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What is multi-jet fusion?

In this introduction to Multi Jet Fusion (MJF), you'll find out all about the tech and applications of this bulk polymer additive workhorse. A powder-based industrial 3D printing process, MJF produces functional nylon prototypes and end-use production parts.

Written by Isaac Maw

A custom designed part printed on MJF printer.

Multi-jet fusion (MJF) is a 3D printing system technology designed by HP Additive. MJF uses an inkjet array to selectively apply fusing and detailing agents across a bed of nylon powder, which are then fused by heating elements into a solid layer. This additive manufacturing system is designed for industrial additive manufacturing, and it has features that support industrial use cases. A selection of materials is available, but polyamide materials work best.

Multi jet fusion printers by HP.
HP Multi Jet Fusion printers

In the 1990’s, when additive manufacturing was beginning its transition from research and patents by private companies and development in university labs to actual real-world applications in manufacturing, one of the key obstacles to adoption was the slow speed. Compared to injection molding or metal stamping, for example, most types of 3D printers—such as the fused filament fabrication printers popular in the rapid prototyping and hobbyist markets—took forever to print a single part. While some early adopters attempted to speed up production by using ‘farms’ or arrays of multiple machines to print higher quantities, a research and development team at HP Additive, based in global additive manufacturing hub in Barcelona, Spain, developed a system which built parts layer by layer in a large bed of powder material, similar to selective laser sintering (SLS) or other powder bed fusion designs.

Find out exactly what Multi Jet Fusion (MJF) is and how it works in this 10-minute video.

Designed for production

By building parts in a powder bed, the need for supports is eliminated, and parts can be nested efficiently in the build area. In addition, unused powder can sometimes be reused for future prints. On top of that, the digital nature of all additive manufacturing processes means that in a single build of 100 small parts, such as eyeglasses frames, the cost and time to print 100 identical parts or 100 unique, customized parts is the same. It’s these distinct advantages which have led MJF to become a serious production technology solution, worth consideration for a wide range of manufacturing challenges.

Printing bed of a multi-jet fusion printer.
Multi-jet fusion print bed

How does multi-jet fusion 3D printing work?

An HP Jet Fusion printer includes a build unit which looks like a rolling cart. The build unit is placed inside the printer before the process can begin. In the printing process, a material recoater carriage moves across the build area, depositing a thin layer of powder material. Next, the printing and fusing carriage moves across the build area. As the printing and fusing carriage passes over the build area, it first preheats the powder to a specific temperature to provide material consistency. Next, an array of inkjet nozzles jet fusing agents onto the powder bed in areas corresponding to the part’s geometry and properties defined at that layer. The printing and fusing carriage then heats the surface of the bed to fuse the material.

Top View of MJF Printer
Top view of MJF printer

After each layer, the build unit retracts downward, creating space for the next layer of material deposition, and the process is repeated. 

By the end of the process, the build unit contains a three-dimensional area filled with unfused powder and the fused part or parts. An operator will remove the build unit from the printer and roll it into a separate processing machine, for cooling, unpacking the parts, and recovering unfused material powder.

Illustration of fused and unfused layer.
Unfused powder and the fused part

Binding agents increase build flexibility

Compared to some other powder bed-based additive technologies, one feature of multi-jet fusion is the use of a variety of binding agents deposited on the material by the inkjet nozzles. For example, consider a selective laser sintering (SLS) machine. SLS is similar to MJF in that powder material is deposited in the build area layer by layer, but in SLS, the material is fused by selectively aiming a laser at the material to sinter the powder particles together and to the underlying layer of the part. The laser power can be varied to alter material properties, but those parameters are limited. Because HP has an array of chemical agents that can be deposited on the material before fusing, there are more opportunities to alter the properties of each voxel of the part. According to HP, these properties may one day include:

  • Dimensional accuracy and detail 

  • Surface roughness, texture, and friction coefficient 

  • Tensile strength, flexibility, hardness, and other material properties 

  • Electrical and thermal conductivity 

  • Opacity or translucency in plastics 

  • Color: embedded and at the surface

Applying fusing and detailing agent

Multi-jet fusion print parameters

Currently, MJF is a proprietary system owned by HP. While there are other powder bed fusion systems in the market, such as those that use laser or electron beams to fuse material, the parameters of MJF printing are limited to three machines: the HP Jet Fusion 5210 pro, the HP Jet Fusion 5200, and the HP Jet Fusion 4200. There are also the HP Jet Fusion 500/300 series printers, which are smaller and were designed for prototyping applications. These full-color printers have been discontinued by HP, but will still be supported for years to come. 

