Molds & Tooling
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Warpage from one-size-fits-all shrinkage rate in PP molds
Fenómeno del problema
For molds designed with a uniform shrinkage rate, the product warps and dimensions are out of tolerance, improving after mold modification.
Análisis de la causa
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%.
Solución
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.
Conclusión clave
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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