Hardware & Maintenance

Heat creep: why your 3D printer stops extruding mid-print

Heat creep is when heat travels up past the heat break into the cold side of the hotend, softening the filament too early. It swells, sticks to the walls and jams, so the extruder can't push it. The printer keeps moving with no error, the temperature reads normal, and nothing comes out. The usual fix is better hotend cooling, a cooler chamber for PLA, and shorter retractions.

8 min read Updated October 2026 Intermediate Hardware & Maintenance · 3 of 6

Quick fix

Check the hotend fan (the one cooling the heatsink, not the part fan) spins whenever the nozzle is hot, and blow dust out of the heatsink fins.
Printing PLA in an enclosure? Open the door or lift the lid, and drop the bed temperature 5–10 °C.
Shorten retraction: about 0.5–1 mm on direct drive; on Bowden, the least that stops stringing.
Clear the jam: heat, unload, cut off the swollen end, and do a cold pull if needed.
Check the hotend build: PTFE tube seated with no gap, nozzle and heat break tightened as the maker specifies.

What is heat creep?

A hotend has three zones. The heater block and nozzle melt the plastic. Above that, the heat break is a thin tube that blocks heat from travelling upward. Above that, the heatsink, cooled by the hotend fan, keeps the filament solid so it works like a piston.

When the heatsink can't shed heat fast enough, the soft zone climbs into the heat break. Softened filament swells against the walls and grips them. The extruder can't push past it, and retractions drag more soft plastic up into the cold zone, making it worse. The thermistor sits in the heater block, so the temperature reads perfectly normal the whole time.

Heat creep symptoms (and how it differs from a clog)

Sign	Heat creep	Nozzle clog
When it fails	Starts fine, stops partway through	Usually from the start
After cooling down	Often extrudes again for a while	Still blocked
Unloaded filament	Swollen bulb or "mushroom" on the end	Normal end, maybe dark residue
Most common with	PLA, warm rooms, enclosures, slow or small prints	Dirty or wet filament, abrasives, debris
Extruder	Clicks or grinds once it jams	Clicks or grinds

Prusa notes that heat creep is only one way a hotend clogs, and often not the most likely one. If the symptoms don't match, start with the clogged nozzle guide.

Cause 1: Hotend fan not cooling the heatsink

The hotend fan must run whenever the nozzle is hot, whatever the part-cooling fan is doing. Klipper's heater_fan section, for example, turns it on whenever the extruder heater is on, and by default keeps it running until the hotend drops below 50 °C.

Check it spins at full speed and blows air into the heatsink. Prusa points out that a fan mounted backwards (sticker facing out) can't cool it.
Clean the fins. Dust and filament fluff build up over time; remove the fan and blow it out with compressed air.
Replace a noisy or slowing fan with one of the same voltage, size and airflow. A quieter fan with less airflow can bring the problem back.
Heed fan errors. Most current printers, including Bambu Lab and Prusa models, monitor fan speed and stop with an error if the hotend fan fails. Don't bypass it.
Keep cables and ducts clear of the fan intake and the heatsink.

Cause 2: Hot room or enclosure (PLA's weak spot)

The heatsink can only cool to the temperature of the air around it. PLA softens at a low temperature; Bambu Lab puts most PLA's softening point at around 45 °C. So in a hot chamber, a working fan still can't keep the filament stiff.

Prusa lists rooms above 35 °C (30 °C for some filaments) and fully closed, unventilated enclosures as the most common causes.
Bambu Lab recommends opening the door or removing the top cover for PLA, unless you're printing on a cool plate at about 35 °C. It also says to keep chamber heating off for PLA and PETG.
Lower the bed temperature 5–10 °C for PLA (Prusa's tip); a 60 °C bed in a closed box heats the whole chamber. If adhesion suffers, add a brim.
PETG, ABS and ASA soften at higher temperatures, so they're much less prone. ABS and ASA actually want a warm enclosure. See the PLA guide and ABS and ASA guide.

Cause 3: Too much retraction

Every retraction pulls soft plastic from the melt zone up into the heat break, where it cools and sticks. Long or frequent retractions build that up over a print.

Direct drive: Ellis' guide starts at 0.5 mm and suggests staying under 1 mm; 2 mm is an absolute ceiling.
Bowden: start around 1 mm and increase only as far as stringing needs. Longer tubes need more. Tuned pressure (linear) advance usually lets you retract less.
Lots of tiny islands? Many short travels mean many retractions. Enable your slicer's "avoid crossing walls" or combing options to cut the count.

For stringing that tempts you to retract more, check stringing and oozing first. Wet filament and high temperature are often the real cause.

