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Peel Ply techniques

Old 04-22-2005, 10:01 AM
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TGDF
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Default Peel Ply techniques

What, if any, is the best way to use peel ply? What do YOU use peel ply/release film for? I know what it is supposed to do. What was the last/most interesting actual application where you used it?

In one of Rutan's books he recommends using peel ply over an overlap of fabric so that the edge of the fabric will stay down and so the resin will fill in a bit under the peel ply. That will give a smooth, usually hump free overlap.

But how do you hold down the peel ply? I find I only get this smooth an overlap when I vacuum bag. Sometimes if I carefully wrap and secure a small piece I can get the peel ply tight enough for marginally acceptable results, but nothing beats the finish, adhesion, uniformity of lamination thickness and perfect resin content I get with bagged peel ply.

Ordinarily I lay up the work with a moderately generous amount of resin (not sloppy and dripping) and apply the peel ply dry. Is there a circumstance when one might want to wet out the peel ply?

Also, I reuse peel ply where the finish of the work is not critical. I get small thin pieces of the prior lamination that pull off the peel ply and may be a different color that the piece at hand. Does anybody else reuse the stuff? Its expensive!

Old 04-22-2005, 10:26 AM
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Default RE: Peel Ply techniques

I re use the perforated peel ply all the time. The only thing I use it for is when I layup a part in a female mold that I intend to vacumm bag. AFter I am done laying it up, I put the peel ply down on the resin/glass and then a layer of good quality paper towels to soak up the excess resin that comes through the perforated holes when I apply vacumm.

Works good and removes excess weight.
Old 04-22-2005, 10:48 AM
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Default RE: Peel Ply techniques

I guess I should have added that all my work is sandwich construction. Fabric over foam. And all the peel ply I use so far is dacron polyester fabric.

But great, I'd love to hear how anyone uses any release film/fabric in any application. Sandwich, mold, plug, whatever.

Dave, does perforated release film give a flat, almost mirror finish inside the molded part, an image of the fabric texture or a wavy glass texture like un perforated film? Can you reuse release film or does it tear up when you remove it?

Thanks.
Old 04-22-2005, 10:54 AM
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Default RE: Peel Ply techniques

I am doing wings in a female mold using sandwiched Rohacell, so I don't get a mirror finish. After the resin cures, the rohacell is smoother than it was, but I definately wouldn't call it perfectly flat.

I don't think it would leave a mirror finish using just glass though.
Old 04-22-2005, 11:08 AM
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Default RE: Peel Ply techniques

I use peel ply (tight weave dacron fabric) on the bag side of my sandwich panels (wing skins, fuselage parts... flat panels, etc.). I use peel ply when I want to remove excess epoxy, but primarily to result in a bondable surface where panels will be joined, internal parts will be added and so on. The perforated release film leaves a surface much like nonporous film, with the addition of little dots at the location of each perforation.

I don't usually wet-out the peel ply, as that would reduce its ability to wick epoxy out of my layup.

Keep in mind that Rutan was building full scale light aircraft... so the scale of things is a little different. I suspect the strip of peel ply he suggested using would have been wet out to use surface tension to hold it flat. It won't be as flat as a bagged surface though. There's no way around that, so Long-Eze builders do a lot of sanding.

The most "interesting" use I've found for peel ply is to use it as the hinge material in a skin hinge. Cheaper and easier to deal with than Kevlar and works just as well.

-David
Old 04-22-2005, 12:10 PM
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Default RE: Peel Ply techniques


ORIGINAL: davidfee

I use peel ply (tight weave dacron fabric) on the bag side of my sandwich panels (wing skins, fuselage parts... flat panels, etc.). I use peel ply when I want to remove excess epoxy, but primarily to result in a bondable surface where panels will be joined, internal parts will be added and so on. The perforated release film leaves a surface much like nonporous film, with the addition of little dots at the location of each perforation.
I've got some perf release film. Dying to try it but haven't had the chance.

I don't usually wet-out the peel ply, as that would reduce its ability to wick epoxy out of my layup.
I thought that's what would happen with vacuum.

Keep in mind that Rutan was building full scale light aircraft... so the scale of things is a little different. I suspect the strip of peel ply he suggested using would have been wet out to use surface tension to hold it flat. It won't be as flat as a bagged surface though. There's no way around that, so Long-Eze builders do a lot of sanding.
Yes. And I've read that Rutan never bags anything. You're right. As I recall the context now, he was probably referring to a 3-4 in strip along the full leading edge of a full sized aircraft. Perhaps that would work if you wet out the peel ply.

The most "interesting" use I've found for peel ply is to use it as the hinge material in a skin hinge. Cheaper and easier to deal with than Kevlar and works just as well.
I've seen you write that before and I can't imagine that the stress at the hinge would not cause delamination over time. But I haven't been able to visualize just exactly how a fabric hing of any material would work right. Can't visualize how the resin would score and crack without cutting/breaking the hinge material. Except perhaps kevlar. (sigh) I'm sure the cure for that, like many other things is just to build one and see how it comes out.
Old 04-22-2005, 02:11 PM
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Default RE: Peel Ply techniques

The way I've done the peel ply skin hinges, the peel ply (dacron/nylon fabric) goes between the outer glass (or carbon, whatever) layers and the core material. The epoxy which penetrates the peel ply acts like a million tiny rivets holding the whole thing together. There is a ton of bonding area, relatively speaking.

