# Problem in 3D printing of an empty model

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

• 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 Jul 1 '19 at 11:01
• 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 Jul 1 '19 at 19:15
• The STL does not slice at all in Ultimaker Cura 4.0. This is most probably an invalid model. – 0scar Jul 2 '19 at 6:20
• 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 Jul 2 '19 at 7:23

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.

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)
• 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 Make sure to check the slicer, because we have some strays, visible in red... where are those? They are faces hidden in the body!

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

In the end, it should look like this in cura:

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.

### 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. 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.

• 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 Jul 1 '19 at 15:37
• I used solidify modifire in blender to add thickness to the model. However, my model can not be printed correctly. – sara Jul 1 '19 at 19:21
• @CarlWitthoft extruding along the normals in blender does not create a single body but one body per face. – Trish Jul 1 '19 at 20:44
• you need to apply solidify first, yes. – Trish Jul 2 '19 at 9:52
• @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 Jul 2 '19 at 10:56

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

• 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 Jul 6 '19 at 9:53

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.

• 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 Jul 1 '19 at 5:11
• @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 Jul 1 '19 at 8:27
• @0scar (and Sara) - as well as the Blender forum (link?), don't forget that there is also an excellent Blender Q&A site here on Stack Exchange. – Greenonline Jul 1 '19 at 20:59
• Secret holes are no problem. I'm also critical of the comment about blender being non-optimal. – Sean Houlihane Jul 2 '19 at 9:16
• Blender can be a very powerful tool to fix errors or design organic looking, smoothed parts if you know how to use it. – Trish Jul 2 '19 at 10:03