ISO/ASTM definition: “binder jetting, —an additive manufacturing process in which a liquid bonding agent is selectively deposited to join powder materials.”
Binder Jetting can also be known as (in alphabetical order):
➢ Three Dimensional Printing or 3DP[2,3]
➢ ColorJet Printing or CJP (3D Systems Corporation)
➢ Digital Metal® (Höganäs AB)
➢ Ink Jet Printing or IJP
➢ Multi Jet Fusion™ (Hewlett-Packard Development Company)
➢ Plaster-based 3D Printing or PP
➢ Powder Bed And Inkjet Head 3D Printing or PBIH
In 1989, researchers at Massachusetts Institute of Technology (MIT) developed a process called 3D printing[2,3] and in 1993 licensed it to a number of different companies. These included ZCorp in 1994 (later acquired by 3D Systems in 2011) and Ex One in 1996. In 2014 traditional 2D printer company HP developed a new method of binder jetting with an integrated heater claiming a 10-time speed increase over other AM technologies like laser sintering. Even though binder jetting isn’t the first AM technology to be invented (as that distinction goes to Stereolithography), it has been known as 3D printing the longest. This leads to minor confusion as most instances of 3D printing talked about in the media do not actually mean the binder jetting process, but rather AM in general.This process most resembles traditional 2D printing. First, rather than printing onto a sheet of paper, a thin layer of loose powder between 0.05mm and 0.5mm is smoothly spread flat over a build platform. Then a traditional inkjet print head moves back and forth over this powder and deposits a binding material rather than an ink onto the areas that are to be made solid. This clear or coloured binder is initially a liquid and causes the powder material to bind together usually involving a chemical reaction. This chemical reaction depends on the method and materials used and occurs either when the binder comes into contact with the raw powder, comes in contact with the air and evaporates, contacts another chemical that is mixed into the powder or is activated by heat. In the next stage of printing the build platform changes height, a new layer of powder is deposited, and the process is repeated layer by layer until the part is complete. The print heads used are exactly the same types that exist in 2D printers, namely piezoelectric or thermal print heads. These print heads require the binder to exhibit similar properties to the original ink. Typically, the binder needs to be a low viscosity (usually in the tens of centipoise range) so that it can be jetted through the very small nozzles located in the print head. A piezoelectric print head has a piezoelectric mechanism connected to a diaphragm which pushes material out of the nozzle to form a droplet. Thermal print heads are also known as bubble jet print heads since a thermal heating element starts to boil the binder which rapidly forms bubbles in the print head chamber which cause the binder to be pushed out of the nozzle and form a droplet. In the case of the new HP process, a heat activated binding agent is initially applied with a detailing agent being applied before the heat which neutralizes any potential binding resulting in a very clear separation of fused and unfused material which yields a nicer surface finish. Because HP’s binder is heat activated, the resulting parts have slightly higher material strength than normal binder jetted parts.Advantages of this process are that the materials and binders used can be very inexpensive, which allows a very low cost per part. Traditionally a gypsum powder and water/alcohol mix were some of the very first powder/binder combinations, but now a variety of materials can be printed such as metals like stainless steel alloys and copper, plastics, and glass. Parts can be also created very quickly; nearly at the same speeds a 2D printer is able to print paper. Print heads can also be combined and different colours can be added to the binders in order to create full-colour 3D printed parts. Due to laying down material over the entire build platform at once, there is no need for support structures. This process is also scalable to produce large build envelopes, such as with the Exerial™ machine from ExOne which has a build envelope of 2.2m x 1.2x x 0.7m. It is used in creating sand cores and molds for casting and allows the mass production of traditional castings using AM technology.Disadvantages include a very weak final part, especially when no external energy like heat is used to bind parts. These green parts generally need to be post-processed by infusing them with another material like Cyanoacrylate glue or bronze to give the final part more strength. Parts that are not infused are also not very dense, and if made from metal, are usually sintered in an oven to give some additional strength without adding other materials. Depowdering a part (taking the final part out of the powder bed build area) can be very messy. Also as the powder is very fine and can be an inhalation hazard; special respiratory equipment is usually needed.
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