Why does filament get stuck in extruder like this?

Why does filament get stuck in extruder like this?

It looks as if the heat crept up and the soft filament then filled up any nook it could get into. This is a rather typical case for a jam, though more extreme than most.
Chances are that the cooling just didn't work.

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A couple of strategies:
-insulate the hotend both to keep heat in which maintains a stable temperature and to keep heat from rising up the heatbreak with fibreglass mat, kapton tape or similar
-put thermal transfer compound for a pc CPU heatsink on the threads of the heatbreak between heatbreak and heatsink (NOT between the heatbreak and the heater block).
-use a sufficiently large and powerful fan, I prefer 40mm to 30mm or smaller which only work with a very tight fitting fan shroud such as e3d.
-ensure your filament isn't too wide or had too much variation in diameter
-avoid low ambient air temperatures, when my printers were outside in the cold garage I had 3 nozzle jams in 2 days now they are indoors and no issues since.
-to avoid a complete strip down and clean heat extruder set extruder 10mm then quickly pull the filament out. Then reload and try again, this sometimes clears a mild jam.
-if using abs and its totally jammed dismantle the hotend and soak everything in pure acetone for a day or two.
-PLA jams more than abs

Edited 2 time(s). Last edit at 01/09/2016 04:23AM by DjDemonD.

Nozzle assemblies have zones Hot, Warm and Cold

Upon my Ormerod 2 there is the hot end block, just above that, is a heat sink cooling block to which a 40cm fan attaches.
When the printer is powered on, this fan runs.

One day I started up the printer and turned on the hot end so able load filament, started a print but there was no extrusion, the nozzle was blocked.
I noticed that the fan had stopped turning which caused filament within the length above the hot end to melt above the cooling block, so when got the fan started again.
The filament then become solid again and the heat was not able to get back up it's previous point so couldn't unblock it.

Ended up removing hot end and heating the nozzle with a hot air gun, which in a short time unblocked the nozzle.
One thing I noticed about the bit I pulled out, it had an expanded solid top with an "99 ice cream like" appearance on the bottom.

Filament near the cooling block is cooler than when it's at the hot end.
Filament used was PLA, the best way to fix. Is to remove hot-end from printer and heat with a hot air gun.

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Printer: Ormerod 2 (528.4) Duel extruder set-up with Aluminium X-Rib, RRPro Firmware v1.11-ch (2016-04-08)

With PLA, heat creep back up the thermal break can also be caused by printing too slowly, or if that is an all metal hotend then it may just be insufficient cooling (heatsink fan not working). You would need to give us more information about the hotend, and the circumstances that caused the blockage.