Injection Molding
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Environmental Stress Cracking of Block Copolymer Polypropylene Injection Molded Products
Problem Phenomenon
The product cracks brittlely when contacting chemical media or in the use environment, and the cracks usually propagate from the surface to the inside.
Environmental stress cracking is delayed and may appear after weeks or months.
Cause Analysis
（1）The residual stress in the product is large, and cracks are initiated at the 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 material molecular weight is low or the molecular weight distribution is wide, and the environmental stress cracking resistance is poor
（5）The product contacts incompatible chemical media
Solution
（1）Increase the melt temperature and mold temperature to higher values within the 200-270/40-90°C ranges 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 grades 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 apply a protective coating when 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 contact with media.
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