Piecing it Together: Additive Manufacturing Types and How They Work

Article by Tom Serres

Additive manufacturing, better known to the general public as 3D printing, is a remarkable breakthrough in modern manufacturing technology. Although most people are aware of additive manufacturing technology, many do not realize that there are several different types. They each employ a wide variety of techniques to create three-dimensional objects. Below are descriptions of the major additive manufacturing technologies and what you need to know to understand their workings.

Fused Deposition Modeling (FDM)

Fused deposition modeling is the most common type of additive manufacturing. Many manufacturers use it to make objects from various polymer filaments. Materials used in FDM 3D printing include ABS, PLA, nylon and PET. Blended filaments meant to provide greater strength or flexibility, such as carbon fiber and nylon blends, are also frequently used on FDM machines.
Aside from being one of the most common types of additive manufacturing, fused deposition modeling is also among the simplest. FDM printers use heated extrusion nozzles. These nozzles raise the temperature of a filament until it is soft and pliable. The heated filament is then laid down in layers to create the object being printed. Fused deposition modeling is a relatively quick and inexpensive process compared to other additive manufacturing technologies. As a result, FDM has become by far the most commonly used technology in the 3D printing industry, with an estimated 69 percent market share as of June of 2018.

Selective Laser Sintering (SLS)

Though less common than FDM, many manufacturers still use SLS technologies. While FDM machines melt filaments, SLS devices use focused lasers to sinter granules of material powder together into cohesive layers. Because of this sintering process, SLS prints have very strong cohesion between layers.
The main polymer printed using selective laser sintering is polyamide 12, or PA 12 for short. One of the difficulties of SLS technology is that it can only work with a few polymers, a fact which somewhat limits its use in printing plastic parts.
Where SLS truly sets itself apart from other types of additive manufacturing, however, is in its ability to print objects from metal. When applied specifically to metals, this technology is referred to as direct metal laser sintering, or DMLS. While the range of metals that could be printed was fairly limited in the technology’s early days, it has now grown to include steel, aluminum, titanium, gold, silver, platinum and even advanced nickel alloys.

Stereolithography (SLA)

No discussion of the different types of additive manufacturing can be complete without addressing stereolithography (SLA). First developed in 1983 by Chuck Hall, SLA machines were the original additive manufacturing technology. Then and now firms mostly use it to print objects from various resins. Today, resins with a wide range of physical properties are available for use in SLA-based additive manufacturing.
Stereolithography makes use of a concept known as photopolymerization to create solid objects from pools of liquid resin. Photopolymerization refers to a process by which certain resins solidify when exposed to UV light. SLA machines use lasers to focus UV light onto a build plate in a resin vat. This process solidifies the resin into one layer of the object being printed. The laser focuses on the correct coordinates using a mirrored galvanometer. It slowly moves to cure the resin in the pattern needed to create the design that is being printed. The machine then repeats this process over and over to create a complete object. Through this process, SLA printers are able to create higher-quality prints with greater detail than their FDM counterparts.

Digital Light Processing (DLP)

DLP is closely related to stereolithography but differs from the older additive manufacturing technology in one important regard. Like SLA, DLP involves focusing UV light on liquid resin in order to solidify it through photopolymerization. However, DLP replaces the laser beam used in SLA machines with a single flash of projected light to create an entire layer of solid material at once. The result is a printing process that is much faster than traditional SLA. The downside of digital light processing: the objects it creates are lower in detail than those created using stereolithography.

Electron Beam Melting (EBM)

Electron beam melting is similar in principle to SLS. Like selective laser sintering, manufacturers utilize EBM heavily in producing metal parts. The primary difference between these two technologies is that EBM uses a high-powered electron beam to melt metal powder, as opposed to the laser used by SLS devices. Though EBM can be a useful technology for printing metal objects, it is relatively expensive, usually requiring extensive post-processing work. As a result, firms mostly use EBM to prototype and build very small batches of specialized parts.

Material Jetting (MJ)

Material jetting is an additive manufacturing method that incorporates principles from both FDM and SLA printers. MJ devices deposit droplets of liquid resin, plastic or wax onto a build platform to create the layers of an object. The solidification process can involve either regular cooling or SLA-style curing with exposure to UV light. Because it deposits material in an extremely accurate manner, manufacturers can use material jetting to create high-quality prints with relatively little waste.

Bioprinting

A final type of additive manufacturing worth discussing is bioprinting, which allows for the printing of organic tissues. In bioprinting, tissues can be built up from gels or inks that are interspersed with layers of living cells. The printing medium is imbued with nutrient content that allows the cells to survive and multiply. This then eventually causes them to populate the entire structure to form a single piece of living tissue. Currently, we have only used bioprinting technology in fairly limited ways. Mostly researchers have employed it to create small, relatively simple pieces of tissue for analysis and testing. With the technology behind the process developing rapidly, however, some experts believe that it will soon be possible to print functional organs that could serve as transplants for patients.
Clearly, there are many different types of additive manufacturing, each of which has its own unique attributes and abilities. However, this technology is still in its infancy. As time goes on, the ability to print three-dimensional objects will only improve. This will allow 3D printing to take on a larger role in modern manufacturing and improve logistical processes.

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