When I first started 3D printing, the non-manifold edge was an intimidating concept to grasp. For our purposes, you can think of manifold as something that can be manufactured and become a real-life object. Everything has an inside and an outside, is connected, has a thickness, and would make sense to the 3D printer. Non-manifold, on the other hand, would be a geometry that can't exist in real life.
Finding and fixing non-manifold edges
Exploring examples of non-manifold edges
For a more concrete way of looking at it, all your edges should be connected to two faces with matching normals. Edges that don't meet that criteria will be flagged as non-manifold. Some examples are explained in the following sections:
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