SLA (stereolithography) printing is a 3D printing technology that uses a UV laser to harden liquid resin into solid objects, one thin layer at a time. It was the first 3D printing method ever invented, and it remains one of the most precise, capable of producing details as fine as 20 microns (roughly a quarter the width of a human hair).
How SLA Printing Works
An SLA printer starts with a tank of liquid photopolymer resin. A UV laser traces the shape of each layer across the resin’s surface, and wherever the light hits, a chemical reaction kicks off. Light-sensitive molecules in the resin (called photoinitiators) absorb the UV energy and break apart into reactive fragments. Those fragments latch onto nearby resin molecules and trigger a chain reaction, linking small molecules into long, rigid polymer chains. Once enough chains form and lock together, the liquid resin solidifies into hard plastic.
The laser traces one cross-section of the object, then the build platform shifts by a fraction of a millimeter and the laser traces the next layer on top. This repeats hundreds or thousands of times until the full object emerges from the resin tank. Each layer is typically 50 microns thick for standard prints, though high-detail work can go as thin as 20 to 30 microns. The X-Y resolution (the fineness of detail within each layer) generally ranges from 20 to 90 microns depending on the printer.
SLA vs. DLP vs. MSLA Printers
SLA is sometimes used as a blanket term for all resin-based 3D printing, but there are actually three distinct approaches that differ in how they deliver light to the resin.
- SLA (laser) uses a single UV laser steered by mirrors to trace each layer point by point. This produces consistently high detail across the entire build area, since the laser spot size stays the same no matter where it’s aimed.
- DLP (Digital Light Processing) uses a digital projector to flash an entire layer at once. This makes it faster than laser SLA, but resolution depends on how many pixels the projector has relative to the build area. Larger prints mean each pixel covers more space, reducing detail.
- MSLA (Masked Stereolithography) shines a UV light source through an LCD screen that acts as a mask, blocking light everywhere except the shape of the current layer. MSLA printers are the most affordable option, and their resolution is determined by the pixel size of the LCD panel.
For applications demanding the tightest tolerances, laser SLA tends to edge out the others. One comparative study of dental surgical guides found that an SLA printer achieved deviations as low as 0.078 mm on precision features like drilling holes, consistently outperforming a DLP system on fine details.
Types of Resin
The resin you choose determines the mechanical properties of the finished part. Standard resins produce rigid, opaque objects suitable for prototypes and display models. Beyond standard, several specialty categories exist:
- Engineering resins mimic the properties of traditional plastics. Some formulations behave like polypropylene, others like ABS or polycarbonate. Post-cured engineering resin can reach a tensile strength around 59 MPa and a stiffness of 2.9 GPa, putting it in the range of injection-molded parts.
- Flexible resins produce rubbery or bendable parts for gaskets, grips, or wearable prototypes.
- High-temperature resins resist heat deflection, useful for molds or parts exposed to hot environments.
- Clear resins cure to a translucent or near-transparent finish after polishing.
- Castable resins burn out cleanly in a kiln, making them ideal for lost-wax casting in jewelry and metalwork.
- Biocompatible and dental resins are formulated for safe contact with the human body, used for surgical guides, dental aligners, and custom implant models.
Post-curing (exposing the finished print to additional UV light) significantly improves mechanical performance. One study found that curing roughly doubled the stiffness and tensile strength of standard resin prints.
What Happens After Printing
A freshly printed SLA part isn’t ready to use. It comes off the build platform coated in uncured liquid resin and needs two key post-processing steps: washing and curing.
Washing involves soaking the part in isopropyl alcohol (IPA) to dissolve and remove residual resin. A typical process uses two separate baths. You agitate the part in the first bath for about 30 seconds, let it soak for half the total wash time, then move it to a clean second bath and repeat. After washing, the part needs at least 30 minutes of air drying so the alcohol fully evaporates from the surface.
UV post-curing then finishes the job. The part goes into a curing chamber (essentially a box lined with UV LEDs) where additional light exposure completes the chemical reactions that started during printing. This step hardens the part to its final strength and stiffness. Cure times vary by resin type and part geometry.
Most SLA prints also require support structures: thin scaffolding that props up overhanging features during printing. These supports need to be clipped away afterward, and the contact points typically leave small marks that you can clean up with a craft knife and sandpaper. How much cleanup you’ll need depends largely on decisions you make before printing. Adjusting support density, contact size, and placement in your slicing software can dramatically reduce post-processing effort.
Common Uses for SLA Printing
SLA’s strength is precision, which makes it dominant in fields where fine detail matters. In dentistry, it’s used to produce surgical implant guides, custom aligners, crowns, and bridges. In medicine more broadly, surgeons use SLA-printed anatomical models for pre-operative planning, and researchers print scaffolds from biocompatible polymers for tissue engineering.
Jewelry designers use castable resins to print intricate patterns that get cast in gold or silver. Engineers print functional prototypes that closely mimic the look and feel of final production parts. Tabletop gaming enthusiasts print highly detailed miniatures. And in manufacturing, SLA produces master patterns, jigs, and fixtures with tolerances tight enough for production use.
Health and Safety Considerations
Liquid photopolymer resin is a skin sensitizer and irritant. Wear nitrile gloves when handling uncured resin or freshly printed parts, and avoid skin contact entirely. If resin gets on your skin repeatedly, you can develop a lasting allergic sensitivity that makes future exposure increasingly problematic.
Volatile organic compounds (VOCs) are another concern. Resin printing releases VOCs during every stage of the process, but the post-processing phase is the most significant: washing and curing can release roughly twenty times more VOCs than the printing stage itself. Even pouring resin into the tank produces some of the highest short-term concentrations despite being the quickest step. These emissions include compounds linked to respiratory irritation, decreased lung function, and in cases of prolonged high-level exposure, increased cancer risk.
Good ventilation is essential. Print in a well-ventilated room or use an enclosure with a filtered exhaust. Wear a respirator with organic vapor cartridges when handling resin or during post-processing. Dispose of uncured resin and contaminated IPA as hazardous waste, not down the drain.
A Brief Origin Story
Chuck Hull developed the first working stereolithography prototype in 1983 and filed his patent for the technology in August 1984. His company, 3D Systems, released the SLA-1 in 1987, making it the first commercially available rapid prototyping system. That machine established the basic architecture that SLA printers still use today: a UV laser, a vat of photopolymer resin, and a platform that descends layer by layer. What’s changed since then is mostly scale and accessibility. Desktop SLA printers now cost a few hundred dollars, and the resins have expanded from basic prototyping materials to specialized formulations for engineering, medicine, and art.