Injection Molding
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Environmental Stress Cracking of High-Density Polyethylene Injection-Molded Products
Fenómeno del problema
The product undergoes brittle cracking when contacting chemical media or in the use environment, and the cracks usually expand from the surface to the inside.
Environmental stress cracking is delayed and may appear weeks or months later.
Análisis de la causa
(1) The residual stress in the product is large, and cracks are initiated at stress concentration points under the action of environmental media
(2) Low melt temperature and low mold temperature lead to increased internal stress
(3) The product structure has stress concentration parts (sharp corners, notches, etc.)
(4) The molecular weight of the material is low or the molecular weight distribution is wide, and the environmental stress cracking resistance is poor
(5) The product contacts incompatible chemical media
Solución
(1) Increase the melt temperature and mold temperature to the higher values within the ranges of 180-260°C/30-80°C to reduce internal stress
(2) Avoid sharp corners and notches in product design, and use large fillet transitions at corners (R ≥ 0.5 times the wall thickness)
(3) Select a grade with higher molecular weight and narrow molecular weight distribution to improve stress cracking resistance
(4) Perform annealing treatment on the product after demolding to eliminate internal stress
(5) Avoid the product contacting incompatible media, and coat a protective layer when necessary
Conclusión clave
Environmental stress cracking is the result of the synergistic action of internal stress and chemical media, and needs to be prevented from two aspects: reducing internal stress and avoiding contact with media.
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