Below is a brief look at the parameters of each solution. This information is sourced from the HP Additive website.

It is important to note that the 5210 Pro and the 5200 are the same machine and can produce the same amount of parts per week. The difference between these two is the pricing structure given by HP. 5210 has a much higher price tag but in turn offers much lower material costs which is beneficial in the long run for printing services with a high up-time.

HP Jet Fusion 5210 Pro HP Jet Fusion 5200 HP Jet Fusion 4200
Ideal role in production Mid-volume production environments producing over 550 parts per week Mid-volume production environments producing over 200 parts per week Industrial prototyping and final part production environments producing up to 200 parts per week
Print color Gray Gray Gray
Materials HP 3D HR PA 11 HP 3D HR PA 12 HP 3D HR PA 12 glass beads (GB) HP 3D HR PP enabled by BASF BASF Ultrasint® TPU01 HP 3D HR PA 11 HP 3D HR PA 12 HP 3D HR PA 12 glass beads (GB) HP 3D HR PP enabled by BASF BASF Ultrasint® TPU01 HP 3D HR PA 11 HP 3D HR PA 12 HP 3D HR PA 12 glass beads (GB) HP 3D HR TPA enabled by Evonik ESTANE® 3D TPU M95A
Effective Build volume (x, y, z) 380 x 284 x 380 mm (15 x 11.2 x 15 in) 380 x 284 x 380 mm (15 x 11.2 x 15 in) 380 x 284 x 380 mm (15 x 11.2 x 15 in)
Software HP 3D API / HP 3D Process Control6 / HP 3D Center / HP SmartStream 3D Build Manager / HP SmartStream 3D Command Center HP 3D API / HP 3D Process Control6 / HP 3D Center / HP SmartStream 3D Build Manager / HP SmartStream 3D Command Center HP 3D API / HP 3D Center / HP SmartStream 3D Build Manager / HP SmartStream 3D Command Center
Print time (Average results assuming a full job from “print” to “job finished” using HP 3D High Reusability PA 12 material.) 11.5 hrs (Balanced print mode) 9.5 hrs (Fast print mode) 11.5 hrs (Balanced print mode) 9.5 hrs (Fast print mode) 16.5 hrs (Balanced print mode) 11.5 hrs (Fast print mode
Doorway clearance 2320 mm (91.3 in) 2320 mm (91.3 in) 2320 mm (91.3 in)
Operating footprint 21.5 m² (232 ft²) 21.5 m² (232 ft²) 21.5 m² (232 ft²)
Supported Industrial Management systems 3D Control Systems, AMFG, LINK3D, Siemens NX AM, Siemens Opcenter 3D Control Systems, AMFG, LINK3D, Siemens NX AM, Siemens Opcenter 3D Control Systems, AMFG, LINK3D, Siemens NX AM, Siemens Opcenter

What materials are used in MJF 3D printing?

The Jet Fusion system focuses largely on polyamide materials, including a proprietary PA12, PA11 and glass bead reinforced PA12. Currently, all materials for the machine are developed by HP and its partners. In addition to these PA materials, polypropylene, flexible TPU and TPA are available. 

HP 3D printing materials portfolio selection guide

Multi-jet fusion post processing

Like most manufacturing processes, multi-jet fusion requires printed parts to undergo processing steps before they are complete. However, compared to many other additive technologies, the post processing required for MJF is relatively light.

When a print job finishes, the build unit is filled with a three-dimensional bed of unfused powder, and the parts sit buried in the powder. 

The first step in MJF post-processing is cooling. This can take place within the build unit, or HP also offers a system which provides modular units which can be placed on a rack to naturally cool, allowing the build unit to go back into use for a new print without additional wait time. 

Once the build chamber is sufficiently cooled, an operator moves it to the processing station and vacuums the unfused powder material into a container for recovery. 

Vacuuming of the unfused powder
Vacuuming of the unfused powder

The printed parts must then be bead blasted, air blasted or water blasted to remove any remaining powder. This can be done manually or automatically, using a tumbler, ultrasonic cleaner, or vibratory finishing machine.

