Bottle Defects Analysis - Why My Bottles Are Not Clear After Blowing Process

2022-10-17 15:55


Sometimes we can see that the PET bottles we blow out from the blowing machine have poor clarity, there is a milky coating appeared on the corner of the bottles. It's also known as stress whitening.



1. The stretching speed of molecules is faster than they can respond, exceeding to their natural stretching limits.

2. The production temperature is too cold.

3. The material of the bottles is overstretched (The stretching ratio of the preform to bottle is too high). It can be checked by testing the thickness of the preforms and finished bottles.


1. Find the optimal temperature for processing conditions – not too hot, not too cold.

2. Check the nozzles, pre-blow valves and blow valves to see if there any leak if the haze appeared on the bottle shoulder.

3. If the haze appeared on the bottle body, we can try to reduce the pre-blow flow, increase the pre-blow time, also reduce the pre-blow pressure.

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