Injection Molding
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Roundness out of tolerance of random copolymer polypropylene injection molded products
Fenómeno del problema
The cross-section of round products is not round, with ovality out of tolerance, affecting sealing performance and fitting accuracy.
Roundness out of tolerance is common in round products such as tubular products, bottle caps and gears, and is closely related to uneven shrinkage and cooling differences.
Análisis de la causa
(1) Mold cavity and core cooling is uneven, with large shrinkage rate differences in different directions
(2) Random copolymer polypropylene has anisotropic shrinkage; shrinkage differs between the flow direction and the perpendicular direction
(3) Improper gate position; single-side feeding causes asymmetric molecular orientation
(4) Holding pressure distribution is uneven; shrinkage compensation is inconsistent in all cavity directions
(5) Product wall thickness is uneven; shrinkage differs greatly between thick and thin areas
Solución
(1) Optimize the mold cooling system, with cavity and core cooling water channels symmetrically arranged
(2) Control mold temperature uniformity, with temperature difference of each part not exceeding 5°C (40-80°C range)
(3) Use multi-point gates or ring gates to make the melt fill the mold uniformly and reduce orientation differences
(4) Optimize the holding pressure scheme to ensure uniform shrinkage compensation in all cavity directions
(5) Optimize product design, keeping wall thickness as uniform as possible, with inner and outer circles concentric
Conclusión clave
Roundness out of tolerance is caused by non-uniform shrinkage and needs to be improved from three aspects: cooling symmetry, gate design and wall thickness uniformity.
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