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Mature additive manufacturing technologies present new opportunities for R&D prototypes, high-end manufacturing facilities, and hobbyists alike.

Layerwise 3D-Printed Jaw Bone

The first 3D-printed full jaw replacement was made in laser-sintered titanium by the Belgian company LayerWise. Image: LayerWise

The additive manufacturing industry is populated by a broad family of technologies and some high-end systems can achieve impressive results with metals and polymers. Developments in ceramics may soon make a big impact. The low end of the market has recently been shaken up by the entry of some very low-cost systems that are causing a lot of excitement in the hobbyist market.

Metal parts made by laser sintering of powders top the list in performance. A wide range of stainless and tool steels, titanium and nickel alloys, and cobalt-chrome, as well as copper, aluminum, and precious metals can all be formed in machines built by companies such as EOS (Munich), Concept Laser (Lichtenfels, Germany), Renishaw Inc. (Wotton-under-Edge, U.K.), and Phenix Systems (Riom, France). Metal parts are fully dense, with a uniform microstructure due to the localized melting of a static powder bed. Titanium parts meet American Society for Testing and Materials (ASTM) standards for wrought titanium and exceed the strength and toughness of cast materials.

Laser cladding systems, such as those built by Optomec (Albuquerque, N.M.) and POM Group Inc. (Auburn Hills, Mich.), operate by jetting metal powders through a nozzle directed at a focused laser spot. These systems are able to build up parts from different metals in different locations, and are also able to effect repairs on damaged parts.

Medical implants are a very lively market for additive manufacturing metal parts. Recently, a complete lower jaw was fabricated in titanium by the Belgian company LayerWise on an EOS machine, and subsequently coated with a bioceramic by plasma spraying. Smaller custom-fit cranial implants, as well as dental implants and copings, are becoming more and more common.

More than 30 different systems make plastic parts of some type. Unlike laser-sintered metals, polymeric parts generally don't meet the same standards as conventionally processed materials. This shortcoming has relegated most processes to design prototyping and display models.

The additive manufacturing industry was founded in the mid-1980s by 3D Systems, Rock Hill, S.C., with a technology called stereolithography, which is still one of the most widely used and profitable methods. It is moderately fast, accurate, and very reliable. It is also laser-based, but rather than directing the laser onto a bed of powder, the laser is focused on the surface of a bath of photopolymer that is selectively cured in layers. The resulting parts—mostly epoxies and acrylics—are transparent and relatively tough. Stereolithography parts are useful for displaying the internal components of assemblies.

Extrusion technologies such as fused deposition modeling (FDM) build objects from engineering plastics. Stratasys, Eden Prairie, Minn., was the original developer of the technique and is still the industry leader in cumulative unit sales. In 2010 they were second only to 3D Systems in revenue. In operation, a polymeric filament, usually acrylonitrile butadiene styrene (ABS) or polyamide, is fed into an extrusion head that travels over a build plate, dispensing a bead of molten plastic. Because it relies on a single forming tool, the build rate is relatively low and the accuracy isn't as good as competing additive manufacturing systems. But the parts are nearly as tough as molded parts, and so they can be built right in to prototype assemblies and accurately simulate the function of a mass-produced molded plastic component.

From high end to hobbyist
In the past few years, a handful of new companies have redefined the low end of the 3D printer market. Previously, printers in that segment started at $15,000 and ran up to about $25,000. Now, BitsFromBytes (Clevedon, U.K.), MakerBot (New York), and Delta Micro Factory all produce machines priced below $5,000; RepRap and Fab@Home provide instructions for users to build their own 3D printers. These machines mostly use the same architecture that Stratasys developed, but with a cheap and modern design developed at University of Bath, U.K. While the quality of parts produced by these systems is quite variable, the low price point attracts a large number of hobbyists, 'makers,' and development engineers who are on a tight budget.

These low-end systems represent a new generation of 3D printers that have become possible because many of the original technology patents are starting to expire, allowing inventors to develop new machines based on variants of these older technologies. Unlike laser-based technologies that carry a higher equipment cost, those based on extrusion, inkjet printing, and projection technologies seem to be poised to join the revolution at the low end of the market.

