I often make designs which are easiest to make as a number of separate closed NURBS surfaces which overlap with each other. For example, something made out of sticks which meet each other at their endpoints is easy to make by drawing straight lines for the sticks, then using Rhino's "Pipe" command, which makes cylinders (with end caps) around the lines. When I turn the surfaces into meshes, what I get is a bunch of overlapping meshes. Rhino sometimes fails when taking unions of the meshes, or the NURBS surfaces, so what I've been doing is leaving it to shapeways to figure out the overlaps. (And according to http://www.shapeways.com/blog/archives/515-Website-Update-Ne w-Volume-Calculation.html they are doing it sensibly.) Sometimes however, I get a message from Shapeways along the lines of "too many shells". Does anyone know what this means? How many shells can we use? Unless I'm doing something wrong with my normals or something, and I really have more shells than I think I do, it seems very inconsistent. I just had a model rejected which (as I understand it) has two shells. I've also had things printed with hundreds of shells. Does "too many shells" really mean "our boolean union algorithm gave up"?