Extrusion & Post
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PET Blowing Milky White Whitening (Overheated Crystallization)
Problem Phenomenon
Porcelain-white turbid areas appear on the bottle body, mostly at the bottom or thick-wall areas; the whitened areas have increased brittleness and decreased strength.
Cause Analysis
The preform is locally overheated, and PET has thermally crystallized before stretching; the crystallized zone cannot be normally oriented and stretched, and appears milky white after blowing with deteriorated mechanical properties.
Solution
①Reduce the temperature of the corresponding heating zone and shorten heating time; ②strengthen preform temperature measurement and thermal image monitoring; ③check lamp aging and reflector condition; ④reject crystallized preforms.
Key Takeaway
Milky white whitening is caused by overheated crystallization, and its machine adjustment direction is opposite to that of pearlescent whitening, so they must be accurately distinguished first; thermal imager monitoring of heating uniformity is an effective preventive means.
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