Is This Double Vacuum Bag Technique Valid? 10

[KirbyWan]

Howdy all,

So two expericenced engineers are arguing about whether this method is valid or not. The issue is the foaming of AF191K adhesive when bonded under full vacuum. We do not want to have AF191K cured under full vacuum to avoid foaming of the adhesive in the bond line. Will the technique as sketched in the attachment work? Here is the process:

1. We create an inner bag pull full vacuum on the inner bag.
2. We create an outer bag and pull full vacuum on the outer bag.
3. We release the vacuum on the inner bag so it achieves equalibrium with the atmosphere.
4. We apply heat to cure the panel.

Assumptions:
We use normal shop practice for vacuum bagging with peel plies, release films, breather cloths etc.
The vacuum bags are well sealed to the atmosphere and to each other.
Full Vacuum is considered to be 22"Hg.

The question is what pressure will this panel be cured at? One of us believes that this set up will allow the panel to be cured at full vacuum pressure while the adhesive will not foam because it is at atmospheric pressure. The other one beleives that panel will experience no pressure after the inner vacuum bag is released. This is a conceptual problem that I was hoping this community of engineers could help sort out. Thank you all for your input.

Kirby Wilkerson

Remember, first define the problem, then solve it.

 

[VEBill]

Step 3a. Apply a very slight positive pressure to the inner bag to lift it clear away from the work piece so that the first engineer will be able to see that the vacuum between the two bags is failing to apply pressure to the work piece.

Disclaimer: not my area. Wait for others.

 

[MintJulep]

The purpose of the vacuum bags is to apply force.

Vacuum bags apply force by differential pressure over an area.

There is effectively no differential pressure in the areas were the inner and outer bags overlap.

Therefore there is no force applied after the inner vacuum is released.

 

[berkshire]

Kirby Wan,
To do what you want, the outer bag has to cover a greater area than the inner one.
B.E.

 

[rb1957]

1) double bagger eh ? (sorry, couldn't help myself)

2) the 2nd bag is making a vaccuum over the first. when you vent the 1st bag, what's the 2nd bag doing ?

Quando Omni Flunkus Moritati

 

[KirbyWan]

Berkshire,

As per my sketch the outer bag does cover a larger area then the inner bag. How does this change anything?

RB1957,

So what are you saying. I'm trying not to argue for or against either side since I'm trying not to put up a paper tiger for my opposition. I also don't want to guide the debate one way or the other, (yeah like I could if I wanted to.) I'm also the far less experienced engineer in this arguement and want the strongest arguements to present if I'm correct.

VE1BLL,

I appreciate your empiricism and think this should settle the arguement right out, however the interlocutors are not colocated.

-Kirby

Kirby Wilkerson

Remember, first define the problem, then solve it.

 

[MintJulep]

Think of your vacuum bags as a cylinder with two pistons.

 

[berkshire]

Kirby,
As per my sketch the outer bag does cover a larger area then the inner bag. How does this change anything?

Your sketch is the right way to do it, although when I do them I run the vacuum tube from the inner bag horizontaly out under the perimeter of the outer bag, with an extra wad of bag seal, so I do not have to cut and seal holes in the outer bag, but thats just me.
Not sealing over the perimeter of the inner bag simply lets the inner and outer bags come together. with no differential pressure applied to the part when the vacuum of the inner bag is released.
B.E.

 

[blakmax]

I first used the double vacuum method in the early 1980's, well before it became popular overseas. The purpose of the double vacuum is to enable entrapped gasses to escape from the laminate or adhesive bond. If you use two flexible bags, then the effect is minimal. Atmospheric pressure on the outer bag will still apply force to the laminate, so the volatiles will still be trapped. If as suggested you apply a small pressure to the inner bag then the volatiles will still not escape. There is no pressure differential to encourage out-gassing.

The secret of success is to use a RIGID outer bag. I made mine from thick perspex. Apply vacuum to the inner bag and outer box at the same time. The pressure on the inner bag will equalize so the inner bag will not apply pressure, but the laminate will be under vacuum, thus enabling the volatiles to escape. Occasionally I would find that the inner bag clings to the laminate, so I used a rod sealed through the lid of the rigid box and at the end of the rod was a ball of vacuum bag tape adhesive. I would raise the rod to pull the bag off the laminate surface.

