<p>You have modeled your bird. So far so good, but you likely only modeled a single surface and not a closed surface body. The crucial step was forgotten, as your pictures 1 and 2 show: you have designed a single surface for most of the object, not a body. To turn the bird into a printable object needs it not to be a single surface but a surface enclosing a volume that has some thickness.</p>

<p>At the moment, it looks like this: 640 vertices, NO enclosed space.</p>

<p><a href="https://i.sstatic.net/UzNV4.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/UzNV4.png" alt="Model as shared"></a></p>

<p>To achieve an even thickness object in blender:</p>

<ul>
<li><code>A</code> to choose the whole model</li>
<li><code>E</code> for extrude Region</li>
<li><code>Z</code> <code>Z</code> to constrain movement to Z axis</li>
<li>type in the wanted thickness

<ul>
<li>remember, that the grid in Blender is usually in cm, while slicing programs reference in mm!</li>
</ul></li>
<li>close the edges by creating faces there (chose 3 and <code>F</code>)
<a href="https://i.sstatic.net/1Lg9T.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/1Lg9T.png" alt="Fix step 1"></a></li>
<li><code>A</code> to grab everything</li>
<li><code>W</code> then <code>R</code> to <em>remove doubles</em>, increase the merging distance to 0.05 (it takes away hundreds of superfluous, slightly shifted vertices!)</li>
<li><code>CTRL</code>+<code>N</code> to <a href="https://all3dp.com/2/blender-recalculate-normals-simply-explained/" rel="nofollow noreferrer">recalculate normals</a>
<a href="https://i.sstatic.net/ErQqb.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/ErQqb.png" alt="With normals turned visible - spiky = good!"></a>
Make sure to check the slicer, because we have some strays, visible in red... where are those? They are faces hidden in the body!
<a href="https://i.sstatic.net/8XS9I.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/8XS9I.png" alt="enter image description here"></a></li>
</ul>

<p>Hide the underside (Select nothing, allow viewing through the object, <code>3</code> > <code>B</code> > draw a box around the lowest layer > <code>H</code>)</p>

<p>If you have the normals visible, you'll see the iffy areas now. Fix them by removing the superfluous faces and flipping those that are not superfuous but just inside out (<code>W</code>><code>F</code>). One example area I highlight in the next picture
<a href="https://i.sstatic.net/MgF9E.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/MgF9E.png" alt="This is the inside, we don&#39;t want long lines here!"></a></p>

<p>In the end, it should look like this in cura:
<a href="https://i.sstatic.net/Ohylv.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/Ohylv.png" alt="Hunt for flipped vertices done"></a>
<a href="https://i.sstatic.net/qxAij.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/qxAij.png" alt="It slices..."></a></p>

<p>Make sure to check layer view and possibly thicken some areas manually - or make a box-part for the top, so you can ensure printability. As you'll see, at some scales, some walls are too thin due to how we extruded along Z only.</p>

<p><a href="https://i.sstatic.net/nLbEp.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/nLbEp.png" alt="A too thin wall"></a></p>

<h3>Alternate ways</h3>

<p>As noted in the comments, instead of the Z-Extrude, a model with very vertical walls could benefit from using the <a href="https://docs.blender.org/manual/en/latest/modeling/modifiers/generate/solidify.html" rel="nofollow noreferrer"><code>solidify</code> modifier.</a> You will have to add it via object mode, modifiers and then choosing <code>solidify</code> and setting a positive thickness. To properly convert the visible modifier into an actual change of the model for the export, you will have to <code>Apply</code> the modifier.</p>