If you think about the loads on the hinge, you can break them down into shear and tension loads. In shear, there's no way you're ever going to pull the hinge out with flight loads... the core material would fail first. In tension, practice peeling up a fully saturated strip of peel ply from a cured laminate. First you have to get it started... and then the forces required are still very much greater than anything a flying surface will see in use.

The trick is to scribe through the outer layers of glass/carbon/epoxy and stop just shy of the peel ply. Then scribe the inside also (need to cut the lower surface free first). Then you gently flex the hinge to fracture the epoxy along the hingeline. The more you flex, the more free the hinge becomes. I typically cut the peel ply on a bias to further reduce hinge stiffness.

Just make yourself a test article like Mike James did... I think you'll be sold. He used Kevlar, I believe, but the principle is no different.

-David
Old 04-22-2005, 02:55 PM
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Default RE: Peel Ply techniques


ORIGINAL: davidfee

The way I've done the peel ply skin hinges, the peel ply (dacron/nylon fabric) goes between the outer glass (or carbon, whatever) layers and the core material. The epoxy which penetrates the peel ply acts like a million tiny rivets holding the whole thing together. There is a ton of bonding area, relatively speaking.

If you think about the loads on the hinge, you can break them down into shear and tension loads. In shear, there's no way you're ever going to pull the hinge out with flight loads... the core material would fail first. In tension, practice peeling up a fully saturated strip of peel ply from a cured laminate. First you have to get it started... and then the forces required are still very much greater than anything a flying surface will see in use.
I hear you saying that, I know how tough it can sometimes be to separate a peel ply, still in my mind I see the thousands of times that hinge will flex and I couldn't use peel ply. Glass or better kevlar so you don't cut it making the score. That's just me, you understand.

The trick is to scribe through the outer layers of glass/carbon/epoxy and stop just shy of the peel ply. Then scribe the inside also (need to cut the lower surface free first). Then you gently flex the hinge to fracture the epoxy along the hingeline. The more you flex, the more free the hinge becomes. I typically cut the peel ply on a bias to further reduce hinge stiffness.
Ah, so you do have to fracture the matrix. I couldn't see...... I'd have to lay it all up with whatever I need for the wing and control surface and then just one ply of kevlar for the hinge. To lay up the wing and control surface as one lamination I'd have to have some kind of precision machine to cut through all but one layer of laminate and leave the last one intact. Again, that's just me.

Also I couldn't see how to keep the matrix out of the hinge cloth. I figured I'd have to just score and break it like cutting a piece of window glass.

As you say, just go make one. It helps to boot El Diablo out of the details.
Old 04-22-2005, 03:07 PM
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Default RE: Peel Ply techniques

To lay up the wing and control surface as one lamination I'd have to have some kind of precision machine to cut through all but one layer of laminate and leave the last one intact. Again, that's just me.
It's really not that difficult. I've used thin dremel cut-off wheels (hand held, not motorized), the edge of a triangular file.. even the corner of a single-edged razor blade... all guided by a steel ruler... to score through the outer layers. Just work slowly. Make a test piece so you get the feel for it.

Also I couldn't see how to keep the matrix out of the hinge cloth. I figured I'd have to just score and break it like cutting a piece of window glass.
Right, again it's not so complicated.. no waxing of the hingeline with a crayon, no score & snap, etc. It's easy peasy. Bag the peel ply (or Kevlar) right in with the skin and core materials. Score through the outer layers when you demold (or when you remove the mylars for a regular baged wing). Flex the hinge to free it up (I apply pressure along the hingeline with a steel ruler again to hold the skin materials in place).

As you say, just go make one. It helps to boot El Diablo out of the details.
Yup.

Old 04-28-2005, 11:24 AM
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Default RE: Peel Ply techniques

Peel ply lays down quite well all by itself when it is wet out. I use it all the time on my Long EZ, and yes, it is different on full scale. I use it when I want to suck epoxy out of a layup. Put it on the layup, heat it and squeegee it and it takes the extra resin out. You don't want to starve a layup with it though. I have only vacuum bagged one set of wings on the full scale, and it was a huge pain. I would like to learn it with smaller stuff. We don't use vac much on the full scale EZs. The Berkut used it, but like I said, it was a huge pain on that bird. We did vac bag the wings on it, but something that large is tough to get all set up.

Peel ply works great for making a surface uniform so you don't have to sand as much on something that large, given you start with a good surface on the foam. If it is wavy to start with, you will get a bad surface. All my surfaces were cut over size and sanded to final size with straight bars so they are dead nuts! When the glass goes on, it is darn near perfect. Looks great when its done.

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