Hi Larry, so for the MeshMixer unexpected seam display, I closely examined your file impeller05.obj which was this one:



There is indeed a seam here:



There is no seam or "island separation" to be found in the other area you circled though. I tested this by isolating the bottom face in Rhino and pulling the points, each one is a single point connected to everything else and they don't pull apart. Here they are each one point that has a long edge coming from it pulled slightly to the outside of the original circle:



So there is no seam to be found at that spot, there is however some very skinny triangles and a particularly skinny one in that area. It is just quite skinny though, not reversed.

So my best guess is that MeshMixer does not like such skinny triangles, maybe it considers it within some tolerance of being non-manifold.


re:
> This one here : https://grabcad.com/library/impeller-design-02-2-1 doesn't appear to be a false
> positive due to skinny triangles. I've uploaded both its .step and .obj here :
>
> https://drive.google.com/open?id=1VRlXPAT2w7gAgpBk9cftSscmdqZ5r-8I

I took a close look at the flat face of impeller02.2.obj using the above process and could not find any seams other than the one horizontal one.



re: saving as v2 - that's too bad that wasn't without it's own issues. I tested loading in your cage.only.3dm file into Rhino and also saw various meshing artifacts in there as well, here's what it looks like in Rhino:



So the save as v2 won't be viable, it has a different kind of messed up UV curves. So the problem in the original .3dm file is that MoI isn't able to use the UV trim curves on cylinders, cones, and spheres special case type objects. That means it has to make its own UV curves but there's a bug in that process somewhere. When you do a "save as v2", those special objects are converted into regular NURBS surfaces which means that MoI is able to use the UV curves from the v2 file. But the conversion of the UV curves from analytic UV space to NURBS UV space is not very good in the OpenNURBS toolkit. The weird artifacts in both MoI and Rhino are because of poor quality UV curves.

I'll investigate some more to try and see if I can figure out why MoI's UV trim curve generator isn't doing too well on your original converted 3dm file. It looks like it's sometimes going in the wrong direction when the first or last points of the 3D edge curve are on the seam edge of a closed surface.

Thanks,
- Michael