CHAPTER-20 Additive Manufacturing PDF

M20 KALP2244 08 GE C20 page 600 Chapter 20 Additive Manufacturing 20.1 Introduction 601 20.2 Additive Manufacturing Methodology 603 20.3 Extrusion-based Processes 606 20.4 Photopolymerization 608 20.5 Material Jetting 611 20.6 Powder Bed Processes 612 20.7 Laminated-object Manufacturing...

M20 KALP2244 08 GE C20 page 600 Chapter 20 Additive Manufacturing 20.1 Introduction 601 20.2 Additive Manufacturing Methodology 603 20.3 Extrusion-based Processes 606 20.4 Photopolymerization 608 20.5 Material Jetting 611 20.6 Powder Bed Processes 612 20.7 Laminated-object Manufacturing 617 20.8 Miscellaneous Processes 617 20.9 Emerging AM Applications 619 20.10 Direct Manufacturing and Rapid Tooling 619 20.11 Design for Additive Manufacturing 624 20.12 Additive Manufacturing Economics 626 Case Studies: 20.1 Functional Prototyping 602 20.2 Production of Athletic Shoes 610 20.3 Casting of Plumbing Fixtures 622 20.4 Implications of Powder Reuse 627 This chapter describes the technologies associated with additive manufacturing (AM), sharing the characteristics of computer integration, production without the use of traditional tools and dies, and the ability to rapidly produce a single part or small batches of parts on demand. All have the basic characteristics of producing individual parts layer by layer. Classes of processes used in additive manufacturing are reviewed, which include extrusion- based methods, photo polymerization, powder bed processes, sprayed powder approaches, and lamination-based methods. 600  M20 KALP2244 08 GE C20 page 601 Introduction 601 The practice of applying additive manufacturing techniques to the production of tooling that can be used in other manufacturing processes is described. The chapter closes with a summary of additive manufacturing design, opportunities, and eco- nomics. Typical parts made: A wide variety of metallic and nonmetallic parts for product design analysis, evaluation and finished products. Alternative processes: Machining, casting, molding, powder metallurgy, forging, and fabricating. 20.1 Introduction Making a prototype, the first full-scale model of a product, has traditionally involved flexible manufac- turing processes, and often required several weeks or months. Prototypes can now be quickly produced through subtractive processes (basically involving computer-controlled machining operations, described in Chapters 21–25) or by virtual prototyping (involving advanced graphics and software). An important advance is additive manufacturing, by which a solid physical model of a part is made directly from a three-dimensional CAD drawing without the use of tools, and allowing for extremely complex geome- tries (Fig. 20.1). Additive manufacturing is a suite of processes using different approaches, including photo polymerization, robot controlled extrusion, selective sintering, etc. This chapter describes additive manufacturing, formerly called rapid prototyping, whereby parts are built in layers. Developments in additive manufacturing began in the mid-1980s. The advantages of this technology include: Physical models of parts, produced from CAD data files, can be manufactured in a matter of minutes to hours, and thus allow the rapid evaluation of manufacturability and design effectiveness. In this way, additive manufacturing serves as an important tool in the product development process. A wide variety of materials are available, ranging from compliant rubber-like polymers to stiff polymers, metals, and ceramics. (a) (b) (c) Figure 20.1: Examples of parts made by additive manufacturing processes: (a) a selection of parts from fused-deposition modeling; (b) full-color model of an anatomical model; and (c) a speaker cover produced by the CLIP process. Source: (a) and (b) Courtesy of Stratasys, Inc., (c) Courtesy of Carbon, Inc.  M20 KALP2244 08 GE C20 page 602 602 Chapter 20 Additive Manufacturing Additive manufacturing operations can be used in some applications to produce actual tooling for manufacturing operations (rapid tooling, see Section 20.10). Thus, one can make tooling in a matter of a few days. Case Study 20.1 Functional Prototyping Toys are examples of mass-produced items with universal appeal. Because some toys are actually com- plex, the function and benefits of a computer-aided design (CAD) cannot be ensured until prototypes have been made. Fig. 20.2 shows a CAD model and a rapid-prototyped version of a water squirt gun (Super Soaker Power Pack Back Pack R water gun), which was produced on a fused-deposition model- ing machine. Each component was produced separately and assembled into the squirt gun; the prototype could actually hold and squirt water. The alternative would be to produce components on CNC milling machines or fabricate them in some fashion, but this can be done only at a much higher cost. By producing a prototype, interference issues and assembly problems can be assessed and, if neces- sary, corrected. Moreover, from an aesthetic standpoint, the elaborate decorations on such a toy can be more effectively evaluated from a prototype than from a CAD file. Also, they can be adjusted to improve the toy’s appeal. Each component, having its design verified, then has its associated tooling produced, with better certainty that the tooling, as ordered, will produce the parts desired. Additive manufacturing has now been transformed from a prototyping technology to a viable strat- egy for product production. In addition to traditional approaches to manufacturing, the use of additive manufacturing introduces opportunities, including the following: 1. A part produced from additive manufacturing can itself be used in subsequent manufacturing op- erations to produce the final parts. Also called direct prototyping, this approach can serve as an important manufacturing technology. 2. Mass customization can be achieved, where every part can be tailored to a particular user or an appli- cation. For example, it is possible to create prosthetic devices that are tailored to individual patients, based on scanned measurements of the person to produce an optimum fit and function. These pros- thetics have the ability to allow bathing and are more comfortable, and have fewer complications than cast supports. (a) (b) Figure 20.2: Additive manufacturing of a Super Soaker R squirt gun. (a) original CAD description of a toy; (b) fully functional toy produced through fused-deposition modeling. Source: (b) Courtesy of Rapid Models and Prototypes, Inc., and Stratasys, Inc.  M20 KALP2244 08 GE C20 page 603 Additive Manufacturing Methodology 603 Table 20.1: Characteristics of Additive Manufacturing Technologies. Supply Layer creation Type of Process phase technique phase change Materials Stereolithography Liquid Liquid layer curing Photopolymerization Photopolymers (acrylates, epoxies, colorable resins, and filled resins) CLIP Liquid Liquid layer curing Photopolymerization Similar to stereolithography Multijet/PolyJet Liquid Liquid layer curing Photopolymerization similar to stereolithography Material jetting Liquid Droplet deposition Solidification Polymers and wax Fused-deposition Solid Extrusion of melted Solidification Thermoplastics such as ABS, modeling polymer polycarbonate, and polysul- fone Binder jetting Powder Binder-droplet No phase change Ceramic, polymer, or metal deposition onto powder; sand powder layer Selective laser Powder Layer of powder Sintering Polymer powder such as nylon sintering Selective Laser Powder Layer of powder Solidification Metal powders such as stain- melting less steel, titanium, copper, and aluminum Electron-beam Powder Layer of powder Solidification Titanium and titanium alloys, melting cobalt chrome Laminated-object Solid Deposition of sheet No phase change Paper and polymers manufacturing material Laser-engineered Powder Injection of powder Solidification Titanium, stainless steel, alu- net shaping stream minum 3. Widespread application of additive manufacturing allows distributed manufacturing, so that parts can be produced anywhere and not only in factories. For example, prosthetics and braces can be produced at a hospital, avoiding time associated with orders and shipping. Thus, a child with a broken arm can be brought to a hospital and fixed with an optimum brace essentially as quickly as a typical conventional treatment. Almost all materials can be used through one or more additive manufacturing approaches, as outlined in Table 20.1. However, because their properties are more suitable for these unique operations, polymers are the most commonly used material today, followed by metals and ceramics (see Table 20.2); still, new processes are being introduced continually. The rest of this chapter serves as a general introduction to the most common additive manufacturing operations, describes their advantages and limitations, and explores the present and future applications of these processes. 