Need a non-conductor that is stable at high temp

[kingnero]

I have got a small automatic MIG welding machine that is to be used in the field with a stick-out of 30 mm.

Problem is, from the moment everything is powered on (using a mobile generator), there is a certain arc voltage. The manufacturer gave a very useful gauge which can be used for several things, however as it is made out of Aluminum you cannot easily check the stick-out as it sparks heavily the moment you touch the torch (and the workpiece of course).

I want to have something small, handy, non-breakable (or relatively sturdy), and not electrically conducting that can handle temperatures of about 500 °C (930 Fahrenheit). Preferably easy to machine, and low-cost or good availibility.

I was thinking about bakelite, however I do not have any experience in plastics. Maybe there are better alternatives?

Other things that have crossed my mind:
Rock: ideal however cannot be shaped to form
Ceramics: too brittle.
Wood: probably my best bet so far. No problem if it won’t last long as it’s cheap and easy to acquire. Tolerance = +/- 1 mm, so when charred => bin and replace.



Thanks in advance for your input...

 

[Ron]

My thought was a ceramic, but you indicate that it's too brittle for your application. There are composite ceramics that are not so brittle.

I think your thermal range is too high for generally available thermal resistant plastics.

If you can use wood, then you might consider getting refractory brick and slicing it into layers for disposable use. Yes, it is brittle, but could be used a few times before throwing away.

 

[moltenmetal]

Wood becomes conductive when charred...

If wood had a hope, Teflon probably will too. It WILL NOT withstand continuous exposure to 500 C, but intermittent exposure it may take. It's a poor conductor of both heat AND electricity, and doesn't char.

Barring that, some kind of ceramic fibre millboard or even cellulose fibre-reinforced concrete (i.e. Hardiboard) would do the trick. Readily machinable? Not really, but you can bludgeon either of these materials into reasonably decent shapes if you don't mind dulling your tooling or using abrasives and taking care to manage the dust.

 

[kingnero]

Thanks for the ideas so far, I'll check for the tips!
Hadn't really thought about brick or concrete, but if it works fine, I have no problem using that.


Moltenmetal: It does not need to be exposed for long time, only intermittantly and for a short time, indeed.

I am not a enthousiast of the brick idea, as the dust may get on the base material. Wood may have the same problem. No compressor on site, only a hammer and a wire brush.

I will see if I can get some teflon to test.
Will bakelite hold against these temperatures?

 

[IRstuff]

I think you need to revisit ceramic. WHY is it too brittle? How big is your object? There are literally dozens of ceramics that can withstand your temperature range.

Bakelite is probably not going to survive your temperature requirement, and Bakelite is probably one of the more brittle plastics that you could have possibly chosen.

TTFN

FAQ731-376

 

[ScottyUK]

Some of the alumina-based ceramics are pretty tough. So are the beryllium-based types although there are 'issues' with beryllia that you might want to avoid.


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If we learn from our mistakes I'm getting a great education!

 

[Ceramicguy]

If you want a tough ceramic, go with partially stabilized zirconia. It has a mechanical toughness similar to tungsten carbide. Frankly, I think it's overkill for your application. 500 C is cold for most ceramics. If you think you need ceramic, then try contacting someone like Red Seal Electric in OH. They have a whole stable of refractory materials to choose from. It sounds like you need the piece to set up your torch so the contact will be short duration and minimum contact pressure. therefore, thermal transfer should be minimal. Teflon will probably do well. I believe the continuous use temp is in the neighborhood of 260 C with an intermittent use of 400 C. The benefits are ductility and easy machinability. There are other high temperature plastics to consider too. Kapton comes to mind.

Please stay away from the beryllia ceramic option. A piece of that material rattling around in a toolbox will generate respirable dust and if inhaled can result in a serious lung condition in susceptible people.

Bruce

http://www.accuratus.com

 

[Pud]

Warning! Teflon (PTFE) degradation compounds contain fluorine and other nasties like HF - not nice if you like your bones or corneas...

If you need to try polymers, try PEEK (with glass reinforcement)

H

http://www.tynevalleyplastics.co.uk

 

[Ceramicguy]

I agree with PUD. PEEK is a good choice. It competes effectively against my ceramics at lower temps. (Wish I *had remembered the acronym in my original post)

Bruce

http://www.accuratus.com

 

[Ron]

Don't use concrete! When exposed to high temperatures, the interstitial moisture will expand and "blow up". Not good!

 

[MintJulep]

Glass.

 

[berkshire]

kingnero
An old tig torch ceramic cup may be what you need.
B.E.

 

[MikeHalloran]

For a simple U-shaped depth gage sort of thing, say 6mm thick, I think Bakelite is a pretty decent choice.



Mike Halloran
Pembroke Pines, FL, USA

 

[SnTMan]

A phenolic laminate will work if wood will work.

Regards,

Mike