FDM vs SLA vs SLS: Which 3D Printing Process to Choose

FDM, SLA, and SLS, with MJF as a close cousin of SLS, are the three polymer 3D printing families that cover almost every plastic part a designer or buyer will specify. They differ on the five things that decide whether a part works: dimensional accuracy, mechanical strength, surface finish, need for supports, and cost. The right choice is rarely about which process is best in absolute terms, because each one wins on at least one of those five axes. FDM is the cheapest and prints the widest material range. SLA is the most accurate and smoothest. SLS and MJF produce the strongest, near-isotropic parts with no supports at all.

This comparison lays out how each process works in one line, then compares them head to head on accuracy, strength, finish, cost, and supports, and closes with a verdict matrix that tells you which to pick for a given job and when to step outside 3D printing entirely.

The three families at a glance

FDM

FDM, or fused deposition modeling, melts a thermoplastic filament and extrudes it through a nozzle, one track and one layer at a time. It is the cheapest process, prints the widest material range, and is the workhorse for prototypes, jigs, and housings. Its weakness is anisotropy: parts are about 20 to 30 percent weaker across the layers than within them.

SLA

SLA, or stereolithography, uses a light source, either a laser or a projected image, to cure a liquid photopolymer resin one layer at a time. It is the most accurate and leaves the smoothest surface, which makes it the choice for fine detail, cosmetic prototypes, and masters for casting. Its weakness is that the cured resins are relatively brittle and degrade in sunlight.

SLS and MJF

SLS, selective laser sintering, and MJF, multi jet fusion, both fuse nylon powder across a full build bed without supports. SLS uses a moving laser; MJF jets a fusing agent and fuses a whole layer with infrared. Both produce strong, near-isotropic parts and need no support structures, which is why they own functional nylon parts, snap-fits, and small-batch end-use components.

How each works, in one line

In FDM, a heated nozzle extrudes melted filament along a toolpath, building the part layer by layer from the bottom up. In SLA, a light source cures liquid resin in a tank, building the part either upside down from the bottom of a tank or right way up from a descending platform. In SLS and MJF, a recoater spreads a thin layer of nylon powder and a heat source fuses the cross-section, with the unfused powder left in place to hold up every feature above it.

Accuracy compared

Tolerance ranking

Accuracy ranks SLA first, then MJF, then SLS, then FDM, and the gap between SLA and FDM is large. SLA holds about plus or minus 0.05 to 0.15mm, which is tight enough for mating features and fine cosmetic detail straight off the printer. MJF holds about plus or minus 0.2 to 0.3mm, and SLS about plus or minus 0.3mm for features under 50mm and about 0.5mm above that. FDM runs the loosest, from about plus or minus 0.1mm on a small ABS part to about 0.5mm on a larger nylon or polycarbonate part, with the tolerance set mostly by the material and the geometry rather than the machine.

Practical consequence for fits

The practical consequence is that only SLA will reliably hold a close fit as-built. MJF and SLS land in the same functional range and are interchangeable for most fits, while FDM is best treated as a near-net process whose mating faces will need machining or drilling to size. For example, a printed locating hole that must hold a bearing should be modeled slightly undersize and reamed after printing in any of the four processes, but only SLA will get the surrounding geometry close enough that the reaming cut is light.

Strength and isotropy compared

Strength splits the four processes into two clear groups. SLS and MJF nylon parts are strong and near-isotropic, meaning the part takes load from any direction with no marked weak axis, because each fused layer bonds to the next across a full powder cross-section held near the melting point. FDM is anisotropic: the bond between layers is weaker than the material within a track, so a printed part is about 20 to 30 percent weaker along the Z, or build, direction than within the XY plane. SLA resins are stiff and can be quite strong in bending, but they are relatively brittle, so they crack rather than yield under impact.

The design consequence is direct. If a part takes functional load and the load direction is unknown or varies, SLS or MJF nylon is the safe default. If the part is FDM, it must be oriented so the load runs along the layers, never across them, or the section must be thickened to compensate. SLA is reserved for parts where detail and finish matter more than toughness, such as a cosmetic housing or a casting pattern that will not see impact in service.

Surface finish compared

Finish ranks SLA first by a wide margin, then MJF and SLS together (comparable, with MJF marginally smoother and more consistent), then FDM. SLA leaves a smooth, almost injection-molded surface at about Ra 0.5 to 2 micrometers, with layer lines so fine they are often invisible. SLS leaves a slightly grainy, matte surface around Ra 5 to 12 micrometers, which improves with tumbling, dyeing, or vapor smoothing but never reaches the gloss of SLA as-built. MJF is comparable and marginally more consistent, around Ra 4 to 9 micrometers, and comes out a characteristic gray that is usually dyed black. FDM is the roughest, at about Ra 4 to 12 micrometers, with visible layer lines whose prominence depends on the layer height and the orientation of the face.

The consequence is cosmetic. A part that must look finished straight off the printer, or that needs a smooth mating or sealing face, calls for SLA. SLS and MJF parts look functional rather than cosmetic as-built and need post-processing to look finished. FDM parts almost always need sanding, filler, and paint to look cosmetic, and even then a curved or sloped face shows stair-stepping.

