Extrusion & Post
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PET Bottle Pearlescence (Stress Whitening)
Problem Phenomenon
Pearl-like white small circles appear locally on the bottle body, mostly on the inner side of the bottle, with high occurrence at the shoulder/bottom.
Cause Analysis
When PET is over-stretched, the microstructure ruptures producing stress whitening (pearlescence); areas with large wall thickness but low temperature have high stretching resistance, and whitening occurs once the natural stretch ratio is exceeded.
Solution
①Increase the heating lamp power of the corresponding zone (raise the upper-layer lamps for the shoulder, the middle-layer lamps for the body, and the lower-layer lamp temperature for the bottom); ②for thin-wall whitening zones, lower the temperature and transfer the heat to adjacent zones; ③check preform wall thickness and heating matching; ④optimize stretch rod speed and blowing timing.
Key Takeaway
Pearlescence = cold stretching; give the heat to the whitening area.
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