Foam-less molds

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BobT

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May 18, 2008
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Hi Duncan,

Thanks for the welcome!

I'm glad to hear of your continued progress. It sounds like you are using techniques similar to those shown on the Glen-L site that Orion referenced.

My initial bias was C-Flex, which I dismissed because of possible issues of availability (they are a US based company) and weight. I haven't looked at the material in years, but they have two weights available, so maybe weight isn't a problem. I don't know enough about composites in aircraft to judge. As for availability, I can't speak to the distribution channels. Below are a couple of links to Seeman Composites' website for anyone who might be interested. I think you'll find some similarities to your approach.

Cheers,

Bob


C-Flex website: http://seemanncomposites.com/cflex.html
C-Flex manual: http://seemanncomposites.com/CFLEX MANUAL-WEB.pdf
 

rtfm

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Bob,
Hi. I had come across C-Flex before but it has (as far as I can tell) three drawbacks for my purpose:
  1. It is not available locally in NZ
  2. Ar 12" wide, I think the strips are too large to easily fold over some of my curves
  3. It is about twice the weight of my own solution (65lbs vs 33lbs) for the fuselage.
However, I think the concept is excellent. If only they made the strips in half weight, and maybe even 6" wide planks.

Cheers mate,
Duncan
 

AVI

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Duncan:
You might want to check out this extremely informative site. It's obviously heavily influenced by the Vision site but there's lots of good stuff with excellent pictures and instruction manual.

http://krsuper2.com/index.html
 

rtfm

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Hi,
Thank you for one of the most interesting and informative sites I've had the pleasure of visiting. Of special interest to me is the fact that the very first plane I bought plans for and actually started building (before I got the design bug) was the KR2.

Love the site, and it is firmly bookmarked.

Cheers,
Duncan
 

Greg Mueller

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At one point I made a few wind tunnel models for a company on the airport at Arlington. Most of the parts were solid aluminum and done on the CNC mill. On one plane they got to the nose and were running out of time, but they didn't have the dough for a lot of machine time. So I took the 3D cad drawing of the nose and sliced it into sections that were about 1" thick. Kind of like slicing bologna at the deli (only done with cad) That gave me a profile of each of the sections. I was able to cut them out of 1" thick foam pretty quickly and I put two reference holes (in the same relative location) on each one. All they had to do was slide the foam sections onto long rods and they had a nose that looked like steps or saw teeth. Then all they had to do is glue them together and "file" away the steps. Because the sections were rather thin, the resolution was pretty good.
Maybe this idea is something you can adapt
 

orion

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This used to be a fairly common way to do tooling plugs by those companies that couldn't afford or didn't have access to large gantry mills for tooling fabrication. If I recall right, I think even the first tools for the ViperJet were made in this fashion. But the process has a few drawbacks, namely in that the person with the sanding board needs to be very careful and needs to have a really good eye in order to achieve a fair line and reasonable symmetry.

For developing a fair tooling plug, a variation on the same theme would be to do the stepped cross sections, then use a filler to achieve the shape, rather than sanding down the corners. Pulling the fairing compound with a flexible tool would allow you to get the net shape in only a few cycles and done right, should involve only minimal sanding. One example that I saw of this was using a plaster with short glass fiber reinforcement for the large fills, then finished off with just the plaster alone. Final shaping was done with a sponge - only a small amount of actual sanding was needed. The finishing was done using Duratec sanding primer (after sufficient drying of the plaster).

But for a one-off, the stepped process could work if done carefully. One idea might be to cut the outer line as described, then cut an inner line but in a dashed pattern. This way you can slide the whole thing onto an inner tube structure but when the outside is all glassed, the dashed pattern would allow you to easily remove the inner foam.
 

Greg Mueller

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The guy they had doing the shaping was a real pro. A lot of times he just used metal templates that he would hold up to the shape to see where the light came through. His talent along with the predefined "steps of foam" made the process very quick and they said "It worked great!"
 

Greg Mueller

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Yep
I used high density foam. It's usually a yellow or tanish color. Before sacrificing an expensive piece of micro cast aluminum (when making female molds) it's a lot less expensive to try the program out (and get approval) with a piece of relatively inexpensive foam. This was all back about 20 years so probably they have something different now.
 

rtfm

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Hi,
Rather than starting another thread, I though I'd tack a question onto the end of this one, since it is related...

