Stereolithography (SLA): What It Is, How It Works

This 3D-printing technique uses photopolymer resins to fabricate everything from dentures to aircraft parts.

Written by Brooke Becher
Published on May. 30, 2024
Stereolithography (SLA): What It Is, How It Works
Image: Shutterstock

One of the most preferred 3D printing techniques is called stereolithography (SLA), a process that builds objects one layer at a time by curing liquid resin with an ultraviolet laser. While SLA may be more costly than its peers, the method is known for producing highly accurate, functional prints that feature smooth surfaces and fine details.

Stereolithography Definition

Stereolithography is a 3D printing technique that uses UV light to cure photopolymer resins into three-dimensional objects. 

What Is Stereolithography?

Stereolithography (SLA) is a type of 3D printing process that uses a ultraviolet light to cure liquid resin into solid objects. Most often, SLA printers create models upside down, featuring a build platform that descends into a vat of photopolymer resin. These models are built in consecutive layers that are thinner than a single strand of hair, resulting in extremely high-resolution prints.

The stereolithography process is known for its “exceptional detail and smooth surface finish,” Paul Chow, chief technology officer and co-founder of 3DGearZone, told Built In, “making them ideal when you need high-precision, detailed parts.”

That’s why SLA is used in prototyping across a variety of industries, from medical devices and instruments to aerospace and automotive parts.

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What Materials Does Stereolithography Use?

Stereolithography uses photopolymers, or resins, which are photo-reactive liquids that come in a variety of compositions with blended formulations based on their intended application. Additives, like glass, silicone or ceramic, may be mixed in to enhance specific properties of a material, increasing its heat deflection or water resistance.

 

How Does Stereolithography Work?

Like every other 3D printing method, the first step to stereolithography is uploading a computer-aided design (CAD) file, providing a slice-by-slice blueprint of the 3D model.

“This software cuts the part into layers as well as adds any support structures to anchor the part to the machine during the printing process,” Robbie Long, a product marketing manager at Fictiv, explained. The tank is then filled with liquid photopolymer resin that’s either manually poured or automatically dispensed.

A build platform descends into the vat of resin, leaving a thin layer of resin between the platform and the bottom of the tank. Then, a UV laser shoots through a transparent window to trace the base layer of the model. As the resin chemically reacts to the heat, it hardens and attaches to the build platform.

Once a cross section is drawn and the material cured, the platform slightly lifts, allowing the tank to restore a fresh coat of resin, then lowers again. The next layer is built on top of the previous and repeats until the part is complete.

After printing completes, the model is removed from the build platform and transitions into post production. Any wet resin is cleaned in a solvent solution, and support structures are removed from the print. The model is then placed in a UV chamber for further curing, and the material’s formulated properties are sealed in.

“When fully hardened,” Long said, “the print is sanded, bead blasted, painted or otherwise finished to achieve the desired surface finish for the prototype.”

 

Take a closer look at stereolithography in action. | Video: Formlabs

How Is Stereolithography Used?

Below are some of the ways stereolithography is used across a wide range of applications.

Prototyping and Product Development

Engineers and designers rely on SLA printers to create accurate, finely detailed concept models as well as functional, high-fidelity prototypes to test form, fit and function during the product development process before graduating a project into mass production. Its ability to quickly turnaround iterative designs enable rapid prototyping, expediting a project’s lifecycle and reducing its time-to-market.

Manufacturing

This ability to create high-precision prototypes and custom parts prepares a project for mass production, while also producing detailed master patterns and molds for processes like injection molding and casting. SLA is also used to directly fabricate custom jigs, fixtures and tooling in house, which can greatly enhance the efficiency and accuracy of the overall manufacturing process.

“With the ability to print functional prototypes in house,” Alec Rudd, engineering lead of Formlabs’ SLA print process, told Built In, “SLA fast tracks the product design cycle and allows for more design freedom to produce new models daily rather than waiting for outsourced parts to come in.”

Automotive

SLA takes part in nearly every step of the automotive lifecycle, from printing out conceptual models to fabricating aftermarket parts. Engineers and designers rely on SLA to produce lightweight and complex parts, such as brackets, housings and interior components, when testing and validating final designs as well as low-volume customizations per order.

Aerospace

SLA-printed components have made it to the International Space Station and on commercial airlines. Typically, SLA’s role in aerospace is fabricating lightweight parts with intricate geometries and assisting in prototyping, testing and design validation. But these machines may be able to bioprint organs in case of mid-flight emergencies and architect moon bases in the near future.

Dental

Dental professionals use SLA to create models, crowns, bridges and clear aligners tailor-made to the anatomies of each individual patient. Beyond finding the perfect fit, SLA is also used to print custom surgical guides that aid in the precise placement of dental implants and fabricate highly detailed casting patterns for prosthodontics.

