Injection Molding
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General-purpose polyethylene injection molded products weld lines
Problem Phenomenon
Linear marks appear on the product surface where two or more melt fronts converge, usually showing a V-shaped groove or color difference line, with low strength.
Weld lines not only affect appearance, but are also the weak link of the product, easily cracking from this location under stress.
Cause Analysis
（1） Multiple gate design or complex product structure causes the melt to split and then converge; the melt temperature at the convergence is low and fusion is insufficient\
（2） Injection speed too slow, the melt temperature drops too much during filling, and cannot fully fuse when converging\
（3） Barrel temperature too low, melt flowability poor, insufficient pressure at the convergence\
（4） Mold temperature too low, the melt cools rapidly after contacting the mold wall, and the fusion strength at the convergence interface is low\
（5） Poor venting, gas at the convergence cannot escape forming a barrier layer
Solution
（1） Increase melt temperature to the higher value within the 160-260°C range, improve melt flowability\
（2） Increase mold temperature to the higher value within the 30-80°C range, delay melt cooling\
（3） Increase injection speed, shorten filling time, reduce melt temperature loss\
（4） Adjust gate position and number, minimize melt splitting; if necessary add overflow wells to eliminate weld lines\
（5） Improve mold venting, set vent grooves or vent pins at weld line locations
Key Takeaway
Weld lines are a manifestation of poor melt convergence and fusion, improving melt fusion quality by increasing melt temperature, injection speed and mold temperature.
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