Shells are a special type of surface or collection of surfaces you can use for special modeling operations, or for export to solid modeling packages.
Shells are collections of adjacent NURBS surfaces. Every surface stitched into a shell must meet the edge of another surface in the shell at some point.
Shells are stored as a single node in the DAG.
Shells can be open or closed. For closed shells, the normals should always point outward. This is necessary for the Boolean operations.
The main uses of shells are:
Some CAD packages deal with shells much better than normal trimmed NURBS surfaces.
The Boolean tools (Shell subtract, Shell intersect, and Shell union) only work on shells. Often you simply stitch surfaces into shells, apply a boolean operation, then unstitch back into surfaces.
Surfaces can only be stitched into shells if they are within an adjacency tolerance.
If the tolerance is set correctly, you can easily check whether a group of surfaces will export or build properly by checking whether they stitch together into a shell.
Use Object Edit > Query Edit to check for open edges in shells. Red arrows clearly mark gaps in the shell.
Shells have the following limitations:
In this case, unstitching does not produce surfaces that match the originals exactly either.