Problem with "Shell" command on Mechanical Desktop

[olga]

I am working with hollow plastic part produced by rotational molding. I make a design of the parts in Mechanical Desktop 4. For machining molds from my models I don’t need to make the models hollow, but I would like to calculate the models with FEA program. I have tried to use “shell” command to convert solid model to hollow parts close to real, but on big models around 6-8MB I always have message: “Failed to compute Thin Wall Solid, Gap cannot be filled”

Does somebody have an idea what this message means and how to accomplish my objective-which is make it “Hollow”?

Thanks in advance.

Olga