What watertight and manifold mean
A geometry is watertight when its surface completely encloses a volume with no gaps, holes, or overlaps — pour water in and none escapes. Closely related is manifold: every edge is shared by exactly two faces, with no dangling faces, no T-junctions, and no self-intersections. Together these properties give a mesher an unambiguous answer to the only question it really asks: for any point, is it inside the domain or outside?
Why leaks break meshers
Volume meshers work by filling the space inside a closed surface. When the surface is not sealed, that fill has nowhere to stop:
- A missing face or a gap between parts lets the volume mesh leak out of the intended region.
- Overlapping or self-intersecting surfaces make inside/outside undefined, so cells form in the wrong place.
- Tiny slivers and near-coincident faces create degenerate cells that wreck mesh quality.
CAD that looks perfect on screen is often not watertight underneath — imported STEP/IGES parts, tessellated STLs, and multi-body assemblies routinely hide micron-scale gaps and duplicate surfaces.
The cleanup tax
Because of this, engineers spend a large share of meshing time on geometry repair, not meshing: stitching gaps, removing duplicate and free edges, defeaturing tiny fillets and holes, and re-checking for leaks. It is tedious, easy to get wrong, and has to be redone every time the CAD changes.
What a “watertight mesh” is
The same idea applies to the mesh itself. A watertight surface mesh is closed and manifold — no holes, no non-manifold edges — which is what a downstream volume mesher (or a 3D printer) needs as a clean starting point.
How AutoMesh-Geo helps
AutoMesh-Geo is built to conform to real, imperfect geometry rather than assuming a flawless watertight input, reducing the manual repair loop between CAD cleanup and a usable boundary-layer mesh.