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Electrically-insulating material selection

B

Borealis

Aluminum
Joined
Jul 22, 2021
Any suggestions for a material that is:

--Machinable with conventional carbide/hss tooling
--Electrically insulating
--Has a coefficient of thermal expansion ideally below 25ppm/degF (~45ppm/degC)

...and then it'd be nice if it...

--costs less than $5/cubic inch in quantities of ~20 cubic inches.


This is a low-voltage device, less than 10V across the part. Macor would work but it's overkill.
It'd be good to discover something less expensive for this application.
 
How thin can it be? for 10 volts I would think any plastic,dry wood or marble would work. wiring is pvc, nylon, teflon. I bet pex, polyethelene, rubber hose would all work. Also superglue seems to insulate.
I will be following this thread because I need to make a new bushing for my self igniting propane torch. The spark wire comes through a metal handle casting. That bushing is a hard black plastic that cracked before. When it cracked the spark just shorted out, well below the gas flow. I took it out and super glued it back. It worked fine for a few years until I kicked the torch over and cracked the plastic. Lost a piece so I need to make a complete bushing this time. Think I will try something more flexible like nylon tubing.
Bill D
 
I think the first elctro magnet used wire insulated with strips of silk ripped from his wife's wedding dress. Later they would dip the wire in shellac.
 
The part is rectangular and needs to be about 0.15" thick. The need for a low coefficient of thermal expansion rules out many materials that would otherwise work fine. Marble is an interesting idea and has a CTE in the right range...wonder how hard it'd be to cut some 4-40 threads in it.
 
Some variants of polyimide will have thermal expansion that low, but you'll have to read spec sheets.

Many species of wood also meet the spec, but you probably want something homogenous and isotropic, like MDF. You can laminate 1" layers of MDF, or get it in thicker sheets (harder to source).

[Added in edit] Oh, now I see you want it thin, not thick.
 
Most all of the usual suspects in the plastics world will do the job if you get glass fiber reinforced versions of the polymer, it really brings the CTE down (into the 30s (x10e-6/°C) for the most part, down to the 20s for a few of them).
 
CalG said:
Micarta doesn't fit the bill? or G10?
Click to expand...

The CTE on phenolic-type materials tends to be closer to 100ppm/degC. I'm not sure about G10...do they make that in thicker sections? The only stuff I have in the shop is the typical 0.06" circuit board stuff...but it's glass-filled, so maybe the CTE is good.
 
Conrad, thanks for bringing my attention to Aremco. Never heard of them before but bookmarked it immediately after seeing all the unusual materials they offer.

Magnetic, great idea worth looking into... Didn't know soapstone & talc are the same thing. Looks like it's easy to find online and the prices are reasonable.

Boxcar, thanks for the suggestion to look for glass-filled versions of the plastics.
 
I do not have a specific material to recommend, but I would urge you to be sure whatever you pick does have it's electrical resistance/conductance specified. A well respected company that built professional audio equipment ordered some plastic panels to use as terminal boards. They assumed the plastic they specified would be an insulator. Turns out it was a black color and that was produced by adding carbon powder to the mix. It drove them nuts trying to find where the cross talk was being introduced.
 
I think PEEK fits the bill, minus the price... Although I understand the industrial grade stuff is quite a bit cheaper than the medical grade I'm used to?
 
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