Injection Molding
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Dimensions of general-purpose polyethylene injection molded products continue to change after demolding
Fenómeno do problema
The dimensions of the product 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.
Análise das causas
（1）General-purpose polyethylene has high crystallinity, and continued crystallization after demolding causes post-shrinkage
（2）The demolding temperature is too high, the product has not been sufficiently cooled, and it continues to shrink at room temperature
（3）The cooling time is insufficient, and the crystallization process is not completed
（4）The ambient temperature varies greatly, and the dimensions of the product fluctuate at different temperatures
（5）The release of internal stress causes the dimensions to change slowly
Solução
（1）Extend the cooling time to ensure that the demolding temperature of the product is more than 10°C below the heat distortion temperature
（2）Appropriately lower the mold temperature to the lower value within the range of 30-80°C to ensure sufficient crystallization
（3）Anneal 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, avoiding drastic temperature changes
（5）Establish a dimensional stability inspection system, recording the dimensional changes at 24h, 48h, and 72h after demolding
Conclusão principal
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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