Hi MrBraun, yes MoI will only understand how to automatically cap an extrusion if it is of a planar curve.

If you extrude a non-planar curve you will get only the side parts and no capping surface.

There are a few different ways to get what you want though.

Like Pilou shows, one method is to trim the cylinder with your projected curve (projection is actually built in to the trim command directly, so you don't really need to do a separate projection step if you want to do this), then move the trimmed fragment over and that can be your cap.

Another method is instead of doing the extrude on the curve, you can trim out the surface piece, and then do an extrude with that small surface selected instead of the curve. When extruding a surface, MoI knows how to make an end cap for it, because it can be done by just copying the base surface over.

But generally it can be easier to keep caps on things if you work with solids throughout instead. By this I mean instead of doing a projection to generate a curve, extrude your circle into a cylinder and boolean it on to the other cylinder, then use a boolean to carve away the end with a curve.

This method is illustrated here:
http://moi3d.com/forum/index.php?webtag=MOI&msg=1359.2

It can be easiest to end up with a closed solid if you keep things as a solid more throughout the process and use booleans to cut pieces away.

- Michael