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The top shape is the one I try to replicate. I can use the boolean modifier to create those shapes, but as long as I put the mirror modifier below the boolean modifier, the faces, for some reason, are lost. Does anyone know what caused this problem?

enter image description here My Blend File

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  • $\begingroup$ Is it a stacking order issue? move the mirror modifier in the stack maybe $\endgroup$
    – barkest
    Aug 5, 2019 at 10:25
  • $\begingroup$ @barkest: I want to mirror the boolean modifier, so that I dont have to do it twice. Can I technically do that operation? Anw, I have just updated the question with my blend file. $\endgroup$
    – LOTR_7193
    Aug 5, 2019 at 10:31
  • $\begingroup$ The mirror can't merge as the empty is misplaced to allow it. In consequence, both mirrored parts are 'opened' as they are disconnected. That's why the bool op can't work as you whish. And of course, you need to place mirror before the booleans $\endgroup$
    – lemon
    Aug 5, 2019 at 10:42
  • $\begingroup$ @lemon: Would you mind elaborating on why the empty is misplaced? I put it in the center of the mesh. Also, I dont understand this sentence "both mirrored parts are opened as they are disconnected". I still dont understand the underlying reason why the faces are lost. Could you please write for me another comment? $\endgroup$
    – LOTR_7193
    Aug 5, 2019 at 10:49

2 Answers 2

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If you take the mesh with only the mirror, you can notice that the mirror will not merge the parts:

Here in the top part of the image, the mesh before the mirror is applied and on the bottom, after its application (both parts are not merged).

enter image description here

As they are not merge, Blender can only see opened meshes, so the boolean differences operate on open meshes too and that's why the result is opened too.

To fix that, just move the empty a bit along X.

Secondly, and for the same reason, the mirror needs to be set on top of the stack (at least before the booleans), as if it is not, when a boolean operates, the mesh is opened and you have the result shown in your question.

So the best is to set the mirror on top, then the subdivision and last the booleans:

Here is the result (after the empty is moved a bit along +x and the modifiers reordered:

enter image description here

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    $\begingroup$ Thanks a ton for your time and knowledge, lemon. You're very helpful, as always. $\endgroup$
    – LOTR_7193
    Aug 5, 2019 at 11:54
  • $\begingroup$ Hi, I've reviewed this problem and found that this solution only solves a part of the problem. Even if I mirror the object perfectly, as long as I put the mirror modifier after the boolean modifier, the faces will be lost. I think there's no reason for the mirror modifier to be before the boolean modifier, because users should be able to mirror the boolean modifiers as well. If you create a cube and put the mirror below the boolean modifier, it works. I just dont know why it doesn't work in this case. $\endgroup$
    – LOTR_7193
    Aug 6, 2019 at 10:18
  • $\begingroup$ I've tried it again in another layer with a cube. It seems that this is an universal issue, in that if the cube has a hole in it, we cannot mirror the boolean modifier properly. Now the question is, what should we do to achieve that result? $\endgroup$
    – LOTR_7193
    Aug 6, 2019 at 10:26
  • $\begingroup$ @ĐặngHảiPhụng, there is a reason to place the mirror before the boolean: if you don't do it, it doesn't work. I can't say more... boolean will make open cuts on opened meshes... $\endgroup$
    – lemon
    Aug 6, 2019 at 10:28
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One way that might work is to mirror the boolean around the original object as picture below.

Mirror Boolean

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