How to Separate an Imported STL into Two Parts in Fusion 360
When you import an STL into Fusion 360, it comes in as a mesh body (a bunch of triangles). For proper mold design, it’s usually best to convert this mesh into a solid (BRep) body and then split it.
Method 1 (Recommended): Convert to Solid → Split Body
This is the standard workflow for mold-making.
Step 1 — Import the STL
- Go to Insert → Insert Mesh.
- Select your STL file and place it in the workspace.
Step 2 — Convert Mesh to Solid (BRep)
Fusion works best if the mesh has a reasonable number of triangles. If it’s very dense, reduce it first.
- Switch to the Mesh workspace.
- With the mesh selected, go to Modify → Reduce and lower the triangle count (aim for around 20k–50k triangles if possible).
- Then go to Modify → Convert Mesh and choose BRep as the conversion type.
- After conversion, the body appears as a solid (BRep) body instead of a mesh.
Step 3 — Create a Cutting Plane
You need a plane that defines where the object will be split (this is often your mold parting line).
- Use Construct → Midplane to split exactly in the middle, or
- Use Construct → Offset Plane to position the plane where you want, or
- Create a custom sketch and use it to define a splitting plane.
Step 4 — Split the Solid Body
- Go to Modify → Split Body.
- Body to Split: select your solid body.
- Splitting Tool: select the plane you created.
- Confirm the command.
Fusion will create two separate solid bodies (e.g. Body1 and Body2), which you can then use as mold halves or further modify.
Method 2: Split the Mesh Directly (Plane Cut)
Use this if the STL is too heavy to convert, or you just need a simple split and don’t plan complex mold features.
- Import the STL so it appears as a mesh body.
- Stay in the Mesh workspace.
- Go to Modify → Plane Cut.
- Position and rotate the cut plane where you want the split.
- Set the operation to Slice (not just cut one side).
- Confirm the command.
This will slice the mesh into two separate mesh bodies. Note that mesh bodies are less flexible for precise mold design compared to solid bodies.
