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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.
 “ISO/ASTM 52900:2015(en), Additive manufacturing — General principles — Terminology,” International Organization for Standardization (ISO), Geneva, Switzerland, 2015.
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 Crump S. S., “Apparatus and method for creating three-dimensional objects,” U.S. Patent 5,121,329, 09-Jun-1992.
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 Postiglione G., Natale G., Griffini G., Levi M., and Turri S., “Conductive 3D microstructures by direct 3D printing of polymer/carbon nanotube nanocomposites via liquid deposition modeling,” Composites Part A: Applied Science and Manufacturing, vol. 76, pp. 110–114, Sep. 2015.
 Morissette S. L., Lewis J. A., Clem P. G., Cesarano III J., and Dimos D. B., “Direct-Write Fabrication of Pb(Nb,Zr,Ti)O 3 Devices: Influence of Paste Rheology on Print Morphology and Component Properties,” Journal of the American Ceramic Society, vol. 84, no. 11, pp. 2462–2468, Nov. 2001.
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Additive manufacturing (AM) has been called the next Industrial revolution–. It is a recent technology that encompasses a wide range of different processes and materials. It is changing industries like aerospace by adjusting how engineers think about designing complex parts–. This revolution can take designs that have gone unchanged for decades and improve them.
AM was first conceived of in the 1970s with the first parts made in the 1980s and has had many names over the years.
- Layer manufacturing or LM
- Rapid manufacturing or RM
- Rapid tooling
- Rapid prototyping or RP
- Laser rapid prototyping
- Solid freeform fabrication or SFF
- Direct digital manufacturing or DDM
- 3D printing or 3DP
Initially, AM was used as a tool to create prototypes much more rapidly than could be done by other means and aided in the development of bringing new products to market, . It then helped create tooling, tooling inserts, jigs and fixtures that were used in the manufacturing of end-use parts, . It has continued to evolve and now is being used in making end-use parts such as low-volume production plastic casings, high-end consumer in-ear music monitors, GE LEAP jet engine fuel nozzles, and SpaceX SuperDraco rocket engine chambers and Falcon 9 main oxidizer valve bodies which have gone into space.
While AM has been around for three decades, it has yet to be utilized in the manufacturing of electric motors. Since the late 1800s, electric motors have seen only a small number of improvements, most of which have come from new and improved materials and new manufacturing techniques–. Yet the core design of motors has been relatively unchanged –. There is an evident opportunity to research how AM can change electric motor design. AM has the potential to produce new electric motors that can increase motor efficiencies and power densities.
Review of Additive Manufacturing
There are many different types of AM processes. Each has specific strengths and capabilities with unique areas of specialization. Regardless of the process, there are some fundamental principles that all follow.
Subtractive manufacturing usually starts out with a solid chunk of material that is larger than the final desired shape or part. Then using different tools, material is removed (or subtracted) until the final shape or part is achieved. It is important to note that casting is not considered additive or subtractive, but rather a formative process, as an existing mould or pattern is needed to create the final part.
Compared to subtractive, AM works in the opposite way. Instead of removing material to get the final desired shape or part, material is added to a build platform bit by bit. Most forms of AM follow these basic steps:
- Take a 3D model
- Slice model into layers and generate computer code
- Print first 2D slice and supports (if needed)
- Increment height
- Print next layer
- Repeat steps 4-5 until finished
- Post process (if needed)
A minor exception to these steps is when an AM process can deposit material in three-dimensional space. It is then not limited to just printing 2D layers one at a time–. AM can be a faster and more economical way to make parts, especially when the part is complex and/or made from an expensive material. These steps are still quite broad and have many details that can provide deeper insight.
AM process steps
Step 1: 3D model
The process begins with a computer-generated 3D model of the desired final part. The model needs to be capable of being printed, which means that it needs to occupy a defined volume. The part can’t be a single surface with a wall thickness of zero. Once it has thickness and volume, it then needs to be an enclosed watertight solid. This means that if water is put in the interior volume of the model, there are no holes from the inside volume to the outside surface. Even objects like a Möbius band or Klein bottle can be printed as long as the single surface is thickened to have a defined watertight volume.
