# Creating a hollow solid object

Continuing the discussion from Problem creating hollow solids with a wall thickness:

Rhino doesnât create âsolidsâ per se. It is a surface modeller. So if you have your two spheres of different diameter, thatâs all it needs to know. You can group or block them if you want to keep them together.

No man, in Rhino V5, if you do as I described, you get a so called âsolid polysurfaceâ if you check the property of the object by running the âWhatâ command. And thatâs exactly what Iâm looking for as and end result so I can export a stl. file for printing a hollow object. Does it make sense?

So the exercise is pretty easy as I posted, if you just go through the steps youâll be able to replicate exactly what I meant. Itâs just with complicated surface conditions I cannot rely on offset surface to automatically calculate a valid offset surface for me to create that solid. And after post processing the offset surfaces, I couldnât find the right tool to combine it back to the original surface to create that so called âsolid polysurfaceâ. And you know that in rhino you cannot flip the normals of a closed polysurface. So maybe you can instead explain to me if you have those two individual polysurfaces how can you combine them into one geometry that I can send off to 3D print?

There you go - learn something new every day. Iâve never heard of such a thing in Rhino.

Anyway itâs not important for what you want to do.
All that a 3D printer wants to see is closed meshes with the normals pointing in the right direction (out for outside faces and in for inside faces).

First create your outer skin polysurface. Then shell it for the inner surface and clean up any horrors that appear and join that up into a polysurface. So far so good, except of course the normals on the inner skin are pointing in the wrong direction and as you rightly point out you canât flip normals on a polysurface. With meshes you can though and youâre going to need a mesh to export anyway, soâŚ
So now make good meshes of both polysurfaces at a density that reflects the 3D printer youâll be useing. Get the normals pointing the right way useing the DIR command and you are good to go.

Hope that helps, Steve

You can do this in Rhino with polysurfaces. It is generally not recommended, since the result may not behave as expected in other Rhino operations, but should be fine for your purposes.

Make the inner and outer polysurfaces. Call NonmanifoldMerge selecting both, Call CreateRegions on the result. Delete the inner region.

Chuck Welsh
Robert McNeel and Associates

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Thank you Steve, Thank you Chuck, it does the trick.