Blobs and zits on a 3D print: where they come from

How they form

A layer is printed in a loop and at the end it has to join up with the start. At that point, called the seam, the pressure in the nozzle changes and a small blob forms. Zits are similar small bumps that can appear anywhere, when the nozzle oozes or the plastic bubbles from moisture.

Molten plastic is a thick liquid with a high viscosity, so the pressure in the nozzle evens out with a delay after the movement changes.

Common causes

• Badly set retraction and nozzle pressure.
• A high temperature: runnier plastic is harder to control, see nozzle temperature.
• Damp filament, see moisture.
• Too much plastic, see over-extrusion.

With colour changes in a 3MF there are more places where the nozzle stops and starts again, and so more chances for small bumps. More colours mean more such places.

What you can do in the model

A seam cannot be removed, but it can be positioned. When a part is rounded, the seam shows up along one line, and in a sharp corner it is almost invisible. If it matters, design an edge or a groove at a place where the seam does not matter.

Very small, fine details are more sensitive to blobs than large surfaces.

For your order

Small zits are normal in FDM and we do not count them as a flaw until they are pronounced. If you want a perfectly smooth surface, FDM is not the best technology, and we do not do surface finishing such as sanding. We photograph every batch before dispatch.