The fixture before the sentence
Every conversion note starts with a file that can be opened again. The test record for this site holds nine fixtures, including a 20 mm cube, a two-shell plate and a package whose production extension is required. The 20 mm cube leaves as 12 triangles and a 684-byte STL; the same cube wrapped in a 3MF is 1,042 bytes because the package also stores a model part, relationships and content types. Those two numbers are read from the files, not estimated from a screenshot.
The two-shell fixture is the more useful one. Its 3MF is 1,284 bytes and its STL is 1,147 bytes with 24 triangles. The smaller STL is not a better file: it has lost the object boundary. Both shells still arrive in one mesh and one build item, which is exactly the kind of detail a conversion note has to say out loud.
What gets checked
- The declared unit and the factor applied to every coordinate.
- Triangle count, bounding box and shell count after the STL is parsed again.
- Whether a required extension changes the result or produces a partial read.
- Claims about PrusaSlicer and Bambu Studio against those applications' own documentation.
The extension test is deliberately split. One fixture declares a production extension as required and converts cleanly. A second, split production package fails with Could not convert this file: Maximum call stack size exceeded in the browser, while PrusaSlicer 2.9.7 opens both. That disagreement is published as a disagreement, not rounded into “the converter works”.
Pictures are not proof
Technical posts use diagrams drawn from the fixture record. Experience posts use generated photographic illustrations. Neither is a photograph of a printed part, a light-box test or a caliper reading; the site has no workshop evidence and does not imply one. Screenshots of the result panel come from a browser run, and the caption repeats the numbers visible in that run.
Who answers for it
The Editor Team writes and maintains these pages. There is no freelance byline and no invented specialist. A correction sent to $editor@3mftostl.io goes to the same people who can change the page. Substantive changes are dated; the page is not silently rewritten.
The limit stated up front
An STL cannot carry the 3MF unit declaration, object names, colour or print settings. A note that says otherwise is wrong even if the mesh looks right. The test record lists the fixtures and the parsed outputs; the format reference lists what each container can hold.