Machinery's Handbook, 31st Edition
1564 METAL ADDITIVE MANUFACTURING PROCESSES Complex Fins and Blades: Axisymmetric parts, such as impellers made up of fins or blades on a flat or cone shape, are costly and time-consuming to machine by CNC but are relatively easy using AM. This is one of the rare cases when it is cost-effective to produce an existing design by AM rather than a conventional process. As part complexity is on the extreme right side of the curve, the cost of production by AM will be lower, as shown in Fig. 30b. Internal Channels: Internal fluid flow paths, in parts such as heat exchangers, fuel injec - tors, and sensors, normally are accomplished by assembling tubes, or by drilling and cap- ping internal passages. These internal pathways usually are round and straight, because tub- ing is most easily produced with round tubes, and drilling uses cutting surfaces in circular motion. Use of AM processes enables formation of channels in any cross section and along non-straight paths. As a result, heat exchanger configurations can be designed and made by AM that allow for more effective and thus efficient heat transfer between hot and cold fluids. A notable example of AM for flow control—and one of the first examples of AM used in serial production quantities—is a fuel injector produced by GE for the LEAP gas turbine engine; nineteen fuel injectors are required for each engine. Redesigning the component for AM reduced flow losses and, at the same time, reduced the parts count from twenty to one. Another example of the benefits of producing internal channels by AM is injection mold tooling. Rapid removal of heat from the injected polymer is key in improving production rate of injection molded parts, as this step takes up about half of the total time for each in- jection cycle. Internal cooling channels frequently are used to increase heat removal rates; however, such channels are made by drilling straight channels that intersect internally and capping holes that penetrate the outside of the mold. With conformal cooling chan- nels (channels that conform to the outline of the part), heat can be removed more quickly, decreasing the injection cycle time. In one test of injection molding a non-symmetric part in polyethylene and polycarbon- ate, the cooling time was reduced by 30 percent, resulting in a 15 percent reduction in cycle time. While this seems a small saving for each cycle, consider the cumulative effect over millions of cycles typically used in injection molding of plastic consumer goods. Internal Structures and Topological Optimization: One of the unique capabilities of AM processes is the ability to produce lattice structures, cellular structures, and organic ge- ometries. Lattice structures involve a network of struts inside a part that remove a large amount of material yet maintain strength of the part. Cellular structures build up a part as an assembly of cells, usually hexagonal in shape. Organic structures are unusual shapes that appear similar to natural materials, such as bone or trees. Such geometries can be developed using topological optimization computer programs to analyze stresses in the part under load and remove sections that transmit little or no stress. Assemblies and Part Integration: Part count reduction can be accomplished by design- ing assemblies as one component. This reduces the need for assembly operations, such as brazing or riveting. In addition, by producing the part in one piece, blend radii can be made at joining surfaces, reducing stress concentrations and potential sources of fatigue failure. While the cost of each part of the assembly may be small, the cumulative cost of tooling for each part, keeping an inventory of each part, plus the cost of assembly, can be greater than the cost of producing the assembly as one component by AM. Reasons to Choose AM: Current AM processes cannot be expected to compete economi- cally with traditional metal manufacturing methods, unless: (1) you absolutely need it as soon as possible without waiting for tooling fabrication or CNC programming, and cost is not an issue; (2) complexity of the part is so great that it actually is less expensive to make by AM than by conventional processes; (3) AM processes provide unique geometric or mate- rial characteristics, such as internal conformal cooling channels or part consolidation, that improve performance, so lifecycle cost savings by AM exceed its premium production cost. To take advantage of opportunities afforded by metal AM processes, it is necessary to consider the entire system, rather than each individual part, for part reduction possibilities and/or potential improvements in system performance. As continuous improvements are made in AM materials, machines, and software, this innovative manufacturing method will become increasingly viable for large-scale production of automobile components and other commodity items.
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