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Chapter 5 Introduction to Rapid Prototyping Eef Moeskops and Frits Feenstra, TNO Science and Industry, Netherlands Abstract The term rapid prototyping (RP) refers to a class of technologies that are used to produce physical objects layer-by-layer directly from computer-aided design (CAD) data. These techniques allow designers to produce tangible prototypes of their designs quickly, rather than just two-dimensional pictures. Besides visual aids for communicating ideas with co- workers or customers, these prototypes can be used to test various aspects of their de- sign, such as wind tunnel tests and dimensional checks. In addition to the production of prototypes, rapid prototyping techniques can also be used to produce molds or mold inserts (rapid tooling) and even fully functional end-use parts (rapid manufacturing). Because these are nonprototyping applications, rapid prototyping is often referred to as Figure 5.1. Prototype of a talking barcode scanner 100  5 Introduction to Rapid Prototyping solid free-form fabrication or layered manufacturing. For small series and complex parts, these techniques are often the best manufacturing processes available. They are not a so- lution to every part fabrication problem. After all, CNC technology and injection molding are economical, widely understood, available, and offer wide material selection. In rapid prototyping, the term “rapid” is relative; it aims at the automated step from CAD data to machine, rather than at the speed of the techniques. Depending on the di- mensions of the object, production times can be as long as a few days, especially with complex parts or when long cooling times are required. This may seem slow, but it is still much faster than the time required by traditional production techniques, such as ma- chining. This relatively fast production allows analyzing parts in a very early stage of designing, which decreases the resulting design cost. The costs can also be reduced be- cause rapid prototyping processes are fully automated. and therefore, need the skill of individual craftsmen for no more than finishing the part. General advantages: · Freedom of design: The production of complex parts is reduced to the accumulation of layers. · Well automated: No supervision is needed during the build process. · Relative easy to employ: Only little preparation and postprocessing are required. · Avoiding the high cost of prototype tooling, allowing (more) design iterations. · Physical models are easy to check for errors. General disadvantages: · Accuracy generally >0.1 mm. · Material properties: products can be very fragile, and some need postprocessing be- fore they can be handled (as with 3-DP). · Staircasing effect. Because an inclined surface is constructed using several layers, staircasing will occur. 5.1 The Basic Process Rapid prototyping techniques share the following process steps (see Figure 5.2): 1. Creating a CAD model either by designing a new or scanning an existing object. 2. Converting the CAD data to STL format. Because the various CAD packages apply a number of different algorithms to represent solid objects, the STL format (Standard Triangulation Language) has been adopted as the stan- dard of the rapid prototyping industry to establish consistency. This STL file is a concrete visualization of the product geometry, built up from triangles. Using triangles to describe a surface, curved surfaces can only be ap- proached. Increasing the number of triangles (i.e., increasing the resolution) yields a better approach. However, it also enlarges the STL file. So, one has to find the optimum balance between file size and part accuracy. 5.1 The Basic Process  101 Figure 5.2. The basic RP process (FDM). A color reproduction of this figure can be seen in the Color Section (pages 219–230). 3. Slicing the STL file into thin cross-sectional layers. After the STL file has been sized and oriented, it is sliced in layers with a predefined thickness. 4. Generation of a support structure. This additional step is not required for all techniques. Because the model is built up in layers, there may be areas that could float away or have overhanging features, which could distort the re- sulting model. A base and support structures have to be added, which can be easily removed after the building step. 102  5 Introduction to Rapid Prototyping 5. Producing the model layer-by-layer. The generated slices are reconstructed in the machine by building one layer at a time. This can be fully automatic. 6. Postprocessing. This step enhances cleaning and finishing the model and (if a base or support structure was built) removing the support structure. Some materials need to be postcured or infiltrated to achieve optimal properties. 