A stereolithography prototype is the closest thing to a moulded part that can exist before a tool does, at least to look at. That makes it powerful for decisions taken by eye and hand, and dangerous for decisions taken about strength and durability, because the material underneath the excellent surface behaves nothing like the production resin.
What it settles quickly
Whether the product looks right. Whether it feels right in the hand. Whether parts fit and assemble in the intended order. Whether a graphic or a texture reads correctly at real size. These are the decisions that stall in review meetings and that a screen cannot resolve, and an appearance model resolves them in one sitting.
As a master for tooling
The smooth surface is why stereolithography parts are the standard master for silicone tooling: the tool copies the master exactly, so effort spent finishing the master is repaid on every cast part. It is also why castable resins exist, allowing a printed pattern to be burned out and replaced by metal in investment casting.
Where it misleads
Stiffness and impact behaviour are not those of the production thermoplastic. Snap fits that work on a printed part may fail in the moulded one, or vice versa. Thin ribs behave differently. And nothing about a printed part indicates whether the geometry can be moulded, because printing happily produces zero draft, varying wall thickness and undercuts everywhere.
Getting the most out of one
Decide before ordering what the model is for, because an appearance model and a fit check are finished differently and priced differently. Order the finish that answers the question rather than the best one available. And run a manufacturability review against the production process in parallel, so the two kinds of question are both being answered while the design is still cheap to change.
Questions people ask about sla models
What is an SLA model used for?
Appearance models, fit and assembly checks, and masters for silicone tooling or investment casting. It is the best surface available before a mould exists.
Can I test strength on an SLA prototype?
Not reliably. Photopolymers are more brittle than moulded thermoplastics, so results mislead in both directions. Test function on sintered nylon or a machined part.
Why are SLA parts used as casting masters?
Because the silicone tool copies the master exactly, so a smooth accurate master produces smooth accurate cast parts with no further work.
How should I finish an appearance model?
According to the decision it has to support. A painted and polished model costs several times a sanded one, and a fit check needs neither.