SLA and DLP: A Process Engineer's Guide to Vat Photopolymerisation

Vat photopolymerisation is the oldest commercial AM process — Charles Hull's 1987 SLA-1 predates every other AM technology. It's also the process most often treated as a black box: operators adjust exposure settings until parts look right, without understanding the physics that governs cure depth, dimensional accuracy, and final mechanical properties.

This guide covers the engineering fundamentals, process differences between SLA and DLP variants, resin selection for different applications, and the post-processing workflow that largely determines final part properties.

The Jacobs working curve: cure depth physics

Jacob's working curve, developed by Paul Jacobs at 3D Systems in the late 1980s, remains the foundational model for understanding cure depth in photopolymerisation:

Cd = Dp × ln(E₀ / Ec)

Where:

Cd = cure depth (mm) — how deep the resin polymerises in a single exposure
Dp = penetration depth (mm) — a resin-specific constant, the depth at which incident irradiance drops to 1/e (~37%) of surface value
E₀ = surface irradiance dose (mJ/cm²) — energy delivered to the resin surface
Ec = critical exposure (mJ/cm²) — minimum dose at which gelation (solidification) begins

What Dp means physically: Photons are absorbed and scattered by the photoinitiator and resin matrix. Dp is determined by Beer-Lambert law — the higher the photoinitiator concentration or the more opaque the resin, the smaller Dp. Clear resins have Dp of 0.15–0.35 mm; pigmented or filled resins have Dp of 0.05–0.15 mm.

What Ec means physically: Ec is the dose at which the photoinitiator generates enough free radicals to initiate a propagating polymerisation network. Below Ec, the resin remains liquid. Above Ec, a gel forms; above ~2×Ec, a mechanically rigid solid forms. Ec is typically 1–15 mJ/cm² for standard resins.

Practical use of the working curve:

If you know Dp and Ec for your resin (obtained from a cure depth test or datasheet), you can calculate the required scan speed (for SLA) or exposure time (for DLP) to achieve a target cure depth.

For SLA with a 50 mW laser at 355 nm, 0.1 mm spot:

E₀ = P × 2/(v × d × √π/2)

Increasing scan speed v reduces E₀ and thus Cd. Decreasing layer thickness requires that Cd exceeds the layer thickness (with ~20–40% overcure margin) for interlayer bonding.

The overcure margin: Cd must exceed layer thickness by at least 20% to ensure adequate interlayer bonding. At Cd = 1.2 × layer thickness, bonding is marginal and delamination risk is high. Most systems operate at Cd = 1.5–2.0 × layer thickness.

SLA vs. DLP: process physics compared

Stereolithography (SLA)

SLA uses a UV laser (typically 355 nm solid-state or 405 nm violet) steered by galvanometer mirrors to scan the resin surface point by point. Each point is cured sequentially as the laser passes over it.

Characteristics:

XY resolution is determined by laser spot diameter — typically 0.05–0.2 mm
Resolution is uniform across the entire build platform
Large build platforms (300 × 300 mm to 650 × 750 mm for industrial systems) don't sacrifice XY resolution
Throughput scales poorly with part count — each additional part adds proportional laser scan time
Industrial examples: 3D Systems ProX 800, DWS DFAB 020, Lithoz CeraFab (ceramic)

Digital Light Processing (DLP)

DLP uses a Digital Micromirror Device (DMD) — an array of individually addressable mirrors — to project a full layer image onto the resin surface simultaneously. The entire layer is exposed at once.

Characteristics:

XY resolution is determined by the pixel size of the projected image, calculated as: pixel_size = build_area_width / x_pixel_count
Resolution varies with build platform size — larger platform = larger pixels (lower resolution) unless lens system is upgraded
Typical pixel size: 25–100 µm for prosumer/professional machines
Throughput per layer is independent of part geometry — a full-platform layer takes the same time as a single-part layer
Especially fast for large XY cross-sections, but no advantage for tall thin parts

Pixel pitch limitation in DLP: A 50 µm pixel pitch means the minimum feature size is 2–3 pixels = 100–150 µm. Compare to SLA at 0.1 mm spot: similar minimum feature size but true Gaussian beam vs. square pixel. DLP edges have a staircase artefact at pixel pitch; SLA edges are slightly smoother.

