filament guide

This snap-on filament-guide fits the Prusa I3mk3 spool holder. The main reason for having it is to prevent filament to fall off the spool and build knots. This happens mainly on fresh (full) rolls, and some materials, eg PP, is more prone to it, but might happen also to PLA if the print head makes large movement during print. An opening with a edge-guide makes loading/unloading trivial. The material used for print must be slightly flexible, both for the snap-on/snap-off, as well as for the load/