Heat creep: when filament softens too early in the hotend

This guide is for anyone whose prints stop extruding partway through, especially PLA prints on enclosed printers. It explains what heat creep is, the symptoms and causes printer and filament makers list, the checks Bambu Lab, Prusa, and Creality document for their own printers, and how to tell heat creep from an ordinary clog.

What is heat creep?

A hotend has a hot side, where filament melts, and a cold side above it, where filament should stay solid so the extruder can push it. A heatsink and a hotend fan keep the cold side cool. Bambu Lab says there "should be a sharp temperature transition between the hot side of the hotend and the cold side." Heat creep is what happens when that transition moves upward.

Polymaker describes it as heat traveling "up the heat break" and warming the cold end. Filament there "softens and expands, creating a plug that jams the filament path before the nozzle." Prusa says that when the heatbreak gets too hot, "the filament starts to soften higher inside the hotend and causes a clog."

Low-softening materials are most at risk. Bambu Lab says the problem occurs most often with PLA and TPU, which have a lower glass transition temperature, and that it also applies to PETG "if the enclosure temperature is too high." Bambu calls PLA "the most common filament that gets clogged."

What are the symptoms of heat creep?

• Extrusion stops mid-print, with no error. Prusa says a printer that "suddenly stops extruding filament, yet continues the print moves without reporting any error" may have heat creep.
• A good start, then a clog. Bambu Lab describes a heat-creep failure where the printer "started printing normally," and "after some time, the nozzle developed a partial clog."
• Clicking from the extruder. Prusa says the extruder motor grinds into the filament and can make "a 'clicking' or 'ticking' sound" as it tries to push.
• Under-extrusion and gaps first. Polymaker lists thin or inconsistent lines, gaps, and clicking or grinding among the signs of a partial or full clog.
• Warm rooms and closed enclosures. Bambu says that "in hot summer or hot environments," filament can soften even when the printer's own temperature control is working properly.

Prusa adds an important caveat: heat creep "is only one of the ways by which your hotend can clog," and more often than not "it is not the most probable explanation." Rule out the simpler causes too. Our guide to a printer that stopped extruding mid-print covers them.

Causes

A hotend fan that's failed, blocked, or dusty

The hotend fan (Creality's K2 Pro guide calls it the heat break fan, Prusa the hotend cooling fan) is the main defense. Polymaker lists "fan not running, wrong fan direction, weak airflow, or blocked ducts" as causes. Bambu Lab's example failure happened because the heatsink overheated, "likely due to either a high ambient temperature inside the enclosure or a malfunctioning hotend cooling fan." Prusa says dust on the heatsink fins "decreases heatsink's effectiveness at dissipating the heat."

Creality's K2 guide also names the extruder cooling fan. It says that if that fan isn't spinning, "heat accumulation in the extruder motor will cause the filament to soften inside the extruder, resulting in a clog."

A warm chamber or enclosure with PLA

This is the cause makers write about most. Prusa lists a room above 35 °C (30 °C for some filaments), or a printer in an enclosure, as a cause, and says "printing PLA in a fully closed and unventilated enclosure is very likely to cause problems." Bambu Lab says heat buildup inside the enclosure is "the most common reason for hotend or extruder clogs" on the X1 Carbon and P1S. A hot bed adds to that heat. Bambu recommends keeping the temperature inside an enclosure "at least 10 degrees lower than the glass transition temperature of the filament." Our PLA printing problems guide collects the other PLA-specific fixes.

Long or slow retractions

Polymaker lists "slow retractions with long dwell times" as a cause, because the filament spends longer in the warm transition zone. On Bowden printers, it says long retractions "can pull semi-molten filament into cooler zones, where it solidifies and creates plugs."

Slow printing, thin layers, and high temperatures

Moving filament carries heat away, so low flow lets heat build up. Prusa says thin layers mean "not much filament flowing through the nozzle, which allows the heat to move up inside the extruder," and that "a slow print can cause heat creep for the same reason." Polymaker says that printing too hot can make filament "ooze back up into the heat break, where it cools and forms a plug."

Prusa also lists filament with metal particles, which conduct heat up the hotend, and poor heat transfer between the nozzle, heatbreak, and heatsink.

Checks and fixes by maker

First, clear the existing jam using your printer maker's unclogging procedure for your exact model. Our clog diagnosis guide links to them. Then work through the causes below so it doesn't come back.

Check the fans (all printers)

• Is the hotend fan spinning? Polymaker says to make sure the hotend fan "is always on when the hotend is hot." On the K2, Creality says to check that the extruder cooling fan is spinning once the nozzle reaches 100 °C. For the K2 Pro, Creality says the heat break fan turns on when the nozzle starts heating or goes above 50 °C.
• Is it blowing the right way? Polymaker says to verify airflow direction through the heatsink. On its MK2.5 and MK3-family printers, Prusa says you shouldn't see the sticker on the fan, because that means it's mounted backward. The fan "must push air inside, not outside."
• Is it fast enough? On its MK2.5 and MK3-family printers, Prusa says to check the fan speed under LCD Menu > Support > Extruder info > Fan speed during a print, where the nozzle fan should read 4000–4400 RPM. Other printers differ, so check your maker's documentation.
• Is it clean? On the same printers, Prusa suggests removing the hotend fan and blowing dust off the heatsink and fan with canned air. Creality's K2 Pro fan guide says to check for foreign objects and clean them out, retighten loose fan screws, and contact customer service if the fan is broken or its wires are interfering.