Bottom-up vs. top-down configurations

Bottom-up (upside-down DLP/LCD — the dominant consumer/prosumer architecture): The resin vat has a transparent FEP or nFEP film at the bottom. The projector is below the vat. Each layer cures against the FEP film, then the build platform pulls the cured layer up (peel step) to separate it from the film before the next layer.

Peel force is the critical constraint: the suction between cured resin and FEP creates significant force during peel. Large cross-sections can cause part failure or print failure. Mitigation: tilt peel (rotating peel), antialiasing, and PDMS/FEP material choice.
Resin depth is minimal — only enough to coat the FEP surface.
Examples: Formlabs Form 4 (uses parabolic mirror + LED), Phrozen Sonic, Elegoo Saturn, Bambu Lab Photon series.

Top-down (right-side-up SLA — dominant for industrial systems): The laser or projector is above the vat. The build platform descends into a deep resin vat after each layer.

No peel force — fresh resin flows over the surface under gravity.
Requires a deep resin vat (cost, waste for small runs).
Better suited for large flat cross-sections and fragile structures.
Examples: 3D Systems ProX 800 (industrial SLA), DWS systems.

Resin families and selection guide

Standard photopolymer (clear, grey, white, black)

Properties: Brittle (elongation 5–10%), moderate stiffness (E = 2.5–3.5 GPa), limited impact resistance.

Applications: Concept models, fit and form verification, flexible tooling (short-run vacuum forming), display models.

Do not use for: Snap-fits, living hinges, mechanical load-bearing prototypes, parts requiring impact resistance.

Tough / ABS-like resins

Properties: Higher elongation (20–40%), improved impact resistance (notched Izod 25–50 J/m), stiffness similar to standard (E = 2.0–3.0 GPa). Formlabs Tough 2000, Siraya Tech Blu, resin equivalents from each DLP supplier.

Applications: Engineering prototypes, snap-fit testing, functional jigs, enclosures, parts where brittle failure is unacceptable.

Note: "ABS-like" is marketing — these resins do not match the impact resistance or chemical resistance of injection-moulded ABS. They are substantially better than standard photopolymer, not equivalent to thermoplastic ABS.

Flexible and elastic resins

Properties: Shore A 50–80, elongation 100–200%, low modulus (E = 0.5–2 MPa).

Applications: Gaskets, over-mould simulations, wearable prototypes, footwear components, flexible fixtures.

Limitations: Poor tear resistance; most flexible resins degrade in UV exposure over time; not suitable for dynamic sealing applications.

High-temperature resins

Properties: HDT 200–300°C (under load at 0.45 MPa), achieved through high cross-link density or filled formulations.

Applications: Mould masters for silicone casting, housings for electronics at elevated operating temperature, automotive underhood prototypes, wind tunnel models.

Constraints:

Brittle (elongation typically <2%)
Requires heated build enclosure or elevated post-cure temperature (typically 120–160°C post-cure to achieve full HDT)
Warp-prone in large cross-sections due to high cure shrinkage
Examples: Formlabs HT Resin (HDT 238°C), DSM Somos Taurus, carbon fibre-filled high-temp DLP resins

Castable resins (wax-filled)

Properties: Wax-filled photopolymer formulated to burn out cleanly in investment casting (burnout temperature 700–900°C), leaving minimal ash residue.

Applications: Jewellery casting, dental casting, prototype investment casting where silicone tooling is not appropriate.

Constraints:

Extremely fragile (handle with care before casting)
Requires specific investment materials (gypsum-bonded investments cause issues — use phosphate-bonded)
Burnout protocol must be followed precisely — rapid heating causes resin expansion before burnout and cracks the investment shell
Not suitable for structural prototype applications

Dental resins (biocompatible)

Properties: ISO 10993 tested for cytotoxicity (typically classes I and II); some formulations tested to ISO 10993-4 (haemocompatibility) for short-term mucosal contact. Specific mechanical properties determined by application (denture base, surgical guide, crown and bridge, splint).

Applications: Surgical guides (bone-supported, tooth-supported), orthodontic aligners, night guards, temporary crowns, study models.