<p>Afterwards, go back to hunting stray internal surfaces and flipped faces.</p>

CC BY-SA 4.0
2019-07-01T15:37:03.410Z
For thickness, wouldn't you want the extrusion to be "Normal" rather than "Z axis" ?  I'm not familiar with Blender but that's where I'd be headed in MeshMixer.
Carl Witthoft
CC BY-SA 4.0
2019-07-01T19:21:54.653Z
I used solidify modifire in blender to add thickness to the model. However, my model can not be printed correctly.
sara
CC BY-SA 4.0
2019-07-01T20:44:22.617Z
@CarlWitthoft extruding along the normals in blender does not create a single body but one body per face.
Trish
CC BY-SA 4.0
2019-07-02T09:49:21.317Z
To address the point raised by Carl, after the extrude-Z (by desired layer thickness, around 1mm), I would scale in X-Y (shift-Z) which isn't quite the right transformation. @sara, did you 'apply' the solidify? I think blender needs this when exporting as stl.
Sean Houlihane
CC BY-SA 4.0
2019-07-02T09:52:03.683Z
you need to apply solidify first, yes.
Trish
CC BY-SA 4.0
2019-07-02T10:30:35.427Z
Thank you so much. Did you check the preview of the model in cura? After these changes, it can be printed correctly? I was almost disappointed in printing this model :)
sara
CC BY-SA 4.0
2019-07-02T10:56:33.290Z
@sara according to cura, it should print with the fixes - you might want to tweak the "upper" in some areas to beef up the walls in some areas as the last picture shows.
Trish
CC BY-SA 4.0
2019-07-11T16:13:17.780Z
@anaximander added a path for that.
Trish
CC BY-SA 4.0
2019-07-01T11:13:32.090Z
2019-07-11T16:12:51.690Z
2019-07-11T16:12:51.690Z
Trish
<p>after a lot of search, i used the autodesk Meshmixer to add thickness to the model. I used the blender only for modeling and didn't use the solidify modifier to add thickness to the model. Then in autodesk Meshmixer, using select→ edit → Extrude, i added thickness to the model (the new stl file is presented). It seems that the new model can be printed. However, the blue lines can be seen in the print preview of Ultimaker Cura and i dont know how to remove them. Unfortunately, i haven't a 3d printer to test the model, but it seems that it is printable.
<a href="https://www.dropbox.com/s/u32zb0egiyrgpvo/%E2%80%8F00.stl?dl=0" rel="nofollow noreferrer">the link of repaired stl file</a></p>

<p><a href="https://i.sstatic.net/JzW7Q.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/JzW7Q.png" alt="repaired model in cura"></a></p>

<p><a href="https://i.sstatic.net/iMild.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/iMild.png" alt="the bottom of the repaired model in print preview of cura"></a></p>

CC BY-SA 4.0
2019-07-06T09:53:22.837Z
The blue lines are moves, there is a tiny gap at the tail end though (the same area I had without throwing a fix over the top part), otherwise, it's a good and working fix. Good job finding a way that works for you!
Trish
CC BY-SA 4.0
2019-07-06T09:45:19.080Z
2019-07-06T10:20:36.707Z
2019-07-06T10:20:36.707Z
sara
<p>Slicers don't do well with &quot;empty&quot; (hollow) bodies, or bodies with secret holes in it. You need to supply a filled body<sup>1</sup>, the slicer will make it mostly empty anyways (depending on the infill percentage). Also, you need to have infill, else the top cannot be printed as it does not have any support.</p>
<hr />
<h3>Footnote</h3>
<p><sup>1</sup> This means the body has to have one continuous surface without gaps or holes that encloses a volume. Among the most simple examples are spheres and cubes.</p>

CC BY-SA 4.0
2019-07-01T05:11:19.833Z
Thank you. I'm new to 3d printing. Could you please give me more guidance? In blender, i can not supply a filled model. I only can change the infill parameter in ultimaker cura.Based on the last image, it seems that the software can not detect the walls of my model and only it fills the middle of the model. Can i solve my problem by increasing the infill percantage in cura?
sara
CC BY-SA 4.0
2019-07-01T08:27:11.537Z
@sara Blender is not an optimal tool for creating models for printing. Blender support should be asked elsewhere, e.g. Blender forum? Similar issues have come along the past years, you could try to search for Blender specific questions. I'm not a Blender user; I just share slicer specific issues with hollow bodies. I have had issues with hollow bodies created in other tools.
0scar
CC BY-SA 4.0
2019-07-01T20:59:20.640Z
@0scar (and Sara) - as well as the Blender forum (link?), don't forget that there is also an excellent [Blender](https://blender.stackexchange.com) Q&A site here on Stack Exchange.
Greenonline
CC BY-SA 4.0
2019-07-02T09:16:35.373Z
Secret holes are no problem. I'm also critical of the comment about blender being non-optimal.
Sean Houlihane
CC BY-SA 4.0
2019-07-02T10:03:47.250Z
Blender can be a very powerful tool to fix errors or design organic looking, smoothed parts if you know how to use it.
Trish
CC BY-SA 4.0
2019-06-30T21:46:50.630Z
2019-07-01T21:00:49.540Z
2020-06-18T08:29:41.667Z
0scar
<p>I'm new to 3D printing. I modeled an empty bird in Blender (the stl file of model is presented). I tested the model in Blender (using 3D printing tool) and also the Netfabb software. They don't show any error. However, when I load this model in Ultimaker Cura for printing, as shown in the last image, the result is only a cylinder shape bird. I have seen many 3D printed empty models on internet.  Why can't my model be printed correctly?
<a href="https://www.dropbox.com/s/ymhpc8c1qne1i2c/bird_model.stl?dl=0" rel="nofollow noreferrer">the download link of the model</a></p>

