Nothing comes out. Or a thin, curled ribbon emerges and wraps itself around the outside of the nozzle instead of going down onto the print.
Before dismantling anything, note that a full blockage and a partial one behave differently: a partial clog produces under-extrusion that comes and goes, while a full one stops flow dead and usually makes the extruder click or grind as it pushes against it.
What the plug is actually made of
Three different things, and they call for different work.
Cooked plastic. Polymer left in a hot melt zone slowly darkens, cross-links and stops flowing. It is the residue of the last material at a temperature above what that material tolerated, and it is why the classic clog appears when you switch from a high-temperature filament back down to a low-temperature one: the leftovers never melt again at the new setting.
Something solid that will not pass. A dust particle drawn in on the filament, a fragment of carbonised material shed from the block, or an abrasive lump in a filled filament. A 0.4 mm orifice is a small hole; visible dust is comparable in size.
Filament softened in the wrong place. If the section above the melt zone gets warm, the filament swells there and locks itself into the path. That is not really a clog at all and clearing the nozzle will not stop it coming back — it is heat creep, and it has its own diagnosis.
Triage before dismantling
Heat to the material's normal temperature, remove the filament, and push a fresh piece through by hand.
- Comes out easily, straight down: not a blockage. Look at the extruder.
- Comes out but curls sharply to one side: the orifice is partly obstructed or the tip is damaged. A curl means the plastic is meeting an asymmetric restriction on the way out.
- Will not move at all under firm hand pressure: a genuine full blockage.
The cold pull
This works because a semi-crystalline plastic gripped in a cooling nozzle grabs the debris around it and takes it with it. Nylon or purpose-made cleaning filament is used because both stay tough at the pull temperature; PLA works acceptably and tends to snap.
- Heat the hotend to the cleaning material's normal printing temperature and push it through until the extrusion runs clean-coloured.
- Let the temperature fall to roughly 90 °C for PLA, or 120 °C for nylon. The filament must be solid but still slightly soft.
- Pull firmly and steadily, straight up. Release the extruder idler first so you are pulling by hand, not through the drive gear.
- Inspect the tip that comes out. A clean cone with a moulded impression of the nozzle bore means it worked; a stub with black flecks means repeat it.
Three or four pulls is normal. The block is at printing temperature during step one — burns here are common and unpleasant, so move the toolhead somewhere you can reach it without leaning over the bed.
The liner limit that causes repeat clogs
Many hotends have a short PTFE tube running down to meet the top of the nozzle. That design is fine within its limits, and outside them it is the usual explanation for a printer that blocks every time a high-temperature filament is loaded.
Treat 240 to 260 °C as a band, not a line — this page is where that band is defined and the rest of the site defers to it. PTFE's published continuous service limit is 260 °C, and it is a genuine limit: measurable fume release begins around there, though the decomposition associated with polymer fume fever needs closer to 350 °C. Inside a hot zone the polymer is treated far more harshly than a datasheet assumes — clamped against the melt chamber, held at temperature for the whole print, and abraded by filament dragged through it — so liners shorten and discolour in service from around 240 °C, well below the number on the material's own sheet.
That is why quoted figures differ, and it is worth knowing which one you are reading. A datasheet ceiling, a fume threshold and a printer owner's practical experience are three measurements of three different things, and none of them is the others rounded off. For deciding whether your machine can print a material, the low end of the band is the one to work to.
The failure it produces is mechanical before it is chemical. As the tube shortens, a gap opens between the liner and the top of the nozzle; molten plastic fills it, cooks, and eventually blocks the path. If you print PETG hot, polycarbonate, ASA or any polyamide, the machine wants an all-metal hotend rather than a replacement liner every fortnight.
Check which one you have before assuming a temperature is safe. It is printed in the machine's specification as a maximum hotend temperature, and a stated ceiling around 250 °C nearly always means a lined hotend.
Stopping it happening again
- Purge properly when changing materials downward in temperature. Coming from a polyamide to PLA, run the higher material's temperature for the purge, not the lower one.
- Fit a dust filter — a sponge clipped over the filament path. Free, and it removes the whole second category above.
- Use a hardened nozzle for anything filled. Carbon fibre, glass fibre, wood flour and metal powder all cut brass, and a nozzle whose orifice has been eroded out of round both clogs and prints badly.
- Dry hygroscopic filament. Wet nylon and wet PVA foam in the melt zone and leave residue behind that later carbonises.
- Do not leave a hot nozzle loaded and idle. Material sitting still at temperature is material being slowly cooked; see jam after a pause.
On the question of poking a cleaning needle through the orifice, opinion is genuinely divided. A correctly sized needle used gently from the outside is standard practice in some workshops; others avoid it because a steel wire in a brass bore can score the orifice into an oval that never prints cleanly again. If a nozzle has already resisted several cold pulls, replacing it costs less than the evening you are about to spend on it.