Additive manufacturing is a process of making a three-dimensional solid object of virtually any shape from a digital model. Additive manufacturing is achieved using an additive process, where successive layers of material are laid down in different shapes. Additive manufacturing is also considered distinct from traditional machining techniques, which mostly rely on the removal of material by methods such as cutting or drilling (subtractive processes).
The first working additive manufacturing process was created in 1984 by Chuck Hull of 3D Systems Corp. Since the start of the 21st century there has been a large growth in the sales of these machines, and their price has dropped substantially.
Additive manufacturing technology is used for both prototyping and distributed manufacturing with applications in architecture, construction (AEC), industrial design, automotive, aerospace, military, engineering, civil engineering, dental and medical industries, biotech (human tissue replacement), fashion, footwear, jewelry, eyewear, education, geographic information systems, food, and many other fields.
The term Additive Manufacturing refers to a collection of technologies where materials are selectively accumulated to build, grow, or increase the mass of an object layer-by-layer until a three-dimensional object conforms to its digital model. Objects that are manufactured additively can be found throughout the product life cycle, from pre-production (e.g. rapid prototyping) to full-scale production (e.g. rapid manufacturing), in addition to tooling applications and post-production customization.
ASTM F42 Committee on Additive Manufacturing Technologies publishes the official terminology standard for the industry. Active Standard F2792-12a "Standard Terminology for Additive Manufacturing Technologies defines Additive Manufacturing as the process of joining materials to make objects from 3D model data, usually layer upon layer, as opposed to subtractive manufacturing methodologies. Synonyms: additive fabrication, additive processes, additive techniques, additive layer manufacturing, layer manufacturing, and freeform fabrication. ASTM F2792-12a generically defines seven process classifications for additive manufacturing, specifically Binder Jetting, Directed Energy Deposition, Material Extrusion, Material Jetting, Powder Bed Fusion, Sheet Lamination, and Vat Photopolymerization.
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A number of additive processes are now available. They differ in the way layers are deposited to create parts and in the materials that can be used. Some methods melt or soften material to produce the layers, e.g. selective laser melting (SLM) or direct metal laser sintering (DMLS), selective laser sintering (SLS), fused deposition modeling (FDM), while others cure liquid materials using different sophisticated technologies, e.g. stereolithography (SLA). With laminated object manufacturing (LOM), thin layers are bonded and cut to shape and joined together (e.g. paper, polymer, metal). Each method has its own advantages and drawbacks. The main considerations in selecting a system or process for a given part are based on functional goals. The materials that are joined by these additive manufacturing processes are specialized to perform in the dedicated apparatus on which they are to run. For example; powders to be fused must be capable of absorbing energy, jetted binders must be dispensable and polymers must respond to controlled activation. In general terms, one must select from the material choices that are offered by a given system menu. System manufacturers support the distribution of these materials to their user community.
The binder jetting process was developed by MIT professors Dr. Emanuel Sachs and Dr. Michael Cima in the late 1980’s, with their patent being published in 1993. MIT licensed this technology to a number of companies, including Z Corporation. Z Corporation was later acquired by 3D Systems in 2012.
Binder jetting is also known by the names inkjet powder printing and 3D printing. MIT trademarked this specific process under the name “3D printing," which was later popularly adopted to describe most additive manufacturing processes versus just binder jetting. It should be noted that binder jetting, or 3D printing, is merely one of seven additive manufacturing processes. For this reason, 3D printing is not technically synonymous with additive manufacturing.
The binder jetting process shares some similarities with document printers, which is what gave rise to the name 3D printing. The most notable of these similarities is the jetting process. While a document printer will selectively jet ink onto a page, the binder jetting process selectively jets adhesive onto a bed of powder. A binder jet machine can have as many as five print heads, with each print head boosting as many as 300 jets.
Like all additive manufacturing processes, it begins with the processing of a digital geometric model into layers of finite height. The binder jetting machine will selectively bind (glue) one layer of powder at a time, each succeeding layer being swept over the previous one (sweep, bind, sweep, bind, etc.). The end result will be a cube of power with a number of solid parts inside. These parts must be excavated from the cube of powder and post-processed with an air compressor to remove any residual powder. In some cases, a heat treatment is also used in the post-processing. Any unused powder can often be used again. The most common material used is a gypsum (chalk) based powder. Other materials range from plastic to metal.
