modeling - What is non-manifold geometry? - Blender Stack Exchange
Fixing Non Manifold parts in Blender 3d - YouTube
Tips for a successful manifold 3D model – Help Center
What is a non-manifold mesh and how to fix it - BlenderNation
3D Printering: Blender Tips For Printable Objects | Hackaday
What is a non-manifold geometry and how to avoid it - crwjaakko
How do I fix these manifold issues? And, should I have to "fix" the model in Simplify3d every time, or can I address that in Blender? (Blender -> Simplify3d) : r/3Dprinting