The Libro-Struder filament pushing monster
Of all the things I invented while working at the library district, this has got to be my favorite! The Libro-Struder is an answer to the many problems that most 3D printers encounter when trying to push a plastic filament.
Most 3D printers have a small metal wheel that pushes the filament with tiny sharp teeth. It works fine until the filament jams for a second or so - once this happens, those sharp little teeth that once did the gripping simply grind out a little trench for themselves, and now they do nothing but slip.
The other issue I have had with 3D printer extruders is that they usually have several miniature bearings, springs, some odd screw sizes etc. The parts list generally a pain to assemble because of all the special sizes. I wanted to make an extruder that could be darn-near 100% printed, wouldn't slip, and used easy to find parts. I succeeded - partially!
The LibroStruder (yes, because it was created at a library) uses friction as the grabbing force to move the filament, rather than mechanical digging-in of teeth. It does this by pulling the filament through a groove in the outside of a large gear, surrounded by eleven smaller gears on spring-arms. The small gears hold the filament in tightly, and there are small pegs that keep the filament from buckling out under extreme pressure. The plastic-on-plastic friction is more than enough to couple the motor's output shaft to the filament velocity.
On the up side, it actually does operate quite nicely if everything is put together correctly. It uses small 1/8" pop rivets as the shafts and bushings for the small gears. They are easy to find and add lots of strength once all of the small metal nails are pushed into place. The extruder runs in both directions efficiently, and I have printed a number of things with a RepRap fitted with it. It is also such a minimal print that it takes only around 2 hours to print the whole thing, gears and all.
The problems I encountered were mainly the following:
There is a LOT of torque on the big gear. This means the shaft must be firmly seated or it will start to work loose and create a ton of slop in operation.
The motor used is expensive. I chose a Nema 17 stepper with a 27:1 gearbox. It cost around $40USD, vs a $15 motor of the same kind but without the gears.
Finally, the system provides so much feeder force that it can actually damage the tube fittings as well as 'cold-fill' the hotend by jamming 10-12 inches of cold filament right into the top half. If you are looking for sheer filament pushing power, you got it!
I put together some plans and commentary as well as a published set of print-ready models on Thingiverse.com. I am continuing the research a little by integrating a printed gearbox into the design. So far, the mechanics haven't held up to the forces needed. I think it's a valuable concept, especially when you need an extruder but can't get special parts.
Here is a video of loading filament into the Libro-Struder. The pressure gears (red) appear to undulate in a traveling wave, while the center gear carrying the filament shows continuous motion.