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.

 

[berkshire]

Kirby,
Composite pro explained it in his ballon analogy. All the inner bag does is to allow you to take off the below gage reading.
B.E.

 

[KirbyWan]

So I think it's safe to say we are all in agreement that for the stated purpose of maintaining bond pressure on the panel using vacuum while keeping the adhesive from foaming by equalizing it with atmospheric pressure this set up is totally ineffective. When the inner bag is vented to atmospheric pressure the panel is no longer experiencing pressure from the outer bag and it would be the functional equivalent of bonding the panel without a bag.

Thank you all for your time and sharing your knowledge and experience, and feel free to carry on the conversation if there are usefull additions to throw in there.

-Kirby

Kirby Wilkerson

Remember, first define the problem, then solve it.