Infusion with a double bag achieves constant vacuum and pressure on the laminate. When the flow is opened and infusion begins a double bag with breather material allows much more compaction on the first bag.
With infusion you do not use breather… With a double bag you should. Using a double bag and infusing has allowed the use of infusion on many projects you will see 20 years from now. Consolidation is greatly increased and also lowers resin content so much that it can starve the laminate.
so does it mean that the first bag gives 14psi then the second puts another 14psi on top of that? what is the function of the breather? I can see how it could be a pressure intensifier in cerain places but over all…?
thanks Rotor
Dbl bagging is relevant if you are going to infuse. You set up a regular infusion, pull vacuum and get all of your consolidation/compaction done. Then cut a float piece of bagging material and back it with breather so that vacuum will “see” the entire laminate. (The float piece of bagging material is so that if there is a small leak in the first bag, the float piece will keep resin from being pulled up into the breather cloth which would potentially degrade the clamping ability of the “dbl” bag setup) Lay that into place, bag it all with new vacuum line(same original source). You will accomplish a couple of important things here. The laminate will never suffer the chance of small air leaks(very important in infusion) and next, without the dbl bag -> as the resin front travels accross the laminate, compaction is potentially lost because there is no vaccum or “clamp” behind the front which can allow the shifting around of the laminate. The dbl bagging maintains a proper clamp. This isn’t so much of a problem with a flat laminate laying face down on the mold or glass or whatever (unless you flood it with resin), but once you get away from the simple laminate everything changes. Dbl bag is my preference and for such a small part, I agree as suggested earlier that you shouldn’t need flow media. You will get a much better part than any of the other methods other than maybe autoclave in my opinion.
Build a proper infusion type mold and use the dble bag method. You’ll get the results you require.
Thanks finite. Essentially you loose the effects of full vacuum at saturation. You’ll always have 14.5 psi regardless but with a double bag you have full vac and ambient pressure at all times. Or less if need be. Double bag infusions can and have been proven to result in less resin to fabric than conventional pre pregs in comparison. Also tight corners usually do not build up excess resin with double bags as the breather allows the double bag to clamp the laminate. Ever notice how the debulk seem phase has no bridging and boom u infuse to find an early Christmas present? You loose clamp at infusion.
The dbl bag infusion method will produce parts(cf/epoxy) that when tap tested(after full post cure) will sound like you’re tapping on a piece of glass or metal as opposed to the sound of a plastic part. This is the mark of a good fabric to resin ratio part. This is the road to light weight high strength parts and is the difference between all of the carbon fiber “stuff” made with good intentions and the rare real thing outside of the aerospace and other high end industries.
The second bag does not change any physik! It makes no difference using one ore two bags.
If a tank is parking on you, it makes no difference if there is one or two sheets of paper between you and the tank!
On the contrary, once infusion is complete there is no more work being done in a conventional bag by vacuum. With a double bag the vacuum assists in the compaction of the fibers before, during and after infusion. It is no more than a constant debulk. The second bag de bulks the infused fabric. That’s how you achieve the resulting parts.
Vacuum is a measurement of hg is a gas atmosphere. Once there is no more gas there is no more vacuum. The second bag allows vacuum to stay in the bag. That is the physics of it basically.
There are case studies that prove it works with results of greater than 70% fabric to resin.
http://www.compositesworld.com/articles/double-bag-infusion-70-fiber-volume
Here is an article about double bag you might find helpful in understanding.
Boeings patent application for “dbvi” process wich is a modified “Capri” process.
DC: When infusing, the resin line side will always be thicker than the vent side. That is why there has been much research in using membranes. This way you have vacuum being pulled from the entire part at once. Think about it, the resin side is open to atmosphere. Even if you clamp the resin line early, there is a chance the resin flow will not even out. Double bagging will compact the entire mold at the same pressure, regardless if you have full vacuum on the vent side, and 1atm on the resin side. The part itself, will be compacted EVENLY at 14psi (not saying the part itself doesn’t still have the pressure differential.)
if bag number one is exposed to atmosphere it will ballon up the second bag. so once resin is introduced in atmospheric conditions the effects are the same as one bag as DD as mentioned. the benifits to two bags are vacuum integrity. also when doing DBVI you attached your vent directly to the laminate and purge the resin until all the air is out reducing void content, at that point vent is closed and bag number two keeps the laminate consolidated.
http://www.compositesworld.com/columns/single-bagging-can-do-the-job
Here is the quote from that article that is important. “All of the issues raised in the double-bagging article can be addressed with a single bag if the correct procedure is used. The main reason the double-bag method works is that it improves the total vacuum integrity of the job”
This gentleman’s article is fine if you want to infuse and aren’t concerned with a good compaction or consolidation before introducing resin resulting in a potential bad part. The other issue with the membrane and single bag is that if the resin front travels all of the way to the vent line before the resin cooks off you will loose the effect of the clamp above the membrane resulting again in a potential bad part.
