CAD to .STL in Rhino

Create a solid object and check that it’s closed

Create a solid object in Rhino
Check the object using the “what” command; look for the words "closed" and "valid".

Results of "what" command for solid

Create a mesh version of the object and move it to its own layer

Select the object
Create a mesh version (Mesh>From NURBS Object)
In the dialog box, move the slider all the way to the right (More polygons) and click OK
Select the mesh object (not the original object) and move it to a new layer
If the mesh object is not resting in a stable position on the construction plane, reorient it
Check to be sure the mesh is closed by running the “what” command; look for language like “Closed polygon mesh” (See note below)
Save your Rhino model

Results of "what" command for mesh

Export the mesh object to an STL file

Select the mesh object only
File>Export selected
In the dialog box, Name the file and in the dropdown list “Save as Type”, select “STL (Stereolithography) (*.stl)”. Then click Save
In the dialog box that appears, choose Binary; also be sure the “Export open objects” box is NOT checked. Click OK

Dialog for export to STL

Note: Your 3D print job will succeed only if the object defined by your STL file is “closed”. Conceptually, this means that your mesh has no gaps or holes; that it is “watertight”.

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CAD to .STL in Rhino