Over-extrusion

More plastic is coming out than the model needs. It shows as rough bulging walls, parts that measure oversized, blobs where the nozzle ploughs through its own excess, and top surfaces that look overfilled. Fixing it is mostly calibration, done in a fixed order.

Looks like: Walls look fat and bumpy, dimensions come out oversized, and the nozzle drags through excess plastic.

Fixes, in the order worth trying

1 · Try this first

Check the filament diameter setting

Check this before anything else — it is a one-line fix that invalidates every other calibration if wrong. The slicer computes volume from this number, so a profile left at 2.85 mm while you run 1.75 mm filament will massively over-extrude.

Diameter: Cura → Material → 1.75 mm
Filament diameter: OrcaSlicer / PrusaSlicer → Filament → Filament → 1.75 mm — measure your spool with calipers to confirm

2 · Try this first

Calibrate E-steps, then flow

E-steps make the extruder push exactly the length it was told; flow rate then trims for material differences. Doing them the other way round means the flow value is hiding a hardware error and you have to redo it for every filament.

Mark 120 mm of filament above the extruder, command a 100 mm extrusion, then measure what is left. If 30 mm remains you extruded 90 mm, so new E-steps = old × (100 ÷ 90). Save with M92 and M500.

3 · Common cause

Tune flow rate with a single-wall cube

The definitive measurement. Print a hollow cube with one wall and no top or bottom, then measure the wall with calipers. It should equal your line width — if it measures 0.46 mm against a 0.42 mm target, reduce flow by that ratio.

Flow: Cura → Material → Adjust by measured ÷ expected, typically 94–100 %
Flow ratio: OrcaSlicer → Filament → Filament → 0.94–1.00 · OrcaSlicer has a built-in Flow Rate calibration

4 · Less common

Lower the temperature

Hotter plastic is less viscous and keeps flowing after the extruder stops, which reads as over-extrusion even when the flow number is right. If you also have stringing and blobs, temperature is the shared cause.

Printing Temperature: Cura → Material → −5 °C per test

Change one thing at a time. If you adjust temperature, retraction and speed together and the problem goes away, you have learned nothing and you will be back here next month. Re-print the same test object after each single change.

Frequently asked questions

How do I know whether it is over-extrusion or a Z-offset problem?

Look at where it happens. Over-extrusion affects the whole part — every wall is fat and the dimensions are out top to bottom. A Z-offset that is too low only affects the first layer or two, which is elephant foot instead.

Should flow rate be different for each filament?

Yes, slightly. Different materials and even different brands vary in true diameter and melt behaviour, so a value tuned on one PLA may be 1–3 % off on a PETG. Set flow per filament profile, not globally.

My parts measure oversized but the walls look fine. Is that over-extrusion?

Possibly not. Consistent oversizing with clean walls is more often elephant foot at the base, or the natural "bulge" of an extruded line on outer surfaces. Measure at several heights and check horizontal expansion compensation before touching flow.