Never tried it, I might in the future.
http://www.compositesworld.com/articles/double-bag-infusion-70-fiber-volume
I use this process-
Here’s the write up from boeing.
Great read
I never thought of it as creating a constant compaction over the part. If I do it, it’s because I had a big leak and didn’t want to rebag the part 
I think I will ask about this process and try to compare the 2 versions.
When you put that extra floating bag between the first / second with some sort of breather over top of the float and the vacuum is balanced from the first and second bag, the whole process seems to come together really well. -clamp off both drain and feed side resin lines after infusion, maintain the secondary vacuum until you would normally take vacuum off.
I’m curious about this process. Would you need two separate vacuum pumps/sources to pull this off?
Here is a recent article that comments on the one posted by 316Composites. http://www.compositesworld.com/columns/single-bagging-can-do-the-job
No just one.
Interesting that the article speaks about microporuous films. I’ve done a lot of work in that area and trying to stop it!
I like the double bag process when I have a mold with a lot of little details that need compacted and worked into place after the first bag is drawn down. The first bag is at the point of no return if you don’t want to loose the work in compacting and is certain to have very small holes poked into it that are most impossible to manage after infusion is started and will deliver a bad part. The second bag, “floating membrane” breather and vacuum are applied after that compaction work giving vacuum integrity and a “clamp”
Other than that, the single bag with two “chambers” gets the job done nicely which is discussed in the boeing write up as well. This process requires a separation membrane with breather that is vacuum sourced on the resin drain side of the mold. The membrane is perimeter sealed except for vacuum source area at the resin drain of the mold. This works unless resin reaches the drain and starts to backfill the breather over top of the membrane in which you have no more clamp. That may or may not be an issue and you can cut little “witness holes” in the breather to keep track of the resin front.
The single bag method can be just fine as that guy mentions but the bagging materials as well as every aspect of the process have to be near perfect or the odds of a bad infusion and part are greatly increased.
It all comes down to what is required to get the desired part quality.
He does mention vacuum integrity as the highest priority with problems occuring in the infusion process. This is why I like the full control of the dbl bag process after applying however many hours into setting up a mold before starting the infusion and finding one little flaw in vacuum integrity that provides a ruined part.
You are trying to stop your work in membranes??? or stop the resin going THROUGH it?
I do a lot of working making barrier films - so stopping permeation in various membrances.
It seems that the use of the double bagging has many advantages over using a single bag approach, but it does not seem to be too widely used for the advantages it seems to bring, very likely I will use it in my project
I don’t think double bagging has THAT many advantages. It gives you better vacuum integrity, that’s about it. This can yield a slightly higher quality part, but whether that justifies the extra consumable cost is debatable. Hence why it isn’t used as regularly.
Hi Hanaldo, I understand your point of view, but considering that infusions are made mainly to obtain better quality parts, and considering the cost of consumables are a quite small portion of the total cost of materials to obtain a piece, I would say it is worth, it allows to control the volume fiber ratio and compaction without subjecting the resin to (almost) any low pressure and this reduces the size of any gas bubble to an absolute minimum.
It would be interesting to know which vacuum level should be applied to the second bag to achieve a not too dry finished part, maybe if full vacuum is applied to the second bag the exerted pressure may produce a too dry piece … any tips in this regard ???
It doesn’t though. The compaction of the fibres is determined by the first bag, you are forgetting that it is a pressure differential. It is the weight of the atmosphere pushing against the pressure inside the bag, the pressure inside the bag is the same whether you have one bag or 50 bags. There is no compound effect from the bags, so you don’t gain any extra compaction. Same with the volume fibre ratio, that is determined by the amount of resin you let into the laminate, doesn’t matter how many bags you have.
Vacuum is a strange force of physics to get your head around, but you need to remember that you are only working with the weight of one atmosphere. That weight is ~14.7psi, you can’t change that by adding more bags.
I think you need to have a good understanding of the process and really stay on top of the details when you set up the laminate and vac bag. If you are so unsure of your skills to do the job right then get better at it. You should know before you mix resin if it all ok with drop tests and leak detectors etc.
I concur. Though double bagging is a useful way to partially dodge around getting a 100% seal, which can be handy while learning the ropes, I do think it is ultimately better to master the basics.
Hanaldo, I might be wrong with my assumptions but I believe that after resin entered under the plastic film, most of the compaction is lost, precisely because what was vacuum and compacted is now filled with resin almost at ambient pressure so the pressure diferential behind the resin front is gone almost completely, In my humble opinion, the second bag restablishes the pressure differential compressing the laminate stack against the mold and it does so without subjecting the resin to any low pressure so any gas bubble size is minimized, it does not seem to be just an insurance for vacuum integrity … it seems to bring the advantage of reduced size gas bubbles as well …
On the other hand unless there exists a transparent breather material to be placed between plastic films, I do not like the idea of only seeing how the infusion is going on thorough some “windows” made in the breather material, any ideas to overcome this …
Think about what you are saying. It is a pressure DIFFERENTIAL. Yes your laminate pressure tries to equalise with the pressure outside the bag, but it does not reach ambient pressure unless you completely flood the bag with resin. What you need to understand is that the pressure inside the bag is pushing up against the atmosphere pushing down on the outside of the bag. So no matter how many bags you have, that pressure DIFFERENTIAL is the same. The only way to reduce the pressure inside the bag again is to remove the force pushing up on the bag, ie. the resin.
You should watch DD-Compound’s MTI valve video. The animation explains the physics of it very well.