Over-extrusion: when a printer lays down more plastic than needed

What it looks like

The surface is uneven, bulging lumps form on corners and small details get flooded. The dimensions of the print are slightly larger than in the model, which matters for parts that must fit precisely. In FDM printing plastic is laid along lines of a defined width, and when there is too much of it, it has nowhere to go.

On top surfaces plastic can pile up and the nozzle drags it on its next pass, which looks like scratches.

The difference from a correctly set print is small, often just a few hundredths of a millimetre per wall. On shape-wise simple parts such as a holder or a stand you will not notice it. On joints, threads and sliding parts it decides the outcome.

Why it happens

• A wrongly set flow rate or a wrong filament diameter, see filament diameter.
• A temperature that is too high, so the plastic is runnier and spreads more.
• A worn or widened nozzle.
• A first layer set at the wrong distance, see Z-offset.

Over-extrusion and under-extrusion are two opposite ends. With correct settings the slicer stays in the middle, but every filament behaves a little differently.

What makes the difference for fit

If a part fits into another, a positive deviation of a few hundredths of a millimetre is normal, while lumps in the corners are worse. So for joints add a gap of 0.2 to 0.3 mm, see tolerances. Details smaller than 0.4 mm will not print accurately even without over-extrusion, with more under smallest detail.

For your order

We take care of the flow and temperature settings, and we photograph the batch before dispatch. We do not promise accuracy to a tenth of a millimetre, so for precise joints add a gap to your model and check the dimensions on one part before a larger batch. We do not offer sanding of the surface after printing.