<blockquote>
  <p><a href="https://i.sstatic.net/EB3Tq.png" rel="nofollow noreferrer" title="The top view of the model in Blender"><img src="https://i.sstatic.net/EB3Tq.png" alt="The top view of the model in Blender" title="The top view of the model in Blender"></a> </p>
  
  <p><a href="https://i.sstatic.net/5Xu4F.png" rel="nofollow noreferrer" title="The bottom view of the model in Blender"><img src="https://i.sstatic.net/5Xu4F.png" alt="The bottom view of the model in Blender" title="The bottom view of the model in Blender"></a></p>
  
  <p><a href="https://i.sstatic.net/9bVef.png" rel="nofollow noreferrer" title="The 3D printing parameters in Blender"><img src="https://i.sstatic.net/9bVef.png" alt="The 3D printing parameters in Blender" title="The 3D printing parameters in Blender"></a></p>
  
  <p><a href="https://i.sstatic.net/fh4q8.png" rel="nofollow noreferrer" title="The model in Cura"><img src="https://i.sstatic.net/fh4q8.png" alt="The model in Cura" title="The model in Cura"></a></p>
  
  <p><a href="https://i.sstatic.net/evekb.png" rel="nofollow noreferrer" title="The 3D printing preview of model in Cura"><img src="https://i.sstatic.net/evekb.png" alt="The 3D printing preview of model in Cura" title="The 3D printing preview of model in Cura"></a></p>
</blockquote>

CC BY-SA 4.0
2019-07-01T11:01:42.443Z
by empty you mean hollow? Or is your object just a top surface? Did you close the bottom of the object so it has one contiguous surface all over?
Trish
CC BY-SA 4.0
2019-07-01T19:15:14.737Z
I named the model empty because i didnt close the bottom of the object. I would like to use the printed object as a mold. Therefore the bottom of the model should not be closed.
sara
CC BY-SA 4.0
2019-07-02T06:20:36.743Z
The STL does  not slice at all in Ultimaker Cura 4.0. This is most probably an invalid model.
0scar
CC BY-SA 4.0
2019-07-02T07:23:03.483Z
Thank you. I have seen this problem. Sometimes the ultimaker cura can slice the model incorrect as shown in figures. But most of the times it is considered as an invalid model in cura and can not be sliced. I used netfabb to repair stl file. It shows the file as a valid stl file. However it can not be printed using cura. I think this a simple model and i cant realize what is the problem.
sara
<ultimaker-cura><3d-design><blender>
Problem in 3D printing of an empty model
sara
# Problem in 3D printing of an empty model

(Asked by: username_0 on 2019-06-30)

I'm new to 3D printing. I modeled an empty bird in Blender (the stl file of model is presented). I tested the model in Blender (using 3D printing tool) and also the Netfabb software. They don't show any error. However, when I load this model in Ultimaker Cura for printing, as shown in the last image, the result is only a cylinder shape bird. I have seen many 3D printed empty models on internet. Why can't my model be printed correctly?
[the download link of the model](https://www.dropbox.com/s/ymhpc8c1qne1i2c/bird_model.stl?dl=0)

