Multi Jet Fusion (MJF), the technology developed by HP, is the main alternative to SLS for functional nylon parts. We print only SLS, on Formlabs Fuse printers, so keep that in mind as you read. We have tried to describe both processes fairly and to say clearly when MJF is the better choice. This guide continues the comparison in the guide to SLS, FDM, SLA and MJF.

How each process works

SLS (selective laser sintering)

The chamber and the powder are heated to just below the melting point of the polymer. For each layer, the laser traces the outline of the cross-section, then hatches it and melts the powder only where the part has to be. The platform drops, a blade spreads a new layer of powder and the cycle repeats. On Formlabs Fuse printers the layer is 110 µm thick. The unsintered powder supports the part, so no support structures are needed.

MJF (Multi Jet Fusion)

MJF also starts from a layer of heated powder. An inkjet print head passes over the whole powder bed and deposits 2 liquids: a black fusing agent where the part has to be, and a detailing agent along the contour, which stops the powder around the part from fusing. Infrared lamps then heat the whole layer at once. The powder that received fusing agent absorbs more heat and melts, and the rest stays as powder. The layer is usually 80 µm thick. MJF does not need supports either.

What the difference means in practice

The laser works point by point, so in SLS the time per layer grows with the area that has to be sintered. In MJF, the agents and the heat are applied to the whole layer on every pass, so the time per layer depends little on how many parts it contains. In both processes, the total time depends heavily on the number of layers, that is, on the height of the build.

Materials

Both technologies work mainly with polyamides. The HP material list for MJF includes PA12, PA11, PA12 filled with glass beads (PA 12 GB), polypropylene, several types of TPU and a white PA12 used on the printer dedicated to white parts. Our SLS range, with Formlabs powders, covers PA12, PA12 White, PA12 Tough, PA11, PA12 GF, PA11 CF, TPU 90A and polypropylene while stock lasts.

The same name does not mean the same material. PA12 from HP and PA12 from Formlabs are different powders with their own data sheets, and an MJF service may stock only part of the HP range. If the part has precise requirements, compare the data sheets of the actual materials and ask the supplier what is available. The HP list we checked when writing this guide does not include a carbon-fibre-filled polyamide, which we offer as PA11 CF.

Surface finish and colour

After blasting, parts from both processes are matte and slightly grainy. In a Formlabs study, MJF parts straight out of the printer were somewhat smoother. After blasting, the roughness Ra ranged from 4.1 to 20.9 µm for MJF and from 8.0 to 16.4 µm for SLS, depending on the face. The ranges overlap, so the difference depends more on the face measured than on the technology.

Colour is the most visible difference. The fusing agent is black, so standard MJF parts come out grey and are usually dyed black. HP also has a printer dedicated to white parts, the Jet Fusion 5420W, which uses a white PA12, but not every MJF service has one.

In SLS, the colour comes from the powder. PA12 comes out grey and can only be immersion dyed black. PA12 White comes out white and can be immersion dyed black, red, blue, green, yellow or orange. We print PA12 White only on the Fuse 1+ 30W, so the part has to fit within 165 × 165 × 300 mm. For a specific RAL colour, parts from either technology can be spray painted. The colour options we offer are described on the finishing page.

Detail and accuracy

The thinner MJF layer (80 µm against 110 µm on the Fuse) makes the steps between layers somewhat less visible on gently sloping surfaces. For the Fuse printers, Formlabs publishes minimum feature sizes measured on PA12: vertical walls of 0.6 mm, holes of 1 mm and pins of 0.8 mm. MJF services have their own design guides, and the minimum values cannot be carried over directly from one technology to the other.

The same applies to tolerances. For PA12 on the Fuse, Formlabs gives ±0.3 mm or ±0.5% in X and Y, whichever is larger. For an MJF part, ask the supplier what tolerance they give for the material you choose. How to design around these values is explained in the guide to SLS tolerances.

Mechanical behaviour

For the same type of material, SLS and MJF parts have comparable mechanical properties. In both processes, the part is somewhat weaker along the Z axis, between layers, than in the XY plane. For Formlabs PA12, for example, elongation at break is 11% in XY and 6% in Z. The exact values differ from one powder to another, so compare the data sheet of the actual material, not just its name.

In practice, the choice of material matters more than the choice between SLS and MJF. A PA11 clip behaves differently from a PA12 one, whatever the technology. The differences between them are explained in the guide to PA12 and PA11.

Typical uses

Both processes are used for the same kinds of parts: functional prototypes, enclosures, jigs and fixtures, air ducts and small runs of end-use parts. MJF is often used for series production of black parts, at services with several printers. We use SLS for functional prototypes, low-volume production, white or coloured parts and PA11 CF parts.

Batch cost

With both technologies, a build heats, prints and cools the whole volume of the chamber, and parts are nested in 3D, next to and on top of each other. The cost per part falls as the chamber gets fuller, because the cost of the build is shared between more parts.

Unsintered powder is partly reused in both. For PA12, HP gives a reusability ratio of up to 80%, which means the mix contains at least 20% fresh powder. Formlabs gives 30% fresh powder per build for PA12. Actual consumption also depends on how full the chamber is, so compare the final price per part, not the percentages.

Industrial MJF systems are built for high volumes. For thousands of identical parts a month, an MJF service with several printers can offer a better price per part. For one-offs and runs of tens or hundreds of parts, the price depends on how full that supplier can make the chamber, on the material and on the finish. Compare quotes for the same part, with the same material, colour and lead time.

When MJF is the better choice

You need thousands of identical black parts a month and work with an MJF service that has several printers.
The part is already validated on MJF. Changing technology, even for the same type of material, means validating the part again.
You have an MJF supplier you are happy with for PA12 parts. In that case there is no significant technical reason to switch.
The material you need is in the HP range but not in ours.
The part has gently sloping surfaces on which the steps between layers should show as little as possible.

When we recommend SLS

You need white parts, or parts immersion dyed in colour, in PA12 White.
The part has to be stiff and withstand heat under load, and PA11 CF, filled with carbon fibre, meets the requirements.
You are working on prototypes or small runs that change often, and you want to discuss each version directly with the workshop that prints the part.

If you have a part designed for MJF, you can send it to us as it is. Both processes use powder without supports, and the design rules are similar. We check the model against the SLS minimums and tell you before quoting if a feature is borderline.

SLS design limits are described in the SLS design guide. When your file is ready, you can send it for a quote.

Sources

HP, Introduction to 3D printing processes (how MJF works)
HP, 3D printing materials (MJF materials and powder reusability ratios)
HP, Jet Fusion 5420W (white MJF parts)
Formlabs, Comparing Surface Finish: Fuse Series SLS, EOS SLS, HP MJF, and Form 4 SLA (surface roughness)
Formlabs, Nylon 12 Powder technical data sheet (TDS) (PA12 properties)