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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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Unread postAuthor: flamerz14 » Sun Sep 30, 2007 3:39 am

OKay...but remember this is bluefoam and it is WAY stronger and harder than ordinary foam cups..
@cappy333: PVC rubbing on bluefoam (or vice versa) does not have much effect so long as there is no other partcles between.Yes, probably it will melt slightly, but over long usage.

Bluefoam can withstand much higher pressures as its pieces are closer together, making it more rigid and less flexible,thus giving it more tensile strength.I'm planning to use about 5cm thick bluefoam for the piston.
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Unread postAuthor: battlemonkey » Sun Sep 30, 2007 1:29 pm

try using builders bog and pour it into a piece of pvc to give u the shape u wont. its hard and easy to work with.
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Unread postAuthor: silverdooty » Mon Oct 01, 2007 12:30 pm

flamerz14 wrote:OKay...but remember this is bluefoam and it is WAY stronger and harder than ordinary foam cups..
@cappy333: PVC rubbing on bluefoam (or vice versa) does not have much effect so long as there is no other partcles between.Yes, probably it will melt slightly, but over long usage.

Bluefoam can withstand much higher pressures as its pieces are closer together, making it more rigid and less flexible,thus giving it more tensile strength.I'm planning to use about 5cm thick bluefoam for the piston.


while cappy linked to 20 psi pipe insulation, most bluefoam doesn't exceed 25 psi. styrofoam highload 40 is 40 psi, styrofoam highload 60 is 60 psi.

i had to modify the t&g foam i used. i melted the exterior. i then used bacon fat to prevent more friction melting. given enough pressure it will crush just like a foam cup.
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Unread postAuthor: Gepard » Mon Oct 01, 2007 1:07 pm

You can't compared the polystyrene cups and the "blue foam", they're both polystyrene but different manufacturing processes.

Technically you shouldn't call the cups Styrofoam as thats a Dow Registered Trademark for the blue (and pink) polystyrene boards.

And the Styrofoam boards won't compress anywhere near as easily or as much as the cups since they are much more dense.
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Unread postAuthor: psycix » Mon Oct 01, 2007 3:52 pm

Maybe try combinations of different foams, a light foam in the core and a sturdy and solid foam around it
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Unread postAuthor: Gepard » Mon Oct 01, 2007 3:55 pm

The best way would be too make a polystyrene one then just add a couple of layers of fibreglass (using Epoxy resin as polyester melts polystyrene). Should make them strong and light in fact that's how they make nose cones for rockets for exactly that reason, and it shouldn't cost a alot at all.
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Unread postAuthor: jackssmirkingrevenge » Tue Oct 02, 2007 7:05 am

Gepard wrote:The best way would be too make a polystyrene one then just add a couple of layers of fibreglass (using Epoxy resin as polyester melts polystyrene). Should make them strong and light in fact that's how they make nose cones for rockets for exactly that reason, and it shouldn't cost a alot at all.


That's pretty much how I make my own pistons with epoxy casting around a foam core.
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Unread postAuthor: williamfeldmann » Tue Oct 02, 2007 8:45 am

But if you were coating the entire piston in epoxy or anything other substance, including fiberglass, why would the foam's density matter?

Ideally, the outer coating would be resistant to pressures, especially when dealing with epoxy. The foam is basically only there for shape during the casting process and to reduce overall weight from that of a solid piston.

So why should I buy expensive, dense blue foam when all I really want is shape and I can get that with dixie cups and have a hollow center which is even lighter, or styrofoam cups which are also lighter and could be used to give shape as well?
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Unread postAuthor: jackssmirkingrevenge » Tue Oct 02, 2007 9:25 am

Blue foam a remarkably verastile material, here's a 1/72 scale rubber powered Messerschmitt Me 410 I had started making,a mere 9 inches span and only weighs a few grams (the fuselage and engine nacelles are hollowed out). I never got down to finishing it though as it was around the time when my interest in making holes in stuff took over everything else :roll:
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Unread postAuthor: spudgunnerwryyyyy » Tue Oct 02, 2007 1:54 pm

Do what jackssmirkingrevenge did and cast epoxy around the foam. Creating a light weight piston that fits perfectly and is durable from the epoxy.
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Unread postAuthor: flamerz14 » Wed Oct 03, 2007 1:38 am

yes the foam is the kind jackssmirkingrevenge used for his "plane". And i don't have to buy it cos my school's using it and we have a whole box of scrap bits in class.So technically, its FREE :wink: Anyways, the density does matter to its ability to withstand pressure. The epoxy used for jsr's how-to; Is there another way? I cant get epoxy resin from the school.. :roll:
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Unread postAuthor: jackssmirkingrevenge » Wed Oct 03, 2007 2:06 am

I suppose you could substitue hot glue but I doubt it will be very durable. Do they use fibreglass at your school, perhaps you can obtain some of that resin?
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Unread postAuthor: Novacastrian » Wed Oct 03, 2007 2:28 am

Hey JSR that is one sweet plane, you never fail to amaze :D

Why foam and not epoxy?
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Unread postAuthor: jackssmirkingrevenge » Wed Oct 03, 2007 2:29 am

one word - weight.

Indoor model planes need to be extremely lightweight in order to fly well.
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Unread postAuthor: flamerz14 » Wed Oct 03, 2007 2:33 am

By theway, JSR: what did you use to machine the parts of the plane?
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