What Is 3D Printing? And How Does It Work?

Three-dimensional (3D) printing holds promise for a wide variety of applications, from biomedical implants to space exploration. But when a friend asked me how it worked, I had no idea. It was a perfect excuse to learn something new. And now, dear reader, I can explain it to you.

3D printing is exactly what it sounds like – using a machine to create a three-dimensional object out of thin air. Well, not exactly thin air. They use everything from metals to ceramic powder. But the process does look like magic. (Before I go on, it’s worth noting that 3D printing goes by a number of different names, including: rapid prototyping, direct manufacturing, solid freeform fabrication, and additive manufacturing. I’ve chosen to use the term 3D printing, but they all mean approximately the same thing.)

The 3D printing process begins on a computer. The first step is to create a three-dimensional model of an object using a computer-aided design (CAD) program – laying out the precise height, width and depth of the object you want to fabricate. This could be a surgical implant, an automobile part, an artistic sculpture, whatever you can think up.

The second step is to convert your model into an STL file (the “STL” stands for stereolithography, and is used regardless of whether you’ll be using stereolithography technology). This conversion process takes the 3D model you made in CAD and maps its surface using a series of connected triangles.

The STL file is then run through a program that “slices” the model into wafer-thin layers. This program also maps where the plane of each layer intersects with the “legs” of the various triangles. Each of these intersections is represented by a dot. By connecting the dots, you can see the profile of that particular layer of the 3D model. This profile tells 3D printing machines how to “draw” each layer. The layers pile up on top of one another and – presto! – you have a finished product. This (extremely short) animation does a pretty good job of illustrating the process.

But how do the machines work? It depends on which machine you use (there are LOTS to choose from).

Stereolithography is the oldest method, dating back to the late 1980s. Stereolithography machines contain a table that sits in a bath of resin. The resin solidifies (or “cures”) when exposed to a particular wavelength of light – usually ultraviolet (UV) light. A laser, using the relevant wavelength of light, draws the profile of the first layer of your model in the resin, and then scans back and forth, filling the profile in. Like a very attentive grade-schooler, coloring in a picture. The area  being filled in by the laser responds to the light and solidifies. The table then lowers a little bit further into the resin bath (maybe 0.1 millimeters), and a blade sweeps across the top to even out the resin. This process repeats itself until you have a finished product, made of a hard, plastic-like epoxy. This video (also short – and there’s a skull at the end!) shows the stereolithography process in action.

Other machines use different technologies, but operate on the same general principle of building one layer on top of another. Some machines operate like inkjet printers, but print UV-curable resin instead of ink. As the machines print each layer of resin, it is scanned with UV light, curing it. Other machines print using a binding agent instead of ink, and print into a bed of ceramic powder. Where the binding agent is applied, the ceramic powder sticks together. The powder sits on a table, which lowers with each layer, allowing the binding agent to add additional layers. The finished product here is a plaster-like ceramic.

Still other machines use extruder nozzles to deposit layers of plastic (or anything else that can be squeezed out) directly onto a plate.

But the really cool machines use metal. Electron beam melting (EBM) machines use electron beams to melt metallic powders into layer after layer of the appropriate shapes (other machines do the same thing, using lasers). These machines can be used to create functional metal parts for everything from hip implants to aerospace equipment. Depending on the part, some finishing work may need to be done (such as smoothing or polishing rough surfaces), but the ability to create custom-made metal parts out of metal powder has a lot of potential.

For one thing, this technology would come in handy when you don’t know exactly what replacement parts you’ll need and both space and payload are limited. For example: at sea on an aircraft carrier; at a remote research facility in Antarctica; or in a spaceship headed for Mars. In any of those situations, it’s unlikely you’ll be able to pack every possible spare part you could possibly need. But, theoretically, you could include the electronic 3D schematics for all of those parts, an EBM machine and a big bucket of metal powder. Break a cog, gear or widget? No problem. Print out a new one.

3D printing technology also opens the door to creating entirely new products, because it allows the creation of shapes that cannot be made with conventional manufacturing technology – such as light, but strong, 3D lattice structures (see the image below).

Researchers are continuing to move forward to explore these potential new designs, as well as how 3D printers such as EBM machines might be able to use different materials (e.g., lunar dust) to fabricate objects.

Note: Many thanks to Ph.D. student Tim Horn and Dr. Ola Harrysson of NC State’s Edward P. Fitts Department of Industrial and Systems Engineering, for taking the time to talk to me about 3D printing. They were patient in their explanations, and kindly answered all of my questions (even the inane ones). Any errors in the above post are mine and mine alone.