MJF 3D Printing: HP Nylon Parts, Tolerances & Design

HP Multi Jet Fusion, or MJF, is a powder-bed process that deposits a thin layer of nylon powder, jets a fusing agent and a detailing agent onto it, and then fuses it with infrared energy across the whole layer at once. Where SLS traces each cross-section with a moving laser, MJF fuses the entire bed in one pass, which makes it faster and gives very consistent mechanical properties. The result is a functional nylon part, similar in strength to SLS, with slightly better detail and consistency.

Like SLS, MJF needs no supports, because the unfused powder bed holds the part as it builds. That makes it strong for complex, interlocking, and densely nested geometry, and the parts come out strong and largely isotropic. The trade-offs are a characteristic gray as-built color that is usually dyed, a slightly rougher surface than SLA, and a material range centered on nylon.

How MJF works

An MJF machine spreads a layer of nylon powder, typically 80 micrometers thick, across the build bed. A printhead then moves across the bed and jets two agents: a fusing agent, deposited where the part should be solid, and a detailing agent, deposited at the boundary to sharpen the edge and limit heat bleed. Infrared lamps then pass over the bed and fuse only the areas that carry the fusing agent, bonding them to the layer below. The bed lowers, a new powder layer spreads, and the cycle repeats.

Because the infrared fuses the whole layer at once rather than point by point, MJF builds faster than a laser SLS machine of similar size, and the full-bed fusion keeps the part density and strength uniform across the build. The detailing agent is what gives MJF its comparatively crisp feature definition, holding finer walls and edges than a pure laser sinter at the same layer height. After the build, the powder cake cools, the part is removed, and the loose powder is recovered and sifted for reuse, just as in SLS.

The powder bed and self-supporting geometry

Like SLS, MJF is self-supporting: the unfused powder holds every overhang, bridge, and cavity, so the process needs no added support material. Overhangs above roughly 30 degrees from horizontal print cleanly, and bridges can span a few millimeters without sagging. This is what lets MJF nest many parts in one build and print complex, interlocking geometry that FDM and SLA cannot produce in one piece.

Materials

The MJF material range is nylon-centered, matching SLS. PA12, nylon 12, is the primary material: strong, stable, and dimensionally consistent, absorbing only about 1 percent moisture, which keeps parts stable in service. PA11 is tougher and more ductile, suited to living hinges and snap-fits that must flex repeatedly. For higher loads, glass- and carbon-filled grades add stiffness and heat resistance, at the cost of a rougher surface and faster tool wear during any post-machining.

MJF also runs TPU powder, including HP MJF TPU powder grades, for flexible, rubber-like parts such as gaskets, seals, and overmolds. TPU on MJF tends to be more consistent than FDM TPU, because the powder bed fuses uniformly without the extrusion variability of filament, which makes it a good choice when a flexible part needs repeatable properties across a batch.

Choosing an MJF material

The simplest way to pick an MJF material is to start from what the part must do. For a general functional bracket, housing, or prototype, PA12 is the default: strong, stable, and the lowest-cost option, with good dimensional stability from its low moisture uptake. For a snap, living hinge, or any part that must flex without cracking, PA11 is the upgrade, tougher and more ductile than PA12. For a part that must carry higher load or run hotter, a glass- or carbon-filled grade adds stiffness and heat resistance, at the cost of a rougher surface and harder post-machining. For a flexible seal, gasket, or overmold, an HP MJF TPU grade gives a repeatable elastomer that beats FDM TPU on consistency across a batch. In every case the dye step is planned in advance, since most MJF parts are dyed black and the color is part of the finishing, not an afterthought.

File format and units

Provide an STL with the units stated, and for hollow parts include escape holes so the trapped powder can drain during depowdering. STL carries no units metadata, so a file without explicit units is read against the supplier default, and a millimeter-versus-inch mistake produces a part 25.4 times the intended scale. State the units in the file and the filename, and confirm them in the order, because this single mistake is the most common and most expensive file-format error in custom manufacturing.

Tolerances

As-built tolerance and surface

Dimensional tolerance runs about plus or minus 0.2 to 0.3mm for features up to 100mm and about 0.3 percent for larger dimensions, with a floor near 0.3mm, which is slightly tighter than SLS on smaller features. As-built surface finish is around Ra 4 to 9 micrometers, and the standard layer thickness is 80 micrometers. For a mating or bearing fit, plan to machine or ream the printed blank, because the as-built tolerance and the gray, slightly textured surface will not meet a precision fit on their own.

Machining and dyeing effects on fit

For example, a locating boss printed at its nominal size and then reamed to a sliding fit lands on dimension, while trusting the as-built surface would leave the fit loose or galled depending on the local powder fusion. The dyeing step adds no measurable thickness, so it does not change a validated clearance, but the as-built surface itself must be allowed for on any critical face.

Post-processing

After depowdering, MJF parts are commonly tumbled to smooth the surface, media-blasted for a uniform matte finish, and dyed in a hot bath, most often black, to set the color and seal the porosity. Vapor smoothing can gloss the surface and seal it against moisture uptake, which improves both appearance and durability. Because dyeing penetrates the nylon rather than coating it, it does not change mating dimensions, which is why it is the standard coloring method over paint. Critical faces can be machined to size or to a tighter tolerance than the as-built process holds.

