Two laser technologies, one additive goal
SLA and SLS are both industrial additive processes that use a laser to build parts layer by layer, yet they sit at opposite ends of the material spectrum. SLA works in liquid photopolymer; SLS works in polymer powder. That single difference cascades into accuracy, strength, finish, and cost.
Understanding the mechanism is the fastest way to choose. If you remember only one thing: SLA cures resin into smooth, brittle, detailed parts, while SLS sinters nylon into tough, support-free, functional parts. Everything else below follows from that split.
How SLA works: photopolymerization of liquid resin
Stereolithography directs a UV laser across the surface of a liquid resin vat, selectively curing a thin slice where the beam lands. The build platform lifts by one layer height, and the next slice is cured on top, gradually pulling a solid object up out of the liquid.
Because the laser spot is tiny and the material starts as a perfectly flat liquid, SLA achieves the finest feature resolution and lowest layer lines of any 3D process. The trade-off is that the chemistry locks in a thermoset-like, inherently brittle solid.
How SLS works: laser sintering of nylon powder
Selective laser sintering spreads a thin layer of nylon powder, then a laser fuses the cross-section where the part exists. The powder bed drops, a fresh layer is rolled on, and the cycle repeats, with the surrounding loose powder supporting the part throughout.
No liquid, no vat, no support structures: the unsintered powder itself acts as the build support. This lets SLS produce enclosed channels, interlocking assemblies, and thin walls that would be impossible or support-heavy in SLA, all in a single unfussed build.
Materials: photopolymer resin versus nylon powder
SLA offers a wide resin menu: standard, tough, clear, castable, and high-temperature grades, each tuning color, clarity, or impact resistance. They share the same brittle, UV-sensitive character and are best where looks or masters matter.
SLS centers on nylon, mainly PA12 and PA11, with glass- or aluminum-filled variants for extra stiffness or heat deflection. Nylon is a true thermoplastic: tough, semi-flexible, and chemically resistant, which is why SLS parts can ship as end-use components rather than just models.
Dimensional accuracy and fine detail
SLA wins on accuracy and surface. Feature sizes down to fractions of a millimeter, crisp text, and smooth curves make it the choice for miniatures, patterns, and presentation models. Typical tolerance is about plus or minus 0.05 mm on small parts.
SLS is accurate but a step behind on fine detail because the powder grain leaves a slightly soft edge and a minimum feature size near 0.3 to 0.5 mm. For most functional parts that difference is invisible; for jewel-like detail it is decisive.
Mechanical strength and durability
SLS nylon is markedly stronger and tougher than SLA resin. It bends rather than snaps, resists impact, and tolerates repeated stress, so SLS parts serve as clips, brackets, and enclosures in real products. Tensile strength sits around 45 to 50 MPa with good elongation.
SLA parts are rigid and dimensionally stable but brittle; a dropped resin model can shatter, and long-term sunlight yellows and embrittles it further. Choose SLA for form and SLS for function whenever the part will be handled, loaded, or assembled.
Surface finish and post-processing
Straight off the machine, SLA shows the smoothest, most finished surface, with fine layer lines easily sanded or polished to a high gloss and paint-ready prime. It is the natural choice when the part is customer-facing.
SLS emerges matte and slightly grainy from the powder, with a velvety touch and visible but benign texture. It is bead-blasted to remove loose powder and can be dyed, smoothed, or coated, but it will not reach a resin's mirror gloss without extra work.
Support structures and design freedom
SLA requires supports for any overhang or enclosed volume, which must be designed, printed, and removed by hand, leaving witness marks that need cleanup. Steep overhangs and internal cavities are constrained by support access.
SLS needs no supports at all, so designers enjoy near-total geometric freedom: hollow forms, interlocking moving parts printed in one piece, and conformal lattices. This freedom is often the deciding factor for complex functional prototypes.
Cost, lead time, and scalability
SLA machines and resin are generally cheaper to own and run, making SLA economical for single detailed models and small batches. Build time scales with part height and cross-section, and labor for support removal adds up on complex geometry.
SLS carries higher equipment and powder cost, but its support-free bed packs many parts at once with almost no per-part labor. At volume, SLS per-part cost drops fast, and lead time per part is often shorter because nothing is hand-removed.
Best-fit applications for each process
Reach for SLA when the goal is a beautiful object: concept models, trade-show displays, dental and jewelry masters, casting patterns, and micro-fluidic devices where detail and surface rule. Appearance is the product.
Reach for SLS when the goal is a working object: functional prototypes, end-use brackets, snap-fit enclosures, jigs, and low-volume production where toughness, complexity, and support-free geometry earn their premium.
A decision checklist and FabProto's guidance
Ask four questions: Is fine detail and a smooth surface the priority? Will the part be handled, loaded, or assembled? Does it have complex internal geometry or overhangs? Is this a one-off model or a functional run? Detail-first and one-off favors SLA; function-first and complex favors SLS.
FabProto operates both SLA and SLS in-house, so you do not guess. Send the CAD and describe the use; we recommend the process, return a tolerance and cost comparison, and print a sample so you can feel the difference in surface and strength before committing.