# Acetal (plastic) piston and/or valves, anyone ever tried it?



## n4zou (Feb 20, 2011)

Acetal (or Delrin) has high natural lubricity and it's easy to machine to high tolerance requirements. I only use compressed air on the "steam" engines I build so heat would never be a problem. It's good for temperatures up to 172 degrees F so I think it would work as a reduced cost material for graphite and brass pistons and valves in Stirling and flame eater engines as well. Enco stocks it. Thoughts? I'm planing on building the rotary valve two cylinder marine engine.

http://www.use-enco.com/CGI/INPDFF?PMPAGE=902&PMITEM=891-6533


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## Metal Butcher (Feb 20, 2011)

I used it successfully on a few air engines. I don't think its suitable for a steam powered engine. It has a tendency to absorb moisture and swell. It won't work on a sterling due to the heat. I tried it and found out the hard way.

Give it a try. Maybe you'll have better luck.

-MB


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## stevehuckss396 (Feb 21, 2011)

n4zou  said:
			
		

> Acetal (or Delrin) I only use compressed air on the "steam" engines I build



I use delrin on air engines. I used it in the Micro V8 (.250 bore) and also in a 5 cylinder rotary (1 inch bore) that was a group build. The rotary runs all day at the shows and never has a problem.


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## fcheslop (Feb 21, 2011)

Ive never used it in a steam engine but Acetal or P.O.M is hygroscopic and requires drying before it can be used in injection moulding also if the temp exceeds 220 deg Celsius it gives of formaldehyde gas so I would imagine the parts will swell 
best wishes frazer


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## Chaffe (Feb 22, 2011)

Acetal is an exceptionally good material to work with, its fantastically easy to machine. The only thing i would suggest, and this may not apply to you personally as you are using only air, it does expand quite a lot with little heat, causing it to bind in the cylinder. I used it once on a dumpvalve for a car, made it a couple of thou clear on the bore and with about 50 degrees C of heat it was siezed solid in the bore.


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## Bluecat (Feb 22, 2011)

A little OT, but how about a plastic called Ertalon 6A? I have a bar of it, it's really cheap compared to brass, it'll withstand the heat by far. Anyone heard of this plastic?


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## Ken I (Feb 23, 2011)

I've used Acetal for a piston ring on an air engine - it works just fine.

But

It tends to "stick - slip" when turning by hand and can be "squeaky" which is really annoying as it sounds and feels like something is wrong when you turn it slowly by hand.

But as I said it runs just fine.

Ken


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## Swede (Apr 19, 2011)

I've often thought that PTFE (teflon) would be an ideal material for steam engine pistons that are not seeing a high load. Virgin teflon is fairly soft and does not machine like acetal, but there are PTFE formulations that are "stiffer" for lack of a better word, that might work. The best part is that PTFE is totally inert, very very slippery, and can handle several hundred degrees without problems.

It might work well in Stirling cycle engines, being very light.


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## Ken I (Apr 19, 2011)

You also get formulations of PTFE with bronze, carbon fibre etc which are much stiffer and stable.

We used to make shockabsorber piston rings out of this stuff - and that's a high cycle / durability application.

I'm not sure how readilly availabe such materials are however.

Ken


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## lordedmond (Apr 19, 2011)

try PEEK very expensive but makes good piston valves for steam , do not make the whole bobbin out of PEEK ( to expensive ) just make the two polo mints for each end and fit them to the bobbin 



Stuart


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## Longboy (Apr 19, 2011)

n4zou  said:
			
		

> Acetal (or Delrin) has high natural lubricity and it's easy to machine to high tolerance requirements........ so I think it would work as a reduced cost material for graphite and brass pistons and valves in Stirling and flame eater engines as well. Thoughts? http://www.use-enco.com/CGI/INPDFF?PMPAGE=902&PMITEM=891-6533


.......Exactly how much money will you save using Delrin over graphite or brass for a model piston and where is the valving in Stirling & flame lickers. ;D


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## Tin Falcon (Apr 22, 2011)

here is a version of the McCabe runner engine I built clear acrylic block with a Teflon (PTFE) Piston and valve button I can blow into this thing and it will run. 




Tin


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## Chaffe (Apr 24, 2011)

Tin Falcon  said:
			
		

> here is a version of the McCabe runner engine I built clear acrylic block with a Teflon (PTFE) Piston and valve button I can blow into this thing and it will run.
> Tin


video of said "Blowing" into this engine and making it run please!


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## Tin Falcon (Apr 24, 2011)

What you do not believe Will see what I can do about a vid clip. 
Tin


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## steamer (Jun 3, 2011)

My Steam Launch has teflon piston rings ( 1.5 x 3.0 x 2.5), runs on 160 psi steam at 600 rpm and has been running fine for some years now.


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