Let us ride the next wave in the industrial revolution by establishing a definition for additive manufacturing (AM). The most simple, but limiting, the definition would be “an ability to print 3D objects via a computer-generated design”. In many scenarios, this definition is appropriate; however, I think we can significantly expand upon this definition. Here is how I would define AM:
Additive manufacturing is the process by which simple or complex 3D objects are rendered from material(s) using a computer-generated design, generally with the intention to limit material waste, accelerate fabrication time, and reduce cost.
Notice that this definition contains the word “material(s)”, which is a key characteristic in AM. In this portion of the blog series, I will mainly focus on the materials aspect of our AM definition. I will cover the history, types of materials and their use in AM.
A quick walk through AM history
The history of materials used in AM starts with one of the most ubiquitous classes of materials – plastics and polymers. It is common for many to point out stereolithography (SLA) as being the first 3D printing approach to polymers.
I, however, believe the history commences with the inception of plastic extrusion as the precursor to mainstream polymer AM, which dates back to the 1820’s . With extrusion, a bulk filament or block is forced through a die with restricted dimensions, for instance, a nozzle, which deforms of modifies the shape of the filament.
Typically the modification is done by mechanical force and/or thermal heating. This is very similar to how most fused filament fabrication (FFF) printers operate, as shown in the figure below.
The metals AM origin story finds its roots in powder metallurgy and the advancement of lasers. The technique used for 3D printing of metals is commonly referred to as selective laser sintering (SLS) or direct laser melting/sintering (DLM/S) and has been around since the 1980’s .
In this approach, specially refined metal powders are placed on a tray, and a laser is used to raster patterns. The heat delivered by the laser is critical as it enables densification of the powder. Once a pass is completed, a new layer of powder is applied and the process is repeated.
I think it is worth mentioning 3D fabrication within the semiconductor industry due to its similarity and influence on AM. The transistors that power the computer I’m writing on are fabricated using light projected onto photosensitive films that are masked to obtain specific patterns. The non-exposed area can then be selectively etched away.
This is the basis for photolithography and has been the workhorse of the semiconductor industry for nearly 50 years . It is closely related to SLA used in AM of photosensitive polymers. The main difference between the photolithography approach in the semiconductor industry and AM is that it is subtractive.
The Material Cookbook
From a materials selection vantage point, AM is really making significant progress. Polymers used to be limited to acrylonitrile butadiene styrene (ABS) and Nylon, but now 3D printing can be done with poly-methyl methacrylate (PMMA), thermoplastic elastomers, and even wood filament.
Metals are showing particular growth since it is now possible to use lightweight structural metals such as Ti and Al. The use of Cu, Ni, and CoCr alloys are even more attractive since the composition can be tailored to achieve better product characteristics (e.g. density).
Ceramics parts have been produced using porcelain, glass-ceramics, and aluminium oxide. Refractory ceramics like silicon carbide are even possible with the addition of proper sintering aids. The material cookbook will most likely continue to grow, and I am sure AM chefs (e.g. scientist and engineers) will make use of those recipes (e.g. materials).
What’s so great about AM?
The applications of AM are considerably diverse . Polymer-based AM approaches have enabled hobbyist and small research endeavours to rapidly prototype components and evaluate their performance. This has led to the quick iterative design and improved engineering throughput.
The AM of metals has taken off with outfits like Desktop Metal™ enabling printing of complex structural components that are suitable for field deployment. This has found applications in the aerospace and automotive industries . Furthermore, AM is anticipated to eventually drive down production costs.
Printable ceramics are finding applications in patient customizable dental and medical implants. This alleviates any of the issues or complications that arise from generic implants. Without any doubt, AM will continue to find applications in many existing and future technologies.
It is surely the case that material scientists and engineers are working towards new solutions to grow the gamut of materials for AM. I am under the opinion that if this does indeed flourish, AM will become a transformative factor, thus propelling us into the next wave of the industrial revolution.
 M. Bauser, G. Sauer, and K. Siegert, Extrusion, ASM International, Materials Park, OH, 2006.
 Agarwala, Mukesh, et al., Direct selective laser sintering of metals, Rapid Prototyping Journal, 26-36, 1995.
 John H. Bruning, Optical lithography: 40 years and holding, Proc. SPIE 6520, Optical Microlithography XX, 652004, 2007.
 Huang, S.H., et al., Additive manufacturing and its societal impact: a literature review, The International Journal of Advanced Manufacturing Technology., 67, 2013.
 Desktop Metal™, press release, Burlington, MA, March, 19, 2018.
* Featured image: 3d Printer at HackIllinois 2016. Image provided by Rauhul Varma CC BY-SA 4.0 from Wikimedia Commons.
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