You need Navier-Strokes for resin flow and saturating, but that has nothing in common with the double bag, may be the resin flow changes a little bit because of the friction between the two bags. That can be explained by Navier Strokes, but not why double bag works.
So Dallas, if you are such a pro, please argument how Navier Strokes makes double bag work and changes the load and Vf.
I am waiting.
Holy mack…can’t believe I missed this discussion, it was a good one…I am confused a bit though…has it been answered whether a double bag produces more force?
whatever the inner bag is set to is going to be the pressure on the lamainate. The second bag can help reduce swelling but will not produce more pressure than the first without higher than atmospheric pressure applied. Second bag is all about vacuum integrity and added stiffness. CAPRI would be a better method to go with for higher compaction. That all being said this is probably the best thread in awhile. Should have more like these.
Lol your too much compound… And you still don’t get it.
I tell you what compound beat me at your own game. Prove ns wrong
With bournolli like you asked me to do earlier in this discussion.
I’m waiting. 
Can you help me get it? :o I still don’t see how the Navier-Stokes equations make double bagging work. I can see how more pressure would slow the flow. I can understand how the theory of double bagging can help reduce bag relaxation through stiffness and thus maintain some of the compaction. I can understand how this retained compaction would slow the flow. I spent 3 hours reading about these equations and the variable they include. I just can’t understand how the Navier-Stokes equations make double bagging work…i.e. increase the Vf and reduce porosity. I can understand how double bagging effects the variables of the Navier-Stokes flow equations but not the other way around. It seems like the atmospheric pressure, vacuum integrity, and the stiffness of the bag coupling that are the driving factors of it’s effectiveness. What am I missing?
A major benifit which I havent seen mentioned here about DBVI is that the vacuum line is hook directly up to the laminate stack and resin is flushed through the laminate untill all visible air is removed from exiting resin, at which point that line is closed. Bag number two ensures that bag number one does not loose vacuum after line is closed.
Sort of semantics at this point guys…
I have never made a double bag infusion, but what I can see on the single bag infusion, is that as soon I let the resin into the laminate, the vacuum drops considerabily near the inlet area. So much that I can lift the bag with less force than where the fibers are still dry.
I think this is normal because in practical terms, I’m just connecting the bag with the outside at athmosferic pressure. I there wasn’t the resin, just air would enter into the bag, destroying the vacuum.
So, I can see that actually the amount of vacuum (compaction) avaiable in the wetted areas is inversely related to the flow of the resin: if I slow the flow of the resin, I get more compaction. If I let a large flow of resin go in, the injection point lift itself off the surface of the laminate.
All of this, to say that the second bag of course could help to mantain a constant pressure over the first bag to avoid major differences of pressures all over the laminate. It has nothing to do with the amount of vacuum into the first bag: think at that just like a weight of 1Kg/cm2 over the first bag.
A question that i make to myself: couldn’t I obtain similar results simply stopping the resin flow before the fibers are completely wetted, so to give time to equalize the amount of vacuum into the laminate? (Using the MTI hose, of course)
How do you figure 1K/cm2 on the first bag?
The CAPRI process applies vacuum to the resin pot. If you have 29.97"Hg at the vacuum end and 15" Hg in the post you still have a pressure differential to migrate the resin. Rather than having a full atmosphere pushing to the resin you have .5" atmospheres pushing on the resin. This will also reduce bag relaxation .
I assume I’m able to make full vacuum in the second bag.
The CAPRI system is a good way to have quality parts, I think.
I’ll try it soon.
The vacuum in the second bag will not press against anything in the inner bag. Vacuum in the second/outer bag will only press the inner and outer bag together.
Exactly! This is how you can throttle the resin through the laminate without clamping and also reverse the flow back into the pot known as fill and drain.
Also with the double bag at full vac and assuming full vac at debulk the inside bag is at all times transferring atmospheric pressure to the laminate during infusion. Vacuum integrity is maintained and compaction is maintained.
Adding vacuum to the resin pot works just as good in a single bag.
But how does double bag add compaction and how can you get a better Vf? Lower pososity yes because you have a better vacuum integrety, but the other things?
And please do not mix other processes where diuble bag is used with this thread. Here we want to talk about a better Vf and more compaction and not other thing like adding vacuum to the resin pot.
I tried to explain by pressure differences by Newton on which Bernoulli is based ( pressure equitation of ideal gases, but that makes no difference because…) but also Navier Strokes is just a simple Newton equitation if you work without the friction parameter.
And compresiion and Vf are static and not dynamic.
Now lets go on talking about the physik in double bag, and please dallas do not just laugh and tell me that I did not get it. Explain it to me that I can get it:)
As long as we can quit going back and forth about this and that sure! Would be happy to!
So as to start the conclusion phase of this discussion, lets put ego’s to the side. Shall we?
The “other process” is the basis for the results (and my defensive posture)of better compaction vf and uniform thickness argument. So if we ignore all the control practices and just focus on using two bags vs one let’s be humble in it’s dissection. At the same time let’s also not forget the controls but keep them in the back of our minds.
Compaction and pressure.
