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High temerature ceramic DIY extruder or: how I plan on distroying a microwave oven

Posted by Lawrence Kincheloe 
Like all good experiments, this one is 89% perspiration, 9% innovation, and 2% desperation.

Basically the goal is to combine multiple material properties to complement each other to produce a high performance heating barrel that is easy to make and requires few tools.

First part involves knowing something about ceramics. Clay based ceramics are a mixture of silicon dioxide, aluminum silicates, and a variety of other hydrophilic compounds. The firing process breaks down these compounds into more stable components, some of which have different melting points. This is useful because the internal structure partially collapses as it liquefies and the solid components are wetted and become cemented by the liquid parts.

Second part involves knowing how the phase change in materials changes their electromagnetic absorption properties. Glass and ceramics are interesting because in their solid state, they let microwave radiation pass through them. However, when they transition to the liquid phase, they can absorb microwave radiation which can help maintain their temperature.

Third part involves nichrome wire, which has a unique properties of having a melting temperature above glass and a resistance to oxidation at high temperatures. This makes it useful for open air heating elements as well as support wire for ceramic kilns.

The process involved, requires using a lost wax casting technique to form the inner barrel to the dimensions required. This means taking a length of plastic rod that is straight and filing it down to a point. Next, we form a nichrome wire coil and attach copper leads with brass collars.

We then form a clay barrel with fine potters clay that has the coil and plastic rod placed appropriately.

Now we have a couple of options. After fully drying, the barrel could be put in a kiln and fired in a traditional manner. However, if you don’t have access to a kiln, you might be able to use the nichrome wire itself as a heating coil to fire the clay barrel.

Also, there is the potential of completing the circuit in the coil and then microwaving the mess to create an oscillating current in the wire. The reason this wouldn’t fry the magnetron in the microwave, is because as the clay fires, it partially converts to glass and absorbs the magnetic radiation. Thus protecting the wire and fully firing the clay.

My plan is to take the safest route and to fully dry, fire using the nichrome wire, and then fire again at higher temperatures using a microwave oven converted for this sort of use.

I’ll take videos…
I've also considered using pottery techniques to create the heater barrel, using nichrome as the heating element (as well as using glasswork techniques on glass tubing). I haven't started looking at using either technique yet though, so I'm very much looking forward to your results on this!

Microwave technique sounds interesting -- my own thoughts were to just put the work into small can, insulating to extreme, and simply using the nichrome to ramp the temp thru a standard kiln heating cycle over 24 hours or so. I believe there are some web articles discussing modifications to microwaves that allow it to be used for metal smelting / jewelry work; one of the things they look at was materials that would absorb the microwave radiation to prevent the magnetron from self destructing.. might be worth looking up for reference at least.
The reason I want to use the microwave is because I don't want to embed nichrome wire throughout the ceramic barrel. I'm hoping to use the barrel itself to act as a thermal barrier.

However, I could just use multiple taps and use all loops for firing and only a select few for actual extrusion.

I've looked at the microwave metal melting methods. It might be a path to explore. Actually, it would be somewhat trivial because I could just graphite coat the nichrome wire. However, I suspect that when the magnetron energizes the nichrome coil loop, the runaway currents generated are going to super heat the clay and hopefully glassify it.

The other thing I'm hoping to accomplish is an extremely high temperature, which might not be possible with the nichrome wire alone.

The main issue is that there is a lot of information but its disjointed between material scientist, ceramic artists, and hobbyist.
Re: High temerature ceramic DIY extruder or: how I plan on distroying a microwave oven
April 11, 2010 09:24PM
I'm also quite interested in this - I just had another PTFE thermal break fail on me, most likely because I mistyped an M104 command and overheated it.

Any idea on the thermal conductivity of the ceramic you're using? PTFE looks to be around 0.25 W/(mK), and it works quite well as a thermal break. Some ceramics seem to go as low as 3 W/(mK), which will need a bit longer/thinner thermal break, but should be possible.
Point of clarification, I don't know a whole lot about firing clay. I took some pottery classes in high school.

Basically, for this to work, this project is going to need a current controlled power supply. If your very careful with the wire lengths and have a good ohm-meter, then you could get away with a constant voltage power supply.

  1. Image One: This is the extracted coil from the second and third image. The coil was still intact and is now in use for a second experiment.
  2. Image Two: This is the before image. The sample is dry and then heated to roughly 1000 degrees centigrade. Unsurprisingly, it exploded.
  3. Image Three: The exploded bits. You'll notice some discoloration, which tends to black. This is normal.
  4. Image Four: This image was the first test barrel produced. The coil burned out because I applied too much current and the temperature swung way past the melting point. If you notice the groove in this image is white as opposed to black. This is because this sample successfully reached past 800 degree centigrade without rupturing. After the coil burned out, I broke it apart to see what happened.

In conclusion, it appears that this method is going to require several long firing steps at a gradual temperature increase. This isn't surprising as this is how most ceramics are fired, but it is reassuring that given long enough firing times, a barrel should be producible.

However, going over these resources.
Nichrome Data
Ceramic Firing Temperatures

It is clear that nichrome won't take us high enough for fully firing these clays to their highest strength levels. Hopefully this won't be a problem.

Edited 1 time(s). Last edit at 04/12/2010 03:01PM by Lawrence Kincheloe.
Success! or something like that.

