SLA 3D Printing: Stereolithography Explained for Engineers and Designers

Stereolithography (SLA) was the world's first commercially available 3D printing technology, invented by Chuck Hull in 1986. Today it remains the benchmark for surface quality and dimensional accuracy in polymer additive manufacturing. If you need fine features, smooth surfaces, tight tolerances, or optical clarity — SLA is where the conversation starts.

How SLA Works: Photopolymerisation Explained

SLA builds parts by selectively curing a liquid photopolymer resin using ultraviolet light. In classic top-down SLA (used by industrial systems like the 3D Systems ProX range), a UV laser traces each layer's geometry on the surface of a resin vat. The build platform descends incrementally, allowing fresh resin to flow over the cured layer, and the process repeats.

In bottom-up SLA — used by desktop systems like Formlabs Form series — the build platform rises upward out of the resin tank, with each layer cured from below through an optically clear FEP film. This inverted approach reduces the volume of resin required and enables more compact machine designs, making high-quality SLA accessible at desktop scale.

MSLA (Masked SLA), used by the majority of affordable resin printers (Elegoo, Anycubic, Phrozen), replaces the scanning laser with an LCD screen that masks a UV LED array. Rather than tracing a path, MSLA cures entire layers simultaneously, making it significantly faster than laser-based systems. The trade-off is that LCD panels have a finite lifespan and resolution is fixed by panel pixel density.

Accuracy and Surface Finish: What SLA Actually Delivers

SLA's defining characteristic is its dimensional precision. Industrial SLA systems achieve tolerances of ±0.025–0.05mm across the build volume — comparable to precision CNC machining on polymer materials. Desktop Formlabs systems deliver ±0.1–0.2mm, still substantially tighter than FDM's typical ±0.2–0.5mm.

Layer lines exist in SLA parts, but at 25–50 micron layer heights they are invisible to the naked eye without close inspection. The result is a surface quality that looks and feels like injection-moulded plastic straight off the machine, often requiring no surface finishing at all for visual prototypes and models.

Feature resolution is SLA's other headline advantage. The technology can reliably reproduce features as small as 0.2mm — fine text, living hinges, mesh structures, and complex organic geometry that would be impossible or impractical in FDM. This makes SLA the technology of choice for jewellery, dental work, hearing aids, micro-fluidics, and consumer product design models.

SLA Resin Types: A Material Breakdown

Standard Resins

Standard photopolymer resins are optimised for detail and surface finish rather than mechanical performance. They're brittle compared to engineering thermoplastics, making them suitable for visual prototypes, presentation models, and concept validation rather than functional testing. Most are available in a range of colours and opacities, including clear formulations that can be polished to full optical transparency.

Engineering Resins

Engineering resins trade some surface smoothness for meaningfully improved mechanical properties. Tough resins (Formlabs Tough 2000, similar) simulate ABS-like impact resistance and are suitable for snap-fit assemblies and connectors. Flexible resins enable shore hardness control for gaskets and overmould simulations. High-temp resins survive continuous temperatures above 200°C, enabling use in under-bonnet automotive testing and thermal cycling environments. Rigid resins with glass-fibre or ceramic fill deliver stiffness comparable to glass-filled nylons.

Dental and Medical Resins

SLA dominates dental additive manufacturing. Biocompatible resins certified to ISO 10993 and FDA Class II standards enable production of surgical guides, splints, denture bases, orthodontic models, and temporary crowns directly from digital scans. The precision of SLA — particularly at the fine feature sizes required for dental anatomy — is not matched by FDM. Dental labs worldwide have transitioned from plaster models to SLA-printed equivalents for speed, consistency, and integration with CAD/CAM workflows.

Castable and Investment Casting Resins

Castable resins (often wax-filled) burn out cleanly in a foundry kiln without leaving ash residue, making them a direct replacement for wax patterns in investment casting workflows. Jewellers use castable SLA parts to produce master patterns for gold, silver, and platinum casting with far greater geometric complexity than hand-carved wax. The same approach is used in aerospace for complex investment-cast titanium and nickel superalloy components.

Post-Processing SLA Parts

SLA parts come off the machine wet with uncured resin and require two mandatory post-processing steps: washing and curing. Washing removes surface resin using IPA (isopropyl alcohol) or a dedicated wash solution in an ultrasonic cleaner or wash station. Incomplete washing leaves a tacky surface. Post-cure UV exposure (in a dedicated cure box or simply in direct sunlight) completes polymerisation and brings the part to its final mechanical properties — parts are significantly softer and more brittle before post-cure.

Support removal leaves small witness marks on surfaces. On cosmetic surfaces, these can be sanded with 400–800 grit. If optical clarity is required, progressive sanding through 2000 grit followed by plastic polish achieves excellent transparency. Most engineering resin parts require no further processing beyond support removal and light sanding.

Limitations of SLA

SLA's weaknesses centre on material properties and operational constraints. Photopolymers remain inferior to engineering thermoplastics in impact resistance, fatigue life, and long-term UV stability — many resins yellow and become brittle with prolonged UV exposure. Resin costs are significantly higher than FDM filament, and unused resin has a limited shelf life. The wash-and-cure workflow adds time and requires IPA handling. Build volumes are typically smaller than FDM. And SLA is fundamentally a single-material process — multi-material parts require assembly.

When to Choose SLA

Choose SLA when surface quality and fine detail are the primary requirements: consumer product visualisation models, medical and dental applications, jewellery and small-scale production, investment casting masters, and any application requiring the finest feature resolution available from a polymer process. If your part needs to survive repeated mechanical loading, significant impact, or elevated temperature service, engineering resins or a switch to SLS should be evaluated. SLA and FDM are highly complementary — many product development workflows use FDM for functional prototyping and SLA for presentation and approval models.