-temperature fusion will form obvious weak weld marks. The distance between multiple gates should not be too close, otherwise the weld lines will be concentrated and the structural strength of the position will drop sharply.

Optimize gate style and cross-section size to improve melt fusion

Small gates generate high shear heat, increase melt fluidity and make weld lines lighter, but excessive shear will cause burning. Large gates deliver stable flow, which is suitable for shear-sensitive materials such as PC, while the weld trace is more obvious. Fan gates and film gates make the melt spread evenly, and the fusion effect at the convergence position is better. Submarine gates are easy to form jet flow, resulting in scattered irregular weld lines, so the injection direction must be strictly controlled when used on appearance parts.

Sequential valve gates for dynamic weld line shifting

For large panels and long shells, sequential valve gates can open in time sequence. The melt is pushed forward continuously, and the weld line is transferred to the overflow tank or exhaust position at the end of the product. This scheme has high cost and strict requirements for timing debugging, and is widely used in high-standard automotive and home appliance shell products.

Auxiliary supporting design

Vent slots and overflow wells are opened at the preset weld line position to discharge trapped air and cold material. Combined with moldflow analysis, multiple gate schemes are compared before mold processing. When the product structure inevitably produces weld lines, local thickening, decorative ribs or texture can be used to cover the traces, so as to balance product quality and mold development cost.