High Temperature (Ceramic) Materials for Thermoplastic Extruders

A common goal among reprappers is to find better high-temperature materials for use in the "hot zone" of the thermoplastic extruder, especially one to electrically insulate nichrome wire. Between the nichrome and the heater barrel, we want a material that:

• can take high heats without degrading (well above the nozzle temperature),
• is electrically insulating,
• and is thermally conductive.

[I'm not having any luck embedding pictures yet, so for now, please see the photosI've uploaded into a public picassa album. Any advise on how to get blogger and picassa to work together (as documented) would be much appreciated. IMHO, I'm following the instructions, but I don't see any images other than my personal head shot :-(
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This is a somewhat tricky task, since most good conductors of heat are also good conductors of electricity. Between the heater coil and the environment, we want a material that insulates both thermally and electrically. For a thermal break, we want something that's a thermal insulator, and whose electrical properties may not matter. For now, I'm focusing on electrically insulating materials, for the nichrome (and thermistor leads, too.)

The materials used so far either don't last as well as desired (JB-Weld, BBQ paint)
cannot really take the temperatures involved (Kapton tape, silicones, epoxies)
or are expensive (per use, or per minimum purchase.)

I found (another) promising material, except that it's expensive,
omega's "CC-High Temp Cement"
$18 for a small (3 oz.) kit.
Expensive, but at least (as separate ingredients) it would have a longer shelf life.
Can we do better -- or at least cheaper?

However, in the online manual
both the ingredients and the proportions are given:

• Zirconium Silicate (4 parts by weight)
• Sodium Silicate Solution (1 part by weight.)

SS is alkaline, but not esp, harmful (used to be used to preserve eggs -- see wikipedia entry
ZS is probably not good for people, but the powder I have doesn't become easily airborne (slightly clumpy.)

Note: other reprappers (Viktor) have previously experimented with sodium silicate, but a good (e.g. thermally conductive) filler (that doesn't cause the SS to harden instantly) wasn't known (to us, at least.) Similarly, we didn't know where to get these chemicals inexpensively.

Armed with search engines, and determined to save money for repstrap components and tools, I did some digging, and found that these chemicals were available from chemical supply houses, but were still relatively expensive (likely for purity levels that don't benefit us much.) Eventually, I found a source for these (ceramic) compounds that was (in hind sight) fairly obvious --for ceramics, try a clay/ceramics/glaze supplier!
I found a number of online sources in the US, but one had particularly helpful information about their ingredients, but continentalclay (in Minneapolis, MN) seemed good(helpful info on their ingredients/uses online.) They had not only the sodium silicate, but three kinds of zirconium silicate powder. I asked them which one was the finest particles, and they answered promptly that it was "Superpax" I ordered a pint of SS and a pound of superpax:

They had other items that seemed useful, a ceramic fiber blanket that can take temperatures found in ceramics kilns (and metal-casting for sculpture, 2300 deg. F.) -- far above any extruder I'm contemplating, near term. They also (I realized after my order) sell alumina, which some other reprapper was looking for as a high-temp cement component. (they have two types, hydrated and not; the latter is more expensive.)

I've done some experimenting by mixing up small batches (5 -- 10 ml) and seeing how well it bonds to materials of interest: brass, aluminum, stainless steel, glass and ceramic fibers. Similarly, I've tried heating it up to see how it holds up.

With the exception of some (apparently insufficiently cleaned) aluminum, this cement adheres quite well to the materials I've tried. It can also be made stronger (and less brittle) by mixing in either glass or ceramic fibers (though this makes it more viscse/dough-like, instead of a thin, glue-like coating.) Metal surfaces need to be very clean and adhesion is better if the surface is roughened (either threading {and extreme case} or just abraded with some sandpaper -- similar to prepping a surface for strain gauges.)

An interesting property of sodium silicate is that it is intumescent-- it swells up when heated -- for this reason it is used in some fireproofing applications. From what I can tell, it swells up more when it is:

• Heated suddenly to high heats (from room temperatures)
• Is applied in a thick layer
• Has not been previously cured with slow heat.
  

I've had better luck minimizing the swelling by gradually heating it up. My working hypothesis (since it becomes porus) is that the swelling is from water being driven out of the silicates, turning into steam bubbles trapped in the still soft silicates. When slowly heat cured, it seems to swell less, probably due to the water being able to escape better during a gradual heating profile.

A main interest I have is to eliminate the "flying lead" connections between the nichrome and the copper. Those make me nervous, because I cannot help but think that the contact resistance will probably increase with time (and thus dissipate more heat at the junction), causing more oxidation, thus more resistance, more heat..... resulting in variable extruder performance.

I looked into insulated standoffs, but was put off by the cost and bulkiness. Using this ceramic cement, I can make insulated standoffs out of (cut-of) screws or short pieces of threaded rod.


(My trials/tribulations brazing nichrome I'll leave for another post. Actually, it's *way* later than I thought. More later.)