SpudFiles

Ok, so about every 150 shots with the decimater, I'm havingto rebuild the piston because the forces subjected to a 1.5" piston at 400-700psi eventually cause the rubber sealing face to fail orthe bumper disentegrates, or the flat washers turn into cones, etc... Has anyone used manufactured urethane rod for pistons(JSR's epoxy pistons aside)???

the rubber I use is about an A65 on the shore scale, but I'm thinking that a solid piston of urethane rod with an A90 hardness would be soft enough for a good seal at the barrel, but durable enough to not self destruct as often... A 36"x1.5" rod would be $48usd shipped, but if it'll cut down on rebuild frequency it'd be worth it...

so about every 150 shots with the decimater...

I don't think I own a launcher that's been fired 150 times.

Between plinking with the minimod and the cane, and blasting with the decimater, I go through about 60lbs of co2 a month...

I've never worked with it but it sounds like it's worth a try. You would still use a bumper with this correct?

What about HDPE or nylon. HDPE is incredibly tough, very impact resistant and machineable (and cheap!)

Ideally you don't want something rubbery like that because it will increase piston bounce.

Nylon would probly split or splinter wouldn't it?

Nylon is great but expensive. I guess it would prove hard to make the piston sealless, as it woudl smush. Otherwise teflon could be used. How about a replacabl tip with screws that can be replaces after every 200 or so shots and is liek 1/4th of an inch thick so that if it wears of there is enough to back it up.

I have some nylon, and I would be happy to supply you with some.

A bumper will prolly not be needed, and piston bounce with the decimater is a non issue because when piloted the piston seals to the pilot port and lets no chamber pressure escape on the pilot side...

use light stuff for the piston like what they told you above.
and the seat where the piston seals , make it 2-3 mm thick (the walls of the seats) ,this will prevent the rubber from cutting .

example

far_cry wrote:use light stuff for the piston like what they told you above.
and the seat where the piston seals , make it 2-3 mm thick (the walls of the seats) ,this will prevent the rubber from cutting .

example

I was actually considering brazing a piece of 1"to1.25" bushing onto the sealing end of the barrel so that it would have a larger seat(to prevent cutting), but at the same time, a one piece urethane piston would be nice for simplicity's sake... I will prolly do the barrel mod fo now and see how well the face holds up before ordering urethane...

Thanx for the input guys...

Nylon, Delrin, LD/HDPE or UHMW are all pretty good materials to use, but for the application you can't beat high impact polystyrene. It's a little more pricey but should stick around a little longer. I haven't looked at the individual material densities, but I believe HIPS may be the lightest. It looks like McMaster sells it by the foot as a sub-category of plane on polystyrene, but I'm not sure what Dia. you'd need so can't give you a price atm.

I've never used polyurethane. The stuff is tough as nails and used on skateboard and inline skate wheels. I would have tried it in my cannon, but I went with HDPE because it was easier to find, could be laminated into the size I needed from my local supply, and machined well.

If you use it, let us know how well it works. I think it is a little more dense than HDPE. For our competition, we went with HDPE because it has low density and floats in water.
http://en.wikipedia.org/wiki/High-density_polyethylene

This gem is a good summary;

LDPE is defined by a density range of 0.910–0.940 g/cm3. LDPE has a high degree of short and long chain branching, which means that the chains do not pack into the crystal structure as well. It has, therefore, less strong intermolecular forces as the instantaneous-dipole induced-dipole attraction is less. This results in a lower tensile strength and increased ductility. LDPE is created by free radical polymerization. The high degree of branching with long chains gives molten LDPE unique and desirable flow properties. LDPE is used for both rigid containers and plastic film applications such as plastic bags and film wrap.

Quote from this page;
http://en.wikipedia.org/wiki/Polyethylene

This means, it is softer and more flexible. Not great properties of a piston. It may make a good valve seat.

Machine a piston that seals inside the porting with an o-ring instead of against the pipe.

Can you buy kevlar rods? if you could it would be very strong