Designing with Weld Lines

Knit lines, known also as weld lines, are formed when two melt fronts converge and joined together, forming a thin, fine line in a plastic injected part. In a plastic injected part, weld lines can also be formed as a result of flow interruptions such thin sections, holes, slots, and other types of cored-out areas. Weld lines affect the strength and aesthetics of plastic injected plastic.

If there are holes, ribs, bosses, and opposite gates in plastic injected parts(Fig 1), the plastic flow in a mold is split by an obstruction such as a pin, insert, corner, or slot, weld lines will usually result when the flow fronts meet, as in Fig 1 a and Fig 1 b. Weld lines are particularly noticeable in transparent and translucent plastic materials.

Weld lines not only distort the aesthetics of plastic injected parts, but also affect the strength of plastic injected part for the point of potential failure. This problem occurs mostly to thermoset  and thermoplastic materials used in injection. Weld lines can be prevented by designing the mold so as to permit the material to move with maximum freedom.

An overflow located at the converging melt fronts is a technique. That permits the plastic material at the weld line to flow into a pocket, allowing the two melt fronts to bond together with higher injection pressure, as Fig 1 c. However, after molding, this overflow may need to be removed manually.

Setting a porous metal insert at the place of melt fronts converge and join is another technique. This can remove trapped gases at the converging melt fronts and providing a higher weld line strength, as Fig 1 d. This is recommended mostly for textured surfaces since on smooth surfaces a slight gloss difference can be found on the plastic injected parts.

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Fig 1 Forming and Solution of weld line