Machinable Thermal Insulator Good to > 500F?

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I am making an annealing "oven" for nylon rod out of a piece of 4" stove pipe wrapped in fiberglass insulation. The rods will be suspecded vetically in the pipe, and it will be heated with a heat gun from the bottom. Nylon anneals at ~ 350F, but the termperature will be a bit uneven, so places may get a good bit hotter.

BEFORE EVERYONE GOES OFF ON A TANGENT ABOUT 16 DIFFERENT WAYS TO DO THIS, DON'T BOTHER. THAT'S NOT MY QUESTION. (Sorry, but I'm tired of everyone trying to ignore my actual question in favor of redesigning _everything_. I've had a cold for days, and I'm a bit testy..).

I will support the pipe from the side with a chemical ring stand setup. To minimize the heat lost through the suspension system, I need something that is a good thermal insulator to go between the 1/2" diameter metal support rods and the stove pipe. My default will be threaded ceramic spacers, which I can pick up from Mcmaster Carr, but they are over $4 each. Not fatal by any means, but mildly annoying.

If I knew of a good insulating material that could take the heat, I could machine the spacers myself, but the only plastic I'm aware of that is good to higher temps is teflon, but McMaster says it is JUST good to 500F. It's also not the best material from a mechanical rigidity standpoint. I have some & might try it, but I was wondering if there was some material that I'm overlooking.

Thanks for comments & ideas.

Doug White

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Fairly loosely crumpled balls of aluminum foil, 1" od or so, if that fits. Use three to wedge the pipe into the ring. Conduction won't be nearly as bad as you think, and the price is right.

----- Regards, Carl Ijames "Doug White" wrote in message

I am making an annealing "oven" for nylon rod out of a piece of 4" stove pipe wrapped in fiberglass insulation. The rods will be suspecded vetically in the pipe, and it will be heated with a heat gun from the bottom. Nylon anneals at ~ 350F, but the termperature will be a bit uneven, so places may get a good bit hotter.

BEFORE EVERYONE GOES OFF ON A TANGENT ABOUT 16 DIFFERENT WAYS TO DO THIS, DON'T BOTHER. THAT'S NOT MY QUESTION. (Sorry, but I'm tired of everyone trying to ignore my actual question in favor of redesigning _everything_. I've had a cold for days, and I'm a bit testy..).

I will support the pipe from the side with a chemical ring stand setup. To minimize the heat lost through the suspension system, I need something that is a good thermal insulator to go between the 1/2" diameter metal support rods and the stove pipe. My default will be threaded ceramic spacers, which I can pick up from Mcmaster Carr, but they are over $4 each. Not fatal by any means, but mildly annoying.

If I knew of a good insulating material that could take the heat, I could machine the spacers myself, but the only plastic I'm aware of that is good to higher temps is teflon, but McMaster says it is JUST good to 500F. It's also not the best material from a mechanical rigidity standpoint. I have some & might try it, but I was wondering if there was some material that I'm overlooking.

Thanks for comments & ideas.

Doug White

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I was thinking of something smallish I can easliy run through the fiberglass, which is already tubular, but it might work.

Hmm. I suspect three springs would also provide similar thermal isolation, especially if they were stainless.

Food for thought...

Thanks!

Doug White

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Doug White wrote:

Raw Teflon moldings are sintered at 350 C (Note the C). So, Teflon is almost loafing at 500F. It does get a bit soft above 300 C, but 500 F is only 260 C - nowhere near the breakdown temp. I tried to make a very complex Teflon molded piece some years ago, but was not able to fill the mold uniformly, so the parts were misshapen when they came out of the sintering oven. But, I got some experience with Teflon at high temperatures.

Jon

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I _thought_ Teflon would handle it, but I was a bit surprised to see McMaster only rated it to 500F. They don't exactly say what parameter degrades by what amount to determine that, so it sounds like it would be OK.

Given that I have some on hand, that's probably the place to start.

Thanks!

Doug White

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There is machineable ceramic, but that is probably even more expensive. Can you cast cement into the correct shape? Embed nuts if you need threads at the ends. Mix in shredded fiber glass for extra bending strength if needed. Can you do something with glass tubing? Use something like thinset tile cement to bond nuts in place if needed. You can also make pretty decent thermal insulators from a piece of stainless steel that has a narrow cross-section at some point.

