Guide to Stereolithography (SLA) 3D Printing

Stereolithography belongs to a family of additive manufacturing technologies known as vat photopolymerization, commonly known as resin 3D printing. These machines are all built around the same principle, using a light source—a laser or projector—to cure liquid resin into hardened plastic. The main physical differentiation lies in the arrangement of the core components, such as the light source, the build platform, and the resin tank.

Watch how stereolithography (SLA) 3D printing works.

SLA 3D printers use light-reactive thermoset materials called “resin.” When SLA resins are exposed to certain wavelengths of light, short molecular chains join together, polymerizing monomers and oligomers into solidified rigid or flexible geometries.

SLA parts have the highest resolution and accuracy, the sharpest details, and the smoothest surface finishes of all 3D printing technologies, but the main benefit of the stereolithography lies in its versatility.

Material manufacturers have created innovative SLA resin formulations with a wide range of optical, mechanical, and thermal properties to match those of standard, engineering, and industrial thermoplastics.

Compare stereolithography 3D printing to two other common technologies for producing plastic parts: fused deposition modeling (FDM) and selective laser sintering (SLS).

See how to go from design to 3D print with the Form 3 resin 3D printer. This 5-minute video covers the basics of how to use the Form 3, from the software and materials to printing and post-processing.

The SLA 3D printing process first appeared in the early 1970s, when Japanese researcher Dr. Hideo Kodama invented the modern layered approach to stereolithography, using ultraviolet light to cure photosensitive polymers. The term stereolithography was coined by Charles (Chuck) W. Hull, who patented the technology in 1986 and founded the company 3D Systems to commercialize it. Hull described the method as creating 3D objects by successively “printing” thin layers of a material curable by ultraviolet light.

SLA 3D printing, however, was not the first 3D printing technology to gain widespread popularity. As patents began to expire at the end of the 2000s, the introduction of small format, desktop 3D printing widened access to additive manufacturing, with fused deposition modeling (FDM) first gaining adoption in desktop platforms.

While this affordable extrusion-based technology sparked the first wave of wide adoption and awareness of 3D printing, FDM machines did not satisfy the spectrum of professional needs—repeatable, high-precision results are crucial for professional applications, as are biocompatible materials in the dental industry and the ability to create fine features for industries like jewelry and applications like millifluidics.

SLA printing soon followed FDM to the desktop, when Formlabs adapted the technology in 2011. Small format desktop SLA printers brought the promise of high resolution 3D printing—previously limited to monolithic industrial systems—in a much smaller and more affordable setup with a wide range of print materials. These capabilities expanded access to 3D printing for a variety of custom and high precision applications across disciplines, including engineering, product design, and manufacturing, as well as dental, jewelry, and other industries.

In 2015, Formlabs released its next generation resin 3D printer, the Form 2, which became the industry-leading desktop 3D printer, with parts printed in the field ranging from affordable custom prosthetics to a customizable line of razor handles.

The Form 2 reset the conversation for SLA 3D printing, popularizing a “distributed” model of production, where companies can scale output incrementally, adding more small format printers as demand increases with the flexibility to print in different materials on each printer. The maturity of materials over time has only increased the usage of this model, as more advanced resins have unlocked applications beyond prototyping and into production and end-use parts across industries.

In 2019, Formlabs made another step change in the industry with the launch of the Form 3 and Form 3L, two new hardware products that set a new standard for SLA with systems built on a completely new print process.

Low Force Stereolithography (LFS) technology is the next phase in SLA 3D printing, meeting the demands on today’s market for scalable, reliable, industrial-quality 3D printing.

This advanced form of SLA 3D printing drastically reduces the forces exerted on parts during the print process, using a flexible tank and linear illumination to deliver incredible surface quality and print accuracy. Lower print forces allow for light-touch support structures that tear away with ease, and the process opens up a wide range of possibilities for future development of advanced, production-ready materials.

Inverted SLA printing introduces peel forces that affect the print as it separates from the surface of the tank, so the build volume is limited and sturdy support structures are required. The Formlabs Form 2 is heavily calibrated to account for the forces of the peel process and produce high quality parts. Learn more about the differences between the Form 2 and Form 3, which uses LFS technology.

LFS 3D printing drastically reduces the forces exerted on parts during the print process, using a flexible tank and linear illumination to deliver incredible surface quality and print accuracy. Learn more about how LFS works in this deep dive video.

Engineers, designers, manufacturers, and more choose SLA 3D printing for its fine features, smooth surface finish, ultimate part precision and accuracy, and mechanical attributes like isotropy, watertightness, and material versatility.

