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Jun 25, 2012, 03:30 PM
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spencer6891's Avatar

Molding foam: Thermaforming vs. liquid urethane

I'd like to experiment with producing my own foam fuses and other parts and I'm looking for opinions on how to go about doing this. It seems the most feasible is either to thermaform depron in a plaster mold or fill a mold with 2-part expanding liquid urethane foam, as seen here:

Moulding Foam demonstration (8 min 47 sec)

If anyone has had experience with these methods, please chime in. I'm especially interested in the urethane casting method, as I haven't come across anyone using this material to cast parts. Maybe there's a reason for that. Maybe not.

Of course, it would seem that thermaforming depron would almost always yield a lighter product, unless urethane foam can be found in a lighter weight than 2lb per cubic foot (this seems to be the lightest available). But the tradeoff might be that thermforming would be more of a hassle and the piece would not be as strong.

Note: I'm not interested in molding wings, just 3D sculpted parts of aircraft that are difficult to form with flat foam and are time consuming to reproduce by carving each piece. An example would be the fuse section of the Batwing I just built:
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Jun 25, 2012, 04:17 PM
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vtdiy's Avatar
I'm interested, too, and have thought about it. However, it looks like it would be easily twice as heavy if of the same thickness as a molded EPS fuselage. But also difficult to restrain to the same thickness, so likely to be heavier than even 2x. Constraint increases the density.

You'd need to build a complex core as well as the mold if you want consistency, and also probably a sprue and gate system to get the foam into narrow passageways. My experience with urethane packing foam makes me wonder whether the light weight 3 lb stuff would have as much strength and stiffness as EPS.

All this aside, I'm sure a flyable plane could be made this way. If it hasn't already. But I do wonder if the performance, weight and strength would compete with EPS, from a manufacturing standpoint. Definitely it would have to be filled and painted, or will turn toadstool brown in short order, just like "Great Stuff"

I'd be curious to see any experiments along these lines, for sure.
Jun 25, 2012, 06:08 PM
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spencer6891's Avatar
Originally Posted by vtdiy
Constraint increases the density.
I didn't think about that, but it certainly makes sense. I'd wager the 2/3 lb urethan foam would be stronger than EPS, but EPS would be the lighter choice every time. Of course, for the average Joe Builder, molding EPS is not an option so we must find a reasonable alternative. I wouldn't even consider any sort of large scale production using urethane foam, but for three or four reproductions of certain parts that are difficult to reproduce, it might suffice. And, since it is a closed cell foam, the surface finish would be superior to that of EPS.
Jun 25, 2012, 07:22 PM
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vtdiy's Avatar
Well, I'd distinguish between molded EPS and EPS sheet insulation material, too. I have a feeling the molded EPS might be heavier, but much stronger as well. Mainly due to the constraint, again, surface finish from the molding process. I've bought heavy power tools packed in molded EPS which was extremely tough. I own an EPS Artech Spitfire that has held up quite well considering the abuse I've put it to.

I wouldn't doubt that you could do some short run airplane parts in PU foam with success, though.
Jun 26, 2012, 12:16 AM
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I've actually tried this to some extent, though not for airplanes. I tried making molded kid sized boats that I could skin quickly with light glass. I tried using a small hull that I built and then a top cover to the hull with male parts glued to that to provide the cockpit of the boat(if that makes sense).
The problem I ran into was that the foam would not consistently fill the space and voids even when sealed off to hopefully create a little pressure within the mold. Also it was very inconsistent in keeping a uniform density and wholey unsuitable for the project that I was trying. I do suppose that it is still possible with smaller parts and somehow getting the PU into the mold by a pressure injection system somehow but it was above my paygrade to try anything more complex.
I'm sure there are a few guys who have tried this on here somewhere, and if asked about it , they will probably report similar results. As for making a very rough blank to use for a fuselage or something, that would be filled with fillers and glassed over with a pretty stout layer of glass, it would probably do the job. Good luck with this, I really do hope my experience with this was a mess only due to my inexperience, and I'll be keeping my eye on this one in case someone else has a better solution
Jun 26, 2012, 10:53 AM
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vtdiy's Avatar
Hey mike!

Didn't know you were a boatbuilder, too. Sounds like a tough experience. I do know they make plastic ice chests that way with internally expanded PU foam, but I'm sure bubble size uniformity and strength aren't top priorities.

Sometimes they pack stuff in cardboard boxes with a bag of foam expanded in place to form protection. In fact I just got one of those for packing with some ceramic tiles in a second box. I took a piece of that foam and am not very impressed with it strength-wise. Easy to bend and break and very non-uniform bubbles. I really don't think it is as strong as the beaded foam in the spitfire by a pretty big margin. I think it's okay as backing or filler (like in the ice chests), but you'd need a much heavier formulation to equal molded airplane grade beaded polystyrene.

Hope the fishing and flying are good!
Jun 28, 2012, 03:18 PM
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Here are a few threads of mine thermoforming FFF (or depron) using Harpye's male mold and packing tape technique.
Jun 28, 2012, 05:29 PM
I don't do Normal
Pinky_d_brain's Avatar
My thinking is that it would definitely work don't no how well but you need a male and female mold and only half at a time think waffle maker. see sketch Incorporate ribs and component areas in the mold

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