Lesson 8.2ShapeChapter 8 · Additive manufacturing

Stereolithography (SLA/DLP)

Hardens liquid resin layer by layer with a laser or projector (vat photopolymerization)

Stereolithography (SLA) turns liquid resin into a solid part by hardening it with UV light, one thin layer at a time. Its cousin DLP (digital light processing) does the same with a projector. Dental labs use both every day to print models for aligners and crowns, because they give the finest detail of the common 3D printing methods.

Photo: The wub, CC BY-SA 4.0, via Wikimedia Commons

Bottom-up resin printer in cross-section, with wash and post-cure stepsCircled numbers match the steps below; steps 2 to 4 repeat for every layer.

How it works

Slice the model. Software cuts the 3D model into thin layers and adds supports (thin breakaway struts) that hold the part to the platform.
Dip the platform. The build platform lowers into a tank of photopolymer resin (a liquid plastic that hardens under UV light). It stops one layer above the tank's clear film floor, typically 0.025–0.1 mm.
Cure one layer. UV light shines up through the film and hardens the thin sheet of resin in the shape of the slice. In SLA a laser traces it with a fast-turning mirror; DLP and LCD printers light the whole layer at once.
Lift and peel. The platform rises and peels the new layer off the film, so fresh resin flows in underneath. Steps 2 to 4 repeat, and the part grows upside down.
Wash. You rinse the part in isopropyl alcohol (IPA) to remove the sticky uncured resin.
Post-cure. A chamber with UV light and gentle heat finishes hardening the part to full strength. Then you clip off the supports and sand the small nubs they leave.

Key facts

Materials: Photopolymer resins: standard, tough, flexible, heat-resistant, castable, dental and ceramic-filled grades
Typical parts: Detailed prototypes, dental models, jewelry patterns, miniatures, small molds, microfluidic chips
Part size: Typically up to 20–35 cm per side on desktop and benchtop printers; large industrial machines reach about 1.5 m
Precision: About ±0.1 mm on desktop machines, finer on industrial ones; details down to about 0.1 mm
Surface finish: Smooth and often glossy, close to a molded part; faint layer lines on curves and small marks where supports touched
Batch size it suits: 1 to a few hundred parts; some dental and consumer products run into the thousands
Tooling and machine cost: No tooling; LCD hobby printers cost a few hundred dollars, professional desktop SLA a few thousand, industrial machines hundreds of thousands
Speed: A few hours to a day per build; DLP and LCD are faster than laser SLA because each layer is lit at once

Strengths and limits

Strengths

Finest detail and smoothest surface of the common 3D printing methods
Watertight parts that are nearly isotropic (equally strong in every direction)
Special resins for casting, dentistry, flexible or heat-resistant parts
DLP and LCD print a full platform as fast as one part

Limits

Most resins are more brittle than molded plastics and can yellow in sunlight
Messy: liquid resin irritates skin, so you need gloves and ventilation
Extra steps: washing, post-curing and removing supports
Limited build size, and resin costs more per kilogram than filament

Where it's used

DentalModels for clear aligners, crowns, surgical guides
JewelryCastable ring patterns, later cast in metal by investment casting
MedicalAnatomical models, custom hearing aid shells
Product designSmooth, detailed prototypes of consumer products
EngineeringSmall molds, microfluidic chips, wind-tunnel models
EntertainmentMiniatures, figurines, film props

How to recognize it

A very smooth, often glossy surface with almost invisible layer lines.
A cluster of tiny bumps or pits on one face, where thin supports were clipped off.
Crisp fine detail, such as readable small text or sharp textures.
Often a translucent or uniformly colored, slightly glassy look.

Compare with

FDM8.1melts and lays down plastic filament; cheaper and tougher parts, but coarser detail and visible layer lines.Material jetting (PolyJet)8.6also cures resin with UV light, but jets droplets like an inkjet, so one part can mix colors and materials.Volumetric printing (CAL)8.10cures a whole part at once inside a spinning vial of resin, with no layers at all.

Watch it

Formlabs · The SLA principle, explained by a printer maker

JLC3DP · The process and its post-processing, for beginners

Adam Savage's Tested · Large industrial resin printers in a factory

MLC CAD Systems · The full print, wash and cure workflow

See real photos

Wikipedia: Stereolithography — principle diagram and printer photos
Wikimedia Commons photo search

Remember

SLA and DLP harden liquid resin with UV light, one thin layer at a time.
They give the finest detail and smoothest finish of common 3D printing, at the cost of messier post-processing.
Every print needs washing and post-curing before it reaches full strength.