(kym) One of the biggest problem i have with my bowden tube feed extruder is plastic that has kinks or bends in to, this gets stuck in my feeder and causes the filament to grind.
So taken from the idea of a wire straightener , here is my filament straightener.
i haven’t had any grinding problem since starting to use it.
I find that after slicing my object, I then know how much filament is needed and then pull that amount through and then back and recoil it on the spool.

http://www.thingiverse.com/thing:30733

When using semi translucent plastics, running them through the straightener will turn them slightly white as is bends the plastic. But this colouration goes away after the plastic has been heated.