Bag two is not working on the laminate. Bag two only works on the laminate if it has no contact with bag one, but than it will discharge the first bag.
What happens when you have full vacuum on the second bag and open the first?
If you have low vacuum in the first bag you infusion is faster because your fibres are not completely compressed. But than your fibre to volume ratio is lower.
The laminate will be thicker at the resin inlet, thats why you should close the inlet befor the whole part is impregnated, so the resin can disperse and you will get a constant thickness.
Thats why we developed the MTI hose.
One bag, full vacuum and a calculated amount of resin and you get a perfect part with a perfect fibre to volume ratio.
The second bag is only to be sure that there are no leaks.
Earlier today at my shop, I popped in on this thread and realized I was thinking way to deep about this topic and not thinking correctly. (couldn’t see the forrest through the trees…)
DDcompound you are correct in what you are saying. I’m getting caught up in the semantics of my own “in house” terms and practices when I refer to “Clamp”. Vacuum integrity is in fact the relevant point of my usage of the term “clamp.” The second bag is working off of the 1st bag(unless there is a leak in the first bag) and not against the laminate/mold.
That said, both single bag with membrane and dbl bag method can provide great vacuum integrity across the laminate during infusion. With your MTI tube, the single bag with membrane would be full proof. I’m drawn to the dbl bag method for the compaction and fine tuning ability after pulling vacuum on the first bag. This will always be something I require with the small intricate molds. I have been doing some work for a customer with a Ford GT and recently got to inspect one of my small parts that has been on his car for a while. It came from a mold with sharp transitions and yet this part is very dense and sounds like a piece of ceramic when tapped. This is due to the attention to detail with regards to compaction around all of the sharp transitions in the mold after the first bagging has been pulled under vacuum( the laminate is pulled around and would otherwise yield a resin rich part in those tight transitions). The second bagging is applied at this point to give that final vacuum integrity. (and usually needed because of my pushing and poking at the first bag.)
I’m doing some parts for the interior of a Porsche Carrera here soon. I’ve already built the molds and they are all going to be difficult pieces to get right due to the sharp transitions and so on. I’ll end up using the dbl bag method for good results with these things. Again the only reason for this is to get tight parts. It has been working great.
Herman:
-prep mold
-kit fabric
-apply bag adhesives to mold
-apply feed and vent lines for infusion
-apply laminate
-apply release ply
-apply flow media if needed
-apply first bag
-crimp feed line
-pull vacuum
-fix any leaks.
-cycle vacuum per compaction needs along with hand adjusting the tight areas
-apply vacuum line for second bagging.(I usually source from the same as the first bagging)
-apply floating membrane
-apply breather material with witness holes cut in the feed path line to monitor the resin front progress
-apply second bagging
-check everything twice, mark the clock, mix resin and begin the infuse. I usually have the vacuum pump on a timer to shut it down.
something like that anyways…
Here’s an old pic of a dbl bag on those small parts.
Hope this thread hasn’t been pulled to far off topic
I’m not complaining. this information is pretty much what I need for another project I am getting less than perfect results with. currently am doing WLUVB and have planed for some time to go the infusion route. so carry on. I am doing the CAD work on the body tool and will present snap shots of that soon for advice on how to configure it for experimentation of different processes
Double bagging has traditionally been used for mitigating leaks. We have found by adding a batting material between the bags you can get higher pressures ( a misnomer I know) applied to the part under vacuum, thereby giving an out of autoclave result improvement on just single bagging. Of course there is more to it than that, but just sayin’.
Double bagging is used now to build aircraft grade out of autoclave parts when single bag infusion won’t work.
The proof is in the pudding! Lol.
I dont think theres any debate regarding the improved quality of this process, just what is really happening during the process.
The biggest problem in the composite industry is that most people do not understand the physik.
Finitetech,
Could you please explain what do you mean by “floating membrane” in your post #42 ??
I could be wrong, but I believe he’s referring to any media that is placed between the bags for padding,compression or added bulk pressure in tight spots.
Perhaps like thick breather cloth,felt,or even caul plates as intesifiers.
I put the piece of “floating” bagging material down against the first bag once initial vacuum is pulled and compaction is completed on the first bag. This is a resin block in case there is a leak in the first bag. We don’t want to leach resin from the layup. Beyond that is breather material (strictly as a medium for vacuum) and then the final second bagging.
It all comes together to give a secondary and perhaps more important role of what Worldwealth is stating.
In short all of the important compaction and detail consolidation is done with the first bagging. Literally the process of organizing, compacting and any work required to make sure the dry(not yet wetted) laminate is “exactly” where it needs to be. Then the second bagging is arranged. The end result without rehashing this thread is that like Worldwealth is submitting, the second bag will help to lock down your effort and if all works correctly, insure your efforts. The floating membrane is a component of this effort. One pin hole in the first bag can allow a starved segment in the finished laminate if allowed to bleed resin into the breather material of the second bagging via pin hole. The floating membrane is a simple gasket. It covers the perimeter of the part over top of the first bagging before laying down breather and the second bag.