20.2 Additive Manufacturing Methodology Additive manufacturing operations all build parts in layers, as summarized in Table 20.1. These processes use various physics to achieve a desired part, and a wide variety of materials can be used. All of the pro- cesses described in this section build parts layer by layer. In order to visualize the methodology employed, it is beneficial to think of the construction of a loaf of bread by stacking and bonding individual slices of bread on top of each other (hence the term additive). The main difference between the various additive processes lies in the method of producing the individual slices, which are typically 0.03 to 0.5 mm thick, although they can be thicker or thinner in some systems. All additive operations require dedicated software. Note as an example, the solid part shown in Fig. 20.3a. The first step is to develop a CAD file description of the part; the computer then constructs slices of the three-dimensional part (Fig. 20.3b). Each slice is analyzed separately, and a set of instructions is compiled in order to provide the AM machine with detailed information regarding the manufacture of the part. A trajectory often has to be planned in order to produce the slice. For example, Fig. 20.3d shows the  M20 KALP2244 08 GE C20 page 604 604 Chapter 20 Additive Manufacturing Table 20.2: Mechanical Properties of Selected Materials for Additive Manufacturing. Tensile Elastic Elongation strength modulus in 50 mm Process Material (MPa) (GPa) (%) Characteristics Stereolithography Accura 60 68 3.10 5 Transparent; good general-purpose ma- terial for additive manufacturing Somos 9920 32 1.35–1.81 15–26 Transparent amber; good chemical resis- tance; good fatigue properties; used for producing patterns in rubber molding WaterClear Ultra 56 2.9 6–9 Optically clear resin with ABS-like prop- erties WaterShed 11122 47.1–53.6 2.65–2.88 11–20 Optically clear with a slight green tinge; mechanical properties similar to those of ABS; used for rapid tooling DMX-SL 100 32 2.2–2.6 12–28 Opaque beige; good general-purpose material for additive manufacturing PolyJet FC720 60.3 2.87 20 Transparent amber; good impact strength, good paint adsorption and machinability FC830 49.8 2.49 20 White, blue, or black; good humidity resistance; suitable for general-purpose applications FC 930 1.4 0.185 218 Semiopaque, gray, or black; highly flexi- ble material used for prototyping of soft polymers or rubber Fused-deposition modeling Polycarbonate 52 2.0 3 White; high-strength polymer suitable for additive manufacturing and general use Ultem 9085 71.64 2.2 5.9 Opaque tan, high-strength FDM mate- rial, good flame, smoke and toxicity rat- ing. ABS-M30i 36 2.4 4 Available in multiple colors, most com- monly white; a strong and durable mate- rial suitable for general use; biocompati- ble PC 68 2.28 4.8 White; good combination of mechanical properties and heat resistance CLIP Rigid polyurethane 45 1.9 100 Wide variety of colors Flexible polyurethane 29 0.86 280 Similar to rubber band Epoxy 88 3.14 5.2 Urethane methacrylate 46 2 17 Selective laser sintering Aluminum AlSi12 alloy 480 — 5.5 Common aluminum alloy for AM 17-4 stainless steel 1300 — 16 Properties are after heat treatment 316L stainless steel 600 190 40 After stress relief Titanium GR.5 1100 120 30 Properties are after stress relief WindForm XT 77.85 7.32 2.6 Opaque black polymide and carbon; produces durable heat- and chemical- resistant parts; high wear resistance. Polyamide PA 3200GF 45 3.3 6 White; glass-filled polyamide has in- creased stiffness and is suitable for higher temperature applications SOMOS 201 – 0.015 110 Multiple colors available; mimics me- chanical properties of rubber ST-100c 305 137 10 Bronze-infiltrated steel powder Electron-beam melting Ti-6Al-4V 970–1030 120 12–16 Can be heat treated by HIP to obtain up to 600 MPa fatigue strength  M20 KALP2244 08 GE C20 page 605 Additive Manufacturing Methodology 605 (a) (b) Side View Model Model Support Support (c) (d) Figure 20.3: The computational steps in producing a stereolithography (STL) file. (a) Three-dimensional description of part. (b) The part is divided into slices; only 1 in 10 is shown. (c) Support material is planned. (d) A set of tool directions is determined to manufacture each slice. Also shown is the extruder path at section A–A from (c) for a fused-deposition-modeling operation. path of the extruder in one slice, using the fused-deposition-modeling operation (Section 20.3.1). Similar paths will also be planned for the traverse of a laser in a powder bed process (Section 20.6). Other pro- cesses, such as binder jetting or CLIPS, do not require a path to be generated, but still need a definition of the desired slice. Triangular tessellation of surfaces has become an industry standard and is widely used for the geometry definition (see Section 38.4.2). The production of a path requires operator input, both in the setup of the proper computer files and in the initiation of the production process. Following this stage, the machines generally operate unattended and produce a rough part after a few hours, or a few minutes for smaller parts. The part is then subjected to a series of finishing operations, such as sanding and painting, in order to complete the process.  M20 KALP2244 08 GE C20 page 606 606 Chapter 20 Additive Manufacturing a Ceiling within Desired part Gussets Island an arch Ceiling (a) (b) (c) (d) (e) Figure 20.4: (a) A part with a protruding section that requires support material. (b)–(e) Common support structures used in additive manufacturing machines. Source: After P.F. Jacobs. The setup and finishing operations are very labor intensive and the production time is only a portion of the time required to make a functional prototype. In general, additive processes are much faster than subtrac- tive processes for limited production runs, taking as little as a few minutes to a few hours to produce a part. Supports. Complex parts, such as that shown in Fig. 20.4a, may be difficult to build directly. For such processes as fused deposition modeling or stereolithography, a common difficulty is encountered once the part has been constructed up to height a. The next slice would require the filament to be placed at a location where no material exists to support it. The solution is to produce a support material separately from the modeling material, as shown in Fig. 20.4b. Note that the use of such structures allows all of the layers to be supported by the material directly beneath them. The support is made of a less dense, less strong, or a soluble material, so that it can be removed after the part is completed. 20.3 Extrusion-based Processes 20.3.1 Fused-deposition Modeling In the fused-deposition-modeling (FDM) or fused filament fabrication (FFF) process (Fig. 20.5), a gantry-robot controlled extruder head moves in two principal directions over a table, which can be raised and lowered as required. The extruder head is heated and extrudes a (usually thermoplastic) polymer filament through a small orifice at a constant rate. The head follows a predetermined path (see Fig. 20.3d); the extruded polymer bonds to the previously deposited layer. The initial layer is placed on a foam foundation or other base. When a layer is completed, the table is lowered so that the next layer can be superimposed over the previous one. When the part is finished, it can easily be removed. In the FDM process, the extruded layer’s thickness is typically 125–325 µm; this thickness limits the best achievable dimensional tolerance in the vertical direction. In the x-y plane, however, dimensional accuracy can be as fine as 0.025 mm, as long as a filament can be extruded into the feature. Close examination of an FDM-produced part will indicate that a stepped surface exists on oblique exterior planes. If the roughness of this surface is unacceptable, subsequent polishing or smoothing with a heated tool can be performed. Also, a coating can be applied, often in the form of a polishing wax. Unless care is taken in applying these finishing operations, the overall dimensional tolerances may be compromised. An extreme application of FDM is big area additive manufacturing (BAAM), which can produce parts as large as 6 m × 2.3 m × 1.8 m, with a positioning accuracy of 25 µm. The feedstock in this process is injection molding compound (pellets, sometimes with carbon fiber reinforcement) instead of a filament, so that material costs are significantly lower than in other additive manufacturing processes. Even so, the filament in FDM can be a low cost material, often around $20–$40 per kilogram of spooled filament. Upon expiration of the initial patents for fused deposition modeling, a large number of machines based on FDM have been developed. Some do-it-yourself machines are now freely available as plans that can be downloaded from the Internet. Alternatively, some very inexpensive desktop machines have been marketed, based on these crowd-sourced designs, such as the system shown in Fig. 20.6.  M20 KALP2244 08 GE C20 page 607 Extrusion-based Processes 607 Thermoplastic filament z y x Plastic model Heated build head created in moves in x –y plane minutes Table moves in z-direction Fixtureless foundation Filament supply (a) (b) Figure 20.5: (a) Schematic illustration of the fused-deposition-modeling process. (b) Removing a part from an F370, a popular fused-deposition-modeling machine. Source: Courtesy of Stratasys, Inc. Figure 20.6: Low-cost additive manufacturing machine. The F1000, based on digital light printing stere- olithography (see Section 20.4.1). The maximum build space is 125 mm × 70 mm × 120 mm. Source: Courtesy of 3D Systems.  M20 KALP2244 08 GE C20 page 608 608 Chapter 20 Additive Manufacturing (a) (b) Figure 20.7: Continuous fiber fabrication (CFF). (a) Motorcycle brake lever produced with CFF using con- tinuous carbon fiber reinforcement. (b) The Mark X CFF machine. Source: Courtesy of Markforged, Inc. The general trend for FDM materials is that the higher strength polymers require a higher processing temperature. Thus, stronger materials are more difficult to process, and warpage will be a greater concern. One of the differences between desktop systems and industrial FDM machines is the ability to process ma- terials with better mechanical properties in the latter. Acrylonitrile butadiene styrene (ABS) and polylactic acid (PLA) are commonly used for prototyping and in desktop machines; nylon, polycarbonate (PC), and polyetheretherketone (PEEK) are common for high-strength components. FDM materials are available in a wide variety of colors. A recent development with FDM is continuous fiber fabrication (CFF), where a first extruder prints nylon in the desired pattern. A second head extrudes a continuous carbon, kevlar, or fiberglass fiber inside the part (Fig. 20.7). Control software allows placement of fiber in locations and orientations desired. Metal parts can be produced through two main methods: A plastic filament impregnated with metal powder can be used to produce the desired part. Once completed, the part is sintered to burn off the polymer and fuse the metal, as with powder injection molding (see Section 17.3.3). A metal paste can be extruded. This is commonly combined with a second print head that deposited a thermoplastic, allowing for direct inclusion of conductors inside a polymer part. Low-cost machines have enabled the development of maker spaces, where individual designers (typi- cally high school students) are given access to FDM equipment, sometimes for a nominal fee. Along with Internet-based services that accept CAD files, this trend has brought additive manufacturing capabilities to the general public. Moreover, because of the low cost and availability of these machines, researchers are now able to apply new and innovative materials to rapid prototyping machines. Recent novel approaches include printing of food or biological materials for making medical implants, printing of artificial organs (bioprinting), clothing, and shoes (Section 20.9). 