Molds & Tooling
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Warpage from one-size-fits-all shrinkage rate in PP molds
Problem Phenomenon
For molds designed with a uniform shrinkage rate, the product warps and dimensions are out of tolerance, improving after mold modification.
Cause Analysis
The shrinkage rate of PP in the flow direction is 40~60% higher than in the perpendicular direction; the mold not being differentially scaled is the primary cause of flat part warpage; when the wall thickness difference is greater than 2mm, the shrinkage difference between thick and thin areas reaches 0.5~1%.
Solution
①Differential scaling for homopolymer PP: 2.2~2.5% along the flow direction and 1.5~1.8% perpendicular to the flow direction; ②random copolymer PP takes 1.2~1.5% for approximately balanced scaling; ③glass fiber reinforced PP takes 0.5~0.7%; ④reserve a 0.1~0.2% post-shrinkage margin.
Key Takeaway
PP mold scaling must distinguish material and direction; differential scaling plus aging verification is the fundamental way to solve warpage and dimensional problems.; after confirming effectiveness, solidify the parameters and practices into the work instruction to prevent recurrence
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