SnappyHexMesh, meshing problem with multiple stl's
 

Hey everybody,

i'm trying to mesh an empty space within multiple geometries, different stl's. There is a closed hollow cylinder (tube-like), built up with four stl files. The fifth geometry is embedded in the inside. In fact, the internal mesh is correctly snapped (see attached picture).
The embedded geometry is very close to the walls of the cylinder, but they do not touch or intersect each other.

The problem that occurs: At some positions the internal mesh is interrupted, the wall from the cylinder is meshed together with the embedded geometry. It looks like a gate system (see second picture). The distance between the geometries is nearly constant but it only occurs at a few positions. The rest of the cylinder is fine.

Does anybody have an idea where this problem may come from?
Thanks a lot,
Frank