About the Libro Struder
The Libro-Struder is an original 3d printer extruder created for the Pueblo City-County Library District by David Hartkop. It is part of the RepRap Libro project but is a general purpose filament extruder for all your filament-pushing needs. It uses plastic-on-plastic friction forces rather than toothed-gear biting forces to grip and advance a plastic filament. The Libro-Struder is less prone to jamming and slipping than the more common Wade or Bowden-type extruders. It can also be created with fewer specialized parts. The design explores the idea of using taken-apart pop rivets as tiny bushings on shafts. The current model does require a stepper motor that has an orbital gearbox attached. I have had much success with a Nema 17 stepper with a 27:1 gearbox, Part No. 17HS19-1684S-PG27.
Hackers encouraged to print their own gearbox. :-)
Nema 17 motor, 27:1 gearbox #17HS19-1684S-PG27
4 x M3x8mm screws
Push-in pneumatic fitting for 4mm od nylon tube
18 gage steel wire, 3 inches
22 x 1/8” x 1/4” long pop rivets, steel.
nut #10 (M5)
set screw #10 x 3/16" long (M5x5mm long).
All printed parts can be downloaded from  on thingiverse.com.
Videos of LibroStruder in action:
See video of printing a Pyramid: []
See video of printing a part for the RepRap Libro: []
See video the Libro-Struder power loading here: []
See video of the Libro-Struder extruding here: []
See video of dragging filament out of the Libro-Struder: []