How to Convert 3D Models into STL Files

Exporting from Rhino to .stl
To export your object(s) to a steroelithography (.stl) file format, select your object(s) and go to File, Export Selected. You can also use the Export command. In the Save as type option drop down menu, select .stl as the file type.
If your object(s) are NURBS poly surfaces the first dialog box that comes up will ask you for tolerance input regarding the meshing of your NURBS objects. The number you input for the meshing tolerance will determine the number of mesh t
STL file mesh export option
To have more detailed control of your export meshing options, click the Detailed Controls button. A Polygon Mesh Detailed Options dialog box will appear. Here you can set the mesh density, maximum angle, maximum aspect ratio, minimum edge length, maximum edge length, maximum distance edge to surface and the minimum initial grid quads. A good rule of thumb for inputs here is to leave them all blank but the maximum distance edge to surface. The maximum distance edge to surface should be less than half of the 3D printer’s resolution. Click OK.
Polygon mesh option for STL file
The STL Export Options dialog box will appear. Set the file type as Binary. If you leave the Export open objects box checked then the Export command will export all objects regardless of whether they have open edges, gaps or holes. This is not ideal. Uncheck this option so that you are unable to export objects that will not 3D print without error. Click OK.
STL file export option
If your object(s) are meshes (or if you have converted your original NURBS poly surfaces to meshes using the Mesh command) you will be immediately presented with the STL Export Options dialog box.
If there is a problem with your object(s), a window will pop up that indicates that your object(s) are not adequate for 3D printing. Take this warning seriously. If you decide to Export Anyway you risk 3D printing objects that will have printing errors and potentially waste a lot of 3D printing material and money.
STL file export warning
Click Cancel and fix your modeling errors within Rhino. Mitigating Export Errors
Common sources of error:
Surfaces not being joined together
Double or overlapped surfaces
Holes in a poly surface
Rebuilt edges of surfaces which look like they are coincident but are not identical