Make sure you have a hole that extends to the outside that connect the inner and outer surfaces. If you were to use the shell command on a sphere and uploaded it to Shapeways the automated software at Shapeways will delete the inner sphere. So, you need to connect the inner sphere with the outer sphere thus making a hole. Also too, you need to have a hole or multiple holes so Shapeways can remove unused material. Check the specifications for the material you want to use to find out the specific specifications for these support material removal holes.