<p>Some STLs aren't exported properly from the CAD software.</p>

<p>Use a repair tool or service like <a href="https://modelrepair.azurewebsites.net" rel="nofollow">https://modelrepair.azurewebsites.net</a> .</p>

<p>The repaired STL shouldn't have those issues anymore.</p>

CC BY-SA 4.0
2018-08-02T06:40:04.540Z
Thank you! I had a model that was driving me insane and this site fixed it perfectly.
Kenny Wyland
CC BY-SA 3.0
2016-01-13T14:28:22.347Z
This doesn't seem to answer the "why".
Tom van der Zanden
CC BY-SA 3.0
2016-01-13T14:34:53.763Z
That's getting into the inner workings of CAD tool export logic, which I don't know. My response to 'why' was high level and sufficient for practical purposes.
kareem
CC BY-SA 3.0
2016-01-13T22:37:49.107Z
Honestly, I feel like this is not the kind of quality that we want here, especially at this time. Could you improve your answer, and add more detail? You're probably better off to have this as a comment. -1
Zizouz212
CC BY-SA 3.0
2016-01-13T14:21:37.690Z
2016-01-13T14:21:37.690Z
kareem
<p>In the cad software, it could be that your normals are reversed or inconsistent. Recently I used Cinema4D to export some models, and the faces were acting super weird in the slicer.</p>

<p>What I mean by 'reversed normals' is that your polygon faces could be oriented inward. Some slicing softwares don't care about that, and treat both sides of each face equally. In those that observe reverse normals, faces may be completely invisible, or appear with a lack of shading or strange shading.</p>

<p>There should be tools in your cad software to align the normals, or flip them on each face at least. Alternatively, some tools like MeshLab and Meshixer I believe have tools to fix this. Also of course make sure your model is watertight and that you don't have too many errors like duplicate vertices, faces, etc.</p>

CC BY-SA 4.0
2018-05-27T16:47:04.853Z
2018-05-27T16:47:04.853Z
Evan
<p>Had similar problems with blender &lt;-> Slic3r, modified a stl in blender then slic3r started acting up, showing strange faces where it shouldn't.
The solution was (as suggested by @tbm0115) to solidify the exported object.
Just add a Solidify modifier to the object(no need to apply), and when exporting to stl make sure to check the "Apply modifiers".</p>

CC BY-SA 4.0
2020-02-11T06:23:05.263Z
2020-02-11T06:23:05.263Z
Otto Imre
<p>My understanding is that this occurs when the object is not a true solid. Since an STL holds the triangulation of each face and spline, the slicing engine is not &quot;smart&quot; enough to determine if there is a gap in the model and therefore if it should be filled in and how. When the slicer encounters a gap, it will either treat the endpoint as the end of the feature or navigate to the next point on the layer, resulting in either gaps in the print or extra inclusions that don't make sense.</p>
<p>I've noticed that my models will fail as a true solid when I use complex solid tools such as Union, Subtract, and Trim. A lot of times there will be a rounding error in how these tools interact with the solid model that will result in a small (sometimes not very small) gap in the outer shell of the solid model. When exported to an STL, the gap is retained.</p>
<p>@kareem mentioned it in their answer, but Microsoft does provide online tool(s) for 3D Printing including a solid repair tool. Use <a href="https://tools3d.azurewebsites.net/" rel="nofollow noreferrer">Microsoft 3D Tools</a> to upload your STL and try to automatically detect and fix issues with your file(s).</p>

CC BY-SA 3.0
2016-01-13T18:57:53.180Z
`rounding error` That explains a lot. I often have to add or subtract a tiny fudge factor (eg 0.001 mm) when I'm using OpenSCAD to make it unambiguous when two faces or lines would otherwise coincide. Now I know why I need to do that *and* why it works.
Marsh
CC BY-SA 3.0
2016-01-13T19:16:04.970Z
It's been a while since I've had some better CAD tools available, but I think there's an inverse correlation between price of the CAD software and how often this error occurs lol I mostly use the free version of Sketchup and this happens almost every time. I ended up downloading solid checking plugins to avoid bad prints.
tbm0115
CC BY-SA 4.0
2020-06-24T05:52:43.593Z
Another service I use is Autodesk Netfabb at https://service.netfabb.com/
0scar
CC BY-SA 4.0
2016-01-13T15:48:38.257Z
2020-06-23T20:15:32.593Z
2020-06-23T20:15:32.593Z
tbm0115
<p>In my slicing software (Slic3r) some of the vertices/walls of my model seem to have disappeared, so that the inside of the model - which should be solid - is visible, while the surface appears as a thin shell.</p>

<p>Why does this happen? Is it still safe to export the model for printing?</p>

<slic3r><print-preparation><slicing><g-code>
Why do some models appear broken in slicing software?
Tormod Haugene
# Why do some models appear broken in slicing software?

