Molds & Tooling
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PP Sink Mark (Shrinkage)
Problem Phenomenon
Surface depressions behind ribs, bosses, and at thick walls—common on appliance housings.
Cause Analysis
PP shrinks greatly, and volume shrinkage at thick walls gets no holding pressure compensation; excessive rib thickness ratio, insufficient holding pressure/time, and premature gate freeze-off are direct causes.
Solution
①Control rib thickness to 50%~60% of wall thickness; ②increase holding pressure and extend until gate freeze-off; ③reduce material at thick walls; ④place the gate near thick walls; ⑤strengthen local cooling.
Key Takeaway
PP sink mark = high shrinkage meeting insufficient feeding—treat structure and holding pressure together.
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