The three main advantages of binder jetting over other additive manufacturing processes are the ability to create colored objects, speed, and cost. Binder jeters range from no color capabilities to full CMYK. Color is achieved through the combination of colored binders, much like the use of colored ink in a document printer. To include color in the input digital model, a .vrml file is used versus the traditional .stl. Binder jetting is able to build five to ten times faster than most other processes, boasting a vertical print time in the range of 5-28 mm/min. Bear in mind, however, that it is the overall volume of the object being created that will dictate the time requirements. Lastly, most gypsum based materials are relatively less expensive than the material for other processes. The overall low cost of the process is reinforced by the process spreading powder over the entire print bed, eliminating the need for support material, and the ability to recycle unused powder. Overall, binder jetting machines typically have very fine resolution in the z direction. This makes it difficult to observe that the object is layered and creates a more uniform looking object. Layer heights average around 100 microns.
The process comes with its disadvantages as well, the three most notable being the object’s fragility, required post-processing for cleaning and hardening, as well as the bumpy, grain-like surface finish. A hardening agent can be added in post-processing that both strengthens the object and gives a smoother surface, yet can be tedious to apply.
Directed Energy Deposition
Directed Energy Deposition (DED) is the ASTM term used to describe a family of additive technologies, which essentially adapt welding techniques to build up material into three-dimensional near net shapes. Geometry is created when feedstock material (typically metal wire or powder) is fed into and melted by a focused energy source generally resulting in a melt pool. As the melt pool is moved around (“directed”) the wake of molten material left behind quickly solidifies as added material. The energy sources most typically used include arc, laser, or electron beam.
Unlike many additive technologies, DED is most often used to add material to existing parts, preforms, etc. Although cladding and welding have a long history of use to add material, the automated use of these techniques to build up layers into near net shape parts was pioneered by David Keicher of Sandia National Labs who invented Laser Engineered Net Shaping (LENS) which was later licensed to Optomec.
Compared to some other additive processes DED has seen less frequent implementation, however recent hybrid approaches combining it with CNC milling promise more extensive adoption of this technology in the future.
Fused deposition modeling (FDM) was developed by S. Scott Crump in the late 1980s and was commercialized in 1990 by Stratasys. With the expiration of patent on this technology there is now a large open-source development community this type of 3D printer (e.g. RepRaps) and many commercial and DIY variants, which have dropped the cost by two orders of magnitude.
Fused deposition modeling uses a plastic filament or metal wire that is wound on a coil and unreeled to supply material to an extrusion nozzle, which turns the flow on and off. The nozzle heats to melt the material and can be moved in both horizontal and vertical directions by a numerically controlled mechanism that is directly controlled by a computer-aided manufacturing (CAM) software package. The model or part is produced by extruding small beads of thermoplastic material to form layers as the material hardens immediately after extrusion from the nozzle. Stepper motors or servo motors are typically employed to move the extrusion head.
Various polymers are used, including acrylonitrile butadiene styrene (ABS), polycarbonate (PC), polylactic acid (PLA), high density polyethylene (HDPE), PC/ABS, and polyphenylsulfone (PPSU). In general the polymer is in the form of a filament, fabricated from virgin resins. Multiple projects in the open-source community exist that are aimed at processing post-consumer plastic waste into filament. These involve machines to shred and extrude the plastic material into filament.
FDM has some restrictions on the shapes that may be fabricated. For example, FDM usually requires a support structure for over-hanging geometries which can be broken away during finishing.
The material jetting process was first developed by Japanese company Brother Industries, Ltd. in the late 1980’s, with their patent being published in 1991. Similar patents followed from Richard Helinski in 1992, Sanders Prototyping, Inc. (which changed its name to Solidscape, Inc. in 2000) in 1996, and Objet Geometries Ltd. (which merged with Stratasys, Inc. in 2012) in 2001. It was Objet who would become world-renown for their quality PolyJet technology.
Material jetting uses a number of print heads (similar to a document inkjet printer) to selectively jet build material and support material into place on an xy layer before raising in the z direction to build up the object layer by layer. This differs from the binder jetting process as the build material itself it dispersed versus an adhesive. This also differs from fused deposition modeling as only drops of material are jetted versus strands. Post-processing is usually required and consists of removing support material with a water-jet.
The most common material used in material jetting is a liquid photopolymer. This material will remain a liquid until exposed to UV light, upon which it hardens into a solid. UV lights are typically attached to either side of the print head, so that the material will solidify the instant it is laid down. While other additive manufacturing processes also utilize photopolymers, material jetting differs by jetting the material versus it resting in a vat.
The main advantage that material jetting holds over other processes is its ability to selectively distribute and mix materials. Whereas in the photopolymerization process the vat can only hold a single material, material jetting can selectively place a number of materials in different parts of the model as well as blend these photopolymers to create composites called “digital materials”. There are nearly 1,000 different digital materials provided through Stratasys’ polyjet line. Materials range from rigid to flexible, high temperature to biocompatible. Let also this advantage, most machines are able to print in an incredible z-resolution of 16 microns, resulting in extraordinarily smooth parts. The two main disadvantages of objects created through this method is that they tend to be brittle as well as degrade when left in the sun. Material jetting is also slower than most other processes and the material is relatively more expensive.
