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does anyone know anything about different types of plastic? Answered

I am building a small compressed air engine, (3/8" dia pistons). I am looking for a plastic that will be cheap, smooth, and easy to manipulate, but not flimsy at all. It will not have very much stress from torque, tension, or compression. It will have a little pressure inside. I will use this plastic for the cylinders, pistons, and some valves. I am going to buy my plastic from www.mcmastercarr.com I am thinking PETE, UHMW, or PVC. Does anyone have any particular suggestions? feel free to ask for more details.

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Gorfram

Best Answer 10 years ago

I wouldn't go with PVC for even a low-pressure vessel: it can be brittle and throws nasty shards if it fractures. PTFE has all the merits Lemonie mentioned, but may be susceptible to scratches & marring. Maybe a little pricey as well. Polypropylene might be a good bet - tough, smooth, good to around 300 F/150 C, readily machinable, generally inexpensive.

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GorframGorfram

Answer 10 years ago

Don't know of any PETE applications in anything other than thin sheets like Mylar or thin-walled vessels like pop battles. No idea why that is: it never occurred to me to wonder until just now. :)

But UHMW polyethylene might be a good way to go, if prices are comparative. It is likely to have a somewhat lower temperature tolerance than the polypropylene - what's your thermal operating range?

While I'm at it, what are your expected operating pressures?
And what sort of chemical or UV exposures do you anticipate?

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GorframGorfram

Answer 10 years ago

Er, make that "pop bottles." I don't even want to think about what "pop battles" might be. :)

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jtobakoGorfram

Answer 10 years ago

Think 'menthos injection system" ; )

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MechEngineerMikeGorfram

Answer 10 years ago

lolol my thermal operating range is room temperature. And your comment further above about potato cannons only devoloping 20 psi max, has made me sure of my design. Because of the motion of the engine, it shouldnt have to deal with too much pressure, even if i put a lot of pressure in it. And as for chemicals, just some clear household oil lube, and UV exposures wont be significant. Im goin with UHMW!! Thanks!

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MechEngineerMike

10 years ago

updating you guys, i have been working with polyethylene UHMW plastic for my project, and it is Amazing and perfect for this project. It was not too expensive, and it is very machinable and durable. thanks again!

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lemonie

10 years ago

I agree with Kiteman's answer, but for 3/8" I'm wondering whether you want some pre-formed pieces or a block you can machine?
PTFE is thermally-stable and machinable, it's also relatively low friction. I've known diaphragm pump-heads made of the stuff but not piston-heads.

L

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MechEngineerMikelemonie

Answer 10 years ago

wow ptfe must be some good stuff, it is a little pricey. It also only comes in thin sheets and u-shaped channels from mcmaster. ill keep it in mind for future applications tho.

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lemonieMechEngineerMike

Answer 10 years ago

No it's not cheap. Are you planning to assemble this or machine from lumps? L

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MechEngineerMikelemonie

Answer 10 years ago

i have built engine models in the past from bulk steel, and it SUCKed. It took me 3 months, of 5 days a week, 2 hours a day of machining to finish my current masterpiece. For the fantastic contraption that im building now, i will make it easy on myself, one of my goals is to minimize the build time. I will have to do some moderate machining, but not nearly as much as using solid lumps.

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Kiteman

10 years ago

PVC is probably the way to go - it is available in a range of complementary diameters, reasonably rigid, easy to work and cheap enough to make mistakes. Since spud-gunners regularly pressurise it to 100PSI, it will probably hold up to "a little pressure".

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GorframKiteman

Answer 10 years ago

Okay - according to this site -
"Testing of hairspray spud guns revealed they can develop no more than 10-20 PSI chamber pressure. Most are built from schedule 40 PVC pipe. 3" PVC pipe has a burst pressure of 260 PSI.*"

So, yeah, it will probably hold up to "a little" pressure. Assuming no fatigue effects from repetitive pressure cycling over a long period, and no UV damage, it will most likely be just fine.

As for me - "That's okay, I can see everything just fine from over here behind this nice big rock. But you go ahead and stand right next to it if you like."

(*As a Registered Materials/Metallurgical Engineer, I just can't seem to let myself post this without pointing out that I did not perform the above burst pressure calculation, I have no idea whether it is or is not correct, and I do not endorse it one way or the other.)
(You guys understood all that already, but 20 years of training is hard to shake.)

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KitemanGorfram

Answer 10 years ago

(I didn't mean combustion spud-guns, I meant pneumatic.)

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GorframKiteman

Answer 10 years ago

I have to confess that I'm not really up to speed on my tuberous-projectile weapons systems: combustion-, pneumatic-, or beauty-product-based. :) PVC pipe in any application that involves repeated cycles of rapid pressurization and depressurization poses a risk of failure that is fairly low; but the risk that if a failure did occur, its consequences would be catastrophic, is rather high. That is the basis of My Humble Engineering Opinion that this is a bad idea. (But it is an opinion - another equally qualified engineer looking at the same data might disagree.) (And I hope it doesn't sound like I'm picking on you or anything - that's certainly not my intention.)

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MechEngineerMikeGorfram

Answer 10 years ago

yea i have heard that it is strongly suggested that people use ABS pipe instead of PVC. ABS is supposed to have a higher pressure rating that the same size of PVC, and ABS is supposed to have a higher resistance to cheical exposure AND when is fails from pressure it spilts open instead of shattering. (and its more expensive)

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redsuit09

10 years ago

CPVC may be a bit better... they use it for piping in sprnkler systems