Be careful in using this approach on thin sandwich structure because there is a residual force on the surface caused by the atmospheric pressure trying to push the structure into the rigid box. On thin sandwich structure you may disbond the face sheet from the core.

Regards

Blakmax

 

[berkshire]

with double bagging, you allow the resin and your lay up to be at atmospheric pressure not below gauge. while applying differential pressure with the outer bag. So the pressure of the lay up is positive . This of course presumes that the surface you are bagging against is air tight.And that you have pulled a vacuum on the first bag. If you release the vacuum on the first bag all bets are off, the air simply fills the bag as though you have a hole in your lay up.
B.E.

 

[VEBill]

I think I see what some are saying.

If the dimensions are correct, or by using a rigid plate, the outer bag may be compressed downward by the vacuum thereby applying simple physical force or tension against the workpiece.

 

[rb1957]

blakmax,

ok, so you're the guru, having done this before. your rigid box is quite different to the original proposal.

sounds like you build a small pressure vessel to contain everything in a vaccuum, but i thought the purpose of a vaccuum was to clamp everything together ... using atmospheric pressure on the other side of the bag.

i guess i don't see the point to the first bag ?

Quando Omni Flunkus Moritati

 

[blakmax]

rb1957

The purpose of the first bag is that when you are satisfied that the volatiles have had sufficient time and vacuum to escape, you then release vacuum on the outer box and remove it, thus leaving a conventional vacuum bag for the rest of the curing process to continue.

Regards

Blakmax

 

[Compositepro]

Not everybody has the same definition of "double vacuum-bag process", so you have to be careful. The diagram linked to in the OP is clear, however, and it will not work as intended. Two films stuck together by vacuum behave exactly as one film.

The process using the rigid shell or chamber on the outside can be very useful for removing air and volatiles, with or without an autoclave. If you place pressure intensifiers (solid molds), between the two films it can be helpful to preventing radius bridging.

 

[tbuelna]

Double vacuum bags are used with resin infusion processes such as VARTM. The inner bag uses less vacuum and is used to control the resin flow and extract volatiles. The outer bag uses high vacuum and is used to apply compaction force. The drawback to using a single vacuum bag with infusion is that due to the relatively high viscosity of resin there can be a substantial difference in hydrostatic pressure as you move away from the vacuum ports. On large parts, there may be a difference of 10-15 in.Hg. at a distance of only 2-3 ft. from a vacuum port.

Here's a short article on the subject.

 

[Compositepro]

There was a very lengthy discussion of that article on LinkedIn. There was basically no science in the article, only vague assertions. The outer bag can only apply a different compaction force than the inner bag through the use of solid pressure intensifiers (essentially creating a matched mold surface in limited locations).

 

[KirbyWan]

Blakmax,

In my review of literature I did read about the process you are describing for cureing polyimide and phenolic resisns which, being a condesation reaction, produce volitile components that need to be removed as the part is cured. This is for a different, almost opposite purpose, where we want to have the adheisve at atmospheric pressure to prevent foaming of the adhesive, so it is more like we're trying to keep volitiles in while maintaining pressure with vacuum only.

This is also not for resin transfer molding, in-fact it is for metal faced, metal honeycomb structure on a CF6-50 TR. I suppose I could work the rigid box design in reverse where I apply positive pressure to a box that is clamped to the part, but that has all kind of issues with sealing and not damaging structure. I've even thought about performing the heat blanket run in our autoclave for the positive pressure and venting the bag to atmosphere, which will work, but I don't know how to power the heat blanket inside the autoclave and don't want to apply heat to the entire part to prevent more voids from being created, particularly on an unrestrained translating sleeve.

Thanks for everyone's comments this is guiding me in the direction I expected.

-Kirby

Kirby Wilkerson

Remember, first define the problem, then solve it.

