Thursday, February 02, 2006


Replacing PTFE with concrete

The PTFE rod used in the MK 2 Extruder design is a tricky item to come by. I've been wondering if it might be possible to create a replacement part from concrete. It's readily available, stable up to 290C and doesn't emit nasty fumes when overheated.

It's not very good under tension though, and needs to be kept moist during the curing process - small amounts of concrete are notoriously difficult to cure. I suspect a dampened ,sturdy cardboard tube may be a good former.

Assembling the brass nozzle into a cement holder should be easy enough - either directly or by adding a couple of nuts to it so it can be removed for maintenance. End plugs left as an exercise to the dilligent researcher (Polymorph, gaffer tape, whatever...)

I'm considering putting M3 thread or long bolts down the full length of the cylinder, and making the cylinder roughly 30mm in diameter, 60mm high. I'd be surprised if a 12W heater could warm the entire thing.

The protruding M3 thread would be useful in attaching the holder to the extrusion mechanism, dispensing with the current clamp entirely.

I also wonder if embedding fibreglass insulation would increase the strength of the concrete in tension? It wouldn't help with insulation obviously.

Another tack might be to embed perlite or vermiculite in the concrete to reduce its thermal conductivity.

I feel messy experimentation coming on...

Vik :v)

Strictly speaking, you want to use "cement"; "Concrete" is cement mixed with agregate, and the rocks and gravel might get in the way of fine detail work.

Now, as to this idea that cement is no good in tension, that's not *quite* true; Research has shown that if you can keep the gas bubbles out, so that they don't nucleate cracks, it has quite respectable tensile strength. Probably a bit complex to accomplish at the home scale, though...
Not all concrete formulations use an aggregate in the sense of stones.
If I put vermiculite, perlite, or even pummice chunks in it then I guess it's some kind of concrete then, right?

Vik :v)
Actually, if you use something like pearlite, and replace the plain cement with furnace cement, you've got a homebrew refractory that'll make a furnace you could melt metals in...
The problem with furnace cement is that it is not very strong. I am investigating the use of "silica fume" based cements. These should be widely available.

Vik :v)
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