5.2 Current Techniques and Materials A wide range of techniques and materials can be used for rapid prototyping. There are more than ten commercial rapid prototyping processes and more than five concept modeling processes; all have unique properties. Due to worldwide re- search, this range is growing quickly. Commercial techniques are available to pro- duce objects from numerous plastics, ceramics, metals, and wood-like paper. Among these techniques are · · · · · · · Stereolithography Selective laser sintering Fused deposition modeling Three-dimensional printing Laminated object manufacturing Multijet modeling Laser-engineered net shaping 5.2.1 Stereolithography Stereolithography (SLA-stereolithography apparatus), launched by 3D Systems Inc. in 1987, is the first and most commercially used rapid prototyping method. A platform is placed in a bath of photosensitive UV-curable resin at a level that leaves a small layer of resin between the top of the platform and the surface of the bath. A laser (often He-Cd or argon ion to produce UV radiation of about 320–370 nm wavelength) then strikes the desired areas, thereby curing the resin selectively. As the layer is completed, the platform descends allowing liquid resin to flow over the previously cured area. A wiper blade clears the excess fluid from the top of the surface. This sweep is essential to achieve consistent layer thickness and prevent air entrapment. As the new layer is cured, it sticks to the preceding layer. This process continues until the object is completed. On completion, the object raises above the fluid, so that resin can drain out. The object is carefully removed and washed in a solvent to remove uncured resin. The cleaned object has to be placed in a UV oven to ensure that all resin is cured. During the process, features that lean over have to be supported. This support structure can easily be gener- ated by software and consists of a series of slender sacrificial columns or lattices. 5.2 Current Techniques and Materials  103 Figure 5.3. Schematic of the SLA process Figure 5.4. A detailed DMD mirror reproduction using microSLA A lattice structure is also created as a base to prevent the model from sticking to the building platform. Thus, additional hand-finishing will be needed to remove these supporting structures and to sand any small stubs from the surface. A large variety of photosensitive polymers is commercially available, includ- ing clear, water resistant, and flexible resins that simulate the properties of, for example PA, ABS, PP, and rubber-like materials. Process times, tolerances, and surface finish depend on layer thickness, which is controlled by the amount the platform is lowered into the resin. Generally, layer thicknesses vary from 0.05– 0.5 mm. Thinner layers can be applied with digital light processing using a tech- nique called perfactory, which is based on the standard SLA process. Instead of describing a cross section with a laser, a normal beamer covers the entire cross section at once. Due to the high resolution of the beamer (pixel size: 39 ìm) and the accurate positioning system of the platform (layer thickness: 25 ìm), the parts produced can contain highly detailed features. Characteristics: · · · · Long-term curing can lead to overcuring which leads to warpage. Parts can be quite brittle. Support structures are required. Uncured material can be toxic. 104  5 Introduction to Rapid Prototyping 5.2.2 Selective Laser Sintering Selective laser sintering (SLS) is a process that was patented in 1989 by Carl Deck- ard, University of Texas. A layer of powder (particle size approximately 50 ìm) is spread over a platform and heated to a temperature just below the melting tem- perature. A carbon dioxide laser needs to raise the temperature only slightly and selectively to melt the powder particles. As the layer is finished, the platform moves down by the thickness of one layer (approximately 0.10–0.15 mm), and new powder is spread. When the laser exposes the new layer, it melts and bonds to the previous layer. The process repeats until the part is complete. On completion, the built volume has to cool down to room temperature after which the processed objects can be removed from the powder bed by brushing away excess powder. Sandblasting the objects removes all unsintered particles. Surrounding powder particles act as supporting material for the objects, so no Figure 5.5. Schematic of the SLS process Figure 5.6. Accurate positioning elements with internal hinges produced by SLS 5.2 Current Techniques and Materials  105 additional structures are needed. Furthermore, more objects can be built at the same time because they can be meshed above/in each other. Excess powder can be reused. However, it needs to be mixed with virgin powder to guarantee good part quality. Commonly used materials for SLS are nylon (polyamide-12), glass- filled nylon, and polystyrene. The method has also been extended to direct fab- rication of metal and ceramic objects and tooling inserts. Characteristics: · · · · · Key advantage of making functional parts in essentially final materials. Good mechanical properties, though depends on building orientation. Powdery surface