Injection Molding
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Crazing/Microcracks Appearing on the Surface of High-Density Polyethylene Injection-Molded Products
Fenómeno del problema
Fine network cracks or linear microcracks appear on the product surface, mostly near the gate, at corners, or at ejector pin positions.
Microcracks expand under stress or environmental stress, eventually causing the product to crack and fail.
Análisis de la causa
(1) The internal stress is too large; the ejection force during demolding is uneven or the product structure has stress concentration
(2) The melt temperature is too low, the molecular chain orientation is severe, and the residual internal stress is large
(3) The mold temperature is too low, and the melt cools rapidly, generating large temperature gradient stress
(4) The holding pressure is too high or the holding time is too long, and over-packing generates residual stress
(5) The draft angle is insufficient or the mold surface is rough, and the large demolding resistance produces microcracks
Solución
(1) Increase the melt temperature to the higher value within the range of 180-260°C to reduce molecular orientation
(2) Increase the mold temperature to the higher value within the range of 30-80°C to reduce the cooling rate and reduce internal stress
(3) Appropriately reduce the holding pressure and holding time to avoid over-packing
(4) Increase the draft angle; the draft angle for polyolefin materials is at least 1-2°, and polish the mold surface
(5) Optimize the ejection system, increase the number or area of ejector pins, and ensure uniform ejection
Conclusión clave
Microcracks are a manifestation of excessive residual internal stress, and need to be prevented by optimizing temperature parameters, reducing internal stress, and improving demolding conditions.
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