Laser-based techniques have an inherent limitation in the rate they can build parts because there is only one forming tool, the laser spot, that must address all points in the object being built. Envisiontec, Germany, uses a video projection digital light processing (DLP) device to project an optical image onto a window on the bottom of the photopolymer bath. The build rate is high, but the resolution is limited by the pixel density of the projector. A 2011 infringement lawsuit, judged in favor of 3D Systems against Envisiontec, hasn't prevented small developers from creating platforms using this technology.

The earliest additive manufacturing system based on inkjets was developed at the Massachusetts Institute of Technology (MIT) in the early 1990s, and some of these patents have recently expired. Z Corp. is the most successful licensee, using off-the-shelf bubble-jet printheads dispensing water-based ink onto a water-reactive powder. It is the fastest machine on the market and, by virtue of the inkjet technology, it is the only full-color system in the world today. Although Z Corp. has firm patent coverage on their own implementation—and their recent acquisition by 3D Systems ensures their vigorous defense—the opening up of the MIT base patents offers developers some intriguing options. [Editor's note: James F. Bredt, author of this article, served on the team that developed the inkjet system at MIT.]

A somewhat later entry with an inkjet technology is Objet, Billerica, Mass. They use a drop-on-demand inkjet head to dispense photopolymer onto a build plate, which is cured in a layer-wise manner by an ultraviolet lamp. Although each droplet is small, the high degree of parallelism in the forming operation makes Objet faster than stereolithography, while giving it higher accuracy. It is probably the most accurate system around that still builds parts at a reasonably high rate. They are able to dispense multiple materials at the same time through different channels of their head, allowing for functionally gradient materials to be produced. The most widely advertised entry is an elastomeric system in which the stiffness can be tailored in different regions in a part. One example is a plastic electronic housing with a rigid body and flexible button pads built in.

Materials matter
Ceramic materials haven't achieved the wide use of metals and polymers in the additive manufacturing industry. They don't respond well to laser sintering, but the porous materials produced by other processes have found some specialty applications. The inkjet-over-powder process developed at MIT was originally conceived for refractory ceramics, and several companies have continued with that application: Z Corp., EX-One (Irwin, Pa.), VoxelJet (Augsburg, Germany), and Viridis3D LLC (Lowell, Mass.) all practice some type of ceramic process. Viridis3D, a materials developer for additive manufacturing applications, has introduced a ceramic material that is currently being used by Shapeways. Their materials are also being examined for use in fuel cells, aerospace, and molten metal processing.

By dispersing ceramic powders in photopolymer, several companies have introduced ceramic-filled materials that can be processed into dense ceramics by furnace processes, albeit with large amounts of part shrinkage. Similar issues attend extrusion techniques with ceramic powders dispersed in wax.

Research efforts at universities abound, and are too numerous to discuss in depth. To cite one example, a research group at the University of Illinois, Urbana-Champaign—headed by Jennifer Lewis, Hans Thurnauer Professor of Materials Science and Engineering and director, F. Seitz Materials Research Laboratory—has created an impressive array of concentrated inks and gels that dispense by extrusion through micronozzles. Their platform spans the whole gamut: Metals, ceramics, solvent-based polymers, biopolymers, and microfluidics.

Besides looking for industrial applications, a big driver in academic research is the quest for tissue and organ replacement. A scaffold of a biocompatible polymer is built and infiltrated with stem cells, which grow into the artificial organ. The technical problems are huge, but research efforts such as those being carried out in the Lewis Group are making steady progress in fabricating nanostructured lattices for tissue scaffolds, creating branching microvascular architectures, and dealing with the inoculation and incubation of the living cells.

The best of old and new technologies
The thrill of taking a concept to a physical model in a matter of hours is quite intoxicating, as thousands of new designers at the low end of the market are discovering. 3D printing is simultaneously a new and an old technology. Many people are just learning about the industry because of systems that have only just arrived onto the market.

Most consumer products today are developed with aid of 3D-printing techniques that have been available for nearly three decades. The older practitioners of the art are just coming to realize that they are no longer driving the industry that has now become accessible to hobbyists, students, and amateur developers who now call themselves 'makers.'

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