Under-Extrusion: Why Your Printer Prints Too Little Plastic

If the walls are thin, there are gaps in the top surface and the part breaks easily in your hands, the printer is putting down less plastic than it should. This is called under-extrusion. The cause is most often a partly clogged nozzle, low temperature or an extruder gear that cannot grip the filament. Calibration settings are the last place to look.

Symptoms

• Gaps and holes in the walls and top surface.
• Thin lines that do not touch each other.
• Visible gaps between layers, and the part breaks easily.
• Infill looks sparse and broken.
• At some point in the print the layers suddenly thin out and stop.

Under-extrusion or over-extrusion?

Over-extrusion is the opposite: blobs on the edges, bumps on the surface and parts larger than the dimensions. In both cases the final step of the fix is flow rate calibration. But with under-extrusion you should deal with hardware problems first.

1. Check the filament path

Start with the simple causes:

• Is the spool catching? The spool should turn freely on its own axis. Winding that overlaps and tangles (a snarl) will pull on the filament.
• Is the filament diameter right? Make sure the slicer is set to 1.75 mm. Measure at several points with calipers. In quality filaments the deviation is around ±0.03 mm.
• Is the Bowden tube seated? The tube must sit fully against the nozzle. If a gap remains between the tube and the nozzle, plastic collects there and disrupts the flow.

2. Rule out a clogged nozzle

A partial clog is the most common cause. Take the filament out of the extruder and push it by hand. If the plastic does not come out straight and easily, the nozzle is partly clogged. See the Clogged nozzle guide for the fix.

3. Check the temperature

If the filament does not melt enough, it cannot flow through the nozzle at the speed it needs. This limit is reached sooner in fast printing.

• Test by raising the temperature 5–10 °C.
• If you print fast, you need to raise the temperature or lower the speed.
• A thermistor or heater fault can also cause low temperature. If the temperature on the screen swings up and down instead of holding steady, do a PID tune.

4. Check the extruder gear

• Is the gear dirty? If the gear teeth are full of plastic dust, they cannot grip the filament. Clean with an old toothbrush.
• Is the tension right? If the extruder arm holds the filament too loosely it slips, and if too tightly it crushes the filament. Set it to the manufacturer's setting.
• Are there crush marks on the filament? Deep notches form where the extruder has "eaten" the filament. This shows the filament cannot advance to the nozzle.
• Do you hear clicking? This is covered in detail in Extruder clicking.
• On plastic-body extruders the arm can crack over time. If there is a crack, consider replacing it with a metal one.

5. E-steps and flow calibration

If the hardware is fine, calibrate how much filament the printer pushes:

1. E-steps calibration: You tell the printer to push 100 mm of filament and measure how much it actually pushed. There is a separate guide for this: E-steps calibration.
2. Flow (flow ratio) calibration: A fine adjustment in the slicer specific to the filament you use. Details: Flow rate calibration.

Do e-steps first, then flow. If you mix up the order, one hides the error of the other.

6. Bring the speed to a reasonable level

Every hotend has an upper limit on how much plastic it can melt per second. This is measured as volumetric flow, in mm³/s. Standard hotends are usually around 10–15 mm³/s, and high-flow hotends can go several times higher.

A quick calculation: with a 0.2 mm layer, 0.45 mm line width and 100 mm/s speed, the flow is about 0.2 × 0.45 × 100 = 9 mm³/s. If the speed doubles, the flow doubles too, and a standard hotend hits its limit. If under-extrusion happens only in fast sections, you have reached this limit.

Many slicers (PrusaSlicer, OrcaSlicer) have a maximum volumetric flow setting. If you set this value to a limit suited to your hotend, the slicer limits speed automatically.

7. Damp filament

Damp filament foams and flows unevenly, creating gaps here and there. If you see popping sounds and bubbles, dry the filament.

Frequently asked questions

Under-extrusion happens only on the first layer. Why?

The nozzle may be too close to the bed and blocking the plastic from coming out. Check the Z offset.

Will raising flow to 120% fix it?

In the short term it may close the gaps, but it hides the real cause. As long as the clog, low temperature or gear slip continues, results will be inconsistent. Check the hardware first.

Under-extrusion suddenly started in the middle of a print. Why?

The spool may have caught, the filament may have tangled, the extruder may have overheated or the nozzle may have partly clogged. Pause the print and check them in turn.