Hi Anis, the geometry library that MoI uses has some bugs in it when it comes to shelling certain surfaces that have their ends collapsed down into a single "pole" type point like there is in this situation.

Currently to get a good shell of a shape like this, you have to model it in a slightly different way - instead of making those curves come to a point at the top and bottom, make them into a more square kind of sheet. Then you can trim away the corners of that sheet with a rounded curve.

A larger rectangular sheet that is then trimmed will be a better input shape for shelling than what you've currently got.

Even aside from bugs in the geometry library, it is kind of the nature of such pinched together shapes to have a sort of bumpy surface normal as the shape approaches the tip, when all the control points for the surface come together it is just not unusual for any small variations in them to produce kind of "micro" lumps in the surface - those are the kinds of things that are generally difficult for the offset and shelling functions to process.

Meanwhile a larger rectangular sheet has a much, much more smooth surface quality to it, those kinds of surfaces are just more suited for offset and shell calculations.

Please let me know if you would like some visual examples on the above.

- Michael

Edit - oops I see that Fredrik has already answered with the same advice - yes the image he shows there is the same kind of thing.