BTW, I juste designed a new extruder, based on a NEMA14 geared motor (
http://www.omc-stepperonline.com/gear-r ... p-136.html). This motor has 0.9Nm tork, which is twice the tork of a NEMA17, for the same weigth.
I attached an Openscad screenshot. I printed it, but I still have to order the motor. I designed it especially for Filaflex, which needs a very good filament guiding. Here, I use a PTFE tube, 6/2 (OD=6mm, ID=2mm). The tube is sharpened to a point, so it can be very close to the driving point. The pusher part is also very simple, without spring (I used such pusher on another extruder for months, without any problem); the rotation shaft of the pusher is also pretty crude (the extruder could be printed in 1 part, but would need support, and I don't like support; 2 parts also allow better access by removing the pusher).
I will soon post it on Thingiverse, but if you want the Openscad file, just ask. The hotend mount is made for a Paoparts hotend (
http://www.paoparts.com/fr/extrudeur/90 ... t-3mm.html), but very easy to adapt for other hotends.