Injection Molding
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Roundness out of tolerance of homopolymer polypropylene injection products
Fenómeno do problema
The cross-section of round products is not round, ovality out of tolerance, affecting sealing performance and fitting accuracy.
Out-of-tolerance roundness is common in tubular products, bottle caps, gears and other round products, closely related to uneven shrinkage and cooling differences.
Análise das causas
(1) Uneven cooling of mold cavity and core, large shrinkage rate differences in different directions
(2) Anisotropic shrinkage of homopolymer polypropylene; different shrinkage in flow direction and perpendicular direction
(3) Improper gate position; single-side feeding causes asymmetric molecular orientation
(4) Uneven holding pressure distribution, inconsistent shrinkage compensation in each cavity direction
(5) Uneven product wall thickness, large shrinkage differences between thick and thin parts
Solução
(1) Optimize mold cooling system, arrange cavity and core cooling water channels symmetrically
(2) Control mold temperature uniformity, temperature difference of each part not exceeding 5°C (40-90°C range)
(3) Adopt multi-point gates or ring gates, make melt fill the mold uniformly to reduce orientation differences
(4) Optimize holding pressure scheme, ensure uniform shrinkage compensation in all cavity directions
(5) Optimize product design, wall thickness as uniform as possible, inner and outer circles concentric
Conclusão principal
Out-of-tolerance roundness is caused by non-uniform shrinkage; it needs to be improved from cooling symmetry, gate design, and wall thickness uniformity.
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