Guide to Stereolithography (SLA) 3D Printing

SLA 3D printing uses a light source to cure liquid resin into three-dimensional objects by exposing a vat or tank of resin to a light source, which hardens it. Traditional, top-down SLA 3D printers had that light source positioned above the vat of liquid resin. Inverted stereolithography, first introduced in 2011 by Formlabs co-founders Max Lobovsky, David Cranor, and Natan Linder, positions the light source below the vat of resin; the cross-section is traced on the bottom-most layer of resin, which is backfilled as the build platform lifts up and allows liquid resin to flow underneath the previously cured layer.

There were several important innovations that led to the invention of inverted stereolithography, including the transparent (and eventually flexible) bottomed resin tank. This new tank design enabled the creation of larger inverted SLA 3D printers because peel forces were mitigated by the flexible surface.

SLA 3D printers use light to cure 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.

In the last decade, several new types of resin 3D printing processes have been developed, primarily differentiated by the type of light source they use. These include: laser-powered stereolithography (SLA), digital light processing (DLP), or masked stereolithography (MSLA, also often used interchangeably with LCD 3D printing).

Regardless of the direction from which the light source shines or the type of light source, the SLA 3D printing workflow is straightforward. When the part has finished printing, there is a necessary alcohol or ether wash step to remove excess liquid resin from the surface of the part. Then, depending on the material, a post-cure step may be necessary to complete the polymerization of the part, helping it reach its optimal material properties. Further post-processing methods like coloring, coating, or plating may be used for specific applications or aesthetics.

The SLA 3D printing process first appeared in the early 1980s, 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 3D Systems to commercialize it. Hull described the method as creating 3D objects by successively “printing” thin layers of material curable by ultraviolet light. These first SLA 3D printers were large, industrial systems, often priced above $100,000, that required complex infrastructure and maintenance. 

SLA 3D printing, however, was not the first 3D printing technology to gain widespread popularity. As patents for different types of 3D printing technologies began to expire at the end of the 2000s, the introduction of small-format fused deposition modeling (FDM) 3D printers widened access to additive manufacturing. While this affordable extrusion-based technology sparked the first wave of wide adoption and awareness of 3D printing, FDM 3D printers did not satisfy the spectrum of professional needs.

Formlabs released Form 2 in 2015 and, with the subsequent release of a wider range of materials, SLA 3D printing became much more accessible for professionals in a range of different environments. In 2019, Formlabs introduced Low Force Stereolithography™ (LFS) with the Form 3 and Form 3L SLA 3D printers, which use flexible-bottomed resin tanks to drastically reduce the forces exerted on parts when peeling the cured part away from the bottom of the tank.

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. 

As applications grew, the technology became more popular and widely accepted. Today, stereolithography is one of the three most established plastic 3D printing processes, the other two being fused deposition modeling (FDM) and selective laser sintering (SLS). Successive iterations have made the Form Series the world’s leading professional resin 3D printers with 140,000 units sold and over 400 million parts printed as of 2024.

In 2024, Formlabs took powerful, accessible resin 3D printing a step further, releasing desktop-sized Form 4 and its biocompatible version, Form 4B, as well as Form 4L and Form 4BL large-format resin 3D printers. The newest iterations of Formlabs’ Form Series resin 3D printers are powered by a next-generation MSLA print engine, called Low Force Display™ (LFD).

The accessibility and affordability of high-powered desktop and benchtop units have made new modes of production possible. Companies can scale 3D printing output incrementally, bring their supply chain in-house, and increase flexibility and adaptability in the face of uncertain market conditions. By adding new materials, companies can unlock new applications.

At the core of the LFD Print Engine is the Backlight Unit. The ultra-high power light source uses LEDs and collimating lenses to emit a uniform area projection of light. The light passes through the lens array, making it more collimated (parallel) and uniform, eliminating dark or bright spots.

From here, the light passes through the Light Processing Unit (LPU) 4, where filters and masks form the light into the shape of the printed layer. Once the light reaches the liquid resin inside the tank, the full area turns into a solid layer. The Build Platform then rises out of the resin and a precision Z-axis peels away the layer from the bottom of the resin tank.

In the past, peel forces have been a major hurdle for resin 3D printing, forcing users to sacrifice either part quality, reliability, or printing speed when selecting a printer. With Form 4 and Form 4L, peel forces are minimized using a redesigned Flexible Film Resin Tank and a new Release Texture, a proprietary, microtextured optical film that introduces airflow to prevent the resin tank from suctioning to the LPU.

