As cool as 3D printing is, there is a lot of hype around it, which sometimes causes confusion. Before starting to design for 3D printing, it's best to know a little bit about 3D printing technologies.
3D printing is a limitless technology in the sense that there is no end to the things it can make. Still, that doesn't mean that it can make anything without limitations. 3D printing can make things that no other manufacturing method can, but it has rules that need to be followed to ensure success. There are different types of 3D printing as well, and each type comes with its benefits and drawbacks:
In this chapter, we'll discuss:
What is 3D printing?
What types of 3D printing are there?
How do FFF printers work?
The anatomy of an FFF print.
Supportless 3D printing and YHT.
Wall thickness and tolerances.
3D printing is cool. It seems as if not a day passes without another mention of 3D printing online in the news and media. Everyone is getting excited about 3D printing. But when you look deeper, it seems as if everything is being 3D printed, and anything could be. Does 3D printing something make it better? What exactly is 3D printing?
In many ways, 3D printers are just tools, the same as any that you'd find in a wood shop or garage. These tools make cool things, but not on their own, and just because something is made with, say, an electric drill press, that doesn't automatically make it better than something that isn't. It's the things that people, like you, are doing with these tools that make them cool.
I'm not saying that 3D printing isn't cool by itself. 3D printing lets you create things, test them, change their design, and try something new quickly until you get it right. It makes things of incredible complexity and, because it's additive manufacturing, generates comparatively little waste. The availability of cheaper and faster 3D printers means that there's a chance that there's a 3D printer near you.
There are many different types of 3D printers, but what makes them all similar is that they build solid shapes from layers of materials, starting with an empty build area and filling it with the print. This is called additive manufacturing, and it produces less waste than other techniques, such as starting with a base material that is cut away to make the thing.
3D printers also benefit from being computer-controlled machines, also known as computerized numerical control (CNC) machines, meaning they do what they do with minimal human interaction after the design work is done. They can make many identical copies of a thing one right after the other, and the design can be shared online so that others can make their own copies.
While all 3D printing shares come common features, there are several distinct types of 3D printing that vary in how they produce the print. Fused filament fabrication (FFF), powder bed, or light polymerization, for example, all accomplish 3D printing in very different ways, and each with their own strengths and weaknesses. What works in powder bed 3D printing might not work with FFF 3D printing, and the part you get from light polymerization might not be suitable for the same usage as those made with the other techniques.
It is the best practice to always design towards the strengths and weaknesses of the medium you'll be using. The projects in this series of books will focus on designing for FFF 3D printers, because they're inexpensive and more readily available than the others, and the parts made with FFF 3D printers are suitable for a wide variety of functional uses. Also, many of the techniques for FFF design transfer to the other types of 3D printing. But because FFF 3D printers have limitations, there will be some things you need to know first.
There tends to be a lot of variation within the family of FFF 3D printers. Some have their mechanisms exposed to the environment so that they're easy to repair, while some are protected with fancy covers so that they look good. Some have one extruder, while some have two or more. Some have fancy interface screens, and some require you to use a computer to access even the most basic functions. Yet, for all their variations, there are many similarities that all FFF printers share which define their type. Being familiar with how FFF 3D printers work will help you guide yourself while designing for them.
For FFF 3D printers, a computer takes a 3D model and translates it into commands that the printer can follow. The printer then takes a roll of plastic filament on a spool and uses a feeder mechanism to feed it into the hot end, where the plastic filament is melted and squirted out at a controlled rate onto the print bed, where the print is built up. The extruder head and print bed are moved relative to each other in 3 dimensions, using some sort of movement system in order to create the 3D model:
Drawing a print layer by layer like this takes, as might be expected, a little bit of time. The larger the object, the longer a print will take. FFF 3D printing isn't a fast process. But once the process is done, a new thing will have been created.
Now that the mechanics of FFF 3D printing are clear, it's time to take a look at how a print is built. If an FFF print is stopped partway through, or observed during printing, the following can be seen:
Layers: FFF prints in layers, with each layer sitting on the one below it. Prints can be made with thicker layers so that they print faster or thinner layers so that they look better.
Outlines: When starting a layer, the outline of that layer will usually be printed first. FFF prints often have two or more outlines so that the outside of the print is strong.
Infill: once the outline is done, the rest of the layer is filled in. If an area of the print will not be seen from the outside when the print is done, a loose infill is used to save material and give layers above something to sit on. Top layers are filled in completely. Most FFF prints are largely hollow.
The basic limitations of FFF printers stem from the fact that most FFF 3D printers are developed by people who have very little accountability. To the people creating and manufacturing these printers, if the printer can print a thing most of the time, then that's probably good enough. In this way, FFF printers are more like garage tools than desktop machines. For those unfamiliar with FFF printers, there are some drawbacks that need to be taken into account.
