Sometimes when the filament is feeding incorrectly, the drive gear will begin to strip the filament, shredding it into filament dust that contaminates the extruder assembly. The causes of stripped filament vary and thus must be dealt with differently, but the filament dust must always be cleaned up after as it can continue to cause problems for your Solidoodle.
Cleaning the Drive Gear
To clean the drive gear, first remove the filament. Clean the drive gear by sticking a tooth brush through the access port to the left of the drive gear and scrubbing the drive gear. Pressing extrude on Repetier Host while no filament is loaded will spin the drive gear and allow you to clean the entire gear. Scrubbing the drive gear while it runs should not damage your extruder motor. Alternatively, if your printer is off or if you hit the STOP MOTORS button on Repetier Host, you will be able to spin the drive gear yourself using the knob that sticks out.
It may be neccessary to dig out particularly packed filament dust from between the teeth of the drive gear with a pointy object like a pair of needle nose tweezers or a needle. Filament dust packed in the drive gear will prevent the drive gear from properly gripping the filament and may affect your prints.
Finally, to ensure all filament dust is removed from the extruder assembly, you can use a can of compressed air to flush out any remaining filament dust.
Dealing with the Cause
There are several reasons that the drive gear will start stripping filament. Each requires a different method to deal with it.
The drive gear can only pull on the filament so hard before the filament itself starts stripping. First, make sure you have your included spool mount properly set up, following the spool mount set up guide. Pulling on filament from the wrong direction may cause the filament to hop off the spool and wrap around the PVC pipe, eventually constricting enough to prevent filament from feeding. If filament continues to fall off the spool, as is most common with full spools of filament, you might want to print make yourself a filament guide, such as the one found here.
The filament can also knot itself around the spool. Instead of easily coming off the spool, the filament goes under another loop and tightens into a knot as the filament is pulled by the extruder. Inspect the filament for any more possible knots, then reinsert the filament and continue printing.
When you have two separate pieces of filament in the extruder, when you insert a new piece of filament for example, you hope that the end of one will be able to properly push the end of the other through the extruder. Within the confines of the acrylic extruder assembly, however, there is enough space that filament may sometimes slide past each other. Because the new filament has no where to go, the drive gear will begin stripping the filament in an attempt to cram it in further. If you can still see the old piece of filament, follow the instructions for clearing stubborn filament, then reload the filament and try again.
If the filament rolls freely off the spool and you cannot see a stuck piece of filament in the extruder, you may have a clog somewhere in your extruder. To deal with the clog, go through the unclogging procedures.