Hi dron - it can help a lot if you would post the 3DM model files of your objects instead of only screenshots. Then that makes it possible to examine the actual geometry and zoom in on it, etc... all of which helps with answering questions about the geometry.
But I can tell you that it's likely that the difference between those is that your first closed curve is made up of many different segments and the segments have sharp corners where they touch each other. If you use Edit > Separate on that curve you will see it break into the individual segments.
On your second one that curve is made up of one single segment that is all one smooth curve, there are no sharp corners within the path. That's the difference.
When a sweep path is made up of multiple segments with sharp corners between them, the sweeper will make a separate sweep surface for each surface, and extend those separate sweep pieces at their ends and try to intersect them with each other to produce mitered corners. In some cases the intersection and mitering can be difficult to calculate, especially if the sweep paths are not planar or if they are tightly curved such that the sweep is bunching up.
When the miter intersection mechanism fails to intersect pieces properly you will be left with the extended sweep pieces and you'll need to find some way to trim the pieces yourself.
If you were to edit the curve, possibly using the Rebuild command, to make it all one smooth piece then there would not be any mitering happening, mitering will only happen at sharp corner points within the path curve.
If the path curve is not totally flat try flattening it down (select it and use the edit frame in a side view with "flat snap" as shown here: http://moi3d.com/forum/index.php?webtag=MOI&msg=3378.4) and that may help.
It's difficult for me to see on a screenshot if your curve is totally flat or not, that's one of the things that would be easier to determine if you could post the 3DM model file along with your question.
- Michael
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