> [![The top view of the model in Blender](https://i.sstatic.net/EB3Tq.png "The top view of the model in Blender")](https://i.sstatic.net/EB3Tq.png "The top view of the model in Blender")
>
> [![The bottom view of the model in Blender](https://i.sstatic.net/5Xu4F.png "The bottom view of the model in Blender")](https://i.sstatic.net/5Xu4F.png "The bottom view of the model in Blender")
>
> [![The 3D printing parameters in Blender](https://i.sstatic.net/9bVef.png "The 3D printing parameters in Blender")](https://i.sstatic.net/9bVef.png "The 3D printing parameters in Blender")
>
> [![The model in Cura](https://i.sstatic.net/fh4q8.png "The model in Cura")](https://i.sstatic.net/fh4q8.png "The model in Cura")
>
> [![The 3D printing preview of model in Cura](https://i.sstatic.net/evekb.png "The 3D printing preview of model in Cura")](https://i.sstatic.net/evekb.png "The 3D printing preview of model in Cura")

**Tags:** ultimaker-cura, 3d-design, blender
> **Comments:**
> **username_1**: by empty you mean hollow? Or is your object just a top surface? Did you close the bottom of the object so it has one contiguous surface all over?  
> **username_0**: I named the model empty because i didnt close the bottom of the object. I would like to use the printed object as a mold. Therefore the bottom of the model should not be closed.  
> **username_2**: The STL does  not slice at all in Ultimaker Cura 4.0. This is most probably an invalid model.  
> **username_0**: Thank you. I have seen this problem. Sometimes the ultimaker cura can slice the model incorrect as shown in figures. But most of the times it is considered as an invalid model in cura and can not be sliced. I used netfabb to repair stl file. It shows the file as a valid stl file. However it can not be printed using cura. I think this a simple model and i cant realize what is the problem.  

---

## 3 answers

### Answer by username_1 on 2019-07-01. Score: 4

You have modeled your bird. So far so good, but you likely only modeled a single surface and not a closed surface body. The crucial step was forgotten, as your pictures 1 and 2 show: you have designed a single surface for most of the object, not a body. To turn the bird into a printable object needs it not to be a single surface but a surface enclosing a volume that has some thickness.

At the moment, it looks like this: 640 vertices, NO enclosed space.

[![Model as shared](https://i.sstatic.net/UzNV4.png)](https://i.sstatic.net/UzNV4.png)

To achieve an even thickness object in blender:

* `A` to choose the whole model
* `E` for extrude Region
* `Z` `Z` to constrain movement to Z axis
* type in the wanted thickness
  + remember, that the grid in Blender is usually in cm, while slicing programs reference in mm!
* close the edges by creating faces there (chose 3 and `F`)
  [![Fix step 1](https://i.sstatic.net/1Lg9T.png)](https://i.sstatic.net/1Lg9T.png)
* `A` to grab everything
* `W` then `R` to *remove doubles*, increase the merging distance to 0.05 (it takes away hundreds of superfluous, slightly shifted vertices!)
* `CTRL`+`N` to [recalculate normals](https://all3dp.com/2/blender-recalculate-normals-simply-explained/)
  [![With normals turned visible - spiky = good!](https://i.sstatic.net/ErQqb.png)](https://i.sstatic.net/ErQqb.png)
  Make sure to check the slicer, because we have some strays, visible in red... where are those? They are faces hidden in the body!
  [![enter image description here](https://i.sstatic.net/8XS9I.png)](https://i.sstatic.net/8XS9I.png)

Hide the underside (Select nothing, allow viewing through the object, `3` > `B` > draw a box around the lowest layer > `H`)

If you have the normals visible, you'll see the iffy areas now. Fix them by removing the superfluous faces and flipping those that are not superfuous but just inside out (`W`>`F`). One example area I highlight in the next picture
[![This is the inside, we don't want long lines here!](https://i.sstatic.net/MgF9E.png)](https://i.sstatic.net/MgF9E.png)

In the end, it should look like this in cura:
[![Hunt for flipped vertices done](https://i.sstatic.net/Ohylv.png)](https://i.sstatic.net/Ohylv.png)
[![It slices...](https://i.sstatic.net/qxAij.png)](https://i.sstatic.net/qxAij.png)

Make sure to check layer view and possibly thicken some areas manually - or make a box-part for the top, so you can ensure printability. As you'll see, at some scales, some walls are too thin due to how we extruded along Z only.

