When you are working with PET plastics, one of the benefits is that the plastic can level itself. As one part of the preform begins to stretch, that part will begin to harden so it cannot stretch as much. The nearby parts will then begin to stretch. This process repeats itself until the entire piece has hardened, and the final piece may show a difference of as little as 0.001 inches.
However, when you are working with stretch blow molding, you may find that you have a larger difference than that around the outside of your bottles. If your machines do not meet all of the right conditions, the bottle may not be even. There are several potential issues that may cause uneven thickness in the wall of the bottle.
One of the most common reasons for an uneven bottle is a gate that is off-centre in the stretch blow molding machine. The machine must centre the moulding process’ vestige. This vestige must remain firmly in place during the entire process. If it is not in place, the thickness of the wall will follow the gate and result in an uneven bottle. The self-levelling process will not prevent this problem.
If the stretch rod does not pin down the preform properly, an uneven bottle can result. Adjusting the distance between the mould’s bottom and the stretch rod can help keep the preform from slipping during the process. The smaller the preform that you use, the smaller the gap must be.
Uneven Heating and Cooling
Another common problem is that the bottle is not heating or cooling evenly. If the thickness of the bottle’s walls is uneven, it may be due to one side of the preform being a different temperature than the other side of the preform. The cooler side will not stretch as much, so it will end up being thicker than the other side. It can happen in both the single-stage and the two-stage stretch.
This may be because the air continues to blow on the preform, even after they have stopped rotating. Using a thermal camera can help you determine where the temperature difference is occurring. This can help you locate the source of this issue.
Turn to a Trusted PET Stretch Blow Molding Machine Source
Using the right machinery can go a long way towards ensuring that each of your bottles comes out with an even thickness. Pet All Manufacturing Inc. offers a stretch blow molding machine for you, no matter what your needs are. We can help you create the right bottles for you. Contact us today to learn more information.