issue with stl when imported as mesh

[rdfry]

i have an stl of a case for an electronic device. it is a top and bottom combined as one body. i divide it into 2 separate bodys. it shows that it is hollow like i need it. however i can only work with the outside of each body. the internal sides act as if they are invisible. if i export back out as an stl and slice them they print completely solid. 

[TheCADWhisperer]

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@rdfry wrote:

... it shows that it is hollow like i need it.

I see infinitely thin walls.

In the real world don't walls have thickness?

Can you File>Export your *.f3d file to your local drive and then Attach it here to a Reply?

[rdfry]

here is the file. thank you for helping