At 01:56 AM 6/6/2013, you wrote:
>Hi all,
>
>I'm making a small melting tube that can quickly melt wax and pour on
>demand. It's simply 3" long piece of 3/4" copper pipe with uninsulated
>NiCr wire to be wrapped around it. What's the best way to wrap the NiCr
>around the tube without shorting it? One option is high temperature
>manifold paint, but I'd rather find a more thermally conductive (yet
>electrically insulated) solution.

Commercial heaters generally use ceramic (eg. ceramic powder in a
swaged construction) if they are high watt-density, or mica if they
are inexpensive low watt-density types. Some very high watt density
heaters (eg. for nozzles) use a small-diameter swaged MgO insulated
Inconel-sheathed heater wound into a helix. Not inexpensive, but
can be long life (since the thermal contact with the wire is superb)
assuming the termination doesn't fail.

I'm not optimistic about the chances of plastic working in this
application- the thin NiCr wire will get very hot.. For a homebrew type, a
fiberglass sleeve may work okay (but it will be slow because the watt
density and thermal conductivity will be low). Cheap immersion heaters
sometimes use this construction. Did I mention it's cheap?

Any reason you don't want to just buy a suitable heater? There are
dozens of companies out there.

For world-class 'quickly', how about a thin-wall stainless steel tube with =
an
insulated copper coil wrapped around it?  (Just add XX kHz AC power
to the coil). Maybe dig inside an inexpensive PC supply for the isolated
low-voltage AC coming out from the xmfr for proof of concept.

--sp =20

--=20
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