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Vacuum or Press?

Old 12-21-2007, 12:18 PM
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mmattockx
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Default Vacuum or Press?

I am trying to decide between buying a vacuum bagging system or making a press for doing wings. Costs will be similar, but I think the press will be a touch cheaper in the end. Also, the press has no consumable costs once it is built. Attached is a pic of a press Dick Hanson mentioned using for big wings and liking better than bagging for the big stuff.

Any thoughts/comments/experiences on one versus the other? Versatility, storage, etc.?

Thanks,
Mark
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Old 12-21-2007, 12:21 PM
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fancman
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Default RE: Vacuum or Press?

my vote is for the vaccum bag system. easy to store and more efficient. More even pressure also.
Old 12-21-2007, 01:16 PM
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tggilkey
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Default RE: Vacuum or Press?

I have not gone back to a press type of system since going to the vacuum bag set I got from Aerospace Composites 10 plus years ago. I still need a flat surface and flat piece weighted just enough to keep the sandwich of shucks, skins and core from turning into a banana (the weight is external to the vacuum bag), but I know I use less glue and get good bonding at leading edges because of the over-all nature of the vacuum "pressure." The only consumables I have had to purchase with all these wings has been some of the nylon bagging material to make a "tube" large enough to handle the 18+" root chords of the 2 meter pattern planes. That fabricated tube gets used over and over... The nylon "tube" that came with the kit was sufficient for the .60 size wings but not for the later models. For storage everything goes back in the original box it was shipped in where it gathers dust until the next building season. Aerospace's little system works well for this application.

Enjoy!
Tom
Old 12-21-2007, 01:31 PM
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mmattockx
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Default RE: Vacuum or Press?

Tom,

You can use the bags repeatedly? That was my one issue with vacuum, was having to buy bagging material on a regular basis. If that is a non-issue, it makes vacuum look very good. And storage in the shipping box is also a VERY nice bonus. I do not have a huge shop area for building (yet, I'm working on that) and being able to just pack it all away again is a great feature. I don't expect to ever do wings bigger than maybe a 96" for a 1/3 scale Laser, so a couple different sizes of bag should do me for quite some time. Is it reasonable to lay up two wings, stack them with MDF boards and a bit of weight, then use a tee fitting to draw down both bags simultaneously? Or is it too hard to set up two wings fast enough while using polyurethane glues?

You may have just sold me...

Thanks,
Mark
Old 12-21-2007, 02:40 PM
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dreadnaut
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Default RE: Vacuum or Press?

ORIGINAL: mmattockx

Tom,

You can use the bags repeatedly?

You may have just sold me...

Thanks,
Mark
I reuse the same bags over and over. If you buy the special bag material from CST, you can get a good seal with clothes pins. I don't use a ''bag'' per se. I take a large enough sheet to fold in half over the sheeted core. Just roll the side and end seams and clip it about every three inches. Twist the bag around the suction tube and clip it real good. Resin does not stick to it either. I doubt that you will get the same kind of even pressure with a press.
Old 12-21-2007, 03:06 PM
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Jason Arnold
 
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Default RE: Vacuum or Press?

Hi,

Something to consider between the two systems is honeycombing. If you're going to do honeycombing on the cores the vacuum bagging may not be appropriate.

I used vacuum bagging on a set of Stylus wings and tail cores recently and it worked great. Very easy to use. There are a few good videos on how to do it on the web too. I used Bolly general purpose epoxy which is very easy to work with and gives you a good working time. BTW with vacuum bagging you'll be amazed at how little adhesive you need! The vacuum also provides far greater and more uniform pressure all over the wing. Be careful not to use too much pressure or you'll crush the wing! Try doing that with bricks etc.. lol

For the bags I used plastic pallet covers cut down to size and re-sealed with a sealer/welder.

Cheers
Jason.
Old 12-21-2007, 03:27 PM
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tggilkey
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Default RE: Vacuum or Press?

Hi Mark –

A couple of things:

The bagging material is reusable from one job to the next – the nylon material is fairly thick and does not stretch much. I was surprised when I first encountered it. This material is unlike vapor barrier material used in home construction or painting drop cloths. I have used the same bag for wings of different planforms with no problems. Little if any glue will get on the bag so it doesn’t get gobbed up, and the edges of the foam offcuts and whatever balsa (like leading edges) that sticks out will not snag or puncture the bag requiring patching or disposal. The glider guys that bag foam wings are usually (from what I know) bagging epoxy wetted glass onto foam -- they will use mylar sheets against the glass to get the finish – no foam offcuts like we would use. Their bags may well be messy after one or more uses and require replacement. Ours should last for some time. If a tiny leak can be located it can be fixed with a square of 3M shipping tape. The current bag – which is 10+ years old has no patches. The stab bag which dates from my first use of vacuum bagging does have a patch or two but there is no reason to replace it because it doesn’t leak – if it works, don’t fix it!

