What is a 3MF file?

3MF is a container: a wrapper that holds streams encoded by something else. It stores shapes and coordinates rather than pixels, so it stays sharp at any size — a logo scaled to a billboard loses nothing. It is used for 3D printing.

The extension is .3mf, and the full name is 3D Manufacturing Format. Both matter less than what the file can hold, which is what the rest of this page is about.

3MF Consortium published it in 2015. The specification is 3MF Core Specification.

Age is worth knowing here for one practical reason: the older a format is, the more programs have had time to learn it.

It is published in full, so anyone can implement it from the document rather than by inspection, which is why it turns up in so many programs and why files written twenty years ago still open. A published specification is not the same thing as a royalty-free one: where a format wraps a codec, the patent licensing is a separate question the standard does not settle.

3MF stores its content exactly. Saving it again changes nothing, so it can be opened, edited and re-saved as often as you like without accumulating damage — which is what makes it a working format rather than a delivery one.

PrusaSlicer, Cura and Microsoft 3D Builder read it, and so do most programs of the same kind.

If a file will not open, the format is rarely the problem — it is more often that the program predates it. Converting to something older is the reliable way past that, and it is what the rest of this site is for.

No browser reads it.

That is the single most common reason to convert it: not that the format is bad, but that the place you want to show the file cannot read it.

3MF is meant to be opened and changed. Keep the file in this format for as long as the work is going on, and export from it whenever a finished copy is needed.

STL dates from 1987 and holds a list of triangles: no units, no colour, no materials, no structure, no metadata. Every one of those omissions has become a daily problem as 3D printing left the prototyping lab, and 3MF was published in 2015 by a consortium of printer and software makers to close them.

A 3MF states its units, so a model cannot arrive twenty-five times too large. It carries colour and material assignments, so a multi-material print is describable. It holds several objects with their positions, so a plate of twelve parts is one file that opens arranged. And it can carry print settings and supports alongside the geometry, which turns a shared file into something that prints the way the author intended rather than the way the recipient’s defaults happen to be set.

A 3MF is an OPC package — the same container idea as DOCX and XLSX — holding XML that describes the model plus optional parts for materials, thumbnails and vendor extensions. Rename it to .zip and it opens.

The choice buys three things. Compression, so a mesh is typically a fraction of the equivalent STL. Extensibility, so a printer manufacturer can add its own data without breaking readers that ignore it. And a thumbnail, which is why a folder of 3MF files shows previews where a folder of STLs shows identical grey icons.

STL allows anything: holes, inverted normals, self-intersections, surfaces with no thickness. A slicer then has to guess what was meant, which is where most failed prints begin.

The 3MF specification requires manifold geometry — every edge shared by exactly two triangles, consistent orientation, no degenerate faces. A conforming writer produces a printable model or refuses, which moves the problem from the printer back to the program that created it. That is a considerably better place for it to be found.

Inertia, and it is a good deal of inertia. Nearly forty years of model libraries are STL. Every tutorial says STL. Every slicer, viewer, repair tool and mesh utility written since 1990 reads STL, and a format that everything reads is difficult to displace with one that is merely better.

The direction of travel is nevertheless clear. Prusa, Bambu and other current slicers use 3MF as their native project format, which is what people are actually sharing when they publish a "ready to print" file with supports and settings included. Sharing platforms accept it. The change is happening through slicer project files rather than through anyone abandoning STL.

This causes real confusion. A 3MF exported from a modelling program is a model. A 3MF saved as a project from PrusaSlicer, Bambu Studio or Orca contains the model plus that slicer’s complete configuration — printer profile, filament, supports, modifiers, and per-object settings — in vendor-specific extensions.

A project file opened in a different slicer usually loads the geometry and ignores everything else, sometimes with a warning and sometimes silently. So a file advertised as containing tuned settings may arrive as a bare model. If the settings are the point, the file has to be opened in the slicer that wrote it.

To STL, everything except the geometry is discarded: units, colour, materials, object arrangement, settings, metadata. The shape survives exactly, since both store triangles, and the conversion is worth doing only when something in the chain will not accept 3MF.

In the other direction nothing is invented. An STL converted to 3MF is a 3MF with no colour, no materials and whatever units you tell it — a container upgrade rather than a restoration. Worth doing when a model is entering a workflow that expects the newer format, and pointless as a way of "improving" a file.

Use 3MF for anything with more than one part, more than one colour, or settings worth preserving; for a plate you want to arrive arranged; and for anything you will still be printing in three years, since the metadata is what makes a file comprehensible after you have forgotten it.

Use STL when uploading to a service or a library that asks for it, when the recipient’s software is unknown and old, or when the model is a single grey object and none of the extra information exists anyway. It remains the safe lowest common denominator, which is exactly the role it has played since 1987.