PTFE Coupler Ring to Prevent Back Flow of Filament into the Heater Block.
Please note this is only compatible with Intergrated Heater Block + Nozzle and not Compatible with Olssen Block. Therefore you can only use this on the Ultimaker 2, Ultimaker 2 Extended machines only. If you have upgraded your Ultimaker 2 to a 2+ you can not use this part.