Boolean operation on two objects in Blender
Check your settings in the slicer software you use: if you look at the sliced print carefully, you see only one line in each layer that crosses the inner section. The lines are also yellow, not green. This strongly hints that these are movement commands, not actual print commands.

On the other hand, you might want to carefully check in blender: at times, the normals in places get swapped inside out and then mess up the whole geometry without being visible in blender. Recalculate Normals can fix that at times, other times choose the messed up vertices and flip normals.

Make sure not to include voids in the model.

The model in question

Looking at the model, I also spotted that there was a lingering solidify modifier. Applying that resulted in this:



This is a particularly bad thing. Fixing is easy though: remove the offending inner vertices. 



Atop that, the center points are a clustered mess of vertices - surely you want to merge them into just two vertices.

Applying the Boolean modifier now created a solid shell.



Before exporting, all other items had to be removed (deleted), then the export gives this nice model: