Injection Molding
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Cracking/micro-cracks appearing on the surface of random copolymer polypropylene injection molded products
Fenómeno do problema
Fine reticular cracks or linear micro-cracks appear on the product surface, mostly near gates, corners or ejector pin positions.
Micro-cracks propagate under stress or environmental stress, eventually causing product cracking and failure.
Análise das causas
(1) Internal stress is too large; ejection force is uneven during demolding or product structure has stress concentration
(2) Melt temperature is too low; molecular chain orientation is severe and residual internal stress is large
(3) Mold temperature is too low; rapid melt cooling produces large temperature gradient stress
(4) Holding pressure is too high or holding time is too long; over-packing produces residual stress
(5) Draft angle is insufficient or the mold surface is rough; large demolding resistance produces micro-cracks
Solução
(1) Increase melt temperature to the higher value within the 190-260°C range to reduce molecular orientation
(2) Increase mold temperature to the higher value within the 40-80°C range to reduce cooling rate and internal stress
(3) Appropriately reduce holding pressure and holding time to avoid over-packing
(4) Increase draft angle, at least 1-2° for polyolefin materials, and polish the mold surface
(5) Optimize the ejection system, increasing the number or area of ejector pins to ensure uniform ejection
Conclusão principal
Micro-cracks 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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