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On 12/7/2012 4:03 PM, anorton wrote:

Wonder if you could make 'em out of Sculpey?

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On 8/12/2012 8:03 AM, anorton wrote:

"Macor"

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    [ ... ]

    Hmm ... while ceramic is an excellent *electrical* insulator, I'm not that sure about its thermal insulation properties.

    Well ... if ceramic is good enough, but you want something you can machine yourself, have you looked at "lava" (Alumina Silicate L911A). You machine it and then fire it, and it turns from a gray machinable material to a pink material which you can't touch with any metal tools. Here is a site about it:

    <http://www.professionalplastics.com/LAVAa>

(And no, it is *not* a plastic. :-) It is fired at 1850 and 2000 F (sorry if the degrees symbol does not come out right on your computer. On mine, it is an underlined superscript '0', but it is what I got with cut-and-paste from the web page. Based on the color, it is used to make the "gas lens" for TIG welding guns.

    No -- I don't know what it costs, but the $4.00 each ones from McMaster Carr may be affordable by comparison -- especially if you don't have the ability to fire it already at hand.

    Enjoy,         DoN.

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wrote:

I did a little digging. Steatite ceramic is the off white stuff typically used in electrical insulators. It has 1/8th the thermal conductivity of stainless steel. However, teflon is a factor of 8 better than Steatite, so that looks like the winner.

Doug White

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as

I think Teflon generates Phosgene when over heated. So not something I would use.

Vermiculite and Perlite mixed with portland cement is one possibility.

Or mixed with Waterglass ( Sodium Silicate ).

Or insulating fire brick.

Or look at the A.P. Green catalog for castable refractories.

Your local hardware store probably has furnace cement.

Dan

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wrote:

High molecular weight Teflon only does that above about ~800F. I shouldn't get anywhere near that hot. Low molecular weight Teflon can give off fumes that kill birds down around 375F. I am a featherless biped, with much larger mass, but I may run this where there is at least a little ventilation to be safe.

Doug White

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Could you suspend the ring with fiberglass thread taken from an auto body patch?

Spark plugs are ceramic insulators and some have threads on both ends.

jsw

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Now that is thinking out of the box! I'll probably stick with Teflon, but that is a very clever idea.

Doug White

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On Fri, 07 Dec 2012 23:31:25 GMT, the renowned Doug White

How about McMaster 89835K74 1/2" Ti tubing with 0.035" wall? $8.77 for 1'

Thermal conductivity of Ti is 22 W*m^-1* K-1

Cross sectional area is (6.35^2-5.461^2)* pi = 33mm^2 = 3.3E-5m^2

Thermal conductivity of the tube is 7.3E-4 W*m*K-1

Say 50mm pieces = 0.05m Say 250C difference

Heat flow is 7.3E-4 * 250 / 0.05 W = 3.65W

Four pieces will conduct about 15W

Not bad, IMHO, only 1% of a 1500W heat gun.

Best regards, Spehro Pefhany

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Doug White wrote:

Does the support ring the pipe? Maybe something like the ring on the top of a clay flower pot?

Or get a couple of fire bricks and cut and form them as needed. The ones I used to line my melting furnace could be cut and shaped easily with common woodworking tools.

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'Twere me, I wouldn't worry about it - I'd just use bolts. Given that you're using a heat gun and not heating elements, _most_ of the heat is going to be blown out the exhaust anyhow. Even if you used elements and sealed it up, I don't think that the loss through the supports is worth worrying about.

Bob

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On 12/8/2012 9:20 AM, Bob Engelhardt wrote:

My main point here was "use bolts", but it should have been "you're wasting most of the heat". With a heat gun, air is blown over heating elements and onto/into the heat-ee. And then continues on its way! 90% (95? 98?) of the heat is just going out the end of the pipe that you're using as an oven. The minuscule amount that conducts down & out via the supporting structure is irrelevant.

Bob

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On 12/7/2012 6:31 PM, Doug White wrote:

Foamglas.

<http://www.foamglas.us/building/products/product_overview/foamglas_slabs/

Kevin Gallimore

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email.me:

Interesting stuff, but absolutely no indication where to buy it...

Doug White

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