Injection Molding
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Environmental stress cracking appears in general-purpose polyethylene injection molded products
Problem Phenomenon
The product cracks brittlely when in contact with chemical media or in the use environment, and the cracks usually expand from the surface to the interior.
Environmental stress cracking is delayed and may appear only after several weeks or months.
Cause Analysis
（1）The residual stress inside 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 is in contact with incompatible chemical media
Solution
（1）Raise the melt temperature and mold temperature to the higher values within the range of 160-260°C/30-80°C to reduce internal stress
（2）Avoid sharp corners and notches in the product design, and use large fillet transitions at corners (R ≥ 0.5 times the wall thickness)
（3）Select grades with higher molecular weight and narrow molecular weight distribution to improve stress cracking resistance
（4）Anneal the product after demolding to eliminate internal stress
（5）Avoid the product contacting incompatible media, and apply a protective coating if necessary
Key Takeaway
Environmental stress cracking is the result of the synergistic action of internal stress and chemical media, and needs to be prevented from both aspects of reducing internal stress and avoiding contact with media.
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