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Filament entanglement is a very common cause of air prints (3D printer extruder can't pull the filament when it gets stuck in a knot). The most common underlying reason is that the filament tip at some stage becomes loose and get twined. In the long run this causes a knot to form.

We designed the arm on the filament spool holder to give an adequate entrance point for the filament for most sizes of spools. So far no knots have occurred.