by canadianavenger » Wed Apr 02, 2014 3:29 pm
The key here is keeping the filament under tension so the coils can't shift while on the spool. The "knot" is not a true knot as the end never passes through, but rather the coils become crisscrossed when the coils become loose, and effectively locking up when tension is re-introduced.
One solution here is to transfer the filament from the spool to a barrel cartridge. In the cartridge the filament is removed from the inside of the winding and works it's way out. As a result the coil is always pushing outward on the walls of the cartridge thus keeping the windings tight preventing shifting.
This is harder to do with a spool. You can increase the tension with the spool [thus keeping the filament in tension], but then you place extra load on the extruder, which is not ideal. With filament retraction and systems where the extruder moves relative to the spool this won't work, as the motion inevitably introduces slack into the filament, allowing for this type of tangle to happen. Another solution is to add a second motor as a pre-feeder [that only moves in one direction] that keeps the filament under tension on one side, and maintains a free open loop between itself and the extruder. A simple sensor set-up can be used to determine when the pre-feeder needs to advance more filament off of the spool.