Injection Molding
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Environmental stress cracking appearing in random copolymer polypropylene injection molded products
Problem Phenomenon
The product undergoes brittle cracking when contacting chemical media or in the service environment, with cracks usually propagating from the surface inward.
Environmental stress cracking is delayed and may not appear until weeks or months later.
Cause Analysis
(1) Residual stress inside the product is large; cracks initiate at stress concentration points under the action of environmental media
(2) Low melt temperature and low mold temperature increase internal stress
(3) The product structure has stress concentration points (sharp corners, notches, etc.)
(4) Material molecular weight is on the low side or molecular weight distribution is wide, resulting in poor environmental stress cracking resistance
(5) The product contacts incompatible chemical media
Solution
(1) Increase melt temperature and mold temperature to the higher values within the 190-260°C/40-80°C ranges to reduce internal stress
(2) Avoid sharp corners and notches in product design, using 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) Perform annealing after product demolding to eliminate internal stress
(5) Avoid product contact with 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 two aspects: reducing internal stress and avoiding media contact.
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