hi michael

why this happen

i make this
https://imgur.com/3WaeNXF

and connect it with network this look like this

https://imgur.com/MU33Yl1

and i mark all with ctrl + a and click on OFFSIET and click on SHELL, and i give thnickness and i put 0.07 direction is normal

when i export it .obj and open it in 3d coat see what happen inside is transparent, how to fix it.
https://imgur.com/u77Id7X outside

https://imgur.com/X3fZfEl inside

here is 3dm file
https://files.fm/u/xzmfe54s