 

[KirbyWan]

Berkshire,

You seem to be the one person in favor of this method but your second message confuses me. To quote:
...
with double bagging, you allow the resin and your lay up to be at atmospheric pressure not below gauge. while applying differential pressure with the outer bag. So the pressure of the lay up is positive . This of course presumes that the surface you are bagging against is air tight.And that you have pulled a vacuum on the first bag. If you release the vacuum on the first bag all bets are off, the air simply fills the bag as though you have a hole in your lay up.

...

How is the resin and layup at atmospheric pressure as you indicate in your first sentance, if I do not release the vacuum on the first (inner?) bag which you indicate I should not in your last sentence? The whole point of the inner bag is so the resin (well film adhesive) and panel are at atmospheric pressure, which means before applying heat the vacuum must be released.

The one thing I can see that would make what you are talking about work is using two rigid molds where there must be bridging of the bagging material to force two plates significantly larger then the panel together. But I don't see how this can be actualized easily.

Once again this forum has provided solid guidance on a murcky subject. Thanks for everyone's submissions.

-Kirby

Kirby Wilkerson

Remember, first define the problem, then solve it.

 

[Compositepro]

In conventional autoclave processing, venting the bag to atmosphere is a common procedure. The purpose is to prevent resin from foaming and boiling under vacuum. But in an autoclave you have greater than atmospheric pressure so there is still compaction pressure. Also, resin will not foam simply because there is a vacuum in the breather under the bag next to it. The hydrostatic pressure of the resin must fall. It cannot both support the bag and be under vacuum. The resin has to bleed (flow into the breather) until something else is supporting the bag, and then the hydrostatic pressure will fall and the resin will foam.

If you want to vent the vacuum without an autoclave you simply have to reduce the amount of vacuum to, say, 1 in. of Hg. from 15 in. Hg. The compaction pressure is still 15 in. Hg.

What confuses people about vacuum bags is that there can never be a "pressure" difference between the two sides of a thin film. The film is intended to be highly flexible so it cannot support any load. It is simply a gas barrier. The confusion is in the different definitions of "pressure" and "stress". Both terms have units of force per unit area. But the stress in solids behaves completely differently that the pressure in fluids. And an additional complication is that that resins sometimes are solids and sometimes they are fluids. It depends on the temperature and on the time scale.

Air pressure on a vacuum bag applies a certain force per unit area. This is balanced at all times by an equal and opposite force per unit area. Fiber compaction is measured in force per unit area but, technically it is not a pressure but a stress.Fiber cannot flow or create equal pressure in all directions, as a fluid will.

A vacuum bag cannot remove an air bubble encapsulated in viscous resin, just as it cannot remove air from a latex balloon.
Try putting breather and a vacuum bag around a party balloon. The vacuum bag will suck tightly to the balloon, but will have zero affect on the balloon. Your double vacuum bag is exactly the same arrangement.

At sea level everything is under 15 psi at all times, whether there is an vacuum bag involved or not. All that a vacuum bag allows is for removal of air from the spaces between objects. This also requires that there be an open flow path for the air to get to the vacuum. That is why porous breather materials are used.

 

[WKTaylor]

From a 3M AF 191K data sheet [attached]...

3M™ Scotch-Weld™
Structural Adhesive Film AF 191
Technical Datasheet

Cure Cycle A cure of 60 minutes at 350°F and 45 ± 5 psi positive pressure is suggested when
maximum performance is desired.
Cure Cycle (Autoclave or Platen Press)
The following cure cycle has been used to obtain dense glue lines.
Cure Cycle (Autoclave, Vacuum Bag, or Platen Press)
1. Apply vacuum or pressure to keep assembled parts in place.
2. If using autoclave: Apply positive pressure slowly until 14 psi positive pressure is
applied. Once reached, dump vacuum bag.

3. If using vacuum bag only, limit vacuum to 5-10 inches of mercury. This is necessary to prevent frothing in the bondline.

4. Increase bondline temperature rise rate. 4-5°F/min.
5. Increase bonding pressure. 45 ± 5 psi
6. Cure. 350 ± 10°F
7. Cool. 5-10°F/min.
8. Decrease pressure when temperature is below 200°F.

Regards, Wil Taylor

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