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cool foam cutters

 
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drcrash
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PostPosted: Wed Nov 01, 2006 4:25 pm    Post subject: cool foam cutters Reply with quote

I don't know if this would be interesting to anybody here, but I'm eventually going to try this for making parts of shapes for armor-like things.

The basic idea is to make shapes in foam using a hot-wire foam cutter, and splice them together. From that you could make a mold and cast a stronger plug, in plaster or fiberglass. (Or you might be able to use a dense enough foam that you could use it as a buck directly. I've heard that you can use a hot-wire cutter on non-foam plastics, too, but I don't know how tricky that is.)

The RC model plane guys use hot wire cutters for making foam wing cores, and they've got some ingenious simple rigs to do it. (Some also have homebuilt computer-controlled foam cutters.)

You basically make a bow to hold a piece of straight nichrome wire taut, power it with a transformer, and you can cut through very thick styrofoam. (Like feet thick, cutting wings the long way.) The hot wire vaporizes the styrofoam before it touches it, and if you move it smoothly at about the right speed, it makes very clean cuts. By using templates glued to both sides of the foam block to guide the wire, you can make some nice curves and tapers. You can even make some compound curves, if they're kinda saddle-shaped, or simple twists of straight or tapered. The profiles don't have to be the same, or at the same angles, on both ends. (For example, it's common to make wings that are twisted a little as well as tapered, so that they meet the air at different angles near the fuselage and toward the tip. The airfoil shapes may change as well, as long as it's okay to just do a straight interpolation between the end shapes.)

One difficulty in doing this is moving the wire at a consistent speed, and about the right speed. If you move it too slowly or too quickly, it vaporizes more or less styrofoam around the wire and you can get ridgy cuts where the speed varies.

This article (with a how-to video) shows how to make a simple rig for cutting tapered curved shapes with each end of the wire moving at the right speed for that end. It just uses a weight pulling on two cables that run through a pair of pulleys each. Seems pretty neat.

http://www.rcgroups.com/forums/showthread.php?t=462759

I was wondering if this might be useful for making the simply-curved parts of trooper armor, like the main tapered part of a shin guard, or big parts of the backpack. If so, you might be able to snarf the cross-sections at the right points from trooper templates, fairly directly, and use them to make the templates for guiding the hot wire cutter. (I don't know though; maybe they bulge compoundly a bit, as well as curve and tapering.) Or it might be useful for props.

I've also seen hot wire lathes and other cutters that use stiff, bent piano wire to make non-straight cutting edges. You can bend a profile in piano wire and make figures of revolution, etc.
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jegner
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PostPosted: Thu Jan 04, 2007 3:11 pm    Post subject: Reply with quote

Very cool.

Sculpting 4lb foam has long been a project I wanted to try, but alas, the cost of that material is so high, I doubt I'll do it any time soon. The hobby store foam is just not dense enough to hold a shape, but the cutter would make it easy to flesh out a rough shape quickly.

Here is the 4lb stuff I'm wanting to try:

http://balsafoam.com/balsafoam/default.asp
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drcrash
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PostPosted: Mon Jan 08, 2007 10:17 am    Post subject: Reply with quote

I would think that 2 lb (per cubic foot) pink (Owens Corning) insulation foam would hold its shape while you're shaping it, but maybe scrunch if you try to vacuum form directly over it.

If so, you could then make an intermediate plaster mold and cast the actual buck in that.

It may not be strong enough to do fine sanding, though; most of the RC modelers who use it seal the pink foam with epoxy and cover it in Bondo. I'm not sure if that's just so that they can vacuum form over it, or because they need something harder for fine sanding. I suspect it's both.
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crashmann
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PostPosted: Mon Jan 08, 2007 4:14 pm    Post subject: Reply with quote

I used the pink foam for carving a Clone trooper knee. It held up fairly well under a shop vac pulling .060 HIPS, but any hard corners were rounded off by the heat of the plastic. Also, some of the foam surface stuck to the plastic. My goal was to do this initial pull, then pour plaster into the plastic shell, creating the production buck.

Before (sorry it's a huge picture)


And after:


Adding some epoxy or bondo overcoat would certainly help prevent the melting and rounding of the corners.

Do you have a link that shows examples on the RC Car forums?

Charlie
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drcrash
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PostPosted: Mon Jan 08, 2007 7:32 pm    Post subject: Reply with quote

Quote:
Do you have a link that shows examples on the RC Car forums?


Here's one doing a canopy for an RC plane:

http://www.rcgroups.com/forums/showthread.php?t=489215&highlight=vacuum+form
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crashmann
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PostPosted: Mon Jan 08, 2007 11:02 pm    Post subject: Reply with quote

Hah-hah, very cool! I see you also threw your 2 cents into the thread Smile

One day I'm going to return to the pink foam and add layers of fiberglass and bondo to make them more durable.

You have really come a long way from your first theoretical discussion posts on the board, to implementing and testing your ideas and sharing the results with everyone here as well as other boards.

Thank you for your contributions and research!

Charlie
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badger
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PostPosted: Tue Jan 09, 2007 1:37 am    Post subject: Pink Foam? Reply with quote

I've been thinking about this Pink Foam stuff a bit recently. It seems like it could be fun to work with.

what is it called, so I can find it at Home Depot?

badger
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crashmann
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PostPosted: Tue Jan 09, 2007 2:45 pm    Post subject: Reply with quote

Owens Corning Foamular Rigid Foam Insulation

http://www.owenscorning.com/around/insulation/products/foamular.asp

Available in two foot widths for ease of transport, or bring a utility knife to cut the boards down to fit in your car.

Charlie
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drcrash
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PostPosted: Wed Jan 10, 2007 6:22 am    Post subject: Reply with quote

From T_om over on rcuniverse.com:

Quote:

"Pink Foam" is made by Owens-Corning, "Blue Foam" is made by Dow.

Both are extruded Styrofoam (polystyrene) panels. Both come in varying densities. The Pink Foam you have available will have a number printed on it "Foamular XXX" where XXX is the compression density of the foam. For example. if it says Foamular 250, then the foam can stand 25.0 PSI compressive pressure.

The weight of the foam is determined by the strength (compressive density). Some Blue Foam is heavier than Pink Foam and vice versa.


So look for the denser foam, if you have a choice, pink or blue.

I have heard that while they're very similar, pink works a little bit better than blue for hot-wire cutting. If you're bondoing and sanding, I wouldn't expect that to matter much.

If you're getting a very good surface from hot-wire cutting or other shaping, I'd go easy on the bondo or whatever, and make a plaster cast to cast the buck, rather than trying to vacuum form directly over the foam plug.

25 psi compressive strength is not much. It's more than a full vacuum, but not more than the pressures exerted by the vacuum when stretching the plastic over protrusions. Vacuum forming is going to scrunch the foam unless you put thick enough Bondo on it that you've got to smooth the bondo, even if you had smooth foam under it. (And remember, don't put bondo directly on foam; seal with epoxy first.)

One way to make a plug would be with a waste mold---i.e., one you mean to break getting it off---made of plain plaster of Paris. You could put an impression coat of Bondo in that, then back it up with a some laid-up fiberglass mat & resin.

On the other hand, for stormtrooper armor parts, you've probably got easy enough shapes to get the plug out of the mold without breaking it. (But if you do break it, it shoudn't be a big deal, because it'd be really easy to make another plaster cast off the foam. Plaster shouldn't crush the foam.)

The nice thing about this is that you don't have to sweat much over mold release for polyester. If it doesn't release, chip it off. Making a new plaster mold is easier than worrying about saving the old one.

Paul
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