Double Bagged Infusion - Why does it work?

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#1

Alright, I’m trying to understand how double bagging is capable of improving the Vf of infused laminates. I don’t build “bling”. The things that I build are structural so the topic is interesting to me. I’ve been reading about claims of 70% Vf which is hard to believe.

So why does it work? I’m trying to understand the advantage. I’ve recently done some small test panels (2x2) and seen absolutely zero improvement in the Vf.

Facts: The outer bag is unable to exert a force on the laminate by a function of atmospheric pressure. If the inner bag is vented to atmospheric pressure you have 14.7 psi pushing out and 14.7 psi pushing in = a net force of zero. If I apply vacuum only to the outer bag then I just increase the pressure on anything between the two bags but no pressure is exerted to anything inside the inner bag.

If I apply 29.97" Hg to the inner bag and 29.97" Hg to the outer bag is have 14.7 psi pressing on the laminate. When i open the infusion inlet I have 14.7 psi pushing the resin in. The theoretical pressure against the laminate is now zero and the pressure applied to the breather between the two bags is 14.7 psi.

So, is it the friction between the two bags that holds the shape of the bags to hold the pressure against the laminate when the inner bag pressure is reduce during infusion (14.7 psi pushing the resin in)?

#2

You can reach 70%Vf also with single bag, but that is not good for you part because there is no resin between the fibres. They are compacted so hard that the resin is not allowed to flow between the fibres.
The classic double bag without any pressure pieces between the two bags is just for the vacuum integrity. The friction between the two layers may hold the bags in position, but nothing more, no relevant pressure.
The only relevant thing for the compaction forces is the pressure difference between the ambient pressure and the first bag.
If you open the inner bag when the second bag has full vacuum the ambient pressure works from two sides to the second bag, but that means that the second bag is compacted, but there are no forcec working on the laminate.

#3

I have never gotten a 70% vf with infusion and carbon. That’s a 80:20 ratio by weight. I often weigh the fabric before infusion and then weight the final part weight. The only time I’ve seen an 80:20 by weight is when there are dry spots within the laminate. The parts also tested very poorly.

#4

This only works if you infuse more resin and after infusion you suck resin out through you feed line, a little bit like the SLI system.
Autoklav part with such a high Vf also have pinholes and a high porisity.

#5

I’ve read about doing that but have yet to try it. Applying vacuum to the feed line makes sense to reduce the amount of resin on the feed line end of the system. Otherwise the extra resin in the feed line end has to migrate all the way to the vacuum end to escape.

I tested sample of autoclave parts with Vf of 70%. They can be quite porous and explosive on failure.

#6

70% Vf are never good. All good Vf 55-60% can be reached by classic infusion. On monday I will be back in office and post a graphic I made where the physiks of double bagging is shown.

#7

I typically get a Vf of 50% (60:40 by weight with carbon) with infusion unless it’s UD. I can get 60% Vf with UD. I recently read some impact tests with carbon and the testers found 58% to optimum Vf for carbon in respect to impact.

#8

The optimum Vf depends on the fabric you use. The 58% are the optimun with a non crimped multiaxial, using Spread Tow the optimum Vf is about 60% using plain or twill the optimum is about 55%
But that also depends on the aplication, if you need more flex a lower Vf is better than a high Vf.

#9

I am thinking that with such a small flat part there would be no benefit. I would think that making a large part with much depth and corners, that of a boat, the benefits would be more noticeable.

I am curious though, everyone says that if the first bag is vented, and the second bag is a 29", there is no pressure exerted on anything inside the first bag or pressure exerted to the part. I don’t think this is true. It’s just like having a single bag setup with the inner bag just being another ply with hoses on it, the second bag is compressing everything.

I am thinking the affects would be similar if the first bag was pulled down, then second bag pulled down, the first bag vented, then resin is injected through the mold with positive pressure from the resin cup.

I am not an engineer, just using logic and common sense here.

#10

The second bag won’t compress anything inside the inner bag if the inner bag is vented. You have one atmosphere pushing in through the vent and one atmosphere pushing on the outer bag. All you do is push the inner and outer bag together. Anything between the inner and outer bag undergoes 14.7 psi. Anything inside the inner bag under goes no pressure. Put a soft sponge in the inner bag and apply vacuum to the outer bag and have a see. The sponge will not get compressed. Apply full vacuum to the inner bag and the sponge will go flat as pancake. Then apply vacuum to the outer bag and release the vacuum on the inner bag. The sponge will only rebound a little bit. Grab the bags and you can pull the pull the two sides of the inner apart where the sponge is as you suck air into the inner bag.

