Keep your stored filament from unwinding and breaking.  My filament was breaking when I stored it with the end of the filament looped out and back in the spool side holes - even when I dried it - due to the sharp bend.  No more with this simple clip.

UPDATE: V1.1 fits a little tighter on the spool and holds the filament in place better.

Print Details

Material: Same as spool it will be used on (or higher temperature material) if filament will be dried.  If not, PLA works.

Layer Height: 0.15 or 0.20 mm

Infill: 15% or greater

Supports: None

Brim: None

Design works on cardboard spools.  Let me know in a comment if an adjusted design would work better for the spools you use.