Most slicers are capable of analyzing a model (mesh), identifying mesh issues and solving them automatically if necessary. However, it may happen that the slicer does not solve the problem or that it solves it but not in the correct way.
As already mentioned, creating three-dimensional models for printing requires the use of some care if you do not want to risk creating a model that, in the worst case, may not even be printable.
Mesh issues: What if the slicer fails to correct the mesh
If the slicer does not automatically solve the problem, then you will have to proceed manually. The most suitable software to solve this type of problem are modeling software and tools for mesh correction. In particular, it is absolutely necessary for “mechanical” models. Repairing a mesh visually does not mean maintaining consistency with the original model. A mechanical model is absolutely precise, and is passed to machines that do nothing but reproduce it, in detail, in order to obtain a real object (3D printing is an example). Obviously the machines can be more or less precise, so the same model, worked by different machines will result in a real object, a faithful copy of the model, whose final dimensions depend on the precision of the machine you are using.
Professional modeling programs have a whole range of features to completely control every aspect of a model (mesh). Obviously, they could be, at least initially, more difficult to understand and/or manage, but once you have learned some basic concepts, there is nothing that these software can not model, color (texture), animate, etc. etc. etc.. So you’ll have everything you need to create, repair, modify, etc., your models, in a single program.
Of course, the tools also work well, but they could be limited, so not only will you have to learn how to use the tool, but you’ll probably have to learn how to use more than one, which is equivalent to wasting a little more time just once. If you don’t want to create your own templates, then you can easily choose the tool that best suits your needs.
Manifolds and non-manifolds can be given this simple definition.
A solid manifold is a solid that really exists.
A solid is not a manifold, it is not a real solid.
In free three-dimensional modelling, nothing is forbidden. Let’s explain better, in free modeling you can create any type of geometry, rotating, scaling, moving every single face and/or vertex, eliminating faces etc., but this does not mean that this type of geometry (mesh or model) has no problems for one reason or another.
Usually you may have problems during the creation of the model itself, during its visualization (rendering) and not least, since this is the case that interests us, during printing.
Two planes positioned in this way form a Non-Manifold object ( Software Blender ):
An empty cube without one of its faces is a Non-Manifold object ( Software Blender ):
If you want to create a hollow cube, add the missing faces to create the inside.
Upper face removed ( Software Blender ):
Added thickness ( Software Blender ):
Solving the problem is simple, just delete, add, solder, the faces too many, missing, or not properly connected to the rest of the model.
High Valence Vertex
This is a problem you might have if you decide to “pass” a model previously made with polygonal modeling, to a software for sculpting modeling, perhaps to add details. This means that more than a precise number of “segments” are connected to a vertex.
It is necessary to create subdivisions, other polygons and try to reduce the number of “segments” that connect to the same vertex.
Mesh issues: Overlapping faces
Two faces can’t be on the same plane. If we think of a real object, such as a cube on a table, we already know that the cube rests on the table. In three-dimensional modeling, nobody forbids us from pushing the cube into the table top. The two objects will look like one, but in reality they will not.
If you really want to “merge” the two objects and print them correctly, then you will need to merge them. This operation will eliminate the parts of the object that are not visible, i.e. the recessed parts. In reality, it is not possible for part of a cube to become part of a plane simply by pushing it into it. They will still be two separate objects. The same goes for printing, it is always better to merge the two objects so that they become one only if you do not want inconveniences. On the other hand, the recessed portion of the cube would still not be visible.
If only the two faces shared the same space (think of two sheets of paper in a horizontal position, it is not possible that both share the same space. They will always be one above the other or one below the other) we would have two overlapping faces. This is not possible. Once again, no one forbids us from having two overlapping faces in our model, but this situation could create problems during printing. ( In the jargon of Rendering we define the problem Z-Fight or Z-Fighting )
To solve the problem it will be sufficient to remove or move the face that is not in the correct position (one on horseback of another), or if this should not be possible, it will be necessary to melt the objects using the Boolean operators or, again, manually modify the model. The aim is to make sure that two faces do not share the same space.
Mesh issues: Face orientation ( Normals – Face normals – Vertex normals )
When making a three-dimensional model it is important that every single face is “oriented” in the right way and in the right direction. That is, not seeing a face or seeing a “hole” in a mesh does not mean that the hole really exists.
You should always keep in mind that you are modeling an object within a software that does not impose physical rules (an object can easily pass through another one) but has rules, and violating one of these rules can create problems. Nothing that can’t be solved…
The software you are using, are not created only for 3D printing, but especially for rendering, or for creating models for video games, etc.. Each one of these purposes is subject to certain limitations… For example, to create “fluid” scenes, the number of triangles to be displayed cannot be infinite. The more triangles the software will have to display or redraw (since the scenes are moving), the longer it will take. In addition, the “fluidity” also depends on the hardware (graphics card) you are using.
Let’s analyze the problem
A stratagem adopted by many software to overcome this type of inconvenience is precisely that of not displaying the triangles “not oriented in the right way” (Back-face Culling), therefore not visible.
Think about looking at a cube. Not all of its faces will be visible simultaneously in reality. Similarly, the software “chooses” which are the faces to hide using as a reference the orientation or direction of the face itself (In a mesh is also important to specify the order in which the vertices of each triangle CW – Clockwise – Clockwise or CCW – Counter Clockwise – Counter Clockwise – but you do not have to worry about this).
Nobody forbids us to “reverse” a face, but the result will be an unwanted “hole”.
In this image you can see a cube, but one of the faces that should be visible is not. Also note how the faces that should be hidden are not, visible just as if you were looking at a real cube (although in this case you can see through the cube, but only because a face is not visible) :
The same cube with the face that must be visible, but was not, oriented in the right way (if there are no defects what you will see is equivalent to observing a real object in reality):
This type of problem is by no means serious and easily solved.
Just locate the hidden face and click on Flip Normals, i.e. reorient the face itself. Obviously the name is the way you can reorient the faces of a mesh depending on the software you are using.
Mesh issues: The insights can be different, more or less complete.
Vertex normal ( Source Wikipedia)
Face Normals (Source Wikipedia)
Back-face Culling ( Source Wikipedia)
Z-Fight, Z-Fighting ( Source Wikipedia)
Operatori booleani ( Source Wikipedia)
Mesh issues: Useful sites and guides (if you want to give a reading):
Shapeways Fixing non manifold modes