Injection Molding
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Cracking/microcracks appearing on the surface of polypropylene injection molded products
Problem Phenomenon
Fine mesh-like cracks or linear microcracks appear on the product surface, mostly near the gate, at corners, or at ejector pin positions.
The microcracks expand under stress or environmental stress, eventually causing the product to crack and fail.
Cause Analysis
（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, producing large temperature gradient stress
（4）The holding pressure is too high or the holding time is too long, and over-packing produces residual stress
（5）The demolding draft is insufficient or the mold surface is rough, and the demolding resistance is large, producing microcracks
Solution
（1）Raise the melt temperature to the higher value within the range of 200-280°C to reduce molecular orientation
（2）Raise the mold temperature to the higher value within the range of 40-90°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 demolding draft; the demolding draft for polyolefin materials is at least 1-2°, and polish the mold surface
（5）Optimize the ejection system, increasing the number or area of ejector pins to ensure uniform ejection
Key Takeaway
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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