Injection Molding
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Stress whitening appearing near the gate of random copolymer polypropylene injection molded products
Fenómeno do problema
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.
Análise das causas
(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
Solução
(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
Conclusão principal
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.
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