Double Bagged Infusion - Why does it work?

[DDCompound]

"Emanis believes double bagging overcomes this"
You can believe in a lot of things, but that does not mean that it is true.

[wyowindworks]

Originally Posted by 316Composites

All the info and pics more in detail are found here!!

http://www.compositesworld.com/artic...0-fiber-volume

I've read that article which inspired me to experiment. It's full of false assumptions though like this one:

"Emanis believes it is essential to split the two main vacuum functions, assigning volatiles extraction (removal of entrapped air, ambient moisture and/or solvents) to the inner bag and then using the outer bag for compaction."

The outer bag cannot provide any compaction. Applying vacuum to the outer bag will only determine the compaction of the breather between the two the bags. It cannot apply pressure to the laminate via atmospheric pressure. To say that the outer bag can provide compaction to the laminate is like saying that an envelope bag sitting on your bench can provide pressure to the bench.

The only feasible explanation I can comprehend is that the inner bag applies the initial compaction of the laminate then vacuum is applied to the outer bag. As the vacuum reduces in the inner bag during infusion the inner and outer bags pressed together hold their form and reduce SOME of the fiber rebound in the laminate.

[HeyBen]

I still think the inflatable bladder would overcome this. I think if the bladder were inflated above the mold flange, the second bag would use leverage to pull the bladder down towards the mold. Just like the sponge has pressure exerted on the mold surface.

Am I wrong? I think you could get some decent pressures out of it too.

[wyowindworks]

I can't see how a bladder would help. The bladder would just push the two bags apart.

[HeyBen]

Exactly!

The fulcrum for the second bag of this setup is the mold flange. The first bag is pulled against the mold. The second bag is pulling the inflated bladder into the mold.

These are the exact same forces being exerted when a single bag is compressing a sponge or stack of fibers.

Yes, the bags are being pushed apart, but the first bag has nowhere to go but down, farther into the mold. The second bag is being pushed away from the mold, with the bladder trying to separate the vacuum between the bags, not going to happen.

Forgive my lack of technical writing skills.

On a typical setup, the bag "pulls" the layers of fiber into the mold, compressing them, correct?

On a typical setup, if you replace the layers of fiber with an inflatable bladder the bag is pulling the bladder into the mold. If you inflate the bladder the pressure increases exponentially per psi.

The forces exerted in an atmosphere with lower than atmospheric pressure are astounding.

Tank car implosion d'un wagon citerne - YouTube

Tank car implosion d'un wagon citerne - YouTube

The vacuum on a mold is no different. So, pull a vac on a bladder setup and you then have a tremendous amount of force contained under the bag. It would probably warp even a strong mold.

When adding the bladder, the first bag now becomes the mold surface so to speak. The second bag is trying to compress all of the air within the bladder, just like the sponge, forcing it into the mold. After the vacuum pump runs out of lungs you could crank up the bladder psi. However I'm not sure what kind of pressures would be in the bladder after a vac was pulled, if you started at say 5psi. 10? 100? 5.1?

If my theory is correct you could then vent the resin lines and set your vf to whatever you like using the bladder and weighing the excess.

Of course I could also be totally, 100% wrong. Which is usually the case, or so she says.

[HeyBen]

Here, I baked you a pie.




In the above engineering masterpiece you can see the "pie". Black is mold, yellow is the fibers, red is bag one, green is bladder and blue is second bag.

I think the key to success with this setup would be to have air pockets (purple dots) to give the bag something to pull down on, otherwise maybe the bag and bladder are pulling themselves together like on a typical 2 bag setup.

[HeyBen]

I bet if you did this setup on a piece of glass it would explode when air is added to the bladder.

[DDCompound]

It will not explode. There are no forces working on the Mould. If you want to create forces you have to pump the bladder with more than the atmospheric pressure, and that will not work with a bag.
The other problem by reducing the first vacuum is that you raise your maximum porosity!
Than smaller your absolute pressure, than smaller the maximum mistake.

The maximum compression you can reach is the difference from absolute vacuum to atmospheric pressure, not more.
And your Vf depends on the ammount of resin in your laminate and to the pressure your laminate is compacted.
And than smaller the absolute pressure in the infusion chamber/ bag1 than smaller the voids by enclosed air.

[dallasb84]

I baked you a pie.

And it exploded!

[HeyBen]

Hence the reason I am a forum troll not an engineer. lol

Thanks for entertaining my stupidity.