Hi OSTexo - you're not getting caps on the extrusion because your curves there are not planar - MoI only knows how to automatically cap something if it is closed and planar.

Usually you would probably want to have extruded the original planar curves (instead of projecting them on to the curved surface) to make a cutting object. But if you want something that goes in by a certain distance hugging the contour of the surface instead of a straight distance in only one direction, then you would probably want to build an offset surface like Danny mentions.

I think you may also have the same curve replicated several times here? Some of them are not actually hugging the surface 100% because there is a bit of a bend to the surface there. So that's making an alternate method of using Trim + extrude the face difficult in this situation.

Do you have the original curve before you projected it? If you could post that I can show you another possible way to do this.

- Michael