Non-manifolds: Your Worst 3D Printing Nightmare

Objective: Learn what manifolds are, how they affect 3D printing, and how to recognize what type of non-manifold problem may occur.

To learn how to fix non-manifolds see:

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The 3D Printing Ninja

3DNinjaneer@gmail.com

Step 1: How a Model Is Made?

3D Printing starts by first having a model designed virtually on a computer. Then the model file needs to be in a format that a printed can read. This is done by creating geometrical points, lines, and faces which is called a mesh. The mesh is then sliced by another program, which creates a code or instructions that tell the printer where to go. The code then gets printed and the object appears.

Step 2: What Is a Manifold?

In order for the program to correctly make the code for the printer, your file should be a manifold. Imagine a balloon... A manifold is the rubber that makes up the balloon. Manifolds are good. A manifold is our geometry.

Step 3: What Is a Non-manifold? (Self-intersecting)

If geometry intersects or is directly occupying the same space this is a non-manifold. This is a very common error, an example would be that one vertex is in the same place as another. Manifolds are selfish they never share the same place with other geometry. Each part of geometry should have their own place.

One experience that I had was an orthodontist who came to print a mold of a patient's teeth. The model was made by a 3D scanner. The program allowed you to move each teeth individually and then to create a mold to make an invisilalign retainer. However the teeth where seperate objects and penetrated into the gums. This was an example of self intersecting non-manifolds. I simply joined the teeth with the gums to create a single object and a manifold.

Step 4: What Is a Non-manifold? (Separate Object)

Imagine the balloon and except that now you put your sunglasses on the balloon. This is called a floating piece or separate object non-manifold. I often see this type of non-manifold with teeth, hair, and glasses on a 3D model.

Step 5: What Is a Non-manifold? (Hole)

Imagine the balloon again, but with a hole in it. The balloon would explode or all of the air would escape because there is a hole in the balloon. This is a common non-manifold error that happens when designing. Manifolds don't explode or lose air and inflate.

Step 6: What Is a Non-manifold? (Inner Faces)

Imagine the balloon had a manufacturing error so that the balloon had extra rubber inside the balloon. When I printer reads the code it gets confused because there should only be on outer surface area not two. The inner faces confuse the printer where to Print. The surface should never have a dead end because it is continuous. The inner face makes the surface a dead end, which makes it non-manifold.

Step 7: What Is a Non-manifold? (Overlapping Geometry)

The last type which accounts for many errors in printing is when the same geometry occupies the same space. In the picture above we see that four faces occupy the space of another. This could also be two points or vertices that are in the same place, or two edges on top of each other. This is very easy to accidentally do in modeling so check often for non-manifolds.

Step 8: How Do I Fix Non-manifolds?

There are many ways of fixing non-manifolds. This process may take a few seconds to a few hours depending on what type and how many non-manifolds there are. I recommend always manually fixing your non-manifolds, but it is easier and sometimes more feasible to do it automatically. See these posts to learn how to fix your model:

I have seen many prints with lots of non-manifold errors print fine. On the other hand I have seen one tiny non-manifold completely destroy the print to be unrecognizable. So for the most part it is a risk that has to be decided if it’s worth the time to fix it or hope it works.

Step 9: As a Rule of Thumb

Low Risk:

Holes depend on the size of the hole. Often holes are so small they are insignificant. You should easily be able to tell on the virtual model if the hole is small enough to be printed fine. Otherwise if the hole is large an visible then there will be a big hole there and possibly worse. Layers print on layers, so the next layer can't print on nothing where the hole is.
Separate Objects Often these print fine, sometimes there is a gap between the mesh and the separate object, but otherwise it's almost like printing multiple parts at once

Medium Risk:

Inner Faces - These can cause major trouble or do nothing at all.

High Risk:

Self-Intersecting faces will cause missing plastic around the intersection or just be skipped. Once a layer is skipped the next layer is unable to print on top of the previous because it was skipped. Everything printed afterwards is almost certain to fail. Sometimes a layer corrects the missing layer, but still it causes a significant error.

How to Avoid Non-Manifolds?

As one of my favorite sayings goes, "It's better to maintain than regain" manifolds are the same way. If you constantly check for a non-manifold your life will be easier than realizing you have a non-manifolds from an hour ago. Non-manifolds have a snowball effect. Find and fix them early or pay big later on. On blender this can be done in edit mode with finger yoga (ctrl+alt+shift+M).

Check for Non-manifolds Before Printing

Use the program you are modeling with to check for non-manifolds before you export. In addition there are programs such as Autodesk Meshmixer that has an Inspector tool to find non-manifolds that I highly recommend.