Autodesk Inventor allows you to save both individual parts and assemblies in STL format, at all design levels. This details how to save out all parts and assemblies.
From the Tools menu, select Rebuild All. This ensures that the design data contains recent changes, and that it is not corrupt.
From the File menu, select Save Copy As
In the Save As Type field, select STL Files (.stl), and click Options. The STL Out Save Options dialog box opens.
Select High (for highest quality surface) and click OK
Note: The “High” setting will also produce the largest file size. ie. From Low, Medium to High, the hairdryer sample file in Inventor went from about 6.7MB to 17.6MB to 50MB.
In the Save Copy As dialog box, click Save