Effect on preform due to higher chiller tempreture

Hello,

We are having some problems with our 33 gram 28mm preform which is used to make 1 litre Cola bottle.

I was hoping someone can help me.

We blow bottle using our own 28mm 33g Preform.

Once the bottle goes to our customer and he fills it, after filling, and after a few days the bottle starts to expand, it expands so much that it becomes completely deformed.

Now our customer says that other vendor bottles have no issues, only our bottle expands.

We have tried many many things, we added RIBS on the bottle to give it more strength, we checked
material distribution during blowing, checked the base weight. We checked and rechecked if our customer has made any change to there process.
But the bottle still bottle expands after it is filled and in there warehouse, while other vendor bottle filled with the same Cola doesn't expand

We then purchased preform from a very reputable preform manufacturer, which is the largest manufacturer of preforms and PET resin in Pakistan.

We blew our bottle using that preform and sent it to our customer, this time the bottle didn’t expand and it is now 10 days and the bottle is holding its shape.

We are very confused as to what is the problem, and what to do next, because now we think we need to go one step back and look at our preform manufacturing process.

One change that we have noticed that we did make is that in the last two months which are high humidity months which means there is a lot of moisture build up on the mold. So to counter that we increased our chiller temperature to around 25/26 deg C. We had Mold Dryers but disconnected them many years ago, so we are not using that equipment.

Can this higher water temperature travelling in the mold have an effect on the quality of the preform and thus effect its strength?? The cooling time goes up, but are there other adverse effects on the polymer chain?

I hope you can shed some light on this as we are completely stuffed as to why the bottle expands.

Faraz Shaukat