Modifier : Shell
Works on: Splines, Meshes, Polys, Patches, NURBS
3ds Max Description : The Shell modifier “solidifies” or gives thickness to an object by adding an extra set of faces facing the opposite direction of existing faces, plus edges connecting the inner and outer surfaces wherever faces are missing in the original object. You can specify offset distances for the inner and outer surfaces, characteristics for edges, material IDs, and mapping types for the edges.
Also, because the Shell modifier doesn’t have sub-objects, you can use the Select options to specify a face selection for passing up the stack to other modifiers. Please note that the Shell modifier doesn’t recognize existing sub-object selections, nor does it pass such selections up the stack.

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