Boolean union two closed polysurfaces and getting an open polysurface?


Here I added the model and image to show you my problem. I want to apply boolean union command to solidify green structure and orange border together and even though both are closed solid polysurfaces when I join them boolean union command works and gives me a open-surface which is non-sense. Why to closed polysurfaces become an open surface?
R12.3dm (4.8 MB)

Hi -
It looks like those objects were previously used in a Boolean Difference operation - perhaps with slightly different input objects. All surfaces of the orange object are coinciding with the surfaces of the green object, except for an area to the right (along the x-axis). Here, surfaces create a wedge with some surfaces that get trapped inside the main object, creating a non-manifold object.

To avoid issues when using Boolean operations, always make sure that the objects cleanly intersect one another.

Also, that file was created in a very old version of Rhino 5. At least update that to the latest SR14 for Rhino 5.