modling tube for 3d printing I'm trying to create a coupling in blender, that I can export to cura slicer program.

My problem is the slicer program fills in the center of my tube as if it is a wall.

I have tried creating it from a cylinder and just a plain circle/circles (inside and outside walls).

I exported both .obj and .stl files with the same result.

I'm uploading my file here once I post my question.
Check your 3D printer settings and limitations for how much empty space you are permitted between structures, sometimes you will have to model in support beams in hollow areas and you will have to remove them once printed. You could print a mold and use latex and resin or print and glue together two halves.