Wednesday, February 07, 2007


Concrete as replacement for PTFE insulator in MK2 extruder?

I noticed Vik tried concrete and had successfully used it to extrude parts. Later in a presentation Vik had mentioned concrete on the what doesn't work side of the slide panel. I've been thinking about alternatives for PTFE and thought that concrete as replacement for PTFE insulator in MK2 extruder might be ok. So I'm looking for some input on concrete simply as the thermal insulation between the low thermal inertia nozzle and the rest of the extruder mechanism.

I've got no first hand knowledge of it... but a friend of mine does testing on that sorta stuff (union, does foundations/etc), and he's told me about some temperature resistant concrete... it was a few inches thick, but they had a steel cutting torch directly heating one side, and the other side was cold enough that you couldn't feel any heat with your bare hand...

Considering the amount of nichrome wire I've got, and the fact that I want to extrude teflon/etc, I'm hoping to do some testing with that stuff... Once I get some circuitry working in that department, I might get the chance to go with him to the lab at his company and mess around with some stuff...

The only issue I recall reading about with using it on the extruder head was that it was so heavy and had so much inertia... Given my design, the extruder head is fixed and the platform moves below it, that's not an issue...
I'm building my own Extruder right now, actually the mechanical part is nearly finished. Now I'm planning some experiments with different plastics. Since all have higher melting points than Polcaprolactone (partially >200°C), it will need more heat-resistant thermal and electrical insulation.

For thermal insulation I will try casting conrete in the next days. For electrical insulation, which has na to be stiff I thought about wrapped around glass fibre (silk) or better a silicone tube to put under the nicrome wire.
I've been using standard quickset concrete as an insulator for temps up to about 350C. It seems to work pretty well, though it is very heavy, and hard to make thin. This results in a base about 3/4" thick, any thinner and the concrete wanted to crack when I bolted down the extruder.

I haven't noticed any damage to the concrete or the board it is bolted onto, but I've only gotten the chance to fire the heater up for a few hours.
Two points:

Mix vermiculite in to reduce mass and increase insulating properties.

Put silicone aquarium tubing up the middle so plastic doesn't rub on/melt into the concrete.

Vik :v)
If you're worried about the concrete not being thin enough you might try silica fume as a cement replacement. In construction you use it where you have high strength required in situation where there is a LOT of reinforcing steel and very little concrete. It flows into tight places a lot better than conventional concrete.
Great responses guys. Thanks! And Vik, that idea about the silicone tubing is absolutely brilliant!
Some people make their own hi-temp foundry bricks, it's not hard at all, just a bit of a hassle to buy big bags of this and that for a small reprap extruder!

Has anyone looked into firebrick repair? I'm thinking someone might have a little kit that would be perfect for a reprapper. ;) So far I've found this:

Seems like a few of these products are worth investigating more.

There is a 1200°F paint in there, could it be better than the stuff Forrest has had problems with?

BTW They also have vermiculite, but no price... I've looked for this before, but only found bigger bags.
although if you end up buying a largeish bag of cement other supplies, then you will have the supplies onhand to make more for friends! or for mistakes... =)
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