I plan to cover my wings in sheets of glassfibre previously vacuum infused on a flat surface. ie I'll lay up 2x 10oz satin weave cloth on a 9ft x 14ft flat surface. This will give me a mirror-finish, light and strong glassfibre sheet, easily able to be bonded to my wing ribs in a single piece. I've tried this method on a 4-ply layup 3ft x 3ft and it easily bends round the radius of the wing LE.

Someone mentioned a while ago in a previous post that I would run into the problem of the composite skin shrinking, forming hollows in the wing between the ribs. This has caused me to think again about the method.

First, is this shrinking really an issue? I'll be using the West system epoxy, and the wing skin will be cured to the manufacturer's specifications.

If shrinkage is going to be an issue, I may have to consider a glass/foam sandwich - which will not lend itself to the mothod described above.

Any comments?

Duncan
 

rtfm

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Hi,
One of my mission goals is to fly fast. I want to do some serious cross country flying, including a NZ - Australia crossing. I had originally targeted 120kts cruise - but I'm hopeful of even higher cruise numbers. To do this I will need a low drag laminar flow wing - and fabric just won't do.

Otherwise, no objections at all...

Cheers,
Duncan
 

Rom

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I have discussed this method of doing a wing skin in a previous post. Using a core with fiberglass on both sides is a much better method.
"I plan to cover my wings in sheets of glassfibre previously vacuum infused on a flat surface. ie I'll lay up 2x 10oz satin weave cloth on a 9ft x 14ft flat surface. This will give me a mirror-finish, light and strong glassfibre sheet, easily able to be bonded to my wing ribs in a single piece. I've tried this method on a 4-ply layup 3ft x 3ft and it easily bends round the radius of the wing LE."
The consenses among the experienced composite builders in this forum was that a composite skin with no core backing is subject to oil-canning.
I am getting ready to do a layup in a similar manner, but am going to apply a thin layer of lightweight syntactic foam to the sheet befor making the bend. After that apply another layer of glass the the inside of the wing skin.
 

wsimpso1

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Rom understands.

The first issue with the thin skin method that rtfm mentions is that the ribs would have to be pretty close together becuase the skin stiffness in bending will be pretty low. It might oilcan anyway.

The second issue with the method rtfm mentions is that of secondary bending. When you wrap something flat around a curve, it sort of goes saddle shaped. The reason is simple, the stuff on the inside has to get shorter in the chordwise direction, and it responds by getting longer in the spanwise direction. The stuff on the outside has to get longer in the chordwise direction, so it tries to get shorter in the spanwise direction. So, it will try to sag between ribs and/or rise up at the edges. I am not sure how much of an issue this will be, but I am suspicious that on a single skin for a whole wing, you could have trouble. Make it a little thicker, and it become more and more of an issue.

So, one way to do it is to do what Rom suggested. Thin skin, put it on the ribs, lay on a core (he is using syntactic foam which is microballoons and epoxy) or bond on a foam panel. Trouble with these ideas is that the foam has secondary bending too, and syntactic foam is heavy and hard to get on and sand out to a smooth even shape for glassing on the inside, but it can be done. How you do the second surface requires ingenuity... In any event, I am suspicious that laminar flow will be tough to maintain with the irregularities that will result, unless of course, you go micro and big sanding sticks...

Personally, I would just build molds. My wing skins molds were hotwired from flotation billet, sealed with wet micro and a glass tape around the edge, the working skin is roof flashing but some folks prefer Formica style laminates. Then I was able to vacuum bag the whole skin in one big layup. Do it with vacuum infusion (look up Corsair82's website) and it could be really slick.

Billski
 

rtfm

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Hi,
OK - I get it. My *brilliant* idea for getting glass-finish surfaces on my wing skins seems not be be such a hot idea after all. I'll not pursue it any further.

Personally, I would just build molds. My wing skins molds were hotwired from flotation billet,
By this I take it to mean that you shaped a full wing out of foam?

sealed with wet micro and a glass tape around the edge,
I don't follow what you mean.

the working skin is roof flashing but some folks prefer Formica style laminates.
Ditto the above

Then I was able to vacuum bag the whole skin in one big layup. Do it with vacuum infusion (look up Corsair82's website) and it could be really slick.
Aha - this part I *do* understand. I will be using vacuum infusion as far as possible on all my parts.

