Double vacuum bagging

Yes. One needs a seal on both sides of the layup. There is a whole thread here about "strechalon" molds. Those molds can kill details in soft foam molds. Another method of cheap one off molding is to harden the foam surface with something that isn't porous like a layer of epoxy or filler, paint, whatever. Molding is tough depending on the level of detail like joggles and rebates for covers, etc... For simple parts there is no reason to vacuum a layer over a mold. You can just put the layup in a bag and use both halves of the foam to hold it in position while it cures. You can get good compaction and resin fractions this way but you can also get wrinkles and bunching. So it has it's uses but it certainly isn't a production technique. What I have do quite often to get a prototype skin is to make a foam mold by hot wiring and carving, cnc cutting sections of the mold. Then you can cut mylar or thin aluminum to line the mold. Then you wet layup onto those shiny surfaces and put the whole layup on one layer of bagging material and put peal ply and wicking material on top of that and another layer of bagging material on top of that. Then put the other half of the hot wired foam on top of that, weigh it down and pull the vacuum. Even multi tapered parts or parts that have other than flat wrapped can be done to the point where you have eliminated say 90% of the filling and sanding. You are only limited by your imagination. A mold as one carved part is something that stops people from being clever. A mold can be made up of many pieces carved separately using different techniques. Again not for production work.

 

Billski, have you done this? I can't imagine one bag atop another would give any more squish...? Is this some reduction in permeability of thin materials? I have never seen any evidence of this. In fact the second layer would have to have some sort of manifold in it to even distribute the flow so it would seem counter productive.

 

While infusing, 2 bags are advantageous, higher pressure in the infused part, lower pressure in the outer bag keeping the fibers compact and in place. Once you shut off the resin supply however, there is no more advantage.

With plenty runners, actual infusing should be fast enough not to make this an issue.

 

Never done it so this is all speculation.

Here is a patent that presents some of the advantages:
Patent US7413694 - Double bag vacuum infusion process - Google Patents

And my interpretation of one situation where it might be useful:

 

yES IT TAKES A WHILE TO READ REREAD AND READ YET AGAIN AND EVENTUALLY THE PENNY DROPS AS TO HOW ITS WORKS !!

Its the filing with resin that's the problem and who ever noticed what was happening must have been clever to work out how to solve the problem

 

Big deal. Drop the vacuum pressure til infused then shut the resin line and dial it up. What is the earth shattering break through. Drama queen.

 

Oh dear you got it wrong again !once it filled with resin you disconnect and leave the end open so the outer bag can squeeze some of the surplus resin that's there out !

 

Me???

 

iT UPS THE GLASS TO RESIN RATIO FROM 50 % T0 70% !! SO IF YOU REALLY INTO WEIGHT SAVING SPECIALLY ON BIG PARTS ITS WORTH THE EFFORT !!
Once you shut off the resin supply however, there is no more advantage.You do shut it off yes !!but disconnect both ends to let out the resin that's not needed !!

 

I asked around about this, and it was commented that is was uses quite a bit in production runs.

Here is the take on what I was told, benefits are product constancy, when you open the valve as tunnels mentions, you are actually guaranteeing infusion is completed, now the second bag acts as a continuum of pressure and drains excess resin out of the fabric. On a part to part quality control, it appears that there is greater yield and less delta of deviance from weight and measure strength , saving quite a bit of money for production runs that require at interval x number of parts made, a sample is to be tested until destruction, it appears that double bagging allows for >x intervals and better parts.

Having said that, I was given a look and said, now you would never do that in a home production, even less so in a one off, unless you are looking for extra quality, which there is little room for further gains if done correct with a single bag, however you could save on resin if you are infusing multiple pieces back to back... but not much. ara:

 

Will have to look into it more but I don't believe it. Infusion is about the rate at which the resin can travel the torturous path through the cloth. So if you have a vacuum bag holding down the cloth and you use it to pull the resin through isn't it about just design the ins and outs and the amount of resin and the overage trap? Seems this is just manufacturing planning or methods if you prefer. Double bagging is more steps and more disposable materials. How is that a good thing?

 

From what I understand if your doing vac infusion the molds used need to be somewhat stronger, well sealed if multiple parts and also need a decent sized flat flange around the perimeter to apply sealant/clamps etc. This double bag tech may allow a slightly lighter construction of the mold and once its fully infused the taps are opened and the double bag applied to exterior of both mold and the part being made, the vac is applied effectively over both those parts which should create a lighter part than would otherwise be possible. With vac applied on both sides there should also be less distortion of the mold than if it had vac applied only over the part being made.

 

Jay, what don't you believe? That it's used? That it may have benefits? That some component yields require a different approach? Or just me?

I can tell you I had never heard of it since it was mentioned here and never gave it much thought and I asked around in the Aero-Industrial space from top manufacturing facilities in NL, France, and the UK to understand the debate. Take it at face value, try it out to see what you get out of it and compare, that's my own take.

Would I use it? only if I can't get a part produced satisfactory, but I'm not making much of anything as of lately. :cry:

 

I'm still failing to grasp how pulling two vacuums is better than one in one atmosphere.

I could see pulling a vac then pressurizing the outer layer well beyond one atmo, is that more what we're talking about?

 

Does Double Bag Infusion Really Work

 

I don't believe that you cannot achieve the same results with a single bag and mold. If the mold isn't a mold then another interpretation can be a use for one off and that I have done, but you have to be clever to not create a lot of finishing issues. Can be done well. I am not refuting any opinions. Like I said I have to look into it more. But it seems to me unnecessary complexity.

 

That is a flawed experiment/demonstration and is valid only for relatively flat and small infusions. In another thread Norman posted a video of a Feynman lecture on the scientific principle. One experiment with limited variables means nothing other than to establish one set of data that says that this method is of no value in this particular situation.

 

Sorta what I was trying to allude to. Atmospheric pressure is pretty much a constant. A mold is a mold unless there isn't a mold. I think what the concept is about is separating the clamping function from the infusion differential pressure. But I am not sure why you would have to or what benefit you get from trying other than complexity or some ease of turning one circuit on and off while not messing with the other. With overflow pots and careful measuring and planning you pretty much get all this functionality out of a standard mold under full pressure. The viscosity of the resin and the weave of the cloth only provide one rate of flow at full vacuum. For production you would have done your tests and dialed in the perfect technique with a standard mold and single vac bag layer right? For one offs you hope and pray that you get full saturation if you are cutting it close. Isn't that why we have wicking and peel layers on the non-pretty side?