How do you evaluate an extruder?

The printer kit I got a few years back came with what looks like a Greg-Wades if I have that right. The steppers that came in the kit are 2.4A models (these). The a4988 drivers I have cant drive near that so I'm running them at about half their rated current and so as far as I understand steppers I'd get about half the torque or 2400g*cm yet still it idled along. I wanted to try to lighten the extruder without for an upcoming build without resorting to a bowden setup so I tried to lighten the extruder. I opted for a nema 14 belt driven with a ratio of 2.5:1. I got the motor locally for $20 and had the rest of the parts so I figured I'd just try it; and since people are using direct drive extruders successfully I thought a nema 14 with a 2.5 advantage would be pretty equal... at least says the math.

So I made myself an extruder with the nema 14 motor, belt drive, and changed a few things I didn't like about the greg-wades. Excluding the hot end, my greg-wades weighed in at 540g and my design is 325g - maybe not light but I think 215g would be a significant weight savings. It's also giving me better prints even after increasing printing speeds by 50% afforded by the weight savings. So I guess it was a successful experiment. For real world printing there are no issues but when I click the fast extrude in Repetier Host (where it extrudes 100mm of filament in 1s) it choked a bit, though I wonder if the small thermal mass of the hot end was the weak link and not how fast the extruder could push in filament. Who's extruding that fast anyway?

I'm just wondering if there is some set benchmark procedure to rate the extruder to be able to directly compare it with other popular designs?

Thanks,
Ryan

The fast extrusion in repetier is meant to feed filament through a Bowden tube before it enters the hotend. I doubt you could melt 100mm filament in 1 second

Edited 1 time(s). Last edit at 02/16/2017 03:17AM by o_lampe.

No benchmark that I know of. The criteria are that the extruder needs to grip the filament firmly enough to push it through, preferably in a way that doesn't squash or grind the filament, with a motor that has sufficient torque to accomplish that without skipping steps, and to be able to retract quickly. You are right that when extruding, the speed will be limited by how fast the hot end can melt the filament. As a rule of thumb, gearing of about 3:1 seems the optimum compromise between providing sufficient torque whilst allowing fast enough retraction. Low gearing or direct drive may not provide sufficient torque and/or require a larger motor. Too high a gearing may result in too slow a retraction speed. I guess the best extruder will be one that fits those criteria but has the lowest mass if it needs to move. If the extruder is static, then mass is unimportant.