Hi Alexander - so what's happening in this case is that your original surface is kind of angled in those corner areas instead of coming to a full 90 degree vertical end in those areas.
When you shell by thickening an open surface, the result follow the surface normal. So for example if you have a shape that has a side profile that looks like this:
When the surfaces are offset it will go perpendicular to that, making something like this:
That's what is producing the little raised areas - the profiles in the corners are kind of stretched out. You may want to build the corners of things like this by filleting an initial blockier shape instead, instead of trying to build a long single surface that goes through a sudden bend like that.
But there is a different way that you can use Shell that I think will give you what you want - instead of trying to thicken an initially open surface, you can instead use Shell to hollow out a solid.
When you use it to hollow out a solid, it will make a result that is flush to the face that you pick to be the opening, regardless of what is happening along the side wall portions (bits of the side walls that would have been sticking up past the open face will get trimmed off).
So for your case here, you want to build a solid out of your base shape instead of having it only open surfaces. To do that, select your base surfaces here:
Then run the Construct > Planar command, that will fill in a planar cap along the top to make it into a solid, it will look like this:
Now select that planar face, you do this by a 2nd click on the object, the first click selects the whole object and then a 2nd click on it will "drill down" to select edges or faces, in this case select the face.
When you use Shell with faces selected it will hollow out the object and leave that selected face as the opening. So your selection should look like this now:
Then when you run Shell it will produce a result like you were wanting I think:
The result is attached here as top_shelled.3dm
Hope this helps!
- Michael
