Saturday, June 04, 2005


Yet another polymorph extruder

Encouraged by my previous polymorph extrusion experiments, I designed a much simpler extruder. Though I used a lathe to make a part of it, with care it ought to be possible to build this design using just hand tools as the only operations were drilling holes and tapping threads (which I did by hand anyway).

I started with a brass M6 countersunk screw:

I cut the head off, and drilled a 3mm hole right down the middle:

One could instead use M6 brass studding, or one could cut a thread on the end of a piece of brass tube (only the last 5mm or so actually needs to be threaded, as will become apparent, though the rest of the thread is useful to guide the heater wire, as will also become apparent). I drilled out the end, tapped it M4, and fitted the nozzle described in the blog for 28 May (q.v.).

Then I wrapped some PTFE thread-sealing tape (obtainable from all good plumbers' suppliers...) round the thread...

...and wound a 300 mm length of 0.2mm diameter nichrome wire into the PTFE-insulated valleys of the thread as a heater. The PTFE needs to be thin to allow heat conduction, but thick to prevent shorting with the brass. About two or three thicknesses seemed to work well. I then put a thermistor (RS 484-0149; cut and paste that part number into the box top-left on the RS main page) against the thread at the nozzle end and wound more PTFE tape round it to keep it warm.

I drilled a 3mm hole down a 40mm length of 10mm diameter PTFE rod, then drilled out and tapped M6 the last 5mm or so:

I then screwed the whole assembly together:

The thermistor is the bump at the right hand end; this was positioned past the end of the heater windings to get temperature readings from the brass, not the nichrome heater wire; before putting the thermistor on I cleared a patch of PTFE away using a wall file to expose the brass, so the thermistor is resting right on the metal; I wrapped a little PTFE tape round the thermistor leads to stop them shorting. The heater wires are the ones heading off top right.

I rolled up some 3mm diameter polymorph rod, and switched the heating current on. 300 mm of 0.2mm nichrome wire has a resistance of about 12 ohms, and I was running it at 12 volts (i.e. 1 amp / 12 watts). This brought it up to temperature (about 130 degrees C, or a thermistor resistance of about 350 ohms) so quickly (less than a minute) that I had to turn the voltage down to prevent overheating. This is good, as it means it should be easy to do PWM temperature control with a PIC and a power transistor.

(Note the extremely rusty retort stand; this was done on a Saturday morning in my lab at home, not the well-equipped ones at Bath University...)

The multimeter on the left is measuring the thermistor's resistance, and the bench P.S. on the right is supplying the heater.

I pushed the polymorph rod through. A force of between 20 and 30N gave extrusion at 2mm per second.

The PTFE rod stayed nice and cool, and should be easy to clamp into the pinch-wheel feed that Vik is designing. It may be necessary to put a hot-air heating jacket round the extruder, not to keep it hot, but to direct hot air down to soften the polymer that has already been deposited to get good adhesion.

Unrelated to polymorph extruders, here are a few thoughts on deployment logistics. I haven't seen this topic discussed much elsewhere.

It might be good to work with some organization like the Peace Corps to deploy the RepRap gadget in the developing world, once it's ready for deployment. The Peace Corps is still around, but they're nowhere near as popular or well-known today as when they were heavily advertised in the 60s and 70s. Maybe the RepRap project could give them a new spurt of popularity, and in exchange they provide the organizational support to get it deployed. Deployment will require foreign language skills and training skills, and the Peace Corps has been doing both for decades.

It might make sense to set up regional centers that local people can come to for RepRap training. You want to get to a point where locals are being trained by other locals, and you want deployment to be geographically uniform. So from the day of arrival, you recruit local people for the regional center who understand and agree with the GPL commonweal vibe, so that vibe continues to inform deployment after you leave the region.

So then a town or village sends some people to their regional center to get trained, and part of their training is to complete a half-dozen RepRaps to bring home with them, and to become competent at fixing broken RepRaps. Those villages then become local centers, and get some kind of subsidy for training other nearby villages and supplying them with RepRaps. There might be a volunteer who visits the local center to (a) help them if they run into any trouble, and (b) make sure they're keeping up their end of the subsidy bargain. The subsidy might be paid in raw materials.

