26-07-2012, 02:18 PM
LASER SINTERING OF METALS USING RAPID PROTOTYPE TECHNIQUE
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INTRODUCTION
RAPID PROTOTYPING
Rapid prototyping is the automatic construction of physical objects using additive manufacturing technology. The first techniques for rapid prototyping became available in the late 1980s and were used to produce models and prototype parts. Today, they are used for a much wider range of applications and are even used to manufacture production-quality parts in relatively small numbers. The use of additive manufacturing for rapid prototyping takes virtual designs from computer aided design (CAD) or animation modeling software, transforms them into thin, virtual, horizontal cross-sections and then creates successive layers until the model is complete.
With additive manufacturing, the machine reads in data from a CAD drawing and lays down successive layers of liquid, powder, or sheet material, and in this way builds up the model from a series of cross sections. These layers, which correspond to the virtual cross section from the CAD model, are joined together or fused automatically to create the final shape. The primary advantage to additive fabrication is its ability to create almost any shape or geometric feature.
STEREO-LITHOGRAPHY
Principle and Process
This is based on selective polymerization of a photosensitive resin using ultraviolet light. In this system, an ultraviolet laser beam is focused on the top layer of photo sensitive resin contained in a vat.The beam is positioned and moved in horizontal X and Y directions to polymerize the resin within the boundary a particular cross-section. The cured layer of polymer is lowered by a platform attached to it, so that a fresh layer of liquid resin covers the cured layer.
LASER SINTERING PROCESS
The geometry of a part to be made by LS or any SFF technique is converted to the STL format. In the STL format part geometry is approximated by a triangular surface mesh that bounds the solid volume. Before the geometry is fabricated, the STL file is “sliced” into a series of cross-sections of a given thickness using a slicing algorithm. Each of these cross-sections is then built in succession to form the part. The laser sintering process, schematic shown in Figure 1.1, starts by repeatedly depositing smooth, uniformly dense layers of powder into the part bed until a powder base of a set depth is deposited.
The temperature of the part bed is ramped up to its operating temperature as the base layers are added; heaters are positioned above the part bed. The powder is delivered from heated feed cylinders (only one is shown in the figure) on either side of the part bed via a counter-rotating roller. The part bed is then raster scanned by a CO2 laser beam, fusing corresponding areas in the powder bed into a solid image of a given slice. The part bed is then lowered a given distance, (3-6 thousandths of an inch is typical), and additional powder is added to the part bed on top of the layer previously scanned. The next slice of the STL file is then fused onto the previous slice by raster scanning the laser beam. Successive layers are then deposited and fused in the same manner until the desired part geometry is complete.
Part Bed Heating
Initially, the part bed is heated to a temperature where un-scanned powder does not consolidate over the time scale of a part build. For semi-crystalline polymers the part bed temperature is above the glass transition temperature, Tg, and below the melting temperature, Tm. For amorphous polymers the part bed temperature is less than or approximately equal to the glass transition temperature. Maximizing the part bed temperature: 1) minimizes the amount of laser energy needed for consolidation, 2) minimizes the thermal gradient between the consolidated and unconsolidated powder and 3) minimizes the thermal expansion due to heating by the laser. Part bed heating is one
aspect of LS where profitability and part quality are at odds; the expense of powder is such that reusing unconsolidated powder is necessary.
DIRECT METAL LASER SINTERING (DMLS)
Direct Metal Laser Sintering (DMLS) is a revolutionary technology that produces metal components that are 99.99% dense, directly from your 3D CAD data. The parts produced are comparable to a good investment cast part and the mechanical properties are comparable to those of a cast or machined component. The DMLS process is not restrictive in its application and the components produced can be used in place of almost any conventionally manufactured part, whether they would normally be machined or cast. The advantage of the process is that the more complex or feature rich the component, the more economical the process becomes. This fast, flexible and cost-effective method enables you to produce prototype or production parts without the investment in time and money of conventional tooling. DMLS is fast becoming a recognized manufacturing method for the fast, accurate production of one-off prototype components or for the economical manufacture of small series parts for testing purposes or as final products for use in many different environments. More and more industries are using physical models throughout their product development cycle.