Stereolithography (SLA)
Stereolithography cures liquid photopolymer layer by layer with a scanning UV laser, giving the smoothest surfaces in additive manufacturing.
- Part
- Forming
- Revised
- 2026-08-11
At a glance
- Family
- Additive Manufacturing
- Typical tolerances
- ±0.006 in (±0.15 mm) over the first inch, then about ±0.001 in/in (±0.1% of nominal); ±0.004 in (±0.1 mm) on small, well-supported features. Vendors quote between ±0.1% and ±0.2% of dimension.
- Surface finish
- Ra 20–100 µin (0.5–2.5 µm) as built on up-facing and vertical surfaces, the smoothest of the additive processes. Sanding and polishing reach below Ra 10 µin (0.25 µm). Down-facing surfaces are rougher where supports contact.
- Typical volumes
- 1–100 parts; a few hundred when the SLA part serves as a master for urethane casting
- Lead time
- 1–3 business days for typical prototypes, same day on desktop machines. A 6 in (150 mm) tall build runs 8–20 hours plus 1–2 hours of washing and post-cure.
- Materials
- Plastic
What it is
Stereolithography (SLA) builds parts by curing liquid photopolymer with a scanning UV laser, one 0.001–0.004 in (25–100 µm) layer at a time. It gives the smoothest as-built surface of any additive process — commonly Ra 20–100 µin (0.5–2.5 µm) — and holds roughly ±0.006 in (±0.15 mm) over the first inch, which is why it remains the default for appearance models, master patterns for urethane casting, fluidic prototypes, and any part where a visible layer line counts as a defect.
Feedstocks are acrylate and epoxy photopolymers, not true thermoplastics: standard rigid, tough (ABS-like), elastomeric, castable, biocompatible dental grades, and high-temperature resins with heat deflection near 460 °F (238 °C). Build envelopes run from about 5.7 × 5.7 × 7.3 in (145 × 145 × 185 mm) on desktop machines to roughly 30 × 30 × 22 in (750 × 750 × 550 mm) on large industrial frames.
SLA is economical from one part to a few hundred. Photopolymers yellow and embrittle under UV and creep under sustained load, so long-life structural parts usually belong in SLS or molded thermoplastic.
How it works
- Slice and support. The model is oriented, sliced at 0.001–0.004 in (25–100 µm) — 0.002 in (50 µm) is the common default — and a support scaffold is generated. Contact points are typically 0.4–0.6 mm across so they snap off cleanly.
- Recoat. The platform steps by one layer thickness and a recoater blade (top-down machines) or the resin film itself (bottom-up machines) re-establishes a flat layer over the last cured cross-section.
- Expose. Galvanometer mirrors sweep the laser across the cross-section. Industrial systems use a 355 nm solid-state laser with a 0.003–0.010 in (80–250 µm) spot; desktop machines use 405 nm diodes. Cure depth follows the Beer–Lambert relationship between exposure energy and the resin's penetration depth, so each pass deliberately over-cures into the layer below to weld the two together.
- Peel or dip. Bottom-up machines separate the part from the vat film every layer, and this peel force is what limits large flat cross-sections. Typical vertical build rate is 0.2–0.8 in/h (5–20 mm/h).
- Drain and wash. The build drains, then washes in isopropyl alcohol or TPM for 5–20 minutes to strip uncured resin. Over-washing swells and softens thin features.
- Remove supports. Cut or snap supports while the part is still green and comparatively soft; it is far harder after post-cure.
- UV post-cure. 15–60 minutes of UV at 60–80 °C (140–175 °F) completes crosslinking. This step is not optional — a green part can be 30–50% weaker and noticeably more compliant, and every published resin datasheet is quoted post-cured.
Design guidelines
Wall thickness
Supported walls print reliably at 0.020 in (0.5 mm). Give unsupported or tall walls 0.040 in (1 mm); thinner sections flex under recoating and peel forces and finish wavy.
Hollowing and drainage
Hollow any section thicker than about 0.4 in (10 mm). Trapped resin adds weight, cures unevenly, and can crack the shell weeks later. Use a 0.080 in (2 mm) shell with at least two drain holes of 0.14 in (3.5 mm) or larger at opposite ends, so air can enter as resin leaves.
Minimum features and text
Embossed and engraved detail needs 0.016 in (0.4 mm) of width and 0.016 in (0.4 mm) of height or depth. Text below about 0.08 in (2 mm) cap height fills in during washing.
Holes and channels
Keep through-holes at or above 0.020 in (0.5 mm) — smaller ones cure shut. Internal channels below 0.040 in (1 mm) are difficult to clear of resin at any length, so plan a flush port.
