Geometry & modeling
Manifold
Definition
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.
Also known as: manifold mesh, non-manifold, 2-manifold
Updated
Why non-manifold geometry causes trouble#
The 3MF Core Specification states the rule directly: every triangle edge must share its endpoints with the edge of exactly one other triangle. Edges shared by three or more triangles, or used by only one, make the inside of the shape ambiguous.
The OpenSCAD manual notes that a non-manifold STL may preview fine and then fail as soon as you combine it with other geometry. Most slicers and mesh tools include repair functions, but repairs guess at intent and can change small features.
Sources
Related terms
Watertight
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.
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.
3MF
3MF is an XML-based file format for additive manufacturing maintained by the 3MF Consortium. Unlike STL it can carry units, colors, materials, and multiple objects in one package, but its geometry is still a triangle mesh, not editable CAD geometry.
STL
STL is a simple mesh file format that stores a surface as a list of triangles, each with a normal and three vertices. It is the long-standing default for 3D printing, but it has no units, colors, or exact geometry.