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Foam pistons

Post questions and info about pneumatic (compressed gas) powered cannons here. This includes discussion about valves, pipe types, compressors, alternate gas setups, and anything else relevant.
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Foam pistons

Unread postAuthor: flamerz14 » Fri Sep 28, 2007 3:49 am

I wonder if its possible to make a piston out of Bluefoam? Its almost like normal foam but denser, harder and less flexible for the same weight compared to ordinary polystyrene..
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well, this isn't much isn't it?

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Unread postAuthor: psycix » Fri Sep 28, 2007 5:45 am

Only one way to find out:

Try it :)
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Unread postAuthor: jackssmirkingrevenge » Fri Sep 28, 2007 5:55 am

great for carving lightweight indoor r/c planes but I doubt it would withstand the rigours of being thrown about by compressed air.
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Unread postAuthor: williamfeldmann » Fri Sep 28, 2007 8:00 am

I would be somewhat suspicious of it's ability to take impact or pressures on its total surface area. However, with washers on the ends and a bolt running through it, the foam would make an excellent pilot space reducer. It would be incredibly light weight and still somewhat rigid.

That stuff would be easier to get to the desired shape than Great Stuff, and less messy, and cheaper too, especially if you can find scraps laying around from broken sheets that you can get for free.
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Unread postAuthor: Fnord » Fri Sep 28, 2007 8:17 am

Keep in mind what happens to styrofoam cups when they're taken to the bottom of the ocean. The same thing would likely happen to a foam piston inside of an air chamber.
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Unread postAuthor: silverdooty » Fri Sep 28, 2007 8:41 am

the 2" foam works pretty well in sub 100 psi pressures if 2 or more layers are used. 2 worked ok, 3 work better to stabilize it. i still needed to glue a rubber seal on the face. it did make my 2" tee valve longer than i wanted and was unsightly. i've got 75+ shots on it and it is still working. if you use it make sure you have a lubricant that does not eat the foam. i used bacon fat. i also have a small washer on the rear to prevent air injection into the piston.
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Unread postAuthor: Fnord » Fri Sep 28, 2007 9:00 am

Well, I guess it works anyway^
You can always put some foam in a 2L bottle and pressurize it to see the actually effects.

i used bacon fat.

eeww.
Don't leave it in the sun! :)

"Spudfiles: Lubricating guns with fluid from dead pigs since 2004!"
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Unread postAuthor: williamfeldmann » Fri Sep 28, 2007 9:22 am

Let me hear a WOOT for pork!

Back on topic, so did your piston have anything else stabilizing it or is it just foam and a sealing face? I would think it would need some sort of support otherwise the repeated impact would crush it a bit even if the pressure did not.
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Unread postAuthor: silverdooty » Fri Sep 28, 2007 9:25 am

my valve is not VEGAN and never will be :wink:
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Unread postAuthor: silverdooty » Fri Sep 28, 2007 10:07 am

valve is a 2" tee with 1 1/2" port.

piston is:
3 layers rubber cemented of 2" styrofoam t&g compressed to 4 1/2". then with a hot air gun i 'melted' the circumference, just enough to give it a little hardness. then glued on washer on the back and innertube sealing face with rubber cement. no through bolts.

it probably will fail at some point in time.
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Unread postAuthor: MrCrowley » Fri Sep 28, 2007 11:48 am

I've used it as a bumper, but not as a piston.
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Unread postAuthor: williamfeldmann » Fri Sep 28, 2007 1:46 pm

What kind of pressures do you guys with these pistons run at? Would piston made of only foam hold up above 100?

@silverdooty
When you say you used a heat gun to melt the edges you are talking about around the cylinder, right? Did you have to smooth the sides out after that to make it slide without binding? Everytime I have done anything with foam and heat it melts and gets knotty and hard and would seem like it would hang up in the tee.
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Unread postAuthor: silverdooty » Fri Sep 28, 2007 3:01 pm

yes, around the cylinder.

the first one i got way too hot and it ended up a mess. the second one i rotated it quicker while heating it and it turned out better. by the 5th one i had something that worked,slid in and out of the tee nicely with some grease.

i'm very arthritic and it is very hard for me to accomplish anything with finesse. it might take a younger, healthier fellow less tries.

i don't believe they would hold up to any real psi over 100. and i really don't believe they would work in anything other than 1 1/2" or 2". you could possibly melt the entire sheet into a piston like hot glue, whether that would be cheaper i don't know.

or it might work in a coaxial with just one layer.
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Unread postAuthor: veginator » Fri Sep 28, 2007 4:00 pm

This gave me an idea :idea:. Maybe you could make one out of spray foam.
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Unread postAuthor: Cappy333 » Sun Sep 30, 2007 2:16 am

Extruded polystyrene (insulation sheets) does not have enough compressive strength. This density styrofoam will crush at 20 psi.

http://www.dow.com/PublishedLiterature/dh_0053/0901b80380053c3d.pdf?filepath=styrofoam/pdfs/noreg/179-08040.pdf&fromPage=GetDoc

For the piston you need to use a much denser, stronger foam; or solid plastic or metal.

The 16 lb (0.26 g/cc) polyurethane foam on this page should work. Still, I would be worried about friction causing the surface to melt or abrade (polyurethane is friable).

http://www.uscomposites.com/foam.html[/url]
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