What is the difference between SLA, SLS, and FDM 3D printing technologies?

The short answer

Three processes, three mechanisms

The three names are abbreviations for three completely different physical events, and every difference in the table further down follows from that.

SLA stands for stereolithography. A build platform sits one layer below the surface of a vat of liquid photopolymer, and an ultraviolet light source traces the cross-section of the part, curing the resin where it touches. The platform drops, fresh resin flows across, and the next layer is traced. Because the part is cured from a liquid as one solid, there are no weld lines and no direction of weakness. The trade-off is that it needs support structures, which leave contact marks that have to be sanded off.

SLS stands for selective laser sintering. A roller spreads a thin layer of nylon powder, a laser fuses the cross-section into it, the bed drops, and the roller spreads the next layer. The unfused powder around the part supports it, so no support geometry is needed at all, which is why SLS can produce internal channels, undercuts, living hinges and interlocking assemblies that the other two processes cannot build in one piece. Reused powder is blended back in at a controlled refresh ratio, because the chemistry of the powder changes as it is heated repeatedly.

FDM stands for fused deposition modelling, also called material extrusion. A filament of thermoplastic is fed through a heated nozzle which draws the cross-section bead by bead. It is the only one of the three that builds with real engineering thermoplastics rather than resin or nylon powder, which is why the material list runs to ABS, polycarbonate, ASA, PETG and carbon-filled nylon. It is also the only one where the bond between layers is a genuine weak point.

Head to head on the numbers that decide

Read down the tolerance column and the decision for a mating feature is already made: only SLA comes close to the fit a machined part holds, and even it stops an order of magnitude short of CNC. Read across the support row and the decision for a part with an internal channel is made too, because SLS is the only one of the three that can print it without geometry that must be removed afterwards.

Where the precision actually sits

Layer height is the number buyers ask about first and the number that explains the least, because it sets surface finish and vertical detail rather than dimensional accuracy. A 25 micron layer produces a smoother flank and a crisper curve; it does not make the part more accurate in X and Y, and it roughly doubles the build time for the same height of part. Layer height trades time for finish, while tolerance is set by the machine, the calibration and the material's behaviour after the build.

What each one costs

Price follows material cost, machine cost and how much labour the post-processing takes. FDM uses cheap commodity filament and needs only manual support removal, so it is the cheapest. SLS uses a machine that costs as much as a house and consumes nylon powder at industrial prices, so it is the most expensive of the three per part, but it also needs the least post-processing because there is nothing to remove. SLA sits between the two on price and, in practice, is the one where the post-processing step is most likely to be underestimated.

Choosing in one question

The three are not interchangeable, and the choice is usually settled by a single requirement rather than by a weighted score.

• Does the part have to look finished? Choose SLA. It is the only one of the three that produces a smooth surface and fine features straight off the machine, and it is the standard route for appearance models, dental and medical models, and master patterns for casting.
• Does the part have to work, and work in every direction? Choose SLS. Sintered nylon is the strongest of the three, it is isotropic, and the free powder support allows snap fits, hinges and internal geometry that the other two cannot produce.
• Does the part have to be cheap, fast or large? Choose FDM. Commodity filament, the largest build volumes and the shortest lead time make it the right answer for concept models, jigs, fixtures and large enclosures where layer lines do not matter.

When two answers apply, the usual resolution is to print early iterations in FDM, move appearance parts to SLA, and switch the parts that will be loaded to SLS or to machining. Running the same geometry through more than one process is also how tolerance gets settled empirically: print it, measure the features that matter, and correct the model rather than arguing with the datasheet.

Limits: where each one fails

• SLA parts are brittle. Standard resins break at 4 to 6 percent elongation, so thin snap fits and clips crack. Tough and engineering resins improve this to 10 to 20 percent, but they cost more and still behave like a photopolymer rather than like nylon.
• SLA parts change in daylight. Photopolymers degrade under prolonged ultraviolet exposure, discolouring and embrittling within weeks outdoors. Anything that lives in sunlight needs a specific resin grade or a protective finish.
• SLS surfaces are matte and grainy. As-built roughness sits at Ra 6 to 12 micrometres, which is fine for function and wrong for a customer-facing appearance part. Dyeing and blasting change the look, not the texture, and the parts are slightly porous unless they are sealed.
• FDM is weakest where the load crosses the layers. In-plane strength is good, build-direction strength is 50 to 70 percent of it, and layer lines are always visible. It is the wrong choice for fine detail, for fluid-tight parts and for anything judged on appearance.
• None of the three is a production process. They produce prototype quantities. Above a few hundred parts the economics move to moulding or casting, and the design rules change with them, so a printed part is validated for the process that printed it and not for the one that will make the production units.
• No certification is implied here. Which quality system applies to a given build is confirmed per programme and per factory in writing before an order is released.

How to get a quoted comparison

Send the model, the feature that matters most and the quantity, and the same geometry can be quoted across all three processes so the choice is made on your part rather than on a table. See SLA 3D printing for fine detail and smooth surfaces, SLS 3D printing for durable nylon parts and FDM 3D printing for economical large prototypes, then send a file for a free engineering review.