what makes a good extruder?

I've been reading about the extruder. I've seen designs with a DC motor, and I've seen designs with a stepper. I cannot find any discussion about drive strategy. I'm interested in that. What are the requirements to the drive part of the extruder?
I guess that a good extruder gives an exact amount of molten material per time-unit. Since the width of the output is only determined by the size of the nozzle (is it?) the speed with which the material leaves the nozzle is crucial.
If the filament is fed into the melter with a constant speed, the output speed shoud be constant also. But the melter should not be the bottleneck then.

So my conclusion (without ever having seen a extruder in real life): the drive of the filament must give the filament a constant speed.

Is this correct?

Yep, true.

The load can vary because of the temperature of the nozzle, the shape of the filament, where the filament is melting, and starting / stopping.

This means a simple solution (PWM with a DC motor) can work OK, but is not ideal.

Adding an encoder to control the speed of the DC motor helps a lot. It allows running the motor at lower speeds, and compensates for the variable load.

Steppers have been used - they have more rate control and better torque, but are more expensive.

The key requirement is driving the filament with high torque at a low speed (1mm/sec?). For the DC motors, (and some of the stepper designs) this means a reduction geartrain (complex and not easily reproduced).

Grip on the filament is also a problem - various methods have been tried (screw drive, pinch wheels of various types, stacked pinch wheels). Nophead has some research on drive systems and grip. Some of the plastics are slippery and the force to push them through the nozzle can be quite high.

I can't talk with too much authority, because I still haven't built an extruder that works reliably. However, I know a lot of things that don't work properly...
:-)

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Reprapping blog and other rants: [renoirsrants.blogspot.com]
My Reprap: [sites.google.com]

Steppers get heavy, which is a problem, but don't have the sparking problem which creates electrical interference that brushed, DC motors do. Nophead has recently done a GM-17/tin can stepper hack which is both light, powerful and relatively cheap. That's the one I'm trying to work with right now.

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Hell, there are no rules here - we're trying to accomplish something.

Opportunity is missed by most people because it is dressed in overalls and looks like work.

Thomas A. Edison

Why is a heavy stepper a problem? If the extruder doesn't move, it does not really matter how heavy it is, or does it?

While I write this, I realize this touches the basic design (architecure?) of the Reprap. I have not been able to find the "design document" in which design choices are motivated. For example: in the Darwin Reprap, I believe the extruder moves in X and Y direction, and the table only in Z. Why was that choice made? And why are new choices made in the Mendel design?
Is there such a design document/website/wiki?

First understand that eD's Wedge design is NOT Mendel, at least not yet. That said, the Wedge design requires that the extruder move in Z and X. Mind, that isn't a big deal for either Darwin or the Wedge until you get to printing at high transition speeds. Then the inertia of a heavy extruder design becomes important. Darwin at Bath is already doing 30 mm/sec and headed for 50.

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Hell, there are no rules here - we're trying to accomplish something.

Opportunity is missed by most people because it is dressed in overalls and looks like work.

Thomas A. Edison

We can run at 64mm/sec (possibly higher) with a stepper driven screw drive extruder, probably the heaviest combination and it is not a problem, it does not print so well due to thermal stuff etc but not mechanical problems, head wobble, or inertia.

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Ian
[www.bitsfrombytes.com]