What is Stereolithography?

Stereolithography (SL) is one of several methods used to create 3D-printed objects. It's the process by which a uniquely designed 3D printing machine, called a stereolithograph apparatus (SLA) converts liquid plastic into solid objects. 

The process was patented as a means of rapid prototyping in 1986 by Charles Hull, co-founder of 3D Systems, Inc., a leader in the 3D printing industry.

How it works: From CAD to STL

There are many different ways to 3D print an object. But nearly all of them utilize computer aided design (CAD) files.

CAD files are digitalized representations of an object. They're used by engineers and manufacturers to turn ideas into computerized models that can be digitally tested, improved and most recently, 3D printed.

In 3D printing — or additive manufacturing — CAD files must be translated into a "language," or file type, that 3D printing machines can understand. Standard Tessellation Language (STL) is one such file type and is the language most commonly used for stereolithography, as well as other additive manufacturing processes.

Since additive manufacturing works by adding one layer of material on top of another, CAD models must be broken up into layers before being printed in three dimensions.

STL files "cut up" CAD models, giving the 3D printing machine the information it needs to print each layer of an object.

How it works: SLA

Unlike the desktop printer you use to print documents, SLA machines don't extrude ink or some other liquid onto a surface. Instead an SLA machine starts with an excess of liquid plastic, some of which is cured, or hardened, to form a solid object. 

SLAs have four main parts: a tank that can be filled with liquid plastic (photopolymer), a perforated platform that is lowered into the tank, an ultraviolet (UV) laser and a computer controlling the platform and the laser. 

In the initial step of the SLA process, a thin layer of photopolymer (usually between 0.05-0.15 mm) is exposed above the perforated platform. The UV laser hits the perforated platform, "painting" the pattern of the object being printed. 

The UV-curable liquid hardens instantly when the UV laser touches it, forming the first layer of the 3D-printed object.

Once the initial layer of the object has hardened, the platform is lowered, exposing a new surface layer of liquid polymer. The laser again traces a cross section of the object being printed, which instantly bonds to the hardened section beneath it.

This process is repeated again and again until the entire object has been formed and is fully submerged in the tank. 

The platform is then raised to expose a three-dimensional object. After it is rinsed with a liquid solvent to free it of excess resin, the object is baked in an ultraviolet oven to further cure the plastic. 

Objects made using stereolithography generally have smooth surfaces, but the quality of an object depends on the quality of the SLA machine used to print it. 

The amount of time it takes to create an object with stereolithography also depends on the size of the machine used to print it. Small objects are usually produced with smaller machines and typically take between six to twelve hours to print. Larger objects, which can be several meters in three dimensions, take days.

What gets made

Stereolithography is an ideal solution for creating prototypes because it creates highly accurate, durable objects fairly quickly and relatively inexpensively. SLA machines can even create oddly shaped objects, which can be difficult to produce using traditional prototyping methods.

Many industries — from medical to manufacturing — use stereolithography to build prototypes and, on occasion, final products.

A car manufacturer, for example, might use stereolithography to create a prototype casting of a car door handle. Such a prototype can be tested for fit and form and, once perfected, can serve as the master pattern for a machined auto part. 

Companies using SLA

Stereolithography was the first process developed for rapid prototyping, and though it is among the oldest 3D printing methods, it is still very popular today. 

3D Systems Inc., the company that pioneered stereolithography still uses this process to build prototypes for clients. The company also sells SLA machines for use by businesses and manufacturers. 

Other leading 3D printing companies have their own preferred methods for 3D printing prototypes and finished parts. For example, Stratasys, another leading 3D manufacturer, patented the Fused Deposition Modeling process for 3D printing in 1992. 

But there are many companies around the United States that use SLA machines to provide their clients with rapidly produced and relatively cheap prototypes.

SLA at home

Most personal 3D printers on the market today use additive manufacturing processes other than stereolithography to create 3D-printed objects.

However, there are options for those that want to create their own SLA-printed objects at home. Formlabs, a startup based in Cambridge, Mass., recently developed a desktop machine that prints objects using stereolithography. 

And if you're more of a do-it-yourselfer, there are others out there trying to master the art of this high-tech process all on their own, as well. 

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Elizabeth Peterson

Elizabeth is a former Live Science associate editor and current director of audience development at the Chamber of Commerce. She graduated with a bachelor of arts degree from George Washington University. Elizabeth has traveled throughout the Americas, studying political systems and indigenous cultures and teaching English to students of all ages.