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Old 05-22-2010, 06:28 PM   #1 (permalink)
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Foam core board vs. Coroplast

Coroplast is good stuff, tough and durable, but essentially two-dimensional.

Foam core board is very light and stiff, so is used for hand made RC airplanes, etc., but off the shelf is also two-dimensional.

However, some say that by removing the paper from one side, it is much moe flexible and can be formed around gentle compound curves, perhaps like the curvature of car hoods, fenders, etc..

So, anybody here used foam core board?

Workability, formability, etc.?

What techniques, adhesives, and paints work, which do not?

Advice?

Also, Dow BlluCor is polyethelene foam, can be hot-wired to sculpt, light, and popular among RC aircraft hobbyists. Any thoughts on use of that for our purposes?

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Old 05-22-2010, 11:21 PM   #2 (permalink)
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Coroplast can be bent. You can score it with a razor blade and make bends. But there really is no easy way to make round features.

I've been carving pink polystyrene insulation foam to make single-use molds for fiberglass + epoxy resin. Polyester resin eats styrofoam. Fiberglass works great for me. It should be possible to make professional-looking parts with it, but I've mostly stopped short of doing that. You can get a smooth finish and paint it with automotive paint, but mine is somewhat wavy.

I haven't used foam core board, but I'm sure it would be easy to lay up a sheet of fiberglass over it to make a more durable part that holds its shape well.
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Old 05-22-2010, 11:34 PM   #3 (permalink)
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The foam core I've used is still limited to 2D curves, because one face is still paper. Re-gluing the other face after bending does sound like a good way to get a smooth curve. However, it would still need epoxy encapsulation for rainy days. For 3-D forming, Coroplast can be vacuum-formed, but it takes some technique along with the usual equipment. For a gentle shape like an overlapping wheel skirt, you can persuade Coroplast with a hot air gun.
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Old 05-22-2010, 11:45 PM   #4 (permalink)
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Early airplanes used cloth over frames for wings.
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Old 05-23-2010, 02:08 PM   #5 (permalink)
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Quote:
Originally Posted by RobertSmalls View Post
Coroplast can be bent. You can score it with a razor blade and make bends. But there really is no easy way to make round features.

I've been carving pink polystyrene insulation foam to make single-use molds for fiberglass + epoxy resin. Polyester resin eats styrofoam. Fiberglass works great for me. It should be possible to make professional-looking parts with it, but I've mostly stopped short of doing that. You can get a smooth finish and paint it with automotive paint, but mine is somewhat wavy.

I haven't used foam core board, but I'm sure it would be easy to lay up a sheet of fiberglass over it to make a more durable part that holds its shape well.
An idea that may be useful to you comes from Alex Strojnik, designer of the S-2 homebuilt sailplane.

Wanting to avoid all that sanding and filling of wing skins normally needed to make them smooth enough for laminar flow, Strojnik did fiberglass layups on flat sheets of new plexiglass on a table, so the smoothness of the plexi was imparted on the resin.

He waited until the resin cured to rubbery consistency, but not yet fully hardened, then peeled the skin section off the plexi and draped it over his wing panel, leading edge up, and used vacuum to suck and hold the still-curing but rubbery layup against the wing foam. It fully hardened in that shape, yet was still as smooth as the plexiglass upon which it had been laid at the outset. Sanding and filling was limited to narrow strips at the joints of skin panels, eliminating ~95% of that tedious chore. And, since sanding may mean sanding through the fibers which are the strength of the panel, his panels had no risk of compromised strength, since the fiberglass was not touched.

So, I'm wondering if Strojnik's partial-cure trick might work on compound 3 dimensional curves in addition to simple 2 dimensional curves like his wings.
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Old 05-23-2010, 03:38 PM   #6 (permalink)
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Fiberglass cloth will print through and do all sorts of irregularities if messed with when the resin is green enough to accomodate a 3-D shape. You can use stretched membranes to improve the finish on male shapes, but I've not heard of a vacuum bag being used to give a molded part two good surfaces.