20.4 Photopolymerization 20.4.1 Stereolithography A common additive manufacturing process, one that actually was developed prior to fused-deposition modeling, is stereolithography (STL), a term coined by Charles W. Hull in 1986. This process (Fig. 20.8) is based on the principle of curing (hardening) of a liquid photopolymer into a specific shape. A vat, containing a mechanism whereby a platform can be lowered and raised, is filled with a photocurable liquid-acrylate polymer. The liquid is a mixture of acrylic monomers, oligomers (polymer intermediates), and a photoinitiator (a compound that undergoes a reaction upon absorbing light).  M20 KALP2244 08 GE C20 page 609 Photopolymerization 609 Platform motion UV light source UV curable liquid Liquid surface c Formed part b Vat a Platform Figure 20.8: Schematic illustration of the stereolithography process. At its highest position (depth a in Fig. 20.8), a shallow layer of liquid exists above the platform. A laser, generating an ultraviolet (UV) beam, is focused upon a selected surface area of the photopolymer, and then moved around in the x–y plane. The beam cures that portion of the photopolymer (say, a ring-shaped portion), and thereby producing a layer of solid body. The platform is then lowered sufficiently to cover the cured polymer with another layer of liquid polymer; the sequence is then repeated until level b in Fig. 20.8 is reached. A cylindrical part, with a constant wall thickness, has thus been generated. Note that the platform has now been lowered by a vertical distance ab. At level b, the x–y movements of the beam define a wider geometry, thus a flange-shaped portion is being produced over the previously formed segment. After the desired thickness of the liquid has been cured, the process is repeated, producing another cylindrical section between levels b and c. Note that the surrounding liquid polymer is still fluid (because it has not been exposed to the ultraviolet beam), and that the part has been produced from the bottom up in individual slices. The unused portion of the liquid polymer can be used again to make another part or another prototype. Note that the term stereolithography as used to describe this particular process comes from the observa- tions that the movements are three dimensional (hence the word stereo) and the process is similar to lithog- raphy (see Section 28.7). Note also that, as in FDM, stereolithography will sometimes require a support ma- terial, depending on geometry. In stereolithography, this support often takes the form of porous structures. After its completion, the part is removed from the platform, blotted, and cleaned ultrasonically with an alcohol bath. The support structure is then removed, and the part is subjected to a final curing cycle in an oven. The smallest tolerance that can be achieved in stereolithography depends on the sharpness of the focus of the laser, typically being around 0.0125 mm. Oblique surfaces also can be produced, with high quality. Solid parts can be made by applying special laser-scanning patterns to speed up production. For exam- ple, by spacing the scan lines in stereolithography, volumes or pockets of uncured polymer can be formed within cured solid shells. When the part is later placed in a postprocessing oven, the pockets are cured and a solid part is produced. Similarly, parts that are to be investment cast (Section 11.3.2) will have a drainable honeycomb structure, which permits a significant fraction of the part to remain uncured. Total cycle times in stereolithography range from a few hours to one day, without requiring post- processing steps, such as sanding and painting. Depending on their capacity, the cost of the machines is in the range from $100,000 to $400,000. The cost of the liquid polymer is on the order of $80 per liter. The maximum part size that can be produced is 0.5 m × 0.5 m × 0.6 m. The layer height in STL is 25–100 µm, depending on the machine, and use a laser spot size of 50–150 µm. Stereolithography has been used with highly focused lasers to produce parts with micrometer-sized features. The use of optics required to produce such features necessitates the use of thinner layers and lower volumetric cure rates. When used to fabricate micromechanical systems (Chapter 29), this process is called microstereolithography.  M20 KALP2244 08 GE C20 page 610 610 Chapter 20 Additive Manufacturing Build platform UV curable resin Build direction Cured voxel Resin Dead zone Oxygen permeable Dead zone window Oxygen Projector permeable UV light window Figure 20.9: The CLIP process. An array of micro mirrors directs light to cure a layer, but the photopolymer next to the oxygen-permeable window does not cure because oxygen is a curing inhibitor. The cured poly- mer can be pulled out of the liquid photopolymer, with liquid photopolymer flowing into the interface to replenish the build layer. Another form of stereolithography is mask projection stereolithography or direct light processing (DLP) with the advantage of much higher rate of part production. In this process, a DLP device made up of millions of microscopic mirrors is used to direct UV light from a lamp or light emitting diode to expose the entire layer at once. 20.4.2 Continuous Liquid Interphase Production The continuous liquid interphase production (CLIP) process is illustrated in Fig. 20.9. CLIP uses a special window that is transparent to light and is permeable to oxygen, much like a contact lens. By controlling the oxygen diffusing through the window, a “dead zone” is created in the resin pool just tens of microns thick where photopolymerization cannot occur, as oxygen acts as an inhibiter. This ensures that a liquid layer will persist adjacent to the optics, regardless of light exposure. The projector transmits light in the desired pattern into the resin pool from underneath, curing the polymer above the dead zone. The build plate pulls the printed physical object out of the vat, at a speed low enough so that the cured material maintains contact with the uncured liquid and new liquid flows into the curing zone. The CLIP process is continuous, but does require discretization of layers from a CAD file and exposure of layers. The light is projected an entire layer at a time, not in raster fashion as in selective laser sintering or conventional stereolithography. The layer is produced through digital light processing (DLP) hardware that is also common in projector systems and some televisions. A DLP device consists of an array of micromirrors, each of which can direct light towards the build chamber if activated; by activating selected mirrors, “pixels” in the build space are activated, curing the polymer into voxels. A typical voxel dimension is 75 µm. Parts produced through the CLIP process must undergo secondary operations, consisting of, at least, cleaning and either a secondary UV flood cure or thermal cure in an oven. A variety of polymeric material chemistries are now available, and CLIPs can achieve production rates two orders of magnitude higher than other additive manufacturing processes. Case Study 20.2 Production of Athletic Shoes CLIP represents a breakthrough additive manufacturing process in that it allows manufacture of parts at high quantities, and provides a strategy for mass production for certain parts (see Section 37.2.2). Carbon3D, the developer of CLIP, has also developed designs and software to produce a metamaterial (Section 6.16) that has similar mechanical properties as polymer foam, but is easier to clean (Fig. 20.10).  M20 KALP2244 08 GE C20 page 611 Material Jetting 611 By placing more material or changing the metamaterial design where a higher stiffness is desired, it is possible to produce a shoe insole with tuned stiffness, that can also be varied by location. Adidas, well known for its athletic products and materials, partnered with Carbon3D to produce high performance footware, called the Futurecraft 4D (Fig. 20.11). Adidas had collected and maintained athlete data, which was used for the development of a stiffness-tuned midsole for the new shoe design. Adidas and Carbon3D have developed a digitized footwear-component creation process that elimi- nates the need for traditional prototyping or molding. CLIP also allowed Adidas to create a monolithic midsole that addresses precise needs related to movement, cushioning, stability, and comfort. Further, over the course of product development, CLIP enabled Adidas to evaluate more than 50 design itera- tions, a substantial increase when compared with what is achievable with traditional injection molding in the same amount of time. Moreover, engineers from both companies collaborated closely and tested nearly 150 resin iterations. The final midsole material is made of a dual cure resin that generates a polyurethane upon thermal cure. It is a stiff elastomer printed in a lattice structure to develop a high-performance midsole that also offers excellent durability and is aesthetically pleasing. Adidas expects to produce 100,000 such annually. Source: Courtesy Steven Pollack, Carbon and Adidas. Figure 20.10: The use of a