This part is useful for upgrading from 4mm to 6mm OD Bowden tube. It plugs into the end of a Festo fitting for 6mm OD tubing and provides a smooth transition to 4mm. It is compatible with PTFE tubing commonly sold for 3mm filament, e.g., https://amzn.to/2H6VFrK.
6mm tubing is desirable in situations where there is a long or not straight route between the filament spool and printer because it adds much less friction to the path. I use 6mm PTFE between my drybox and MMU2S filament buffer (see https://www.thingiverse.com/thing:3399360).
This part works with both passthrough and non-passthrough fittings, for example PC6-M101 style (https://www.captubes.com/shop/#!/4-Pack-PC6-M101-Fittings-For-2-85mm-Bowden-Tubing/p/103148418/category=33913105).
Print at ultra-fine resolution and 100% infill due to the thin object wall. ABS or PET filament preferred. PLA might be too brittle for this part. You may need to clean up the chamfer with a drill bit after printing.
Insert the tubing into the Festo fitting first, then firmly push the plug into the fitting.
Any ABS or PET preferred