Mike Rife
Ansys Employee

Hi, it depends on the CAD program you are using.  Usually there is some feature to extract a mid-plane surface from a thin body.  Or create a "surface body" from the face(s) of a solid body.  This chapter from the AIS course on analyzing thin bodies may be of help:

https://courses.ansys.com/index.php/courses/geometry-representation-using-ansys-mechanical/lessons/analyzing-thin-structures-efficiently-lesson-2/

Mike