Injection Molding
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Cracking caused by internal stress of high impact polystyrene products
Problem Phenomenon
Products crack during use or storage; the root cause is internal stress generated during processing.
Cause Analysis
(1) Injection speed too fast, molecular chain orientation produces orientation stress
(2) Holding pressure too high or holding time too long
(3) Mold temperature too low, melt rapidly cools producing thermal stress
(4) Uneven product wall thickness, stress concentration at thick-thin junctions
(5) Mechanical stress during demolding
Solution
(1) Reduce injection speed, adopt multi-stage injection
(2) Appropriately reduce holding pressure
(3) Increase mold temperature to the higher value within the 40-80°C range
(4) Optimize product design, uniform wall thickness, enlarge fillets at corners
(5) Ensure uniform demolding force
Key Takeaway
Internal stress cracking is a manifestation of processing stress exceeding material strength.
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