Injection Molding
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Environmental stress cracking of homopolymer polypropylene injection products
Problem Phenomenon
The product cracks brittlely when contacting chemical media or in the use environment, with cracks usually propagating from the surface inward.
Environmental stress cracking is delayed and may appear weeks or months later.
Cause Analysis
(1) Large residual stress in the product; cracks initiated at stress concentration points under environmental media action
(2) Low melt temperature and low mold temperature cause increased internal stress
(3) Stress concentration locations exist in the product structure (sharp corners, notches, etc.)
(4) Material molecular weight too low or molecular weight distribution wide, poor environmental stress cracking resistance
(5) Product contacts incompatible chemical media
Solution
(1) Increase melt temperature and mold temperature to the higher values within the 200-280°C/40-90°C ranges, reduce internal stress
(2) Product design avoids sharp corners and notches, use large fillet transitions at corners (R ≥ 0.5 times 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 product contact with incompatible media, apply protective coating when necessary
Key Takeaway
Environmental stress cracking is the result of the synergistic action of internal stress and chemical media; it needs to be prevented from two aspects: reducing internal stress and avoiding contact with media.
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