Powder Bed Fusion
Another additive manufacturing approach is the selective fusing of materials in a granular bed. The technique fuses parts of the layer, and then moves the working area downwards, adding another layer of granules and repeating the process until the piece has built up. This process uses the unfused media to support overhangs and thin walls in the part being produced, which reduces the need for temporary auxiliary supports for the piece. A laser is typically used to sinter the media into a solid. Examples include selective laser sintering (SLS), with both metals and polymers (e.g. PA, PA-GF, Rigid GF, PEEK, PS, Alumide, Carbonmide, elastomers), and direct metal laser sintering (DMLS).
Selective Laser Sintering (SLS) was developed and patented by Dr. Carl Deckard and Dr. Joseph Beaman at the University of Texas at Austin in the mid-1980s, under sponsorship of DARPA. A similar process was patented without being commercialized by R. F. Housholder in 1979.
Selective Laser Melting (SLM) does not use sintering for the fusion of powder granules but will completely melt the powder using a high-energy laser to create fully dense materials in a layerwise method with similar mechanical properties to conventional manufactured metals.
Electron beam melting (EBM) is a similar type of additive manufacturing technology for metal parts (e.g. titanium alloys). EBM manufactures parts by melting metal powder layer by layer with an electron beam in a high vacuum. Unlike metal sintering techniques that operate below melting point, EBM parts are fully dense, void-free, and very strong.
Another method consists of an inkjet 3D printing system. The printer creates the model one layer at a time by spreading a layer of powder (plaster, or resins) and printing a binder in the cross-section of the part using an inkjet-like process. This is repeated until every layer has been printed. This technology allows the printing of full color prototypes, overhangs, and elastomer parts. The strength of bonded powder prints can be enhanced with wax or thermoset polymer impregnation.
In some printers, paper can be used as the build material, resulting in a lower cost to print. During the 1990s some companies marketed printers that cut cross sections out of special adhesive coated paper using a carbon dioxide laser, and then laminated them together.
In 2005, Mcor Technologies Ltd developed a different process using ordinary sheets of office paper, a Tungsten carbide blade to cut the shape, and selective deposition of adhesive and pressure to bond the prototype.
There are also a number of companies selling printers that print laminated objects using thin plastic and metal sheets.
Stereolithography was patented in 1987 by Chuck Hull. Photopolymerization is primarily used in stereolithography (SLA) to produce a solid part from a liquid. This process dramatically redefined previous efforts, from the Photosculpture method of François Willème (1830-1905) in 1860 through the photopolymerization of Mitsubishi`s Matsubara in 1974.
In digital-light processing (DLP), a vat of liquid polymer is exposed to light from a DLP projector under safelight conditions. The exposed liquid polymer hardens. The build plate then moves down in small increments and the liquid polymer is again exposed to light. The process repeats until the model has been built. The liquid polymer is then drained from the vat, leaving the solid model. The EnvisionTEC Perfactory is an example of a DLP rapid prototyping system.
Inkjet printer systems like the Objet PolyJet system spray photopolymer materials onto a build tray in ultra-thin layers (between 16 and 30 µm) until the part is completed. Each photopolymer layer is cured with UV light after it is jetted, producing fully cured models that can be handled and used immediately, without post-curing. The gel-like support material, which is designed to support complicated geometries, is removed by hand and water jetting. It is also suitable for elastomers.
Ultra-small features can be made with the 3D microfabrication technique used in multiphoton photopolymerization. This approach traces the desired 3D object in a block of gel using a focused laser. Due to the nonlinear nature of photoexcitation, the gel is cured to a solid only in the places where the laser was focused and the remaining gel is then washed away. Feature sizes of under 100 nm are easily produced, as well as complex structures with moving and interlocked parts.
Yet another approach uses a synthetic resin that is solidified using LEDs.
In this technique a 3D digital model is sliced by a set of horizontal planes. Each slice is converted into a two-dimensional mask image. The mask image is then projected onto a photocurable liquid resin surface and light is projected onto the resin to cure it in the shape of the layer.
In research systems, the light is projected from below, allowing the resin to be quickly spread into uniform thin layers, reducing production time from hours to minutes.
The technique has been used to create objects composed of multiple materials that cure at different rates.
Commercially available devices such as Objet Connex apply the resin via small nozzles.
Images courtesy of (left to right): 1. Renishaw 2. Stratasys 3. EOS 4. Solid Concepts 5. EOS