STL, STEP and 3MF: Choosing a File for the Next Handoff

Choose a format around what the next person needs to do: slice a shape, work with CAD geometry, or reopen a prepared slicer project. Keep the native CAD file as your design master, and confirm the receiving application's supported formats before exporting.

Choose by the next task

• STL: send a surface mesh when the receiver asks for a model to slice. State the units separately and check the imported size.
• STEP: exchange CAD geometry when the receiver needs bodies or surfaces rather than a triangle mesh. Keep the native file if they also need your sketches and feature history.
• 3MF: exchange a model package, or save a slicer project when the receiving workflow supports that project's settings. Ask which kind of 3MF the receiver expects.

These choices describe the handoff. They do not establish that a file is accepted by every print service or ready to run on a particular printer.

STL: a surface mesh with an explicit size check

STL describes the object's surface with triangles. It is useful when you have finished the design and the next step is slicing, provided the receiving software accepts it. PrusaSlicer 2.9 documentation lists support for both binary and ASCII STL in its supported formats.

STL is unitless. Autodesk's Fusion export guidance explains that an export/import unit mismatch can change the apparent size. Export in the units the receiver requests, write those units in the handoff, and compare an imported dimension with the design. For example, if your bracket is designed to be 40 mm wide, the imported model should also measure 40 mm wide before slicing.

A triangle mesh does not carry the original CAD sketches and feature history. If a curved edge appears visibly faceted, inspect the mesh export settings and re-export from the design master. Changing the filename extension does not restore the original CAD construction.

STEP: CAD geometry without a promise of design history

STEP is a neutral CAD exchange format. It is a useful candidate when another designer or a service needs CAD geometry for the next operation. Ask which STEP variant or export settings their application expects.

Importing STEP does not necessarily recreate the model's original parametric design. Autodesk's Fusion STEP guidance says STEP imports in Fusion typically produce geometry without the original editable sketches, features and timeline. Send the native CAD file as well when those are part of the agreed handoff.

STEP support in a slicer also has a specific meaning: PrusaSlicer 2.9 documentation states that STEP import triangulates the geometry. You are then slicing a mesh representation, rather than continuing the original CAD feature history. Check the imported shape and dimensions just as you would after a mesh export.

3MF: distinguish a model package from a slicer project

The 3MF Core Specification describes mesh objects, resources and build information, with a model unit that defaults to millimetres when no unit is specified. A package can contain multiple objects. The 3MF specifications also define extensions, including materials and properties. What survives a handoff depends on what the exporter writes and the receiving application supports.

A 3MF exported as a model is different from a slicer project saved as 3MF. PrusaSlicer 2.9 documentation describes a File → Save Project as workflow that saves objects, settings, modifiers and their parameters in one file. That makes it useful for reopening a prepared job in a compatible workflow. See Saving projects as 3MF.

Before using someone else's project, confirm the slicer and version, printer, nozzle, filament and print profile. Inspect orientation, supports and sliced layers. A filename ending in .3mf alone does not tell you which settings are included or whether they match your machine. For layer-height choices, use the profile and preview checks.

Send a print service the file it requests

Start with the provider's published requirements, then confirm any uncertainty for your process. As one concrete example, Protolabs Network's 3D printing page lists STEP/STP and STL among its upload formats. 3MF is not included in that published list; confirm acceptance before relying on it. This is a published requirements example, not a test of the upload portal.

Use the print-farm directory to find a provider, then send a short handoff:

1. Identify the part and revision, with the requested file format.
2. State the units and one reference dimension so the receiver can check scale.
3. Specify quantity, requested material/process, finish and the date you need the parts.
4. Call out mating features, critical dimensions and any drawing or fit requirement you want reviewed.
5. Ask the provider to confirm the file opens correctly and the requirements are understood before production.

If you include a slicer project, label it as a reference unless the provider agrees to use that setup. Let them prepare machine instructions for their equipment. A model or project handoff and printer-specific output are different stages.

Check the exported file before sharing

Open the actual export in the receiving application when available. Confirm the intended bodies or objects, overall size and revision. For a slicer project, check the loaded profiles and preview the job before printing. Keep the native design and export together so a later change can start from the design master.

The practical choice is the format that carries the information needed for the next task and opens correctly in the agreed workflow. FilamentMap's settings hub provides related setup guidance once the model handoff is settled.

Sources

• PrusaSlicer supported file formats — STL support and STEP triangulation; PrusaSlicer 2.9 documentation, currently categorized as Legacy.
• Autodesk Fusion: STL/OBJ exports scaled down — unitless STL and export/import scale checks.
• Autodesk Fusion: importing STEP — CAD exchange and limits on retained feature history.
• 3MF Core Specification and specification index — units, objects and extensions.
• PrusaSlicer: saving projects as 3MF — project objects, settings and modifiers; PrusaSlicer 2.9 documentation, currently categorized as Legacy.
• Protolabs Network: 3D printing — the provider's published upload-format list.

Sources checked 9 October 2026. The file and project checks above are a workflow checklist; no sample part was sliced, printed or uploaded to a provider for this guide.