Blowing pressure and the new Krones Ultravent Design
We do not sleep well with the unknown in the back of our heads. We all are human and human nature is to find explanations. Our success as a species developed with our cognitive skills and that imprinting urges us to understand things. There is, however, limited information about many things and there are of course –especially when things get complicated- limited possibilities to get the whole truth. As we clearly like to sleep well we will find ourselves simple solutions that can go as far and be false as superstition.
What does that have to do with our industry you might ask? Even in our modern and fact based PET world we sometimes have cases of this cognitive bias. Simple and plausible theories regularly override the more complex and more difficult to understand reality. One example is the pervasive hype about special preform base geometries from a couple of years ago (yes, I agree that a lighter preform makes a lighter bottle but there are different opinions on how to best go about this).
A similar topic is today´s – blowing pressure.
Necessary blowing pressure is a more complex topic than it might appear at the first glance.
There are suggestions in the market that claim the reduction of blowing pressure can be targeted easily with an improved venting of the base of the blow mold. When you take a closer look it becomes obvious that the topic is not such an easy one. Necessary blowing pressure is a more complex subject than these claims make it seem.
For making a PET bottle a preform is heated , stretched and blown.
You can get a nice personal impression of the shaping that takes place when you blow a rubber balloon into a PET bottle.
The material is blown towards the wall of the blow mold and then touches the walls of the mold and “freezes”.
The necessary blowing pressure is basically influenced by four things:
The shape that has to be blown:
bottle and base design, of course more remote edges of the bottle shape, like the “feet” of a carbonated soft drink (CSD) bottle base, are reached later by the plastic and need more pressure. Krones owns a patent on the smooth shaping (“freeform”) of our “Carbo Classic Base” and smooth shaping is key
Cooling time (where the material touches the wall and “freezes”):
PET that is not “completely frozen” by wall contact shrinks back. Higher machine speed leaves less time for cooling.
Especially with CSD bases, an apparently good bottle base might be too brittle when the cooling time was not enough and the requirements on the bottle (“specification”) might not be fulfilled.
Higher blowing pressure sometimes is needed to have enough wall contact and to make a “bottle in specification”, although with lower pressure optically fine bottles can also be produced
The restoring force that the PET material has:
a balloon wants to shrink back to the initial shape, this force has to be overcome. Tougher PET-types (“higher IV”) need more pressure – but do also deliver a more pressure resistent bottle. Also higher “stretch ratios” (making the “same” bottle from a smaller, more thick-walled, preform) make to a certain extent stronger bottles, but need more heating energy and more blowing pressure
The resisting force by air, that is unintentionally captured between blow mold and developing balloon, creates improper venting:
air trapped between the developing balloon and the blow mold can be the reason for a not fully blown bottle and the need for a higher blowing pressure. The blow mold should have in the areas where the balloon would otherwise trap air (that is usually where the material arrives last) venting holes to let the air out. If there are no venting holes or they are not at the right spots a higher pressure is needed.
We at Krones have –with our experience of more than 1000 blowing tests each year- developed our own base venting solution that ensures 100% proper venting and keeps the hardware affordable with a solution that is also suitable for manufacturing.
Krones solution for perfect venting of the base mold, Carbo Classic Base in Ultravent design:
This new Ultravent design, applied on our proven Carbo Classic Base, comes as a standard for CSD bottle mold orders from September on.
These complex correlations make it very difficult to predict the necessary blowing pressure, especially when not yet all boundary conditions are known.
Due to many relevant influences and constraints blowing pressure is difficult to predict.
Different factors might limit reduction of the blowing pressure.
Process “optimizations” ignoring inconvenient parts of the bottle specification which may be difficult or very time consuming to test (like stress crack or thermo stability) often lead to “solutions” that seemingly allow to make good bottles with a lower blowing pressure.
You see that with a closer look at it the blowing pressure topic is not only about resin or venting, but also about bottle and preform design, machine speed and specification, which is an important requirement from customer side.
And as you can also see, the process requiring the lowest blowing pressure does not necessarily make the best performing bottle.
Co-author for this post: Simon Fischer, Krones AG.