Medical

Similar to the dental field, medical professionals use SLA to print custom surgical guides and anatomical models that allow for precise planning and execution of complex surgeries. It can also fabricate patient-specific implants and prosthetics for superior fit and function. Additionally, SLA is utilized to create high-fidelity prototypes of medical devices in a fraction of the time and often at lower cost.

Jewelry

With high customization and the ability to create almost any design down to the finest detail, jewelers use SLA to fabricate intricate, one-of-a-kind pieces with the perfect fit. It can also be used to produce large batches of consistent, cast-ready pieces.

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Advantages of Stereolithography

Stereolithography prevails as a choice 3D printing technique for the following reasons: 

High Precision and Detail

SLA produces prints accurately and with extremely high resolution. Its ability to capture fine details makes this method ideal for materializing intricacies and complex geometries that manage well under stress and would otherwise be too difficult to replicate with traditional manufacturing practices or even other 3D printing techniques.

Speed

Depending on the project and choice of resin, stereolithography can produce attractive, functional parts within a few hours — a bonus for designers and engineers who need to quickly iterate during the product development process.

Quick Post Production

Of all 3D printing processes, SLA is “the gold standard” for delivering products with smooth surfaces and fine features hot off the printer, thanks to its thinly cut cross sections. This creates prototypes that are both aesthetic and functional without the need for extensive post-production, and more time for secondary tooling processes.

Versatility

Resins used in SLA printing come in a variety of formulations that can make parts as rigid, flexible or tough as needed, with special properties, like withstanding high-temperatures, heavy impact or biocompatibility. This extends the use of SLA to a wide range of applications across various industries, from surgical guides and dentures to jewelry casting patterns and aerospace parts.

Minimal Material Waste

Only the necessary amount of resin is used for each part in SLA prints, resulting in minimal material waste next to traditional, subtractive manufacturing processes.

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Disadvantages of Stereolithography

Even with its advantages, stereolithography is not meant for every 3D printing project.

Expensive

When compared to alternative 3D printing methods, SLA printers and photopolymer resins often come at a higher price. The printer itself can range anywhere from $3,000 to $200,000 for higher-end, industrial-grade machines. Then, there’s the resin, with an average price point landing between $150 to $200 per liter, according to Formlabs.

That said, desktop SLA printer models are a new, more affordable trend. Meant for small-scale projects, they typically cost less than $300.

Fragile

SLA primarily uses photopolymer-based resins, which tend to be less durable than other 3D printing materials like thermoplastics used in fused deposition modeling (FDM) or selective laser sintering (SLS). SLA-produced parts are sensitive to sunlight — so they’re not suitable for outdoor applications — and unfit for mechanical testing, as they can crack or shatter under too much stress or impact.

Limited Build Size and Volume

SLA printers typically have smaller build volumes compared to other 3D printing technologies, which can be restrictive for printing larger parts. Currently, the largest printer on the market is the ProX 950 by 3D Systems, which can materialize parts up to five feet in length.

Difficult Handling

Sticky photopolymer resins can spill and require careful handling in order to avoid potential health hazards. They also need to be washed, dried and further cured once printing is completed. Proper ventilation, protective equipment and safe disposal procedures are necessary to ensure safety.

 

SLA vs. SLS: What’s the Difference?

Stereolithography is often compared to another popular additive manufacturing technique known as selective laser sintering (SLS). Similarly, they both create complex, high-precision, three-dimensional parts layer by layer. But they use different materials and methods to get there.

While SLA uses a UV laser to harden liquid resin, SLS sinters powdered particles — typically thermoplastics like nylon — together. As a result, SLA produces high resolution products with smoother finishes and fine details, and SLS produces rougher, more robust parts with superior mechanical functionality.

The durability of SLS prints makes the approach “a popular choice among engineers and manufacturers for functional prototyping and limited-run or bridge manufacturing,” Rudd said.

There’s also less post-production fuss with SLS. It doesn’t require dedicated support structures as part of the build that have to be manually removed once printing completes, like SLA does.

“SLA’s main advantage is definitely the stunning print quality,” Chow said. “However, it can be slower, require more post-processing and have a smaller print area compared to some other methods, like SLS.”

Frequently Asked Questions

Stereolithography is a 3D printing technique that uses UV light to cure photopolymer resins into three-dimensional prints. 

Stereolithography (SLA) uses a UV laser to cure photopolymer-based resins while selective laser sintering (SLS) works with thermoplastic powders. As Robbie Long, a product marketing manager at Fictiv, explained: “SLA parts typically have smoother finishes and are better for fine details whereas SLS parts are typically rougher, but are very strong and produced from powdered production grade thermoplastic.”

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