Over-extrusion: too much material, bulges and blobs
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With over-extrusion, the printer pushes more plastic out of the nozzle than the model needs. The excess material has to go somewhere: it forms bulges on the walls, blobs at corners and rough top surfaces. Parts also come out too large and no longer fit together.
How do you recognise over-extrusion?
- Walls look bulgy, the individual layers stand out like small ribs.
- The top surface is rough or wavy, the lines are raised instead of lying smoothly next to each other.
- Blobs and zits at corners and at the Z seam.
- Dimensions too large: a test cube measures more on the outside than intended, holes are too small.
- The nozzle drags across the top surface because too much material already lies there. You hear a faint scraping, and sometimes small parts tip over.
The opposite, meaning gaps, thin walls and a net-like top surface, is under-extrusion. Both problems often come down to the same adjustments, just in the other direction.
The causes, sorted by frequency
1. Flow value too high
The flow factor (flow ratio or extrusion multiplier) determines how much material the printer delivers relative to the calculation. If it is set too high, a little too much comes out on every line. Values above 1.0 are rarely needed. According to Bambu Lab, a filament only very rarely needs more than 1.00.
Solution: run a flow calibration. Bambu Studio and OrcaSlicer offer a built-in test for this: first a coarse pass in 5 per cent steps, then a fine one in 1 per cent steps. You pick the patch with the smoothest top surface. If you work manually, reduce the flow in small steps of 2 to 5 % and print a test cube each time. More on the procedure is in Calibration.
2. Wrong filament diameter in the slicer
The slicer calculates with the diameter that is set. If it says 1.75 mm but the filament is thicker, more material comes out than expected. Conversely, a value entered too large leads to under-extrusion.
Solution: check the diameter in the filament profile; it must match the spool (1.75 mm or 2.85 mm). If in doubt, measure at several points with callipers. If the value fluctuates strongly, the problem lies with the filament itself.
3. Temperature too high
Very hot plastic flows more easily and runs on in curves and at corners. This looks like over-extrusion even though the flow is right. Typical companions are stringing and shiny, soft corners.
Solution: lower the temperature in 5-degree steps until the edges become clean. A temperature tower shows the best range at a glance, see Setting the print temperature.
4. Unsuitable slicer profile
A profile for a different material or a different nozzle can throw a lot off:
- Wrong nozzle size: if 0.6 mm is set in the slicer but a 0.4 mm nozzle is fitted, line widths and flow are no longer correct.
- Line width too large for the nozzle or layer height.
- Pressure advance not calibrated: at corners and when slowing down, pressure remains in the nozzle, producing blobs at the corners while straight stretches look good.
Solution: choose a profile that matches the printer, nozzle and material. For blobs at corners, calibrate pressure advance. With third-party filament, the profile of a similar material is a good start; after that, check flow and temperature.
5. Too much infill overlap
Rare, but possible: if infill and walls overlap strongly, material builds up at the transitions. On the top surface this shows up as a bulge along the edges.
Solution: reset the overlap between infill and walls in the slicer to the profile’s default value. In the Prusa profiles, for example, that is 10 %.
Under- or over-extrusion? How to tell them apart
| Feature | Under-extrusion | Over-extrusion |
|---|---|---|
| Top surface | gaps, infill shows through | rough, raised, bulges |
| Walls | thin, fragile | bulgy, ribs |
| Dimensions | tend to be too small | tend to be too large, holes too tight |
| Sound | extruder clicks | nozzle scrapes across the surface |
| Common cause | nozzle partly clogged, too fast | flow too high, wrong diameter |
If you are not sure: print a 20 mm test cube and measure it with callipers. If it is clearly larger than 20 mm and the top surface is rough, it is over-extrusion.
Quick checklist
- Does the profile match the printer, nozzle and material?
- Is the filament diameter entered correctly in the slicer?
- Flow calibration done? A flow above 1.0 is rarely needed.
- Temperature at the upper end of the manufacturer’s range? Then reduce it by 5 degrees.
- Blobs only at corners? Calibrate pressure advance.
- Measure the test cube and reprint it after every change.
Always change only one value per test print. If the widening only occurs right at the bottom, it is probably elephant’s foot.