Injection Molding
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Flow marks appearing on the surface of polypropylene injection molded products
Fenómeno do problema
Concentric circular or wavy stripes centered on the gate appear on the product surface, that is, 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.
Análise das causas
（1）When the melt flows in the cavity, the front temperature repeatedly rises and falls, forming flow marks of alternating hot and cold
（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 slug layer
（4）The barrel temperature is set improperly, and the nozzle temperature is too low, producing cold slugs
（5）The mold gate size is too small, and the melt generates shear heat and pressure loss when passing through the gate
Solução
（1）Increase the injection speed, using multi-stage injection to maintain a stable speed front during mold filling
（2）Raise the mold temperature to the higher value within the range of 40-90°C to reduce the melt temperature drop
（3）Raise the nozzle temperature to be consistent with the front barrel temperature or 5-10°C higher to prevent cold slugs
（4）Increase the gate size to reduce the shear and pressure loss of the melt when passing through the gate
（5）Appropriately raise the barrel temperature to the higher value within the range of 200-280°C
Conclusão principal
Flow marks are a manifestation of an unstable melt flow front, and the flow front is stabilized by raising the injection speed, mold temperature, and melt temperature.
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