Meshes, Manifolds and Normals

What a mesh is

STL, OBJ and 3MF files describe a shape as a mesh: a list of points (vertices) and a list of flat triangles that join them. The 3MF specification puts it plainly: a mesh is a set of vertices and a set of triangles. There is no "solid" stored anywhere. The slicer works out what is inside the part from the way the triangles close around a volume, so the quality of that closed surface decides whether the part slices correctly. File types are compared in STL, 3MF, OBJ, STEP and G-code.

Watertight and manifold

People use "watertight", "manifold" and "solid" for the same idea: the surface closes completely, with no gaps, so if you filled it with water nothing would leak. Prusa's modeling guide says a printable model must be solid, with manifold geometry that can exist in the real world, and that a model with holes in its surface or internal geometry cannot be sliced. Blender's manual defines non-manifold geometry the same way, as geometry that cannot exist in the real world, and lists 3D printing among the jobs it is not suitable for because the inside and outside of the volume must be known.

The 3MF specification gives the precise rules for a printable model mesh. Each one is something you can check:

• Manifold edges. Every triangle edge is shared with exactly one other triangle. An edge with only one triangle is a hole; an edge with three or more is a fin or a wall stuck inside the part.
• Consistent orientation. Neighboring triangles agree on which side is the front.
• Outward-facing normals. Every triangle faces away from the inside of the object.

Together, the specification says, these make a continuous surface with no holes, gaps or open edges. A mesh that meets them is what slicers expect.

Normals: which way a face points

A normal is the direction a face points, its "front side". Slicers use normals to decide which side of a surface is the part. A face whose normal points inward is called flipped or inverted. One flipped face can confuse the slicer locally; a whole object with inward normals is "inside out".

In Blender, the Face Orientation overlay in the 3D Viewport shows which faces point the wrong way. To fix them, enter Edit Mode, select all, then use Mesh, Normals, Recalculate Outside (Shift-N), which flips the selected faces where needed so they all point outward. The manual notes the mesh does not need to be a closed volume for this. Flip reverses only the selected faces.

The common faults

Blender's Select, Select All by Trait, Non Manifold (Edit Mode, vertex or edge select) finds each kind of fault separately. Its options double as a checklist:

Fault	What it is	What it does to a print
Boundaries	Edges at the rim of a hole or an open surface	The slicer cannot close the volume; layers can come out missing or as a thin skin
Wire	Edges that belong to no face	Nothing printable; usually leftover construction lines
Multiple Faces	Edges shared by three or more faces	An internal wall or fin; the inside and outside become ambiguous
Non Contiguous	Edges shared by two faces with opposite normals	A flipped region; may print as a gap or a void
Vertices	Isolated points, or a single vertex joining two otherwise separate pieces	Stray specks, or two parts touching at a point that will not hold

Other programs use different names for the same faults: open edges, holes, bad edges, inverted faces. Your slicer's warning is usually the first place you hear about them; Repairing Broken Meshes covers what to do next.

Separate pieces and overlaps

A file can hold several closed pieces (shells). That is fine, and it is how Print-in-Place Hinges and Joints parts work: each moving piece is its own closed shell with a gap around it. Problems start when shells overlap or are not closed. The 3MF specification says items should not overlap, and that if they do, the printer software must unite them, so two overlapping shells print as one lump where they meet. If you meant them to be separate, leave a real gap.

Curves are flat facets

A mesh has no true curves. A cylinder is a ring of flat faces, and the smooth shading in your modeling program can hide how few there are. Prusa's guide warns that the print can turn out as a polygon rather than a circle, so check the number of sides before you export. How many triangles to use is covered in Converting CAD Models and Exporting for Printing.

A quick check before every print

1. Open the file in your slicer and read any warning next to the object name.
2. Step through the sliced preview. Missing walls, layers that vanish, or a part that prints as a hollow skin point to a mesh fault.
3. For files you made or edited, run a non-manifold selection in your modeling program and recalculate normals outward.
4. Keep the original file. Repairs change geometry, and you may want to start again from the source.

Files made for CNC carving often fail the first rule because they have no back at all; see Printing a CNC Relief STL.