Use any CAD software or 3D scan data to design your model, and export it in a 3D printable file format (STL or OBJ). Then, import the digital design into print preparation software to specify printing settings and slice the digital model into layers for printing. Formlabs’ print preparation software, PreForm, is free to download and can create automated supports and print orientations. 

More advanced users may consider specifically designing for SLA, or taking steps like hollowing parts to conserve material.

The print preparation software will send the part to the printer, typically over a wireless internet connection, USB, or ethernet. 

Inverted SLA printers use removable tanks and build platforms that make it easy to change materials and start a new print. More advanced SLA printers like Formlabs’ Form Series printers also use a cartridge system that automatically refills the material during printing. This means that after a quick confirmation of the correct setup, the printing process begins and the machine can run unattended until the print is complete.

For Formlabs SLA 3D printers, an online Dashboard also allows you to remotely manage printers, materials, and teams.

After removing parts from the build platform, parts require rinsing in isopropyl alcohol (IPA) or ether wash to remove any uncured resin from their surface. Formlabs’ Form Wash and large-format Form Wash L have been engineered to streamline the wash process, easily removing excess resin and shortening overall post-processing time.

After rinsed parts are dry, some materials require post-curing, a process that helps improve parts' strength and performance, and reach their optimal material properties. Formlabs’ Form Cure and large-format Form Cure L precisely control temperature and light to provide a consistent, high-intensity cure.

Finally, remove supports from the parts and sand the remaining support marks for a clean finish. SLA parts can be easily machined, primed, painted, and assembled for specific applications or finishes.

Some parts could also benefit from further steps like sanding, coating, plating, or media blasting. These advanced resin 3D print post-processing methods can achieve a wide range of results, such as making parts better suited for outdoor applications through UV protection, or increasing the mechanical strength of a part by electroplating it in metal or a ceramic coating solution like Cerakote.

When that speed is compounded day by day and week by week, the throughput benefits are extraordinary. Form 4 and Form 4L can now match the speed of high-throughput technologies such as injection molding. Printing full build chambers over the course of a few hours, multiple times a day, can rival the output of a medium-volume injection molding machine, without the high up-front cost of tooling.

SLA resins are incredibly versatile, and there are hundreds of different formulations available. They can be 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. Materials range from industry-specific, like dentures, to those that closely match final materials for prototyping, formulated to withstand extensive testing and perform under stress.

Many manufacturers of SLA printers also formulate and manufacture their own resins for use in a closed system, though some offer an open platform for any resin to be used, and others white-label other companies’ resins.

Because these resins are formulated specifically for SLA 3D printers, they are not completely analogous to the familiar thermoplastics like nylon or ABS used in traditional plastic manufacturing methods like injection molding. Though learning which resin is best for a specific application may take testing and consideration of data sheets and application guides, there is an SLA resin for almost every application possible — the enormous variety of mechanical and aesthetic properties available make it easy to create an optimized and efficient workflow.

Accuracy and precision are paramount for all industries from manufacturing to dentistry, and SLA printing is one of most accurate 3D printing solutions on the market currently.

Accuracy refers to how closely you match the dimensions of the CAD model, and precision is defined as how repeatedly you can produce the same dimension. Compared to machined accuracy, professional SLA 3D printers are somewhere between standard machining and fine machining. However, accuracy does vary between resin 3D printer manufacturers and can be dependent on the type of light source used to cure the resin, the quality of the components, and the engineering and calibration that go into making those components function together. Accuracy is also dependent on the material — rigid materials are more accurate and easier to print with than flexible materials.

For example, Form 4/B and Form 4L/4BL can print Precision Model Resin — a high-accuracy material for creating restorative models — with >99% of the printed surface area within 100 μm of the digital model. For larger models (81-150 mm features) the XY dimensional tolerance is typically ±0.3% (lower limit: ±0.15 mm) when using Grey Resin.

The heated and enclosed build environment of Formlabs SLA 3D printers 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 close to room temperature, and printed parts don't suffer from thermal expansion and contraction artifacts.

Form 4 and Form 4L's LFD Print Engine, especially the high-resolution liquid crystal display and collimating lenses included in the LPU 4, create hyper-accurate cross-sections of each part. The low peel forces made possible by the Release Texture and Flexible Film Resin Tank make accuracy repeatable, leading to highly precise parts.

SLA 3D printers are considered the gold standard for creating parts with smooth surface finishes and fine features. Resin 3D printed parts can easily have appearances comparable to traditional manufacturing methods like injection molding with barely any post-processing. Conversely, FDM 3D printed parts often have visible layer lines, and SLS 3D printed parts often exhibit a grainy, slightly rough texture on the surface. 

