Molds & Tooling
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Sink marks on thick-wall PP parts (freeze-off type)
Problem Phenomenon
The holding pressure time for thick-wall PP parts has been set very long, but the sink marks are still obvious, and extending holding pressure is ineffective.
Cause Analysis
The gate is too small and freezes off early, so the holding pressure can no longer be transmitted to the thick-wall area, and no matter how long the holding pressure time is, it is 'ineffective holding pressure'; the shrinkage of the thick-wall core is not compensated and the surface dents.
Solution
①Appropriately enlarge the gate to delay the freeze-off time; ②arrange the gate near the thick-wall area; ③core out the thick-wall area to reduce material and shrinkage; ④conduct a gate freeze-off study to determine the effective holding pressure window; ⑤add auxiliary gates when necessary.
Key Takeaway
For PP thick-wall sink marks, first check 'gate freeze-off': after freeze-off, holding pressure is given for nothing; enlarging the gate, moving it closer to the thick wall, or directly reducing material is far more effective than blindly extending the holding pressure time.
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