Double Bagged Infusion - Why does it work?

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#86

Thanks for the article. I appreciate it.

A quote from the article:

In the article, Russel Emanis credits his double-bag process with improving the consolidation of the part due to increased pressure, but because we only have 1 bar/14.7 psi at sea level to compress our part, it doesn’t matter whether we have one or two bags, the minimum thickness achievable is the same. The Emanis process uses two steps: 1) an inner bag at low vacuum pressure to fill the part and then 2) the outer bag and full atmospheric pressure to consolidate and bleed the part. This is similar to the way prepregs are processed.

How the heck can the outer bag at full atmospheric pressure consolidate and bleed the part if the inner bag is at a lower pressure. I’m shocked that Composites World keeps this utter myth alive. I’m also shocked that none of their writers has called B.S. on this concept. None of them seem to understand that it is only the bag stiffness of the inner and outer bag coupling that can retain any compaction if the inner bag starts operating at a lower vacuumm.

#87

Thank you hojo, I does not want to start a discussion about loosing pressure at the sidewalls if using a pressure piece. THANK YOU:)

@ wyo: he says that that with the two bags written by Emanis is wrong.

#88

Thanks for the article. I appreciate it.

A quote from the article:

In the article, Russel Emanis credits his double-bag process with improving the consolidation of the part due to increased pressure, but because we only have 1 bar/14.7 psi at sea level to compress our part, it doesn’t matter whether we have one or two bags, the minimum thickness achievable is the same. The Emanis process uses two steps: 1) an inner bag at low vacuum pressure to fill the part and then 2) the outer bag and full atmospheric pressure to consolidate and bleed the part. This is similar to the way prepregs are processed.
How the heck can the outer bag at full atmospheric pressure consolidate and bleed the part if the inner bag is at a lower pressure? I’m shocked that Composites World keeps this utter myth alive. I’m also shocked that none of their writers has called B.S. on this concept. Even this author who in a proponent of single bagging seems to give Emanis a pass. None of them seem to understand that it is only the bag stiffness of the inner and outer bag coupling that can retain any compaction if the inner bag operates at a lower vacuum.

#89

Anyone that has setup a DBVI setup can quickly see that just by lowering vacuum level in the inner bag during dry debulk, the laminate springs back regardless of the vacuum level on bag two.

#90

The author sure seems to give Emanis a pass when he says, “This is similar to the way prepregs are processed.” As if it was somehow possible.

Also, from the picture in the first article, how does putting an intensifier between the two bags intensify anything?

#91

The hard piece hold the fabrics in place and eleminates bag relaxing, but the maximum pressure is the ambient pressure;)
If you want to intesify you have to play with different surfaces, but than you will have anywhere in the mould less pressure.
If you press the bridging fabrics in the corners I would worry that they will be damaged, and that worries me more than a little bridging and more resin.
The best thing is to work perfect!

#92

I think you might want to vent to atmosphere and lower to what is typical pressure loss during simple infusion and also see if there is difference when inside bag is lowered to 20 -25 hg. If you can negate any benefit I see crow on the menu:) :slight_smile: :slight_smile:

#93

It can reduce relaxation but it cannot intensify anything. With a typical intensifier the bag presses against the intensifier to attempt to eliminate bridging by pushing the intensifier into the mold. With the DB method If the first bag is bridging and the intensifier is put into position the second bag cannot push it any further to reduce bridging. The vacuum in the second bag just squeezes the intensifier rather than pushing it into position. That article seems to have the underlying thread that the outer bag can create a compacting force via external atmospheric pressure.

#94

Yes, like I said. It holds the fabrics in place and does not produce any extra load.
If the “intensifier” is not placed correct it will discharge the laminate, and than the Vf is really bad at a place where you want it perfect (thats why using a intensifier)

#95

I tought about it. I think you are right.:smiley:

#96

A year later … has anyone changed their opinion on sb v. db? Or is the jury still out?

Is the db process capable of producing parts nearly as good as from the autoclave?

#97

quote ;Is the db process capable of producing parts nearly as good as from the autoclave?

autoclave is seven time more pressure than atmospheric pressure , so if you looking at fiber/resin volume autoclave is better
now if you are talking about Aesthetics, you can made very nice parts without autoclave . as for mechanical properties also .

the concept of the double bag was devellop for infusing poly or vinyl resin without outgassing the resin and keeping full atmospheric pressure on the bag.

#98

Pressure on the laminate will be determined by the pressure within bag number 1 only, bag 2 does not contribute any more consolidation. Two bags are better than one for vacuum integrity reasons.

#99

If infusing correct you can reach the same VF than by autoclave process but less voids. The pressure in the autoclave is not important for the VF, it is used to minimize the entrapped air in the laminate.
And just like hojo said, the number of bags does not change any pressure, its just for the vacuum integrity and may it helps a little to hold the bags in place because of the friction between the bags.

#100

From what I understand Double bagging for most people is indeed for vacuum integrity, but something I read… trying to remember where I read the article but it had to to with Boeing’s double bagging method.

I remember reading the double bag method also prevented the swimming effect that the infusion method has when resin is infused to the laminate. Usually you can see this at your inlet area. The second bag was supposed to compact the laminate to prevent the swimming effect. I cant remember if they said the second bag was compacted all at the same time or if full vacuum to the second bag was applied after the part was infused.

I have basically been applying light vacuum to the inlet hose once the part is fully infused. Ive been getting very good results so far with this method.

#101

Exactly, Its is all about vacuum integrity!

Have to disagree about having the same VF as using prepreg in a clave, some of the PrePreg systems that we use are Just 35% resin content, we could never dream of such low resin content from infusion.

#102

You can reach a VF of 65% by infusion, but thats nit really good. Same in prepreg, such a high VF can be used for very light covers. For dynamic load parts this VF is not recommended.

#103

So, after reading the whole thread as carefully as I could my final conclusion would be that double bagging due to the relatively counterintuitive physics involved: would do nothing or practically nothing to provide a more compacted laminate when compared to single bag practise, and that if more compaction is needed all that can be done is to suck resin on both ends of the infusion bag, the vacuum hose and the resin inlet hose applying vaccum or gravity force by lowering the resin pot one meter, it seems that if instead of this the resin inlet is closed before the part gets fully wet some compaction could be achieved as well, so the double bag should be considered mainly as a mean to guarantee vacuum integrity … amazing how the couinterintuitive matter of the double bag beast could lead so many composites prefessionals into fooling themselves … Am I right with my conclusion ???

#104

You are right.

#105

How long is your infusion taking and how long before your resin begins B-staging ?