Two powder bed fusion technologies dominate nylon 3D printing production. Selective laser sintering (SLS) scans a CO₂ laser across a bed of nylon powder, fusing particles point by point, layer by layer. Multi Jet Fusion (MJF) prints a fusing agent onto the powder bed with an inkjet array, then sweeps an infrared lamp across the surface — the agent absorbs the IR energy and locally melts the powder where the image was printed. SLS has 30 years of production history. MJF has roughly 10 years. Both produce functional PA12 and PA11 parts with mechanical properties approaching injection-molded nylon. But the differences — in surface finish, part isotropy, material options, and per-part cost — determine which technology wins for a given production volume and part geometry. This article compares the two technologies across the dimensions that matter for production part quality and unit economics.
How Each Technology Fuses Nylon Powder
SLS heats the powder bed to just below the nylon's melting point — typically 165 to 175°C for PA12 — then scans a 30 to 100-watt CO₂ laser (10.6 µm wavelength) across the surface, tracing the part cross-section at 5 to 15 meters per second. The laser energy melts the nylon particles where they touch, and the molten polymer flows and coalesces into a solid layer. Unfused powder surrounding the part serves as the support structure — no dedicated support generation needed. After the layer completes, the bed drops by 100 to 120 µm, a recoater spreads fresh powder, and the cycle repeats.
MJF spreads a layer of PA12 or PA11 powder at 80 to 100 µm thickness, then an inkjet carriage — similar in concept to a 2D document printer — jets two fluids onto the powder surface: a fusing agent (black, IR-absorbing) where the part cross-section should form, and a detailing agent (clear, IR-reflecting) around the edges of the part to define sharp boundaries by cooling the adjacent powder. A high-power IR lamp then passes over the entire bed. The fusing agent absorbs the IR energy and heats the powder to melting temperature. The detailing agent — and the bare powder where no agent was deposited — stays below the melting point. The result is a layer fused selectively not by a scanning beam but by a printed image — an area-wide process that builds the entire cross-section simultaneously.
Mechanical Isotropy: The Core MJF Advantage
SLS parts exhibit measurable mechanical anisotropy — the Z-axis tensile strength and elongation are typically 15 to 30% lower than the X-Y plane values. The cause is thermal history: each new layer re-melts the upper surface of the layer below, but the melt zone depth is limited to roughly 1.5 to 2 layer thicknesses. Polymer chains at the bottom of each layer never re-melt after deposition, so interlayer bonding is weaker than in-layer bonding. A PA12 SLS part might test at 48 MPa UTS in the X-Y plane and 38 MPa in Z.
MJF parts are measurably more isotropic — Z-axis strength within 5 to 10% of X-Y — because the IR lamp's heat penetrates more deeply into the powder bed than the laser's surface-limited energy input. The fusing agent absorbs heat throughout the printed area simultaneously, and the longer dwell time (milliseconds for the IR lamp pass versus microseconds for the laser spot) allows heat to conduct deeper into previously fused layers. For parts loaded in multiple directions — brackets, manifolds, snap-fit enclosures — this isotropy reduces the need to orient parts for strength on the build plate.
Surface Finish and Dimensional Accuracy
SLS produces as-built surface roughness of 8 to 15 µm Ra on PA12 with a 100 µm layer height. The laser spot size (typically 400 to 500 µm) determines the minimum feature size; narrow ribs below 0.8 mm are unreliable. Dimensional accuracy is ±0.3% with a minimum of ±0.3 mm for well-designed parts — but SLS parts are subject to "growth" or "shrink" depending on geometry, and each new material batch typically requires a scale-factor calibration build.
MJF produces noticeably smoother surfaces — 5 to 8 µm Ra as-built on PA12 — because the fusing and detailing agent chemistry creates a sharper boundary at the part surface than a laser spot. Dimensional accuracy is comparable at ±0.3% (minimum ±0.3 mm), but MJF holds tighter tolerances on small features because the thermal field is more uniform across the build area. The detailing agent actively cools the powder outside the part, preventing the partial sintering and "orange peel" surface texture common on SLS down-facing surfaces.
One practical difference: MJF parts emerge from the build with a uniform gray color (from the fusing agent), while SLS parts are naturally white. If part color matters — consumer-facing products, medical device housings — MJF parts require dyeing or painting to achieve aesthetic finishes; SLS PA12 can be dyed as well, but starts from a neutral white base that accepts a wider color gamut.
What does each technology cost per part?
SLS machine cost ranges from $100,000 to $500,000 for production systems; MJF from $200,000 to $400,000. The hardware cost difference is less important than the per-part economics at production volumes. MJF's area-wide fusing process delivers a build speed essentially independent of part count per layer — one part or fifty parts, the IR lamp pass takes the same time. SLS build time scales linearly with the total cross-sectional area the laser must trace. For a build packed with 200 small nylon brackets, MJF can complete the layer in 8 to 12 seconds regardless of part count, while SLS might need 40 to 60 seconds to trace all cross-sections with the laser — a 4 to 6× per-layer speed advantage for MJF on densely packed builds.
The per-part cost crossover varies with part geometry and build density, but at typical production packing densities of 8 to 15%, MJF delivers PA12 parts at $0.30 to $0.80 per cubic centimeter, versus $0.50 to $1.20 per cm³ for SLS. The gap widens at high production volumes where MJF machines are routinely run with full build chambers on multi-shift schedules.
Material Options: SLS Has the Wider Palette
SLS can process PA11, PA12, PA6, polypropylene (PP), thermoplastic polyurethane (TPU), polyether block amide (PEBA), carbon-fiber-filled and glass-filled nylon composites, and — critically — flame-retardant grades meeting FAR 25.853 for aircraft interiors. MJF's material portfolio is currently limited to PA12, PA11, PA12 with glass beads, and a TPU — each controlled and supplied by HP. If your application requires a specific filled nylon grade, a flame-retardant material, or a non-nylon polymer, SLS is the only option. If PA12 or PA11 meets your material requirements, MJF delivers the faster cycle time and lower per-part cost.
For PA12 and PA11 production parts where isotropy, surface finish, and high-volume unit cost matter, MJF is the stronger choice. For applications requiring specialized materials — filled nylons, FR grades, elastomers beyond TPU — or for low-volume production where the machine cost premium cannot be amortized, SLS remains the more flexible platform. The decision is not about which technology is newer — it is about which technology processes the material your part actually needs.