Because 3D printing creates parts one layer at a time, completed prints may have variations in strength based on orientation of the part relative to the printing process, with different properties in X, Y, and Z axes.

Extrusion-based 3D printing processes like fused deposition modeling (FDM) are known for being anisotropic due to layer-to-layer differences created by the print process. This anisotropy limits the usefulness of FDM for certain applications, or requires more adjustments on the part geometry side to compensate for it.

Read our in-depth guide about FDM vs. SLA 3D printers to learn how they compare in terms of print quality, materials, applications, workflow, speed, costs, and more.

In contrast, SLA resin 3D printers create highly isotropic parts. Achieving part isotropy is based on a number of factors that can be tightly controlled by integrating material chemistry with the print process. During printing, resin components form covalent bonds, but layer to layer, the part remains in a semi-reacted “green state.”

While in the green state, the resin retains polymerizable groups that can form bonds across layers, imparting isotropy and watertightness to the part upon final cure. On the molecular level, there is no difference between X, Y, or Z planes. This results in parts with predictable mechanical performance critical for applications like jigs and fixtures, end-use parts, and functional prototyping.

Because they are isotropic, SLA printed parts like this jig from Pankl Racing Systems can withstand the variety of directional forces they undergo during high stress manufacturing operations.

SLA printed objects are continuous, whether producing geometries with solid features or internal channels. This watertightness is important for engineering and manufacturing applications where air or fluid flow must be controlled and predictable. Engineers and designers use the watertightness of SLA printers to solve air and fluid flow challenges for automotive uses, biomedical research, and to validate part designs for consumer products like kitchen appliances.

OXO relies on the watertightness of SLA printing to create robust functional prototypes for products with air or fluid flow, like this coffee maker.

Industries from dental to manufacturing depend on SLA 3D printing to repeatedly create accurate, precise components. For a print process to produce accurate and precise parts, multiple factors must be tightly controlled.

Compared to machined accuracy, SLA 3D printing is somewhere between standard machining and fine machining. SLA has the highest tolerance of commercially available 3D printing technologies. Learn more about understanding tolerance, accuracy, and precision in 3D printing.

The combination of the heated resin tank and the closed build environment provides almost identical conditions for each print. Better accuracy is also a function of lower printing temperature compared to thermoplastic-based technologies that melt the raw material. Because stereolithography uses light instead of heat, the printing process takes place at close to room temperature, and printed parts don't suffer from thermal expansion and contraction artifacts.

Low Force Stereolithography (LFS) 3D printing houses the optics inside a Light Processing Unit (LPU) that moves in the X direction. One galvanometer positions the laser beam in the Y direction, then directs it along across a fold mirror and parabolic mirror to deliver a beam that is always perpendicular to the build plane, so it is always moving in a straight line to provide even greater precision and accuracy, and allows for uniformity as hardware scales up to larger sizes, like Formlabs larger format SLA printer Form 3L. The LPU also uses a spatial filter to create a crisp, clean laser spot for greater precision.

The characteristics of individual materials are also important for ensuring a reliable, repeatable print process.

Formlabs Rigid Resin has a high green modulus, or modulus before post-curing, which means it’s possible to print very thin parts with precision and a lower chance of failure.

SLA printers are considered the gold standard for smooth surface finish, with appearances comparable to traditional manufacturing methods like machining, injection molding, and extrusion.

This surface quality is ideal for applications that require a flawless finish and also helps reduce post-processing time, since parts can easily be sanded, polished, and painted. For example, leading companies like Gillette use SLA 3D printing to create end-use consumer products, like 3D printed razor handles in their Razor Maker platform.

Leading companies like Gillette use SLA 3D printing to create end-use consumer products, like the 3D printed razor handles in their Razor Maker platform.

Z-axis layer height is commonly used to define the resolution of a 3D printer. This can be adjusted in between 25 and 300 microns on Formlabs resin 3D printers, with a trade-off between speed and quality.

In comparison, FDM and SLS printers typically print Z-axis layers at 100 to 300 microns. However, a part printed at 100 microns on an FDM or SLS printer looks different from a part printed at 100 microns on an SLA printer. SLA prints have a smoother surface finish right out of the printer, because the outermost perimeter walls are straight, and the newly printed layer interacts with the previous layer, smoothing out the staircase effect. FDM prints tend to have clearly visible layers, whereas SLS has a grainy surface from the sintered powder.

The smallest possible detail is also much finer on SLA, given 85 micron laser spot size on the Form 3, in comparison with 350 microns on industrial SLS printers, and 250–800 micron nozzles on FDM machines.

