Not enough plastic: gaps and thin walls (under-extrusion)

Symptom

Material is missing from the part: thin or separated lines, gaps between perimeters, incomplete layers or porous top surfaces. Parts are weak and break easily.

Likely causes, from most to least likely

1. Partially clogged or dirty nozzle.
2. Speed too high for what the hotend can melt (maximum volumetric flow), especially on fast infill.
3. Dirty or misaligned extruder gears, or idler tension set wrong.
4. Wet or inconsistent-quality filament.
5. Nozzle too close to the bed on the first layer, restricting plastic flow.
6. Uncalibrated extruder (steps per millimetre or rotation_distance) or extrusion multiplier set too low.

Checks to tell which one it is

• Note where it fails: only on the first layer (height), only at high speed (flow) or all over the part (clog, extruder or calibration).
• Listen to the extruder: if it clicks, check for a clog and the gears.
• Try another spool: if it prints fine, the filament is the problem.
• Measure how much filament really goes in: with the hotend hot, mark the filament about 70 mm from the extruder inlet, request 50 mm at a slow rate and measure how far it moved (method from the Klipper documentation).

Before you start

Before taking anything apart, cleaning or adjusting it, switch the printer off and unplug it, unless a step says otherwise, and do not touch the nozzle, the heater block or the bed until they have cooled down. If in any doubt about electricity, heat or moving parts, stop and ask a professional.

Step-by-step fix

1. Use the right profile

   Start from the manufacturer's profile for your printer, nozzle and material. With special nozzles (hardened or stainless steel), Prusa says the temperature may need raising by 5-10 °C.

   Check: Lines come out continuous and at the expected width.

   If that does not fix it: Lower the speed or maximum flow.

2. Lower the speed or maximum flow

   Reduce print speed or the profile's maximum volumetric flow and raise it again gradually: the hotend and the material limit how much plastic can be melted.

   Check: Fast infill comes out complete.

   If that does not fix it: Unclog and clean the nozzle.

3. Unclog and clean the nozzle

   If the purged filament comes out thin or in spurts, clean the nozzle: push fresh filament by hand, thin needle from below or cold pull.

   Check: Filament comes out straight and steady.

   If that does not fix it: Check the extruder.

4. Check the extruder

   With the printer switched off and unplugged, clean the gears, check their alignment and set the idler tension as the manufacturer says.

   Check: The gear grips the filament without slipping or chewing it.

   If that does not fix it: Calibrate the extruder.

5. Calibrate the extruder

   With the hotend at printing temperature, mark the filament, extrude 50 mm slowly (on Klipper, G91 then G1 E50 F60) and measure what went in. On Klipper, new rotation_distance = previous × actual distance / requested distance; on Marlin, set the extruder steps with M92 E and save them with M500.

   Check: Repeating the test feeds 50 mm.

   If that does not fix it: Adjust the extrusion multiplier.

6. Adjust the extrusion multiplier

   If everything above is fine and material is still missing, raise the filament's extrusion multiplier (flow) in 1-2% steps (Prusa's guideline) until the top surface has no gaps.

   Check: The top of a test cube is closed and smooth.

   If that does not fix it: If it persists, check the hotend assembly with the manufacturer's guide or ask for technical support.

Prevention

• Keep filament dry.
• Don't print faster than your hotend can melt.
• Clean the extruder gears from time to time.

Still not fixed?

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Printers it applies to

Any FDM (filament) 3D printer, from any brand.

Official sources

This guide summarises the official documentation linked here:

• Under-extrusion · Prusa Research
• Max volumetric speed · Prusa Research
• Rotation distance · Klipper
• Extrusion multiplier calibration · Prusa Research