By burning vegetable oil on the metal, you'll be creating a slippery, very thin, heat resistant polymer coating that makes it very much easier to remove the filament in the future.
It's best if you do this when the hotend is new so the seasoning is right on the metal without bits of plastic stuck to it. If you have already used it, it would be good to use this guide to clean it first if you can.
These instructions are what I did after reading them online. This isn't my invention, just what I read.
NOTE this is ONLY for all-metal hotends. Please don't try to season a hotend with a PEEK/PTFE/other plastic liner such as the JHead. Those don't need this treatment and it might cause trouble.
- Get a couple of lengths of filament, about 20cm long each, and a small quantity of vegetable oil.
- If there is filament in the hotend already, try to do a clean pullout of the filament that's in there. typically for PLA that means heating to 100 to maybe 120 and pulling the filament as soon as it can be pulled.
- Heat the hotend to about 230
- Dip one piece of filament in the veg oil and insert it into the hotend. It will sizzle. Push until you get a couple of centimeters of filament extruded.
- Remove that filament and push through a couple centimeters of the other bit of filament (without dipping it in oil). Remove that bit of filament.
- Repeat with the oil and the unoiled bit at least 2 or 3 times.