Orientation
Tilt the part 10–20° off the platform. That shrinks each layer's cross-section (lower peel force), moves supports off cosmetic faces, and avoids the stair-stepping that shows on shallow slopes. Up-facing and vertical walls come out smoothest; down-facing surfaces carry support witness marks.
Fits and clearances
Allow 0.004–0.008 in (0.1–0.2 mm) for a slip fit and 0.012 in (0.3 mm) for a moving fit. Cured photopolymer has little yield plateau, so interference fits crack rather than deform — the ISO 286 fits and tolerances chart shows how these compare with machined clearances.
Threads
Model threads only at M4/#8 and above. Below that, print a pilot hole and tap it, or design for a heat-set insert; the tap drill chart gives pilot sizes. Printed threads in brittle resin strip after a few cycles.
| Feature | Recommended | Limit | Why |
|---|---|---|---|
| Supported wall | 0.040 in (1.0 mm) | 0.020 in (0.5 mm) | Thin walls flex during recoat and peel |
| Unsupported wall | 0.060 in (1.5 mm) | 0.040 in (1.0 mm) | No backing against separation forces |
| Hole diameter | 0.060 in (1.5 mm) | 0.020 in (0.5 mm) | Smaller holes cure shut during the wash |
| Embossed detail | 0.024 in (0.6 mm) | 0.016 in (0.4 mm) | Laser spot size and cure bleed set the floor |
| Drain hole | 0.20 in (5 mm) | 0.14 in (3.5 mm) | Resin viscosity limits drainage |
| Clearance, moving | 0.016 in (0.4 mm) | 0.008 in (0.2 mm) | Cure bleed closes tight gaps |
| Layer height | 0.002 in (50 µm) | 0.001 in (25 µm) | Build time doubles at half the layer |
Cost drivers
SLA pricing is driven by build height first and resin volume second. Laser SLA time scales with number of layers times scanned area, so the same part standing 6 in (150 mm) tall costs far more than laid flat. Resin is the next lever: engineering, castable, and dental grades run several times the price of standard resin, and support structures are consumed at full resin cost.
Labor is the third. Support removal, sanding of witness marks, and painting or clear-coating are hand operations that often exceed machine cost on cosmetic parts.
Volume breakpoints: SLA is usually cheapest from 1 to roughly 50 parts. From about 50 to 500, printing one SLA master and casting the rest in urethane (vacuum casting) is normally less expensive. Above 1,000–2,000 parts, tooling for injection molding starts to pay back.
- Lay the part down or tilt it; never build tall unless a cosmetic face demands it.
- Hollow bulk sections and add drain holes — you are billed for resin volume.
- Nest several parts into one build rather than ordering them separately.
- Accept 0.004 in (100 µm) layers on non-cosmetic geometry; it roughly halves build time versus 0.002 in (50 µm).
- Specify standard resin unless a datasheet property is genuinely required.
Questions
7 questionsWhat tolerance can SLA hold?
Plan on ±0.006 in (±0.15 mm) over the first inch and roughly ±0.001 in per additional inch. Small, well-supported features can come in at ±0.004 in (±0.1 mm). Accuracy degrades on tall, thin geometry because post-cure shrinkage accumulates over the Z height.
What layer height should I use for SLA?
0.002 in (50 µm) is the standard compromise. Drop to 0.001 in (25 µm) only for fine detail or curved cosmetic surfaces, since it roughly doubles build time. 0.004 in (100 µm) is fine for bulky non-cosmetic geometry.
How thin can an SLA wall be?
0.020 in (0.5 mm) if the wall is supported along both edges, and 0.040 in (1 mm) if it stands alone. Below that the wall deflects under recoating and peel forces during the build and comes out wavy.
Why do SLA parts need post-curing?
Laser exposure only partially crosslinks the resin. A 15–60 minute UV post-cure at 60–80 °C (140–175 °F) completes the reaction. Without it a part can be 30–50% weaker and measurably more compliant than the datasheet values, which are always quoted post-cured.
Are SLA parts suitable for outdoor or long-term use?
Generally no. Photopolymers yellow and embrittle under UV and creep under sustained load. For functional parts that must last, use a sintered nylon process such as SLS or MJF, or move to molded thermoplastic.
Is SLA stronger than FDM?
SLA is isotropic — there is no weak Z axis — and stiffer, so it wins on small, thin, detailed geometry. But standard SLA resins are brittle, often under 10% elongation at break, while FDM in ABS, PC, or nylon absorbs far more impact. Choose SLA for accuracy and finish, FDM for toughness.
Why does my hollow SLA part need a hole in it?
Hollow parts trap liquid resin, which adds weight, cures unevenly, and can crack the shell. Design in at least two drain holes of 0.14 in (3.5 mm) or larger at opposite ends of the cavity so air can enter as resin drains.