Injection Molding
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Flow Marks Appearing on the Surface of Low-Density Polyethylene Injection Molded Products
Fenómeno del problema
Concentric circular or wavy stripes centered on the gate appear on the product surface, i.e., flow marks.
The surface at the flow marks is rough, and the color may be slightly different, affecting the appearance quality of the product, mostly seen in high-gloss products.
Análisis de la causa
（1）When the melt flows in the cavity, the front temperature repeatedly rises and falls, forming alternating hot and cold flow marks
（2）The injection speed is too slow or unstable, and the melt front stagnates intermittently during mold filling
（3）The mold temperature is too low, and the melt cools rapidly after contacting the mold wall, forming a cold material layer
（4）The barrel temperature is set improperly, and the nozzle temperature is low, producing cold material
（5）The mold gate size is too small, and the melt generates shear heat and pressure loss when passing through the gate
Solución
（1）Increase the injection speed, adopt multi-stage injection, and maintain a stable speed front during mold filling
（2）Increase the mold temperature to a higher value within the 30-60°C range to reduce the melt temperature drop
（3）Increase the nozzle temperature to be consistent with the front barrel temperature or 5-10°C higher to prevent cold material
（4）Increase the gate size to reduce the shear and pressure loss when the melt passes through the gate
（5）Appropriately increase the barrel temperature to a higher value within the 160-220°C range
Conclusión clave
Flow marks are a manifestation of an unstable melt flow front, and the flow front is stabilized by increasing the injection speed, mold temperature, and melt temperature.
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