metamaterial shoe sole. Note that the pattern in the metamaterial changes with location to obtain a desired mechanical performance. Source: Courtesy of Carbon, Inc. 20.5 Material Jetting Material jetting (MJ) is a class of additive manufacturing processes that include drop on demand (DOD) and the Polyjet related processes, also known as multijet modeling (MJM). The main difference is that PolyJet uses photopolymer feedstocks; DOD uses thermoplastics or wax. In both cases, a low viscosity is needed to produce droplet jets, which may require preheating of the build material. The PolyJet process is a form of material jetting where print heads deposit a photopolymer on the build tray. Ultraviolet bulbs, alongside the jets, instantly cure and harden each layer, thus eliminating the need for any postmodeling curing that is required in stereolithography. PolyJet results in a smooth surface with layers as thin as 16 µm that can be handled immediately after the process is completed. Two different materials are used: the material for the actual model, and a gel-like resin for support, such as shown in Fig. 20.4. Each material is simultaneously jetted and cured, layer by layer. When completed,  M20 KALP2244 08 GE C20 page 612 612 Chapter 20 Additive Manufacturing Figure 20.11: The Adidas Futurecraft 4D shoe, using a CLIP-produced metamaterial sole. Source: Courtesy of Carbon, Inc. the support material is removed by soaking in an aqueous solution. Build sizes have an envelope of up to 500 mm × 400 mm × 200 mm. The PolyJet process has capabilities similar to those of stereolithography and uses similar resins (Table 20.2). The main advantages of this process are the capabilities of avoiding part cleanup and lengthy post-process curing operations and the much thinner layers produced, thus allowing for better resolution. In DOD, a stream of a material droplets are ejected through a small orifice and deposited on a surface (target), using an ink-jet type mechanism. A second print head deposits a support material that is solu- ble in water or related solvent. DOD is commonly used for producing investment casting patterns (see Section 11.3.2). DOD is considered the most accurate form of 3D printing because of the absence of thermal stresses and because it is mainly used for small parts. Generally, a tolerance of ±0.1 mm can be achieved. A recent innovation is the nano particle jetting (NPJ) process, which uses a suspension of nanoparticles in a liquid carrier as the liquid printed in material jetting. It was noted previously that particles smaller than around 20 µm are difficult to spread onto a build chamber because they can easily become airborne and interfere with optics and lasers. The liquid carrier prevents the entrainment of small particles into air, and therefore allows the incorporation of much smaller particles than other processes. The build chamber is heated sufficiently to evaporate the carrier and bond the nanoparticles. Once completed, the particles are sintered to create fully dense parts. The use of small particles allows printing of detailed features and the development of superior mechanical properties. 20.6 Powder Bed Processes Powder Bed Processes involve a number of approaches that utilize powder as the workpiece material, and where the powder is deposited layer-by-layer in a bed or build chamber. Several powder application systems are used, but they typically involve a counter-rotating roller or a wiping mechanism. The deposited powder has limited green strength but can serve as a support for complicated parts. Powder spreading is a critical step in these processes. Some of the considerations associated with powder spreading are as follows:  M20 KALP2244 08 GE C20 page 613 Powder Bed Processes 613 1. The powder must be capable of spreading into thin layers. Because this can be compromised by moisture, polymer powders may need to be dried before they can be effectively spread. 2. Mean particle sizes of the powder are around the layer thickness, or slightly smaller; there is a range of powder sizes that can be used. Particles larger than the layer thickness will be pushed ahead of the wiper or the roller and are less likely to be part of a spread layer; particles that are too small are likely to become airborne and adhere to exposed surfaces. 3. The powder may be preheated to reduce laser or electron beam power required for melting. 4. Powder explosions or fire can result from static electric discharge; thus, safety protocols regarding equipment and worker grounding, oxygen-free shielding gases, and increased humidity must be carefully followed. 20.6.1 Selective Laser Sintering Selective Laser Sintering (SLS) is a process based on sintering (Section 17.4) of nonmetallic powders selec- tively into an individual object. Direct Metal Laser Sintering (DMLS) or Selective laser melting (SLM) is a related process used with metals; in both cases, material is always at least partially melted. The basic ele- ments in this process are shown in Fig. 20.12. Note that the bottom of the processing chamber is equipped with two cylinders: 1. A powder-feed cylinder, which is raised incrementally to supply powder to the part-build cylinder. 2. A part-build cylinder, which is lowered incrementally as the part is being shaped. In the SLS process, a thin layer of powder is first deposited in the part-build chamber. Then a laser beam, guided by a process-control computer using instructions generated by the three-dimensional CAD program of the desired part, is focused on that layer, tracing and sintering a particular cross section into a solid mass. The powder in other areas remains loose, but this powder can support the sintered portion; with some processes, separate support structures may still be needed. Another layer of powder is then deposited, and the cycle is repeated continuously until the entire three-dimensional part has been produced. Galvanometers Sintering laser Laser Optics Environmental- control unit Process chamber Roller mechanism Powder- Part-build Process-control feed cylinder computer cylinder Motor Motor Figure 20.12: Schematic illustration of the selective-laser-sintering process.  M20 KALP2244 08 GE C20 page 614 614 Chapter 20 Additive Manufacturing The feed chamber contains the desired part supported by unfused powder, called the cake, which has very low green strength. The loose particles are shaken or brushed off, and the part is recovered. The part may not require further curing to develop strength. The supports are removed or machined, and the part is then post-processed as required. A variety of materials can be used in this process, including polymers (such as acrylonitrile butadiene styrene, polyvinyl chloride, nylon, polyester, polystyrene, and epoxy), wax, metals, and ceramics (with appropriate binders). Generally the feed and build chambers are preheated. A major concern is the thermal management in the build chamber, as is the control of the powder during spreading and lasing. Powder that becomes airborne in the build chamber could coat sensors or optics and compromise productivity. Modern SLS ma- chines therefore use a constant flow of shielding gas that directs the powder away from sensitive machine elements. Shielding gases vary with the powder material; argon and nitrogen are common options. SLS parts are susceptible to shrinkage and warpage due to thermal stresses. Each layer that is built is produced on a previous layer; as the new layer cools, it shrinks, which can cause a part to curl upwards. In extreme cases, the part can collide with the wiper or roller depositing a new powder layer, necessitating a build to be aborted. Despite thermal effects, dimensional tolerances of ±0.1 mm can be achieved with well- designed parts. Layer thickness range from around 30–100 µm depending on the material. Stainless steel and titanium alloys generally produce the best part fidelity because of their lower thermal conductivities. SLS has a number of advantages over other AM processes. The material is generally isotropic and accu- rate (although not as good as stereolithography or material jetting) with very good mechanical properties. SLS does not usually need support materials, except for metal parts that are bonded to a build plate by support material to prevent part curl. The main drawback is the high cost of machines; metal-capable SLS machines cost around $300,000 for low-end machines, and can cost almost $1 million. 20.6.2 Electron-beam Melting A process similar to selective laser sintering and electron-beam welding (Section 30.6), electron-beam melting (EBM) uses the energy source associated with an electron beam to melt titanium or cobalt-chrome powder to make metal prototypes. The workpiece is produced in a vacuum, making the part build size limited to around 200 × 200 × 180 mm. Electron-beam melting is up to 95% efficient from an energy standpoint, as compared with 10–20% efficiency for selective laser sintering. In EBM, the supply powder and the build chamber are heated to near the material’s melting point, significantly reducing the energy needed to melt the metal and also reducing thermal stresses. Because the build chamber is at an elevated temperature, the melted metal solidifies more slowly and results in more fully dense parts. A volume build rate of up to 60 cm3 /hr can be obtained, with individual layer thicknesses of 0.050–0.200 mm. Parts may also be subjected to hot isostatic pressing (Section 17.3.2) to improve their fatigue strength. Although applied mainly to titanium and cobalt-chrome alloys to date, the process is being developed also for stainless steels, aluminum, and copper alloys. 