Processing Machinery
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COC Melt Temperature Too High Yellowing
Problem Phenomenon
COC products show overall or local yellowing, transparent parts have increased yellowness index, optical appearance unqualified, batches with long barrel residence time are especially obvious.
Cause Analysis
Thermal oxidation occurs when COC melt temperature exceeds the grade upper limit or residence time is too long, cyclic olefin structure chromophore groups form; different grades have different temperature resistance upper limits, e.g.
8007 grade not exceeding 250℃.
Solution
①Control melt temperature upper limit according to the grade manual; ②shorten molding cycle to reduce residence time; ③lower the barrel to 170℃ for heat preservation during brief cycle interruptions; ④thoroughly clean the machine before production after yellowing appears.
Key Takeaway
COC yellowing is the signal of dual control failure of temperature and residence time; each grade has different temperature upper limit and must be confirmed one by one; optical parts have zero tolerance for yellowing, temperature management starts from actual barrel measurement.
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