What is a .3mf file?

What 3MF fixes

3MF was designed by the 3MF Consortium (Microsoft, HP, Autodesk, Dassault, Ultimaker, Prusa and others) specifically to replace STL for 3D printing. It fixes STL's three failings directly:

• It stores a unit. <model unit="millimeter"> is right there in the

file. A 3MF exported in inches opens at the correct physical size everywhere.

• It stores colour and materials, per object and per triangle.
• It stores multiple parts with their positions on the build plate, and it

can reference one object several times instead of duplicating its geometry.

It also stores print settings, supports and slicing metadata through extensions, which is why Bambu Studio, PrusaSlicer, Cura and Orca all use 3MF as their native project format.

What is inside the file

A .3mf is a zip archive, an OPC package like the ones Microsoft Office uses. Rename one to .zip and open it and you will find:

• [Content_Types].xml, declaring what the parts are
• _rels/.rels, pointing at the model
• 3D/3dmodel.model, which is the geometry as XML
• optionally thumbnails, textures and slicer-specific extension parts

The geometry itself is a vertex list and a triangle list, conceptually the same as STL but shared instead of repeated. That sharing, plus zip compression, is why 3MF files are commonly five to ten times smaller than the equivalent STL.

The build plate and the components tree

3MF handles multi-part prints with two structures.

Objects hold geometry. Components let one object include another with a transform. So a tray of twelve identical clips is stored as one clip plus twelve placements, not twelve copies.

Build items say what goes on the plate. A 3MF can contain objects that are not printed (a reference part, an alternative version) and the build section decides.

This site preserves that structure when converting to a format that can hold it, and tells you when converting to one that cannot.

Colour

3MF stores colour as sRGB, either as a colour group indexed per object or per triangle, or as base materials with a display colour. Multi-material printers use it directly.

Converting a coloured 3MF to STL discards all of it, because STL has nowhere to put it. The conversion report on this site says so.

Compatibility

Every current slicer reads 3MF. Support in CAD is good and improving: Fusion, SolidWorks, Rhino, Blender and FreeCAD all import and export it.

Where you still need STL is with older firmware-driven workflows, some online printing services, and any tool that has not been updated since about 2018.

Which should you send to a print service?

Ask them. If they accept 3MF, send 3MF: the unit is unambiguous, colour survives, and the file is smaller. If their upload form only lists STL, send STL and state the units in your order notes.