Manifold models: why a model must have no holes

What manifold means

Imagine the model as a watertight container. If the surface has a hole, the water would run out, and the printer cannot tell where the inside is. In a manifold model every edge is shared by exactly two faces and there are no gaps, which is why it is also called watertight.

Technically such a model is a solid and its surface is a closed polygon mesh.

Which errors appear

• Holes. Missing triangles in the surface, often after an export or after merging models.
• Edges with three faces. So-called non-manifold edges, where three or more faces meet.
• Flipped faces. The normal points inwards, so inside and outside are swapped.
• Self-intersection. Two parts pass through each other without being joined.

Why this matters for printing

The slicer cuts the model into layers and at each one it has to know where there is plastic and where there is empty space. With a faulty mesh you get holes in the walls, missing faces or a print with a strange interior.

With models from CAD errors are rare. They show up most often with downloaded models, sculpted models and when several bodies are combined.

What we do

After you upload, we repair small holes in the mesh automatically. For bigger errors that would change the shape, we will write to you. See model repair.

If you check the model yourself before uploading, you save time. A check in Blender, or in the slicer when you open the file, will do.