How to fix non manifold edges in a hollow cylinder?

Question

I'm trying to mesh this model via tetgen, but tetgen says the geometry is non manifold. Blender does say so too.

The model is created by a couple of extrude nodes in geometry nodes.

I've tried the brute force method of using the clean up which just merges the faces and hence closes everything up. This you can see in the second picture.

I would want a hollow cylinder like shape like you can see in the first picture though.

I don't see any unnecessary faces i could remove unfortunately and also edge splitting and merging by distance doesn't work as it just opens up new non-manifold edges for Blender to find and disrupts the integrity of the structure.

The problem is the inner faces created by the extrusion node. How can i create a hollow body via extrusion that is not manifold because of the inner faces? Solidify modifier does work for blender but not for the python meshing module tetgen that i use.

Answer 1

In this case, you would have to combine the original mesh with the extrusion, reverse the normals of the faces with Flip Faces, and connect the points with Merge by Distance:

It always behaves the same here: the node Extrude Mesh extrudes the selected faces, but always outputs only the extruded faces at the Mesh output.

I would honestly be happy sometimes too if the ability to keep and connect to the original mesh was available as an option.

Unfortunately, that's not the case, so you'd have to go that route.

Answer 2

I downloaded your Blend.file and checked it with the 3D-Print addon. After converting the curve data, it showed 57 non-manifolds. It is important to give your mesh some depth so that it is physically possible.

• Add a Solidify modifier 
• Apply it
• Check the mesh with the 3d-print addon
• Click make manifold