Watertight Mesh

Theory

A watertight mesh has no boundary edges: every edge is shared by exactly two faces and the mesh forms a completely closed shell enclosing a finite volume. The term is equivalent to manifold in terms of edge valence, but emphasizes the volume-enclosing property. Slicers calculate infill and support by determining which voxels are 'inside' the mesh; an open mesh has no well-defined inside, causing slicer errors or incorrect toolpath generation.

Application

Achieve watertightness by: (1) repairing in Meshmixer (Analyze → Inspector → Auto Repair All), (2) using Microsoft 3D Builder (import → fix → export), (3) using the `manifold` Python/C++ library for programmatic repair, or (4) re-exporting from source CAD with sufficient tolerance. For sculpted meshes, ZBrush's DynaMesh and ZRemesher always produce watertight output. Always validate watertightness before sending to a resin printer where internal voids cannot drain.

Common mistakes

Visually seamless meshes can have microscopic gaps invisible at normal zoom. A 0.001mm gap is enough to make a mesh non-watertight. When combining meshes from different sources, always check for gaps at junctions. Some CAD surface modelers export open shells by default: confirm 'solid body' export option is selected (not 'surface' or 'shell').

Related terms: Manifold Geometry, Non-Manifold Edge, STL