3MF viewer for the format that fixed what STL dropped

The 3D Manufacturing Format, published by the 3MF Consortium — Microsoft, HP, Autodesk, Ultimaker, Prusa and others — as an explicit replacement for STL. It is a ZIP container holding XML that describes the mesh, the units, materials and colours, and the arrangement of objects on a build plate. PrusaSlicer, Bambu Studio, Cura and Windows 3D Builder all read and write it natively, and it has become the default export of most slicers rather than an alternative to them.

For everything except universal acceptance, yes, and the gap is not small. 3MF states its units, so no one has to guess between millimetres and inches. It carries colours and materials, which STL has no way to express. It stores multiple objects with their placement, so a plate survives as a plate. It shares vertices instead of writing every corner three times, so files are smaller. And its XML schema forbids several kinds of malformed geometry that STL happily encodes. The one thing STL still wins is that literally everything reads it, including software abandoned a decade ago.

Two compounding reasons. STL repeats each corner once per triangle that touches it, so a typical solid writes every vertex about six times over; 3MF keeps one vertex list and refers into it by index, which removes that duplication outright. On top of that the XML is deflate-compressed inside the ZIP, and coordinate text compresses well. Halving is common and a factor of five is not unusual — with no loss of geometry, because the triangles are identical.

Yes, where the file defines them as part of the model. 3MF supports base materials, colour groups and a full materials-and-properties extension, and the ones that map onto a renderable material are applied here. The line under the file name states whether what you are seeing is textured, coloured per vertex, or plain. Slicer-specific settings that some tools tuck into a 3MF — supports, per-object print profiles, paint-on modifiers — are extensions outside the core model and are not read.

The geometry, yes; the project, no. Those tools save their projects as 3MF with extra private parts inside the ZIP for print settings, supports and plate configuration. This page reads the standard model part and ignores the rest, so you will see the objects correctly placed and measured but nothing about how they were going to be printed. That is the format working as designed — the standard part is the interchange, and the extras are the tool's own business.

If both ends of the exchange are modern, there is little argument left for it. Send 3MF to your own slicer, to a print service that lists it, and to anyone whose software you know. Keep STL for uploads to model repositories, for print farms with old workflows, and for the case where you cannot ask. The practical middle ground is to author in 3MF and export STL only when a specific destination requires it — losing units and colour on the way out is fine when you are the one who knows what they were.