20.6.3 Binder-jet Printing In the Binder-jet Printing (BJP) process, also known as Binder Jetting or Three-Dimensional Printing, a print head deposits an inorganic binder material onto a layer of sand, polymer, ceramic, or metallic powder, as shown in Fig. 20.13. A piston, supporting the powder bed, is lowered incrementally and with each step a layer is deposited and then fused by the binder. Binder-jet printing allows considerable flexibility in the choice of materials and binders used. Common powder materials are polymers (sometimes blended with fibers), metals, and foundry sand. Since multiple binder print heads can be incorporated into one machine, it is possible to produce full-color prototypes by having different color binders (Fig. 20.14). The effect is a three-dimensional analog to printing photographs using three ink colors (red, cyan, and blue) in an ink-jet printer.  M20 KALP2244 08 GE C20 page 615 Powder Bed Processes 615 Roller mechanism Powder Binder 1. Spread powder 2. Print layer 3. Piston movement 4. Intermediate stage 5. Last layer printed 6. Finished part Figure 20.13: Schematic illustration of the binder-jet printing process. (a) (b) Figure 20.14: Full color parts produced by binder jet printing. (a) A simple model of toy truck wheel; (b) a more detailed model of a human hand, with transparent and colored components. Source: Courtesy of Stratasys. A typical part produced by BJP from ceramic powder is a ceramic-casting shell (see Section 11.2.4), in which aluminum-oxide or aluminum-silica powder is fused with a silica binder. The molds are post- processed in two steps: (a) curing at around 150◦ C and (b) firing at 1000◦ to 1500◦ C. Printing of sand molds is a common practice; molds can be produced with blind risers and with cores, thus avoiding the complicated assembly operations associated with copes and drags (see Section 11.2.1). The parts produced through the BJP process are somewhat porous, and thus may lack strength. Three- dimensional printing of metal powders can also be combined with sintering and metal infiltration (see Section 17.4) to produce fully-dense parts, using the sequence shown in Fig. 20.15. Here, the part is pro- duced as before by directing the binder onto powders. However, the build sequence is then followed by sintering in order to burn off the binder and partially fuse the metal powders, just as is done in powder in- jection molding (Section 17.3.3). Common metals used in 3DP are stainless steels, aluminum, and titanium.  M20 KALP2244 08 GE C20 page 616 616 Chapter 20 Additive Manufacturing Binder deposition Infiltrating metal, permeates into PM part Microstructure detail Unfused powder Binder Metal powder Particles are loosely sintered; Infiltrated by binder is burned off lower-melting-point metal (a) (b) (c) Figure 20.15: Three-dimensional printing using (a) part-build, (b) sinter, and (c) infiltration steps to produce metal parts. Source: Courtesy of Kennametal Extrude Hone. The infiltrating materials typically are copper and bronze, which provide good heat-transfer capabilities as well as wear resistance. This approach represents an efficient strategy for rapid tooling (Section 20.10). Dimensional tolerances vary widely by machine manufacturer and feedstock in BJP. Sand molds and cores are commonly produced with layer thicknesses of 240–380 µm, but layers may be as low as 50 µm with some materials. A more recently developed process, known by its trade name of jet fusion, is based on BJP, but with a number of unique features. In jet fusion, A powder of polymer is spread in a build chamber. Binder is jetted onto the polymer to fuse the powder as desired in the layer. A detailing agent is jetted adjacent to the regions where the binder had been applied. The layer is then subjected to a heat source that cures the polymer containing binder. The function of the detailing agent needs some clarification. In any thermal curing approach, temper- atures are difficult to control and can lead to poor part resolution. The detailing agent prevents curing, so that the boundary between the cured part and the unaffected polymer has very good definition with sharp and smooth edges. In addition, the jet fusion process uses an array of sensors to determine the temperature distribution in the build chamber. If the sensors determine an area of the bed has too high or too low of a temperature compared to the optimum, the intensity of the UV light over the build chamber is varied accordingly, leading to improved mechanical properties and dimensional accuracy.  M20 KALP2244 08 GE C20 page 617 Miscellaneous Processes 617 Laser Optics X–Y positioning device Layer outline Laminating roller and crosshatch Sheet material Part block Platform Take-up roll Material supply roll (a) (b) Figure 20.16: (a) Schematic illustration of the laminated-object-manufacturing process. (b) Turbine proto- type made by LOM. Source: Courtesy of M. Feygin, Cubic Technologies, Inc. 20.7 Laminated-object Manufacturing Lamination involves laying down layers bonded adhesively to one another. Several variations of laminated- object manufacturing (LOM) are now available. Producing parts by LOM systems can be elaborate, where the more advanced systems use layers of paper or plastic with a heat-activated glue on one side. The desired shapes are burned into the sheet with a laser, and the parts are built layer by layer (Fig. 20.16). On some systems, the excess material must be removed manually after the part is made; the removal is simplified by programming the laser to burn per- forations in crisscrossed patterns. The resulting grid lines make the part appear as if it had been constructed from gridded paper, similar to graph paper. 20.8 Miscellaneous Processes 20.8.1 Laser-engineered Net Shaping Laser-engineered net shaping (LENS), also known as laser powder forming (LPF) involves the principle of using a laser beam to melt and deposit metal powder or wire, layer by layer, over a previously deposited layer (Fig. 20.17). The heat input and cooling have to be controlled precisely to develop a favorable microstructure. The deposition process is carried out inside an enclosed volume and in an argon environment, to avoid the adverse effects of oxidation, particularly on aluminum. It is suitable for a wide variety of metals and specialty alloys for the direct manufacturing of parts, including fully-dense tools and molds. The process can also be used for repairing thin and delicate components. There are other, similar processing methods, including controlled-metal buildup (CMB) and precision-metal deposition (PMD, a trade name). LENS has been found suitable for incorporation into hybrid machines that have both additive and sub- tractive (machining) manufacturing capabilities. The advantages are that complex shapes can be quickly produced without refixturing, with high dimensional tolerance and surface finish, and with little scrap. Usually, this operation involves the incorporation of a LENS deposition head in combination with a ma- chining or turning center (see Section 25.2). This is a compelling combination, since LENS on its own does not maintain tight tolerances; ±1 mm is typical of the process limitations.  M20 KALP2244 08 GE C20 page 618 618 Chapter 20 Additive Manufacturing Laser Powder supply Lens Focused Powder supply tube laser beam Powder nozzle Converging powder streams Deposited material Build table Figure 20.17: Schematic illustration of the laser-engineered net shaping (LENS) process. 20.8.2 Friction Stir Modeling The friction stir modeling (FSM) process shares several similarities with friction stir welding (Section 31.4). In this process, powder is delivered to a build location by pushing it into a rotating tube. The friction between the powders and the substrate are sufficiently high to densify the powder and develop a solid material. Because the process is at solid state, there is no appreciable heat-affected zone. Friction stir modeling has been successfully applied to magnesium, aluminum, and titanium, and has the advantage of being able to change the deposited material during a build. For example, a lightweight aluminum part can possess an integral hardened surface for wear resistance. The equipment used for FSM involves conventional CNC milling machines (Section 24.2), modified to deliver the desired powder. Typical layer thickness is around 100 µm, and surface finish is generally poor, requiring subsequent machining to achieve smooth surfaces; a machining allowance is therefore essential. 20.8.3 Wire and Arc Additive Manufacturing As described in Section 30.4.3, gas metal arc welding and gas tungsten arc welding are commonly incor- porated into robot welding systems; they involve material transfer from an electrode or filler material into a weld joint. The same approach can be used to deposit material in a controlled manner; a welding end- effector on a robot provides a platform for large volume, large deposition rate additive manufacturing. This arrangement is known as wire and arc additive manufacturing (WAAM), with a unique feature of the ability to produce designs that are not based on layers; the robot can follow any trajectory. 20.8.4 Hybrid Approaches Additive manufacturing has specific advantages in certain applications, but one of the drawbacks com- pared to machining is the inability to hold tight tolerances or to achieve a desired surface finish. One solution is to combine additive and subtractive processes in the same machine. To date, the most common hybrid approaches involve combining either laser engineered net shaping or selective laser sintering with a CNC machining center. With LENS, a part will generally be produced with a generous machining allowance, since the process can rarely hold tolerances better than a millimeter over 50 mm. The constructed part is then machined without refixturing. With SLS, the machining operations are performed after a layer or a group of layers is produced. Even though the build chamber can be disturbed by machining, fresh powder fills in machined areas while the wiper spreads the powder into smooth layers.  M20 KALP2244 08 GE C20 page 619 Direct Manufacturing and Rapid Tooling 619 A wide range of capabilities have recently been developed into the machinery, including the ability to combine materials to produce composites or to tailor mechanical properties, the incorporation of sensors to detect defects during printing, and automated handling of completed parts. 