Why do PET plastic bottles have bubbles during the production process

We often encounter bubbles on the bottle body during the production of PET plastic bottles. What is the reason for this? Let's analyze from several aspects.
1、 Process factors:
1. The speed is too fast, causing the molten plastic to undergo significant shear and decompose, producing gas.
2. The local cooling of the product is too fast and the shrinkage is uneven, especially when the wall thickness of the product is large, the surface cooling speed is faster than the central part. During the cooling process, the plastic edge in the central part shrinks and expands towards the surface, causing insufficient filling in the central part and forming a vacuum bubble.
3. Insufficient pressure and holding, causing the melt to not adhere closely to the surface of the mold cavity, which is not conducive to the discharge of air.
4. Insufficient back pressure controls the speed at which the screw retracts. When the back pressure is low, the speed at which the screw retracts will be faster, which can lead to insufficient feeding. The air introduced into the material is too high to be discharged from the feeding port in time, resulting in a large number of bubbles.
2、 Material factors
1. The raw material contains moisture.
2. Mold factors
3. If the mold is trapped, it should be maintained or an exhaust slot should be added.
So it is important to pay attention to these aspects during the production process in order to create a perfect bottle.