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I´m looking at many tutorials on preparing things to send as .STL to a printer, but I keep on having the same doubt:

All the tutorials say that, in order to save money, you have to make a hole in the mesh so that the program interprets it as hollow, but at the same time the program wants you to have a totally closed mesh, so it is "manifold".

I really don´t understand how can I do a manifold mesh and leave a hole in it at the same time to be interpreted as hollow.

How is it done? If any of you know any tutorial where they explain it well, it would be a good answer also.

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As someone working in 3D printing, I can tell you that those tutorials are wrong, and making a hole inside a mesh to make it 'empty' inside is only going to create troubles.

Basically, to 3D print, your STL file goes through a program called a 'slicer', which will analyse and slice the mesh in layers before sending it to the printer for the actual printing layer by layer.

When the slicer analyses the mesh, it will identify the skin/walls of the mesh, determine what is outside and inside, which will also help it determine if supports are needed or not.

If, for example, you want to print a cube, making the mesh hollow inside means that the slicer will add support for the internal roof of the cube, support that you'll never be able to take out because it's a cube, and you cannot reach inside unless you break the cube open.

Plus, having mesh with 'holes' inside can sometimes confuse the slicer, especially if the walls of the mesh are thin, and prevent it from correctly interpreting what is inside and outside, or mess with the layers.

Besides, making a mesh empty is useless because all slicers have a setting called 'Infill' which determine the shape and percentage of the internal structure. Depending on your mesh, and how confident you are in the material being used, you can set it to 0%, meaning the printer will only print the walls of your mesh, up to 100% if you want to print a solid block.

So, to sum it up, making a hole inside a mesh is a bad idea, it will create problems, it won't really save you money nor time, especially since there is a setting in the slicer that does the same while ensuring proper printing.

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That is indeed misleading explanation, by "making a hole" the author probably means making it hollow, but not paper thin.

That means turning the object into a "shell" with an unspecified but non-zero thickness, making it manifold, but not solid.

Quickest way would be to use a Solidify Modifier, but it may not work well on all cases, like complex meshes, intricate details, tight curvatures or other discontinuities.

See Adding thickness to a mesh in Blender, otherwise you may have to do it by hand.

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  • $\begingroup$ I see... so you are saying that, in order to set the mesh right for printing, what I need to do is use the solidify modifier (I´ve already applied it in the mesh) but no making any hole. With the solidify modifier applied in a totally closed mesh is enought. Is it? (thanks a lot for your answer) $\endgroup$ – Ana Apr 11 '18 at 16:51
  • $\begingroup$ (by the way, they do say that you have to make a hole in the mesh for the printer to know that it is hollow. I don´t know if they are right or not but they do show that in their tutos) $\endgroup$ – Ana Apr 11 '18 at 16:53
  • $\begingroup$ The hole is probably to let air escape or some other structural purpose specific to the 3D printing technology. You should follow their recommendations. I'm not into 3D printing and obviously haven't read any guidelines you are quoting. $\endgroup$ – Duarte Farrajota Ramos Apr 11 '18 at 19:10

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