Sunday, May 28, 2006


Speculations on RepRap 2.0

Discovering plateaus of very smooth performance (reported in our sourceforge forum) of the threaded rod drive axes last night got me to thinking about what a next generation RepRap might look like.

I hadn't expected to get a Godzilla axis running at 38 mm/sec under any circumstances, never mind smoothly enough to think about extruding at that speed. Actually, using our dodge of extruding on a diagonal we would get 38*2^.5 or about 54 mm/sec. I spent a little time this morning wondering what a RepRap operating at those extrusion speeds would look like.

First off, a long time ago I figured that the Mk II would extrude 2.83 cubic centimers of polymer/hr operating at 4 mm/sec. I most likely underestimated the extrusion rate in that I assumed, those being very early days, that the polymer thread would not expand after having left the 0.5 mm extruder orifice. Subsequent experience has shown that the thread expands as much as a third. That would mean that in reality the Mk II was extruding something like 3.8 cm^3/hr.

What would a RepRap capable of putting polymer down at a rate of 54 mm/sec perform like?

At a 100% duty cycle (unrealistic), it would put down 50 grammes of polymer/hour...

...or 1.2 kg/day

...36.5 kg/month

...438 kg/year

In home use such a RepRap would likely see an 8 hour duty cycle daily, which would mean 12 kg/month or 146 kg/year.

That is still a lot of polymer seeing that it costs about US$12/kg.

When I first built Godzilla I did a volumetric quantity survey of the structure. A one-for-one poplar to polymer replacement meant that it would take about 7 kg of polymer to replace its structure.

Godzilla could replicate itself in 17 days or make 21 copies of itself in a year.

That would be pretty much unstoppable as a diffusing technology.

Just for fun I tried to scale up the Mk II to something that would be able to put out polymer at this rate to see what the design envelope looked like.

Using 3 mm filament the feed mechanism would have to run at a rate of .15 mm/sec or 9 mm/minute. Given that the M3 studding rod we are using as a drive to feed filament into the extruder barrel has a 0.5 pitch we are looking at a rotational rate of something like 18 rpm out of our little G4 gearmotor. That doesn't look to me like something that it couldn't do as it since it is rated at 70 rpm.

Where the Mk II would have to be upgraded, however, is in the nichrome wiring for the extruder barrel. Right now you're using about 300 mm of .2 mm diameter nichrome to provide about six watts to the barrel. You'd need 13.5 times that, or about 80 watts, to run at 54 mm/sec. That's 4 metres of nichrome. You might want to think about one of those ISM sleeve heaters, except that you'd have to redesign the extruder barrel since the minimum diameter on one of those is about 20 mm.

Last I checked, longer nichrome -> higher resistance -> lower current at the same voltage -> lower power input.

So it's more that you need either wider nichrome, or shorter nichrome, if you're keeping voltages the same.
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