Filament grinding — 5 causes of a worn groove in the filament

Symptom: the extruder runs loudly and clicks at regular intervals, and the amount of material reaching the print drops. Opening the feeder reveals a flat, ground-away patch on the filament and fine plastic dust on the knurl and in the bottom of the extruder housing. At that point the filament is noticeably thinner on one side.

Filament grinding is the situation in which the drive gear keeps turning although the material is standing still. The knurl teeth stop biting into fresh material and start working the same fragment over and over until they cut a groove into it. From that moment grip disappears completely and feeding ends for good, even if the original obstruction clears. This is a secondary fault: it is almost always the consequence of something else rather than a defect in its own right. The task is therefore to establish why the filament met resistance.

Most common causes and fixes

1. Retraction is too long or happens too often

Every retraction drags the same length of filament back through the knurl in the opposite direction. On a model with hundreds of small islands, where there are several dozen retractions per layer, the same centimetre of material can pass through the teeth a dozen or more times before it ever reaches the nozzle. Sooner or later it will be reduced to dust.

How to check: count the number of retractions per layer in the slicer preview. If it runs into the dozens, and the groove always appears at the same point in the print, the cause is obvious.

What to do: shorten the retraction distance by 0.5 mm and check whether stringing has come back. Far more effective, though, is reducing the number of retractions themselves: enable the minimum travel distance below which no retraction is performed at all, and set a maximum number of retractions per unit length of filament. In practice the latter parameter solves the problem in most cases without degrading surface quality.

2. Extruder tension is too high

Intuition suggests that if the filament is slipping it needs to be gripped harder. Usually the opposite is true. Excessive tension drives the teeth deep into the plastic and deforms its cross-section, and deformed filament meets greater resistance in the tube and in the hotend. Resistance rises, slipping increases, so the tension is turned up further still and the vicious circle closes.

How to check: take out the section that passed through the extruder. The tooth marks should be shallow and regular. If the filament is clearly flattened or has one side of its cross-section shaved off, the tension is too high.

What to do: set the lowest tension at which the material feeds without slipping in normal printing. Check this on the material you actually use, because PLA and TPU need entirely different settings. While you are there, inspect the knurl: teeth clogged with dust have no grip, however perfect the tension. Clean them with a small brush or a pin.

3. The requested flow exceeds the hotend's capacity

A hotend has a finite melting power. If the slicer demands more material per second than the heater block can melt, the excess simply has nowhere to flow. Pressure rises, the filament stops and the knurl grinds it. Classic situations are a thick layer combined with high speed, wide infill printed faster than the walls, and an over-enthusiastic fast-printing profile carried over to an older printer.

How to check: run a flow test, gradually increasing extrusion speed into the air and noting the value at which the flow starts to break up. Enter the figure you get into the slicer as a flow limit.

What to do: lower the speed or the layer height, or raise the nozzle temperature within the range given by the manufacturer. A higher temperature increases melting capacity, but it has its price in stringing and poorer overhangs, so look for a compromise. Remember that high-temperature materials need more energy: HT PETG runs in the 270–300 °C range, and nylon 260–290 °C.

Porima material	Nozzle	Flow and feeding notes
PLA	200–230 °C	The easiest to feed, the greatest tolerance of mistakes
Tough PLA	210–240 °C	Requires 100 % cooling, flow as for PLA
Hyper PLA+	220–250 °C	A higher temperature makes fast printing easier
PETG	240–260 °C	Sticky; with heavy retraction dust in the knurl comes easily
HT PETG	270–300 °C	High demand for heating power, do not speed up without headroom
ABS / ASA	250–280 °C	Moisture-sensitive, wet material breaks up the flow
PA (nylon)	260–290 °C	Drying 8–12 h at 65–80 °C is mandatory
TPU Flex 98A	230–260 °C	Slow printing, direct drive recommended

Source: the official FDM technical data table from the manufacturer, Porima. All materials: 3D printing temperature table.

4. The nozzle is partially clogged

A narrowed hole raises flow resistance just as effectively as excessive speed. The difference is that here you changed nothing in the settings, and the problem grows from print to print. The typical scenario: first the walls get thinner, then under-extrusion appears, and finally the extruder starts clicking and grinding.

How to check: heat the hotend and extrude material into the air with the Bowden tube removed. The flow should come out vertically downwards, at a constant thickness and with a diameter close to that of the nozzle hole. Curling sideways, pulsing or a distinctly thinner trickle confirms a restriction.

What to do: do several cold pulls until the tip you draw out is clean. If that does not help, replace the nozzle. With abrasive materials replacement is part of normal operation: PLA/CF and ABS/CF have very high abrasiveness and require a hardened nozzle. We cover the details on the clogged nozzle page.

5. Soft flexible filament in a Bowden setup

TPU behaves like a spring in a long tube. Instead of transmitting movement from the extruder to the nozzle, it compresses and expands, so the response to the slicer's commands is delayed and unpredictable. On retraction the material would rather coil up in the tube than come back, and when extrusion resumes the knurl presses it against the obstruction it created itself. Porima classifies the printing speed of TPU Flex 98A as slow and recommends direct drive.

What to do: print TPU with a direct drive if you can. In a Bowden setup shorten the tube to a minimum, use the shortest possible retraction, reduce speed and make sure the material has no point anywhere along its path where it could bulge out. Extruders that guide the filament over a short distance between the knurl and the inlet work far better here than designs with a large gap. See also flexible filaments.

Why knurl dust has to be removed straight away

Ground-off plastic does not disappear. It settles in the grooves of the knurl and between the teeth, and from there it gets back onto the filament and on into the hotend. Clogged teeth lose grip, so slipping begins at ever lower resistance, while the dust drawn in accelerates the build-up of deposit in the nozzle. Two problems start to feed each other, and after a few prints it is hard to tell which came first.

So after every grinding episode, take the feeder apart, clean the knurl with a brush and cut off the damaged length of filament with some margin. Do not try to salvage the worn piece: the constriction at that point will give uneven extrusion anyway, and it is also one of the favourite places for the material to snap in the tube.

Related problems

• Filament jam — the most common underlying cause, which grinding merely reveals.
• Clogged nozzle — a partial clog raises resistance to the level at which the knurl stops holding.
• Under-extrusion — the first visible consequence of losing grip on the filament.
• Stringing — fighting stringing by increasing retraction leads straight to grinding.
• Printer stops extruding mid-print — the final stage, once the groove is fully cut.
• Weak infill — infill, printed fastest, is the first to suffer when flow is short.