A bead of plastic appears where the nozzle meets the heater block. Over a few prints it grows into a dark, burnt mass that engulfs the nozzle, drips onto the part, and eventually swallows the silicone sock.
Where the seal actually is
This is the piece of information that makes the whole fault make sense: the joint that keeps plastic in is not the nozzle's threads.
Inside the block, the nozzle screws up towards the bottom of the heatbreak. The seal is made where those two components meet face to face, metal on metal, clamped together by the nozzle being tightened against the heatbreak. The threads locate the parts; they do not seal them.
So a leak means those two faces are not pressed together hard enough at printing temperature — either because the nozzle was not tightened against the heatbreak, or because one of the faces is damaged, or because thermal expansion has opened a gap that was only just closed when cold.
Why hot tightening is not optional
Steel, brass and aluminium all expand when heated, by different amounts. A joint tightened cold slackens as the assembly comes up to temperature. That is why every hotend manufacturer specifies tightening at temperature, and why a machine tightened cold leaks on its first hot print.
The procedure, with the parts that matter:
- Back the nozzle off a fraction of a turn while cold, so it is not seized.
- Heat the hotend to the highest temperature you print at.
- Hold the heater block with a spanner and tighten the nozzle with a second one. Holding the block is not optional — without it the torque goes into the heatbreak and the mounting, and into the heater and thermistor wires, which is how a maintenance job becomes an electrical fault.
- Tighten firmly, not violently. Roughly the force you would use on a small bolt: enough to feel the joint seat, well short of what it takes to strip brass threads or snap a heatbreak.
- Reassemble the sock and check the wires before printing.
This is done on a block at printing temperature and burns are the routine hazard here. Use tools with insulated handles, work with the toolhead where you can reach it without leaning over a hot bed, and accept that this is a two-spanner job rather than a one-hand job.
The other causes
A heatbreak that is not fully seated. If the heatbreak is not threaded far enough into the block, the nozzle can bottom out on the block's threads before it ever reaches the heatbreak's face. The joint is then tight and not sealed, which feels correct and leaks anyway.
A damaged sealing face. Repeated cold tightening, or grit trapped at the joint, scores the face. Once scored, it will not seal at any torque, and the nozzle is scrap.
Thermal cycling on a joint that was marginal. A hotend tightened once and never re-checked will eventually loosen. It is worth re-tightening at temperature after the first few prints following any hotend work.
What happens if it is left
The leak is not self-limiting. Molten plastic escaping into the block cavity cooks against the heater, so the mass grows and darkens. Three things follow.
It insulates the block, so the thermistor's reading no longer represents the melt and print quality drifts for no visible reason. It fouls the silicone sock and the part-cooling airflow. And eventually a lump breaks free and lands on the print — or the mass grips the thermistor so firmly that removing it later damages the sensor, which is exactly the sequence described on thermal runaway.
Deal with it at the bead stage rather than the blob stage.
Cleaning up properly
Heat the block, peel and pick off as much as will come away while soft, then let it cool and remove the remainder with a brass brush. Do not gouge at the block with a steel screwdriver, and do not attempt to clean around the thermistor with anything sharp.
If the mass is large, the honest answer is to dismantle: remove the nozzle and heatbreak, clean each part separately, inspect both sealing faces, and reassemble hot. It takes half an hour and it is the only way to be sure the joint underneath is sound.
When it recurs
A leak that comes back after a correct hot tighten means a damaged face or a worn thread. Replace the nozzle first, since it is the cheap part; if it recurs again, replace the heatbreak. Chasing it with more torque destroys the block, which is the expensive part, and high-temperature materials make everything on this page happen sooner because they run the joint hotter and softer.