Once you convert a thin solid to a midsurface, it shows as a zero thickness surface, but the elements that mesh that surface are assigned a thickness value. There is a button on the Display tab in Mechanical to Show Thick Beams and Shells, and when you click that, you can see the original thickness if the part has been meshed.
To do Response Surface Optimization, you need to add some parameters. The thickness assigned to the midsurface can be one. If you go back to SpaceClaim, you can add a parameter for the length of the part, the width of the piece the belt wraps around.
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