Figure 1: Klein bottle and Mobius band show a surface with no thickness
Once the model is generated correctly, it needs to be saved to a specific file format. These specific file formats are needed in order for it to be prepared properly for printing. Thus it needs to be saved as either an STL file, AMF file or 3MF file. The STL file format has been the de facto standard since it was created in the late 1980s. However, it does have limitations which cause some problems. It does not store the units of measurement of the original model so they need to be assumed. Also, the file size becomes very large when trying to save a model with a high level of surface curvature. In response to these limitations, the American Society for Testing and Materials (ASTM) introduced the AMF file format in 2011. Then in 2013, Microsoft intended to do the same with its own 3MF format. This format became natively supported in all Windows operating systems since Windows 8.1. 3MF has since garnered considerable support from large companies such as HP, 3D Systems, Stratasys, GE, Siemens, Autodesk and Dassault Systems although it is unknown how many actively use this file format. Thus STL is still the file format of choice for almost all 3D printing. However, a newly signed liaison agreement between ASTM and the 3MF Consortium may bridge some differences between AMF and 3MF and create one new standard file format to replace STL.
Step 2: Prepare for printing
Secondly, special software is needed to turn that 3D model into data that a 3D printer can recognize and use. This usually involves cutting or slicing the model into many digital layers. Each layer is then converted into either a 2D image or into a set of 2D tool paths. The spacing between these slices will determine the printed thickness of the layers that will be seen in the final part. These layers partially determine the final surface quality and surface roughness of the final part, as well as how long the part will take to print. This height is an important compromise between print speed and surface quality, thus they are generally very thin. An average across several processes is around 100 µm or 0.1mm. Depending on the AM process, the range of layer heights is vast. In two-photon polymerization, layer features as small as 40nm or 0.00004mm can be created which is smaller than the wavelength of ultraviolet light. Microwave sintering can create layers up to 5cm or 50mm in depth. Regardless, when working in a process, the thicker the layer, the faster the build speed but the rougher the exterior becomes. Hence layer height is a trade-off between speed and quality.
Figure 2: T shaped structure with supports
Depending on the geometry of the model and the AM process involved, support structures may be needed. These support structures anchor overhanging areas of the final part to the build platform or other solid portions of the part. Imagine printing a 3D letter “T” starting from the bottom moving up to the top. The 3D printer would be able to print the main body of the letter without any issue. But as soon as it gets to the top, it would have a significant challenge to print the rest properly. The reason being there is nothing that would support the outreached arms of the letter. Thus some type of support structure is needed to be built up at the same time as the main body. When it reaches the point to print the arms, it would then be able to print on top of the main body and the support structure. An alternative option is to design the part for AM so that it does not need supports. For example, if the arms of the “T” were angled upward from the main body, the main body then becomes the support structure. The part would look more like the letter “Y”. Thus the letter Y could be considered a 3D print-optimized version of the letter T that doesn’t need support.
Steps 3-6: Printing
The specifics of printing depend on the process involved and will be described in much more detail further on in the report. Regardless, the 3D printer uses the computer code generated in the previous step to create an initial solid layer of material onto a build surface. The build surface could be a solid platform onto which the part is printed or a layer of unsolidified material that will support the part. This first layer is the first slice of the original 3D model that was calculated by the computer software in step two. Once the first layer is printed, the machine increments to the next height ready to print the next layer. This new height corresponds to the second slice of the original 3D model. The printer then creates a new solid layer based on that second slice. This new solid layer bonds to the previous layer making it one solid piece. The process repeats layer by layer until the final layer is finished. If the bonds between layers are weak, the build could fail or result in a structurally weak part. These anisotropic material properties can manifest as a weaker bond between layers than within layers– but can lead to beneficial properties for magnetic applications. There are some processes strategies,  and research projects that are addressing this concern with strength.
Step 7: Post-process
Finally, once a part is finished, it is removed from the build chamber and post-processed. This post process could take any number of forms. Some processes require removing support materials, . A few need to post cure the material to ensure a fully solid part. Laser melted parts need heat treating to relieve internal stresses that build up in the build process. Binder jetted parts can be placed in an oven to remove a sacrificial binder used in printing. Others are placed in ovens and sintered to increase the strength and density of the part. Infiltrants like glues or metals can be added to give more strength and higher density to the part, . Parts can undergo a chemical reaction to change the material for different material properties. Parts can be smoothed by chemicals, by blasting or tumbling to remove layer lines. Or parts can have some artistic flourish through hand painting, hydrographics, or even electroplating.
Methods and processes
Only recently was a standard designed for classifying the different ways something can be made using AM. In 2009, the American Society for Testing and Materials (ASTM) created a committee to define standards in AM technology. In 2010 they defined seven main technologies used in AM. This was given the standard designation: F2792–12a. As of December 2015, these ASTM standards were replaced with a new standard. ASTM joined with the International Organization for Standardization (ISO) to form ISO/ASTM 52900:2015. Despite having seven uniquely defined categories, there are many different processes within each category. Regardless, the overall method and the underlying principles discussed previously still apply. Variance exists only with the materials, deposition of layers, and methods of adhesion.
