ISO/ASTM definition: “material extrusion, —an additive manufacturing process in which material is selectively dispensed through a nozzle or orifice.”
Material Extrusion can also be known as (in alphabetical order):
➢ Direct Ink Writing or DIW
➢ Extrusion Freeform Fabrication or EFF
➢ Fused Deposition Modeling or FDM® (Stratasys Inc.)
➢ Fused Filament Fabrication or FFF
➢ Glass 3D Printing or G3DP
➢ Liquid Deposition Modeling or LDM
➢ Micropen Writing
➢ Plastic Jet Printing or PJP (3D Systems Corporation)
➢ Robocasting or Robotic Deposition, 
In 1988, Scott Crump invented a new AM process based on candle wax and a hot glue gun while making a toy for his daughter in the kitchen. The next year he started the company Stratasys, which became one of the largest AM companies in the world. In 2005, in the United Kingdom, Adrian Bowyer at the University of Bath started the RepRap project based on the technology that made Stratasys so successful. His goal was to be able to make use of expiring patents that would make FFF available to everyone, and create an open source 3D printer that was capable of replicating rapidly (RepRap) itself, or at least make as many parts for itself as it could. This first open source printer was released in 2008 and inspired many companies to make their own versions based on the RepRap platform. One company, MakerBot, was founded in 2009 and later acquired by Stratasys in 2013. This open-source design along with the expired patents allowed hundreds of different printer designs and companies to emerge since then. This recent development has contributed to the public’s general awareness of AM technology, even though the core technology started over 30 years ago. Most desktop 3D printers in the world are of this type and are what most people think of when they think 3D printer.The core principle of this technology is that any material that is in a semi-liquid or paste form can be pushed through a nozzle and used to draw the 2D cross-sections of a sliced 3D model. Similar to how a hot glue gun heats a rod of glue and the trigger selectively pushes the material through the nozzle, material extrusion works exactly the same way. The material that is extruded doesn’t need to be plastic or even heated. While the vast majority of these printers use a plastic like ABS (Acrylonitrile butadiene styrene) or PLA (Polylactic acid), any material that can be pushed through a nozzle (heated or not) and afterwards retain its shape can be used. Other examples include cement, chocolate, ceramic pastes or slurries, metal clays and metal filled plastics, ground-up and blended food, or even biocompatible organic cellular scaffolding gel. The technology is scalable and is only limited by nozzle size and supporting machine structure. This supporting machine structure can take many different shapes such as a delta robot configuration or multi-jointed robot arms. This printer structure can also be built using traditional scaffolding structures to create some of the largest printers in the world. Two examples are a 2014 Chinese built 12m x 12m x 12m printer in the city of Qingdao, and a 2016 12m tall delta printer in the Italian town of Massa Lombarda, both of which are large enough to print a small house. There are plans to build printers that move on a rail system enabling an almost infinite build length in one direction. Multiple print heads can be installed on the same machine thus enabling multi-material printing, but there can be challenges with calibration between heads; thus, more than 2 heads on a machine is rare.The greatest advantage of this process is the extensive range of materials it can use. Almost all types of thermoplastics can be used, from the standard plastics like ABS to more engineering plastic grades like nylon, all the way up to advanced engineering plastics like polyether ether ketone also known as PEEK. These plastics have superior dimensional stability and can be used as actual end-use parts like in the Boeing 787 where many parts (mostly air ducting) are 3D printed from FDM processes. The mechanics of this type of printing are fairly simple and easy to modify especially due to the availability of open source designs; thus people have taken these principles to print anything that can fit into a syringe or that can be made into a filament.Some disadvantages are that this process is slow as only one nozzle operates at a time and the entire layer must be subdivided into actual tool paths to trace out the whole 2D slice. This tool path causes the fill factor to be less than 100% due to geometric constraints and nozzle diameter. Parts generally have anisotropic material properties, and the same part can exhibit different strengths depending on how it was printed. Layer heights are generally larger than other AM processes and are thus more visible and contribute to a higher surface roughness. Support materials and structures need to be used, otherwise, considerable sagging can occur depending on geometry. Removing these supports is either a manual and labour intensive process, or a process which requires dissolving and rinsing of parts in a chemical bath of some sort. Generally, only one material is used, with one main material and one support material being quite common. Anything more than one material and support is rare, it usually requires specialised print heads or specialised calibration techniques.
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