Injection Molding
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Dimensions of random copolymer polypropylene injection molded products continue to change after demolding
Problem Phenomenon
Product dimensions continue to change within 24-72 hours after demolding, with obvious differences between final dimensions and dimensions at demolding.
Dimensional instability affects assembly accuracy and product quality, and is common in crystalline materials.
Cause Analysis
(1) Random copolymer polypropylene has high crystallinity; continued crystallization after demolding causes post-shrinkage
(2) Demolding temperature is too high; the product is not yet fully cooled and continues to shrink at room temperature
(3) Cooling time is insufficient, and the crystallization process is incomplete
(4) Ambient temperature changes greatly; product dimensions fluctuate at different temperatures
(5) Internal stress release causes slow dimensional changes
Solution
(1) Extend cooling time to ensure product demolding temperature is more than 10°C below the heat deflection temperature
(2) Appropriately lower mold temperature to the lower value within the 40-80°C range to ensure full crystallization
(3) Perform annealing after product demolding, holding at 30-50°C below the melting point for 2-4h to eliminate internal stress
(4) Control storage environment temperature stability to avoid drastic temperature changes
(5) Establish a dimensional stability testing system, recording dimensional changes at 24h, 48h and 72h after demolding
Key Takeaway
Post-shrinkage is an inherent characteristic of crystalline materials; dimensional stability is ensured through full cooling, annealing and environmental control.
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