Heat creep — why extrusion stops mid-print

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.

How heat creep happens

A healthy hotend has three distinct thermal zones:

• Cold zone — the top of the heat break + the heatsink above. Held at ambient (max 50 °C) by the hotend fan. Filament here is solid.
• Heat break — a thin metal tube that transitions sharply from cold to hot. About 5 mm tall in a V6, 3 mm in a Bambu hotend.
• Melt zone — the heater block + nozzle. Filament melts here, gets pushed out the nozzle.

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:

1. Expands radially against the inside of the heat break (forming a plug).
2. Can't be pushed past the plug by the extruder.
3. Can't retract because retraction pulls the plug deeper, jamming it tighter.

End result: extrusion stops. No error. The hotend reads fine because the thermistor is still in the melt zone (correctly hot).

Cause 1: Hotend cooling fan failure

The #1 cause. Verify visually that the fan spins at 100% throughout the print.

• Hotend fan should run anytime the hotend is above ~50 °C, regardless of part-cooling.
• Common failure modes: clogged with PLA shavings (clean with compressed air), bearing dry (replace fan), wire pulled loose (re-seat connector).
• Stock fans on cheap printers die quickly — budget $5 for a Noctua / Sunon replacement after the warranty expires.
• On Bambu / Prusa, the firmware reports fan RPM. Look for Err: EXTR. FAN ERROR — that's heat creep waiting to happen.

Cause 2: Ambient temperature too high

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.

• Don't print PLA in a closed enclosure at chamber temps above ~30 °C. Vent the door or take the side panels off.
• PETG and ABS are less prone to heat creep because they soften at higher temps; ABS prints fine at 55 °C chamber.
• Summer / hot rooms: if room is above 28 °C, expect more heat creep. Cool the room or move the printer.

Cause 3: Over-retraction

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.

• Direct drive: keep retraction at 0.4–1.0 mm. Anything beyond ~1.5 mm risks heat creep.
• Bowden: 4–6 mm is enough. The Ender 3 community advice of "more retraction" caused millions of heat-creep clogs.
• Long print durations: if heat creep only appears on prints over 2 hours, the issue may be a slow-accumulating plug. Drop retraction by 0.5 mm.

Cause 4: PTFE tube degradation (Bowden / hybrid)

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.

• Replace PTFE every 6–12 months on Ender 3 / CR-10 / Voxelab class machines.
• "All-metal" hotends (no PTFE liner in the heat break) can do higher temperatures but are more sensitive to heat creep on PLA.
• Cut PTFE perfectly square (use a proper PTFE cutter, not scissors). A 1 ° angle creates a gap that creeps.

Cause 5: Heatsink airflow blocked

Even with a working fan, airflow can be obstructed:

• Silicone sock wraps that block fins (rare).
• Cable management blocking the fan intake or exhaust.
• Dust accumulation on the heatsink. Vacuum with a brush attachment.
• Wrong heatsink orientation after a rebuild — fins should be vertical, not horizontal.

Recovering from a heat creep clog

If extrusion has already stopped:

1. Cancel the print. Don't let it keep running with no extrusion.
2. Heat hotend to 250 °C.
3. Try unloading filament normally. If it comes out with a teardrop or mushroom shape at the top — classic heat creep plug — cut it off cleanly above the deformed section before reloading.
4. If unloading fails: remove the hotend assembly and do a proper cold pull (see Clogged nozzle).
5. Address the root cause before printing again or it'll happen again within a few hours.

Related articles