Pairing Unopposed Surfaces
If you want to create shell elements but have a surface with no opposing surface, you have three options. The option you choose depends on your particular modeling needs.
Use one of the following methods to resolve unopposed surfaces:
• Suppress the feature—Suppress the feature that causes the problem. This saves both modeling and calculation time, but also sacrifices the most in model accuracy.
• Create an opposing surface—Create a small feature to give the unopposed surface a pairing mate. Note, however, that this method has a dimensional impact on the finished mesh.
• Split the surface—Split the surface and distribute it among its neighboring surface pairs. This procedure, while sometimes difficult, does the best job of retaining model accuracy.