Cause 4: Low flow from slow, small or thin-layer prints

Moving filament carries heat out of the hotend. When very little is moving, heat soaks upward. Prusa lists "not enough filament flowing through the nozzle" as a cause, with these fixes:

Use thicker layers where detail allows (0.15–0.20 mm instead of 0.05–0.10 mm).
Print a little faster, about 10%, rather than very slowly.
Don't print hotter than needed. Stay within the filament's range (PLA 200–220 °C).
Watch long ironing passes and tiny parts, both of which move very little plastic for a long time.

Free PrintPal toolVolumetric Flow CalculatorSee how much plastic per second your settings move; very low flow on long prints is a heat creep riskOpen tool

Cause 5: Hotend assembly, PTFE-lined vs all-metal

Hotends come in two broad types, and they fail in different ways.

	PTFE-lined	All-metal
Design	A PTFE tube runs down through the heat break to the nozzle	Metal heat break; no plastic in the hot path
Examples	Many budget Ender-style hotends, E3D Lite6	E3D V6 and Revo, most current Bambu Lab and Prusa hotends
Temperature limit	About 240 °C (E3D rates the Lite6 to 240 °C)	Set by the heater, thermistor and maker's rating; commonly around 300 °C
With PLA	Forgiving: PTFE is slippery, so soft plastic grips less	More sensitive: keep cooling strong and retraction short
Typical failure	Liner wears, shrinks or gaps at the nozzle; plastic leaks into the gap and jams	Poor heat break seating or contact; jams higher in the heat break

PTFE and high temperatures: PTFE starts to break down above about 250 °C, and its fumes are especially dangerous to pet birds. On a PTFE-lined hotend, stay at or below about 240 °C, and upgrade to an all-metal hotend for nylon or PC. More in the 3D printing safety guide.

PTFE-lined: replace the liner when it looks brown, squashed or worn at the nozzle end. Cut it perfectly square with a tube cutter and seat it firmly against the nozzle.
All-metal: tighten the heat break and nozzle as the maker specifies, and use thermal paste only where the design calls for it. On E3D V6-style hotends, Prusa notes there should be about a 0.5 mm gap between the nozzle hex and the heater block.
Metal-filled filaments conduct heat up the hotend, so Prusa lists them as a heat creep risk too.

How to clear a heat creep jam

Cancel the print rather than letting it run with no extrusion.
Heat to the filament's normal printing temperature (10 °C above is fine, staying at or below 240 °C on PTFE-lined hotends).
Unload the filament. If the end has a bulb or mushroom shape, cut it off cleanly above the swollen part before reloading.
If it won't unload, do a cold pull or follow the clogged nozzle steps for a full jam.
Fix the cause (fan, chamber, retraction) before printing again, or it will jam again.

You're printing PLA in a closed enclosure on a 60 °C bed, and extrusion stops after about an hour. What's the best first fix?

Raise the nozzle temperature 10 °C so the plastic flows better
Open the door or lid and lower the bed temperature a little
Increase retraction to relieve pressure in the nozzle

The warm chamber stops the heatsink cooling the filament. Venting the enclosure and lowering the bed temperature tackle that directly. A hotter nozzle and more retraction both make heat creep worse.

How to prevent heat creep

Hotend fan spins at full speed whenever the nozzle is hot.
Heatsink fins are clean. Blow them out during routine maintenance.
Chamber kept cool for PLA: door open or lid off, bed toward the low end of 55–65 °C.
Retraction kept short, tuned after pressure advance.
No very slow, thin-layer marathons unless the detail needs it.
PTFE liner and tubes seated with no gaps, and replaced when worn.
Filament unloaded while hot after the print, as Prusa recommends.

Frequently asked questions

What causes heat creep in a 3D printer?

Heat travelling up past the heat break into the cold side of the hotend. The usual causes are a weak or dirty hotend fan, a hot room or closed enclosure (especially with PLA), too much retraction, very slow or thin-layer printing, and a poorly assembled hotend.

Is heat creep the same as a clogged nozzle?

It causes a jam, but in a different place. A nozzle clog is debris at the tip and usually shows from the start of a print. Heat creep softens filament higher up, so prints start fine, then stop extruding partway through, and the unloaded filament often has a swollen bulb on the end.

Can I print PLA in an enclosed 3D printer?

Yes, but keep the chamber cool. Prusa lists rooms above 35 °C (30 °C for some filaments) and closed enclosures as common causes of heat creep. Bambu Lab suggests opening the door or removing the top cover for PLA, unless you print on a cool plate at about 35 °C.

Do all-metal hotends cause heat creep with PLA?

They're more sensitive to it, because there's no slippery PTFE liner in the heat break. With good hotend cooling and short retractions, all-metal hotends print PLA reliably; most current printers from Bambu Lab and Prusa use them.

Clogged nozzle Under-extrusion 3D printer calibration guide Maintenance schedule