3MF vs STL — what changes when you export

Both formats describe the same geometry, and that is where the similarity ends. STL stores a list of triangles and nothing else: no units, no colour, no name, no parts. 3MF is a zip archive with an XML model inside, and it can carry per-triangle paint, multiple objects, plate layout and print settings alongside the triangles.

Why it happens

That difference is why multicolour work lives in 3MF. Bambu Studio stores AMS paint as attributes on individual triangles — data an STL has nowhere to put. Export a painted model to STL and the colour is gone before the file is written; convert it back to 3MF and it does not return.

How to fix it (about 30 seconds)

1. If the model has paint or multiple parts, keep it in 3MF. Repair it as a 3MF so the paint attributes travel with the geometry.
2. If it is a single unpainted part, either format repairs the same way — STL is smaller and universally readable.
3. When someone sends you an STL of a painted model, that paint is already lost. Ask for the 3MF before you plan a multicolour print.
4. To check either format before printing, drop it into the Ferdelance checker: .3mf and .stl both inspect free, in the browser, with nothing uploaded.
5. After a repair, export in the format you started with — a 3MF in, a 3MF out, with paint intact.

More fix guides

• How to fix non-manifold edges
• Your 3MF isn’t watertight — here’s the fix
• Fix holes and missing faces in your model
• Repair a 3MF without losing your AMS colors
• Free STL repair that never uploads your file
• Your 3MF won't open in Bambu Studio
• Clearing a non-manifold warning in OrcaSlicer
• Check the file before you trust it with a print
• Bambu Studio on Mac has no Fix Model
• Manifold, watertight, and what the slicer is really checking