About Shells
Use shells to model a thin layer of a defined thickness for your part. If your part is relatively thin compared to its length and width, shell modelling is more efficient.
You have several options for including shells in your model:
• Create a single shell as a layer on a solid. In this case, you can define the shell as a composite of layers, or specify the material and thickness.
• Create a shell pair based on the surfaces in your part. The software compresses the pair to a midsurface that it uses in the analysis.
• Create a volume-based shell that has specific material or thickness properties. In this case, the software compresses your model to a midsurface. But, it uses the material and thickness definitions of the shell rather than that of the surfaces associated with the volume for the midsurface thickness.
You create both single shells and shell pairs using the Refine Model tab. The command you select differs depending upon whether you want to create single shells or midsurface compressed shells:
• If your design includes shells that are made of the same material as your part, use the > 
Shell Pair to create shell pairs. To create a shell pair in this way your model must include at least two surfaces on opposite sides of a volume.
Shell Pair to create shell pairs. To create a shell pair in this way your model must include at least two surfaces on opposite sides of a volume.When Creo Simulate analyses a shell model, it compresses shell pairs to a midsurface or set of midsurfaces. To perform midsurface compression, Creo Simulate requires a pairing scheme. To ensure the successful analysis of your shell model, you should check the shell compression before you run an analysis.
• If you want to create a single shell, for example as a layer on a solid, or if you want to create a shell pair that is composed of a different material than the rest of your part, or if you want to include properties such as laminates or material orientation, use the > 
Shell.
Shell.You may choose to design a model solely with midsurface compressed shells, or your design may benefit from a mixture of shells, solids, beams, and other idealizations. If you are working with midsurface compressed shells, before you begin analysis, you should define your model as a solid, as a shell, or as a mixed model. The model type determines how Creo Simulate analyzes your part.
Before you create shells on your model, read about recommended geometry for your model.
Shells are displayed in shaded, outline, transparent or wireframe modes. You can select these display modes from the Shells drop-down list on the Settings tab of the Simulation Display dialog box that opens when you click 
on the Graphics toolbar.
on the Graphics toolbar.After you create a shell, you can edit or delete it by right-clicking on the shell idealization—either Shell Pair or Shell—in the model tree and using Edit Definition or Delete command, as appropriate. If you are deleting a shell, Creo Simulate asks you for confirmation first.