Here is a new extruder developed by Richard Horne that can print directly from pellets. It is much cheaper to print from pellets then to buy filament. One could also grind disposed or failed prints and recycle them directly.
There are several well developed projects for DIY filament extruders but their main purpose is to produce filament and not directly print from it.
There are also new machines like Sculptify David that print from pellets but nothing open sourced and adapted for variety of small home 3d printers.
This is project under development and Richard is asking for feeedback, so go and drop a comment on his video:
https://www.youtube.com/watch?v=nW4kmJAypZM
Excellent development! Kudos Richard!
Update:
Richard released his extruder and details about it can be found on his blog:
http://richrap.blogspot.co.uk/2014/12/no-more-filament-quest-for-universal.html
Here is detailed video presentation of the pellet extruder by Richard:
All the files to make the extruder can be found at:
https://www.youmagine.com/designs/universal-pellet-extruder-reprap-3d-printing
