2. Basic Principles of 3D Printing

All 3D printing works on the same basic principle, but there are several variants and sub-variants. In this chapter we’ll explain the basic principle and then go on to discuss the variants.

Okay, so you can’t afford several thousand dollars’ worth of engineering software. No problem; Autodesk has free software that you use online at www.123dapp.com. This even includes a killer app called 123D Catch that will produce a 3D model using 20–30 digital photos from an ordinary camera, tablet, cell phone, and so on.

In a nutshell, that is the essence of 3D printing because all 3D printers work basically the same way. Their software takes in the 3D model from your design software, creates a cross-section view just a hair’s width up from the bottom of the part, and then prints it as a paper-thin layer of solid material onto a flat platen. The print head lays down a very narrow stripe of material as it traverses back and forth, advancing sideways a tiny amount between each traverse.

Because the printing sequence for the mug shown in Figure 2.1 is more complex, let’s begin by examining a simpler model of the letter H.

Figure 2.2 shows the first three stripes in blue while the yellow print head is working on the fourth stripe, shown in green. In the real world all stripes would normally be the same color and would not have dark edges. I added the edges and the green to make the stripes more obvious.

Figure 2.3 shows the half-finished first layer. The print head is working from right to left on the green stripe.

Moving right along, Figure 2.4 shows the completed first layer.

The printer lowers the plate with the first layer on it by an amount equal to the thickness of the layer and moves the cutting plane in the 3D model up by the same amount. It then prints the new cross-section on top of the previous one, as shown in Figure 2.5.

The process continues. Figure 2.6 shows the half-finished second layer, and Figure 2.7 shows how the first two layers look when finished.

The process repeats, layer after layer, as shown in Figures 2.8–2.19, as the part seems to grow downward from the print head.

I won’t bore you with figures of every layer being created, so we’ll jump ahead a bit to the tenth layer being started in Figure 2.11.

If you want to get a better feel for how the 3D printing process operates, all you need to do is to tear the preceding pages out of the book, cut out the figures, and staple them together to make an animation flipbook. When you’re finished, go buy another copy of the book because you just ruined the first one.

The next morning we can remove the finished part from the 3D printer, as shown in Figure 2.20.

Instead, printing time depends mostly on the thickness of the part. Your car speedometer shows miles and/or kilometres per hour, but 3D printer speeds are typically in the range of an inch per hour in the vertical direction. Realigning the part before printing so that its shortest dimension is in the vertical direction can often significantly reduce the printing time.

Figure 2.21 shows this process in the first two layers of our previous example. The third layer would then be applied in the same direction as the first one, and this would repeat all the way through the part. If this was not done, then the part could easily be much weaker in one direction than another, much like the grain in a piece of wood.

Figure 2.23 shows the contents of the next layer in isolation.

In the real world, Figure 2.23 would look almost exactly like Figure 2.22 except that the part would be thicker by the thickness of the layer shown in Figure 2.22. I figure it’s about time for a coffee break, and so Figure 2.24 shows the finished product.

As you have seen, 3D printing is effectively the opposite of sliced bread, but that doesn’t make it the worst invention of all time. Far from it.

How do you carve a statue of an elephant? You get a big block of marble and cut off anything that doesn’t look like an elephant. How do you 3D fusion print a statue of an elephant? You get a big pile of loose raw material and then stick together everything that does look like an elephant.

When you’ve finished printing, the part is fished out of the unused material; the unused material can be reused for the next part.

Fusion printers come in two basic variants. One variant uses a liquid plastic that solidifies where the ultraviolet laser in the print head shines on it. The first commercially successful 3D printers were the StereoLithography machines from 3D Systems, which use the fusion system.

The other variant of fusion machines uses powdered materials. This can range from corn starch, which is glued together using sugar water from the print head, up to steel powder that is fused together using a high-powered laser. A number of brands and models are available.

Relatively inexpensive home-hobbyist machines such as the MakerBot are readily available.

They can use a wide range of thermoplastic (meltable plastic) materials within the same machine.

Parts usually come out ready to use with little or no cleanup required.

Hollow, sealed cavities can be made, such as a basketball.

Now the bad news:

Parts might require temporary support structures that need to be removed later.

Consider the coffee mug shown earlier. What if we want the handle to be open at the bottom, as shown in Figure 2.25, instead of curving back into and joining the body of the mug?

Figure 2.26 shows everything starting out much as we expect it would.

Figure 2.27 shows everything looking good so far...until we reach the bottom of the handle.

As shown in Figure 2.28, there’s nothing to support the first bit of the end of the handle. Figure 2.28 shows how the first layer of the lower end of the handle would simply fall down because we are trying to print on thin air.

This portion of all subsequent layers will do likewise until we reach the point where the upper end of the handle curves back and joins the main body.

Looking back at Figure 2.23, it seems like it would have the same problem. However, there is a difference. Figure 2.23 shows that the next slice being printed is supported by the lower portion of the handle. Now look at Figure 2.27, which doesn’t have the lower handle portion.

There are three possible solutions:

Redesign the part to eliminate the overhangs.

Add temporary support webs that are removed when the part is finished.

Reorient the part for printing. In the case of the coffee mug, you could simply rotate the computer model 90° on its Y axis so it’s lying on its side or 180° on its Z axis so it’s upside-down.

You can usually ignore the overhang problem because the unfused material will support things until the print operation is finished. This makes it easier to design and print complex shapes.

A wide range of materials can be used from liquid plastic to corn starch to powdered metal.

Now the bad news:

You can’t build empty sealed cavities such as a basketball because they will be full of unfused material. If you need such an item, then you’ll have to leave a drain hole that is plugged later.

Any given machine can use only a specific material or narrow range of materials. You can’t run powdered aluminum through a starch machine, for example.

Parts need to be cleaned after printing. Machines that use a fine powder—starch in particular—tend to produce a lot of dust.

Fusion printers tend to be more expensive than deposition ones.

Later chapters in this book provide more specific information on the pros and cons of the various 3D printers, but this gets you off to a good start.

The following chapters explain how to create 3D models with the iPad, a Mac, or a PC.