MJF compared to SLS in detail

MJF and SLS are close cousins and produce interchangeable functional nylon parts in the PA12 and PA11 family. MJF tends to be faster, thanks to full-bed infrared fusing, and marginally more consistent in mechanical properties across a build, with slightly tighter tolerances on small features. SLS offers a comparable native surface and, in some shops, a wider spread of filled grades. Both leave a grainy, matte as-built surface that takes dye well, and both are self-supporting. For most functional nylon parts the choice comes down to which process a given shop runs, what color is needed, and what the lead time allows, rather than a large difference in the finished part.

A worked MJF design example

Consider a batch of snap-fit cable clamps that must clip into a chassis, survive vibration, and run in a device. The requirements point to MJF in PA12, and each choice follows from the process.

Step 1: choose the material and process

First, choose the material and process. The clamps flex on a snap and see vibration, so a tough, isotropic nylon is required, which rules out brittle SLA resin and anisotropic FDM. PA12 on MJF is the natural fit, with the consistency to repeat across a batch, and the parts can be nested many at a time in one build with no supports.

Step 2: set the clearances and features

Second, set the clearances and features. The snap is given 0.4mm per-side clearance so it flexes and clips cleanly despite the as-built surface, and the mounting boss is modeled at 1.0mm and noted to be reamed to size for a true locating fit. Walls are held at 1.0mm, well above the 0.3mm XY floor, for handling strength.

Step 3: handle the dye and finish

Third, handle the dye and finish. The clamps are dyed black for the final assembly, which does not change the validated clearance, and they are tumbled to take the sharp edge off the as-built surface. No supports are needed, so there are no support marks to clean up, and the batch prints in one build.

Applications and use cases

MJF wins on functional nylon parts that need consistency, throughput, or a clean black finish. Snap-fit housings, brackets and mounts, living hinges, complex assemblies with internal channels, ducts and manifolds, and small-batch end-use parts are all strong applications, as are flexible TPU gaskets and seals when a consistent elastomer is needed. The ability to nest many parts in one fast build also makes MJF economical for short production runs of complex geometry.

MJF strengths and limitations

Strengths

MJF is strong where consistency, throughput, and a clean functional finish matter. It produces uniform, largely isotropic nylon parts across a whole build, which is valuable when a batch must perform the same from part to part. The full-bed infrared fusing is fast, so nested production runs are economical, and the self-supporting powder bed handles complex and interlocking geometry with no support removal. The material set covers tough PA12 and PA11, filled grades for higher load, and consistent TPU for flexible parts, which is a useful spread for functional work.

Limitations

Its limitations mirror SLS. The as-built surface is grainy and gray, so cosmetic parts need dyeing, smoothing, or finishing to look finished, and it cannot match the detail or transparency of SLA. The material range is nylon-centered, so there is no rigid cosmetic resin and no broad filament choice. For a single simple part, FDM is cheaper, and MJF carries a higher unit cost than FDM for low-complexity geometry. MJF is also not automatically food-safe or biocompatible, which depends entirely on the specific powder grade and needs supplier confirmation for any regulated use.

Batch consistency

Because MJF fuses the whole layer at once and runs a controlled recyclate refresh rate, the mechanical properties stay consistent across a nested batch and from build to build, which is one reason it is favored for small production runs where every part must perform the same. The gray as-built color is also uniform across the batch, which makes dyeing consistent from part to part rather than a batch-to-batch variable.

When to use MJF, and when not to

Use MJF for functional nylon parts, complex and nested geometry, snap-fits and living hinges, flexible TPU parts, and small-batch production where consistency and throughput matter. Do not use it for the finest cosmetic detail, where SLA wins, or for the lowest cost on simple geometry, where FDM wins. Expect a gray as-built color that is usually dyed, a slightly rougher surface than SLA, and a material range centered on nylon. The full comparison of the polymer processes is on the FDM vs SLA vs SLS page, and the cost drivers are on the 3D printing quote page.

Design rules for MJF

MJF design follows the same self-supporting logic as SLS, with slightly finer feature limits thanks to the detailing agent. Minimum wall thickness is about 0.3mm in the XY plane and 0.5mm in Z for short walls, minimum feature size is about 0.5mm, minimum hole diameter is 1.0mm, and minimum pin diameter is 1.5mm. Self-supporting angles start at about 30 degrees from horizontal, and escape holes for hollow parts should be at least 2mm so powder can drain.

Clearances for assembled and moving parts

Because MJF parts come out of the bed fused together if they touch, any co-printed assembly or moving joint needs clearance. Use 0.3 to 0.6mm per wall for parts that must move relative to each other, with more clearance for parts that will be dyed, since dyeing can slightly change the fit. For a press fit, use a small interference and validate it with a coupon first. e.g., a co-printed hinge with 0.4mm clearance per side moves freely straight off the build, while the same hinge at 0.1mm would fuse shut and need to be separated and reamed.

Escape holes and hollow geometry

Hollow parts save material and weight, but the trapped powder must escape. Add escape holes of at least 2mm and connect internal cavities to the outside, or the powder stays sealed inside the part and adds weight and cost. The same rule applies as in SLS: design the voids to drain, or split the part to allow cleaning.