Mesh looks to have defects after export to STL

I am currently working on a ring, parts of which will be cast in gold.

Something happened after exporting the parts to STL that might be a bug, or it might be something I am doing wrong. Either way I would like to report it. I have uploaded the .3dm file so people can look at it and possibly duplicate the issue.

After successfully modeling the parts, I added the so-called casting sprues, and did a boolean union of the parts to the sprues. I then exported the individual parts as STL files.

When I opened the resulting files back into Rhino and either looked at them in the viewport in Rendered mode, or I rendered them via Render/Render, strange artifacts showed in all three parts in the locations where the sprues meet the parts. This showed up in 2 spots on each part, in the location shown in the image and on the opposite side. In other words in 6 places total.

I then decided to revert back to before I did the Boolean unions, and selected each part with its sprue, and then export to STL. The results were as I would have expected in the first place.

I did not change any of the STL Mesh options. When I did this via Mesh/From NURBS Object the same thing happened. I did not change the Polygon Mesh Options either.

I am running the latest version of Rhino on Yosemite 10.10.1

Is this a bug or should I have done something differently?

BF 03.3dm (1.2 MB)

Hi Abrasha - this is due to the meshing- you may just need to adjust the detailed meshing parameters - in particular, maybe set the Aspect Ratio to something like 5 or even less. See



That does not answer my question why it does not work after boolean union, but does work before boolean union. It seems that my mesh parameter settings did not affect that at all, since it worked fine without the union.

meshing the intersection after the boolean gets a little wonky…

mesh without Union:

mesh after union:

so Pascal’s suggestion is to make a finer mesh which will handle the intersection a bit better (at the expense of, basically, larger file)…

with max aspect ratio set to 4, the intersection looks like:

fwiw, STL is a mesh format so you need to pay more attention to the meshing parameters upon exporting.

[Edit] oh… (i think) i see what you’re saying now.

export as STL then import that back into rhino and it looks bad around the intersection…
not sure what’s going on there…sorry
i’ll try to look at it closer a little later if nobody else has by then

STL import lets you set the weld angle- at the top of that join, the angle between faces drops below whatever the weld angle is in your impirt, and the vertices are welded rather than showing a hard edge… You can weld at a smaller angle on import, or UnweldEdge the mesh edges at that join, or Unweld the object completely (which will show you the actual shape of the mesh, not a tricky display making it smooth.) Either way the shape and accuracy os the STL for RP purposes is unaffected. Keep in mind that STL does not bring vertex normals so Rhino cooks them up on import- the actual vertex normals from the surfaces from which the mesh was originally made is lost in the STL format.


Thanks Pascal,

I do not fully understand your reply. I’m afraid I am too much of a newbie with Rhino to fully get it.

Having said that, I did import the STL file (after boolean union) again, and this time I unchecked “Weld angle”. The blemish did not show this time, and the model looked as I would expect.

"Either way the shape and accuracy os the STL for RP purposes is unaffected."
Am I correct to understand this to mean, that even though the models have those strange artifacts after import back into Rhino, when I upload them to a service bureau (Shapeways for printing in castable wax), my prints will come out OK?

The artifacts are only visual “on screen”. If you want to see EXACTLY how the objects will look when RP’d, turn on Flat Shading in the Rhino viewport and look at it in rendered mode. That turns off the display smoothing (which is determined by how the mesh is welded) and you will see the individual facets as the RP machine will see them.

HTH, --Mitch

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