04-03-2010, 01:05 PM
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ABSTRACT
Selective laser sintering (SLS) is technique by which parts are building layer by layer. The SLS is a free form fabrication method to create components by precise thermal fusing of powdered materials.
This is durable, economical and fast. The SLS is another form of rapid prototyping. This selective laser sintering (SLS) is one among the rapid prototyping which include stereo lithography (SLA). This method has also been extended to provide direct fabrication of metal and ceramic objects and tools. With this method we can make required and different prototype. This SLS can be used as a mass production prototyping.
INTRODUCTION
Rapid Prototyping (RP) can be defined as a group of techniques used to quickly fabricate a scale model of a part or assembly using three dimensional computer aided design (CAD) data. What is commonly considered to be the first RP technique, selective laser sintering was patented by CARL DECKARD A UNIVERSITY OF TEXAS GRADUATE STUDENT. The company was founded in 1989, and since then, a number of different RP techniques have become available. Rapid Prototyping has also been referred to as solid free-form manufacturing; computer automated manufacturing, and layered manufacturing. RP has obvious use as a vehicle for visualization. In addition RP models can be used for testing, such as wl Icrlan airfoil shape is put into a wind tunnel RP models can be used to create male models for tooling, such as silicone rubber molds and investment casts. In some cases, the RP part can be the final part, but typically the RP materia! is not strong or accurate enough. When the RP material is suitable, highly convoluted shape§" (including parts nested within parts) can be produced because of the nature of RP. There is a multitude of experimental RP methodologies either in development or used by small groups of individuals.
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Engineering
WHY RAPID PROTOTYPING NECESSARY
The reasons of Rapid Prototyping are
¢ To decrease development time.
¢ To decrease costly mistakes
¢ To minimize sustaining engineering changes.
¢ To extend product lifetime by adding necessary features and eliminating redundant features early in the design.
Rapid Prototyping decreases development time by allowing corrections to a product to be made early in the process. By giving engineering, manufacturing, marketing, and purchasing a look at the product the design process, mistakes can be corrected and changes can be while they are still inexpensive. The trends in manufacturing industries to emphasize the following
¢ Increasing number of variants of products.
¢ Increasing product complexity.
¢ Decreasing product lifetime before obsolescence.
¢ Decreasing delivery time.
Rapid Prototyping improves product development by enabling better communication in a concurrent engineering environment.
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Engineering
HIGHLIGHTS OF SLS
¢ It is patented in 1989.
¢ Considerably stronger than stereo lithography.
¢ Laser beam selectively fuses powder materials; nylon, elastomers and so on metals.
¢ Process is simple.
¢ There is no milling or masking steps required.
¢ Powdering, porous surface unless sealant is used. Sealant also strengthens the part.
¢ Uncured materials are easily removed after a build by brushing of. SELECTIVE LASER SINTERING
THE TECHNOLOGY
The selective laser sintering is a free form fabrication method to create components by precise thermal fusing (sintering) of powdered materials. Parts of complex geometries can build in successive layers that define subsequent cross sections of the component.
The sinter powder is powder fed to the process chamber from two cartridges flanking the partly built product. This allows for bidirectional powder feeding to the roller that lays powder across the top of the product, thus improving building speed. Unsintered powder is relumed to the powder feeding cartridges, to be/ recycled. The process is a C02 type ol'50watt power. The process chamber is Oiled with nitrogen to obtain safe materials sintering conditions.
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3
Engineering
HOW IT WORKS
The SLS technology uses a C02 laser to sinter (fuse) a variety of thermoplastic and metal powders to "grow" 3D objects layer-by-layer from 3D electronic data (STL files). Because this is an additive process, highly complex geometries can be built without issue; and, because the powder holds the parts, no support structures have to be added and removed. The key advantage of SLS is its ability to rapidly produce durable, functional objects for a wide variety of applications.
¢ Working parts and assemblies with good detail and surface finishing
¢ Variety of material: rigid and flexible plastics, fully dense metal, rubber like elastomer, foundry friendly patterns
¢ Capable of living hinges,, high-flex snaps, high stress and heat tolerance and service as short-run tooling
¢ Can be finished and painted for presentation, demonstration and video reproduction
¢ Dimensional tolerancing with thousandths of a inch
¢ Delivery of most parts and patterns in just a few working days
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Engineering
THE PROCESS
Selective Laser Sintering (SLS) parts are built with successive layers' of powder selectively bound by a laser beam. SLS is also a technique by which parts are built layer by layer. The basic material consists of powder with particle sizes in the order of magnitude of 50 11m. Successive powder layers are spread on top of each other. After deposition, a computer controlled C02 laser beam scans the surface and selectively binds together the powder particles of the corresponding cross section of the product. During laser exposure, the powder temperature rises above the glass transition point after which adjacent particles How together. This process is called sintering.
THE MATERIAL
The parts are built in polyamide (PA). The powder being a solid material has an attractive feature of being self supporting for the, generated product sections. This makes supports redundant. The polyamide material allows the production of fully functional prototypes with high mechanical and thermal resistance. The use of PA powder filled
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Dept. Of Median ieal Engineering
within glass particles (PA-GF) has a much higher thermal resistance and is typically used in functional test with high loads.
The polyamide SLS parts have excellent long term stability and are resistant, against most chemicals. They can be, made water tight by impregnation. The PA material is used as bio compatible, food safe and not harmful to health or environment.
EXPERIMENTAL SETUP OF SLS
Thermoplastic powder is spread by a roller over a surface of a build cylinder. The piston in the cylinder moves down one object layer thickness to accommodate the new
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layer of powder. The powder delivery system is similar in function to a build cylinder. Here a piston moves upwards incrementally to supply a measured quantity ofpowder for each layer.
