GLOSSARY
Watertight Mesh
A watertight mesh is a closed surface with no holes — fill it with water and none would leak out. Combined with manifold geometry, it is the basic condition for 3D printing.
Definition
A watertight (or "closed") mesh has no boundary edges. Every edge is part of a continuous loop of faces. There are no gaps where the inside is exposed to the outside. The mesh represents the complete surface of a solid object.
Tools like Blender's 3D-Print Toolbox, MeshLab, and PrusaSlicer's built-in repair will analyze a mesh and report open edges, holes, and self-intersections. Most can attempt automatic stitching — closing small holes by adding triangles. Big holes need manual repair.
Why it matters
A non-watertight mesh confuses slicers. They will either print stray surfaces with no thickness, fill in unintended areas, or fail outright. Many slicers patch small holes silently, but you should not rely on it for production work.
For AI-generated meshes, watertightness is part of the spec for tools that target 3D printing. Pipelines optimized for textured rendering frequently produce non-watertight output because rendering does not care.
Common confusion
Watertight does not imply printable. A mesh can be watertight but contain non-manifold edges, flipped normals, or self-intersecting bodies. Run all three checks (watertight, manifold, normals) before slicing a critical part.
Boolean operations in Blender or CAD often produce nearly- watertight meshes that fail by tiny amounts at intersection edges. Tools like Blender's "Fill Holes" or the 3D-Print Toolbox patch these reliably.