Hi monica, the problem there is that your surface has bends in it where the radius of the bend is smaller than the offset distance that you want to use.

That creates a "bunching" type problem as illustrated here:





That's what's happened in your attached files there, in some of those tight bends the offset surface has that kind of messed up self-intersecting bunching in it.

Surface offset does not know how to deal with this kind of tight bending self collision, but curve offset does. Curve offset will produce a cleaner result which will have a sharp point in it. So one way forward would be to use curve offset on your profile curves and then construct new inside core piece from those curves, that's what Max's video shows above.

Please let me know if you are still stuck.

- Michael