Composites Central

Go Back   Composites Central > General Composites Discussions > Composites Talk

Reply
Old 02-17-2012, 01:10 PM   #11
nash

Member
 
Join Date: Apr 2011
Location: Italy, close to Cremona
Posts: 80
Thanks: 18
Thanked 10 Times in 9 Posts



Default

Wyowindworks, how about trying your experiment with a partially liquid filled bag as is the case in infusion. It took Boeing over 8 years to win that patent and its a very important work for the infusion process.

The advantages claimed by "double bag" patent are:

1. Vacuum integrity
2. Reduced bag relaxation

However, from what little I can share, based on our process, during infusion we can get very fine control over the vacuum differential for the infusion. Such a differential will be impossible in a fully drawn bag. Which can indeed give a very high quality laminate (Vf in the range of 62-68%). We can also get to above 70% with tightly woven materials and very low resin viscosity, for instance that laminate on our site is spread-tow panel that has a very high Vf. And no, we don't have voids in our laminates.
__________________
www.nashero.com
NASHERO -- Highest quality, safe, sleek, efficient, comfortable, spacious, intuitively easy and fun to fly aircraft
nash is offline   Reply With Quote
Old 02-17-2012, 03:46 PM   #12
wyowindworks

Composites Expert
 
Join Date: Dec 2011
Location: Rapid City, SD
Posts: 690
Thanks: 13
Thanked 237 Times in 124 Posts



Default

Nash, is that 70% by volume or weight?
wyowindworks is offline   Reply With Quote
Old 02-17-2012, 07:44 PM   #13
HeyBen

Member
 
Join Date: Jan 2012
Location: FL
Posts: 64
Thanks: 7
Thanked 8 Times in 8 Posts



Default

Quote:
Originally Posted by wyowindworks View Post
The second bag won't compress anything inside the inner bag if the inner bag is vented. You have one atmosphere pushing in through the vent and one atmosphere pushing on the outer bag. All you do is push the inner and outer bag together. Anything between the inner and outer bag undergoes 14.7 psi. Anything inside the inner bag under goes no pressure. Put a soft sponge in the inner bag and apply vacuum to the outer bag and have a see. The sponge will not get compressed. Apply full vacuum to the inner bag and the sponge will go flat as pancake. Then apply vacuum to the outer bag and release the vacuum on the inner bag. The sponge will only rebound a little bit. Grab the bags and you can pull the pull the two sides of the inner apart where the sponge is as you suck air into the inner bag.

After doing this show-and-tell in my shop I came to the conclusion that the only advantage to the second bag is for vacuum integrity and some form holding power. If you first apply vacuum to the inner bag and then apply vacuum to the outer bag then vent the inner bag via infusion (one atmosphere pressing on the resin) the two bags getting pressed together will hold their shape and prevent some of the rebound of fiber near the resin front. This isn't happening because the atmospheric pressure in pressing against the laminate but rather the two bags are being pressed together so they hold their form.

Ah-ha, now I see it. Great explanation, thanks for that.

In that setup you had a good media between the layers? I think friction would play a part in this type of thing.

Now I wonder (I hate it when I do that, it gets dangerous) what would happen if you were to lay up a 2 bag system with an inflatable bladder between them?
HeyBen is offline   Reply With Quote
Old 02-17-2012, 09:30 PM   #14
dallasb84

Composites Expert
 
Join Date: Jul 2004
Location: va
Posts: 709
Thanks: 23
Thanked 39 Times in 36 Posts



Send a message via Yahoo to dallasb84
Default

Great work Adam. Simply put the second bag acts like an intensifier. Also works better with stretchalon.

If you have some older material or scrap and can lay up a larger piece I'm willing to bet you would also see differences in weight with db vs sb.

We did while r&d in our test lab.

We also saw differences in weight between like infusions that were sb process. Db process with pressure ramp and time consistencies proved consistent. Resins were premixed and resins were warmed to specific temps for viscous control.

Simply put db just allows for more control and repeatability.

Thanks for doing this Adam.
dallasb84 is offline   Reply With Quote
Old 02-17-2012, 10:43 PM   #15
wyowindworks

Composites Expert
 
Join Date: Dec 2011
Location: Rapid City, SD
Posts: 690
Thanks: 13
Thanked 237 Times in 124 Posts



Default

Could you explain further how the second bag functions as an intensifier?
wyowindworks is offline   Reply With Quote
Old 02-17-2012, 10:48 PM   #16
dallasb84

Composites Expert
 
Join Date: Jul 2004
Location: va
Posts: 709
Thanks: 23
Thanked 39 Times in 36 Posts



Send a message via Yahoo to dallasb84
Default

Second bag with breather in-between maintains laminate compaction before and after resin wave infuses. Thus acting as an intesifier.
dallasb84 is offline   Reply With Quote
Old 02-17-2012, 11:13 PM   #17
wyowindworks

Composites Expert
 
Join Date: Dec 2011
Location: Rapid City, SD
Posts: 690
Thanks: 13
Thanked 237 Times in 124 Posts



Default

What is your explanation for the maintenance of the laminate compaction?
wyowindworks is offline   Reply With Quote
Old 02-17-2012, 11:36 PM   #18
nash

Member
 
Join Date: Apr 2011
Location: Italy, close to Cremona
Posts: 80
Thanks: 18
Thanked 10 Times in 9 Posts



Default

Quote:
Originally Posted by wyowindworks View Post
Nash, is that 70% by volume or weight?
Well, I wonder which company will tell you the exact Vf = volume fraction, or Wf = weight fraction that they get based on a proprietary process they've developed. However, here you go: Vf is just what I'm talking about.

