Injection Molding
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Out-of-Tolerance Roundness of Low-Density Polyethylene Injection Molded Products
Fenómeno del problema
The cross-section of round products is not round, with out-of-tolerance ovality, affecting sealing performance and fitting accuracy.
Out-of-tolerance roundness is common in tubular products, bottle caps, gears, and other round products, and is closely related to uneven shrinkage and cooling differences.
Análisis de la causa
（1）The cooling of the mold cavity and core is uneven, and the shrinkage rate differences in different directions are large
（2）The anisotropic shrinkage of low-density polyethylene, with different shrinkage in the flow direction and the perpendicular direction
（3）The gate position is improper, and single-side feeding causes asymmetric molecular orientation
（4）The holding pressure distribution is uneven, and the shrinkage compensation in all directions of the cavity is inconsistent
（5）The product wall thickness is uneven, and the shrinkage difference between thick and thin parts is large
Solución
（1）Optimize the mold cooling system, and arrange the cooling water channels of the cavity and core symmetrically
（2）Control the mold temperature uniformity, with the temperature difference between parts not exceeding 5°C (30-60°C range)
（3）Adopt 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 directions of the cavity
（5）Optimize the product design, make the wall thickness as uniform as possible, and keep the inner and outer circles concentric
Conclusión clave
Out-of-tolerance roundness 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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