20.9 Emerging AM Applications The low cost and high reliability of additive manufacturing has directly led to its widespread application in a number of areas that are far removed from industrial production. Bioprinting. The production of medical devices is well established and can also be done by additive man- ufacturing. An emerging area is bioprinting, involving the printing of living cells. Using processes related to fused-deposition modeling or binder-jet printing, cells are suspended in a liquid carrier or bioink, producing a construct. This approach allows printing of cells in desired structures and concentrations. While applications are emerging, bioprinting has the potential and aggressive goal of producing living functional tissue, such as organ transplants or tissue that can be used in drug studies. Current limitations are the cell survival during and after the printing process. Architectural applications. Processes that are based mainly on fused deposition modeling have been used to build buildings or various structures from extruded concrete. The approaches generally use tower robots that place concrete along the periphery of the desired structure. Trowels can be located near the extruder head to build near-vertical walls that match from layer to layer, a process variant called contour crafting. Alternative approaches use binder-jet printing with sand to produce structures. Permanent habitats on the moon or on Mars are now expected to be produced through additive manufacturing, using as much native soil as possible, given the cost of transport of materials. 20.10 Direct Manufacturing and Rapid Tooling While extremely beneficial as a demonstration and visualization tool, additive manufacturing processes also have been used to produce functional parts. There are two basic methodologies involved: 1. Direct production of engineering metals, ceramics, and polymer components or parts. 2. Production of tooling or patterns by additive manufacturing, for use in various manufacturing operations. Additive manufacturing operations can be also used to manufacture parts directly, referred to as direct manufacturing. This approach includes the case where a part involves a machining or grinding allowance or requires further finishing operations. Thus, the component is generated directly to a near-net shape, from a computer file containing part geometry. The main limitations to the widespread use of additive manufacturing for direct manufacturing, or rapid manufacturing, are as follow: Raw-material costs are high, and the time required to produce each part is too long to be viable for large production runs. However, there are many applications in which production runs are suffi- ciently small to justify direct manufacturing through additive manufacturing technologies, or where the required material properties are attainable. The long-term and consistent performance of rapidly manufactured parts (as compared with the more traditional methods of manufacturing them) should be considered, especially with respect to fatigue, wear, and life cycle. Much progress is being made to address and respond to these concerns in order to make rapid manufacturing a more competitive and viable option in manufacturing.  M20 KALP2244 08 GE C20 page 620 620 Chapter 20 Additive Manufacturing Several methods have been devised for the rapid production of tooling (RT) by means of additive manufacturing processes. The advantages to rapid tooling include the following: 1. The high cost of labor and the shortening supply of skilled patternmakers can be overcome. 2. There is a major reduction in lead time. 3. The integral use of CAD technologies allows the use of modular dies, with base-mold tooling (match plates) and specially fabricated inserts. 4. Chill- and cooling-channel placement in molds can be optimized more easily, leading to reduced cycle times. Conformal cooling is a strategy for producing cooling channels that are located in a way to maximize heat extraction from a mold or die, while preserving mechanical strength (Fig. 20.18). 5. Shrinkage due to solidification or to thermal contraction can be compensated for automatically, through software, to produce tooling of the proper size and, in turn, to produce the desired parts. Large flat parts should be oriented at an angle or vertically to minimize the cross-sectional area of each layer, thereby minimizing warpage. The main shortcoming of rapid tooling is the potentially reduced tool or pattern life, as compared to those obtained from machined tool and die materials, such as tool steels and tungsten carbides (Chapter 21). Temperature (8C) 270 240 (a) (b) 210 180 150 (c) (d) Figure 20.18: The benefit of conformal cooling in molds produced by additive manufacturing. The images on the left show conventional (machined or drilled) cooling channels, and those on the right show con- formal cooling channels that can be produced in additive manufactured molds. The top images depict the channel layout; the bottom images the temperature distributions in the mold during production. Note that the temperature distribution is more uniform on the molds with conformal cooling, leading to less warpage and higher production rates. Source: Copyright image provided courtesy of Milacron product brand DME Company showing their TruCoolTM technology.  M20 KALP2244 08 GE C20 page 621 Direct Manufacturing and Rapid Tooling 621 1. Pattern creation 2. Tree assembly 3. Insert into flask 4. Fill with investment Crucible Heat Molten metal Grinding spatter Workpiece 5. Wax melt-out/burnout 6. Fill mold with metal 7. Cool 8. Finish Figure 20.19: Manufacturing steps for investment casting with rapid-prototyped wax parts as blanks. This method uses a flask for the investment, but a shell method also can be used. Source: Courtesy of 3D Systems, Inc. A number of strategies have developed to incorporate additive manufacturing into mold and die pro- duction. As an example, Fig. 20.19 shows an approach for investment casting. Here, the individual patterns are made in an AM operation (in this case, stereolithography), and then used as patterns in assembling a tree for investment casting (Fig. 11.14). Note that this approach will require a polymer that completely melts and burns from the ceramic mold; such polymers are available for all forms of polymer AM oper- ations. Furthermore, as drawn in CAD programs, the parts are usually software modified to account for shrinkage, and it is the modified part that is produced in the additive manufacturing machinery. Binder-jet printing also can easily produce a ceramic-mold casting shell (Section 11.2.2) or a sand mold (Section 11.2.1), in which an aluminum-oxide or aluminum-silica powder is fused with a silica binder. The molds have to be postprocessed in two steps: curing at around 150◦ C, and then firing at 1000◦ –1500◦ C. Another common application of rapid tooling is injection molding of polymers (Section 19.3), in which the mold or, more typically, a mold insert is manufactured by additive manufacturing. Molds for slip cast- ing of ceramics (Section 18.2.1) also can be produced in this manner. To produce individual molds, AM processes are used directly, and the molds will be shaped with the desired permeability. For example, in fused-deposition modeling, this requirement mandates that the filaments be placed onto the individual slices, with a small gap between adjacent filaments; the filaments are then positioned at right angles in adjacent layers.  M20 KALP2244 08 GE C20 page 622 622 Chapter 20 Additive Manufacturing The advantage of rapid tooling is the capability to produce a mold or a mold insert that can be used to manufacture components without the time lag (typically several months) traditionally required for the procurement of tooling. Moreover, the design is simplified, because the designer needs to analyze only a CAD file of the desired part; software then produces the tool geometry and automatically compensates for shrinkage. In addition to the straightforward application of additive manufacturing technology to tool or pattern production, other rapid-tooling approaches, based on AM technologies, have been developed. Room-temperature vulcanizing (RTV) molding/urethane casting can be performed by preparing a pattern of a part by any AM operation, which is then used to produce an RTV mold. The pattern is first coated with a parting agent, and may or may not be modified to define mold parting lines. Liquid RTV rubber is then poured over the pattern, and cures (usually within a few hours) to produce mold halves. The mold is then used with liquid urethanes in injection molding or reaction-injection molding operations (Section 19.3). One main limitation of this approach is a lower mold life, because the polyurethane present in the mold causes progressive damage and the mold may be suitable only for as few as 25 parts. Epoxy and aluminum-filled epoxy molds also can be produced, but mold design requires special care. With room temperature vulcanizing (RTV) rubber, the flexibility of the mold allows it to be peeled off the cured part. With epoxy molds, their high stiffness precludes this method of part removal, and mold design is more complicated. Thus, for example, drafts are required, and undercuts and other design features that can be produced by RTV molding must be avoided. Acetal clear epoxy solid (ACES) injection molding, also known as direct AIM, refers to the use of additive manufacturing, usually stereolithography, to directly produce molds suitable for injection molding. The molds are shells, with an open end to allow filling with a material such as epoxy, aluminum-filled epoxy, or a low-melting-point metal. Depending on the polymer used, mold life may be as few as 10 parts, although hundred parts per mold are possible. Sprayed-metal tooling. In this process, shown in Fig. 20.20, a pattern is first created through AM. A metal spray operation (Section 34.5) then coats the pattern surface with