FFF 3D printers have to worry about overhang. Overhang is when a part of the design, when it prints, will not have anything between it and the build platform. To compensate for this, the 3D printer can build a lattice of support material up to the overhanging part. After the print, the support material will have to be removed. But since for most FFF 3D printers the support material is made of the same material as the object, it can rarely be removed without a trace that is sometimes difficult to clean up completely and can leave a mess on more complex prints:
Think about building a snowman or sand castle. There's a lot that can be done with the medium of sand or snow, but try to get too fancy with the design and it will fall apart. As long as every part is sitting on top of something, chances are it will hold together. You could even slope gently outwards, as long as you don't push it too far.
It's the same with 3D prints. Because it prints in layers, each layer needs to have something to lay down on. If a design is made so that a part has nothing underneath it and is dangling in the air, then the printer will still extrude some plastic to try to print the part, but with nothing to print on, the plastic will just drool from the extruder until it gets wiped off on some other part, making an ugly mess and ruining the print.
As long as you put some thought into it, you can make designs that will succeed in most cases. There are a few rules that can help, and these rules can be illustrated with the letters Y, H, and T.
As the print gets to the part where the arms of the Y branch out, the change is gradual. It is possible to have the current layer slightly larger than the previous one, provided the overhang is gentle. Generally, a 45-degree overhang is safe. Hence, a shape like the letter Y will successfully print standing up.
However, if the overhang is too great or too abrupt, the new layer will droop, causing a print to fail. Some 3D printer owners pride themselves in pushing their overhang and have seen success with angles as steep as 80 degrees, but to be safe, keep your angles no more than 45 degrees.
Use caution when bridging. The printer makes no special effort when making bridges; they are drawn like any other layer: outline first, then infill. As long as the outline has something to attach to on both sides, it should be fine. But if that outline is too complex or contains parts that will print in midair, it may not succeed. Being aware of bridges in the design and keeping them simple is the key to successful bridging. Even with a simple bridge, some 3D printers need a little bit more calibration to print it well.
Hence, a shape like the capital letter H will successfully print most of the time because of bridging.
The top arms have far too much overhang to print successfully. Of course, the solution to this is simple: when designing, flip the T over or lay it down. In fact, every letter of the alphabet will print successfully if laid on its back, but the letter T illustrates this best. Sometimes, when designing a part for 3D printing, it's good to turn it around and orient it so that it prints well. Not every print needs to be printed in the same way it's going to be used.
There is a minimum size of things that a 3D printer can print. This size is determined by the size of the hole in the nozzle, called the nozzle diameter. The most common nozzle diameter is 0.4 mm; however, most printers will not print a wall with a single extrusion thickness. They require that a wall be at least as thick as two nozzle widths, which in most cases means walls need to be at least 0.8 mm. However, because of the way slicers calculate outlines, 0.8 mm isn't just a minimum wall thickness—it's a target. For instance, if the wall is 1 mm thick, it won't be able to fill in the gap between the outlines, and there will be an air pocket. And while 0.4 mm is a very common nozzle diameter, it is not the only nozzle diameter, so a 0.8 mm wall may still be too thin for some 3D printers.
For thickness, it's best to err on the side of caution. A 2 mm wall is thick enough that slicers can use one or two outlines without conflict and still have room for a little infill, no matter the nozzle diameter. This will make solid prints that will succeed in almost all cases, and 2 mm is still fairly thin, allowing for considerable detail. Unless you are designing for a specific printer or planning to share your model with others, always make your walls a minimum of 2 mm thick to be safe.
Models for 3D printing must be closed, that is to say, they must have no holes in them. In a classic cartoon, there was a scene where bubbles were blown, but they were not bubble shaped. They were square, squiggly, and pink-elephant shaped. But no matter their shape, they were still bubbles. If a hole developed in them, they popped. In the same way, models for 3D printing cannot have holes:
In mathematical terms, holes in models are included in a family of errors called non-manifold. Models for 3D printing must be manifold or else the slicer will have trouble telling what is supposed to be the inside and outside of the model.
In the same vein, a wall by itself, without an inside or outside, isn't printable because a 2D wall has no thickness and doesn't describe a shape that can exist in real life. 3D prints must be part of a three-dimensional shape with a thickness, as described in the previous section.
3D printing is cool and allows the creation of fantastic and detailed objects without needing much interaction with people after the design is done. But designing for 3D printing is a lot like designing for any other type of manufacturing. It helps to know a bit about the process involved and design with that process in mind.
Fused filament fabrication 3D printing, or FFF for short, is one of the oldest, most mature, and cheapest forms of 3D printing, so this series will focus on designing for it. It involves melting a plastic filament and drawing the object layer by layer, with each layer sitting on top of the one below it.
Designing for the most effective FFF printing means thinking about overhangs and supports and about the parts of the prints that don't have anything underneath them when they print. To avoid needing supports when printing, it can help to remember the letters Y, H, and T when designing, in order to remember to consider gradual overhangs, bridging, and orientation. In addition, it's important to remember that details should be, generally, about 2 mm thick.
Now that the mechanics of 3D printing and how they affect design have been covered, the next chapter will deal with the specific software that will be used in this series.