I created my model in rhino with clearly connected solid, but when slice in simply3d, each wall are always seperated by a small seam, which make the model weak and flimsy.
Make sure that the overlapping polysurfaces are combined into one polysurface with BooleanUnion prior to exporting your mesh. I don’t have the 3dm file here to check but in the Rendered mode screenshot I see some display artifacts in these locations that are common when there are coincident surfaces from separate overlapping geometry.