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production speed, and surface finish, applications range from preliminary pro- totypes to mock-ups for concept proposals or marketing models.
5.2.7 Laser-engineered Net Shaping
In laser-engineered net shaping (LENS), a laser beam focuses onto a metal sub- strate to melt the upper surface. A deposition head then applies metal (powder or fine wire) into the molten puddle to increase the material volume. By moving the platform in raster fashion, each layer of the object is fabricated. An inert gas is used to shield the melt puddle from atmospheric oxygen for better control of properties and to promote layer-to-layer adhesion by providing better surface wetting.
Fully dense metal parts (made of stainless steel, aluminum, copper, Inconel, titanium, etc.) can be produced by LENS. It is even possible to change the mate- rial composition dynamically, which lead to objects with properties that might be mutually exclusive using traditional fabrication methods. Although produced parts are near net shape, they generally require postprocessing. Applications of LENS are injection molding tools and aerospace parts.
5.3.1 Rapid Prototyping
As stated before, RP models improve communication to ensure that all parties in a design process have a complete understanding of the design. So RP models provide the ability to manage, control, and detect changes and required modifi- cations. However, RP techniques can also be applied to the needs of disciplines outside of design engineering. Scanned human parts (bones, tissue) can easily be reproduced using additive RP processes. In 1998, the head of a 16-year-old
Figure 5.16. Use of RP models
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Egyptian girl named Sensaos, who died in 109 AD, was reconstructed after re- searchers scanned Sensaos’ mummy with a tomographic scanner. The comput- erized model was used to reproduce the skull using RP at TNO Science and In- dustry (The Netherlands). Realistic facial features could than be added to the skull to reconstruct the girl’s head.
5.3.2 Rapid Tooling
The automated production of hard and wear-resistant parts with low surface roughness using rapid prototyping techniques is referred to as (direct) rapid tool- ing. Metal molds and dies can be produced by layered manufacturing, which in
Figure 5.17. Vacuum casting of a talking barcode scanner. A color reproduction of this figure can be seen in the Color Section (pages 219–230).
turn can be used in limited mass production. The use of conformal-cooling chan- nels is a method of rapid tooling that offers the potential benefit of improved thermal properties which cannot be realized with conventional machine tools.
Because the mold (or mold insert) is produced in layers, cooling channels that closely follow the contours of the part can be incorporated into the mold. So, con- formal cooling removes heat from the mold or die faster than the straight-line channels in machined tools, thereby reducing molding cycle time and tool distor- tion and improving tool life. Rapid prototyping models can be indirectly used for reproduction. Vacuum casting is the simplest and oldest indirect rapid tooling technique. Using this technique, a model is submerged in a bath of liquid silicone.
When the silicone hardens, it is cut open and the model is removed. Reassembling the silicone parts leaves a gap that has the exact shape of the original model. By filling this gap with a two-component resin under vacuum, a reproduction can be created. Silicone molds can last for as many as 20 reproductions.
Investment casting is another indirect rapid tooling process. A wax model is placed in a bed of ceramic powder. When heated, the ceramic forms a hard shell as the model melts. The remaining gap can be filled with liquid metal. After cooling, the shell has to be removed.
5.3.3 Rapid Manufacturing
In general, there are two types of models in free-form fabrication:
• Shape (or concept) model, when using the model to evaluate the shape and dimensions of a product.
• Functional model. If the mechanical behavior of the model suits the de- signer’s wishes for its application, the model is called a functional model.
Combining both types, as in producing fully functional end-use products, is termed rapid manufacturing (RM). The time to market and product expense can be further decreased when using RP to produce finished manufactured parts.
RM has the ability to create end-use products with almost unlimited complexity.
Furthermore, the techniques are suitable for mass customization.
Examples are hearing aids and nearly invisible teeth aligners made by Align Technology (Santa Clara, California). Especially, when only a small number of products is needed, RM can replace conventional production techniques. Be- cause there is almost no limit to the complexity of products in layered manufac- turing, RM can be a very helpful tool in topology optimization (i.e., optimization for weight or material). In topology optimization, an engineer defines the build- ing envelope and the requirements of a part. Commercially available software optimizes the shape according to those requirements. In most cases, the optimal structure becomes very complex, maybe even hollow, which makes it hard (or impossible) to produce this kind of shape using conventional techniques. So the engineer needs to reconstruct the optimized shape. Because of compromises to
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be made, this final shape will not be the best shape, but just an approximation of it. Using layered manufacturing, the last reconstruction operation becomes superfluous because the technique can create this complex shape, probably without any problem.