Hi clegg - for the extrude command, MoI only knows how to put a cap on a closed flat planar end.

One alternative is to use those curves to trim out small surface pieces, and then you can use extrude on the small surfaces instead.

If you extrude a surface (currently it must be made up of a single surface, not more than one surface joined together) instead of a curve, it will be capped because then MoI knows it can copy the surface to the end to construct the cap piece.

- Michael