a zinc-aluminum alloy. The metal coating is placed in a flask, and potted with an epoxy or an aluminum-filled epoxy material. In some applications, cooling lines can be incorporated into the mold before the epoxy is applied. The pattern is removed, and two such mold halves are used as in injection-molding operations. Mold life is highly dependent on the materials used and the temperatures involved, and can vary from a few to thousands of parts. Keltool process. In the Keltool process, an RTV rubber mold is first produced, based on a rapid-prototyped pattern, as described earlier. The mold is then filled with a mixture of powdered A6 tool steel (Section 5.7), tungsten carbide, and polymer binder, and is allowed to cure. The so-called green tool (green, as in ceram- ics and powder metallurgy) is fired to burn off the polymer and fuse the steel and the tungsten-carbide powders. The tool is then infiltrated with copper in a furnace to produce the final mold. The mold can subsequently be machined or polished to impart a superior surface finish and good dimensional tolerances. Keltool molds are limited in size to around 150 × 150 × 150 mm. Thus, a mold insert, suitable for high-volume molding operations, is made and installed. Depending on the material and processing conditions, mold life can range from 100,000 to 10 million parts. Case Study 20.3 Casting of Plumbing Fixtures A global manufacturer of plumbing fixtures and accessories for baths and kitchens used rapid tooling to transform its development process. One of the company’s major product lines is decorative water faucets, made from brass castings that are subsequently polished to achieve the desired surface finish. The ability to produce prototypes from brass is essential for quickly evaluating designs and identifying processing difficulties that may occur. A new faucet design was prepared in a CAD program; the finished product is shown in Fig. 20.21. As part of the product development cycle, it was decided to produce prototypes of the faucet to confirm the aesthetics of the design. Since such faucets are typically produced by sand casting, it was also essential  M20 KALP2244 08 GE C20 page 623 Direct Manufacturing and Rapid Tooling 623 to validate the design through a sand-casting operation, followed by polishing. This approach allowed evaluation of the cast parts in terms of porosity and various other casting defects, and also would identify processing difficulties that might arise in the finishing stages. A sand mold was first produced, as shown in Fig. 20.22. The mold material was a blend of foundry sand, plaster, and other additives that were combined to provide strong molds with good surface finish (see also Section 11.2.1). A binder was printed onto the sand mixture to produce the mold. The mold could be produced as one piece, with an integral core (see Figs. 11.3 and 11.6), but in practice, it is often desired to smoothen the core and assemble it later onto core prints. In addition, slender cores may become damaged, as support powder is being removed from the mold, especially for complex casting designs. Therefore, the core for this design is produced separately and then assembled into the two-part mold. Using 3D printing, the operation produced brass prototypes of the faucets in five days, which included the time required for mold design, printing, metal casting, and finishing. The actual print time of the mold was just under three hours, and the material cost was approximately $280. The production of pattern plates for sand casting is, in general, too expensive for producing prototypes, and would cost over $10,000 and add several months to the lead time. The incorporation of 3D printing into the design process thus provided new capabilities that confirmed the design aesthetics and function, as well as manufacturing robustness and reliability. Source: Courtesy of 3D Systems. Aluminum powder- Metal filled epoxy spray Alignment tabs Flask Pattern Coating Baseplate (a) (b) (c) Finished mold half Molded part Pattern Baseplate Second mold half (d) (e) Figure 20.20: Production of tooling for injection molding by the sprayed-metal tooling process: (a) A pat- tern and baseplate are prepared through a additive manufacturing operation; (b) a zinc–aluminum alloy is sprayed onto the pattern (see Section 34.5); (c) the coated baseplate and pattern assembly are placed together in a flask and backfilled with aluminum-impregnated epoxy; (d) after curing, the baseplate is removed from the finished mold; and (e) a second mold half suitable for injection molding is prepared.  M20 KALP2244 08 GE C20 page 624 624 Chapter 20 Additive Manufacturing Figure 20.21: A new faucet design, produced by casting from rapid-prototyped sand molds. Source: Courtesy of 3D Systems. Figure 20.22: Sand molds produced through three-dimensional printing. Source: Courtesy of 3D Systems. 20.11 Design for Additive Manufacturing Additive manufacturing is attractive because designers are able to easily produce complex geometries. Often, this has been expressed as the notion that complexity is free, which has led to the development of design optimization software. This approach has led to, for example, minimum-weight parts given shape constraints and the loads applied (see Fig. 20.23), as well as the production of parts with inherent aesthetic aims. There are, however, limits to the shapes that can be produced by AM. Several design rules have been developed that are unique for additive manufacturing. Since machines are now available in a wide variety of capacities and capabilities, detailed design recommendations are manufacturer-specific. The following considerations are generic and are considered to be good design practice: 1. Additive manufacturing processes tend to warp the part, because of thermal stresses and shrinkage encountered during production. In general, the design guidelines for plastic parts, given in Section 19.15, are also applicable to parts produced through additive manufacturing.  M20 KALP2244 08 GE C20 page 625 Design for Additive Manufacturing 625 (a) (b) (c) Figure 20.23: Topology optimization to reduce the weight of a bracket. (a) Original bracket design; (b) pre- dicted minimum-weight bracket from topology optimization software; and (c) final bracket, representing a 70% weight loss from the original design, as produced through selective laser sintering. Source: 3D Systems. 2. The dimensional tolerance standard used (see Section 35.8) should involve symmetric tolerances in order to be applied easily to additive manufacturing. 3. The tolerances within a plane can be much higher than those outside of a plane. Therefore, the part should be oriented to place the critical dimension in the plane of a build, not in its thickness direction. 4. Dimensional tolerances and surface finish depend on the particular machine, the material, and part size and its orientation. In stereolithography, tolerances of ±0.05 to 0.1 mm are achievable, or ±0.001 mm/mm for well-designed parts that do not warp excessively. Typical selective laser sintering of  M20 KALP2244 08 GE C20 page 626 626 Chapter 20 Additive Manufacturing polymers yields tolerances of ±0.4 mm, or 0.1 mm/mm, whichever is greater. For metal selective laser systems, tolerances of 0.05–0.125 mm are generally achievable, with roughnesses in the range of 5–40 µm. For better tolerances, a machining allowance of 0.5–1 mm should be provided for post- processing. 5. Steps can be noticeable in an inclined plane; generally, the use of flat planes or planes inclined at not less than 20◦ are producible without noticeable steps. 6. The same considerations as stated for powder injection molding are valid for binder jetting (see Section 17.6). 7. In selective laser sintering of polymers, it is recommended to have a clearance of 0.3–0.5 mm within the plane for surfaces that are not joined together; up to 0.6 mm is required in the build direction. 8. The thinnest wall that can be produced depends on the material and the aspect ratio; common ranges are 0.5–1.5 mm for polymers in selective laser sintering. In fused deposition modeling, it is generally recommended that a wall be at least four times wider than the thickness of the layer. 9. Recognizing that the powder in the build chamber may not be reusable, as well as to maximize pro- duction, it is beneficial to fill a build space with as many parts as possible, and nestable (Section 16.14) parts be used when possible. 10. To reduce costs, the height in the build direction should be low, and stackable parts should be used to increase the amount of powder that is fused in a build chamber. 11. Consideration must be given to the removal of the uncured photopolymer or powder when the parts being made are hollow. 12. Large parts are especially susceptible to warpage; it may be a good strategy to produce a part in components that can be assembled after printing, or else design parts to use as little mass as possible. 13. Plan the part to allow for powder or liquid photopolymer removal when appropriate. 14. Build time depends on the volume of the material that is to be fused in a process. It is therefore beneficial to model an object with solid surfaces, but supported by porous structures or struts, instead of a solid bulk. This approach produces designs that can be optimized to minimize weight by carefully designing the supporting structure. 15. Complexity is free. That is, there is no need to restrict designs to geometries that are easy to manufacture for casting, forging or machining operations. Corner radii, draft angles, accommodations for parting lines, etc., do not need to be included in AM part design. With binder jetting, color can be incorporated into designs easily. 