Injection Molding
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Crazing/microcracks on the surface of homopolymer polypropylene injection products
Problem Phenomenon
Fine mesh-like cracks or linear microcracks appear on the product surface, mostly near gates, corners, or ejector pin locations.
Microcracks propagate under stress or environmental stress, eventually causing product cracking failure.
Cause Analysis
(1) Excessive internal stress; uneven ejection force during demolding or stress concentration in product structure
(2) Melt temperature too low; severe molecular chain orientation, large residual internal stress
(3) Mold temperature too low; rapid melt cooling produces large temperature gradient stress
(4) Holding pressure too high or holding time too long; over-packing produces residual stress
(5) Insufficient draft angle or rough mold surface; large demolding resistance produces microcracks
Solution
(1) Increase melt temperature to the higher value within the 200-280°C range, reduce molecular orientation
(2) Increase mold temperature to the higher value within the 40-90°C range, reduce cooling rate to reduce internal stress
(3) Appropriately reduce holding pressure and holding time, avoid over-packing
(4) Increase draft angle; polyolefin material draft angle at least 1-2°, polish mold surface
(5) Optimize the ejection system, increase ejector pin number or area, ensure uniform ejection
Key Takeaway
Microcracks are a manifestation of excessive residual internal stress; they need to be prevented by optimizing temperature parameters, reducing internal stress, and improving demolding conditions.
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