Autodesk Inventor supports export to plenty of other 3D modeling / CAD formats, including IGES 3D exchange format (.igs, .iges). So the easiest and most reliably way to convert .idw to .igs would be to use the original tool. Some viable alternative would be to use some other, 3rd party 3D modeling or CAD tool with support of Inventor's formats.
Updated: October 29, 2021
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