Hello,

When making a model that is to be shelled then extra care is needed, as it can become a very complex process for calculation of the surface offsets, this applies to any nurbs package. I know some programs such as solidworks will attempt to build and give warnings of surface problems and will attempt to create an approximation, but you do need to spend the extra $.

I will put a post together to show what I did to change your model, which should help to show you how I would construct taking into account a need to try and keep the surfaces smooth.