There are several reasons you may need to clean out your nozzle. Foreign particles, filament contaminants, accumulated dust, overheated and scorched plastic can all become lodged in your nozzle and prevent smooth extrusion. Scorched plastic can also stick to the sides, creating resistance for the plastic traveling past without actually blocking the nozzle. This procedure for clearing your nozzle should be performed any time you are unable to extrude plastic, and periodic cleaning by this method is not a bad idea.

In the early days of home 3d printing, it was much more difficult to clear nozzles. It often required disassembling them and heating with a torch to remove plastic to get obstructions out, but this results in more scorched plastic and risks damaging some of the parts if improper force is applied (stripped threads, nozzles breaking at the narrow point between the threads and the inner bore, etc.) or nozzles being reinstalled without a proper seal. Some people also soak their nozzles in a solvent, but it can take some time for solvents to completely penetrate the plastic inside the nozzle, and even once saturated with a solvent, the plastic can be viscous and difficult to remove entirely.

The safest and most effective way to completely clear the plastic from a nozzle, taking any contaminants with it, is what I call a “cold pull”. The idea behind the cold pull is to pull the filament out of a nozzle at a temperature cool enough to keep it in one piece (rather than leaving molten plastic in the hot zone), but still warm enough to allow the plastic to stretch enough to pull away from the sides of the barrel so that it doesn’t seize up entirely. This is easiest to perform with a polished-smooth stainless steel barrel, with those that have a PTFE liner all the way to the end coming in second, because nozzle pressure can slightly compress the softer PTFE and create a plug that will be difficult to pull out.

The cold pull technique has been successfully done with both ABS (this was the best material to use for a long time, with a cold-pull temperature of about 160-180C) and PLA (much more difficult due to its thermal transition properties, but a cold-pull temperature of 80-100C will sometimes work), but Nylon 618 from Taulman (pull temperature of 140C) is far easier and more reliable to use for this purpose due to its strength, flexibility, and low friction.