Creating Contoured and Plastic Parts - Autodesk Inventor 2010: No Experience Required - page 369

FIGURE 8.4 Extruding the sketch
FIGURE 8.5 Add a fillet to the ends of the feature.
The Shell Tool
When you need a part to be hollow and (usually) have a consistent remaining
wall thickness, you can use the Shell command to remove a large portion of the
body rather than modeling the walls of a model individually. To do this, you select
a face or faces to be removed. The faces that you don't select are thickened and
remain a solid.