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We've been doing some work automatically generating STL files using Python. We've made a ring of cubes like so:

Python generated ring of cubes

Importing it into Cura still makes it look valid, with a hole in the center:

Ring of cubes in Cura

However, post slicing it comes up with saying it will take 14 hours to print! A ludicrous time. Looking at the preview, it seems to be adding supports to the entire inside of the structure:

Slicing preview

And also a top / bottom layer:

Slicing preview - top

Slicing preview - bottom

Why is this? You can tell it's adding supports to the center and is not filled in due to the differing structure midway through the block:

Close up of support structures midway

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    $\begingroup$ This is a broken STL file see e.g. this answer, probably due to incorrect normals or hidden faces, this has been asked before. A very good answer to question "Cura messes up my print. How can I make it print what I see in Blender?" is found here. The question is very good though, nice images, maybe this question deserves an answer to be referenced later, this has no link to Blender, but does hint to a broken STL file. $\endgroup$
    – 0scar
    Jun 28, 2021 at 8:48
  • $\begingroup$ Usually hidden faces have caused that kind of problem for me. It looks like you already have an unwanted surface in the hole after the import. $\endgroup$
    – Perry Webb
    Jun 29, 2021 at 14:40
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    $\begingroup$ I hoped @Trish would notice this question, as this is Trishes specialty, with many excellent answers on this subject. $\endgroup$
    – 0scar
    Jun 30, 2021 at 15:31
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    $\begingroup$ I'm new to this forum, but am quite experienced in generating 3d models. Can you share the STL that is causing the problem? I suspect you have concave surfaces that aren't triangulating as expected. Have you tried opening your STL in Blender and running the 3D print tests that software provides? $\endgroup$
    – Ron Jensen
    Jun 30, 2021 at 22:11
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    $\begingroup$ Any suggestions for a site I can upload the STL file to? I'm happy to, it's just a square so it's not private. @RonJensen $\endgroup$ Jul 5, 2021 at 11:30

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The surfaces are clearly ill defined - it shows the top surface as a bottom one (those are marked red), and so the normals of that surface are pointing to the wrong direction.

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