Molds & Tooling
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Post-shrinkage and dimensional drift of PP products
Problem Phenomenon
PP products that measure qualified at demolding continue to shrink in size after being stored for 24~72 hours; precision mating parts (such as bushings, snap fits, seals) go out of tolerance, and the assembly becomes loose or fails.
Cause Analysis
PP is a semi-crystalline polymer; crystallization is not complete at demolding, and crystallization shrinkage continues afterward (post-crystallization), which is more obvious at higher ambient temperatures; if the measurement timing is not unified (some people measure parts just demolded, others measure stored parts), normal post-shrinkage will be misjudged as process fluctuation.
Solution
①Extend in-mold cooling so that crystallization is completed in the mold as much as possible; ②appropriately increase the mold temperature to promote in-mold crystallization; ③unify the measurement aging (24 hours after molding); ④dimension-critical parts can be annealed at 60~80℃ for about 1 hour to accelerate stabilization; ⑤correct the mold shrinkage rate based on the actually measured 'post-aging dimension'.
Key Takeaway
The core of PP dimension management is 'the post-aging dimension prevails': first unify the measurement time point, then use mold temperature and cooling to 'lock' crystallization in the mold as much as possible, and finally correct the mold with measured shrinkage; otherwise you will forever be fighting a guerrilla war against post-crystallization.
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