What Is an SLA (Stereolithography)?

SLA (stereolithography) is a 3D printing process that uses an ultraviolet laser to cure liquid photopolymer resin into a solid part, one thin layer at a time. It belongs to the vat photopolymerization family and produces smoother surfaces and finer detail than FDM or SLS, which makes it a common choice for cosmetic prototypes and fit checks.

In short: SLA is the 3D printing process to pick when surface finish and small features matter most.

How Does SLA 3D Printing Work?

A laser traces each cross-section of the part on the surface of a vat of liquid resin. Wherever the UV light lands, the resin hardens. The build platform then moves by one layer thickness, fresh resin covers the cured layer, and the next cross-section is drawn on top. Charles Hull's 1986 patent described the method as printing "thin layers of a curable material, e.g., a UV curable material, one on top of the other."

The international standard ISO/ASTM 52900 places SLA in vat photopolymerization, one of seven additive manufacturing process categories. NIST describes the category as forming structures "by curing liquid photopolymer resin with ultraviolet light."

A printed SLA part is not finished yet. It comes off the platform still coated in uncured resin and attached to support structures, which the process needs under overhangs. The part is cleaned, the supports are removed, and it is post-cured under UV light to develop its full mechanical properties.

SLA vs SLS vs FDM

	SLA	SLS	FDM
ISO/ASTM process category	Vat photopolymerization	Powder bed fusion	Material extrusion
Feedstock	Liquid photopolymer resin	Nylon (polyamide) powder	Thermoplastic filament
Support structures	Required	Not required	Required
Surface finish	Smoothest, finest detail	Slightly rough	Visible layer lines
Typical use	Visual models, fit checks, master patterns	Functional parts, snap fits, living hinges	Concept models, large parts, early iterations
Tolerance at NICE Rapid	±0.1 mm (±0.004 in)	±0.2 mm (±0.008 in)	±0.3 mm (±0.012 in)
Layer thickness at NICE Rapid	25–100 µm (0.001–0.004 in)	100–180 µm (0.004–0.007 in)	100–300 µm (0.004–0.012 in)
Lead time at NICE Rapid	2–3 days	2–3 days	1–2 days

Which SLA Resin Should You Use?

Resin choice drives most SLA performance. Common grades:

• Standard photopolymer — smooth and rigid but brittle. Best for visual models, fit checks and master patterns for casting.
• ABS-like resin — tougher and more impact resistant, for functional prototypes, housings and enclosures.
• Flexible resin (Shore A 40–80) — elastic and tear resistant, for soft seals, grips and snap-fit trials.
• High-temperature resin (HDT above 180 °C / 356 °F) — for heat-exposed fixtures.
• Castable resin — burns out cleanly, for investment casting patterns.

When to Choose SLA

• Choose SLA when the part is judged by how it looks: appearance models, presentation samples and parts with fine text, thin ribs or small snap features.
• Use it for master patterns. The high-accuracy master for vacuum casting is typically an SLA print, and its surface finish carries straight into the silicone mold.
• Move to SLS or FDM when the part must take repeated load or impact, or when it is bigger than the SLA build volume. At NICE Rapid, that volume is 300 × 300 × 400 mm (11.8 × 11.8 × 15.7 in).
• Mention critical cosmetic faces when you order. Build orientation decides where the support marks land.

NICE Rapid runs SLA in-house alongside SLS, FDM and metal printing, and checks orientation and support placement on every file. See the 3D printing service for the full resin list.

Frequently Asked Questions

What does SLA mean in 3D printing?

SLA stands for stereolithography, sometimes expanded as stereolithography apparatus. It is the resin-based 3D printing process in which a UV laser cures liquid photopolymer layer by layer. In manufacturing, it has nothing to do with a service-level agreement.

What is stereolithography used for?

Stereolithography is used for parts where surface finish and detail matter: visual and appearance models, fit checks, housings, fine-featured prototypes, master patterns for vacuum casting and castable patterns for investment casting.

How accurate is SLA 3D printing?

SLA is the most accurate of NICE Rapid's plastic 3D printing processes. SLA parts are held to ±0.1 mm (±0.004 in) with layer thicknesses of 25–100 µm, compared with ±0.2 mm for SLS and ±0.3 mm for FDM.

Related Services at NICE Rapid

• 3D Printing — in-house SLA with standard, ABS-like, flexible, high-temperature and castable resins, ±0.1 mm tolerance and 2–3 day lead times.
• Vacuum Casting — SLA master patterns are produced in-house and turned into silicone molds for small series of polyurethane parts.
• Request a Quote — upload an STL or STEP file and our team will confirm the process and lead time within 24 hours.

Sources

1. US Patent 4,575,330 (C. W. Hull), Apparatus for production of three-dimensional objects by stereolithography (filed 1984, published 1986) — origin of the process; printing "thin layers of a curable material, e.g., a UV curable material, one on top of the other" — patents.google.com
2. NIST, Additive Manufacturing: Vat Photopolymerization — the category forms structures "by curing liquid photopolymer resin with ultraviolet light" — nist.gov
3. Koltsaki et al., "A Comprehensive Overview of Additive Manufacturing Processes Through a Time-Based Classification Model," 3D Printing and Additive Manufacturing — ISO/ASTM 52900:2021 defines seven process categories; vat photopolymerization is "commonly known as" SLA; "support requirement" listed among its limitations; FDM = material extrusion — pmc.ncbi.nlm.nih.gov
4. Kafle et al., "3D/4D Printing of Polymers: Fused Deposition Modelling (FDM), Selective Laser Sintering (SLS), and Stereolithography (SLA)," Polymers — "stereolithography apparatus (SLA)"; SLA parts are usually post-cured to enhance mechanical properties; comparison table (SLA high resolution and accuracy; SLS no supports but rough finish; FDM supports required and poor finish) — pmc.ncbi.nlm.nih.gov