Cost compared

FDM versus SLA on size

Cost ranks FDM lowest, then SLA for small parts, then SLS and MJF in the mid-range, with the ranking shifting as part size and quantity change. FDM filament is cheap, the machines are inexpensive, and a single large simple part costs less in FDM than in any other process. SLA resin is more expensive per kilogram, but for very small finely detailed parts the resin volume is tiny, so SLA can undercut FDM on a small jewelry master or a dental part. For larger parts, FDM pulls ahead on cost again because SLA resin volumes and tank costs climb.

SLS, MJF, and the nesting advantage

SLS and MJF sit in the middle on unit cost. A single simple part costs more in SLS or MJF than in FDM, because the powder, the energy, and the slow cool-down all add cost. But SLS and MJF builds nest many parts into one powder bed with no supports, so the cost per part falls sharply as the quantity rises, which is why the two processes own small-batch production runs of complex geometry. The break-even is usually somewhere in the tens to low hundreds of parts, below which 3D printing wins on cost and above which injection molding wins once the tooling cost is spread across the run.

Supports compared

Which processes need supports

Supports split the four processes into two clean groups. SLS and MJF need no supports at all, because the unfused powder bed holds every overhang, bridge, and cavity as the part builds, which is the single biggest reason they own complex, interlocking, and nested geometry. FDM and SLA both need removable supports on overhangs steeper than about 45 degrees from vertical, and on any isolated feature with nothing below it.

Design freedom and finishing labor

The consequence is design freedom and finishing labor. An SLS or MJF part with internal channels, snap pockets, and thin overhangs prints in one piece with no support marks, while the same part in FDM or SLA needs supports inside those features that must be removed and that leave witness marks. For example, a snap-fit housing with a recessed clip pocket prints clean in SLS, but in FDM or SLA the supports inside the pocket must be broken away and the marks sanded out, and the snap may be too brittle in SLA to survive repeated use.

Orientation driven by supports

Supports also drive orientation, and orientation drives accuracy and strength. In FDM, the part is usually oriented to minimize supports and to put the load along the layers, which trades off against the smoothest face. In SLA, the part is oriented so supports land on a non-cosmetic face, because support marks on a visible face are hard to remove without spoiling the finish. SLS and MJF sidestep both problems entirely, since the powder bed removes the support decision and leaves the designer free to orient the part for strength or nesting rather than for support removal.

Verdict matrix: which process to pick

The decision comes down to what the part must do, so the cleanest way to choose is by use case rather than by spec sheet.

When to pick FDM

Pick FDM if cost is the primary constraint, the part is large or simple, the material needs to be tough rather than finely finished, or the job is a quick physical answer to a design question. FDM is the default first print for a new design and the natural choice for shop-floor jigs, drill guides, check fixtures, housings, and any part where surface finish is secondary. It is also the only low-cost route to a very large plastic part, since SLS and SLA build volumes are smaller and more expensive per unit of volume.

When to pick SLA

Pick SLA if the part needs fine detail, a smooth cosmetic surface, tight dimensional accuracy, or transparency. SLA is the choice for a cosmetic prototype that must look like an injection-molded part, a master pattern for silicone molding or investment casting, a dental or medical model, or any small intricate geometry where layer lines would spoil the result. Avoid SLA when the part must take impact, flex repeatedly, or survive sunlight and heat, because the resins are brittle and age in UV.

When to pick SLS or MJF

Pick SLS or MJF if the part must be strong and functional, take load from any direction, flex on a living hinge or snap repeatedly, carry complex internal geometry, or run in small batches. These two processes own snap-fit housings, living hinges, complex assemblies with internal channels, brackets and mounts that carry real load, ducts and manifolds, and short production runs of end-use nylon parts. The choice between SLS and MJF usually comes down to which process a given shop runs, what color is needed, and what the lead time allows, since the two are interchangeable for most functional nylon work.

The matrix is not symmetric. FDM wins on cost and material range. SLA wins on detail, finish, and accuracy. SLS and MJF win on strength, isotropy, and design freedom without supports. No single process wins on more than two of the five axes, which is exactly why all three families coexist and why the right question is never which is best but which fits the job.

When not to 3D print at all

Three signals push the decision out of 3D printing entirely and into CNC machining or injection molding. The first is tolerance. If a mating face needs to hold better than about plus or minus 0.05mm, even SLA will struggle and CNC machining is the reliable route. The second is volume. If the quantity climbs past a few hundred identical parts, the per-part cost of 3D printing falls behind injection molding, whose upfront tooling cost amortizes across the run. The third is material. If the part needs a specified engineering plastic with a property sheet, or a metal with specific properties, or a surface that 3D printing cannot reach as-built, CNC machining from stock or molding is the answer.

A useful rule is that 3D printing wins on geometric complexity, on lead time for one or a few parts, and on low upfront tooling cost, while CNC and molding win on tolerance, on surface, on isotropic strength, and on cost at volume. The boundary between them moves with the part, which is why a clear requirement set, covering the load, the tolerance, the finish, the quantity, and the material, is what makes the decision clean. The dedicated pages for FDM, SLA, SLS, and MJF carry the detailed tolerances and design rules for each process, and the 3D printing quote page lays out how the cost drivers stack up for a given part.