Dear Solidworks Experts,

I extruded a rectangle, to make a simple cube. When I export it to a STEP file: it is not a closed water-tight solid, but 6 individual surfaces, representing the faces of the cube.

How to export STEP file from Solidworks, as a closed water-tight solid?
 

I tried using both STEP AP203 and AP214, but again I get individual surfaces.
I also tried checking/unchecking the "Split periodic faces" and "Export face/edge properties" options:

But still STEP file has 6 individual surfaces.

I did some googling, and someone suggested to export to Parasolid (.x_t) file first. Then to open that Parasolid file, and export it to a STEP file. This also did not work.

I would be very grateful if someone could help me export a STEP file from Solidworks, as a closed water-tight solid.

Thank you in advance for the help.