Cleaning Nozzles
PS always remove the nozzle (very carefully, of course) when it's HOT, otherwise ABS works just like a really strong glue - it's very likely something will break when unscrewing a cold nozzle.

I've tried many ways to clean out a 3D printer Nozzle - this is what I've found works best. . A word of warning - NEVER buy the carbide drill cleaners advertised, they are extremely brittle, and can easily ruin a nozzle if they break off, which I had happen before I changed to HSS cleaners.

Firstly, buy some HSS (High Speed Steel) drills, the same size as each Nozzle you use (usually 0.3mm or 0.4mm) - these are only a few dollars on eBay. The cheaper ones may not be the specified diameter, so check they fit through a clean nozzle first, and put the oversize ones away where they won't be used by mistake.

I found these drills so small they were very hard to handle, so I designed this knurled 3D printed plastic sleeve to make handling.

I alse decided to print my drill sleeves in Orange for 0.3mm & Yellow for 0.4mm - yes - the same as the resistor color code, so I can tell the size very easily. I also color code the cleaned nozzles the same way (using a TextaColor) - it makes finding a working, cleaned out, nozzle a lot easier, rather than digging out a magnifying glass for the very small writing on the nozzle.

To clean the nozzle, first drill out any plastic with a 2mm drill, do it slowly with a battery drill & don't press too hard, you just want to clean out any ABS/PLA plastic, not drill into the metal Nozzle. Then, either by hand or a small drill, clean out the nozzle tip, blow it all clean - voila - you have a perfectly clean Nozzle. PS - it's sometimes a good idea to run an m6 die over the 6mm threads, if they are really gunked up with plastic.