Injection Molding
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Weld Lines Appearing on Block Copolymer Polypropylene Injection Molded Products
Problem Phenomenon
Line-shaped marks appear on the product surface where two or more melt streams converge, usually in the form of V-shaped grooves or color difference lines, with low strength.
Weld lines not only affect the appearance, but are also the weak link of the product, and it is easy to crack from there when stressed.
Cause Analysis
（1）Multi-gate design or complex product structure causes the melt to split and then converge, and the melt temperature at the convergence is low and the fusion is insufficient
（2）The injection speed is too slow, the melt temperature drops too much during mold filling, and it cannot be fully fused when converging
（3）The barrel temperature is low, the melt flowability is poor, and the pressure at the convergence is insufficient
（4）The mold temperature is too low, the melt cools rapidly after contacting the mold wall, and the fusion strength of the convergence interface is low
（5）Poor venting, and the gas at the convergence cannot be discharged, forming a barrier layer
Solution
（1）Increase the melt temperature to a higher value within the 200-270 range to improve the melt flowability
（2）Increase the mold temperature to a higher value within the 40-90°C range to delay melt cooling
（3）Increase the injection speed, shorten the mold filling time, and reduce the melt temperature loss
（4）Adjust the gate position and quantity to minimize melt splitting; add overflow grooves when necessary to remove weld lines
（5）Improve the mold venting, and set vent grooves or vent pins at the weld line position
Key Takeaway
Weld lines are a manifestation of poor melt convergence and fusion, and the melt fusion quality is improved by increasing the melt temperature, injection speed, and mold temperature.
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