Stereolithography (SLA): Everything You Need to Know

Stereolithography is considered the technology that gave birth to 3D printing, with the first equipment patented in 1984 by Charles Hull and the first commercial machine developed by 3D Systems in 1988. Today we will learn more about the stereolithography process to understand what it is.

Stereolithography, also known as SLA (Stereolithography Apparatus), uses the principle of photopolymerization to produce 3D models using UV-sensitive resin. It should not be confused with STL files, which are 3D printing files. Solidification is caused by passing a laser layer by layer. It allows for one of the best printed surfaces compared to existing 3D printing technologies.

From a historical point of view, the first patent was filed in 1984 by Chuck Hull and marketed by 3D Systems in 1988. A few days before the Americans, a trio of French researchers composed of Jean-Claude André, Olivier de Witte and Alain le Méhauté, conceived a similar patent on behalf of the Alcatel group.

SLA 3D printers use a liquid print material and a UV protective coating (usually orange, green, red, or yellow). Stereolithography offers a relatively small build volume compared to other printing technologies. However, some machines like the Mammoth 3D printer can produce parts larger than 2 m.

How Stereolithography 3D Printing Works

As with other 3D printing technologies, a digital 3D file is required, which can be obtained using CAD software (SolidWorks, Sculpt, or SelfCAD, for example). These files (usually STL files), are sent to a “slicer” that cuts the model into thin layers to be printed. The instructions are then sent to the 3D printer.

Stereolithography machines contain a resin plate, a moving platform (Z-axis), a scraper system (X-axis), a UV laser, focusing optics, and a mirror galvanometer (X and Y axes).

The laser beam cures the liquid resin according to the digital 3D model supplied to the printer. After a layer has solidified, the platform moves down one level. The next section is then solidified. There are as many printing cycles as there are layers needed to obtain the full volume of the part.

On some SLA 3D printer models (such as those from Formlabs, for example), the part is produced upside down. The platform is immersed in the resin tray, while the laser acts from the bottom up.

Post-processing

Once the print is complete, a cleaning step of the chamber is required using a solvent (usually isopropyl alcohol, also called isopropanol) to remove excess unsolidified resin. Unlike other technologies such as Selective Laser Sintering (SLS), Fused Deposition Modeling (FDM), and PolyJet 3D printing, an additional UV treatment is usually required to finalize the photopolymerization and optimize the strength of the material.

Like Fused Deposition technology, SLA uses support structures when printing complex shapes. Much like scaffolding, these help support parts as they swing. They are then removed during post-processing.

Stereolithography technology offers a slightly glassy surface finish, but is usually superior to FDM or SLS processes (for layers of the same thickness). It is not uncommon for the different print layers to be barely visible. However, in SLA, few colors are available.

SLA 3D printing has given rise to numerous technologies

Following the failure of the first patents related to stereolithography, several players in the industry have worked to improve the technology. Examples include DLP technology, which uses a video projector instead of a laser to cover a larger area, resulting in faster printing. The French company Prodways has also developed a technique called MovingLight, which uses a video projector to further speed up the production of parts. The British company Photocentric uses an LCD screen as a UV light source.

More recently, manufacturer Carbon3D unveiled its CLIP (Continuous Liquid Interface Production) process, which controls the amount of oxygen during the photopolymerization reaction. This makes printing speeds 25 to 100 times faster. Another interesting initiative: ONO raised $2.3 million for a system that uses light from a smartphone to solidify resin.

Among the leading manufacturers of SLA 3D printers there is obviously 3D Systems, the founder of the technology itself, but also new players such as DWSlab, B9Creator or Formlabs. Based in Boston, the startup Formlabs launched a crowdfunding campaign on Kickstarter in 2011, raising the remarkable sum of 3 million dollars for the Form 1, the first desktop SLA 3D printer. The latter was marketed at around $5,000.

The price of stereolithography 3D printers ranges from €1,000 for low-cost versions to several tens of thousands of euros for professional machines with a large production volume.

Applications of stereolithography

SLA is mainly used for prototyping in all areas of the industry for its speed, but depending on the quality of the printer it is also possible to obtain immediately functional parts. It is also used for the production of molds or forges, in particular for the world of jewelry or dentistry.

Lost-wax casting is an example of an indirect manufacturing process that uses stereolithography. This age-old technique now uses 3D printing to produce accurate replicas of the final product (also called a jewelry mold) in castable wax. Once the shape is created, it is encased in refractory material creating the mold. Molten metal is then poured into the mold to replace the wax. Once the mold is removed, the metal part can be used.

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