After powder material is removed, additional post-processing may be required for parts. For reference, consider the post-processing required for casting processes. Mating surfaces, bores, tolerances exceeding the capabilities of the MJF process, internal threads and other features may require machining. Hand sanding or other surface finishing may be required to meet technical requirements. In addition, other finishing processes such as painting or coating may be needed.

3D printed parts in dyed black + vapor smoothed surface finishing
3D printed part in dyed black + vapor smoothed surface finishing

Advantages of multi-jet fusion

The important advantages to consider when analyzing MJF as a production technology depend on what it’s being compared to for each application. For example, in the past, HP has highlighted Smile Direct Club, a manufacturer of individually-customized dental aligners. In Smile Direct’s process, the aligners are made using molds of each customer’s mouth. To support Smile Direct’s need for mass production of high-precision individual parts (the mouth molds), the Jet Fusion system is ideal. Traditionally, without additive technology, these molds would be manufactured using casting processes, a time consuming and manual process. Smile Direct’s business model, in which a large, distributed base of customers order aligners over the internet, would likely not be feasible. For this user of MJF, the key advantage is that a build volume filled with unique individual parts does not cost more than a run of identical parts. Outside the additive manufacturing space, that’s essentially unheard of, since processes like casting or molding benefit from the economy of scale.

Of course, this “lot size of one” economy is common to all digital 3D printing processes, it’s a universal benefit of 3D printing technology. However, the Jet Fusion systems differ from most other additive technologies and enhance this benefit by design. Because the build unit is a rolling cart and the processing station is included with the printer, the multi-jet fusion systems are designed for manufacturing. Unlike many other additive technologies, such as FFF or SLA, the system can handle production of larger runs of parts, nested optimally in the powder bed and moved to processing using the rolling cart. Parts are post processed in bulk, and depending on the technical requirements of the part, hand finishing may be minimized. This is a distinct advantage over parts printed via filament deposition. For a large run of parts, an array of multiple printers would need to be built, and support structures would need to be removed from each part.

Disadvantages of multi-jet fusion

Users of MJF have highlighted a high cost of hand sanding to improve the finish on parts. Of course, this is only relevant to certain applications. The key disadvantage of multi-jet fusion today is that it is a closed, proprietary system controlled by HP. This is a familiar disadvantage for anyone experienced in selecting a manufacturing equipment vendor or solution. However, HP also controls all the materials for the solution, as well as the software and machines. Compare this to filament deposition printers. Because that market is in some ways more mature, it’s common to find open software ecosystems allowing users to configure the machine with different software, and to use other material suppliers. In the powder material space, it’s still common for 3D printer OEMs to supply the powder materials themselves and through partnerships with materials suppliers like BASF, for example. This is because each platform is designed to use powders with specific parameters. For example, customers of metal additive machines may wonder why they must use powder metal materials designed for the process, rather than buying powdered metals from other suppliers. In many cases, this is because metal powders designed for additive processes are manufactured for high sphericity and flowability, while off-the-shelf metal powders may simply be ground, and will not work in the machine properly. 

As all powder bed fusion processes mature, we may see growth in material selection and material vendor options. However, for the time being, using multi-jet fusion means being restricted to the materials offered by HP.

MJF rules of thumb

Designing for manufacturability is critical for all manufacturing processes. For example, draft angles are a standard design feature in injection molded parts as they allow parts to release from the mold. Similarly, machined parts do not include square inside corners, since the tools have a minimum radius. These are well-established design conventions in manufacturing, honed over decades working with these processes. For additive, the conventions are newer, and in many cases still emerging.

One obvious rule of thumb when designing for additive is to avoid fully-encapsulated or hollow geometry. This is because unfused powder will be trapped inside. Be sure to include a hole or access to allow the unfused powder to be removed from the part, including stubborn, stuck-on powder that will be removed by media blasting.

Another design rule for MJF is to think beyond the design conventions familiar to conventional, subtractive manufacturing processes. For example:

  • Instead of a living hinge, why not design a real hinge which can be printed in one piece? 

  • Instead of designing a simple cylinder, why not choose a more organic shape? 

  • Instead of an assembly of parts, can the entire assembly be printed as one part?

With multi-jet fusion, it’s important to consider how best to nest parts in the build chamber to make best use of each print. While unfused powder can be reused, it’s best to use it efficiently. 

In addition, powder materials, including polymers, can pose a health hazard. When working with the MJF process or with unprocessed parts, always use the appropriate PPE. 

 

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