Because of space constraints I only do one wing panel at a time. Many times I only have time to bag one at a time anyway so it is not an issue. With the time to work through two panels I could see using two separate bags teed into the vacuum source. I’m shooting from the hip now and typing as I think (may get scattered or miss something). I would want a valve in the line leading to the second bag being left off when the first wing goes in the bag – that way the second wing doesn’t have to be hurried – making sure alignment is kept and that LE and TE are flat/straight and that the bag gets pulled down evenly all over, etc. Once satisfied with the first bag, then I would start gluing up the second panel. Once slid into the bag and the bag sealed, I would slowly open the valve to pull down the second bag. Now that I think about it, some kind of one way check valve might be necessary to inhibit the loss of vacuum to the first bag while the second is being pulled down. Maybe the first bag line should just have a valve that can be turned on/off like the one we talked about for the second bag. That way the first could be turned off as the second is turned on (maybe turn on the second after the first is off temporarily sealing the first bag). The pump would not be harmed by a few seconds of being shut off like that. This pump is a constant run oscillating something or other that doesn’t really have any mechanical parts to wear out. Where I build the temps are cool and I have left a panel in the bag with the pump running for 48 hours. There is not much air moving when the pump has pulled the bag tight so a few seconds of having a line shut off will not hurt it. Once satisfied with the pull down of the second bag, the valve to the first could be re-opened. Capacity/volume with the basic vacuum pump from Aerospace might be an issue with two giant scale wing panels in separate bags being pulled down at the same time. It probably would be OK to pull them down one at a time – it might/should be able to maintain both when pulled down one at a time.

I made my current tube from sheet/bulk material I purchased from Aerospace Composites. Determine how much you need for a comfortable circumference for the root end of the panel and bag – just right gives room for a hand or two in the way plus the whole wing panel (top and bottom shucks, core and sheeting). I made it rectangular as opposed to wing planform shaped. One long side will be a fold of the material (no leaks there), one end will be a permanent seal as will the other long side. Seal it up with 2” wide 3M Packing Tape – the original tape is still there. I may have had another set of hands to help with the long side – basically just fold the tape over the joint you are making.

Reinforce the hole in the bag for the vacuum fitting with quality duct tape inside and out. It won’t deteriorate or leak (or put stress on the bag material that could cause problems down the road.

I can’t speak to using the urethane glue/timing for sheeting – I’m just starting to use it for some construction items – haven’t used it for sheeting. I know some pattern builders who use them with great success – I don’t know if they lay up both wings at the same time.

Apologies for the “stream-of-consciousness” discussion/typing!

It was nice to see Aussie Knife Edge’s confirmation of my experience of using less glue!

Sounds like CST is a good source for vacuum supplies also.

That’s all I can think of at the moment. If you have questions or need follow-up or whatever, holler!

Later, Tom
Old 12-21-2007, 05:48 PM
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mmattockx
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Default RE: Vacuum or Press?

ORIGINAL: tggilkey
Apologies for the “stream-of-consciousness” discussion/typing!

It was nice to see Aussie Knife Edge’s confirmation of my experience of using less glue!

Sounds like CST is a good source for vacuum supplies also.

That’s all I can think of at the moment. If you have questions or need follow-up or whatever, holler!

Later, Tom
Thanks for the thoughts, Tom. No problem on the format, I work best that way too!

You can definitely use less glue this way. Even with weights and a flat board I would only use 2oz of epoxy max per 60 size wing panel, and that was lots even for a duffer like myself. The really good builders are under 1oz for the same thing, but I prefer a certain bond over 1oz of weight saved. The polyU glues are lighter than epoxy and I believe Terry Brox says he uses something like 10g of glue per 2M wing panel, maybe per skin not per panel. Either way, it's super light.

Now to go shopping with some of that Christmas $$$.

Mark
Old 12-22-2007, 10:02 AM
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ofremmi
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Default RE: Vacuum or Press?

I prefere 'vacuum', it can be done with very little investment. The setup described in the link below has been in use for 20+ years.

http://home.online.no/~ofremmi/HowTo.../VacuumBag.htm

PS : I wish everybody a relaxing holiday, and all the best for 2008

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