Injection Molding
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Flow marks appearing on the surface of random copolymer polypropylene injection molded products
Fenómeno del problema
Concentric circular or wavy streaks centered on the gate appear on the product surface, namely flow marks.
The surface at flow marks is rough and the color may be slightly different, affecting product appearance quality; they are more common in high-gloss products.
Análisis de la causa
(1) The temperature of the melt front repeatedly rises and falls when flowing in the cavity, forming alternating hot-cold flow marks
(2) Injection speed is too slow or unstable; the melt front stagnates intermittently during filling
(3) Mold temperature is too low; the melt cools rapidly after contacting the mold wall, forming a cold material layer
(4) Improper barrel temperature setting; low nozzle temperature produces cold material
(5) Mold gate size is too small; the melt generates shear heat and pressure loss when passing through the gate
Solución
(1) Increase injection speed, using multi-stage injection to maintain a stable speed front during filling
(2) Increase mold temperature to the higher value within the 40-80°C range to reduce melt temperature drop
(3) Increase nozzle temperature to match the front barrel temperature or 5-10°C higher to prevent cold material
(4) Increase gate size to reduce shear and pressure loss when the melt passes through the gate
(5) Appropriately increase barrel temperature to the higher value within the 190-260°C range
Conclusión clave
Flow marks are a manifestation of an unstable melt flow front; the flow front is stabilized by increasing injection speed, mold temperature and melt temperature.
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