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Old 02-17-2012, 08:32 AM   #1
wyowindworks

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Default Double Bagged Infusion - Why does it work?

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)?

Last edited by wyowindworks; 02-17-2012 at 08:46 AM.
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Old 02-17-2012, 08:51 AM   #2
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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.
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Old 02-17-2012, 09:00 AM   #3
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Quote:
Originally Posted by DDCompound View Post
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.
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.
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Old 02-17-2012, 09:09 AM   #4
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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.
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Old 02-17-2012, 09:30 AM   #5
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Quote:
Originally Posted by DDCompound View Post
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.
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.
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Old 02-17-2012, 09:35 AM   #6
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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.
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Old 02-17-2012, 09:43 AM   #7
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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.
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Old 02-17-2012, 09:52 AM   #8
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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.
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Old 02-17-2012, 11:15 AM   #9
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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.
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Old 02-17-2012, 11:44 AM   #10
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Quote:
Originally Posted by HeyBen View Post
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.
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.

Last edited by wyowindworks; 02-17-2012 at 12:04 PM.
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The Following 5 Users Say Thank You to wyowindworks For This Useful Post:
canyon (02-17-2012), dallasb84 (02-17-2012), DDCompound (02-17-2012), HeyBen (02-17-2012), ovikas (02-17-2012)
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