What is Multi Jet Fusion (MJF) 3D Printing?
Multi Jet Fusion (MJF) is a 3D printing technology for producing rigid and flexible polymer parts of medium size and resolution. It is proprietary technology that has recently been developed by Hewlett Packard (HP). Like SLS, MJF belongs to the category of polymer powder bed fusion processes.
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Materials
The most common material for MJF is Polyamide 12, abbreviated as PA 12. PA 12 is also called nylon. Other popular materials are glass filled PA 12 and PA 11. It is also possible to produce flexible parts with MJF by using Thermoplastic Polyurethane, also known as TPU.
Part Quality
MJF is an excellent process for manufacturing prototypes, tools and functional parts that range from 10’s of millimetres to 100’s of millimetres in size with a dimensional accuracy of up to 0.3 mm. The material properties of MJF parts are almost isotropic. The surface quality is consistent, with a roughness of about Ra ~ 5 µm after bead blasting.
| Build Volume | Up to 380mm x 284 mm x 380 mm |
| Layer Thickness | 0.08 mm |
| Dimensional Accuracy | 0.3 mm |
| Surface Roughness | Ra ~ 5 µm (after bead blasting) |
Build Cycle
The part is produced by adding the powdery build material layer by layer along with the injection of two other components called “fusing agent” and “detailing agent”. The process begins with adding the first layer of material on the build platform which gets exposed to thermal energy. Then the fusing agent is selectively printed where the particles need to be fused, followed by a detailing agent which helps the fusion process at the boundary interface to be either amplified or reduced. Then the layer is exposed to a high-power infrared source, causing the selected areas to be fused together. This process is repeated until the part is printed.
In a HP Jet Fusion 3D printer, dual carriages are used to scan across the working area in perpendicular directions. One carriage carries the fresh material and other carries hp functional agents. The machine is equipped with high intensity and high voltage bulbs that heat the chamber, a powder dispenser and the binding head. In effect, the exposure time per layer is constant, and in turn the build rate is independent from the part volume, it only depends on the number of layers.
Post-Processing
After the build cycle has finished, the entire build unit needs to cool. This can take more than a day, although faster cooling is possible at the possible expense of part quality. After the build unit has cooled down, the parts are removed and separated from the powder. Each part needs to be blasted to remove residual powder from the part surface. After blasting, the parts are cleaned with pressurized air to remove remaining blasting material and loose powder. This is the standard process, and the part is considered finished afterwards. MJF parts with standard finish have a rough surface finish compared to injection molding or Resin 3D Printing.
There are several additional post-processing methods in use to further increase the surface quality of MJF parts. Vibratory grinding is a common finishing option for a smoother surface.
The default color of MJF parts is grey. If the parts need to have a different color, they need to be dyed during post-processing. Due to the natural grey color, it can be challenging to produce bright and vibrant colors. However, darker colors such as black can easily be achieved. There is one type of MJF machine that can print in full color, but it is not yet widely available and has a smaller build volume than the standard machines.
About the author:
Harald Schmid is the founder and managing director of Gramm GmbH. Gramm is located in Regensburg, Germany.
harald@gramm.online
Website: gramm.online
Prototype Hubs Profile: Gramm
