Shell features remove surfaces to hollow out a design model, leaving walls with specified thickness values.
Shells can be created using the Lead or Follow workflow. You can use drag handles or the dashboard to modify the thickness of the shell feature. The Flip icon in the dashboard is equivalent to specifying a negative shell value
Ribs are typically used to strengthen parts. A rib feature is similar to an extruded protrusion, except that it requires an open section sketch. The rib also conforms to existing planar or cylindrical geometry when it is extruded. After you select an open section sketch and set a thickness, Creo Elements/Pro automatically creates the rib feature by merging it with your model. The system can add material above or below the sketch, and the thickness can be applied on either side, or be symmetric about the sketch. The Rib tool enables you to create rib features faster than it would be for you to create and sketch a protrusion