I use Blender to create 3D models for printing. Occasionally when I select mesh by trait - interior faces, the results shows some exterior faces selected.

I tried to use Mesh > Normals > Flip to fix this but nothing happens. Same for recalculate exterior faces.

I've also tried the 3D model plugin to "Make Manifold" but it removes the selected faces.

The solid model looks correct but will not print correctly.

Is there a way to fix this?

Solid model looks correct

enter image description here

  • $\begingroup$ Does it conected to a mesh or is it separated? I mean when you select a vertex and hit L key does it select all vertices or only highlighted part on your screen? $\endgroup$
    – vklidu
    Feb 20 '20 at 6:38
  • 1
    $\begingroup$ I think you would have more than one issue :-) here are nicely documented all possible issues check it out sculpteo.com/en/tutorial/prepare-your-model-3d-printing-blender/… $\endgroup$
    – vklidu
    Feb 20 '20 at 7:00

When you have trouble with Normals, i would highly recommend the following.

  1. Go in the Viewport Overlays and activate "Face Orientation" enter image description here
  2. You should now see red and blue overlay on your object (blue face direction should always point outwards, red the backside of faces and should only point inwards)
  3. Go in Edit Mode and press A to select all
  4. Press Shift+N to recalculate normals outside
  5. If there are still red areas facing outwards you need to select them manually
  6. Press Alt+N and Flip those Normals

Now the entire object should be blue and you can deactivate the "Face Orientation" A little tip, add Flip and show Face Orientation to Quick Favorits per RMB It's so helpfull to have at hand and saves a lot of time.

Important is that only selected faces will be affected by the normal operations.

Interior faces as the tooltip says are faces were all edges have more then 2 face users!

enter image description here

Means faces that cover holes where more geometry is hidden behind.

Here a little visual explanation about it:

enter image description here

It's obvious that there are faces hidden behind the middle one covering a hole. I believe you may have generated overlapping faces that lie above/in front of others that are facing outwards. Try saving the file incremental and then select those interior faces and hide them with H. Should those now hidden faces not affect the mesh, just press Alt+H to show them again and delete them with X, "Faces".

  • $\begingroup$ Thanks, I have done all of that and the blue pointers indicate that the normals are facing outward. But then when I check again with mesh "Select all by trait - interior faces", the same faces are still selected. $\endgroup$ Feb 20 '20 at 2:22
  • $\begingroup$ Those faces should be either unneeded or are in front of faces that are hidden and are not needed. $\endgroup$
    – Xylvier
    Feb 20 '20 at 8:01

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