I have modeled a piece for STL output to be used in 3D printing. In the model, the piece appears to have a smooth surface. When I export, the surface becomes faceted. I have tried shrinkwrapping, welding the edges, changing to a nurbs surface, using SubD, and quad remeshing. Here is an image of how the model appears in rhino, vs the stl output.
First off, the technology:
An STL is a faceted mesh representation of your smooth NURBS surface. The smooth NURBS surface is “tessellated” into discrete flat mesh facets according to your STL export settings. The finer the mesh export settings, the smoother the result will be - but the file will also be heavier because it has more facets. The idea is to export an STL that is fine enough so that the facets will be smaller than your 3D printer’s resolution (so you will not see them), but not finer than that in order to avoid having a huge file. Some experimentation/experience is always necessary.
Now, as to what you are seeing on-screen:
By default, Rhino smooths the display of mesh facets so that they look smoother than they actually are (there are a number of reasons for this). To avoid this, in your display mode, right click the viewport title and check the box “Flat Shade”. This will show you the real faceting of a mesh with no artificial display smoothing.
When you export an STL directly from, there is no good way to see the result without re-importing the STL and looking at it under flat shade. A procedure I like is to first mesh the NURBS object in Rhino using the Mesh command. Then inspect the mesh object you created visually on-screen with flat shade. It is pretty WYSIWYG. If the mesh is too coarse, undo and mesh again with finer settings. When satisfied with the result, export the mesh object - not the NURBS object - as an STL.
Some good information on meshing here:
https://wiki.mcneel.com/rhino/meshfaq
Kyle’s video series on 3D printing: