Will my groups stay separate?

Yes — each becomes its own 3MF object with its name. That is the main reason to choose 3MF over STL here.

What about the .mtl file?

It is a separate file and is not part of this conversion. The geometry is unaffected.

Is my model uploaded?

No. The parsing and rewriting happen in this tab. Models are frequently confidential — an unreleased product, a client’s part, a scan of something proprietary — and none of it is transmitted. Turn your Wi-Fi off and the tool keeps working.

How large a model can I convert?

There is no imposed limit; the ceiling is your device’s memory. A desktop browser handles files of roughly 2 GB and a phone about 400 MB. A 200 MB STL — around four million triangles — converts in a few seconds on a laptop.

OBJ to 3MF Converter — Multi-Part Models for Slicing

Converts OBJ to 3MF. Unlike converting to STL, this keeps your named groups as separate objects — which is what you want when the model is an assembly rather than a single part.

Groups stay separate

Each o or g group in the OBJ becomes its own object in the 3MF, keeping its name. In a slicer that means you can position, scale and assign materials to each part independently — the thing STL makes impossible.

Quads are triangulated first

3MF, like STL, is triangles only. Quads and n-gons are split with a fan, and the count is reported. Nothing else about the geometry changes.

Materials still do not come across

OBJ keeps materials in a separate .mtl file, which is not part of the file you dropped. 3MF can hold materials, but there is nothing to put in them. The geometry and the structure are what transfer.

3MF knows what a millimetre is, and OBJ does not

An OBJ file records numbers with no unit attached. Whether a 40 means 40 millimetres or 40 inches is a convention between whoever exported it and whoever opens it, which is why a model arriving at 25.4 times its intended size is the oldest complaint in 3D printing. 3MF fixes the unit in the document itself, so a slicer that reads it does not have to guess. That, rather than file size, is the reason the printing industry moved to it: STL and OBJ describe a shape, and 3MF describes an object somebody intends to manufacture.

Other names for this

Also searched as “obj to 3mf for slicing”.

Questions

Will my groups stay separate?: Yes — each becomes its own 3MF object with its name. That is the main reason to choose 3MF over STL here.
What about the .mtl file?: It is a separate file and is not part of this conversion. The geometry is unaffected.
Is my model uploaded?: No. The parsing and rewriting happen in this tab. Models are frequently confidential — an unreleased product, a client’s part, a scan of something proprietary — and none of it is transmitted. Turn your Wi-Fi off and the tool keeps working.
How large a model can I convert?: There is no imposed limit; the ceiling is your device’s memory. A desktop browser handles files of roughly 2 GB and a phone about 400 MB. A 200 MB STL — around four million triangles — converts in a few seconds on a laptop.