Convert STL to ASAT in your application
3D InterOp is a CAD interoperability SDK that lets your software read STL files and write ASAT, writing the STL facets as faceted geometry, since STL stores triangles rather than solids.
Spatial does not offer an online converter or a desktop conversion app. 3D InterOp is a component that software vendors embed in their own products.
- Reads STL · Writes ACIS assembly
- Triangle mesh → faceted B-Rep
- C++ / C# APIs
STL to ASAT at a glance
STL stores triangles; ACIS assembly is an exact-geometry format. The mesh is written as faceted geometry: 3D InterOp doesn’t rebuild smooth surfaces from triangles.
| STL (source) | ACIS assembly (target) | |
|---|---|---|
| Full name | Stereolithography | ACIS assembly, text (ASAT) and binary (ASAB) |
| Standard / owner | Open format | Spatial (3D ACIS Modeler) |
| File extensions | .stl | .asat, .asab |
| Geometry | Triangle mesh | Exact B-Rep |
| Assemblies / structure | No | Yes |
| 3D InterOp support | Read and write | Read and write |
What's preserved, what changes
Here is what your application can expect to carry across from STL to ACIS assembly.
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Changes
Faceted geometrySTL stores triangles, so the ASAT file holds faceted bodies, not solids. 3D InterOp doesn’t rebuild exact B-Rep from a mesh.
Why software vendors add STL to ASAT conversion
Hand models to kernel-based applications
Many CAD, CAM and CAE applications are built on the ACIS assembly kernel. Writing ACIS assembly gives them exact geometry they can load without another translation step.
Downstream modeling and analysis
An exact ACIS assembly B-Rep can still be edited, meshed or machined by downstream tools, which a tessellated export would not allow.
Exchange within one ecosystem
Partners whose tools share the ACIS assembly kernel often prefer to exchange ACIS assembly files directly.
Things to check before you ship
- Written ASAT versions: ACIS R18 to 2026 1.0.
- Supported STL input: any STL version.
- Validate a sample of translated files in the target system your users actually work with.
Working with Spatial was a real milestone for us: Thanks to the powerful 3DInterOp library, we were able to successfully bring our complex CAD-based solutions to the cloud while remaining flexible to meet individual customer requirements. For us, Spatial is not just a technology partner, but a true enabler of our product strategy.
Dr. Arno Michelis, Founder, Simus Systems
Read the Case StudyAbout the formats
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Source format
What is an STL file?
STL describes a 3D surface as a set of triangles, with no units, materials or assembly structure. Its simplicity makes it the usual input for 3D printing.
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Target format
What is an ASAT (.asat) file?
ASAT and ASAB are the assembly versions of the ACIS save file, in text and binary form. They store the product structure of models built on Spatial’s 3D ACIS Modeler kernel.
Frequently asked questions
No. STL stores triangles, and 3D InterOp writes them to ASAT as faceted geometry. It doesn’t rebuild exact B-Rep from a mesh.
An ACIS assembly (ASAT/ASAB) doesn’t support layers or colors at instance level, suppressed or hidden instances, or units at instance level.
STL files store no units, so the 3D InterOp STL reader assumes millimeters. Agree on the unit with the application that wrote the file.
3D InterOp reads any STL version, as visualization data.
3D InterOp writes ACIS R18 to 2026 1.0.
Yes. 3D InterOp reads and writes both formats, so ASAT to STL is supported too.
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Building software that needs to convert STL to ASAT?
3D InterOp is an SDK that lets your application read STL and write ACIS assembly alongside 30+ CAD and BIM formats.