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Machined plastic 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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Machined plastic pistons.

Unread postAuthor: Mitchza89 » Fri Jul 17, 2009 7:15 am

Hey guys,

I'm looking to go pro with my cannon building. I'd love to get into machining my own pistons but I have no real idea on what plastic to use. I saw Nylon, HDPE and a few more but I don't know where to look to find the stuff.

What are some recommendations to use?

Thank you,

Mitcho.
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Unread postAuthor: inonickname » Fri Jul 17, 2009 7:35 am

If I had to pick one, it would be HDPE. It has excellent characteristics basically all around.

Though, if you wanted to do an ultra-high performance gun teflon would be excellent due to it's ultra low friction.
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Hell...you could make this piston. I designed it for my soon to be rifle. Teflon or HDPE body, integrated high flow check valve.

Valve in open position, spring not pictured
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Unread postAuthor: Mitchza89 » Fri Jul 17, 2009 7:44 am

That's a great idea. I just have no idea where to get HDPE in Brisbane or how much it is. Tomorrow I may have a fiddle with car body filler. I have heaps of it here. I think it may crack and crumble though... Hmm..
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Unread postAuthor: inonickname » Fri Jul 17, 2009 7:50 am

Mitchza89 wrote:That's a great idea. I just have no idea where to get HDPE in Brisbane or how much it is. Tomorrow I may have a fiddle with car body filler. I have heaps of it here. I think it may crack and crumble though... Hmm..


Bondo, as the Americans would call it. If it's the same type, it's extremely strong (usually a kind of fiberglass resin)

Try a plastic supplier, or one that makes signs or similar.
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Unread postAuthor: Mitchza89 » Fri Jul 17, 2009 7:53 am

Yeah the stuff we have in Aus is like a resin. It's a white paste along with a separate red past. It's got talc powder in it so it can be sanded. I wonder if this is what you guys have..
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Unread postAuthor: MrCrowley » Fri Jul 17, 2009 8:11 am

My hardtech teacher always has teflon rod laying around, up to 40mm. If I ever needed it, i'd ask him to order me some haha. Just check on Google for some websites I guess, maybe even eBay?
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Unread postAuthor: Mitchza89 » Fri Jul 17, 2009 8:13 am

Just as I looked just then, I saw some plastic mobs about 20-30 min from me. I'll be sure to have a good look around. Price is just a bit of a problem for me. An epoxy piston (probably Bondo) might be the best bet for now I think.

Cheers for the tips up Crowley.
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Unread postAuthor: MrCrowley » Fri Jul 17, 2009 8:22 am

What diameter are you looking for anyway?
For diameters about 40mm and up, check out these:
http://www.spudfiles.com/forums/get-you ... 18661.html

Went to a boat-store today and they have solid plastic rods about 4-6" long, so they're not all necessarily in the hour glass shape.
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Unread postAuthor: Mitchza89 » Fri Jul 17, 2009 8:30 am

To fit inside a 1'' Brass nipple. I may duck to BCF (boating-camping-fishing) tomorrow. They may have some thick HDPE or nylon cutting boards for pretty cheap. I could just use the drill press to punch out pistons close to the size I need and just turn them down on my drill press. That may be my best option.
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Unread postAuthor: inonickname » Fri Jul 17, 2009 8:35 am

I am in aus mitch..so ya 'we' have the same stuff :wink: :roll:
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Unread postAuthor: Mitchza89 » Fri Jul 17, 2009 8:38 am

Ohhhh that really helps. Sorry ino, I had no clue :D. I reckon I may go ahead with the car bog idea. My only problem could be it's viscosity and cure time. It's pretty pasty so it's not going to run easily into a mold. I may have to just scrape it in and pray for the best..
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Unread postAuthor: velocity3x » Fri Jul 17, 2009 9:40 am

"DuPont Delrin® acetal polyoxymethylene (POM) resins are highly versatile engineering polymers that bridge the gap between metals and ordinary plastics."

I make pistons using Acetal or Delrin. It has a high natural lubricity and is hard enough to be machineable .....extremely strong stuff!
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Unread postAuthor: inonickname » Fri Jul 17, 2009 9:44 am

Velocity brings up a good point. Delrin (commonly engineering plastic for prototyping) would make an excellent piston. I have long rods of it. It has good properties, and machines excellently.
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Unread postAuthor: velocity3x » Fri Jul 17, 2009 9:54 am

inonickname wrote: Delrin (commonly engineering plastic for prototyping)


It's hard enough that it makes great bearings too. HDPE can't do that!
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Unread postAuthor: Moonbogg » Fri Jul 17, 2009 10:38 am

UHMW all the way. It comes oil impregnated for self lubrication and is specifically designed for impact resistance and bearing usage.
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