STL to 3MF

An STL is a bag of loose triangles with no units and no name. Dropping one on preslice gives you a .3mf that has both.

How do I turn an STL into a .3mf?

Drop the .stl on the viewer, or click to browse. Binary and text STL both work. You get bracket.3mf from bracket.stl, holding one object named after the file, in millimetres.

Do the triangles change?

The vertices do. An STL repeats each corner once per triangle it touches, and the .3mf stores each corner once, which is what slicers expect and what makes the file smaller. The shape is untouched.

Where do the print settings come from?

Nowhere, at first: an STL carries none, so the new file is geometry alone. Open Convert, pick a slicer, and the file gets that slicer's default printer, filament and process settings.

Can I aim it at my own printer?

Yes. In Convert, tick printer, nozzle, bed or filament and choose from the list. Anything you saved under My hardware is marked in it.

What has the .3mf gained?

Units, an object name, a plate and somewhere to keep settings. The new file declares unit="millimeter", one of the six units the 3MF Core Specification allows on a model, so nothing downstream has to guess the scale. That is the whole difference between the two formats, and it is why a slicer would rather have the .3mf. See 3MF against STL.