Injection Molding
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Stress whitening appearing near the gate of random copolymer polypropylene injection molded products (variant: strengthened measures)
Problem Phenomenon
A white misty area appears near the product gate, with obvious color difference from the surrounding normal area.
In the stress whitening area, molecular chains inside the material are severely oriented, mechanical properties decline, and the product easily cracks from there under external force.
Especially, additional attention is needed on top of the original basis; this problem is more prominent and requires focused attention.
Cause Analysis
(1) Gate size is too small; shear rate is too high when the melt passes through the gate, and molecular chains are strongly oriented
(2) Injection speed is too fast; shear stress at the gate is too large
(3) Melt temperature is on the low side; melt viscosity is high, and shear through the gate is greater
(4) Holding pressure is too high; over-packing near the gate produces residual stress
(5) Mold temperature is too low; melt near the gate rapidly cools and freezes the orientation
(6) On top of the original basis, additional attention is needed as process parameter fluctuations or equipment condition changes aggravate this problem
Solution
(1) Increase gate size; gate thickness is at least 50-70% of product wall thickness to reduce shear
(2) Reduce injection speed, especially the injection speed at the beginning of filling
(3) Increase melt temperature to the higher value within the 190-260°C range to reduce melt viscosity
(4) Appropriately reduce holding pressure to avoid over-packing near the gate
(5) Increase mold temperature to the higher value within the 40-80°C range to facilitate orientation relaxation
(6) And strengthen process monitoring; on top of the original basis, additional attention is needed to pay 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.
In different production scenarios, targeted preventive measures need to be taken.
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