# Fixing non-manifold meshes

Hi

I have been reading the Shapeways on how to ensure that your models are manifold before printing. I have two questions

1. Water tight and manifold are two different things right?

(I ask this because http://www.shapeways.com/tutorials/fixing-non-manifold-models says not being water tight is a type of non manifold error)

1. When you go to Edit Mode, and press ctrl-alt-shift-M what are you doing exactly?

Regards

In short: your mesh is manifold when all vertices are connected with edges and those edges are connected to exactly two faces.

ctrl+alt+shift+m tries to select non-manifold parts of your mesh.
Those parts could be: loose vertices, double geometry, inside faces/edges, holes

The term watertight means that your mesh is closed. Pour water in, does it leak? All real life objects have thickness, so the models must have thickness because you can’t 3D print a polygon. If it doesn’t have thickness, it’s also non-manifold.
A plane primitive for example is non-manifold, but if you extrude a cube out of it (thickness), it’s manifold.

So basically, you have to fix them manually (‘them’ being the non-manifold parts). Just out of curiosity, how did people spot the non-manifold parts before ctrl-alt-shift-M, if there was a period where it didn’t exist?

Being water tight and manifold are two different things as you put it. I also understand water tight as not having holes on the surface and manifold as – there are no edges shared by more than two surfaces.

Since you say the highlighted parts can be either (or all four) of the following “loose vertices, double geometry, inside faces/edges, holes”.
Are you sayin’ that the key combination, ctrl-alt-shift-m not only identifies parts that make the mesh non-manifold but also not water-tight?

This might help

You might find this video helpful, it helped me though

Watertight is nonmanifold in laymans’ terms. The result you want is the mesh to be 1 intact uninfied surface with correctly oriented normals, and a well defined volume. The criteria JA12 listed are verts that Select nonmanifold displays as the problem areas, or faces that select interior faces highlights.
If you are new to this type of repair, there is a free program called Netfabb basic which can list problems, display problem areas for a stl or object file . And repair them. When I started making models for 3D printing purposes, I would select nonmanifold in Blender, then export a stl into Netfabb to find out the problems. Then I found out how what repairs were needed in Blender. You better get prepared to understand correctly oriented normals also, this is needed to define material volume