Printer stops extruding mid-print — 5 causes

Symptom: the print ran flawlessly for an hour, or five, and then the head goes on tracing paths but nothing comes out of it. The model breaks off at a flat, smooth surface halfway up its height, with nothing but air above. Sometimes the printer stops by itself and displays an error message.

This is one of the most frustrating scenarios, because the first layers give no warning. Since the print started correctly, the geometry, the levelling and the profile are all in order. The cause must therefore be sought among the things that change over time: material running out, heat building up, electronics temperature or the state of the sensors. The good news is that the moment of stopping carries information in itself. Note how long the print ran before it stopped, and whether it always happens at a similar point.

Most common causes and fixes

1. The filament has run out or tangled on the spool

The simplest cause, and by no means the rarest. The end of the material on a spool without a sensor simply means a quiet end to the print. The more insidious variant is tangling: a winding slips under its neighbour and forms a loop that tightens under tension. The extruder has not the force to break it, clicks for a while, wears a groove in the filament and stops feeding.

How to check: inspect the spool and the filament path. A tensioned length running under other windings, or a spool standing still instead of turning, settles the matter immediately.

What to do: unwind the material by hand and rewind it, and always clip the free end into the hole on the rim of the spool. Check the holder: it should let the spool turn easily, with no perceptible resistance. Before a long print, estimate how much material is left. There is more on the tangled filament on the spool page.

2. A growing clog from heat creeping up the hotend

The hotend fan maintains the boundary between the hot zone and the cool one. If cooling is on the edge of adequacy, everything works for the first hour, but the heat gradually moves up the channel. The filament softens where the channel is at full diameter, swells and after a time forms a plug that nothing will push through. This is the most common technical cause of stoppages mid-print.

How to check: after a failed print, pull the filament out of the hot hotend and inspect the tip. A thickened spot shaped like a droplet, or a neck a few centimetres above the melted end, is this fault's signature. A second clue: the failure always comes after a similar length of time.

What to do: check that the hotend fan runs throughout the print and that the heatsink is not dusty. Ensure free airflow around the hotend. If you print in a closed chamber, required for ABS, ABS/CF, ASA and PC/ABS at 50 °C, feed the hotend fan air from outside the chamber. We describe this mechanism in detail on the clogged nozzle page.

3. The thermistor or heater gives out and the controller aborts the print

The firmware makes sure the measured temperature agrees with the target. A loose thermistor, a chafed wire in the loom or a heater losing contact all cause jumps in the reading. If the difference exceeds a threshold, thermal runaway protection trips and the print is aborted. The quieter variant is more dangerous: a thermistor that has slipped out of its seat reads low, the controller keeps heating, and the actual temperature in the block goes well above target. The plastic then degrades in the channel and clogs the nozzle with burnt deposit.

How to check: watch the temperature graph in the printer interface. A healthy hotend holds the value in a narrow band. Sudden drops or rises of a dozen or more degrees, especially just as the head has accelerated, point to a loose contact in the loom.

What to do: never bypass the thermal protections in the firmware. Tighten the thermistor and heater in the block, inspect the wires along the whole length of the chain, especially where they bend most, and recalibrate the temperature controller after every intervention in the hotend.

4. The extruder motor driver overheats and loses steps

A driver running without cooling eventually reaches its protection temperature and limits its output. The extruder motor loses torque, stops pushing material through, grinds a groove in it along the way, and feeding ends for good. The symptoms are almost identical to a clog, but the difference can be felt by touch: with an overheated driver the hotend is usually still clear, and the filament can be pushed through by hand without much resistance.

How to check: touch the driver heatsinks right after the print stops. The extruder driver should not be noticeably hotter than the others. Also check whether the extruder motor is too hot to touch.

What to do: provide airflow over the drivers and check the motor current setting. Too high a current is as harmful here as too low, because it heats the driver needlessly. If the printer is enclosed together with the electronics, move the board outside the chamber.

5. The filament sensor raises a false alarm

An optical or mechanical filament runout sensor is meant to stop the print when the material runs out. It happens, however, that it reacts to transparent material, to vibration, or to a momentary slackening of tension during retraction. The effect is the same as a real end of spool: the printer suspends work and waits for attention, and if you do not notice, the hotend stands hot over the print, degrading the material in the channel.

How to check: look in the printer's event log. A filament runout event on a spool that still has half a kilogram left points unambiguously to the sensor.

What to do: clean the sensor and check that the filament passes through it without catching. Transparent materials such as PETG Transparent can be troublesome for optical sensors. If the equipment allows, extend the time after which a lack of movement is treated as the end of the material, or switch to a mechanical sensor.

Porima material	Nozzle	Chamber	Risk of stopping mid-print
PLA	200–230 °C	open	low
PETG	240–260 °C	open	low
HT PETG	270–300 °C	closed recommended	high, thermal load on the hotend
ABS / ABS/CF	250–280 / 260–280 °C	closed, 50 °C	elevated, chamber heat loads the cooling
ASA	250–280 °C	closed, 50 °C	elevated
PA (nylon)	260–290 °C	closed and heated	high, the material absorbs water
PLA/CF	220–250 °C	open	elevated, nozzle wear
TPU Flex 98A	230–260 °C	open	elevated in a Bowden setup

Source: the official FDM technical data table from the manufacturer, Porima. The complete set of ranges: 3D printing temperature table.

How to limit the losses once a print has stopped

If you notice the stoppage reasonably early, some prints can be saved. Do not move the model off the bed and do not break adhesion, because without that resuming makes no sense. Prints of many hours are worth protecting in advance: enable position saving on power loss in the firmware if your printer supports it, and get into the habit of splitting very large models into mechanically joined parts. With technical materials, where one failed attempt means a dozen or more hours, that split pays for itself immediately.

It is also worth remembering the prevention that costs nothing. Before starting a long print, check how much material is left, unwind a few metres of filament by hand and leave it slack, so that the first hour is not pulling on a tightened winding. Print hygroscopic materials from a dryer, because with nylon moisture alone can stop a print regardless of the state of the equipment. You will find the recommendations in the filament drying guide.

Related problems

• Clogged nozzle — the most common technical cause of a stoppage after an hour of printing.
• Tangled filament on the spool — a slip loop stops the feed without warning.
• Filament jam — the same problem seen from the feeder side.
• Filament grinding — a groove worn by the knurl closes off the way back.
• Brittle filament — a broken-off piece in the tube gives an identical symptom.
• Under-extrusion — usually precedes a complete stop in extrusion.
• No extrusion at the start of a print — if the problem occurs from the first layer, look there.