By definition, non-manifold geometry is a mesh that cannot be unfolded into a flat surface with one directed normals and can not exist in the real world.
In the case of 3D printing, a non-manifold object is a 3D model with structural issues. Some can even prevent the model from being sliced correctly and then 3D printed.
The most common issues are:
- Overlapping faces or an inner face if more than two faces are linked to a single edge.
- Holes in the object surface, missing faces, or non-welded edges.
- Two or more faces share a single vertex but no face.
- Separates shells in the same object.
- Self-intersecting surfaces.
- Inverted face regular (the normal is the polygon information to define what is the inside or outside direction).
- Non-planar face.
- And more…
Lychee Slicer will alert you if a model is non-manifold or not:
You can select a model with more information in the Layout workspace and repair it if needed.
In the object list, a non-manifold model will have a red warning next to its name. Clicking on the small arrow will give more information about the error.
Keep in mind that you should avoid printing a model flagged as non-manifold or in error as much as possible. Try to repair it as soon as you import it inside Lychee Slicer.
If the automatic repair tool provided can’t fix it, you may have to repair it in a dedicated fixing software or by using 3D modeling software to edit it. Then re-import it.
One major exception is if a clean 3D model is exported with its supports from Lychee Slicer and then imported back, it can be flagged as a non-manifold when you import it back into the Slicer.
It’s a known issue. Your model and support will print fine. No need to repair them. The most common case is purchased pre-supported STL 3D model.