<p>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.</p>

CC BY-SA 4.0
2022-06-22T10:39:12.963Z
2022-06-22T10:39:12.963Z
Trish
<p>We've been doing some work automatically generating STL files using Python.  We've made a ring of cubes like so:</p>
<p><a href="https://i.sstatic.net/rdC1D.png" rel="nofollow noreferrer" title="Python generated ring of cubes"><img src="https://i.sstatic.net/rdC1D.png" alt="Python generated ring of cubes" title="Python generated ring of cubes" /></a></p>
<p>Importing it into Cura still makes it look valid, with a hole in the center:</p>
<p><a href="https://i.sstatic.net/Pw2Pd.jpg" rel="nofollow noreferrer" title="Ring of cubes in Cura"><img src="https://i.sstatic.net/Pw2Pd.jpg" alt="Ring of cubes in Cura" title="Ring of cubes in Cura" /></a></p>
<p>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:</p>
<p><a href="https://i.sstatic.net/6Yo0B.jpg" rel="nofollow noreferrer" title="Slicing preview"><img src="https://i.sstatic.net/6Yo0B.jpg" alt="Slicing preview" title="Slicing preview" /></a></p>
<p>And also a top / bottom layer:</p>
<p><a href="https://i.sstatic.net/VeEjJ.jpg" rel="nofollow noreferrer" title="Slicing preview - top"><img src="https://i.sstatic.net/VeEjJ.jpg" alt="Slicing preview - top" title="Slicing preview - top" /></a></p>
<p><a href="https://i.sstatic.net/iIAac.jpg" rel="nofollow noreferrer" title="Slicing preview - bottom"><img src="https://i.sstatic.net/iIAac.jpg" alt="Slicing preview - bottom" title="Slicing preview - bottom" /></a></p>
<p>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:</p>
<p><a href="https://i.sstatic.net/10RX0.jpg" rel="nofollow noreferrer" title="Close up of support structures midway"><img src="https://i.sstatic.net/10RX0.jpg" alt="Close up of support structures midway" title="Close up of support structures midway" /></a></p>

CC BY-SA 4.0
2021-06-28T08:48:35.320Z
This is a broken STL file see e.g. [this answer](/a/639/), 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?](/q/15868)"* is found [here](/a/15881/). 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.
0scar
CC BY-SA 4.0
2021-06-29T14:40:40.070Z
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.
Perry Webb
CC BY-SA 4.0
2021-06-30T15:16:22.193Z
+1 Nice question and nice images. However, @0scar - should this be closed as a duplicate? If so, against which of the two previous questions..?
Greenonline
CC BY-SA 4.0
2021-06-30T15:31:58.233Z
I hoped @Trish would notice this question, as this is Trishes specialty, with many excellent answers on this subject.
0scar
CC BY-SA 4.0
2021-06-30T22:11:56.670Z
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?
Ron Jensen
CC BY-SA 4.0
2021-07-05T11:30:50.653Z
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
Alexander Craggs
CC BY-SA 4.0
2021-07-05T11:34:31.773Z
Found out you can upload it to GitHub, so here's the code that generates it and the STL file. The GitHub rendering of the STL file seems wrong.  Blender, Cura and MatPlotLib all seem to display it accurately. https://gist.github.com/popey456963/470d995032d321b6e49100c51d65f249
Alexander Craggs
CC BY-SA 4.0
2021-07-05T23:48:19.583Z
Your normals are flipped randomly. When you generate the vertices of the triangles you need to keep the winding order consistent. This is important to Cura so it knows what the "inside" and "outside" of the model is.
Ron Jensen
CC BY-SA 4.0
2022-06-24T04:26:16.813Z
It looks to be a straightforward item - would you like it remade ?   (assuming this is still of use a year later)
Criggie
<ultimaker-cura><stl><python>
Why is this central area of the STL being filled in?
Alexander Craggs
# Why is this central area of the STL being filled in?

(Asked by: username_0 on 2021-06-27)

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](https://i.sstatic.net/rdC1D.png "Python generated ring of cubes")](https://i.sstatic.net/rdC1D.png "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](https://i.sstatic.net/Pw2Pd.jpg "Ring of cubes in Cura")](https://i.sstatic.net/Pw2Pd.jpg "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](https://i.sstatic.net/6Yo0B.jpg "Slicing preview")](https://i.sstatic.net/6Yo0B.jpg "Slicing preview")

And also a top / bottom layer:

[![Slicing preview - top](https://i.sstatic.net/VeEjJ.jpg "Slicing preview - top")](https://i.sstatic.net/VeEjJ.jpg "Slicing preview - top")

[![Slicing preview - bottom](https://i.sstatic.net/iIAac.jpg "Slicing preview - bottom")](https://i.sstatic.net/iIAac.jpg "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](https://i.sstatic.net/10RX0.jpg "Close up of support structures midway")](https://i.sstatic.net/10RX0.jpg "Close up of support structures midway")

**Tags:** ultimaker-cura, stl, python
> **Comments:**
> **username_1**: This is a broken STL file see e.g. [this answer](/a/639/), 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?](/q/15868)"* is found [here](/a/15881/). 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.  
> **username_2**: 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.  
> **username_3**: +1 Nice question and nice images. However, @0scar - should this be closed as a duplicate? If so, against which of the two previous questions..?  
> **username_1**: I hoped @Trish would notice this question, as this is Trishes specialty, with many excellent answers on this subject.  
> **username_4**: 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?  
> **username_0**: 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  
> **username_0**: Found out you can upload it to GitHub, so here's the code that generates it and the STL file. The GitHub rendering of the STL file seems wrong.  Blender, Cura and MatPlotLib all seem to display it accurately. https://gist.github.com/popey456963/470d995032d321b6e49100c51d65f249  
> **username_4**: Your normals are flipped randomly. When you generate the vertices of the triangles you need to keep the winding order consistent. This is important to Cura so it knows what the "inside" and "outside" of the model is.  
> **username_5**: It looks to be a straightforward item - would you like it remade ?   (assuming this is still of use a year later)  

---

## 1 answers

### Answer by username_6 on 2022-06-22. Score: 3

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.


---

