Injection Molding
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Stress Whitening Near the Gate of Block Copolymer Polypropylene Injection Molded Products
Problembeschreibung
A white hazy area appears near the gate of the product, with obvious color difference from the surrounding normal areas.
The molecular chains inside the material in the stress whitening area are severely oriented, the mechanical properties decline, and it is easy to crack from there when subjected to external force.
Ursachenanalyse
（1）The gate size is too small, the shear rate is too high when the melt passes through the gate, 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 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 produces residual stress
（5）The mold temperature is too low, and the melt near the gate cools rapidly and freezes the orientation
Lösung
（1）Increase the gate size, with the gate thickness being at least 50-70% of the product wall thickness, to reduce shear
（2）Reduce the injection speed, especially the injection speed at the beginning of mold filling
（3）Increase the melt temperature to a higher value within the 200-270 range to reduce the melt viscosity
（4）Appropriately reduce the holding pressure to avoid over-packing near the gate
（5）Increase the mold temperature to a higher value within the 40-90°C range, which is beneficial to orientation relaxation
Wichtigste Erkenntnis
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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