Heat Creep
Fix extrusion failures caused by filament softening too high inside the hot end.
Heat creep occurs when heat travels upward from the heater block into the normally cool section of the hot end.
Filament softens too early, expands and becomes difficult for the extruder to push.
A typical sign is a printer that starts normally but begins under-extruding or stops extruding after several minutes.
Check that the hot-end cooling fan is working properly. This fan is different from the part-cooling fan and normally runs whenever the hot end is hot.
Make sure the heatsink and fan are not blocked with dust.
Printing PLA at unusually high temperatures can make heat creep more likely.
Excessive retraction can repeatedly pull hot filament upward into the cooler section of the hot end.
If the printer is inside an enclosure, very high chamber temperatures can also contribute, particularly with PLA.
Allow the printer to cool, remove the filament and inspect it. A swollen section near the end of the filament can indicate heat creep.
Common causes: failed hot-end fan, excessive temperature, excessive retraction, blocked heatsink, high enclosure temperature.