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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