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Belt Driven Ultimaker 2 Extruder
This is NOT an easy (or cheap) upgrade for your printer! See link below to see if you are hitting the same issues before using!
Here is a new extruder design that fixes the underextrusion problem by augmenting the hobbed gear with rubber belts. While not the cheapest solution, I've had no underextrusion issues at all with this extruder design.
For more information, design files, pictures, and full documentation see my post at https://0x7d.com/2014/07/improved-ultimaker-2-material-extruder/.