Hotend

Its job is more delicate than it looks: it has to melt material over a very short distance and, just above that, keep it solid so the filament can push itself like a piston. That boundary is held by the heatbreak, cooled by a heatsink and a fan. When that cooling fails, heat creeps up the tube, the filament softens too early and a 'heat creep' clog forms, one of the most common reasons extrusion stops mid-print. The hotend's maximum temperature decides which materials you can use: those with a PTFE tube inside should not go beyond around 250 °C.