Hi niko - curves will only cap if the curves are closed and planar.

If you have projected your curves on to a curved surface, then you do not have planar curves.

What you can do though is to trim the surface by the curves, and then instead of extruding the curves, extrude the trimmed surface fragment instead.

When you extrude a face rather than a curve, it can be capped because MoI can copy the face to the end to make the cap.

If you just have some arbitrary non-planar curve MoI does not know how to create a surface to fill it in, that's why non-planar curves don't get an automatic cap on them.

Let me know if you need more details.

- Michael