Injection Molding
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Low Weld Strength of High-Density Polyethylene Injection Molded Products
Problem Phenomenon
The strength of the product at the weld line is significantly lower than that of other parts, and it breaks from the weld line when stressed.
The weld strength is usually 50-80% of the base material strength, and affects the safety of use in stressed and sealed products.
Cause Analysis
（1）The melt convergence temperature is low, and the two melt streams are not fully fused
（2）The injection pressure transmission is insufficient, and the holding pressure shrinkage compensation at the weld line is not enough
（3）The mold temperature is low, and the melt convergence area cools rapidly
（4）The cavity venting is poor, and the gas at the convergence area hinders fusion
（5）The weld line is located at the stressed part of the product
Solution
（1）Increase the melt temperature to a higher value within the 180-260°C range
（2）Increase the mold temperature to a higher value within the 30-80°C range
（3）Increase the injection pressure and holding pressure to ensure the weld line is fully compacted
（4）Improve the mold venting, and set vent grooves or overflow grooves at the weld line
（5）Adjust the gate position to move the weld line to a non-stressed part
Key Takeaway
Low weld strength is caused by insufficient melt fusion, and the weld quality needs to be enhanced by increasing the temperature, the pressure, and improving the venting.
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