SLA resins have the benefit of a wide range of formulation configurations: materials can be soft or hard, heavily filled with secondary materials like glass and ceramic, or imbued with mechanical properties like high heat deflection temperature or impact resistance. Material range from industry-specific, like dentures, to those that closely match final materials for prototyping, formulated to withstand extensive testing and perform under stress.

Using Ceramic Resin, you can 3D print parts with a stone-like finish and fire them to create a fully ceramic piece.

In some cases, its this combination of versatility and functionality that leads to companies to initially bring resin 3D printing in-house. After finding one application solved by a specific functional material, it’s usually not long before more possibilities are uncovered, and the printer becomes a tool for leveraging the diverse capabilities of various materials.

For example, hundreds of engineers in the Design and Prototyping Group at the University of Sheffield Advanced Manufacturing Research Centre (AMRC) rely on open access to a fleet of 12 SLA 3D printers and a variety of engineering materials to support highly diverse research projects with industrial partners like Boeing, Rolls-Royce, BAE Systems, and Airbus. The team used High Temp Resin to 3D print washers, brackets, and a sensor mounting system that needed to withstand the elevated, and leveraged Durable Resin to create intricate custom springy components for a pick and place robot that automates composites manufacturing.

Engineers at the AMRC use a fleet of 12 SLA 3D printers and a variety of engineering materials to print custom parts for diverse research projects, like brackets for a pick and place robot (top), and mounts for sensors in a high-temperature environment (bottom).

SLA 3D printing accelerates innovation and supports businesses across a wide range of industries, including engineering, manufacturing, dentistry, healthcare, education, entertainment, jewelry, audiology, and more.
 

Many companies start using 3D printing via outsourcing to service bureaus or labs. Outsourcing production can be a great solution when teams require 3D printing only occasionally, or for one-offs that require unique material properties or applications. Service bureaus can also provide advice on various materials and offer value-added services such as design or advanced finishing.

The main downsides of outsourcing are cost and lead time. Often, outsourcing is a gateway to bringing production in-house as needs ramp up. One of the greatest benefits of 3D printing is its speed compared to traditional manufacturing methods, which quickly diminishes when an outsourced part takes multiple days or even weeks to arrive. With growing demand and production, outsourcing also rapidly becomes expensive.

Because of the rise of affordable industrial-quality 3D printing, today, more and more companies choose to bring 3D printing in-house right away, vertically integrating into existing shops or labs, or in the workspaces of engineers, designers, and others who could benefit from translating digital designs into physical parts or who are involved in small batch production.

Small format, desktop SLA 3D printers are great when you need parts quickly. Depending on the number of parts and printing volume, investment into a small format 3D printer can break even within months. Plus, with small format machines, it’s possible to pay for just as much capacity as a business needs and scale production by adding extra units as demand grows. Using multiple 3D printers also creates the flexibility to print parts in different materials simultaneously. Service bureaus can still supplement this flexible workflow for larger parts or unconventional materials.

Fast turnaround time is a huge advantage to owning a desktop 3D printer. When working with a printing bureau, lead times, communication, and shipping all create delays. With a desktop 3D printer like the Form 3, parts are in-hand within hours, allowing designers and engineers to print multiple parts in one day, helping to iterate faster and drastically reduce product development time and quickly test mechanisms and assemblies avoid costly tool changes.

Owning a desktop 3D printer results in significant savings over 3D printing service bureaus and traditional machining, as these alternatives rapidly becomes expensive with growing demand and production.

For example, to fulfill tight production deadlines, a process engineer and team at Pankl Racing Systems introduced SLA 3D printing to produce custom jigs and other low-volume parts directly for their manufacturing line. While in-house SLA was initially met with skepticism, it turned out to be an ideal substitute to machining a variety of tools. In one case, it reduced lead time for jigs by 90 percent—from two to three weeks to less than a day—and decreased costs by 80-90 percent.

Pankl Racing Systems significantly reduced lead times and costs by 3D printing custom jigs in-house.

With small format machines, it’s possible to pay for just as much capacity as a business needs and scale production by adding extra units as demand grows. Using multiple 3D printers also creates the flexibility to print parts in different materials simultaneously.

The Design and Prototyping Group at the University of Sheffield Advanced Manufacturing Research Centre (AMRC) runs an open-access additive manufacturing station with a fleet of 12 Form 2 stereolithography (SLA) 3D printers for hundreds of engineers working on diverse projects across the site.

Formlabs offers two high precision SLA 3D printing systems, a growing library of specialized materials, intuitive print preparation and management software, and professional services—all in one package.

To continue exploring SLA 3D printing, start with feeling the quality of SLA for yourself: Request a free sample 3D printed part in your choice of material to be mailed straight to your door.