[Failing to get settings right for hollow no top/no bottom]

Thank you so much for your help! Can you clue me in on why a spherical surface creates non-manifold edges?

> Spheres are hard to print without infill or interior support of some kind.
Oh, that makes sense, I'll see what I can figure out. 

[Failing to get settings right for hollow no top/no bottom]

Hi, I am trying to create a resonator object to play with, but am having no success getting the slicer output settings to produce the part that I've designed. I'm getting the "missing or extraneous surfaces" error, whether I use the version of the part where I have "thickened" the surface, or just with the raw surface. (Mesh tools says it's not watertight, but that's not a surprise, it's not supposed to be enclosed to begin with.)

 

I prepared the model on Onshape, and my suspicion is that I have intersecting faces sharing an edge, though I've tried to union all the pieces together before exporting the STL. The end result I'm looking for is solid walls, (i.e. with no infill, approx. 2-5mm thickness, this is just a first pass so I don't have a precise specification for the wall thickness) with no infill in the cavity, no top, and no bottom. Help!

 

 

helmholtz resonator.stl