Injection Molding
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Dimensions of Block Copolymer Polypropylene Injection Molded Products Continue to Change After Demolding
Problem Phenomenon
The product dimensions continue to change within 24-72 hours after demolding, and the final dimensions differ significantly from the dimensions at demolding.
Dimensional instability affects assembly accuracy and product quality, and is common in crystalline materials.
Cause Analysis
（1）The crystallinity of block copolymer polypropylene is high, and continued crystallization after demolding causes post-shrinkage
（2）The demolding temperature is too high, the product is not yet fully cooled, and it continues to shrink at room temperature
（3）The cooling time is insufficient, and the crystallization process is not completed
（4）The environmental temperature changes greatly, and the product dimensions fluctuate at different temperatures
（5）The release of internal stress causes the dimensions to change slowly
Solution
（1）Extend the cooling time to ensure the product demolding temperature is more than 10°C lower than the heat deflection temperature
（2）Appropriately reduce the mold temperature to a lower value within the 40-90°C range to ensure sufficient crystallization
（3）Perform annealing treatment on the product after demolding, holding at 30-50°C below the melting point for 2-4h to eliminate internal stress
（4）Control the storage environment temperature to be stable and avoid drastic temperature changes
（5）Establish a dimensional stability inspection system, recording dimensional changes at 24h, 48h, and 72h after demolding
Key Takeaway
Post-shrinkage is an inherent characteristic of crystalline materials, and dimensional stability is ensured through sufficient cooling, annealing treatment, and environmental control.
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