Double bag and single bag achieve the same load on the laminates at full vacuum. I’m sure we can both agree on this for sure. In double bag setup this is done to set the clamp of the second bag to the inner bag and take the shape of the substrates preform. No infusion has taken place and I’m not discussing it at this point. At debulk and clamp sb and db both share the same load values because of atmospheric pressure. This should be static.
Vf ratio
Without controling atmospheric pressure at resin cup, performing debulk, maintaining clamp, and controlling resin flow… I think it’s safe to say you can throw a consistent vf out the window but I might be wrong. So without incorporating the other process in this… I’ll give it my best shot.
Sb with no control on cup uses vacuum and the small amount of atmospheric pressure that gets applied to the cup to wet out the laminate. Standard practice is to clamp and unclamp resin feed to control infusion. Resin brakes and clamping is used to stop or slow the process helping uniform infusion. Mti hose provides a uniform draw and continuos infusion until saturation is complete. It’s like a resin brake meets spiral wrap meets invisible man taking over while you take a smoke break. How vf is affected (for simplicity’s sake I’m going to use by weight for this comparison) applys to how the bag reacts in a single bag. The bag tends to relax as atmospheric pressure at the resin cup tries to push resin into the bag. Although total equalization does not occur vacuum pressure loss is sometimes experienced as the infusion begins. We can overcome this with valves on resin feed lines and partially clamping the hose and so on but… None the less vf and weight depend on the technician or some control not described. Resin rich laminates and non uniform vf laminates can be made with no control on resin flow and bag relaxation.Simple example would be puddling of resin in tight corners. Which apparently Mti hose can help with though I have no experience trying it in this manner. Leaving vacuum on until resin cures is a common practice to evacuate as much resin as possible from the laminate to improve vf… But without no control. Again a skilled technician can produce a high quality part.
Vf and db.
As the mentioned above db process without Including the other process one major thing happens with a properly assembled and debulked db setup. Second bag when clamped is rigid and maintains a relative profile of it’s substrate. Even if no control on resin cup is used there can be improvements in reduction of puddling, relative uniform thickness than in a single bag simply because a like profile is on top of the laminate. With no resin cup control there is risk of the second bag simply floating because atmosphere is trying to equalize inside the inner bag and thus pushing resin into the bag. A resin rich laminate across the board can occur, negatively affecting vf. Evacuation of resin after infusion is common place to reduce the amount of resin in the laminate in this senario.
The other process
Better Vf ratios, compaction, and uniform thickness of the laminate using controls of the Capri process yield my implied result are the basis of my argument defending double bag. Simply using a double bag gives you a rigid top layer of a single bag in my opinion. And not the best option available I might add. But with process controls, Balanced resin feed and speeds, together with closely regulated inner bag pressure the rigid preform will not relax nearly as much as an infusion without such control. A rigid clamp on the laminate and slow introduction of the resin can maintain much of what was achieved at initial debulk. Less resin is introduced into the bag and resin infusion can simply be stopped by balancing full vacuum at both exit and feed. When clamp is maintained during the infusion better vf is achieved as well as compaction.
To sum it up.
Db with out controls is basically infusing with a rigid profile and does not in itself yield better vf, compaction etc. But with controls it does and can do so repetitively. Is it better to do a db vs a single bag without controls by a skilled technician? In my opinion yes… In some situations. Like using a second bag to hold core or a caul plate in place is handy. I also prefer db when infusing parts that have valleys and sharp corners because it helps reduce puddling. I use a very loose outer bag and generally use as much breather as I can. I mentioned before it acts as an intensifier and this is why.
Forgive Me for the long winded post and repeating a bunch of stuff but I bet a humble and detailed response outside of the heated debate might chill things off. Also grammar and punctuation could have been better. 
Compound… Did not at all mean to provoke you btw. You add alot to this forum and to the community with your experience and product. I did address Mti to mitigate some of the drawbacks of sb in my…thesis… Only to show that it indeed helps. cheers.
I left physics and references to derivatives out because… Discussing db vs sb is easier without it in general.
But compound… Vf being simple ratio of fibers and matrix is static. But how the ratio occurs and is achieved is not static. That’s physics
Can we both agree that db might have it’s uses and benefits? I find it useful when necessary most of the time but not all.
I read all you have written. And I read the same thing I said, there is no plus using a double bag, might be the less relaxing by the friction of the two bags.
All pros you wrote are because of changing process parameters and complete independent from double bag.
Working with a core or a caul plate is a simple version of a closed mould, not of double bagging.
So we can fix that there are no big advantages (stiffness of two evacuated bagging films is the only one) on double bag.
No more compaction, no better Vf, no better flow. Might be a little less bag relaxing, but that is just cosmetic.
DDCompound, it seems like reducing bag relaxation has a larger range of effects than just cosmetics. I would think it could reduce resin pooling which would increase the Vf. I’m planning on running a batch of experiments to see how much pressure the coupling of the inner and outer bag can actually maintain. I planning on vacuum bagging a scale with a double bag and releasing the inner bag pressure to see how pressure can be maintained.