Under-Extrusion and Over-Extrusion: Diagnosing and Fixing

Extrusion consistency is the foundation of FDM print quality. The slicer calculates exactly how much filament should be deposited at each point in the print. If the actual amount varies from the calculated amount — whether too little (under-extrusion) or too much (over-extrusion) — the entire print is compromised.

Identifying the Problem

Under-extrusion symptoms:

• Gaps between perimeter walls or infill lines when viewed from above
• Fine surface texture showing through infill or solid layers
• Parts that break along layer lines under light force
• Inconsistent line widths that appear visually uneven
• Printing sounds: occasional clicking or skipping from the extruder

Over-extrusion symptoms:

• Blobs and zits on outer surfaces
• Elephant foot (first layer spreading wider than subsequent layers)
• Top surfaces with bumps and ridges
• Dimensions slightly larger than the model
• Parts that feel very solid but have slightly rough texture

The Calibration Hierarchy

Work through these in order. Most extrusion problems are resolved by the first two.

Step 1: Extruder Calibration (E-steps)

The extruder calibration value (steps per mm, or e-steps) tells the printer how many motor steps equal 1mm of filament movement. If this is incorrect, everything downstream is wrong.

To check: Command exactly 100mm of extrusion at a low speed. Measure how much filament actually moved. If 95mm moved when 100mm was commanded, your e-steps are 5% too low.

E-step correction = (current e-steps × 100) / actual movement

Step 2: Flow Rate / Filament Multiplier

The flow rate (or extrusion multiplier) is a percentage scaling factor applied on top of e-steps. It compensates for the actual diameter of your specific filament spool (which varies from the nominal 1.75mm) and the thermal properties of the material.

Measure your filament diameter at several points along the spool with digital callipers. Average the readings. Enter this in your slicer's filament profile.

For fine-tuning beyond diameter measurement: print a single-wall cube and measure wall thickness. Compare to your configured extrusion width. Adjust flow rate proportionally.

Step 3: Print Temperature

Insufficient temperature causes under-extrusion by increasing melt viscosity. If under-extrusion gets worse at higher speeds, the hotend is not melting material fast enough — increase temperature.

Step 4: Print Speed

Exceeding the volumetric flow rate of your hotend causes under-extrusion. If under-extrusion only appears at high speeds, reduce speed or upgrade to a high-flow hotend.

Extruder Skipping / Clicking

An extruder that audibly clicks or skips is being commanded to push filament faster than the hotend can melt it. Causes: print speed too high, temperature too low, nozzle partial clog, or retraction settings pulling material that then needs to be re-pushed.

The click is the extruder gear slipping on the filament — visually indicated by small flat ground spots on the filament strand if you pull it back and inspect it.