What this really means is that while in arm chair discussion “yes” it works, the set up requires perfection before the infuse begins. What I don’t care for with the single bag with membrane method is that it doesn’t allow for the ability to completely consolidate/compact the laminate. I prefer the full dbl bag process because once the first bag is under vacuum, you can complete the compaction and consolidation over the most tight detail of the laminate. Something you cannot accomplish with the single bag method(with membrane) or after the second bag is applied.This is the only way to ensure that the part is as free as possible of resin pockets and other silly problems that could be addressed before infusion begins otherwise.
With money riding on the job and getting a perfect part every time being the desired result, the dbl bag process will overcome all of the problems you may encounter with the other methods when setup properly and the compaction is done before the second bagging is applied.
If you aren’t concerned with defects or consistancy out side of visual (“Hey, this looks like carbon fiber…”) then use any method other than dbl bag. You’ll spend less in materials and labor. The dbl bag method is the way to get a consistant perfect part every time when money and time are your accountable factors. 5 perfect parts are better than 5 out of ten when spending money.
We have so many people here coming from different angles. Hobbiest, one timers, professionals, to be professionals, and the rest of us.
Im not saying double bag is not worth doing. but consolidation of laminate will be no different, bag number one with or without bag number 2 will still be exposed to the same atmospheric pressures during infusion. if you set bag number #1 to 15HG, and number two to 29.92HG your laminate will be consolidated at 15HG’s not 29.92HG. the benefit however would be voids that are present are smaller due to the lower vacuum level. The higher quality you mentioned is because a better vacuum integrity is achevied.
There is no differential of vaccum to atmosphere behind the resin front. This is why we refer to the dbl bag as maintaining the clamp after infusion has begun.
If you apply vaccum to the resin supply bucket you will see that the resin no longer travels, draws or feeds in the infusion… I think there is a confusion about what is happening as the resin front travels across the laminate.
If you have any details in your mold that are too tight for the vacuum to consolidate the laminate into you will have a resin rich area. In these applications the compaction and consolidation is by far different because you have to manually work these areas before infusion begins. This is why it is relevant. You can’t put those extra materials into place and expect them to address those areas without manual attention. They must be done by hand and with the single bag and membrane this is not possible because of the breathercloth and extra material in the way. With the dbl bag, you can pull vacuum on the first bagging, do the detail work and then apply the second bagging. No other way to get around this with out resin rich areas on heavy detail transitions in infusion layups. Autoclaves are much better suited otherwise and I don’t have any autoclave experence.
Hojo actually in your example laminate will see 29 hg but resin is slowed to 15…
Here’s a few things that have not been mentioned… And can be compared to an autoclaves run. Vacuum and pressure can be manipulated. Resin flow can be stopped while pressure is still applied. Bag two hg can be bumped. Bumping pressures … Similar to ramp and pressure purges in autoclave (only inside bag) achieves better compaction. Your looking at a 3% on average void, saturation, and wieght reduction difference between standard and dbvi. On complex parts the difference is greater. When looking at most quality aspects 5% or less void content is auto clave standard… Dbvi can achieve that… Single bag cannot. The benefits are… Less weight, uniform part thickness and better mechanical properties.
This is great info boys. hopefully the size of my part will help achieve very low defects
That’s wrong. The pressure difference is between the ambient pressure and the bag the resin is infused. And the pressure difference is the deciesive parameter.
Mmmm so your saying bag two isn’t working on the laminate??? Even after infusion? Or are you saying that the laminate sees only 15hg. Inside bags pressure only works to infuse. I have used the inside bag at such low pressure it took over an hour to infuse to slow for a very consistent thickness. All while the 2nd bag held full vacuum.
On some occasions full vacuum is held on bag one but resin feed is restricted.
FiniteTech:
Which is the EXACT sequence of things to do?
-apply laminate
-apply bag 1
-apply vacuum
etc.