  1. It would be relatively easy to shape an accurate foam wing.
  2. It would also be relatively simple to make a female mold from this
  3. I could then use this mold to make a glass/foam sandwich skin
Is this what you are suggesting? Seems like a lot of work for a one-off part...

Duncan
 

Rom

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I was planning on using a syntactic foam by adding micro polysyrene spheres to the mix. The result will be a syntactic foam at half the weight. The shape will be held in place with female formers spaced at a workable distance.

It goes something like this:
One layer of bid at 45 degrees then one layer of uni along the span on a flat tool. Partial cure.
Apply 1/8" of syntactic foam. Place in form. Partial cure.
Apply layer of bid at 45 degrees inside.
Insert glassed foam ribs. etc.
 

rtfm

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Seems like an awful lot of work, but you'd definitely get a strong skin, that's for sure.

What about this?
  1. Create the flat vacuum infused skin (as in my suggestion above)
  2. In addition to the full-depth ribs attached to the spars, make MANY (ie one every 6 inches or so) hollowed out ribs - to act as stiffeners only. Weight would be negligible, but if made right would stiffen the skins admirably. These stiffeners could be vacuum infused in an "L"-shaped mold, maybe 1 inch per side, which would allow for a good bonding surface as well as a good shear-surface.
  3. Bond skins to ribs and stiffeners as per above.
This would cut out any oil-canning, but still allow for a light and easily manufactured wing skin. And the stiffeners wouldn't interfere with any internal mechanisms or fuel tanks.
 
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etterre

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Aha - this part I *do* understand. I will be using vacuum infusion as far as possible on all my parts.
  1. It would be relatively easy to shape an accurate foam wing.
  2. It would also be relatively simple to make a female mold from this
  3. I could then use this mold to make a glass/foam sandwich skin
Is this what you are suggesting? Seems like a lot of work for a one-off part...

Duncan
Actually, he skips to step #2. The hotwired foam is the female mold and, if I understand him correctly (Billski doesn't upload photos;)) the roof flashing is somehow formed and placed inside the mold to provide a smooth surface for the outside of the skin. So the layup order looks something like:
  1. floatation billet mold (sealed with micro slurry)
  2. roof flashing
  3. knitted fiberglass
  4. peel ply and other vacuum bag materials
 

wsimpso1

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Sorry about not doing photos. I made the upper and lower wing skins by making a female mold for each one. I took about 10 hours to build each one after I made the templates.

I made templates for both ends and two places along the skin, and hotwired them;
Aligned them, bonded and sealed them using micro;
Placed a glass tape with extra wet micro around the perimeter for sealing the bag;
Skinned the mold working surface with 36" wide coated aluminum roof flashing, using wet micro and vacuum bagging the whole thing;
Built up the leading edge area with several plies of duct tape to make room for tapes to close the front. Did the same thing to the oter end to allow the skin tapes for the tip to be blended in. Did the same thing again on the aft edge of the bottom skin to leave room for a tape attaching the bottom skin to the fairing for the slot;

Once the mold was made, I waxed and buffed it three times, and then built a skin in it:

The skin is foam cored with no core at the edges, through where the spar runs, and at any access panels or fuel ports. I made the access covers on the mold, and then taped them to the mold with packing tape, and made the skin right over it, so I KNOW that they fit. My schedule looked like this (Starting at the mold): Peel ply; Perforated ply; Peel ply; Triax; 3/8" PVC foam and cloth tape where there is not foam; Biax; Peel ply; Batting; Bag fim; Apply vacuum.

Biax and Triax work great on simple curved parts, and vacuum bag to very low resin fractions. This stuff is an epoxy sponge otherwise.

Now, I went to this trouble to make wet wings. If I were designing and building a bird like rtfm's, and wanted laminar flow, I think that I would build it using Rutan's methods. This is the lightest way to build a non-fuel carrying laminar flow wing. Yeah, there is all that foam inside, but at 2lb/ft^3, it is lighter than all of the ribs and inner glass layers on the skins and adhesive, etc, that would otherwise be needed. I personally would vacuum bag the spar layups and the skins onto the cores too, and save some more weight... But that is just me.

There are threads already on this forum discussing these very same topics...

Billski
 
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