Injection Molding
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General-purpose polyethylene injection molded products surface cracking/microcracks
Fenómeno do problema
Fine reticular cracks or linear microcracks appear on the product surface, mostly near the gate, at corners or at ejector pin locations.
Microcracks propagate under stress or environmental stress, eventually causing the product to crack and fail.
Análise das causas
（1） Internal stress is too large, ejection force is uneven during demolding, or stress concentrates in the product structure\
（2） Melt temperature is too low, molecular chain orientation is severe, and residual internal stress is large\
（3） Mold temperature is too low, the melt cools rapidly producing large temperature gradient stress\
（4） Holding pressure too high or holding time too long, overpacking produces residual stress\
（5） Insufficient draft angle or rough mold surface, large demolding resistance produces microcracks
Solução
（1） Increase melt temperature to the higher value within the 160-260°C range, reduce molecular orientation\
（2） Increase mold temperature to the higher value within the 30-80°C range, reduce cooling rate to reduce internal stress\
（3） Appropriately reduce holding pressure and holding time to avoid overpacking\
（4） Increase the draft angle, polyolefin materials need a draft angle of at least 1-2°, polish the mold surface\
（5） Optimize the ejection system, increase the number or area of ejector pins, ensure uniform ejection
Conclusão principal
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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