Critical requirements:

ISO 10993 compliance only valid after full post-cure per resin datasheet
Specific post-cure wavelength and duration mandatory (under-cured biocompatible resin is not compliant — residual photoinitiator is cytotoxic)
Machine calibration to validated profiles — don't use dental resin on an uncalibrated desktop DLP
Formlabs Dental SG, Dentca, and SprintRay all supply resin with documented post-cure protocols that must be followed exactly

Layer thickness vs. quality

Layer thickness is the primary lever for balancing surface quality against throughput.

Layer thickness	Application	Z-axis surface texture	Typical build speed
25 µm	Dental, high-precision optical, jewellery	Excellent, Z steps imperceptible	Slow (5–15 mm/hr)
50 µm	Engineering prototypes, medical models	Good, Z steps visible under magnification	Moderate (15–30 mm/hr)
100 µm	Draft prints, fast iteration	Visible Z steps	Fast (30–60 mm/hr)
150–200 µm	Draft/large parts, speed priority	Coarse, steps clearly visible	Very fast

Surface finish achievable:

SLA and DLP produce the finest surface finish of any AM technology:

XY plane: Ra 0.3–1.0 µm (SLA), Ra 0.5–2.0 µm (DLP limited by pixel pitch)
Z plane: Ra 1.0–3.0 µm (staircase effect at 50 µm layers); improves to Ra 0.5–1.5 µm at 25 µm layers

The XY surface finish in SLA is primarily limited by photoinitiator scatter and the Gaussian beam profile. In DLP, the pixel pitch creates a regular pattern that appears as a subtle orange-peel texture on curved surfaces.

Z-staircase effect: The step-like appearance on curved surfaces in the Z direction can be reduced by:

Reducing layer thickness
Orienting curved surfaces to face upward (Z steps are less visible than on steep-angle surfaces)
Sanding (grits 400→800→1200) followed by UV-cure coating for cosmetic surfaces

Support design

Touchpoint geometry

Touchpoint diameter: 0.4–0.8 mm for standard resins; 0.3–0.5 mm for dental/precision (finer detail, easier to remove)
Touchpoint depth: 0.2–0.3 mm penetration into the part surface (insufficient depth = detachment during build; excessive depth = dimple on surface that requires sanding)
Z-tip length: 1–2 mm (the tapered stem connecting the main support shaft to the touchpoint)

Density rules

Overhangs and horizontal faces: Support every surface below 45° from horizontal. Dense support grid (3–5 mm pitch) for large flat overhangs. For small overhangs (<3 mm span), single supports at corners are often sufficient.

Tall thin features: Use sparse supports to avoid over-constraining the feature. A single row of supports along the base is usually adequate for features <5 mm wide. Over-supported thin walls can fracture during peel (bottom-up DLP) because the support itself transmits the peel force.

Large flat surfaces (DLP peel force concern): Large horizontal cross-sections in bottom-up DLP generate significant peel forces. Mitigation:

Tilt the part 10–15° to stagger the cross-section and reduce peel force
Use hollow shell with drainage holes rather than solid sections
Add drain holes (minimum 2 mm) at the lowest point of any hollow features

Wash and post-cure workflow

Wash

IPA (isopropyl alcohol): The standard solvent for most consumer and professional resins. 2–5 minutes agitation in clean IPA, followed by a second 1-minute rinse in clean IPA. Do not over-wash — IPA swells most photopolymers if soaked for >10 minutes. The part absorbs IPA and temporarily softens; dimensional changes of 0.05–0.15 mm are possible in swelled parts.

TPM (tripropylene glycol monomethyl ether): Formlabs' recommended wash solvent for Form 3/4 and many resins. Less aggressive than IPA, longer bath life, slower solvent evaporation. Wash time 5–10 minutes.

IPA vs. TPM selection: For dental and biocompatible resins, follow the resin datasheet exactly — some resins require TPM, not IPA, to avoid surface degradation that affects biocompatibility testing compliance.

Wash station monitoring: Replace wash solvent when it becomes visibly turbid or resin-coloured. Saturated wash solvent re-deposits uncured resin on the part surface, causing tacky spots. Test wash effectiveness by wiping the part surface with a paper towel — the towel should come away clean.

Post-cure

Post-curing with UV light completes the polymerisation that was only partially achieved during build. Under-cured parts have:

Lower stiffness and strength (typically 30–60% of full cure values in extreme cases)
Tacky surface
Non-compliant biocompatibility (for dental/medical resins)
Higher warpage (residual stress from incomplete cure)

Wavelength: Most photopolymers are maximally sensitive at 365–405 nm. A 405 nm UV-A lamp is effective for the majority of consumer resins. Dental resins may specify 380–420 nm broad-spectrum UV.

