What is a non-manifold edge?

A mesh is manifold when it could exist as a real solid object: every edge joins exactly two triangles, one on each side. A non-manifold edge breaks that rule. It is shared by three or more triangles, so the surface around it no longer has a clear inside and outside.

Why slicers care

A slicer cuts the model into layers and, for each layer, decides which areas are solid. It does that by tracing the outline of the surface. Around a non-manifold edge there is more than one way to trace it, so the slicer has to guess. Different slicers guess differently, and the guess can change from one layer to the next.

Typical results are thin gaps in a wall, a missing slice of infill, or a small region printed hollow when it should be solid.

Common causes

• Two parts touching along a line. Two boxes placed edge to edge share an edge with four triangles around it.
• Two parts touching at a single point. The same problem, at a vertex instead of an edge.
• Internal faces. A wall left inside a model after joining two shapes, so three faces meet at the join.
• Merged copies. Duplicating a part and placing it exactly on top of the original.
• Careless boolean operations in modelling tools, which can leave slivers of geometry where shapes meet.

How it differs from other faults

• Open edges belong to only one triangle: the surface has a hole.
• Non-manifold edges belong to too many triangles: the surface is ambiguous.
• Flipped normals have the right number of triangles but one faces the wrong way.

A model can have any mix of the three. See how to check an STL before you print it for the full list.

How to find them

You cannot see a non-manifold edge by looking at a shaded model, because the surface looks closed. You need a tool that counts how many triangles meet at each edge. MeshView Pro’s printability check counts non-manifold edges, highlights them on the model, and includes them in its verdict.

How to fix them

1. Separate touching parts. Move them apart slightly, or overlap them properly so a union can merge them into one solid.
2. Union the shapes in your modelling tool instead of just placing them together, so internal faces are removed.
3. Delete duplicates that sit exactly on top of each other.
4. Use a repair tool for meshes you did not make. Most slicers and mesh editors can resolve simple cases automatically.

When you can ignore it

A handful of non-manifold edges in a hidden spot sometimes print fine, because the slicer happens to guess right. That is luck, not a guarantee. If the part matters, fix the mesh, then check it again before you print.