Heat creep is when filament softens above the melt zone — in the cool section of the heat break that should stay rigid. A soft plug of filament forms, the extruder can't push past it, and the print quietly stops extruding with no error message. The hotend is still hot, the printer is still moving — just nothing's coming out.
Verify the hotend cooling fan is actually spinning at full speed during print. (It's the small fan blowing on the heat break, not the part-cooling fan blowing on the print.) Then reduce retraction distance — over-retraction is the #1 software cause. If you have a Bowden printer, replace the PTFE tube. If your printer is in a warm enclosure printing PLA, vent the enclosure or print outside it.
A healthy hotend has three distinct thermal zones:
When heat creeps up the heat break into the cold zone, filament starts to soften 10–20 mm above where it should. The softened filament:
End result: extrusion stops. No error. The hotend reads fine because the thermistor is still in the melt zone (correctly hot).
The #1 cause. Verify visually that the fan spins at 100% throughout the print.
If the cold zone is at 50 °C and PLA softens at 60 °C, you only have 10 °C of headroom. In a warm room or an enclosed printer, that headroom disappears.
Long retractions pull warm plastic from the melt zone up into the heat break, where it cools and forms a plug. Especially bad on Bowden printers running 6–8 mm retraction.
PTFE liners in the hotend cold zone wear and gap over time. Once there's a gap between the liner and the heat break, melted plastic seeps into it and acts as an insulator — reducing the cold-zone cooling.
Even with a working fan, airflow can be obstructed:
If extrusion has already stopped: