Geometry & modeling
Watertight
Definition
A watertight model has a closed surface with no gaps or holes, so it encloses a well-defined solid volume. Leaky models cause failed booleans, bad 3D printing slices, and solids that CAD programs import only as loose surfaces.
Also known as: closed mesh, closed solid, leaky model
Updated
Meshes and B-Rep#
For meshes, watertight usually goes together with manifold: the OpenSCAD manual asks for STL files that are manifold and have no holes or self-intersections.
For B-Rep models, the same idea applies to faces. A STEP or IGES import with small gaps between faces arrives as a set of surfaces instead of a solid, and the CAD program has to stitch or knit it before features like shell or fillet will work.
Sources
Related terms
Manifold
A manifold mesh is one where every edge is shared by exactly two triangles, so the surface clearly separates inside from outside. Slicers, mesh booleans, and CAD imports rely on manifold geometry to behave predictably.
Mesh
A mesh represents a shape as a surface of small flat polygons, almost always triangles, joined at shared edges and vertices. Meshes work well for rendering, games, and 3D printing, but they have no true faces, edges, or exact curves, so they are hard to dimension or edit as engineering geometry.
B-Rep
B-Rep is the way professional CAD kernels describe a solid: by its boundary, made of exact surfaces (faces), the curves where they meet (edges), and the points where edges end (vertices). Because the geometry is exact and the topology is explicit, B-Rep models can be measured, filleted, dimensioned, and manufactured precisely.