Open the extruder and there is a flat, polished notch cut into one side of the filament, with fine plastic dust packed into the drive gear's teeth. Extrusion has become weak or stopped entirely.
Grinding is never the fault
A drive gear cuts into filament only when the filament is not moving and the motor is. Something ahead of the gear is refusing to accept plastic, the motor has enough torque to keep turning anyway, and the teeth have nowhere to put that force except into the filament's surface.
Once a flat is cut, it is self-perpetuating: the gear has less material to grip, so it slips more, so it cuts deeper. That is why this fault goes from mild to total in the space of a few minutes.
The useful question is what stopped moving.
Read the flat
The damage is evidence, and it takes ten seconds to interpret.
A short, deep flat at one point means the filament stopped dead — a full blockage or a mechanical jam. Look at the hotend.
A long, shallow, polished region means the filament was moving but slower than commanded, over a longer period. That is a flow ceiling or a partial restriction rather than a hard stop.
A flat that starts partway along a print and coincides with a spool that will not turn points backwards rather than forwards, at a tangle or a snagged feed path.
Dust but no visible flat, on a flexible filament means the gear is deforming the filament rather than pushing it — a tension and path problem specific to elastomers.
The four things that stop filament moving
- A blockage in the nozzle. Cooked residue, an abrasive particle, or a plug formed above the melt zone. See clogged nozzle and, if the timing is characteristic, heat creep.
- Demand beyond what the hotend can melt. Here the machine is healthy and the settings are not: past a certain volume per second the filament arrives at the melt faster than it can liquefy, so the drive gear ends up pressing against material that is still solid. The arithmetic for working out whether your profile does this is on under-extrusion.
- The nozzle too close to the plate on the first layer. There is genuinely nowhere for the plastic to go. This is the version that appears on layer one, on a machine that was fine yesterday, right after someone adjusted the offset — see first layer too squished.
- A spool that will not turn. Tangle, binding holder, or a feed tube with a bend the filament cannot round.
The tension trap
The instinctive response to slipping is to tighten the idler, and it makes everything worse in three ways. It presses the teeth deeper into filament that is already damaged, so the flat cuts faster. It deforms round filament into an oval that fits the bore badly. And on flexible materials it squashes the filament until it buckles sideways instead of advancing.
Set extruder tension to the manufacturer's specification, or to the point where the gear grips without visibly embossing the filament, and then leave it alone. Tension is almost never the cause and is frequently the thing that turns a recoverable fault into a jam.
Clearing up properly before the next attempt
- Cut off every millimetre of damaged filament. A flat that reaches the gear again will slip immediately, and the dust it sheds ends up in the hotend.
- Clean the teeth. A brass brush or a pick; packed plastic dust makes a gear grip like a bald tyre. This is the step most people skip and it is why the second attempt fails as fast as the first.
- Check for dust inside the extruder body, which can pack into the idler bearing and add drag.
- Then fix the actual cause before loading anything, because reloading into an unchanged blockage simply grinds a new flat.
Flexible filaments are a special case
TPU below about 90A hardness grinds for reasons that have nothing to do with blockage: the filament is soft enough to deform under the drive gear at normal tension. Those materials want a constrained filament path with no gap between the gear and the hotend entrance, a slower print speed, and lighter tension rather than heavier. An extruder without a constrained path will grind soft TPU no matter what else is correct.
Materials that reveal it fastest
Fibre-filled grades, because they combine a higher melt viscosity with an orifice that is being eroded, so demand rises against a falling capacity. Wood-filled and other particle-loaded materials, which block small nozzles readily. And PLA in a warm environment, which softens above the melt zone and jams while everything downstream looks perfectly healthy.