Injection Molding
4 조회수
Stress Whitening Near the Gate of High-Density Polyethylene Injection-Molded Products
문제 현상
A white misty area appears near the gate of the product, with an obvious color difference from the surrounding normal area.
In the stress-whitened area, the molecular chains inside the material are severely oriented, the mechanical properties are reduced, and it is easy to crack from here when subjected to external force.
원인 분석
(1) The gate size is too small, the shear rate of the melt when passing through the gate is too high, and the molecular chains are strongly oriented
(2) The injection speed is too fast, and the shear stress at the gate is too large
(3) The melt temperature is too low, the melt viscosity is high, and the shear is greater when passing through the gate
(4) The holding pressure is too high, and over-packing near the gate generates residual stress
(5) The mold temperature is too low, and the melt near the gate cools rapidly and freezes the orientation
해결 방법
(1) Increase the gate size; the gate thickness should be at least 50-70% of the product wall thickness to reduce shear
(2) Reduce the injection speed, especially the injection speed at the initial stage of filling
(3) Increase the melt temperature to the higher value within the range of 180-260°C to reduce the melt viscosity
(4) Appropriately reduce the holding pressure to avoid over-packing near the gate
(5) Increase the mold temperature to the higher value within the range of 30-80°C, which is beneficial to orientation relaxation
핵심 요약
Gate stress whitening is caused by excessive shear stress and molecular orientation, and is improved by enlarging the gate, reducing the injection speed, and increasing the temperature.
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