Many variables to control No support required 5.2.3 Fused Deposition Modeling Fused deposition modeling (FDM), developed by Stratasys, is the second most widely used rapid prototyping process. A filament thread of plastic is unwound from a coil and fed into an extrusion head, where it is heated and extruded through a small nozzle. Because the extrusion head is mounted on a mechanical stage, the required geometry can be described, one layer at a time. The molten plastic solidifies immediately after being deposited and bonds to the layer below. Support material is laid down similarly through another extrusion head. The platform on which the object is built steps down by the thickness of a single layer. The entire system is contained within a heated oven chamber which is held at a moderate temperature above the glass transition temperature of the polymer. This provides much better control of the process because stresses can relax. As in the SLA process, overhanging features need to be supported. This support material needs to be removed in secondary operations. Commercially available Figure 5.7. Schematic of the FDM process 106  5 Introduction to Rapid Prototyping Figure 5.8. Scanned archery handle and an FDM reproduction water-soluble support materials facilitate this final step. ABS, polycarbonate, and poly(phenyl)sulfone are commonly used materials in the FDM process. Characteristics: · · · · Office-friendly and quiet. FDM is fairly fast for small parts. Good mechanical properties, so suitable for producing functional parts. Wide range of materials. 5.2.4 Three-dimensional Printing In some textbooks, the term “three-dimensional printing” (3-DP) is used for all rapid prototyping processes. The process developed at MIT is referred to here. In this process, a layer of powder is spread over a platform. The particles are bonded together selectively by a liquid adhesive (binder solution). This liquid is deposited in a two-dimensional pattern by a multichannel jetting head. As the current layer is completed, the platform moves down by the thickness of a layer, so that a new layer can be spread. This process is repeated until the entire object is formed within the powder bed. On completion, the object is elevated and the 5.2 Current Techniques and Materials  107 Figure 5.9. Schematic of the 3-DP process Figure 5.10. 3-D printed landscape extra powder is brushed away, leaving a fragile “green” object. It is necessary to infiltrate the part with another material to improve mechanical characteristics. No support structures are required because the surrounding powder particles support overhanging features. By adding color to the binder solution, objects can be produced in every desired color. Starch, plaster, medicines (for produc- ing controlled-dosage pharmaceuticals), ceramics, and metals are commonly used materials (powders) for 3-DP. Characteristics: · Limitations on resolution and surface finish. · Fragile objects need to be infiltrated. 108  5 Introduction to Rapid Prototyping 5.2.5 Laminated Object Manufacturing In laminated object manufacturing (LOM), a sheet of paper (unwound from a feed roll) with a polyethylene coating on the reverse side is placed on a plat- form. This coating is melted by a heated roller, making the paper adhere to the platform. Then, a carbon dioxide laser cuts out the cross section of the object and a border. The laser also creates hatch marks, or cubes that surround the pattern within the border. These cubes behave as a support structure for the model. When the laser has finished the layer, a new paper sheet is applied. Upon completion, the model is captured within a block of paper. When all of the surrounding cubes have been removed, the unfinished part is sanded down. The humidity and temperature dependency of the paper material can be reduced by coating the model. The finish and accuracy are not as good as with some other methods; however, objects have the look and feel of wood and can be worked and finished like wood. Figure 5.11. Schematic of the LOM process Figure 5.12. Trumpet prototype using LOM 5.2 Current Techniques and Materials  109 5.2.6 Multijet Modeling Multijet modeling (MJM) uses multiple print heads to deposit droplets of mate- rial in successive, thin layers. Two major MJM techniques can be distinguished (see http://www.3dsystems.com/ for more information): ThermoJet?. A 96- element print head deposits droplets of wax. Because of its relatively fast pro- duction, this technique is marketed to the engineering or design office for quick form studies (concept modeling). However, wax models can also be used as master patterns for investment casting, as will be explained later. InVision?. A print head jets two separate materials, an acrylic UV-curable photopolymer-based model material and a wax-like material to produce sup- port structures for the model. Due to the relative good quality of the models, Figure 5.13. Schematic of the ThermoJetTM process Figure 5.14. Wax models produced by M