Ultimaker retraction ring

This is a variation of the extruder wedge thingies that keep the drive-end clamp taught. Most of these are not easy removable, something you want to do when doing multicolered prints or just for switching filament. This one can be undone by a simple twist and slide. On the low side you have enough room to undo the bowden tube.
It does not come loose easily and because it is ring shaped you will not lose it because one of uncaring/sloppy collegues/users.

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