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


Re: Machinable Thermal Insulator Good to > 500F?
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  



Re: Machinable Thermal Insulator Good to > 500F?

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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

Re: Machinable Thermal Insulator Good to > 500F?
Doug White wrote:

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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

Re: Machinable Thermal Insulator Good to > 500F?

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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

Re: Machinable Thermal Insulator Good to > 500F?

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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.  


Re: Machinable Thermal Insulator Good to > 500F?
On 12/7/2012 4:03 PM, anorton wrote:
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Wonder if you could make 'em out of Sculpey?

Re: Machinable Thermal Insulator Good to > 500F?
On 8/12/2012 8:03 AM, anorton wrote:
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"Macor"


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Re: Machinable Thermal Insulator Good to > 500F?
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    [ ... ]

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

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    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.

--  
                  Remove oil spill source from e-mail
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Re: Machinable Thermal Insulator Good to > 500F?

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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

Re: Machinable Thermal Insulator Good to > 500F?

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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

Re: Machinable Thermal Insulator Good to > 500F?

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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

Re: Machinable Thermal Insulator Good to > 500F?
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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  



Re: Machinable Thermal Insulator Good to > 500F?
email.me:

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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

Re: Machinable Thermal Insulator Good to > 500F?
On Fri, 07 Dec 2012 23:31:25 GMT, the renowned Doug White

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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
--  
"it's the network..."                          "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
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Re: Machinable Thermal Insulator Good to > 500F?
Doug White wrote:
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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.

--  
Steve W.

Re: Machinable Thermal Insulator Good to > 500F?
'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

Re: Machinable Thermal Insulator Good to > 500F?
So skip the washer and use SS tubing for the rod and wrap the first couple  
of inches with insulation before it reaches the clamp.  Slit and fold the  
end over to make a vertical tab for the screw to go through with a nut on  
the backside, and put a cork in the cold end to limit internal convection  
losses.  I guess one thing we don't know is if this is a one-use lash up or  
something to be used for a long time that needs to be efficient and sturdy.  
I'm assuming few uses, or else you'd wrap the tube with heating tape under  
the insulation for a more uniform temp distribution, but you said we weren't  
allowed to start redesigning that part :-) :-).

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


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and

Rather than making a completely insulating threaded Teflon spacer, I may
start with a 6-32 SS screw & a Teflon washer.  I don't actually have true
ring stand rings at this point, just rods & clamps.  I was planning on
just running a 1/2" rod out from a clamp, with a screw in the end through
the stove pipe.  The heat flow would be from the pipe to the end of the
rod.

Doug White  



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