I had questions similar to yours before studying the boeing patent with care and trying out their ideas first. I thought that the outer bag and inner bag once compacted against each other was just one thick bag. But that was only part of the story...

Another thing regarding your experiment, a laminate stack is not like a sponge, dry laminate has very little spring-back, and if its liquid filled, which allows the inside surface of the inner-bag a liquid to flexible film contact with the outside surface of the mold, a liquid to solid contact, things are very different from your experiment.

If you think of it a bit more carefully, there is flange side where there's an overlap in all double bags, that overlap is approximately 10cm between bags, If you have a breather between the two bags, there's always a differential even if your inner bag is at atm pressure. Once the outerbag has taken its shape, and the pressure is not released, it will try to keep that shape (less the slippage between the breather, outer and innerbags). That differential is proportional to the innerbag-projected area on the solid mold divided by the outerbag-projected area on the mold. What this does is that it provides a constant debulking force on the stack to not losen itself.

Assuming that the inner bag is at atmospheric pressure is wrong, since its the pressure differential that is important and not the absolute pressure in the inlet. So your inlet pressure will almost always be a slight vacuum.

I'd suggest you to give it a shot to see if it will work for you. Like I said, we use both methods, but for structurally important things where laminate quality is important we prefer to use doublebag.

Hope it clarifies.
__________________
www.nashero.com
NASHERO -- Highest quality, safe, sleek, efficient, comfortable, spacious, intuitively easy and fun to fly aircraft

Last edited by nash; 02-18-2012 at 12:00 AM.
nash is offline   Reply With Quote
Old 02-18-2012, 12:30 AM   #19
DDCompound

Forum Donor
Composites Expert
 
Join Date: Aug 2011
Location: Germany
Posts: 890
Thanks: 44
Thanked 134 Times in 120 Posts



Default

Quote:
Originally Posted by nash View Post
Well, I wonder which company will tell you the exact Vf = volume fraction, or Wf = weight fraction that they get based on a proprietary process they've developed. However, here you go: Vf is just what I'm talking about.
We often ash a probe to get the exact Vf of the part which is written down in the part book, so everyone can see the VF of a part!
I think the customer is allowed to know this thing about his part if he wants to know.

Quote:
Originally Posted by nash View Post
I had questions similar to yours before studying the boeing patent with care and trying out their ideas first. I thought that the outer bag and inner bag once compacted against each other was just one thick bag. But that was only part of the story...
This is the true part of the story!

Quote:
Originally Posted by nash View Post
Another thing regarding your experiment, a laminate stack is not like a sponge, dry laminate has very little spring-back, and if its liquid filled, which allows the inside surface of the inner-bag a liquid to flexible film contact with the outside surface of the mold, a liquid to solid contact, things are very different from your experiment.
Yes thats correct, but this little spring back decides over 30% or 55% Vf. So it just makes it a bit easyer to calculate when to stop the resin in. In single bag will be a little more resin at the inlet point, but if you know when to stop it will dispers all over. So if you work correct there is no difference at the end.

Quote:
Originally Posted by nash View Post
If you think of it a bit more carefully, there is flange side where there's an overlap in all double bags, that overlap is approximately 10cm between bags, If you have a breather between the two bags, there's always a differential even if your inner bag is at atm pressure. Once the outerbag has taken its shape, and the pressure is not released, it will try to keep that shape (less the slippage between the breather, outer and innerbags). That differential is proportional to the innerbag-projected area on the solid mold divided by the outerbag-projected area on the mold. What this does is that it provides a constant debulking force on the stack to not losen itself.
This works in LRTM/VARTM if working with a hard stabil secondmould piece, but not if working with a bag. We are talking only a few percent VF you want to get with the DB. That for it should not expand for 0.1mm. And even a hard closed mould will expand a little bit, thats why you can not get parts with a high Vf in RTM processes.
And believe me, a bag will move more than 5mm if the inner bag is venten a little too much, even if the resin is in it is vented complete. Where resin is, there is no vacuum, because resin is pushed in by atmospheric pressure.
The best efficent way is to controll your resin in not to able your laminate to swim up. And this can be done by several ways.

Quote:
Originally Posted by nash View Post
I'd suggest you to give it a shot to see if it will work for you. Like I said, we use both methods, but for structurally important things where laminate quality is important we prefer to use doublebag.
I promise, if someone who understands the physik of the process will test both methods he will get the same results
DDCompound is offline   Reply With Quote
Old 02-18-2012, 07:03 AM   #20
316Composites

Forum Donor
Member
 
Join Date: Jun 2005
Location: Connecticut
Posts: 192
Thanks: 2
Thanked 12 Times in 9 Posts



Default

All the info and pics more in detail are found here!!

http://www.compositesworld.com/artic...0-fiber-volume
316Composites is offline   Reply With Quote
Reply

Bookmarks

Tags
bagged, double, infusion, work

Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump


All times are GMT -8. The time now is 07:21 PM.
Powered by vBulletin® Version 3.8.7
Copyright ©2000 - 2018, vBulletin Solutions, Inc.

Inactive Reminders By Icora Web Design