I'll update more later, but it appears that microwaving clay can be done so it is non destructive to the microwave oven. Also, the coils alone get hot enough to literally glass the clay. The only problem is that leaving it in there for even a few minutes can cause it to make some pretty nifty glass chunks which aren't usable.

Also, unfired wet clay makes an excellent packing material as it won't fuse and comes off easily while insulating the inner piece.

So here is a preliminary recipe for a diy ceramic heater barrel.

The easiest way would be to get your local pottery shop to fire your ceramic barrel.

Otherwise, take a coil of nichrome wire and embed it in white/grey clay. Let dry and repair any cracks that are visible. Fire the barrel at low temperatures either by running a low voltage/ low current through the coil or by firing it in a kiln.

Once it has done this for a few hours (you can't over cook it), up the voltage/current to some yet undetermined value. This is when your starting to convert the various clay particles into fused quartz, so go slow or it will crack.

The other things to worry about at this phase are imperfections in your coil. If one part of your coil gets too hot, it can and will melt. This will cause an otherwise good coil to be useless.

The next step is to explore what the easiest way to get to a finished barrel is. The main stumbling block for the microwave kiln fire process, is that we really need to be dancing along the edge of the glass temperature and not right on top of it. Without being able to use ambient oven temperature as a gauge like in a normal kiln, there currently is no good feedback mechanism for improving the firing process. This lack of fine control may be the ultimate reason why microwaving clay doesn't work.
Lawrence Kincheloe Wrote:
> Fire the barrel at low
> temperatures either by running a low voltage/ low
> current through the coil or by firing it in a
> kiln.

running it thru a few cycles in a normal oven (or mini-oven) at ~400F should also help. The point of the initial temp gradient is to slowly boil off any and all moisture so the piece does not explode from moisture / pressure build up.
I am new to reprap and Mendel making. I am working on my frame and haven't really gotten to determining how to do the extruder. I have a ceramic kiln and would be happy to try some experiments with clay and nichrome wire. Are you trying to make the nozzel part of the extruder (the tip through which the plastic is squeezed) or the thermal barrier part (the part normally made from PTFE)?

Clay shrinks when it is fired - the amount varies with the type of clay and any additives. Some clays (for raku) are more resistant to thermal shock and should work better for an extruder part. What are the dimensions of the desired product? I have some very thin, uninsulated nichrome wire from McMasters & Carr, 8880K82 NICKEL CHROMIUM WIRE, .0100" DIAMETER, 1/8-LB SPOOL, 403' SPOOL. Is this a good size to use?
The final dimensions should fit the same diameter of the normal plastic extruder.

The idea is to use the ceramic barrel as both the thermal barrier, the nozzle and the holder for the heating element.

If your firing the barrel I would go the safest route and not embed the nichrome wire into the clay, but etch space to wind it on there. The nichrome can be wound around the barrel along with the temperature gauge later.

Because clay does shrink both during the drying phase and the firing phase, knowing how much it shrinks would be a key part of making this work.

For example, you can use a length of plastic rod whittled to a tip during the drying phase to build in a plastic guide and nozzle for the ceramic barrel. However, when fired, the plastic rod will melt away and the inner void will shrink. How much depends on the clay and the firing temperatures. This has its good side, because the nozzle hole will be even smaller than what you can whittle by hand. However, your going to have to use a length of plastic rod larger than what you'll use to extrude with. This again shouldn't be an issue, but to determine how much you must pad the hole is best determined experimentally.

My biggest worry is that unless you fire the clay to high temperatures, you might not have a barrel that is strong enough to resist cracking under pressure.

So for you, I would get a wood dowel rod, a little bigger than 1/8th inch and form a barrel around it. Whittle one end to a steeped sharp point and position it such that you can just feel the tip of the point. Around the point, make some grooves in the clay so you can wind your nichrome wire around the nozzle. Now, just fire as normal. When fully fired, plastic filament should move easily through the barrel. When nichrome is wrapped around the nozzle and heated to the melting temperature of the plastic, you should be able to hand feed plastic filament through the barrel and melted plastic should flow from the nozzle.

Hope this helps and happy experimenting!
Some additional ideas to try and experiment with, if you're willing to improve community knowledge in this area:

1. Can one embed copper wire to improve heat flow where needed (within hot end to help distribute heat -- within cold end to aid in removing heat)?

2. Experiment with embedded fibers (they too should burn out during firing) to create voids and improve insulating properties at the transition point -- how much strength can be retained?

3. Add ceramic 'fins' on cold end to aid cooling?

4. Use graphite 'mechanical pencil' leads to create the nozzle hole. It should burn away during firing. Variable sizes give lots of options: .7mm, .5mm and .3mm?

5. Experiments with glazes on interior and how that effects extruder strength, performance and reliability?

6. Experiment with tapering the inner bore -- try to get diameter from 3mm at cold end to something slightly greater than 3mm -- 3.2mm or so -- at the hot end, in the finished product?

7. Adding mechanical structure to make it easy to attach and remove the device. Large threads? Lip? Pegs? Holes? What works best?
As an aside, I've found a nifty way of making glass in the microwave using nichrome wire as an antenna which super heats ceramic material to its glass phase which then starts absorbing and shielding the metal. Not practical yet, but interesting.
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