20.12 Additive Manufacturing Economics As in all processes, design and manufacturing decisions are ultimately based on performance and cost, including the costs of equipment, tooling, and production. The final selection of a process or processes also depends greatly on production volume. High costs of equipment and tooling in plastics processing can be acceptable only if the production run is large, as is also the case in casting and forging. However, using additive manufacturing operations makes these processes economical for limited production runs by applying rapid tooling approaches (see Section 20.10), though the tools and molds have limited life. Additive manufacturing operations are suitable for prototypes and limited production runs, but they require expensive consumables, and thus are unsuitable for moderate to high production runs. This situation is complicated by the fact that some processes (such as selective laser sintering and electron beam melting) may require the unfused powder in the build chamber to be discarded. Thus, if only 10% of  M20 KALP2244 08 GE C20 page 627 Additive Manufacturing Economics 627 the build volume is reused, the material cost for the part is ten times the nominal material cost. This is a significant concern; titanium (Ti-6Al-4V), for example, costs over $400/kg for the raw powder. The cost of a part produced by additive manufacturing can be generalized as Cp = Cm + Cs + Ct + Cf , (20.1) where Cm is the material cost, Cs is the setup cost, Ct is the cost of machinery or tooling per part, and Cf is the cost of finishing operations. The material cost is high compared to conventionally produced polymers (such as in injection molding) or metals (such as extrusions). However, Ct in conventional processing is generally much higher, since no tools are required in additive manufacturing. Finishing operations may or may not be necessary, so that Cf may often be ignored. When deciding if additive manufacturing is suitable for production, the cost compared to the conven- tional alternative has to be justified. Consider the case where a part is being considered for either injection molding or selective laser sintering. For low production runs, the high cost of tooling associated with injec- tion molding dominates the part cost. However, for mass production, the tooling cost is amortized over many parts. The higher cost of the materials in additive manufacturing makes mass production a less economical option. Case Study 20.4 Implications of Powder Reuse Powder for additive manufacturing processes can be expensive; for example, $400 per kg of titanium powder is not unusual, and $100 per kg for high quality polymers is common. This high cost is especially important if a powder-bed process is used, as the volume fused in the build chamber may be as little as 10% of the total volume. A common concern is whether or not unfused powder can be reused, that is, taken from the build chamber and then placed into the feed chamber. Often, the unfused powder (or cake) is loosely adhering and has to be sieved or otherwise treated to break up clumps of powder. If powder is not reused, then the cost embedded into the material can be several times the powder cost, thus making AM uneconomical for almost all commercial applications. There are several strategies that can be applied in powder reuse: 1. The unfused powder can be taken as is from the build chamber, then sieved, examined, and placed in the feed chamber. Evaluations are generally associated with powder size distributions and the ability of the powder to flow or spread itself into a continuous and smooth layer. 2. The powder from the build chamber can be blended with virgin powder, often in a 1:1 mixture. Some of the concerns associated with powder reuse are: 1. Additive manufacturing takes place under a controlled atmosphere, generally argon or nitrogen, in order to prevent powder oxidation and also to control any fire or explosion hazards associated with powders. However, when the powder is removed from the build chamber and is sieved, it is exposed to air, and therefore has the potential of oxidation. 2. The additive manufacturing process is rather complex. Videos of selective laser sintering have shown that particles that have been exposed to laser energy jump off the powder layer, the melt pool is highly turbulent, and there is a contraction as the powder melts and then solidifies. 3. The size distribution of powders (see also Section 17.2.2) can change over time. 4. Careful examination of powder size distributions have found subtle changes. When the feed cham- ber piston moves upwards, and the wiper or cylinder moves across the build chamber to create a fresh powder layer, there is always a slight surplus of powder in order to ensure that the build chamber layer is fully developed. The larger particles are pushed by the wiper or cylinder into an  M20 KALP2244 08 GE C20 page 628 628 Chapter 20 Additive Manufacturing overflow trough, leaving the smaller particles in the build chamber layer. These smaller particles are consumed during additive manufacturing, but the overfeed trough is blended with the pow- der reintroduced into the feed chamber. The result is that the mean particle size tends to increase slightly as the powder is reused. 5. There is a major concern that the high temperatures associated with the melt pool could cause par- ticles near the melt pool to fuse. However, sieving eliminates such particles from being introduced into the build chamber. 6. Selective laser sintering and electron beam melting involve preheating the build chamber, in order to have a more robust process and to reduce the power required in the laser. With selective laser sintering, this preheat is much lower than with electron beam melting; still, there is a concern that this preheat can alter the microstructure or the chemistry of the powder. Figure 20.24 shows the effect of reuse on ultimate tensile strength. Note that there is no noticeable reduc- tion in mechanical properties associated with the first powder reuse for any of the metals considered. There is a drop in strength when a nylon powder is reused four times, but there is no further reduction through eight reuses. This is a main justification for the practice of blending virgin nylon powder with reclaimed powder from the build chamber. It should also be noted that the observations regarding Fig. 20.24 may not hold for all materials. It has been suggested that alloys that are especially sensitive to oxygen and water vapor (such as magne- sium alloys) may undergo a degradation in mechanical properties associated with reuse, because of their exposure to humidity during reclamation and sieving. Regardless, the reuse of powders is now seen to be a plausible strategy for cost reduction in AM, and that it could greatly accelerate additive manufacturing application to actual production. Source: Courtesy of the National Center for Defense Manufacturing and Machining, America Makes, and the Air Force Research Laboratory. 1500 Ultimate tensile strength (MPa) 1000 1 Reuse 4 Reuses 8 Reuses Virgin 500 4 Reuses 8 Reuses 4 Reuses 8 Reuses 4 Reuses 1 Reuse 1 Reuse 1 Reuse Virgin Virgin Virgin 0 Nylon 316L 17-4 PH Ti-6Al-4V Figure 20.24: Effect of powder reuse on mean ultimate tensile strength for nylon, 316L stainless steel, 17-4 precipitation hardening stainless steel and titanium alloy Ti-6Al-4V.  M20 KALP2244 08 GE C20 page 629 Key Terms 629 Summary Additive manufacturing techniques have made possible much faster product development times, and they are having a major effect on other manufacturing processes. When appropriate materials are used, additive manufacturing machinery can produce blanks for investment casting or similar processes, so that metallic parts can now be obtained quickly and inexpensively, even for lot sizes as small as one part. Such technologies also can be applied to producing molds for operations (such as injection molding, sand and shell mold casting, and forging), thereby significantly reducing the lead time between design and manufacture. Additive manufacturing continues to grow into a valuable new manufacturing discipline. It is a useful technique for identifying and correcting design errors. Several techniques have been developed for producing parts through AM. Fused-deposition modeling consists of a computer-controlled extruder, through which a polymer filament is deposited to produce a part slice by slice. Stereolithography involves a computer-controlled laser-focusing system, that cures a liquid ther- mosetting polymer containing a photosensitive curing agent. Multijet and PolyJet modeling use mechanisms similar to ink-jet printer heads to eject photopolymers to directly build prototypes. Laminated-object manufacturing uses a laser beam or vinyl cutter to first cut the slices on paper or plastic sheets (laminations); then it applies an adhesive layer, if necessary, and finally stacks the sheets to produce the part. Three-dimensional printing uses an ink-jet mechanism to deposit liquid droplets of the liquid binder onto polymer, metal, or ceramic powders. The related process of material jetting directly deposits the build material. Using multiple printheads, three-dimensional printing can also produce full-color prototypes. Selective laser sintering uses a high-powered laser beam to sinter powders or coatings on the powders in a desired pattern. Selective laser sintering has been applied to polymers, sand, ceramics, and metals. Electron-beam melting uses the power of an electron beam to melt powders and form fully-dense functional parts. Key Terms ACES Direct manufacturing Additive manufacturing Distributed manufacturing Big area additive manufacturing Direct prototyping Binder jet printing Electron-beam melting Bioprinting Friction stir modeling CLIP Fused-deposition modeling Continuous liquid interphase production Hybrid approaches Contour crafting JetFusion Desktop machines Keltool Direct AIM Laminated-object manufacturing  M20 KALP2244 08 GE C20 page 630 630 Chapter 20 Additive Manufacturing Laser-engineered net shaping Rapid tooling Mask projection stereolithography RTV molding/urethane casting Mass customization Selective laser sintering Material jetting Sprayed metal tooling Multijet modeling Stereolithography Photopolymer Subtractive processes PolyJet