Weld Lines in Plastic Injected Parts

Weld lines also called knit lines, are formed when two melt fronts converge and join forming a thin fine line in the plastic injected parts. This can be seen in untextured parts and in highly polished surfaces. The two converging melt fronts cool rapidly and bond poorly when they join. At this melt junction, gas is also trapped by the converging melt fronts. These fine lines cause a major weakness in the part that can caused part failure in its end use. Differences in gloss can also be noticed in weld line areas. Here are some causes and solutions below weld lines of plastic injected parts.

Moisture in the material due to insufficient drying can form at the melt front, forming an interface between the two melt fronts. This creates a weakness in the parts structure. Low melt temperatures cause the melt  fronts to cool faster, causing converging melt fronts to bond poorly. Slow injection speeds will allow the melt fronts to cool early in the part fill, and by the time the two melt front join, poor and weak bonding of the melt front occurs.

Low mold temperature cause the melt front to cool fast and set up before the converging melt fronts join. This creates weak weld lines and more pronounced weld lines. Poor venting at the location where melt fronts come together results in gas trapping at the melt front, preventing the melt fronts from coming together.

Using recommended drying conditions for the material is advised. Increasing melt temperatures increase flow and improve the melt front bonding. High packing and hold pressures are needed to adequately force the two melt fronts to join and bond, and to fill out the part completely to assure strong weld line strength or even to eliminate weld lines. Increasing hold time or packing time will also improve weld strength.

Raising the mold temperature will keep the material molten longer, resulting in stronger weld lines and even elimination of weld lines. Adding vents, using overflows, or using porous metal inserts can force air out, improving the weld line.

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