Been working on these self-rewinding Filament Spool Holders for my highly modified ERCF 3D Printing Setup.
The System uses a Flat Coil Spring made out of 3D printed PETG and a Clutch of the same Material. As the Spool unwinds the tension of the Spring increases until the Windings have fully collapsed onto each other at which point the Clutch will start moving with the Spool until no more Filament is being pulled.
When the Filament is being returned by the ERCFs Gear Motor the built-up tension in the Spring will start rewinding the Spool. With the current design this will rewind the required 1'200mm of Reverse Bowden Length in my System so about 3x full turns on a fresh Spool.
Performance could most likely be improved by a metal Flat Coil Spring allowing for more windings, less fatigue as it keeps getting tensioned, and generally more strength when rewinding - It cannot rewind the Filament itself and is totally reliant on the ERCFs Gear Motor assisting it.
Another "Issue" is the situation once a Spool has been sufficiently emptied at which point the 3x turns of rewinding capacity will no longer suffice but so far this has only proven to be a cosmetic issue as the loose Windings don't seem to exit the Spool.
Something else of note is the mechanism of securing the Spool to the Rewinder allowing for a quick installation of a Spool using a single Nut in the center.
As of now this is still only a Proof of Concept.