I have a 3D mesh that I intend to print, a robot head. The head should be printed in a variety of colors, but the originator of the mesh didn't part it out. Here's an example:

enter image description here

The red section is what I'd like to separate from the rest of the head. When I do that, here's what it looks like:

enter image description here

I know how to separate parts, but after I do so, there's a gaping hole in the main section of the head. It's not manifold (watertight), and therefore not printable. My idea is this: After the two sections are separated, I want to make both parts manifold, and for the gaps in the geometry to be filled while following the original curvature of the head. That way I can print both the head and the "scalp" part (for lack of a better word) in different colors, and then glue the scalp in place.

I hope my explanation makes sense. Here's the link to my .blend: https://www.mediafire.com/file_premium/1edxr0mmubpmh9t/UC_WIP.blend/file

  • $\begingroup$ I succeeded in selecting the boundary of the "scalp" in Edit Mode using Select->Select Loops->Edge Loops, to duplicate it, then to fill it. Unfortunately, the topology is a nightmare and it is not continue with the head. So I was unsuccessful in joining a copy of this new surface with the head. My advice would be to clean the topology on both sides of the cut between head and scalp (probably by brute force and patience) then to try again... $\endgroup$ Apr 20 at 21:34
  • $\begingroup$ I would recommend (1) to merge by distance vertices with a threshold of 0.01 m (for a object of about 30 m...) (2) to limited dissolve the faces because most of the STL triangles are split quads... $\endgroup$ Apr 21 at 7:50

1 Answer 1


Using any mesh that has been triangulated is a huge pain, especially .stl meshes, which are used for 3D printing.
If at all possible, you want to start with something with clean topology.

Nevertheless, start by edge loop selecting the rim of the hole. You can use alt + mouse click to select loops at a time. However, since everything is in triangles, it won't work perfectly. Be patient, and work your way around.
Important: Every part of the edge must be selected. Switching to vertex select can help visualize what parts have not been selected yet.

enter image description here

After you are sure everything is selected, use f to fill it in. If you get a weird stringy result, you missed some edges. enter image description here

After that, select the new face you just made, and either right-click and choose "Triangulate Faces", or use Ctrl + T. enter image description here

Hopefully this is helpful to you!
If the flat top is not giving you enough surface area to glue the other part to, you can use the knife tool to give it some extra geometry.

For the other piece, the steps are the same: enter image description here

(Using Select > Select Loops > Edge Loops, or alt + Click worked perfectly here) enter image description here

Ctrl + T enter image description here

  • $\begingroup$ That was fantastic. Worked like a charm, and I feel like I just leveled up. Thanks so much! $\endgroup$ Apr 21 at 19:57
  • 1
    $\begingroup$ Glad I could help! $\endgroup$ Apr 21 at 20:24

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