Multi-jet 3D fusion 3D printing guide (MJF)

• 1
  Key takeaways
• 2
  Multi-jet 3D fusion 3D printing guide: how the cycle works
• 3
  Materials: what MJF can and cannot print
• 4
  Design rules that keep MJF parts usable
• 5
  Tolerances, anisotropy, and surface finish
• 6
  Build volume economics and nesting
• 7
  Where MJF beats machining, and where it does not
• 8
  MJF compared with CNC machining

Multi-jet fusion builds parts in a powder bed, but the printhead never pushes plastic through a nozzle. Instead, the machine spreads a thin layer of polyamide powder, typically 80 μm, across the build platform with a roller or blade.

An inkjet array then deposits two fluids in the same pass. The fusing agent absorbs infrared energy and converts it to heat. The detailing agent cools the edges and keeps the boundary crisp. Where no agent lands, the powder stays loose and later acts as support.

Infrared lamps sweep the whole layer at once, so every voxel with fusing agent reaches its melting window in a single flash. The part cools slowly inside the powder cake, which keeps internal stress low compared with a laser that heats one point at a time.

At the end of the build, the cake is lifted out, cooled, and broken apart. Unfused powder is vacuumed off and most of it is sieved back into the next job. That powder reuse is the main reason MJF cost per part drops at volume.

The process needs a powder that melts in a narrow window and re-crystallizes predictably. PA12 is the workhorse. It gives good elongation, low moisture uptake, and a slightly grainy surface. PA11 is bio-based, tougher at low temperature, and takes a bit more energy to fuse.

Glass-bead and carbon-fiber filled grades raise stiffness and heat deflection. They also drop elongation, so snap-fit tabs and thin living hinges become risky. If you need a hinge that flexes thousands of times, plain PA12 is usually the safer call.

MJF does not print metal, elastomer, or high-temperature polymers such as PEEK. Those go through other processes. For rigid metal parts with tight tolerances, machining is the normal route.

Color is limited. Parts come out gray, and dyeing adds a batch step that can color the whole build uniformly. Multi-color printing exists in the platform but is not a production-grade option for most industrial buyers.

Minimum wall thickness sits around 0.8 to 1.0 mm for a self-supporting part. Thinner walls can be printed, but they warp when the cake cools. Add ribs rather than thick slabs; thick sections hold heat and shrink differently from the skin.

Clearance for moving parts should be at least 0.4 mm on a side. Powder escapes a gap that wide, and a narrower gap traps cake that will bind the joint. For snap features, design a 1.0 mm deflection allowance and test one build before committing to a mold.

Holes below 1.5 mm diameter tend to close or come out tapered. Drill them after printing if the function demands a precise bore. Same logic for threads: print a pilot hole and tap it, or use a heat-set insert.

Escape holes matter on hollow shells. Any enclosed cavity traps powder. If the powder cannot leave, the part is heavier than the model and can crack as the trapped cake expands during cooling.

Typical MJF tolerance is around ±0.3 mm, or ±0.3 percent of the dimension, whichever is larger. That is fine for brackets, ducts, and housings. It is not fine for bearing bores or mating faces that must seal.

The layer stacking creates direction-dependent strength. A part loaded across the layers can lose 30 to 50 percent of its tensile strength compared with the same geometry loaded in-plane. Orient the build so the main load path runs parallel to the platform.

As-built surfaces sit around Ra 10 to 15 μm with a matte, slightly porous skin. Bead blasting improves the look and removes clinging powder. Vapor smoothing seals the surface, but it rounds sharp edges and softens fine detail.

MJF parts absorb a small amount of moisture, so dimensions move with humidity. If a part must hold size over a season, seal it or specify a machined interface where the fit actually matters.

MJF cost is driven by the build chamber, not the part. A full cake of nested parts spreads the powder, machine time, and labor across every piece in the job. A half-full chamber costs nearly the same to run.

That is why nesting matters. Parts can be stacked in three dimensions with roughly 3 to 5 mm between them, since no support structures are needed. A well-packed build can hold dozens of small brackets or a handful of large panels.

Cost per part falls sharply as quantity rises, then flattens once the chamber is saturated. Below roughly 20 to 50 parts, MJF competes poorly with CNC on price for simple geometry, because the fixed build cost is spread over too few pieces.

Post-processing is the hidden line item. Depowdering, bead blasting, dyeing, and hole reaming are manual steps. A part with many small holes costs more to clean than a smooth enclosure of the same volume.

MJF wins on shape freedom. Lattice cores, internal channels, and organic brackets that would need five setups on a mill come out in one build. There is no tooling, no fixture, and no cutter access problem.

Machining wins on tolerance and material range. A ±0.005 mm bore in 7075 aluminum or 17-4PH stainless is routine on a 5-axis center. No powder process holds that, and no polymer matches metal stiffness.

The practical split is by function. If the part carries load, seals a fluid, or fits a bearing, machine it. If it routes air, covers electronics, or fills space inside a housing, MJF is usually cheaper and faster.

A hybrid route works too. Print the complex shell in PA12, then machine the critical bores and faces as a second operation. Inserts, threads, and sealing lands can all be added after printing.