Sheet Thicken is used to generate a solid body from a sheet body. When sheet geometry is imported from CAD, a sheet body is automatically created with zero mass and mass moment of inertia. If you define the mass, mass moment of inertia and the center of mass of the sheet body in the body properties dialog, the sheet body will be closely modeled as a rigid body. However, if you want to treat the sheet body as a flexible body, the sheet body must first be changed to a solid body by using Sheet Thicken.
