Sunday, December 28, 2008

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 :-(

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.)

Saturday, December 20, 2008

Cerberus, a RepStrap machine: a start at documenting my thoughts and efforts

Greetings all,

This is the beginning of my 'blogged "build journal" of my efforts to build a RepStrap machine.
(Please bear with me; this is my first blog; I'm learning as I go.)

What is a RepStrap? A RepStrap is a machine to bootstrap into RepRap capability. A RepRap is a Replicating Rapid Prototyping machine -- a "three-dimensional-printer," particularly one that can print many of its own parts, and whose design is open-sourced. The RepRap project was started by Adrian Bowyer, in the Mechanical Engineering Dept. of Bath University, UK.
For background details, see the reprap website.

Why build a RepStrap and/or RepRap machine?
  • For the fun,
  • For the technical challenge (motivation to learn new things),
  • To justify buying a bunch of tools I've always wanted, anyway!

What is/will be different about Cerberus vs. other already-extant RepStrap machines?
  • Cerberus is *my* RepStrap, built the way I want it (or as close as my skills, time, tools and budget permit.)

  • Cerberus will be capabile of both additive fabrication and "conventional" subtractive machining. Note: several other repStraps do this, notably nophead's HydraRaptor
    and Forrest Higgs' Tommelise (among others), so this notion isn't new here.

  • Cerberus' design is not constrained to be buildable with simple tools as is the Darwin RepRap, or the McWire-based mechanism often used for RepStrapping. (I'm explicitly going for performance/versatility rather than ease of development/adoption.)

Why name it Cerberus?
  • Cerberus is the name of the three-headed dog that guarded entry into to land of the dead in Greco-Roman mythology. My RepStrap will (eventually) have multiple tool "heads" one for additive fabrication, another for subtractive fabrication (milling), and a third for measuring/probing (~CMM) So that's three, and nophead already used Hydra in the name of his machine. Iff I get to the point of adding a forth head, I may have to rename it. (I considered using the name Janus, a two-headed Greco-Roman deity, but upon investigating, I decided that I liked Cerberus better. Proteus (also from mythology) was also a candidate, for awhile. (Proteus was also the name of the submarine that was miniaturized in the movie _Fantastic Voyage_, one of my childhood favorites. But since this machine won't really change form or size, the name isn't that good a fit. )
Brief outline of Cerberus' design:

The Cartesian(X, Y, Z) mechanism

  • I've based the horizontal (X, Y) degrees of freedom on a "compound table" (AKA XY table, or mill/drill table) from Shars Similar devices are available from other vendors (e.g. Grizzly, Enco, or Palmgren also smaller ones.) Mine is an import from PRC, and the typical caveats re quality apply, though (after dissasembly, cleanup, re-assembly, and adjustments), I think it'll do what I need, and it's good bang/$.
  • After dis-assembly/cleaning, but before re-assembling it, I lapped the linear bearing surfaces (slideways, AKA ways) of the table. There is (to put it mildly) considerable controversy on whether this is a good idea, or a terrible one. "Purists" argue that the proper way to improve ways is by scraping them in (and using a reference surface and prussian blue to find the high spots to scrape.) I agree that (all things being equal, this is the most accurate method.) However, my goal was not mainly to improve geometric accuracy, but to reduce the friction, and thus reduce the torque my actuators (stepper motors) would require to move the table. I compromise was to lap the ways as carefully as I could, using a machinst's (6") parallel to hold the carbide paper flat (at least for local, 6" regions being lapped.) I used a light oil with the carbide and inspected carefully for any carbide grains embedded in the cast iron. (Some advocate using less agressive, non-embedding abrasives; I wasn't patient enough to wait for a mail order; I had the #400 carbide in hand.)
  • I made mounting brackets (out of some scrap Aluminum) to support the stepper motors and couple them to the Acme screws. These may need some iteration; the stepper motors can drive both X and Y, but only to ~50% of the motors' no-load speed. The residual backlash is also not perfect, but may do for now. If I re-design these, I will incorporate better thrust bearings, instead of the solid bushings I made with scrap brass and stainless I had handy.

  • For the vertcal DOF, I was planning to use a smaller linear stage (5" travel) I have in my junk box. Now that I have the XY table, I'm starting to think I should get something a bit sturdier, and maybe with a bit more travel. Stay tuned.
Additive toolhead (thermoplastic extruder)


Subtractive Toolhead

I bought a rotozip (model RZ20) to use as my tool spindle. This has some features I wanted:
  • Variable speed (though only ~factor of two, 15 -- 30,000 rpm.)
  • A three-jaw chuck instead of collets, this permits use of tools with a range of shank diameters, e.g. drill bits.
  • A removable handle -- I hope to either clone the latches, or cannibalize the handle to make a mount for it.

Measurement Toolhead


Real-time controller

Structural Framework