Extrusion Stops Partway Through a Print (Heat Creep)

Likely causes, most common first

1. Warm enclosure or hot room, especially with PLAMost common
   Bambu Lab says heat buildup inside the enclosure is the most common reason for hotend or extruder clogs on its enclosed printers, and that PLA clogs most because it has the lowest glass transition temperature. Bambu Lab also names TPU, and PETG when the enclosure gets too hot.
2. Bed temperature heating the chamberCommon
   Bambu Lab notes that a high bed temperature passively warms the chamber. Prusa suggests lowering the bed 5 to 10 °C Starting point for PLA.
3. Hotend cooling fan failing, blocked or dustyCommon
   Prusa says to check that the fan is not mounted backward, that it spins at the expected speed, and that dust on the heatsink fins is not reducing cooling.
4. Too little flow through the nozzleOccasional
   Prusa notes slow extrusion lets heat rise, because the filament itself carries heat away. Very thin layers and slow prints move little filament.
5. Nozzle temperature set too highOccasional
   Prusa lists a high hotend temperature as a cause.
6. Metal-filled filamentLess common
   Prusa notes filaments containing metal particles conduct heat up the hotend.
7. Poor heat path in the hotend assemblyLess common
   Prusa lists poor thermal transfer between the nozzle, heatbreak and heatsink, and a PTFE tube that is not seated firmly against the heatbreak.

Fix it, one change at a time

1. Clear the existing jam first, using your printer maker's unclogging procedure. A heat creep jam is a clogged hotend, so the same cleaning steps apply. See Nothing Coming Out of the Nozzle for the safe order, and heat the nozzle only through the printer's menu.
2. Let the heat out. For PLA in an enclosed printer, open the door or remove the top cover, as Prusa and Bambu Lab both advise. Bambu Lab's rule of thumb is to keep the enclosure at least 10 °C below the filament's glass transition temperature Rule of thumb. Move the printer away from heaters and direct sun.
3. Lower the bed temperature to the low end of the filament's range. On Bambu Lab enclosed printers, its lower-temperature plate lets PLA print with the enclosure closed. Prusa warns not to change bed temperature on a running print after the first layer, because the sudden change can deform the part; add a brim if adhesion suffers.
4. Check the hotend fan and heatsink. Switch off and let the printer cool fully, then follow your maker's guide to check that the fan turns freely, faces the right way and that the heatsink fins are free of dust. Prusa cleans them with canned air. On Prusa's MK3-series printers the fan should read about 4000 to 4400 RPM Maker spec in the printer's info menu during a print. Replace a failing fan with the maker's part.
5. Lower the nozzle temperature a few degrees within the filament's printed range.
6. Move more filament. Prusa suggests a thicker layer, such as 0.15 or 0.20 mm Starting point, or about 10% more speed Starting point, when fine detail is not needed.
7. Unload filament at the end of a print while the hotend is still hot, as Prusa recommends, so it does not sit softening in a cooling hotend.
8. Check the hotend assembly if none of the above helps: a loose PTFE tube or missing thermal paste is fixed by following your maker's assembly or replacement guide, or by their support.

Confirm it is fixed

Reprint the job that failed, or a test that runs at least as long as the point where extrusion stopped, with the same filament and the same enclosure setup. Extrusion should stay even from start to finish with no clicking. If it passes with the door open but fails with it closed, keep the door open for that material.

Why it happens

A hotend has a hot end and a cold end on purpose. The heater block and nozzle melt the plastic. Just above them, a thin heatbreak tube and a finned heatsink with its own fan keep the filament solid, so it acts like a piston pushing melted plastic out ahead of it. Bambu Lab describes the goal as a sharp temperature transition between the hot side and the cold side.

Heat creep is that transition moving upward. If the air around the heatsink is warm, the fan is weak, or the plastic moves through slowly, heat climbs the heatbreak. The filament softens in the narrow tube above the melt zone, swells against the walls and sticks. The extruder can no longer push it, so it grinds and clicks. Because it takes time for the heat to build, the print starts perfectly and fails later, which is the signature that separates heat creep from an ordinary clog.

PLA is the usual victim because it softens at the lowest temperature of the common filaments; Bambu Lab ties this to its low glass transition temperature. The same printer that runs ABS happily with the door shut can jam on PLA with the door shut. Prusa notes heat creep is only one way a hotend clogs, and often not the most likely one, so if it fails from the start rather than partway through, look at a plain clog first.