Under-extrusion and over-extrusion are opposite problems that sometimes get confused because both can produce a rough, inconsistent-looking print. The distinction matters because the fixes are opposite too: treating one as the other makes it worse.
| Under-extrusion | Over-extrusion | |
|---|---|---|
| What you see | Gaps between lines, thin or missing walls, visible holes in top surfaces, weak layer bonding | Blobs, “zits,” rough bumpy surfaces, dimensions larger than modeled, walls that look swollen |
| Sound during printing | Often a clicking or grinding sound from the extruder motor skipping | Usually quiet: the extruder isn’t struggling, it’s just pushing too much |
| Typical cause | Partial clog, worn nozzle, filament diameter thinner than the slicer’s assumed value, extruder tension too loose, temperature too low for consistent flow | Flow rate set too high, filament diameter thicker than assumed, wrong nozzle diameter selected in slicer |
| First thing to check | Extruder gear tension and a cold pull to check for a partial clog | Flow percentage in the slicer and the actual measured filament diameter |
| Calibration fix | Flow rate calibration, increased if too low; check E-steps if the gap is large | Flow rate calibration, decreased if too high |
Quick diagnostic: the single-wall test
Print a simple single-walled cylinder or box with no top or bottom layers. This isolates flow from every other variable: no infill pattern, no top-layer bridging, just one line of extrusion stacked repeatedly.
- Walls that are thinner than the nozzle diameter, with visible gaps or a “see-through” quality when held up to light, point to under-extrusion.
- Walls that are noticeably thicker than the nozzle diameter or have excess material squeezing out at the corners point to over-extrusion.
Measure the actual wall thickness with calipers and compare it to your nozzle diameter. For a 0.4 mm nozzle printing a single-wall test, you’re looking for wall thickness close to 0.4–0.45 mm, not exactly one to the thousandth, but close enough that a large deviation is informative.
Why both can look “rough”
Extrusion volume that’s off in either direction disrupts how cleanly each new line bonds to its neighbor. Under-extrusion leaves gaps that show up as visible texture and weak points; over-extrusion pushes excess material sideways with nowhere to go, which also shows up as texture, just in the form of bumps and blobs instead of gaps. From a few feet away, both can read as “the surface looks bad.” That’s exactly why the single-wall test matters more than eyeballing the finished part.