Vacuum, Voids, Bags, Liquids, and more...

General Composites Discussions Composites Talk
#1

Im getting a bit sick of reading about the practice of reducing vacuum pressure after infusing a part in hope that it will reduce voids. The reasoning is that with vacuum still acting on the resin after clamping the inlet, any entrapped air will then expand like it does in a degassing pot and create bigger voids. Sounds reasonable doesnt it?

Well lets look at this in a bit more detail…

first, let me ask 2 simple questions;

What happens if you have a bucket full of resin in a rigid degassing chamber, and put a balloon filled with air, tied to the bottom so its suspended in the middle of the resin, and vacuum is applied to the chamber… what happens to the balloon inside?

Now repeat the above apparatus, but instead of a degassing chamber, we now use a plastic bag to put everything inside… so the same resin, and the same little balloon filled with air suspended in the middle of the resin, all contained within a large plastic bag with a vacuum line out the top of the bag. What happens to the little balloon this time when the vacuum is applied?

#2

I like your thinking and have thought about this as well. Let me add and tell me if it makes sense.

It’s a little different from a degass chamber to a bag though.

In the degassing chamber the air pressure is pushing down on the lid right? Try to pull off the lid on the chamber, not gonna happen.

In the bag you have all of the force pushing down on the bag, hence the part and the resin.

If you reduce the vacuum you reduce the amount of force from the atmosphere. I would think that the reduced pressure would allow the voids to expand not contract.

I had one engineer write out some formulas about gases and pressures and how the vacuum creates voids. But he seemed to forget that there was a liquid covering the balloon as you say, so the vacuum would never really touch or effect the balloon via gas/vacuum forces right?

If a void saw actual vacuum, wouldn’t it tend to be sucked out? Once the resin has flowed through and entrapped the void, and sucked out all the air, then really there is no vacuum in the bag anymore since there is nothing but resin and carbon in the bag?

input would be appreciated.

#3

Im gonna wait a bit longer in the hope more people get involved in this, but lets take a look at what happens to the second apparatus…

Do we agree that after the vacuum is applied, the bag will shrink until all the air is evacuated - so it will look like a peice of vacuum bagged meat, with a balloon inside? IF the vacuum is kept on, do we all agree that the resin will start to flow into the vacuum line and start to fill up our catch pot?

Lets assume we have now sucked out half the resin into our catch pot… what has happened to our little balloon, still in the now half volume of resin, at this point?

#4

I answered in the UD thread.