Wyo it cannot be regained. Unless part is placed inside a pressure vessel.
The second bag only maintains what the first bag achieves at initial debulk. I’ve stated that numerous times during this discussion. But to say a second bag does nothing or that double bag process does nothing is absurd.
wyo … You’ve answered your own question btw…
The pressure can be regained by applying max vacuum to the inner bag till the vacuum is equalized on both ends of the system then slowly release the vacuum in the outer bag to allow the bags to reposition and the reapply vacuum to the outer bag.
The second bag cannot maintain the initial debulk. If the inner bag looses vacuum the debulk is affected. SOME of the pressure can be maintained on the preform but it is directly related to the stiffness of the inner/outer bag coupling.
I’ve learned a lot from the personal research and experimentation. I always think it’s important to understand why things work. If you don’t understand why they work you cannot trouble shot or evolve the process very well.
I suppose my debate is not what happens in a 2 bag setup, rather how to overcome it and ADD external forced upon the fiber layers in a regular mold, achieving home-brew autoclave results. The physics already stated are law.
In the below true to scale technical engineering schematic entitled “two layer cake”, bag one is in green, yellow bladder and blue bag two. By keeping the 2 bags apart you create a more “compressible” atmosphere for the system to work. But as stated before, the weak point is the strength of bag two, in blue. Trade the typical plastic material for something with high resiliency and a TON of breather and you have something worth talking about.
I have used pond liner before from home depot. It is rubber and has high elasticity. The bladder can be whatever, like Wyo said, it’s just a membrane. This would add external forces to the stack, controllable by pressures.
Or do the physics prevent it?
Yes, and the stiffnes is so small that you can not stop relaxing the first bag relevant.
If you work with more than atmospheric pressure in the yellow part you can create extra load on the laminate. But that has nothing to do with a classic double bag.
Ben that’s sorta how wyo builds his models. Replace blue with mold 2.
Can somebody post a link to this patent please?
You should read your own thread because that is where I found it. :D:D
But here you go: Boeing Double Bag Patent
Dallasb84
DDcompound was simply trying to explain his knowledge to the best of his ability. Your personal attack on his spelling is unwarranted.
To all, this thread is getting heated, please refrain from personally attacking other members.
I’ve never had to do this yet but if things get out of hand I’ll suspend users. This is a friendly forum for discussion, no one is forcing advice upon anybody. Take it or leave it but don’t disrespect others.
Dallas84, could you further explain how the Navier-Stokes equations apply to double bagging? I’m only slightly familiar with them. What part do they play? How does double bagging play a role with Navier-Stokes equations and not single bagging? Please educate me. :o
It applys to both.
Elaborate explanation. 
I need more to understand
The reason why double bagging works.
Navier/stokes
Could you elaborate a little more? How does the Navier-Stokes equations make double bagging work?
I would also like to know how the Navier-Stokes is working in a infusion. I think he wants to tell that an infusion slows down or speeds up because of a cross section change.
But that would just explain why a DB might have a little faster flow front but not a better Vf.
I would also like to know why that is not only simple static physik?
The resin flow is not simple static, but we are talking about Vf and that depends on the load and compaction, and that is static.
He always says it works, but he never tells us how.
Give me a second fellas.
N/s allows for tensor inputs that aren’t used in classic mechanics. Langarian derivative allows factoring tensors viscosity friction and pressure of a molecule at certain points in time.
How n/s applies to the physics of VIP is. Having a constant pressure variable allows for a constant calculation of the fluid through the infusion mesh.
If we measured the time it takes for the resin wave front to move through a laminate we should see a different value for the pressure because of the loss of compaction in the fibers with a single bag vs a double.
Basically my argument is with N/s we should see that db infusion of the same laminate would be slower than a single bag. This is because the resin after wave front is exterted more pressure than a single bag. The friction of the fluid flow through the infusion mesh should cause this.
That’s how I’m relating n/s to infusion.
Ok, that means you still belive that you have more compaction because of double bag.:sad:
Exactly!!! It’s not that I believe it. It is that sb and db have different velocities. How can they have different velocities with out having different pressure, temp, vacuum, viscosity, or friction? The only one that can change is pressure.
So it must be pressure by elimination.
This must be very interesting argument since its never ending. I retracted from the discussion since it didn’t make sense, however this is truly hilarious. For your information: Darcys’ Law which is an equation based on experimental results that governs the process of infusion, is directly derivable from Navier Stokes equation. If you didn’t know, all the software, such as Polyworx, that do infusion simulation use Darcys’ Law, and hence a reduced form of NS.
Omg Nash!!! I actually asked my co worker tonight who the guy was that calculated groundwater flow! He was like wtf are you talking about? I was like you know the guy… water in sand at the beach?! He was like dude… Whatever your smoking??? I want some! Ha ha ha. Realizing how crazy I sounded I just said nevermind 
But my teacher used the beach and sand to explain it and I couldn’t remember for the life of me!
No joke I was trying for the life of me to remember darcys law.
Dallas84, ROFL, very funny! Well, I studied Navier Stokes in 1986 and then we didn’t have fancy computers as we have today to solve NS. Today we use it very often in our design. There’s a nice opensource NS solver called OpenFoam that you may like to look at.
Thanks alot Nash I’ll certainly check it out! Man it was getting to the point of arguing which came first the chicken or the egg in here… And everyone was saying I said goose!!!
Ha ha ha