Material Drying
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PET preform yellowing
Problem Phenomenon
The preform is overall yellowish; obvious color difference after bottle blowing; brand customers fail the inspection
Cause Analysis
Excessive drying time and excessive temperature trigger oxidation; excessive barrel temperature causes thermal degradation; abnormal mold temperature and excessive melt residence are all sources of yellowing; during on-site troubleshooting, verify item by item according to this mechanism, and avoid blindly adjusting the machine based only on experience
Solution
① Discharge the over-dried material and re-draw new material; ② lower the barrel temperature to 2-5℃ above the temperature that does not produce white haze; ③ air-shot to discharge the high-temperature material in the barrel; after adjustment, continuously sample to verify the effect, and resume mass production only after confirming stability
Key Takeaway
Preform yellowing is investigated along three routes: drying, temperature, and residence; the treatment of both oxidation and thermal degradation is material change plus temperature reduction; after confirming effectiveness, fix the parameters and practices into the work instruction to prevent recurrence
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