Nozzle clogs or stops extruding mid-print (heat creep)

Symptom

The printer keeps moving but little or no plastic comes out, often after printing fine for a while. The extruder may click or grind, lines come out thin or patchy, filament curls around the tip, or it won't load at all.

Likely causes, from most to least likely

1. Heat creep from a hot environment: with PLA or other low-temperature materials in an enclosed printer or a very warm room, heat travels up the hotend, the filament softens above the melt zone and forms a plug. Heat rising from the bed also contributes.
2. Poor heatsink cooling: hotend fan stopped, slow, badly oriented or blocked, or fins clogged with dust.
3. Low flow for long periods: with very thin layers or very slow printing the filament moves slowly and the heat has time to creep up.
4. Dirt, impurities, or wet or degraded filament blocking the nozzle.
5. Wrong retraction settings in the slicer, for example longer than your printer manufacturer recommends.
6. Hotend assembly: PTFE tube not seated, missing thermal paste on the heatbreak thread, or a loose nozzle. Metal-filled filaments also play a part, as they conduct heat upwards.

Checks to tell which one it is

• If the print starts fine and stops extruding after a while with PLA and the door or lid closed, or with the room above about 35 °C (30 °C for some filaments, according to Prusa), suspect heat creep.
• With the hotend hot, check that the heatsink fan spins at its normal speed and air flows freely; with the printer off and cold, look for dust in the fins.
• Check layer height, speed and retraction in the slicer and compare them with the manufacturer's profile.
• Heat the nozzle from the menu and push filament by hand (opening the extruder idler if needed): if nothing or very little comes out, there is a clog.
• A loud click from the extruder points to a clogged or deformed nozzle; a light click points more to the gears.

Before you start

Before taking anything apart, cleaning or adjusting it, switch the printer off and unplug it, unless a step says otherwise, and do not touch the nozzle, the heater block or the bed until they have cooled down. If in any doubt about electricity, heat or moving parts, stop and ask a professional.

Step-by-step fix

1. Push fresh filament through a hot nozzle

   Heat the nozzle only from the printer menu (never with an external heat source), to the material's printing temperature or the one your manufacturer gives for unclogging. Unload the filament and push fresh filament by hand until it comes out clean. Don't touch the hot hotend and wear gloves in case plastic suddenly shoots out.

   Check: Filament comes out straight, steady and at its normal thickness.

   If that does not fix it: Clean the nozzle with a thin needle.

2. Clean the nozzle with a thin needle

   With the nozzle hot, insert a needle or wire thinner than the orifice from below (for a 0.4 mm nozzle Prusa uses 0.3-0.35 mm) up to 1-2 cm, pull it out and push filament again. Repeat several times. Don't do this on high-flow nozzles: their internal geometry doesn't allow it.

   Check: Filament flows normally again.

   If that does not fix it: Do a cold pull.

3. Do a cold pull

   Using the temperatures your printer manufacturer gives, push filament through the hot nozzle (PLA works best), let it cool and, at the stated pull temperature, pull it out firmly, slowly and straight up. Repeat until the tip comes out clean. If your printer has an automatic cold pull, use it.

   Check: The extracted tip shows the shape of the nozzle interior, with no dark residue or other materials.

   If that does not fix it: If flow doesn't come back, replace the nozzle or contact technical support: the plug may be higher up, in the heatbreak or the PTFE tube.

4. Take heat away from the hotend when printing PLA

   With PLA, open the printer door or top lid (Prusa always recommends this for PLA on its enclosed models) and avoid very warm rooms. If adhesion allows, lower the bed by 5-10 °C: its heat also rises towards the extruder.

   Check: Long PLA prints finish without extrusion dropouts.

   If that does not fix it: Check the heatsink fan.

5. Check the heatsink fan

   Switch off and unplug the printer and let it cool. Blow dust out of the heatsink fins and fan with compressed air and check that the fan is correctly oriented and spins freely. When the hotend heats up, it should start at its normal speed.

   Check: The fan spins strongly and air passes through the fins unobstructed.

   If that does not fix it: If the fan is slow, noisy or stops, replace it with the manufacturer's spare before printing again.

6. Adjust the slicer so filament doesn't sit in the heat

   If you don't need that much detail, avoid very thin layers (Prusa suggests trying 0.15 or 0.20 mm) or raise speed by around 10%. Make sure retraction isn't longer than the manufacturer recommends and use the profile for your nozzle type (for example, a high-flow profile only with a high-flow nozzle).

   Check: Parts that used to clog now finish with steady extrusion.

   If that does not fix it: Check the hotend assembly (PTFE tube seated, thermal paste on the heatbreak, nozzle tightened while hot) using the manufacturer's guide, or ask for technical support.

Prevention

• When a print finishes, unload the filament while the hotend is still hot.
• On enclosed printers, print PLA with the door or lid open.
• Keep the heatsink and its fan clean.
• Use good-quality, dry filament and the slicer profile that matches your nozzle.

Still not fixed?

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Printers it applies to

Any FDM (filament) 3D printer, from any brand.

Official sources

This guide summarises the official documentation linked here:

• Extrusion stopped mid-print (Heat creep) · Prusa Research
• Clogged nozzle (CORE One, CORE One L, XL, MK4/S, MK3.9/S) · Prusa Research
• Cold pull (MK3S/MK2.5S) · Prusa Research
• E3D Classics Support: V6 HotEnd · E3D