Keep the original as the source of truth
3MF can contain more than a triangle mesh: colors, material resources, texture references, component relationships and application-specific project settings may travel with it. STL stores triangles and normals. This tool exports supported geometry and a human-readable account of the conversion, but it does not transfer material definitions, color assignments, slicer profiles or support settings into STL.
The report is a geometry and loss manifest, not a complete archival copy of the project. It identifies the exported instances and their placement. To preserve editable settings and material assignments, retain the original 3MF or open it in the application that created it.
Recognize a deliberate refusal
The parser supports Core 2015/02 triangle meshes and the supported Production component-path mechanism. Unknown required extensions, external package relationships, missing geometry references, cyclic assemblies, unsupported geometry elements and non-model helper object types stop conversion. No remote model or texture is fetched to make a partial result appear complete.
The file limit is 32 MiB compressed, with 96 MiB total expanded ZIP data, 24 MiB per entry and 3,000 entries. There can be at most 128 expanded mesh instances and 100,000 source or expanded triangles. Repeated copies count separately. Excessive nesting, encrypted archives and unsupported ZIP structures are also refused.
Choose an appropriate next step
For an oversized model, export a simpler geometry-only copy from the original slicer or CAD application. For an unsupported extension, ask that application for an ordinary Core mesh export. Do not merely rename a file or remove the extension declaration: doing so can conceal information needed to interpret the object correctly.
A successful conversion confirms that this bounded geometry path produced actual STL bytes. It is not a check for every self-intersection, open edge or manufacturing defect. Reopen the result, compare dimensions and orientation, and inspect the target slicer’s layers. Use a dedicated repair workflow if the source mesh needs repair.