Convert 3MF to STL

There are two kinds of .3mf file

The first kind is a plain model, written by Fusion 360, Windows 3D Builder or a plugin that exports the 3MF standard. One shape, sometimes a colour, and the unit written into the file.

The second kind is a slicer project. When you save your work in Bambu Studio, OrcaSlicer or PrusaSlicer, you get a .3mf that holds the whole plate: every object you arranged, the supports, the printer profile and the sliced result. Bambu and Orca go one step further and put each object in its own part inside the package. Many converters only look at the first part they find, so they hand you one object out of four and say nothing about it. This one reads the whole package, follows every reference, and shows you the list.

Choosing what comes out

If the file holds more than one object, a list appears under the 3D view with the name and triangle count of each. Everything is selected to begin with, which merges the plate into a single STL. Turn one off and it disappears from the 3D view straight away, so you can see what you are about to get before you download it.

The names come from the file. A 3MF from the standard carries a name on each object; a Bambu or Prusa project keeps its names in a settings part alongside the model, and both are read. Objects keep the position they had on the plate, so a part that was at the back right of the bed arrives at the back right of the STL.

Units, and why the size is right

A 3MF states its own unit. It can be microns, millimetres, centimetres, inches, feet or metres, and the file says which. STL states nothing at all: it is a list of numbers and every program guesses. That gap is where most bad conversions happen.

Here the unit is read out of the 3MF and shown in the first box below the model. The output is set to millimetres, which is what every slicer expects, and the numbers are converted. A 3MF written in inches comes out at the correct millimetre size rather than being twenty-five times too small.

Painted colours and modifier meshes are not geometry

Bambu Studio and OrcaSlicer let you paint a model: colour for a multi-filament print, or seam and support painting that tells the slicer where to do something differently. None of that is stored as shape. It is stored as a per-triangle attribute pointing at a colour group, or as a separate settings part describing a region.

So a painted model converted to STL arrives as the plain shape it always was. Nothing is damaged and nothing is added — the painting simply has nowhere to live in an STL. The same goes for modifier meshes, those invisible boxes you place to change infill in one region: they are real geometry in the project, and including them in an STL would give you a solid block where you wanted a setting.

Modifier meshes are therefore left out of the conversion rather than merged in. They appear in the object list so you can see they exist, and they are not selected.