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A layer beam is then traced over the surface of this tightly compacted powder to selectively melt and bond it to form a layer of the object. The fabrication chamber is maintained at a temperature just below the melting point of the powder so that heat from the laser need only elevate the temperature slightly to cause sintering. This greatly speeds up the process. The process is repeated until the entire object is fabricated.
After the object is fully formed the piston is raised to elevate it. Excess powder is simply brushed away and final manual finishing may be carried out. No supports are required with this method. Since overhangs and under cuts are supported by the solid power bed. It may take a considerable length of cool down time before the part can be
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removed from the machine. Large parts with thin sections may require as much as two days of cooling time.
OVERVIEW OF SLS
X & Y AXIS CONTROL
MATERIAL SUPPLY
V, LASER
MATERIAL BED BUILD LEVEL
PROTOTYPE TANK
This overview bypasses many intricacies of the processes involved. Selective laser sintering is very similar to Stereo Lithography (SLA) with the exception of using powdered raw material rather than photo-sensitive liquid as the build material. The system consists of an X-Y axis controlled laser, a Z axis controlled table, and a bed of powdered raw material that is maintained at a constant level. Prototypes are created with the table initially positioned below the surface of the powder a distance equal to (he thickness resolution (or the part. The laser traces a two dimensional cross-section of the item at that level, fusing the powder touched by the laser. The table then descends a distance equal to the thickness resolution and powdered media is replenished over the solidified layer. This process continues until the entire prototype has been solidified.. At
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8
Engineering
this time the model is lifted out of the media bed. cleaned of loose material and smoothed to remove the Z axis stair-steps created by the process. "Trapped volumes" are not a problem for SLS models. Support structures arc generally not required. A good selection of materials is available for this process, ranging from a rubber-like polymer to hard plastic.
PROTOTYPES MANUFACTURED BY SLS
AIRFOIL PAINTBALL - MASK MPELLER
GARDEN TRIMMER VALVE BODY DIESEL ENGINE
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9
Engineering
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ADVANTAGES OF SLS
¢ It offers the key advantage ol'making functional parts in essentially final materials.
¢ The system is mechanically more complex than SLA and most other technology
¢ The method has extended to provide direct fabrication of metals and ceramic objects and tools.
¢ Since the objects are sintered, they are porous.
SLS are
Fast
Economical
Durable and functional parts Large and complex parts Small series, produced in one building process No support structure' necessary
All kinds of finishing degrees can be made water light
LIMITATIONS OF SLS
1. Surface finish: The surface of an SLS part is powdery, like the base material whose particles are fused together without complete Melting. The smoother surface of an SLA part typically wins over SLS.
2. Dimensional accuracy: SLA is more accurate immediately after completion of the model, but SLS is prone to residual stresses that are caused by long term curing and environmental stresses. Both SLS and SLA suiler from inaccuracy, but SLS is less predictable because of the variety of materials and process parameters.
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Engineering
3. Accuracy: the accuracy of a rapid prototype is dependent on many factors. Upon completion of the prototype the SLA provides greater accuracy than SLS.
APPLICATIONS
Selective Laser Sintering is the ideal solution for Fully functional prototypes and series. Rapid Manufacturing.
Fully functional prototypes and series
1. Parts for mechanical and thermal tests
The polynmide material allows the production of strong, durable parts that can be used tor extensive functional testing. Sintered products have mechanical properties comparable to those of injection moulded PA 12 parts, typical applications are snap fits but it is also possible to produce working hinges.
Polyamide parts with glass filling have a much higher thermal resistance and are perfectly suited for lighting elements and ventilation systems or products that require high thermal loads. Apart from their use as test products, the functional SLS parts often also need to be used at the same time for a visual/csthetical control or dimensional check.
2. Scries of small plastic parts
SLS is an interesting and cost-effective alternative to injection moulding (Rapid Tooling). With the P 700 machine which has a large build area, a series of small pieces can be built in one single laser sintering process. This dramatically decreases the price, as the cost of an SLS part depends on its volume. Or in other words, the cost is defined by the amount of powder it takes to build it and not by an initial investment in an injection moulding tool. Moreover, series of SLS parts are available in a few days. So no need for
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Engineering
high start-up investments, no long lead times to produce a mould and injection mould the parts, no difficulties in case the parts are complex.
Component of a vacuum cleaner, buiii on the EOSINT P700 with a build volume of700x380x580mm
3, Large and complex functional parts
The EOSINT P 700 machine can build large, complex geometries in one piece, up to 700x380x580mm. The number of layers to be built is significantly reduced as large parts can be built horizontally, which considerably shortens the building process. Parts exceeding the P 700' maximum dimensions .can be built in multiple pieces and put together afterwards. The process of gluing sub-parts and assembling components can be done in the most accurate and secure way using the Rapid Fit system. Rapid Fit allows to firmly position the parts on a unique support system with individualized fixtures, supporting the part on well positioned points.
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12
l)enl. Of M cchanical Engineering
CONCLUSION
Selective laser sintering provides exact representations of yotir complex designs in just days. This means that without delay, you receive a superior design communication tool. Using the physical prototype, you can detect errors early and correct them before it's too late. It all adds up to hitting aggressive deadlines critical to time-to-market reductions.
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Engineering
SELECTIVE LASER SINTERING
REFERENCE
1. howstuffworks.com
2. materialise.com
3. acceIeratedtechnologies.com
4. K. Murali e t al. / Journal of Materials Processing Technology 136 (2003) 179-185
5. Selective laser sintering of metals and ceramics, Int. J. Powder Me tall.
6. Laser sintering, J. Mater. Process. Technol. 111 (2001)210-213
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DepL Of Mechanical Engineering