(Asked by: username_0 on 2016-01-13)

In my slicing software (Slic3r) some of the vertices/walls of my model seem to have disappeared, so that the inside of the model - which should be solid - is visible, while the surface appears as a thin shell.

Why does this happen? Is it still safe to export the model for printing?

**Tags:** slic3r, print-preparation, slicing, g-code

---

## 4 answers

### Answer by username_7 on 2016-01-13. Score: 15 | Accepted answer

My understanding is that this occurs when the object is not a true solid. Since an STL holds the triangulation of each face and spline, the slicing engine is not "smart" enough to determine if there is a gap in the model and therefore if it should be filled in and how. When the slicer encounters a gap, it will either treat the endpoint as the end of the feature or navigate to the next point on the layer, resulting in either gaps in the print or extra inclusions that don't make sense.

I've noticed that my models will fail as a true solid when I use complex solid tools such as Union, Subtract, and Trim. A lot of times there will be a rounding error in how these tools interact with the solid model that will result in a small (sometimes not very small) gap in the outer shell of the solid model. When exported to an STL, the gap is retained.

@kareem mentioned it in their answer, but Microsoft does provide online tool(s) for 3D Printing including a solid repair tool. Use [Microsoft 3D Tools](https://tools3d.azurewebsites.net/) to upload your STL and try to automatically detect and fix issues with your file(s).

> **Comments:**
> **username_8**: `rounding error` That explains a lot. I often have to add or subtract a tiny fudge factor (eg 0.001 mm) when I'm using OpenSCAD to make it unambiguous when two faces or lines would otherwise coincide. Now I know why I need to do that *and* why it works.  
> **username_7**: It's been a while since I've had some better CAD tools available, but I think there's an inverse correlation between price of the CAD software and how often this error occurs lol I mostly use the free version of Sketchup and this happens almost every time. I ended up downloading solid checking plugins to avoid bad prints.  
> **username_9**: Another service I use is Autodesk Netfabb at https://service.netfabb.com/  

---

### Answer by username_1 on 2016-01-13. Score: 3

Some STLs aren't exported properly from the CAD software.

Use a repair tool or service like <https://modelrepair.azurewebsites.net> .

The repaired STL shouldn't have those issues anymore.

> **Comments:**
> **username_3**: This doesn't seem to answer the "why".  
> **username_1**: That's getting into the inner workings of CAD tool export logic, which I don't know. My response to 'why' was high level and sufficient for practical purposes.  
> **username_4**: Honestly, I feel like this is not the kind of quality that we want here, especially at this time. Could you improve your answer, and add more detail? You're probably better off to have this as a comment. -1  
> **username_2**: Thank you! I had a model that was driving me insane and this site fixed it perfectly.  

---

### Answer by username_5 on 2018-05-27. Score: 1

In the cad software, it could be that your normals are reversed or inconsistent. Recently I used Cinema4D to export some models, and the faces were acting super weird in the slicer.

What I mean by 'reversed normals' is that your polygon faces could be oriented inward. Some slicing softwares don't care about that, and treat both sides of each face equally. In those that observe reverse normals, faces may be completely invisible, or appear with a lack of shading or strange shading.

There should be tools in your cad software to align the normals, or flip them on each face at least. Alternatively, some tools like MeshLab and Meshixer I believe have tools to fix this. Also of course make sure your model is watertight and that you don't have too many errors like duplicate vertices, faces, etc.


---

### Answer by username_6 on 2020-02-11. Score: 0

Had similar problems with blender <-> Slic3r, modified a stl in blender then slic3r started acting up, showing strange faces where it shouldn't.
The solution was (as suggested by @tbm0115) to solidify the exported object.
Just add a Solidify modifier to the object(no need to apply), and when exporting to stl make sure to check the "Apply modifiers".


---

