Diagnose by symptom
- Nothing comes out at all: full clog or heat creep
- Comes and goes, sometimes during long prints: partial clog or PTFE gap
- Extruder grinds but no filament: the blockage is upstream, or heat creep softened the filament before the melt zone (extruder feed checks)
- Under-extrusion that comes and goes: partial clog
- Stringing and blobs at the same time: worn nozzle (enlarged orifice)
The cold pull test (works on most FDM printers)
Heat to printing temperature, let the hotend cool to about 90°C for PLA (120–140°C for PETG/ABS), then yank the filament out firmly with pliers — the semi-solid plug pulls debris out with it. A clean, cone-shaped tip means the nozzle is clear; a flattened, oval or scraped tip means the bore is worn and the nozzle must be replaced. Do this before any other teardown — it answers 'clog or worn?' in one procedure.
PTFE gap — the quiet clog cause
On Bowden and many direct-drive hotends, a gap between the PTFE tube and the nozzle lets molten filament pool in the gap instead of flowing through. The pool carbonizes over time and eventually blocks the path. Symptom: a clog that returns shortly after a cold pull, or a clog that appears from nowhere on a machine that printed fine for months.
Fix: push the PTFE tube firmly against the nozzle so there is no daylight between them, and check the heat break is fully threaded into the heat block. Replace the PTFE tube if it is worn, kinked or discoloured.
Heat creep — softening above the melt zone
Heat creep is heat travelling up the heat break and softening the filament before it reaches the melt zone, so the gears push against goo instead of solid plastic. Suspect it when clogs happen after long prints, or on TPU (soft and sensitive to heat creep), or when the hotend fan is weak or blocked.
Fix: confirm the hotend fan spins at full speed with the hotend hot; make sure the heat break is properly seated and the fan duct is clear; for TPU, slow the print and re-check retraction.
Worn nozzle
Abrasive filaments (carbon-fibre, glow-in-the-dark, wood-fill) wear brass nozzles in weeks. An enlarged orifice oozes plastic, causing both stringing and blobs. Suspect wear if problems survive a confirmed-clear cold pull: hold the tip under magnification — a round hole is healthy, an oval or flared one is worn. Replace brass nozzles every 3–6 months with abrasive filaments, and consider hardened steel if you print a lot of abrasive material.
Full clog procedure
- Cold pull first — it clears most partial clogs
- If that fails, heat the nozzle to temperature and use a needle (0.4mm) or nozzle cleaning pin to clear the tip
- Disassemble and soak the heat break in a hot-end cleaner if the clog is deeper
- Check the PTFE gap before blaming the filament — a pooling gap re-clogs every time