Extruder

If heat sink cooling fan breaks, nozzle will get stuck.
To test for cooling fan effectiveness - pushing filament by hand should encounter norubbery springiness in the filament. If it feels springy while pushing manually - the transition zone is too hot (no cooling fan) and the filament will start to bend - feeling springy instead of stiff. Done on Lulzbot Mini.
Nozzle cleaning - heat up nozzle with heat gun, and poke with acupuncture needles, which are .2 - .1 mm wide. Alternately, dip PLA in acetone overnight, and you won't need heat.

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3D Printer Troubleshooting Log