Saturday, April 14, 2007


Testing the envelope of the Mk 1 AEM extruder

I've been running Tommelise pretty much continuously (12-18 hours/day with about a 50-75% duty cycle). The controls are getting more reliable by the day making it possible to do things without hovering over it 100% of the time.

A few days ago I decided that I wanted to run the extruder barrel on the Mk 1 AEM extruder a lot cooler than the 200-odd degrees Celsius that I'd been running it previously. Here is the performance table that I developed.

Gearmotor Heater Temp (C) Amps Flow Rate
12% 60% 132 1.2 ~1 mm/s
25% 75% 152 1.5 ~2 mm/s
36% 75% 152 1.5 4 mm/s
50% 75% 152 1.5 6 mm/s
75% 75% 152 1.5 11 mm/s
100% 100%1952.0 16.8 mm/s

The important thing to remember about these tests are that they were run for periods of just a few minutes. Since then I've been running at settings for hours. Here is what I've discovered.

I started out with a setting of 40% max for the gearmotor and 75% of max for the extruder barrel heater. After about 18 hours of that you start hearing infrequent, intermittent clicks out of the GM3 gearmotor as the clutch slips that drives the Mk 1. The clutch slips at 0.4237 Newton metres (60 oz-in).

The motor for the GM3 will have heated to about 50 degrees Celsius and the 754410 that powers it to about 40. By way of comparison the axes motors run at about 30 degrees as does the 754410 that drives them.

From a cold start the Mk 1 could handle a 75% extruder barrel setting and a 50% gearmotor setting for about an hour and a 60% gearmotor setting for about 10 minutes.

The extruder cooled down completely in about 20 minutes and was able to repeat the cycle.

I tore down the Mk 1 to make sure that nothing untoward had happened inside to confuse matters. There was much less wear on the 1/4-20 threaded rod polymer pump than I expected. The pump was, however, liberally peppered with a fine black dust which I took to a fine powder of either brass or steel. I blew that out insofar as possible. My breaking down the pump, however, disturbed the dust and got it into the extruded plastic thereafter turning the next HDPE raft that I put down grey.

It's worth mentioning again that HDPE and HPP even more resist being cut by the polymer pump's threaded drive rod much more than polycaprolactone. This forces me to use a much tighter setting for these plastics. The tighter setting causes the polymer pump gearmotor to draw more current.

This I think is what I was worried about long ago, but had never heard anything about. How you can relate a linear motion to a volumetric flow based on melting without having some kind of pressure sensor to back off when you try to thread more stock than you are ready to melt and extrude. Even if your flow matched perfectly for a while wouldn't there be some thicker spots of filament here and there that would cause "back up" and result in extra pressure against the drive mechanism. The reason I didn't bring it up myself was for lack of being able to devise some kind of sensor that could be reliably used to limit the driving force. As I think now though, maybe some kind of force sensor integrated into the mounting of the motor (between the motor and a mount) might be suitable :) I'm always thinking.
As a practical matter it doesn't seem to be that big a deal. I've had the Mk 1 running for hours at a time without problems so far.
I think monitoring the motor current would give a reasonable indication of the amount of pressure being applied.

Does anybody know the thoeretical relationship between pressure, nozel size, viscosity and flow rate?
On second thoughts I don't think knowing the pressure is so important. Just use a shaft encoder to ensure the filament if fed at a constant rate. As long as the pressure does not get high enough to stall the motor then all should be well. If the extruder barrel temperature is kept constant then the only variables are the diameter and density of the filament. Presumably these are fairly constant.
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