Three ways to cure a liquid
Stereolithography, material jetting and digital light synthesis all build parts by curing a photopolymer with light. They resolve detail no extrusion process can reach, and they share the same two liabilities: the parts need washing and post-curing before they are finished parts, and photopolymers age under ultraviolet light in a way thermoplastics do not.
SLA — stereolithography
A laser traces each layer on the surface of a vat of resin. The workhorse for appearance models, master patterns, and anything where a smooth surface and crisp edges matter more than long-term mechanical performance. Supports touch the part and leave marks; where they touch depends on the orientation, so the cosmetic face is usually turned away from them.
PolyJet — material jetting
Print heads jet droplets of photopolymer and cure them immediately, which allows several materials — and colours — in one part. It is the only one of the three that can produce a rigid part with a soft-touch, overmoulded feel in a single build. Supports are a dissolvable or removable second material rather than the part’s own.
Carbon DLS — digital light synthesis
A projected image cures a continuous cross-section against an oxygen-permeable window, and a secondary thermal cure develops the final properties. The result behaves much more like an injection-moulded engineering polymer than a typical photopolymer, which is what makes it interesting for end-use parts rather than models.
Choosing between them
Appearance and fine detail at low cost — SLA. Multi-material, multi-colour or a soft-touch feature — PolyJet. Real elastomeric or engineering properties in an end-use part — Carbon DLS.
The caveat nobody puts on the datasheet
Photopolymer parts left in sunlight discolour and embrittle, and a part that is strong on day one may not be on day two hundred. If a resin part is going outdoors or into a window, plan for it in the design. For outdoor parts, an FDM print in ASA is a durable alternative — see PLA vs PETG: which filament?