Injection Molding
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Cracking/micro-cracks appearing on the surface of random copolymer polypropylene injection molded products (variant: strengthened measures)
Problem Phenomenon
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.
Especially, additional attention is needed on top of the original basis; this problem is more prominent and requires focused attention.
Cause Analysis
(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
(6) On top of the original basis, additional attention is needed as process parameter fluctuations or equipment condition changes aggravate this problem
Solution
(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
(6) And strengthen process monitoring; on top of the original basis, additional attention is needed to pay special attention to parameter stability
Key Takeaway
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.
In different production scenarios, targeted preventive measures need to be taken.
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