Non-manifold geometry are shapes and objects that can’t physically exist but can, however, be rendered with a computer. The most common example of a non-manifold geometry is a 2D wall with no thickness. A computer can display this as part of a 3D model with no problem. If you try to print a 3D model with a 2D surface, you’ll run into a whole range of issues. This is because such a surface is impossible in the real world.
Non-manifold geometry will be a familiar issue for many 3D printing enthusiasts. It can cause structural issues, misaligned edges, and other unexpected results. These all result from the slicing software trying and failing to represent impossible geometry. In some cases, the software might not even try at all.
Common types of non-manifold geometry
Disconnected corners and edges: If you have two edges touching from either different parts of the same model, or from two different models, they should merge properly. If these edges are simply overlayed on each other the slicing software can end up printing them next to each other but disconnected. Similarly, if you have two 90-degree corners meeting to make an “+” shape, these can cause non-manifold vertices if the outside edges of the corners just touch but don’t properly merge together. Both of these issues can easily result in structural issues for the print.
Internal faces: All of the walls of a print should be solid with the space inside that solid wall being empty rather than containing further geometry.
2D geometry: 2D surfaces simply aren’t possible in the real world and are impossible for you to print.
Holes: A hole in a 3D model can cause problems, as it can result in impossible walls. For example, imagine a solid cylinder, you can print this as it has a 3D structure. If you take the same cylinder and drill a hole from one end to the other, while ensuring the resulting tube has solid sides, this will again be printable. If you only cut a hole in one end, however, you can end up with a cup where all the surfaces are two-dimensional.
Addressing non-manifold geometry
To address non-manifold geometry in your models, it’s important to ensure that edges and corners that are supposed to touch, are properly merged. If this isn’t the case you may end up with them printed next to each other. It’s also important to ensure that all of your walls have a thickness, especially if you’re hollowing a shape.
Tip: When 3D printing it’s best to ensure that your wall thicknesses are set to a multiple of the diameter of your print nozzle. This is because these are the only thicknesses of filament you can actually extrude.
Many slicers offer tools to address or in some cases even highlight non-manifold geometry. These can be a really big help in trying to find and fix this issue in your models. The specifics vary between software suites, so you’ll need to find the instructions for this functionality for your preferred software.
You should now be familiar with what non-manifold geometry is and how to avoid it in your models, have you got any other tips to share about spotting or avoiding non-manifold geometry? Let us know down below.