Multi Jet Fusion (MJF)

Multi-jet Fusion (MJF) is an additive manufacturing technique invented by Hewlett-Packard that fuses powdered materials into a solid object using heat and a binding material.

What is Multi Jet Fusion (MJF)?

Multi jet fusion is a powder bed fusion technology involving a fine layer of powder spread over a build platform that the MJF printer fuses by selectively applying heat with a fusing and a detailing agent. Therefore, it is a multi-agent process of additive manufacturing.

This additive manufacturing technique produces functional parts with remarkable isotropy and quality. It is fast, and the cost per part is low. All these features make it ideal for applications such as:

• Automotive parts
• Protective equipment such as helmets
• Prosthetics

How does Multi jet fusion work?

Multi jet fusion printer Overview

An MJF printer is usually composed of the following:

• A build chamber.
• A powder bed or build platform.
• A print head array. This array includes the fusing agent heads and the detailing agent heads. The print head also includes fusing lamps.
• An energy source. This source usually comes in the form of heating lamps.
• A cooling system.
• Control systems.
• Post-processing equipment that integrates a thermal camera, a scraper, and powder lifter screws to recover the unfused material.

It is essential to highlight that there are different models of MJF printers. So, the configuration may vary from one to another.

MJF process steps

As with any other additive manufacturing technology, creating a 3D model is the first step to building a prototype or fully functional piece with MJF. Then, it is necessary to load the model into the printer’s software, which creates the layers for the print. After that, the actual printing process begins by following these steps:

1.  A spreader applies a thin layer of powder to the work area. This layer is pre-heated until it reaches a temperature near the sintering temperature to prepare the material for fusing.
2. Nozzles arranged in the printhead spray both the fusing and detailing agents on the build platform based on the data provided by the CAD software regarding the model.
3. Heating lamps with infrared light move over the layer of powder. The multi-agent mixture absorbs the energy, thus generating the reactions between the agents and the material. As a result, the heat fuses the powder covered with the agents while leaving the rest of the material in the powder form. The detailing agent remains on the outer surface and edges of the layer to generate a better resolution and surface finish.
4. The machine repeats the previous steps until the part is complete.
5. Like the parts printed with SLS technologies, the post-processing of MJF parts involves time for cooling and cleaning to remove the unfused powder, usually with a vacuum system.

Advantages and disadvantages of MJF

Advantages of Multi jet fusion

• Low production times – Thanks to shorter cooling and post-processing times compared to similar technologies like SLS
• Outstanding mechanical properties – MJF machines print strong and functional parts. When testing for tensile strength under the ASTM D638 standard, MJF parts have reached up to 48 MPa. Also, it is possible to achieve low layer and wall thicknesses.
• High precision for fine details – MJF can jet material at 1200 DPI and offers tight tolerances of around +/- 0.012 in. (0.30mm).
• Highly reusable material – Regarding HP machines, the manufacturer claims that their machines can recover 80% to 85% of the unused powder for future reuse.
• No support structures required – A portion of the powder remains unfused. This powder acts as a support structure.

Disadvantages of Multi jet fusion

• Initial investment – MJF printers can be costly. So, MJF is a manufacturing process that requires a high initial investment. Material can also be expensive depending on the properties required. However, the reusability offered by the process usually offsets the cost of the material. The main problem is that HP MJF printers mainly use proprietary material. So, no competition might push prices lower.
•  Geometry limitations – In general, MJF is capable of producing complex geometries. However, certain features are not possible, especially cured and hollow ones.
• Warping – Large parts and thin features are prone to suffer from warping.

What materials does MJF use?

Nylon is the preferred material for MJF manufacturing. However, it is not the only one. Here is a list of the materials used in MJF printing.

• PA 12 Nylon – Popular for its robustness, this thermoplastic boasts outstanding mechanical properties and chemical resistance. Suitable for functional parts.
• PA 12 Glass-filled Nylon – The glass-filled PA 12 is a version of the standard PA 12 but is more expensive. However, it offers higher rigidity and dimensional stability thanks to a glass bead reinforcement.
• PA 11 Nylon – Similar to PA 12 but more ductile. Suitable for functional parts.
• PP – This material is a polypropylene with high chemical resistance and low moisture absorption. The resulting parts can be welded and work well as functional parts.
• TPU01 Thermoplastic Polyurethane – This material is a type of elastomer offering an excellent combination of flexibility and shock absorption. Achieving high-resolution features is possible with this material.
• TPU M95A Thermoplastic Polyurethane – Another type of elastomer described by its manufacturers as a material with a high bouncing capacity and significant abrasion resistance.
• TPA Thermoplastic Polyamide – Also in the elastomer category, this lightweight material is an excellent choice for easy processing and flexible pieces due to its properties, such as a high elongation at low temperatures.

MJF vs SLS

MJF 3D printing is similar to SLS 3D printing, with both technologies achieving high-quality functional parts with smooth surface finishes.

Some of their similarities include:

• Additive Manufacturing (AM) category – MJF and SLS belong to PBF or Powder Bed Fusion, a class within additive manufacturing that includes all technologies using a heat source to build a part by solidifying a powder.
• Material and process – Since they belong to the PBF category, MJF and SLS use powder materials and fuse them thermally.
• Support structures – None of these additive manufacturing technologies require support structures since the powder bed serves this purpose.
• Surface finish – Both processes offer a rough and matte surface finish.

However, they have differences that are worth highlighting.

• Multi-agent process – MJF uses a fusing and a detailing agent that offers the opportunity to modify the part properties. On the other hand, SLS uses a laser to fuse the powder particles.
• Heat source – MJF uses infrared light as the heat source to fuse the material, while SLS uses a laser, usually a CO2 laser, as the heat source.
• Dimensional accuracy – Regarding dimensional accuracy, MJF and SLS are comparable. However, MJF offers slightly tighter tolerances at +/-0.2 mm for 100mm, while SLS works better at +/-0.3 mm for 100mm. Also, MJF achieves a print resolution of 80 microns with a minimum feature size of 0.5mm, while SLS reaches 100 microns and a minimum feature size of 0.75mm.
• Production speed – SLS requires more cooling and post-processing time. The faster post-processing is possible thanks to a dedicated station offered by HP.
• Materials – SLS offers more variety than MJF, although this may change in the future.
• Waste – SLS generates more waste than MJF. While SLS can only recover and reuse 30 to 50% of the powder, MJF can save up to 80% of the material that was not fused.
• Cost – MJF has the added cost of the binding material. However, the waste reduction provided by the material reusability offsets the added cost. Also, printing only one part is more expensive when using SLS, but the cost per part is comparable when scaling production.