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.
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 μm) and the accurate positioning system of the platform (layer thickness: 25 μm), 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.
Figure 5.3. Schematic of the SLA process
Figure 5.4.A detailed DMD mirror reproduction using microSLA
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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 μm) 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
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
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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
Figure 5.8. Scanned archery handle and an FDM reproduction
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.
Figure 5.9. Schematic of the 3-DP process
Figure 5.10. 3-D printed landscape
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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
5.2.6 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 MJM
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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.