Power and time:

Resin family	Minimum UV dose for full cure	Typical post-cure setup
Standard photopolymer	80–200 mJ/cm²	405 nm, 40 W, 10–20 min
Tough/ABS-like	100–250 mJ/cm²	405 nm, 40 W, 15–30 min
Flexible	80–150 mJ/cm²	405 nm, 40 W, 10–15 min
High-temp	200–500 mJ/cm²	405 nm, 80+ W, 30–60 min at elevated temp
Dental (biocompatible)	Per IFU exactly	Use calibrated dental curing unit
Castable	80–150 mJ/cm²	405 nm, 40 W, 10–15 min

Temperature-assisted post-cure: For high-temperature and castable resins, post-curing at elevated temperature (60–80°C) while applying UV accelerates cure completion and achieves higher cross-link density. A standard approach is: UV post-cure at room temperature until tack-free, then oven soak at 60°C for 30–60 minutes. For maximum HDT (high-temp resins): UV post-cure first, then oven at 160°C/60 min.

Do not post-cure in direct sunlight. Uncontrolled UV + heat causes rapid yellowing, surface degradation, and uneven cure.

Common failure modes and fixes

Delamination (layers separating mid-build)

Cause: Contaminated FEP film (resin residue from previous print), incorrect exposure (under-cure = insufficient interlayer bond), old/degraded resin, FEP film damage.

Fix: Clean FEP film, replace if scratched, perform cure calibration test, check resin age (most resins expire after 12 months open), ensure resin is homogeneous (mix before use).

Elephant foot (first layers oversized)

Cause: Bottom layers are over-exposed to compensate for the FEP-resin interface in bottom-up systems. The first 3–8 layers cure more than intended, creating a flared base.

Fix: Reduce "base exposure" (burn-in layers) time, or add 0.3–0.5 mm to base layer count to reduce per-layer dose, or use software compensation (Chitubox, Lychee Slicer allow bottom exposure control). Alternatively, raise the part on supports to eliminate direct contact with the build plate.

Suction cup cracking (hollow parts fracture during build)

Cause: In bottom-up DLP, large hollow cross-sections create a sealed chamber with sub-atmospheric pressure during peel. The pressure differential can fracture the part.

Fix: Add drain holes (minimum 2 mm) in the lowest regions of hollow features, or orient part so the hollow opening is accessible during printing.

Warpage (parts curl off supports or distort post-cure)

Cause: Residual stress from cure shrinkage (most resins shrink 2–5% during photopolymerisation), excessive support density constraining shrinkage, or under-cure leaving differential stress.

Fix: Ensure full cure, review support placement to allow some shrinkage freedom, consider tilting large flat parts, use anti-warp settings (slower peel for bottom-up DLP).

Tacky surface after post-cure

Cause: Oxygen inhibition — atmospheric oxygen reacts with surface radicals and prevents surface cure. Uncured monomer migrating from interior.

Fix: Post-cure in a nitrogen purge, submerse in water during UV cure (water excludes oxygen), or apply a thin glycerin layer during final UV exposure. Alternatively, check UV power — a weak lamp at the cure station is a common cause.

SLA vs. DLP at a glance

Factor	SLA (laser scan)	DLP (projector)
XY resolution	0.05–0.15 mm (beam diameter)	0.025–0.1 mm (pixel pitch)
Resolution across large platform	Uniform	Decreases with platform size
Throughput per layer	Proportional to cross-section area	Fixed per layer (platform-independent)
Suitability for large XY parts	Slower than DLP for large cross-sections	Superior
Suitability for many small parts	Comparable	Comparable
Surface quality (Z)	Ra 0.5–2 µm	Ra 0.5–2 µm
Surface quality (XY)	Ra 0.3–1 µm	Ra 0.5–2 µm (pixel artefact)
Industrial build volume	Up to 650 × 750 × 550 mm	Up to 400 × 220 × 300 mm typical
Resin depth (vat)	Deep (top-down) or shallow (bottom-up)	Shallow (bottom-up typical)
Peel force concern	Low (top-down); present (bottom-up)	Present (bottom-up)