Injection Molding
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HDPE injection molded part sink marks
Problem Phenomenon
Depressed sink marks appear at thick walls and behind ribs of HDPE injection molded parts; increasing holding pressure alleviates but does not eliminate them.
Cause Analysis
HDPE has high crystallinity and a shrinkage rate of 1.5~3%; insufficient compensation during cooling shrinkage of thick areas forms depressions; insufficient holding pressure or time and early gate freezing aggravate sink marks.
Solution
①Increase the holding pressure and extend the holding time; ②core out thick areas for uniform wall thickness; ③enlarge the gate to delay freezing; ④appropriately increase the mold temperature to improve compensation.; after adjustment, take consecutive samples to verify the effect, and resume mass production only after confirming stability
Key Takeaway
HDPE's high shrinkage makes sink marks a common defect; structural homogenization is fundamental, holding pressure compensation is a process means, and combining the two gives the best effect.; after confirming effectiveness, fix the parameters and practices into the work instruction to prevent recurrence
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