SLS vs MJF: choosing between nylon powder 3D printing processes?

The short answer

Same physics, different heat source

Both processes are powder bed fusion and both are support-free: the unfused powder around the part holds it up, so undercuts, lattices and nested parts cost nothing extra. The difference is how the heat is delivered. SLS scans a laser across each layer, point by point along the part's cross-section, so the melt pool is local and cools directionally. MJF prints a fusing agent where the part should be solid and a detailing agent at the boundaries, then passes a broad infrared lamp over the whole bed, so every part of the layer is fused at once.

Almost every difference in this answer follows from that one fact. Because MJF heats the whole layer together, the temperature gradient across a part is smaller, the part is less directional, and the build is faster when the bed is full of parts. Because SLS delivers energy through a focused spot, the resolution is limited by the spot size, the cooling is directional, and the same machine can be pointed at a much wider range of polymers, including grades that MJF does not offer.

Mechanical properties: closer than the marketing suggests

On standard PA12 the two are in the same band. Typical tensile strength is about 45 to 50 MPa for SLS and 48 to 52 MPa for MJF, so the difference is smaller than the scatter between powder lots. Elongation runs the other way: SLS is usually the more ductile of the two at roughly 15 to 25 percent against 10 to 20 percent, which matters for snap fits and parts that must absorb impact rather than resist it.

The number that actually decides structural parts is anisotropy. In SLS the laser's directional cooling leaves the Z axis noticeably weaker, typically 80 to 85 percent of in-plane strength. MJF, fusing the layer evenly, reaches about 85 to 90 percent. Neither is truly isotropic, and both are far better than FDM, where the Z axis can be a fraction of XY. If the part is a bracket loaded across the build direction, that five-point gap is worth checking against the safety factor before choosing on surface finish.

Accuracy, minimum feature and minimum wall

Tolerance on both is usually quoted as a combination of an absolute figure and a percentage, and the percentage takes over on large parts. SLS is commonly quoted at plus or minus 0.3 mm or 0.3 percent, whichever is greater; MJF at plus or minus 0.2 mm or 0.2 percent. On a 30 mm feature that is 0.3 against 0.2 mm, a difference you can feel in an assembly. On a 300 mm part both are dominated by the percentage term and by how the part is oriented and cooled in the cake, so the gap narrows.

Feature size separates them more clearly. SLS resolves detail down to roughly 0.3 to 0.5 mm because the laser spot sets the limit, and a reliable minimum wall is around 0.8 to 1.0 mm. MJF, with the detailing agent sharpening the boundary, resolves about 0.2 mm and holds walls down to roughly 0.5 mm, which is why it can print fine embossed text, small holes and thin lattice struts that SLS rounds off. If the design depends on 0.5 mm ribs or 1 mm text, that single difference decides the process.

Surface finish and the post-processing load

Both come out grainy, and both are bead blasted as a standard step. The difference is how much work the blast has to do. SLS has the characteristic sandy, orange-peel surface, with roughness typically in the region of 150 to 300 microinches, and often needs more than a light blast before it can be dyed or painted evenly. MJF comes out noticeably smoother, around 100 to 250 microinches, with crisper edges, and takes dye more uniformly.

That difference has a cost consequence that does not appear in the machine rate. Finishing is labour, and a part that needs less of it is cheaper to deliver even when the printing itself is priced higher. For a batch of cosmetic covers the MJF premium can disappear entirely in the finishing line; for a batch of internal brackets that will never be seen, it is pure added cost.

Throughput and cost per part

MJF wins on batches because the lamp fuses the whole layer at once. A representative comparison for fifty small nylon brackets is roughly 8 to 12 hours on MJF against 16 to 24 hours on SLS, and the gap widens as the bed fills, because MJF's time per layer barely changes with the number of parts on it while the laser has more outline to trace. That is why MJF quotes fall faster with quantity and why the same part can be cheaper on one process at ten pieces and on the other at five hundred.

Powder economics matter too. Both processes blend used powder with fresh, but the ratios differ: SLS commonly runs 30 to 50 percent fresh, while MJF machines typically need around 30 percent fresh for PA12, with better consistency across reuses because the powder sees less thermal exposure. On an expensive nylon that refresh ratio is a real cost per part, and it is one of the numbers a supplier should be able to state rather than estimate.

Material range and build envelope

SLS has the wider palette. Beyond PA12 and PA11 it runs glass-filled, carbon-filled, aluminium-filled, flame-retardant and flexible blends, and it is the practical choice when the part needs a property that standard nylon does not have. MJF is more constrained, centred on PA12, PA11, glass-bead PA12 and TPU, with fewer third-party materials. If the specification names a flame-retardant or carbon-filled grade, the decision is often already made.

The envelope runs the other way. SLS platforms go considerably larger, with build volumes up to about 750 by 750 by 550 mm on the biggest machines, while MJF's common platform is around 380 by 284 by 380 mm and large-format MJF is rare. A part that is 500 mm long, or a batch that only makes sense if it is nested in one tall build, may be an SLS job regardless of which process is better on the numbers.

The last row is not a dodge. For a single part the process difference is usually smaller than the difference between a supplier with a build running tomorrow and one that has to wait for a full bed, so ask for the next build date before arguing about isotropy.

The decision, in four questions

First, how many parts and how soon: a batch that has to ship this week usually goes to MJF, a single large part usually goes to SLS. Second, does the design depend on fine detail: text under 1 mm, holes under 1 mm, walls under 1 mm point to MJF. Third, does it need a resin other than nylon: flame-retardant, carbon-filled or elastomeric blends point to SLS. Fourth, does it fit: anything beyond roughly 380 mm in the MJF envelope has to go elsewhere. If all four answers are neutral, choose on finishing cost and on isotropy, in that order, because those are the two differences that persist into the delivered part.

One caution on comparisons: published property tables are usually tested on optimally oriented specimens, and neither process is isotropic. Ask for the orientation used, and if the part carries load across the build direction, derate the Z figures rather than the XY ones.

Both processes sit inside the 3D printing capability, with powder parts listed under SLS parts; where the part needs metal rather than nylon, CNC machining is the route to compare against.