Vat photopolymerization: SLA, DLP, and MSLA without the marketing fog

01

One process family, several light engines

In vat photopolymerization, light initiates reactions that convert selected liquid regions into a cross-linked solid ¹. The build may progress upward from a vat floor or downward from a free surface, and the cured geometry remains attached to a platform while fresh liquid reaches the next exposure plane. Photoinitiator chemistry, absorber, pigment, wavelength, irradiance, exposure time, temperature, and oxygen conditions help define what cures and how far.

The family is attractive for fine geometry and relatively smooth surfaces, yet the object leaving the machine is normally a green part: shaped, but not necessarily at its final conversion or properties. Supports, liquid resin, wash solvent, and secondary curing are not optional footnotes. They affect dimensions, surfaces, mechanical response, exposure risk, and whether a validated application remains within its approved process ⁴.

02

SLA scans, DLP projects, MSLA masks

Laser-scanned stereolithography, or SLA, steers a light spot along vectors that trace and fill each layer. DLP systems project a two-dimensional image using a digital micromirror device. Masked stereolithography, commonly called MSLA or LCD resin printing, uses an LCD panel as a spatial mask over a broader light source. A projected or masked layer can be exposed at once, while scanned exposure time depends more directly on path length and scanning strategy.

Architecture influences productivity and sampling, but advertised pixel pitch or laser spot size is not identical to minimum accurate feature size. Optical blur, focus, nonuniform illumination, scattering, cure beyond the nominal boundary, mechanics, resin flow, separation, and post-cure movement all contribute. A pixelated exposure can also produce stair-stepping in the plane, while a scanning system can show vector and spot-overlap effects. Compare measured artifacts built under stated conditions, not one isolated specification ¹².

• XY sampling describes the light pattern, not guaranteed dimensional accuracy.
• Layer thickness describes Z sampling, not surface roughness or Z accuracy by itself.
• Build-area exposure time and scan-path exposure time scale differently.
• Resin and optical settings must be matched to the machine’s wavelength and energy delivery.

03

The working curve connects light to cure depth

A common vat-photopolymerization model relates cured depth to radiant exposure through two resin parameters: penetration depth and critical exposure. The relationship helps engineers select enough exposure to bond layers while limiting unintended growth. Underexposure can leave weak or incomplete regions; excessive exposure can close gaps, thicken features, alter supports, and reduce dimensional fidelity. The model is useful only when the exposure and measurement methods are characterized.

A NIST-led study involving 24 laboratories found striking spread in reported working-curve parameters—up to sevenfold for penetration depth and seventyfold for critical exposure—when participants used differing procedures and light sources ². NIST released Reference Material 8047 in 2026 to support more consistent assessment ³. The lesson for a workshop is not that calibration is futile; it is that exposure seconds cannot be copied meaningfully without wavelength, irradiance, geometry, resin condition, temperature, and method.

04

Orientation and separation shape the build

Bottom-up systems must separate each cured layer from a vat interface before replenishing resin. Cross-sectional area, lift motion, film condition, viscosity, part stiffness, and support layout influence that event. Large abrupt areas can create demanding forces and flow conditions. Angling a part can distribute area changes and move support marks, but may increase build height, support quantity, accumulated opportunities for error, and direction-dependent surface texture.

Supports provide attachment and stabilize islands, slender features, and loads during separation. Their tips must be strong enough to survive yet removable without unacceptable damage. The first cured layers may intentionally receive different exposure to secure the build, so they should not be treated as precision evidence. Review every new island and trapped volume, provide drainage, and consider whether uncured resin can be removed from cavities before approving a hollow design.

• Avoid sealed cavities that can trap uncured resin or pressure.
• Place supports away from critical interfaces when possible.
• Balance peel behavior, surface quality, height, drainage, and inspection access.
• Treat film, optics, platform condition, and resin cleanliness as maintained process inputs.

05

Finish, measure, and qualify the result

After printing, allow safe drainage, remove the part using the prescribed method, wash it for the resin-specific time, and let solvent evaporate fully before post-curing. Overwashing, dirty solvent, incomplete drying, unsupported thin features, or an incorrect cure schedule can change surfaces and properties. Remove supports at the stage recommended for the material and geometry. Heated or UV curing still requires suitable ventilation and exposure controls.

Measure critical features after the complete defined post-process, because green dimensions may not represent delivered dimensions. For regulated medical work, the FDA emphasizes material controls, machine parameters, post-processing, verification, and testing across the complete manufacturing workflow ⁴. Hobby and prototype work does not inherit every medical requirement, but the principle is universal: a named resin or printer does not establish final performance without a documented process and application-appropriate evidence. Define whether inspection occurs immediately after curing or after controlled conditioning, and keep calibration features representative of production thickness and orientation.

• Use a dimensional artifact that includes holes, gaps, walls, pins, and features in multiple orientations.
• Separate exposure calibration from support and mechanical troubleshooting.
• Inspect for uncured pockets, delamination, cracks, distortion, and damaged support contacts.
• Retain the manufacturer’s safety data sheet and processing instructions for the exact resin.