Molds & Tooling
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Cavity-to-cavity inconsistency in PP multi-cavity molds
Problem Phenomenon
In a multi-cavity mold, some cavities show short shots and some show flash, and the weight and dimensional differences between cavities exceed the allowable range.
Cause Analysis
The runner system is unbalanced, and the filling pressure and time of each cavity are different; inconsistent cooling water channel efficiency of each cavity also causes shrinkage differences; PP is sensitive to pressure and temperature, and cavity-to-cavity differences are further amplified.
Solution
①Geometrically balance the runners with equal length and equal diameter, or adopt a naturally balanced layout; ②use the short-shot method to test the filling balance of each cavity; ③check and balance the cooling water channel flow of each cavity; ④independently throttle/time-control each gate of the hot runner when necessary; ⑤establish cavity number traceability and SPC.
Key Takeaway
For multi-cavity molds, do a 'filling balance test' first before talking about the process window: when the runners are unbalanced, any process adjustment is just robbing Peter to pay Paul; cavity-to-cavity consistency must be guaranteed by the hardware balance of runners and cooling.
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