These seven categories will be explored in depth with their:
- alternate industrial or trade names
- a brief history
- description of method and materials
- advantages and disadvantages
These seven categories are (in alphabetical order):
- binder jetting
- directed energy deposition
- material extrusion
- material jetting
- powder bed fusion
- sheet lamination
- vat photopolymerization
The following are some common terms that are used when talking about these processes. They come from the ISO/ASTM definitions and are used throughout the seven category descriptions.
3D printer: the machine used for 3D printing.
Build chamber: the enclosed location within the 3D printer where the parts are fabricated.
Build platform: a base which provides a surface upon which the building of the part is started and supported throughout the build process.
Build space: the location where it is possible for parts to be fabricated, typically within the build chamber or on a build platform.
Build surface: the area where material is added, normally on the last deposited layer or for the first layer, the build surface is often the build platform.
Build volume: the total usable volume available in the machine for building parts.
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Lawrence Livermore National Laboratory (LLNL) researchers have become the first to 3D print aerospace-grade carbon fiber composites, opening the door to greater control and optimization of the lightweight, yet stronger than steel material.
The research, published by the journal Scientific Reports online on March 6, represents a “significant advance” in the development of micro-extrusion 3D printing techniques for carbon fiber, the authors reported.
“The mantra is ‘if you could make everything out of carbon fiber, you would’ — it’s potentially the ultimate material,” explained Jim Lewicki, principal investigator and the paper’s lead author. “It’s been waiting in the wings for years because it’s so difficult to make in complex shapes. But with 3D printing, you could potentially make anything out of carbon fiber.”
Carbon fiber is a lightweight, yet stiff and strong material with a high resistance to temperature, making the composite material popular in the aerospace, defense and automotive industries, and sports such as surfing and motorcycle racing.
Carbon fiber composites are typically fabricated one of two ways — by physically winding the filaments around a mandrel, or weaving the fibers together like a wicker basket, resulting in finished products that are limited to either flat or cylindrical shapes, Lewicki said. Fabricators also tend to overcompensate with material due to performance concerns, making the parts heavier, costlier and more wasteful than necessary.
However, LLNL researchers reported printing several complex 3D structures through a modified Direct Ink Writing (DIW) 3D printing process. Lewicki and his team also developed and patented a new chemistry that can cure the material in seconds instead of hours, and used the Lab’s high performance computing capabilities to develop accurate models of the flow of carbon fiber filaments.
“How we got past the clogging was through simulation,” Lewicki said. “This has been successful in large part because of the computational models.”
Computational modeling was performed on LLNL’s supercomputers by a team of engineers who needed to simulate thousands of carbon fibers as they emerged from the ink nozzle to find out how to best align them during the process.
“We developed a numerical code to simulate a non-Newtonian liquid polymer resin with a dispersion of carbon fibers. With this code, we can simulate evolution of the fiber orientations in 3D under different printing conditions,” said fluid analyst Yuliya Kanarska. “We were able to find the optimal fiber length and optimal performance, but it’s still a work in progress. Ongoing efforts are related to achieving even better alignment of the fibers by applying magnetic forces to stabilize them.”
The ability to 3D print offers new degrees of freedom for carbon fiber, researchers said, enabling them to have control over the parts’ mesostructure. The material also is conductive, allowing for directed thermal channeling within a structure. The resultant material, the researchers said, could be used to make high-performance airplane wings, satellite components that are insulated on one side and don’t need to be rotated in space, or wearables that can draw heat from the body but don’t allow it in.
“A big breakthrough for this technology is the development of custom carbon fiber-filled inks with thermoset matrix materials,” said materials and advanced manufacturing researcher Eric Duoss. “For example, epoxy and cyanate ester are carefully designed for our printing process, yet also provide enhanced mechanical and thermal performance compared to thermoplastic counterparts that are found in some commercially available carbon fiber 3D printing technologies, such as nylon and ABS (a common thermoplastic). This advance will enable a broad range of applications in aerospace, transportation and defense.”
The direct ink writing process also makes it possible to print parts with all the carbon fibers going the same direction within the microstructures, allowing them to outperform similar materials created with other methods done with random alignment. Through this process, researchers said they’re able to use two-thirds less carbon fiber and get the same material properties from the finished part… more