The space marked by the selected vertices should be solid, but when faces are added the mesh becomes non-manifold. The manifold property is important for printing.

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The faces should be here:

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and also at the outer edge ring.

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    $\begingroup$ It isn't clear what you mean when you say adding faces. What do you want to accomplish that requires these extras faces? Are you trying to cap off that hex nut looking thing? $\endgroup$ – Rekov Feb 24 at 20:14
  • $\begingroup$ @Rekov I added another image. And yes the solid part is intended to hold a hex bolt, this is to adapt canon foto lenses to a 1.25 cam. $\endgroup$ – stacker Feb 24 at 20:24

Non-manifold geometry basically occurs when you have an edge that is part of more or less than exactly two faces. For the sake of 3D printing this is important because non-manifold geometry is geometry that isn't describing the outer shell of a physical object. And edge that is connected to more than two faces means that you have faces that are 'inside' your mesh, for example.

Basically, if you add that face there, you need to delete some of your older geometry that is becoming internal. I'm not 100% sure this is what you're trying to accomplish, but it should give you the basic idea:

enter image description here

Delete faces that would become 'inside' faces, then select the edge loops around the hole you've created, and fill them. It's possible that you will want to clear away one more faces. As I've done it above, you have a hex shaped indent both inside and outside the cylinder. The same technique works if you want a smooth cylinder on the inside.

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  • $\begingroup$ Thanks, I know what manifold means (blender.stackexchange.com/questions/7910/…) but in this case I can't find the culprit. Did you manage to edit my file and in Edit Mode: Select / Select All by Trait / Non Manifold does not show any vertices? $\endgroup$ – stacker Feb 24 at 20:36
  • $\begingroup$ Your mesh is manifold until you add that face. Think about what adding a single face represents. You're adding an infinitely thin wall there as far as 3D printing is concerned. You have to add two faces, one on either side of the surface you want to 3D print, and then you have to delete the faces you've now shifted to inside your mesh. $\endgroup$ – Rekov Feb 24 at 20:43
  • $\begingroup$ Thank you so much! I just ignored the extra faces, ouch $\endgroup$ – stacker Feb 24 at 20:50

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