Hi, I am having some isses with the draft and shell features on my model.

I am trying to model a rotationally moulded part, hence the part should be hollow and have a uniform thickness everywhere.

I have had a similar shell issue with similar models in the past so I expect it's probably quite a common issue and there is probably already  an answer out there somewhere in the forum but I haven't found it yet. Basically the upper and lower surfaces of my model thicken into each other. In my model there are a series of ribs on the bottom, most of them have shelled exactly the way I want, but the circular rib hasn't and the bottom surface sticks up through the top surface (and vice versa).

I've tried loading it as a parasolid as suggested in a similar post but I get the same result.

The other issue I am having is that the part line draft is not working the way I want for two of the ribs. The last line on the selection is being ignored by the feature and giving me some crazy geometry. The drafted and filleted ribs you can see in the pictures are exactly what I want, the issue is with the highlited faces which are the ones basically being ignored by the feature.

So the features that I am having troubles with are:

shell 5

draft 7 and

draft 9

I'm using the academic SW 2012

(The way the model was made is probably a bit strange, I'm basically self taught with surfaces and the model was made around some 3d scan data)

Thanks in advance