The Tetra mesher automatically attempts to close all holes in a model so special consideration should be given to regions between two surfaces, or curves, that are very close together or that meet at a small angle. (This would also apply if the region outside the geometry has small angles.) If the tetra sizes are larger than, or approximately the same size as, the distance between the surfaces or curves, the surface mesh could have a tendency to jump the gap, thereby creating non-manifold vertices during the meshing process. That is, the small thickness or gap may be interpreted as a hole by the mesher. You should either define a thin cut in order to establish that the gap is not a hole; or make the mesh size small enough so that it will not close the gap when the meshing is performed.

If the local tetra sizes are not small enough to allow at least 2 or 3 elements across the material thickness (or gap), you should select Define thin cuts, in the Volume Meshing Parameters section of the Global Mesh Setup tools. To define a thin cut, the two surfaces have to be in different Parts. If the surfaces meet, the curve at the intersection of the surfaces will need to be in a different part.