For putting a nice skin on foam, here's a new spray-on coating:

Industrial Polymers Corporation-StyroSpray

I've also had good success producing mold bucks (one-use master shapes) from foam covered with easy-sanding wall filler and then painted. I had a mix of latex paint, spackle, and microbaloons that sanded as easy as regular 2-lb insulation foam, but could get a hard, shiny surface by being coated with either PVA or Shellac and polished as usual.
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Old 05-24-2010, 11:22 PM   #7 (permalink)
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Quote:
Originally Posted by Bicycle Bob View Post
Fiberglass cloth will print through and do all sorts of irregularities if messed with when the resin is green enough to accomodate a 3-D shape. You can use stretched membranes to improve the finish on male shapes, but I've not heard of a vacuum bag being used to give a molded part two good surfaces.

For putting a nice skin on foam, here's a new spray-on coating:

Industrial Polymers Corporation-StyroSpray

I've also had good success producing mold bucks (one-use master shapes) from foam covered with easy-sanding wall filler and then painted. I had a mix of latex paint, spackle, and microbaloons that sanded as easy as regular 2-lb insulation foam, but could get a hard, shiny surface by being coated with either PVA or Shellac and polished as usual.
Thanks Bob, StyroSpray or similar is so far my first choice in making a hard, smooth male mold for my proposed Porsche nose. I'd use an armature with hotwire cutting tools for the foam cutting, followed by StyroSpray, then sanding using the same armature, replacing the hotwire cutter fist with a sprayer and/or squeege to apply the StyroSpray, then with a random orbital sander. Or, instead of an armature, fix the cutting/sanding tools on a table, and rotate the foam against it by using a simple pivot.

I already have the extruded blue Dow styrofoam, which then entails something like StyroSpray as a chemical barrier to keep the inexpensive polyester resin from eating* the foam, or else have to go with 3 X as expen$ive epoxy. With StyroSpray, could use cheap polyester resin.

Otoh, if I went with polyurethane foam, I could not hotwire it (toxic fumes) so sanding/shaping would be messier, but then could apply cheap polyester resin and fiberglass without fear of chemically eating the foam. And, I'd have a nice impact absorbent foam pillow in the nose, easily carved for cooling ducts, etc..

Pros and cons of each method. Another post about the Lithuanian guy says go with polyurethane from the getgo.

Got suggestions or advice?

*Cheap polyester resin is about as strong as epoxy when cured at room temperature, but eats styrofoam and is OK with polyurethane foam. Epoxy won't eat styrofoam, but costs 3 x as much as polyester resin. This is a non-structural part, so strength is not a big deal.
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Old 05-25-2010, 12:50 AM   #8 (permalink)
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BTW, webmasters, Styrospray is something I found by clicking an ad on this site, the first time I've responded to a web advert in years. This site is tops in quality flack, too!

Another option for styrofoam and polyester is to use a barrier coat of SB-112 epoxy from System Three. It won't melt the foam, yet the esters stick to it. It is also very handy for any bodywork. It will seal in rust pits, and take bondo on top.

I'd say that Vinylester is roughly the equivalent of decent, lightly baked epoxy. Generally, though, the epoxy is tougher, which is often what matters most. That works against it if you are doing any shaping, sanding, or polishing, though. And when refining a shape, it is very handy to be able to mix up a bit of quick-setting ester. It even works a bit better for green trimming, which is a great time saver. And, of course, you can mix in Bondo (lightweight, for ecomodders :-)

If you have a CAD setup, you might be able to print out shapes for templates that a hot wire can follow, complete with station numbers for synchronization of each end of the wire. Given the accuracy of hot-wire, etc, you'll have some filling to do. It is a great advantage to be sanding only one hardness composing a surface.
For a quick build, do the 1st coat of filler with a notched trowel. That gives much less sanding to establish the contour, and the grooves are then easy to fill. Random Orbit sanders are nice, but for this stuff, longboards are your friend to create fair surfaces. Just glue coarse sandpaper on to strips of 1/4" (6mm) plywood or something similar.
Have a look at Fartstone glass foam blocks, too. The wear into any contour, cut fast, and finish smooth. They get their name from the hydrogen sulphide used to foam the glass, but you have nose protection anyway, na?


Last edited by Bicycle Bob; 05-25-2010 at 01:03 AM.. Reason: more
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