Weld 3D Node

Weld 3D combines vertices that occupy (almost) the same position in space. Typically useful when a mesh should have shared vertices at its junctions but the export didn't do it - each face keeps its own independent vertices, separated by a hair.

This problem is common with OBJ/FBX exports from some applications (especially with unhandled smooth groups). The result looks like a clean mesh but produces all kinds of artifacts as soon as you work on it.

• Effects Library: Tools > 3D > Weld 3D
• Quick add: right-click in the Node Editor → Add Tool > 3D > Weld 3D

Four common artifacts caused by duplicated vertices:

• Unwanted hard edges - visible shading transitions where the mesh should be smooth. Each pair of duplicated vertices has its own normal → a discontinuity.
• Cracks during a Displace 3D - when you deform the mesh along its normals, the duplicated vertices move in slightly different directions → holes appear between the faces.
• Missing pixels in the render - tiny gaps in the final render caused by geometric discontinuities.
• Particles passing through - particles in a simulation slip through the invisible micro-cracks between faces.

If you run into one of these symptoms on an imported mesh, a Weld 3D is probably the solution.

• SceneInput (orange) - required. Mesh or 3D scene to repair.

FBX Mesh 3D → Weld 3D → Displace 3D → Replace Normals 3D → Merge 3D

Place Weld 3D after the import and before any deformation (Displace, Bender, Custom Vertex). Chaining a Replace Normals 3D afterward guarantees clean shading.

Weld Mode (default: Weld)

Two modes:

• Weld - merges nearby vertices based on the tolerance. The standard behavior.
• Fracture - the opposite operation. Splits every shared vertex and removes the polygon adjacency information. Used to deliberately create a shattered/fractured effect.

Tolerance (auto by default)

Maximum distance for two vertices to count as "nearby" and be merged. The node has automatic detection that works in most cases - adjust it manually if the result isn't satisfying.
Pitfall: a tolerance that is too high merges vertices that shouldn't be (details disappear). A tolerance that is too low fixes nothing.

Weld 3D only merges vertex positions. It does not merge normals, texture coordinates or vertex colors.

As a result, even after a Weld, you may still see hard edges because the normals remain different at the merged points. The complete solution is:

Import → Weld 3D → Replace Normals 3D → ...

Replace Normals 3D recomputes consistent normals from the post-weld geometry → clean shading.

Replace Normals 3D has a Pre-Weld Position Vertices option that does essentially the same thing as a Weld 3D, right before recomputing the normals. When should you use which?

Replace Normals alone with Pre-Weld

Faster - a single operation instead of two nodes. Ideal when you just want to fix the shading of a mesh and don't use the welded mesh elsewhere.

Weld 3D + Replace Normals

More control - you control the Weld tolerance independently. Essential when:

• you also want to apply a Displace afterward (the mesh welded upstream benefits the whole chain)
• Replace Normals' automatic detection isn't enough
• you want to view the welded mesh for debugging

Rule: Pre-Weld is enough for a simple shading fix. A dedicated Weld 3D for more complex cases (deformation, debugging).

• FBX/OBJ import with unwanted hard edges - a mesh that looks smooth in Blender arrives faceted in Fusion. Weld 3D + Replace Normals fix it.
• Preparing before a Displace 3D - avoid cracks in the deformed geometry.
• Mesh from a 3D scan - scans often produce duplicate vertices. Weld cleans them up before any processing.
• "Fractured" effect - Fracture mode instead of Weld to intentionally split all the faces (useful for explosion effects, or to simulate a mesh breaking apart).
• Debugging a rendering glitch - temporarily insert a Weld to see whether the gaps in the render come from duplicated vertices.
• Particle simulation - before using a mesh as a collider for particles, Weld it so they don't slip through the micro-cracks.

• Automatic tolerance works almost every time - start with the default. Only adjust it if the result isn't good.
• Tolerance too high = lost detail - on a mesh with fine details (high-resolution mesh), a large tolerance merges vertices that are legitimately separate and flattens the details. In practice: raise it gradually.
• Don't forget Replace Normals afterward - Weld alone isn't enough to fix hard edges (it merges positions, not normals).
• Fracture mode is rarely used - it is the conceptual opposite of Weld. Interesting for creative effects, but be careful: it breaks geometric adjacency.
• For a quick first pass, prefer Replace Normals 3D with Pre-Weld Position Vertices checked - a single node that does both.
• Visual debugging - to find out whether a mesh has duplicated vertices, enable wireframe and zoom in on an edge. If you see two very close edges instead of one, there is duplication.