Injection Molding
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General-purpose polyethylene injection molded products stress whitening near the gate（Variant: Strengthened Measures）
Problem Phenomenon
A white hazy area appears near the product gate, with an obvious color difference from the surrounding normal area.
In the stress-whitened area, the molecular chains inside the material are severely oriented, mechanical properties decline, and cracking easily starts from this area under external force.In particular, extra attention is needed on top of the original basis, this problem is more prominent and requires key attention.
Cause Analysis
（1） Gate size too small, shear rate too high when the melt passes through the gate, molecular chains strongly oriented\
（2） Injection speed too fast, shear stress at the gate too large\
（3） Melt temperature too low, melt viscosity high, greater shear when passing through the gate\
（4） Holding pressure too high, overpacking near the gate produces residual stress\
（5） Mold temperature too low, the melt near the gate cools rapidly freezing the orientation\
（6） On top of the original basis, extra attention is needed: process parameter fluctuations or equipment status changes aggravate this problem
Solution
（1） Enlarge the gate size, gate thickness at least 50-70% of product wall thickness, reduce shear\
（2） Reduce injection speed, especially the injection speed at the initial filling stage\
（3） Increase melt temperature to the higher value within the 160-260°C range, reduce melt viscosity\
（4） Appropriately reduce holding pressure to avoid overpacking near the gate\
（5） Increase mold temperature to the higher value within the 30-80°C range, favorable for orientation relaxation\
（6） And strengthen process monitoring; on top of the original basis, extra attention is needed, paying special attention to parameter stability
Key Takeaway
Gate stress whitening is caused by excessive shear stress and molecular orientation, and is improved by enlarging the gate, reducing injection speed and increasing temperature.Targeted preventive measures need to be taken in different production scenarios.
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