Understanding manifold design

This chapter is all about the 3D digital creation process. This is two components. First, exploring the various ways to create 3D designs, and some tips and tricks to keep in mind for each work flow. Second, introducing concepts that are important for 3D printing. Creating manifold designs, keeping track of units and following design guidelines. Regardless of what software title or 3D printer you plan to use. This chapter will give you a good foundation in design for 3D printing.

The first rule of 3D printing, is that your design must be manifold. And regardless of how you create your design, you'll need to keep this in mind from the beginning. What does manifold mean? Let's take a look. A 3D printed design requires volume. It needs to have a complete outer surface. This is called watertight geometry. Let's think about this in two dimensions for a minute. Let's say we have a square. It has four lines that combine to create a shape.

But if we delete one of those edges, it's just a collection of lines. When we're talking about the concept of manifold in 3D space, the idea is really similar. Just like the lines need to be fully connected to create a shape, and there are lots of ways of doing that. A surface in a 3D design has to be connected on all edges as well. And that's what I'm talking about when I say manifold. More than defining manifold it's important to understand what non-manifold is. Because these are the errors that cause problems for 3D printing.

Before we take a look at the specifics, let's take a look at some definitions. The data in a 3D printing file is called a mesh. It's made up of a collection of faces, sometimes called triangles. The faces are composed of edges. And where the edges meet is a vertex or vertices. When we're talking about more than one. There are a handful of mesh errors to look out for. I'll start with the most important and finish with the more subtle rules. This Stanford bunny originated as data from a 3D scanner. The original design has holes in the base.

Or that's missing faces, so it's not manifold. Even small holes can create problems. Make a design manifold, by closing all the holes. Another common mistake is intersecting geometry. This two-headed bunny looks okay on the surface, but a peak inside shows extra faces. Combine your geometry so the model has one continuous outer surface. What if you want a hollow design? It's common to create a hollow model to reduce print costs when using a commercial 3D printing surface.

But you can't just remove the bottom of the model. 3D printed designs require wall thickness. Make your design manifold by adding wall thickness. Faces have an inside and an outside. Some programs will color the faces so you can tell which is back and front. Make sure that all of your faces point in the same direction. When the faces are oriented the wrong way, it's called inverted normals and it makes it difficult for the printer to interpret the inside or outside of the design. Make sure all your faces are oriented correctly.

There are a handful of more subtle problems to watch out for. Avoid loose faces floating around or inside your model. And avoid overlapping faces, and coincident edges, which are unconnected edges on top of one another. Keep these concepts in mind while you create your design. While minor mesh issues can be fixed by a repair utility after your digital model is complete. It's best to avoid those problems up front and design manifold geometry as you go.