Bambu Lab recommends checking "the printer's heating elements and cooling system" regularly to prevent heat creep caused by equipment failure.

Keep the chamber cool for PLA: Bambu Lab

Bambu Lab's guidance depends on the printer and the plate:

• X1 series: With the Cool Plate, which Bambu says works for PLA at a bed temperature of about 35 °C, you can print PLA with the front door and top cover closed. With the Textured PEI or High-Temperature PEI plate, Bambu recommends printing PLA "with the front door open and/or the top cover removed."
• P1S: It ships with a Textured PEI plate, which Bambu says needs a 55 °C bed for PLA. Bambu recommends the front door open and/or the top cover removed on that plate, and on aftermarket plates needing more than 50 °C. To print with the door and cover closed, it recommends the Cool Plate at 35 °C. The chamber fan turns on for PLA when the bed is set above 50 °C, but Bambu says that in a warm room it "might not be able to keep the temperature inside the chamber low enough."
• P1P: Bambu says the open-frame P1P won't need changes for PLA, TPU, or PETG. If you enclose a P1P, with Bambu's P1S upgrade kit or another enclosure, follow the P1S advice.

Bambu's table also marks PLA, TPU, and PVA as materials to print with the upper glass cover removed, and PETG, ABS, ASA, PC, and nylon as materials to print with it on. In a very cold room (5–10 °C), Bambu says you can close the door and cover for low-softening filaments. If clogs still happen, open the front door slightly to let hot air out.

Keep the chamber cool for PLA: Prusa

On the CORE One and CORE One INDX, Prusa says having the chamber grille open "is important for the chamber ventilation, since low-temperature filaments might clog the nozzle and cause stringing." The printer shows a reminder (warning #31837 on the CORE One) when you start a low-temperature filament after a print that needed the grille closed. Prusa notes that the message "does not sense if the grille is open or not," so check it yourself.

For its MK2.5 and MK3-family printers, Prusa suggests lowering the bed temperature for PLA by 5 or 10 degrees, since heat rising from the bed can warm the extruder. If adhesion suffers, it suggests a brim. It also suggests unloading the filament while the hotend is still hot at the end of a print.

Keep the chamber cool for PLA: Creality

For the K2, Creality says to check that the chamber temperature is within 37 °C when printing PLA, because "excessive chamber temperature will cause the filament to soften prematurely in the extruder." Its K2 Pro fan guide says the exhaust fan runs at full speed when the chamber reaches 35 °C. When the chamber setpoint is below 40 °C, mainly for PLA, chamber heating is turned off "to prevent nozzle clogging." We didn't find equivalent PLA chamber guidance for the K1 series in Creality's wiki, so check your model's manual.

Adjust retraction and flow

• Shorten retractions. Polymaker suggests reducing retraction distance on direct-drive printers, avoiding excessive retraction speeds, and tuning retraction to your hotend.
• Use thicker layers or a little more speed. On its MK2.5 and MK3-family printers, Prusa suggests testing 0.15 or 0.20 mm layers, and raising print speed by about 10%. It says not to exceed 200 mm/s infill speed at layers of 0.20 mm and below, and to test speed changes with the LCD knob first.
• Don't print hotter than you need. Polymaker suggests lowering the nozzle temperature in 5–10 °C steps within the filament's range if you see burnt material, stringing, or fumes.

Don't raise the temperature to fix it

Raising the nozzle temperature is a common fix for a clicking extruder, but not for heat creep. Polymaker says: "If you are experiencing a clog in your barrel due to heat creep, increasing your extrusion temperature will only make the problem worse." It only suggests raising temperature when there's no heat creep and the barrel "remains close to room temperature."

If the nozzle temperature reading itself swings up and down during a print, that's a different problem. See our guide to a fluctuating nozzle temperature.

Heat creep or a full clog?

Heat creep produces a clog, so the two overlap. The difference is where the jam forms and why it keeps coming back.

Polymaker places heat-creep jams "before the nozzle," in the cold end. It describes other clogs differently, such as burnt residue that "sticks inside the nozzle" when filament sits in a hot nozzle too long. Both are cleared with your maker's procedure. Our clog diagnosis guide covers cold pulls and model-specific steps. If the clog keeps returning on long PLA prints after a clean, look at heat creep's causes rather than replacing the nozzle.

A sensible order

1. Clear the jam with your printer maker's unclogging procedure.
2. Watch the hotend fan while the hotend heats. Check that it spins, blows the right way, and isn't clogged with dust.
3. For PLA or TPU on an enclosed printer, open the door, lid, or vent your maker specifies, or use the cooler plate it recommends.
4. Move the printer out of a hot room or direct sun if you can.
5. Shorten retractions, and avoid very thin layers at very slow speeds.
6. Use the filament maker's temperature, not hotter.
7. If clogs continue with the fan working and the chamber cool, follow the clog diagnosis guide for other causes.

When not to buy parts

Hold off on a new hotend, nozzle, or extruder if:

• The clogs only happen with PLA on a closed printer. Opening the door or lid, as your maker documents, costs nothing.
• The hotend fan is dusty. Clean it first.
• You've raised retraction or slowed prints way down recently. Undo those changes and test.

Replacement parts make sense when your maker's checks point to them, such as a hotend fan that doesn't spin or a damaged extruder. On the K2, Creality says to troubleshoot or replace an extruder cooling fan that isn't spinning. Its K2 Pro guide says to contact customer service if a fan is broken. Buy parts that match your exact printer.

Nozzles: what to check before you buy ↗ PLA: what to check before you buy a spool ↗