Molds & Tooling
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COC Sink Marks and Shrinkage Cavities
Problem Phenomenon
COC products have sunken sink marks on the back of thick-wall rib positions, transparent parts sink marks have obvious optical distortion, internal shrinkage cavities visible on sectioning.
Cause Analysis
Although COC shrinkage rate is only 0.1~0.7%, local shrinkage concentration in thick-wall areas still forms sink marks; insufficient holding pressure or too short holding time gives insufficient feeding; early gate freeze cuts off the feeding channel.
Solution
①Holding pressure set in the 300~600bar specific pressure range and extended until gate freeze; ②holding time subject to just eliminating sink marks to avoid over-holding; ③gate near thick-wall area; ④structural design uniform wall thickness.
Key Takeaway
COC low shrinkage does not equal no sink marks; the objective function of holding time is the minimum value of sink mark disappearance; over-holding introduces new stress cracking risk.; after confirming effectiveness, fix the parameters and practices into the work instruction to prevent recurrence
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