The surface quality of SLA 3D printed parts lends itself to the creation of end-use products that look and feel like mass-produced consumer goods. It also makes secondary processes like rapid tooling possible.

SLA 3D printers also can achieve finer features and smaller minimum feature sizes than FDM 3D printers, comparable to SLS 3D printers. The light in resin 3D printers can be controlled in much more precise shapes than a filament extruder and therefore create smaller features or thinner walls. And, because SLA light sources can be lower power than the lasers required to melt powder in SLS 3D printers, they can cure with greater precision, leading to smaller features.

Because 3D printing creates parts one layer at a time, completed prints may have variations in strength based on the 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.

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.

Custom or low-volume watertight and gas-tight parts are also sought after across several industries, such as marine research, underwater robotics, sustainable technologies engineering, oil and gas industries, and defense. Though some 3D printing technologies present an ideal solution for these parts, the common perception of additively manufactured parts is that they are porous and cannot be deployed in pressurized environments.

In recent years, however, this assumption has been thoroughly disproven. SLA printers can create watertight enclosures and completely waterproof parts. Institutions like National Oceanic and Atmospheric Administration (NOAA) and the University of Rhode Island have made incredible strides in marine research by implementing low-cost, high-quality SLA 3D printed testing and research equipment.

Formlabs’ General Purpose Resins are formulated for speed and consistency to create parts in a range of applications and industries. From matte greyscale parts for design review prototypes to transparent parts printed in Clear Resin for see-through models and molds, General Purpose Resins are the workhorses of SLA 3D printing. Formlabs latest General Purpose Resins developed for Form 4 allow new heights to be reached in speed, mechanical properties, and resolution — Fast Model Resin can print at speeds of 100 mm per hour and Grey Resin has 30% higher impact strength.

Formlabs Engineering Resins have been formulated to answer specific needs in engineering and manufacturing workflows, and enable new applications, streamline operations, and simplify field testing. These materials are designed to match or exceed the capabilities of industry-familiar materials like ABS, silicone, or PEEK. From extremely stiff, rigid materials, to robust materials that can handle impacts, to soft and flexible materials that can handle bending and flexing through repeated cycles. Unique specialty materials include ESD-safe or flame-retardant resins, as well as technical materials that haven’t been accessible in desktop 3D printing before, such as true ceramic and silicone 3D printing.

For dental professionals, the Formlabs ecosystems offers a simplified, all-in-one workflow that promises accurate parts, every time — no adjustment or calibration necessary. From large labs and practices with multiple types of appliances to smaller operations that specialize in certain indications, the Form 4B and library of Dental Resins provide solutions for everyone.

For healthcare professionals, Formlabs BioMed Resins are medical-grade materials for a wide range of applications where performance and biocompatibility are critical. Materials in the BioMed Resin family are developed and manufactured in an ISO 13485 certified facility and are compatible with common disinfection and sterilization methods.

Formlabs Jewelry Resins are designed to reliably reproduce crisp settings, sharp prongs, smooth shanks, and fine surface details. They enable anyone from retailers and designers producing custom jewelry to large casting houses manufacturing at scale to produce try-on pieces for customers, ready-to-cast custom jewelry, or masters for reusable jewelry molds.

SLA 3D printing can help businesses eliminate the high costs and long lead times associated with outsourcing or more complicated alternative methods like machining. With 3D printing, there’s no need for costly tooling and setup; the same equipment can be used to rapidly produce different geometries.

3D printing in-house can look different for different types of professionals. Many large businesses, such as Microsoft or Rivian, choose to operate internal service bureaus, where engineers, designers, and manufacturing teams request parts from a centralized lab. Other businesses, especially those that are design and iteration-focused with high degrees of CAD expertise in their workforce, prefer a decentralized approach with a printer on every designer’s desk. Accessible, affordable, desktop-sized machines like Form 4 make these types of workflows possible and can present agile solutions for a changing workforce and office environment. 

Whether centralized or decentralized, 3D printing in-house gives employees greater control over their workflows, and reduces costs and uncertainties for the business as a whole. 

Depending on the number of parts and printing volume, investment into a small format 3D printer can break even within months. 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. 

To make managing multiple resin 3D printers easier, Formlabs customers have access to two software platforms. Dashboard is a free software platform that helps them monitor their printers and streamline management. For added capabilities and advanced management tools, Fleet Control leverages automation to automatically assign prints and make multi-printer management more efficient.