The print pauses — for a filament change, for an insert, or because you stopped it deliberately — and on resume nothing extrudes, or it extrudes weakly and never fully recovers.
Standing still in a hot nozzle is not a neutral state
Filament sitting in the melt zone is being held at printing temperature with nothing moving it along. Two processes then run.
It cooks. Thermal degradation is a function of temperature and time, and a material that is perfectly stable during a print because it passes through in seconds is not stable when held for twenty minutes. It darkens, cross-links and stops flowing.
It swells. Above the melt zone, in the transition region, filament that softens without moving expands slightly against the bore. Once it has, the extruder is pushing a plug that is fractionally wider than the channel it came in through, and the force needed rises sharply.
The two combine, which is why a long pause is so much worse than a short one and why the failure is often unrecoverable without dismantling.
Before a long pause
- Retract the filament out of the melt zone entirely rather than leaving it loaded. Most firmware's filament-change routine does this; a bare pause command often does not.
- Drop the hotend temperature while paused. Some firmware does this automatically after a timeout; if yours does not, set it manually. Even a hundred degrees down makes the difference between cooking and waiting.
- Keep the bed hot. The part must not release from the plate or contract while you are away, so the bed stays at its printing temperature even while the hotend cools.
- Plan the pause at a sensible layer. A pause during a solid infill region leaves a visible line; a pause at a layer change on a flat section is far less obvious.
Resuming without making it worse
- Bring the hotend back to temperature and purge generously before allowing motion — considerably more than the machine's default prime.
- Check the extrusion is clean-coloured and consistent before letting the print continue.
- Wipe the nozzle so the accumulated blob does not land on the part at the first travel move.
- Expect a weak layer at the joint. The layer below has been cooling for the whole pause, so the weld there is poor. On a functional part that plane is a designed-in weakness, for the reasons on weak layer adhesion.
If it has already jammed
The plug is usually at the transition between the cold and hot sections, not at the orifice. A cold pull is the right tool: the procedure is on clogged nozzle, and it wants nylon or cleaning filament rather than whatever was in there.
Do not raise the temperature and push harder. A swollen plug plus force is how a heatbreak gets damaged, and a hotter block softens the filament further up the path rather than clearing what is already stuck.
Machines that pause on their own
Two automated cases deserve attention because neither is under your control at the time.
Filament runout sensors pause a print and hold it until someone loads a new spool. If that happens overnight, the machine has been sitting hot for hours. Configuring the firmware to cool the hotend after a runout pause is worth doing before it happens rather than afterwards.
Power-loss recovery resumes into a nozzle that has been cold and then reheated with old material in it. Purge before it starts printing, and see layer shift after a power interruption for the positional half of that problem.
Which materials will not tolerate it
The soluble supports are the worst: PVA and BVOH degrade quickly at temperature and are notorious for jamming after any idle period, which is why multi-material prints using them are the classic case for this fault. PLA cooks readily too and is the material most single-colour pauses go wrong with.
Polycarbonate and the high-temperature engineering materials are held at temperatures where degradation is fast, so a pause with those loaded is a short-fuse situation. PETG is comparatively tolerant, and ABS sits in between — but no material here is genuinely happy being parked in a hot nozzle, and the habit of retracting out of the melt zone costs nothing on any of them.