If you're vigilant during the modeling process, this step should almost be a non-issue.
- Non-manifold geometry is technically defined as any edge shared by more than two faces.
This issue can occur when a face or edge is extruded but not repositioned—the result is essentially two identical pieces of geometry directly on top of one another. As would be expected, this situation ends up being quite confusing for 3d printing equipment.
A non-manifold model will not print correctly.
One very common cause for non-manifold geometry is when an artist extrudes a face, moves it, decides against the extrusion, and attempts to undo the action. An extrusion is recorded by most software packages as two separate commands:
- The extrusion itself.
- And the repositioning of the face or edge.
Therefore, to undo an extrusion, the undo command must be given twice. Failure to do so will result in non-manifold geometry, and is a relatively common mistake for novice modelers.
It's a problem that's easy to avoid but it's often invisible and therefore quite easy to miss. Be aware of the issue, know how to spot it, and be sure to fix it as soon as you're aware of the problem. The longer you wait to fix non-manifold issues, the harder they are to eliminate.
Spotting non-manifold faces can be tricky.
If you're using Maya make sure your display settings are such that a selection handle (a small square or circle) appears in the center of each polygon when you're in face selection mode.
If you spot a selection handle directly on top of an edge you've probably got non-manifold geometry. Try selecting the faces and clicking delete—sometimes this is all it takes. If this doesn't work, you can also try the Mesh > Cleanup command, making sure non-manifold is selected in the options box.
Although extrusion isn't the only cause of non-manifold issues, it's probably the most common. Shapeways examines non-manifold geometry in great depth here.