Self skinning foam/Rubber parts

Has anyone ever used self skinning foam? Or foam rubber to fill a hollow part? What would you use to fill a large void in a prop but keep the weight down? Insulation foam? Wow thats a load of questions, anyway the basic line is I want something light and pourable/sprayable. Thanks!!!

When I made my jet pack I used bird feeder tubes for the fuel cells and filled them with insulation foam. The foam does have a tendency to keep expanding for a almost day or so, though. After I let it sit for a while I trimmed off the ends flat. It slices easily with a knife.

It looked like two big gnarly roots coming out of each end of the tubes

I Just did some test pours this week of a rigid foam, flexible foam, and lightweight resin of the clone belt boxes. If your prop is rigid, one of the rigid foams seems appropriate for the application. SmoothOn has all the materials on their website for you to look at. I'll try to get a pic up tonight of the three pours I did with their weight. I think the spray insulation foam may be harder to fill voids as it expands quicker than the foam you mix.

Originally Posted by Steelblitz

I Just did some test pours this week of a rigid foam, flexible foam, and lightweight resin of the clone belt boxes. If your prop is rigid, one of the rigid foams seems appropriate for the application. SmoothOn has all the materials on their website for you to look at. I'll try to get a pic up tonight of the three pours I did with their weight. I think the spray insulation foam may be harder to fill voids as it expands quicker than the foam you mix.

Very interested to see your results. I'm curious about those particular products and how they work because I'm looking to do the same thing.

Here you go, three of my clone belt boxes fresh from the mold. From left to right is the lightweight resin weighing 250 grams, in the middle is a flex foam at 200 grams, and then to the far right is a rigid foam at 100 grams. Naturally, the resin gives the finest detail, followed by the flex foam. The rigid foams may be tough for an external surface because if the skin is opened up, say via sanding, it reveals the open cell structure inside. Next experiment I may try brushing some resin into the mold, let it harden, then fill it with rigid foam. Overall, I wouldnt hesitate to use wear either the lightweight resin or flex foam boxes. It's just a slight tradeoff for sharper details versus a little less weight.

Thanks for the info!

Thanks Steelblitz that's exactly the sort of info I was after. The parts I am making are similar to the Clone belt boxes. I was considering using gelcoat(sp?) to give a smooth surface on the piece before back filling with foam. I always had the impression that the foams were air reactive and that they had to be sprayed but seeing as they can be poured this can make things a lot easier.
Looks as though the rigid foam would be best for this route. Some questions though -
What brand of foam do you use?
How is the weight to volume ratio before and after expansion? i.e. how much mix makes how much foam?

I think you will need to check the data sheet for whichever product you are going to use, as foams come in a variety of 'weights' and give a wide range of results. If you plan to use the smooth-on products for example, you can find all the data sheets on their webpage. The polyurethane spray foam insulation in the cans have as much as 10 times the volume depending greatly on the ambient air temp. The pourable foams that I have seen from smooth on usually range from 2-5 times original volume. In the end, pick a product then find out the stats for that product, but keep in mind conditions under which you use the product can greatly impact results. Good luck and let us all learn from what you find out.


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I use flex foam on certain props, just remember to check the spec sheet for expansion rates. Its a urethane rubber that expands when mixed with a catalyst. It self-skins, but can sometimes be crazy to work with and you need a back pressure plate to get the foam to pick up the details correctly and not just expand out of the mold.

Thanks for the info, I'm gonna use smooth-on for reference but there are few if any places in the UK that stock it. I'll try some local alternatives and let you know how it goes.

Ditto on what everybody said here. Smoothon has some good info for you to look over on their website. Using a resin such as their rotocast resin and then backfilling with a rigid foam sounds like will give you what you are looking for. Good luck with whatever you try.

Originally Posted by Steelblitz

Here you go, three of my clone belt boxes fresh from the mold. From left to right is the lightweight resin weighing 250 grams, in the middle is a flex foam at 200 grams, and then to the far right is a rigid foam at 100 grams. Naturally, the resin gives the finest detail, followed by the flex foam. The rigid foams may be tough for an external surface because if the skin is opened up, say via sanding, it reveals the open cell structure inside. Next experiment I may try brushing some resin into the mold, let it harden, then fill it with rigid foam. Overall, I wouldnt hesitate to use wear either the lightweight resin or flex foam boxes. It's just a slight tradeoff for sharper details versus a little less weight.

Who makes the flex foam product?

Smooth-On makes Flex Foam. The higher the number, the less it expands. Sounds backwards, but I believe I got that right.

The number refers to the lbs. density. I believe Smooth on does the 3, 5, and 10 lb flex foams. The higher number means higher mass per given volume. The 3 lb foam weighs 3 lbs/cubic foot. The 5 lb foam is 5 lbs/cubic foot... so on and so forth. A cubic foot of water, by comparison is 62.4 lbs. Hope that helps people wrap their brains around the why. self skinning foams I have seen go as high as 25 lbs. Walcomaterials.com (you have to call for prices, they don't list them on the site) also sells them at fairly competitive prices.

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