Heat Creep: the clog that's not a clog

You ran a six-hour print. The first four hours looked perfect. Then the extruder started clicking, the next layer turned into spaghetti, and by the time you got back the print head was painting air. You pull the nozzle, expect to find burnt PLA and a partial clog — and instead the nozzle is clean. Cold-pull the heatbreak and a soft, slightly mushroomed plug of filament drops out from above the heater block.

That's heat creep. It is the single most misdiagnosed failure mode on consumer FDM printers, and it gets blamed on everything except the actual cause.

And what it actually is

The symptom is under-extrusion or a hard stop mid-print, often after the printer has been running long enough to warm-soak the toolhead. The extruder motor starts skipping or clicking. Filament won't push through. Most hobbyists run the same diagnostic loop:

1. "Must be wet filament" → dry it, problem persists.
2. "Must be retraction" → tune retraction, problem persists.
3. "Must be a partial clog" → cold-pull, find nothing obvious.
4. "Must be a bad nozzle" → swap it, problem comes back two prints later.

Heat creep is none of those things. It's a thermal management failure in the heatbreak — the thin neck between your heatsink and your heater block that's supposed to keep a sharp temperature gradient. When that gradient collapses, filament starts softening above the melt zone, where it has no business being soft. It expands, jams against the inner wall of the heatbreak, and the extruder motor — which is geared for cold-side feeding — can't push it through. The print fails not because anything is clogged, but because the boundary between "solid filament" and "melted filament" walked north.

One paragraph, then a chart

Your hotend is a heat pump in reverse. The heater block puts heat in; the heatsink and its fan pull heat out; the heatbreak — a narrow stainless or titanium tube, sometimes lined with PTFE — keeps the two zones thermally isolated. That isolation depends on three things: airflow over the heatsink fins, the surface area of the fins, and the thermal conductivity of the heatbreak itself. Lose any of them and the gradient flattens. Once the temperature 5–10 mm above the heater block crosses the glass transition of your filament (around 60°C for PLA, 80°C for PETG, 84°C for PCTG), the filament starts softening exactly where you don't want it to.

And why that matters

Heat creep is a time-dependent failure. The heatsink isn't dissipating less heat as the print progresses; the toolhead is accumulating heat from radiant load (heated bed, heated chamber, slow movement), partial fan blockage from dust, or simply running at the edge of its thermal envelope. The first hour is fine. The fourth hour fails. That intermittency is the fingerprint — and it's exactly why the wet-filament theory keeps winning. Wet filament fails immediately. A partial nozzle clog fails consistently. Heat creep waits.

Work it in order

Each step is cheaper than the next. Don't skip ahead.

1. Look at the heatsink fan. Not the part-cooling fan — the small 4010 or 5015 axial fan blowing across the heatsink fins. With the printer at temperature, hold a strip of paper near it. If it doesn't snap to the fan, the fan is dying or has dust in the bearing. Replace it. Total cost: $5 and 15 minutes.
2. Pull the shroud and clean the fins. Print dust, lint, and PLA shavings pack between heatsink fins like felt. A dust-clogged heatsink can lose 30–40% of its dissipation capacity without any visible change to the fan. Compressed air, work outside, takes 3 minutes.
3. Check the fan's PWM curve in your firmware. Some firmware ramps the heatsink fan with hotend temperature. If yours does, force it to 100% any time the hotend is above 50°C. There is no good reason to PWM-throttle a heatsink fan. The noise tradeoff is not worth the failure mode.
4. Inspect the heatbreak. If you have a PTFE-lined heatbreak and you're printing above 240°C — which means most PETG and all engineering filaments — the PTFE is slowly degrading. Above 260°C it off-gases and softens, narrowing the throat and holding heat instead of isolating it. All-metal heatbreaks (titanium or bi-metal) solve this for any filament above 230°C.
5. Check thermal paste at the heater block-heatbreak joint. Rare, but on bi-metal heatbreaks a poor thermal interface lets heat pool at the wrong end.

And yes, this is where PCTG comes in

Lower glass-transition materials soften higher up the heatbreak. That's why PLA (Tg ~60°C) is the canary: if your heat creep starts with PLA and clears up when you switch to PETG, your heatbreak gradient is marginal — you're not solving the problem, you're just running a material that tolerates a hotter cold-side.

What to do right now

If you're mid-failure and reading this: stop the print, let the toolhead cool to room temperature, then heat to 90°C and pull the filament out by hand from above. If it comes out as a thickened plug with a tapered tip, you have heat creep, full stop. Do steps 1–3 above. If it happens again within a week, do step 4.

Don't dry your filament. Don't retune retraction. Don't buy a new nozzle. The problem is six inches above where you're looking.

— DuffAM