Symptom: the walls come out thin and translucent, gaps show between neighbouring lines, the top layer refuses to close up, and in places the infill simply disappears. The print can be snapped apart in your fingers along the layers, even though the geometry of the model is correct.
Under-extrusion (too little material) is the situation where less plastic passes through the nozzle than the slicer assumed. The filament's route has several bottlenecks: the drive wheel needs something to grip, the melt zone has to get the material hot in time, and the nozzle has to be clear. It only takes one link failing to keep up for the shortfall to show on the surface immediately. That is why you work through the chain from the beginning rather than turning up the Flow parameter, which masks the symptom instead of removing the cause.
The most common causes and their fixes
1. The nozzle is partly clogged
A partial blockage can be insidious, because the printer carries on extruding, just less. The culprit is usually charred residue from a material printed at a higher temperature, or a particle from a filled filament.
How to check: heat the nozzle to working temperature, retract the filament and push a good ten centimetres of material out into the air by hand. The stream should fall vertically downwards and be roughly the diameter of the nozzle. If it curls to one side, pulses, or needs noticeable force, the passage is narrowed.
What to do: do a cold pull, and if that does not help, change the nozzle. With abrasive filaments a brass nozzle wears quickly and the orifice loses its shape: PLA/CF requires a hardened nozzle, and PA and PLA Wood also accelerate wear. We have covered the details on the clogged nozzle page.
2. The temperature is too low for the speed you are printing at
Temperature and speed are a pair. A setting from the bottom of the range that works fine on a gentle print stops being enough once you raise the pace. The plastic does not soften in time, resistance rises, the drive wheel starts skipping and extrusion collapses.
How to check: print the same model at half the speed. If the walls suddenly come out solid, the problem is not the slicer but a temperature too low for the flow you are asking for.
What to do: raise the nozzle temperature in 5 °C steps within the range specified for the material, and judge the result each time. We have collected the ranges in the table below. Do not go above the top of the range, because in exchange you will get stringing, blobs and discolouration.
3. Flow and e-steps are not calibrated
The slicer works out how many millimetres of filament to push into the hotend. The controller converts that into motor revolutions using the steps-per-millimetre parameter. If that value is set too low, every single extruder move delivers less material than it should, and the shortfall shows evenly across the whole print.
How to check: mark the filament 120 mm above the entrance to the extruder, tell the printer to feed 100 mm slowly, and measure how much actually disappeared. There should be 20 mm left. A difference of more than a fraction of a millimetre means the e-steps are set wrong.
What to do: recalculate the steps per millimetre from your measurement and save them to EEPROM, and only then turn to the extrusion multiplier in the slicer. The order matters: Flow corrects for the properties of the material, e-steps correct the mechanics. Mixing those two levels ends in a profile you cannot carry across to another filament.
4. The filament diameter is unstable or entered incorrectly
The slicer works out the volume of material from the diameter entered in the profile. If it says 1.75 mm there and the filament is really 1.68 mm, every line gets a few per cent too little plastic. The worse case is filament whose diameter waves up and down along the spool. Extrusion is then correct one moment and too thin the next, and no setting will even that out.
How to check: measure the filament with callipers at five points about a metre apart, on two perpendicular axes. Note the values and work out the mean and the spread.
What to do: enter the measured mean into the profile. If the spread exceeds a few hundredths of a millimetre, the problem is in the filament, not the printer. Porima filaments are produced to a ±0.03 mm tolerance, which with a fixed profile gives a repeatable flow across the whole spool.
5. The extruder pressure is too low
The drive wheel has to bite into the filament enough to drive it, but not so hard that it crushes it. Too slack a spring causes quiet skips you will not hear until you put your ear to the extruder. On the print the effect is irregular gaps in the material, worse at higher speeds.
How to check: retract the filament and look at the length that was inside the extruder. The tooth marks should be clear and even. A polished, shallow indentation means the wheel is slipping over the material, and a hollowed-out groove full of dust is already filament grinding.
What to do: tighten the spring a quarter turn at a time and check after each change. With flexible materials the relationship is the other way round: TPU Flex 98A buckles inside the extruder under too much pressure, which is why it is printed slowly and preferably on a direct drive.
6. The speed exceeds the hotend's melting capacity
Every hotend has a limit on how many cubic millimetres of plastic it can soften per second. Above it, the material leaves the nozzle underheated and there simply is not enough of it, however well calibrated the extruder is. You can work the flow out yourself: extrusion width times layer height times speed. A line 0.45 mm wide at a 0.2 mm layer height and 100 mm/s is 9 mm³/s.
How to check: print a tower in which the speed increases every few layers. The height at which the walls start to thin out marks the real limit of your hotend-plus-material combination.
What to do: enter the value you found into Max volumetric speed with a little margin. Materials with higher temperature ranges, such as HT PETG or PA, need more energy to melt the same volume, so their limit sits lower than PLA's.
| Porima material | Nozzle range | Recommended nozzle | Abrasiveness |
|---|---|---|---|
| PLA | 200–230 °C | 0.4 mm | none |
| Tough PLA (PLA+) | 210–240 °C | 0.4 mm | none |
| Hyper PLA+ | 220–250 °C | 0.4 mm | none |
| Silk PLA | 230–260 °C | 0.4 mm | none |
| PLA Premium | 210–240 °C | 0.4 mm (Granit 0.6) | none |
| PLA Wood | 210–240 °C | 0.6 mm | moderate |
| PLA/CF | 220–250 °C | 0.4 mm hardened | very high |
| PETG | 240–260 °C | 0.4 mm | none |
| HT PETG | 270–300 °C | 0.4 mm | none |
| ABS | 250–280 °C | 0.4 mm | none |
| ASA | 250–280 °C | 0.4 mm | none |
| PA (nylon) | 260–290 °C | 0.4 mm | moderate |
| TPU Flex 98A | 230–260 °C | 0.4 mm | none |
Source: the official FDM technical data table from the manufacturer, Porima. The full set for 25 materials: print temperature table.
Quick checklist
- Push material out by hand and judge the stream — that rules out a blockage in a minute.
- Measure the filament with callipers and enter the real diameter into the profile.
- Check the e-steps with the 100 mm test and correct them before touching Flow.
- Look at the tooth marks on the filament and adjust the extruder pressure.
- Raise the temperature by 5 °C within the material's range.
- Print a speed tower and set Max volumetric speed below the limit you found.
- Only at the very end adjust the extrusion multiplier, and then by single per cent.
The overriding rule: change one parameter at a time. Otherwise you will never learn what worked.
When it is not the settings
If the same profile printed correctly for six months and the gaps appeared overnight, do not look for the fault in the slicer. The most common reasons for a sudden change are a worn nozzle, wet filament and mechanical resistance somewhere on the spool's path. Moisture is treacherous, because the steam it releases creates voids in the stream that look exactly like under-extrusion: an uneven, dull wall and crackling at the nozzle. PETG, PA and TPU absorb water within a few days of the packet being opened, and PA must be dried, 8–12 hours at 65–80 °C. Before you calibrate anything, dry the spool and check that the filament unwinds freely: a tangled coil on the spool gives an identical picture of shortfalls, because the motor has not the strength to pull the material through.
Drying temperatures and times for every material: drying filament.
Related problems
A stable diameter is the foundation of repeatable extrusion. Porima filaments are produced to a ±0.03 mm diameter tolerance, and we publish the manufacturer's full official print data — you don't have to guess it. Our warehouse is near Poznań, in Swarzędz.
See Porima filaments · Print temperature table · All 3D printing problems