Injection Molding
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Stress whitening near the gate of homopolymer polypropylene injection products
Fenómeno do problema
A white misty area appears near the product gate, with obvious color difference from surrounding normal areas.
The material inside the stress whitening area has severe molecular chain orientation, decreased mechanical properties, and easily cracks from there under external force.
Análise das causas
(1) Gate size too small; melt shear rate too high when passing through the gate, strong molecular chain orientation
(2) Injection speed too fast; excessive shear stress at the gate
(3) Melt temperature too low; high melt viscosity, greater shear when passing through the gate
(4) Holding pressure too high; over-packing near the gate produces residual stress
(5) Mold temperature too low; melt near the gate rapidly cools and freezes orientation
Solução
(1) Enlarge gate size; gate thickness at least 50-70% of product wall thickness, reduce shear
(2) Reduce injection speed, especially injection speed at the beginning of mold filling
(3) Increase melt temperature to the higher value within the 200-280°C range, reduce melt viscosity
(4) Appropriately reduce holding pressure, avoid over-packing near the gate
(5) Increase mold temperature to the higher value within the 40-90°C range, favoring orientation relaxation
Conclusão principal
Gate stress whitening is caused by excessive shear stress and molecular orientation; it is improved by enlarging the gate, reducing injection speed, and increasing temperature.
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