Many developing countries have local warlords. They'll want to control the RepRap technology and deny it to everybody outside their direct control. Also, they'll try to use the RepRap to make weapons. I don't think I have any good ideas how to deal with these things. Maybe the Peace Corps has useful insight here.

You also need to get raw materials to all these RepRap users, unless the RepRap reaches the point where it can work from rocks and leaves and twigs. Temporarily, this can be subsidized by charity funding from the developed world, but that won't be sustainable in the long haul. Eventually users will need to exchange something for raw materials. If all users build the same stuff, then RepRap-built finished goods will quickly become cheap commodities, worth little more than the raw materials themselves. It's probably smart to encourage them to diversify and specialize, to produce unique local artifacts that get additional value from their uniqueness.
That's both interesting and useful.

I always intended to approach charities with interests in secondary education in the developing world - I think it would be a lot easier to get 15-year-olds using RepRap than 50-year-olds... We would also need a supply of cheap PCs; fortunately these are now such a throw-away item in the developed world that their disposal is becoming an ecological problem...

As to the point about of people's using RepRap to make weapons, regrettably some will of course - no difference there from any other manufacturing technology. But give people the internal combustion engine and they use it to make many more ambulances than tanks. Most people are rather well intentioned...

And the self-replicating nature of the device makes it rather difficult for warlords (whether in developing countries or in Whitehall/The Pentagon) to control...
I've already e-mailed the MIT $100 Laptop team to ensure that what we're doing will be compatible with what they're doing. No response to the e-mail, but I only sent it 4 days ago.

Adrian, what's a good source of PTFE rod?

I'll have to go get myself a longer M6 bolt, new 0.4mm drill and some finer nichrome, after which I might post some pictures of the 'Afghan Lathe' in action.

Vik :v)
I got my PTFE rod from The Fluorocarbon Co. Ltd. ( They have supported the RepRap project, so, World, buy from them.

RS ( do it too, part number 680-628, though they charge a fortune...
"A force of between 20 and 30N gave extrusion at 2mm per second."

That's with a 0.5mm diameter, right? So, that's, what, about 0.4 cubic mm per second or about 1.4 ml per hour. What's the target deposition rate? I guess the main constraint would be the ability to produce all the parts for a copy in a reasonable period of time.
It'll do until how we figure out how to connect up the turbocharger :)

More seriously, we don't have to do all the depositing with a 0.4mm nozzle. Do the edges with 0.4mm and fill in the centres with a 1 or 2mm nozzle. It might even pay for some parts to infill with liquid resin.

First 0.4mm nozzles, tomorrow the world!

Vik :v)
"Do the edges with 0.4mm and fill in the centres with a 1 or 2mm nozzle. It might even pay for some parts to infill with liquid resin."

I'd wondered if that was being considered. It would certainly speed up the build time. The overall design would be a bit more complex, though.

"I have heard that PTFE (teflon) breaks down at high temperatures and releases chemicals that kill birds and is bad for humans."

There's some evidence to support this. The general concensus is that you'd have to get the PTFE well above its normal operating temperature to generate fumes. The maximum temperature that I see most often is 500F (260C). We have birds, so the only PTFE coated cookware we use is thermostatically controlled. No stove-top pots and pans.
The principal nasty you get from a pyrolytic insult to PTFE is perfluoroisobutene, and it is very nasty indeed. But as Steve says it's fine up to about 250 degrees C. We're operating 100 degrees below that (i.e. cooking oil temperatures...), and we're thermostatically controlled. We'll probably put a 200 degree thermal fuse in the final design for safety. Solder should do quite nicely...
It just occurred to me that you could build this extruder into a hand-held device about the size of a soldering iron. Spring-loaded to push the plastic stock with a fingertip control to adjust the feed rate. I'm not sure what such a thing would be used for, other than maybe doing touch-up work on a finished piece. I suppose you could also use it to bond two pieces together or to repair breaks.
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