ISO/ASTM definition: “material jetting, —an additive manufacturing process in which droplets of build material are selectively deposited.”
Material Jetting can also be known as (in alphabetical order):
➢ Aerosol Jet® (Optomec, Inc.)
➢ Ballistic Particle Manufacturing or BPM
➢ Drop On Demand or DOD
➢ Laser-Induced Forward Transfer or LIFT
➢ Liquid Metal Jetting or LMJ
➢ Multi-Jet Modelling or MJM (3D Systems Corporation)
➢ Multi-Jet-Printing or MJP (3D Systems Corporation)
➢ Nano Metal Jetting© (XJet) or NMJ
➢ NanoParticle Jetting™ (XJet) or NPJ
➢ Polyjet® (Stratasys Inc)
➢ Printoptical© Technology (Luxexcel)
➢ Thermojet Printing (3D Systems Corporation)
In 1984, Bill Masters patented one of the first AM technologies and founded the company Perception Systems, Inc. He later changed its name to BPM Technology and called the technology ballistic particle manufacturing but eventually went bankrupt in 1998. The first commercialized machines were made in 1994 by a company that later became Solidscape. Multi-Jet-Printing is a process from 3D Systems and was commercialized in 1996 which had a few different trade names like Thermojet and MJM. Then in 1998 Polyjet technology was developed by Objet, a company based in Israel which later merged with Stratasys in 2012. Then a founder of Objet started XJet in Israel and in 2015 announced a new material jetting technology that can make fully dense metal parts with a high level of surface quality.
Figure 1: Material Jetting example setup
Material jetting is very similar to the type of technology that exists in a standard home-based inkjet printer and is closely related to the binder jetting process. The main difference is that rather than printing ink or a binder, it prints the specific type of material that will make up the final part. Another difference from standard 2D printing is rather than printing this material into a sheet of paper, the material gets deposited directly onto the build surface and becomes solidified by some mechanism. Then the build platform changes the height and the process is repeated until the final geometry is achieved. The material is deposited drop by drop in a very precise and fine detailed manner; however, the exact mechanism for depositing these drops varies with the type of material being jetted. Some of the print heads used are exactly the same types that exist in 2D printers, namely piezoelectric or thermal print heads. These print heads are the exact same type as described for binder jetting. However, in the case of LMJ where the temperatures involved are too high to either boil the material or have a piezo mechanism operate, a combination of magnetic and electrical forces operate and utilize Lorentz forces to propel droplets of liquid metal to be printed. In the case of LIFT, a laser pulse hits a special film consisting of the desired build material, as well as a carrier substrate, and results in a droplet being formed that falls towards the build surface. Then the film is moved so that the laser can hit a new part of the film and release additional material. Regardless of the drop creation process, once the drop is deposited, it then solidifies either through material cooling (LMJ and LIFT), external curing from a UV light source (Polyjet and Multi-Jet Printing), or by evaporating a liquid transport material by using infrared light/heat (NMJ). Research into reactive jetting using monomers and catalysts to form polymers will increase the strength of parts made in this way. These polymers are generally materials that cannot be easily used in AM technologies but are highly attractive because of the long molecule chains associated with them. By being able to reactively jet these materials, the resulting plastics will have very large cross bonds that are formed within and between layers which will increase the strength of these plastic parts. Parts made this way will have a strength equivalent to injection moulded parts.
One big advantage of this technology is the ability to gang multiple print heads together. Having multiple print heads allows these machines to do unique things, such as print in different colours like traditional ink-jet printers, print faster by printing over the entire build surface in one pass, and print in multiple materials at the same time. The surface quality of these parts is usually quite high due to jetting very small droplets. Similar to how a normal inkjet printer is able to print thousands of different colours using only three different inks, a 3D printer that is able to jet multiple materials can combine these materials in different proportions in order to vary material properties in the finished part and create so-called digital materials. There is also a wide range of potential materials that can be directly jetted from plastics to metals; however, the time to develop new materials can be long. An advantage of using similar UV cured resins to those used in SL is that by incorporating the UV cure immediately after jetting, the parts come out fully cured and do not typically need any sort of post-curing.
Some disadvantages of this technology are the build time can be slow due to the nature of jetting very small amounts of material at a time over a small portion of the build area. Some machines use excess material by purging extra material through the nozzles and lines between layers or when the machine is not printing in order to preserve the print heads and prevent them from clogging up. Support structures are also required, thus one print head is dedicated to jetting only support material. This support material generally has very different material properties from that of the main part. Either it melts at a much lower temperature, is much softer, or is chemically different. Removal of the support material is then a manual step requiring one of the following methods: melting or dissolving away of the support material, spraying away the support material manually with water using a high-pressure wash, or removing them by hand.
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