[![A too thin wall](https://i.sstatic.net/nLbEp.png)](https://i.sstatic.net/nLbEp.png)

### Alternate ways

As noted in the comments, instead of the Z-Extrude, a model with very vertical walls could benefit from using the [`solidify` modifier.](https://docs.blender.org/manual/en/latest/modeling/modifiers/generate/solidify.html) You will have to add it via object mode, modifiers and then choosing `solidify` and setting a positive thickness. To properly convert the visible modifier into an actual change of the model for the export, you will have to `Apply` the modifier.

Afterwards, go back to hunting stray internal surfaces and flipped faces.

> **Comments:**
> **username_3**: For thickness, wouldn't you want the extrusion to be "Normal" rather than "Z axis" ?  I'm not familiar with Blender but that's where I'd be headed in MeshMixer.  
> **username_0**: I used solidify modifire in blender to add thickness to the model. However, my model can not be printed correctly.  
> **username_1**: @CarlWitthoft extruding along the normals in blender does not create a single body but one body per face.  
> **username_4**: To address the point raised by Carl, after the extrude-Z (by desired layer thickness, around 1mm), I would scale in X-Y (shift-Z) which isn't quite the right transformation. @sara, did you 'apply' the solidify? I think blender needs this when exporting as stl.  
> **username_1**: you need to apply solidify first, yes.  
> **username_0**: Thank you so much. Did you check the preview of the model in cura? After these changes, it can be printed correctly? I was almost disappointed in printing this model :)  
> **username_1**: @sara according to cura, it should print with the fixes - you might want to tweak the "upper" in some areas to beef up the walls in some areas as the last picture shows.  
> **username_1**: @anaximander added a path for that.  

---

### Answer by username_0 on 2019-07-06. Score: 3

after a lot of search, i used the autodesk Meshmixer to add thickness to the model. I used the blender only for modeling and didn't use the solidify modifier to add thickness to the model. Then in autodesk Meshmixer, using select→ edit → Extrude, i added thickness to the model (the new stl file is presented). It seems that the new model can be printed. However, the blue lines can be seen in the print preview of Ultimaker Cura and i dont know how to remove them. Unfortunately, i haven't a 3d printer to test the model, but it seems that it is printable.
[the link of repaired stl file](https://www.dropbox.com/s/u32zb0egiyrgpvo/%E2%80%8F00.stl?dl=0)

[![repaired model in cura](https://i.sstatic.net/JzW7Q.png)](https://i.sstatic.net/JzW7Q.png)

[![the bottom of the repaired model in print preview of cura](https://i.sstatic.net/iMild.png)](https://i.sstatic.net/iMild.png)

> **Comments:**
> **username_1**: The blue lines are moves, there is a tiny gap at the tail end though (the same area I had without throwing a fix over the top part), otherwise, it's a good and working fix. Good job finding a way that works for you!  

---

### Answer by username_2 on 2019-06-30. Score: 1

Slicers don't do well with "empty" (hollow) bodies, or bodies with secret holes in it. You need to supply a filled body1, the slicer will make it mostly empty anyways (depending on the infill percentage). Also, you need to have infill, else the top cannot be printed as it does not have any support.

---

### Footnote

1 This means the body has to have one continuous surface without gaps or holes that encloses a volume. Among the most simple examples are spheres and cubes.

> **Comments:**
> **username_0**: Thank you. I'm new to 3d printing. Could you please give me more guidance? In blender, i can not supply a filled model. I only can change the infill parameter in ultimaker cura.Based on the last image, it seems that the software can not detect the walls of my model and only it fills the middle of the model. Can i solve my problem by increasing the infill percantage in cura?  
> **username_2**: @sara Blender is not an optimal tool for creating models for printing. Blender support should be asked elsewhere, e.g. Blender forum? Similar issues have come along the past years, you could try to search for Blender specific questions. I'm not a Blender user; I just share slicer specific issues with hollow bodies. I have had issues with hollow bodies created in other tools.  
> **username_5**: @0scar (and Sara) - as well as the Blender forum (link?), don't forget that there is also an excellent [Blender](https://blender.stackexchange.com) Q&A site here on Stack Exchange.  
> **username_4**: Secret holes are no problem. I'm also critical of the comment about blender being non-optimal.  
> **username_1**: Blender can be a very powerful tool to fix errors or design organic looking, smoothed parts if you know how to use it.  

---