After doing this show-and-tell in my shop I came to the conclusion that the only advantage to the second bag is for vacuum integrity and some form holding power. If you first apply vacuum to the inner bag and then apply vacuum to the outer bag then vent the inner bag via infusion (one atmosphere pressing on the resin) the two bags getting pressed together will hold their shape and prevent some of the rebound of fiber near the resin front. This isn’t happening because the atmospheric pressure in pressing against the laminate but rather the two bags are being pressed together so they hold their form.

#11

Wyowindworks, how about trying your experiment with a partially liquid filled bag as is the case in infusion. It took Boeing over 8 years to win that patent and its a very important work for the infusion process.

The advantages claimed by “double bag” patent are:

  1. Vacuum integrity
  2. Reduced bag relaxation

However, from what little I can share, based on our process, during infusion we can get very fine control over the vacuum differential for the infusion. Such a differential will be impossible in a fully drawn bag. Which can indeed give a very high quality laminate (Vf in the range of 62-68%). We can also get to above 70% with tightly woven materials and very low resin viscosity, for instance that laminate on our site is spread-tow panel that has a very high Vf. And no, we don’t have voids in our laminates.

#12

Nash, is that 70% by volume or weight?

#13

Ah-ha, now I see it. Great explanation, thanks for that.

In that setup you had a good media between the layers? I think friction would play a part in this type of thing.

Now I wonder (I hate it when I do that, it gets dangerous) what would happen if you were to lay up a 2 bag system with an inflatable bladder between them? :smiley:

#14

Great work Adam. Simply put the second bag acts like an intensifier. Also works better with stretchalon.

If you have some older material or scrap and can lay up a larger piece I’m willing to bet you would also see differences in weight with db vs sb.

We did while r&d in our test lab.

We also saw differences in weight between like infusions that were sb process. Db process with pressure ramp and time consistencies proved consistent. Resins were premixed and resins were warmed to specific temps for viscous control.

Simply put db just allows for more control and repeatability.

Thanks for doing this Adam.

#15

Could you explain further how the second bag functions as an intensifier?

#16

Second bag with breather in-between maintains laminate compaction before and after resin wave infuses. Thus acting as an intesifier.

#17

What is your explanation for the maintenance of the laminate compaction?

#18

Well, I wonder which company will tell you the exact Vf = volume fraction, or Wf = weight fraction that they get based on a proprietary process they’ve developed. However, here you go: Vf is just what I’m talking about.

I had questions similar to yours before studying the boeing patent with care and trying out their ideas first. I thought that the outer bag and inner bag once compacted against each other was just one thick bag. But that was only part of the story…

Another thing regarding your experiment, a laminate stack is not like a sponge, dry laminate has very little spring-back, and if its liquid filled, which allows the inside surface of the inner-bag a liquid to flexible film contact with the outside surface of the mold, a liquid to solid contact, things are very different from your experiment.

If you think of it a bit more carefully, there is flange side where there’s an overlap in all double bags, that overlap is approximately 10cm between bags, If you have a breather between the two bags, there’s always a differential even if your inner bag is at atm pressure. Once the outerbag has taken its shape, and the pressure is not released, it will try to keep that shape (less the slippage between the breather, outer and innerbags). That differential is proportional to the innerbag-projected area on the solid mold divided by the outerbag-projected area on the mold. What this does is that it provides a constant debulking force on the stack to not losen itself.

Assuming that the inner bag is at atmospheric pressure is wrong, since its the pressure differential that is important and not the absolute pressure in the inlet. So your inlet pressure will almost always be a slight vacuum.

I’d suggest you to give it a shot to see if it will work for you. Like I said, we use both methods, but for structurally important things where laminate quality is important we prefer to use doublebag.

Hope it clarifies.

#19

We often ash a probe to get the exact Vf of the part which is written down in the part book, so everyone can see the VF of a part!
I think the customer is allowed to know this thing about his part if he wants to know.

This is the true part of the story!

Yes thats correct, but this little spring back decides over 30% or 55% Vf. So it just makes it a bit easyer to calculate when to stop the resin in. In single bag will be a little more resin at the inlet point, but if you know when to stop it will dispers all over. So if you work correct there is no difference at the end.

This works in LRTM/VARTM if working with a hard stabil secondmould piece, but not if working with a bag. We are talking only a few percent VF you want to get with the DB. That for it should not expand for 0.1mm. And even a hard closed mould will expand a little bit, thats why you can not get parts with a high Vf in RTM processes.
And believe me, a bag will move more than 5mm if the inner bag is venten a little too much, even if the resin is in it is vented complete. Where resin is, there is no vacuum, because resin is pushed in by atmospheric pressure.
The best efficent way is to controll your resin in not to able your laminate to swim up. And this can be done by several ways.

I promise, if someone who understands the physik of the process will test both methods he will get the same results :wink:

#20

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

http://www.compositesworld.com/articles/double-bag-infusion-70-fiber-volume