Rapid_Prototype

1.1 Introduction In this chapter the basic definition of Rapid Prototyping and Manufacturing (RP&M) is given. Outline of different systems available in RP& M are presented. Advantages and disadvantages of some of the systems are also highlighted. 1.2 General Being a manufacturing engineer, one is usually concerned with the removal of material from stock or producing a new shape via castings. The removal of material has advanced to machines and virtual manufacturing. All of these processes involve tooling. In late 80’s the concept of Rapid Prototyping (RP) was coming to light, even though the layered method to produce molds for topographical relief maps dates back to 1890 [1]. This reminded me of my grand mother, who used to make mud ovens by layer addition, after preparing the mud from the earth by pouring some water and hay. Rolling a piece of mud as a layer and giving it a round shape by joining the ends like a big circle. The second layer was produced the same way and placed on top of the first layer after wetting the top of the first layer using water as a gluing agent. Leaving it to the sunlight ensured that the layers are cured. The subsequent layers were shorter in diameter and length as shown in figure 1.1. The oven used to take two to three days to complete. The final process was to cure it in the sunlight or on fire. Such ovens are produced in thousands in different parts of the world. There are lot of similarities between producing a mud oven by hand and producing complicated shapes via RP processes. 1.3 Basic Definition of RP RP may be defined as rapid generation of complex physical objects by layer deposition of material, directly from the CAD model without using tools. Other terms usually used for RP are layer manufacturing, automated fabrication, materials in cress manufacturing, solid free form fabrication, instant manufacturing, fast free form fabrication, rapid fabrication technologies, desktop manufacturing, direct metal deposition, 3D printing technologies, direct light fabrication, direct layer deposition, time compression technologies and many others. RP involves generation of a CAD model of the desired product. The model is then tessellated and converted into Stereo Lithography file format. file is simply the conversion of the object surface into a connected array of tiny triangles. file/object is then sliced by passing horizontal planes mathematically through the tessellated file, depicting the layer thickness. The information regarding the border geometry of each layer and the scanning pattern to scan internal area of each layer is also contained in the SLICE file. The build file is now used to control the laser. In the case of stereo lithography the laser is used to cure the photo curable liquid resin in a vat. The laser beam first traces the boundary of the section and then it solidifies the interior cross-sections. Once one layer is completely traced, the movable table is lowered in the vat for a distance equal to the layer thickness. The process is repeated until the part is completely fabricated. Supports for the overhang portions are also built along with the main part, which are then cut-off before curing. After completion the part is post cured. About 90% of all RP&M systems sold in 90’s were using the system similar to as described above. [2] 1.4 Systems of RP & M [3] There are many diverse RP systems developed over the years. Different types of materials, from plastic to ceramics and from metallic powders to water particles have been used and tried out by different researchers. Figure 1.2 gives the different systems available in RP now a day. Figure1.2 Classification of RP methods (adapted from [3] 1. Stereo Lithography ( was the first commercial system available to produce physical objects directly from the CAD model by layer additive process. The working is the same as explained in the introduction section 1.3. The out line is shown in figure 1.3. Reported advantages are good surface finish, greatest accuracy and large object sizes are possible. Figure 1.3 Simple out line of the process 2. Liquid Thermal Polymerization Infrared laser is used to cure the thermostatic resin. The fabrication sequence is the same as that of 3. Beam Interference Solidification (BIS) Two laser beams are used to polymerase the resin. Both the beams are having different frequencies and are mounted at right angle to each other. Transparent vat is used in this process. The resin is polymerized only at the intersection of both the beams as shown in Figure1.4. The solidification occurs in two stages, in the first stage the resin in the path of laser 1 is excited to a metastable state and the process is completed only after it is subjected to laser 2. No commercial system is available so far and it is still being researched. The process is limited due to shadows of the solidified portions, and drop in laser intensity due to increased depth [3]. Figure 1.4 Beam Interference Solidification 1.4.4 Object Quadrant Process The Object Quadrant Process utilizes ink jet technology by incorporating 1536 nozzles to deposit photosensitive resin. Two UV lamps fully cure the resin immediately following droplet deposition allowing faster building time. No post curing is required. A layer thickness of 20(m can be achieved by this system. The system is shown in figure 1.5 [4]. [pic] Figure 1.5 Object Quadrant Tempo TM 1.4.5 Solid Ground Curing In the first step a coating of photo polymer resin is applied to the object under construction (A) as it is passed under the resin applicator station (B) shown in figure 1.6. It is then shifted to the exposure cell (C). An electron gun is used to write a charge pattern on a glass plate (D). A mask is then generated on this glass plate by electrostatics transferring toner to produce a negative impression, using the data from the CAD model. The mask is then moved to the exposure cell where it is positioned above the object under construction. Opening a shutter, allows a high intensity light to pass through the exposed cross sections in the mask and cure the resin. The entire exposed surface is cured at once. After cleaning of the glass plate new mask is generated for the next layer. The under construction object is moved to the aerodynamic wiper (E) where the unhardened resin is vacuumed off. It is the transferred under the wax applicator (F) where the voids are filled after the removal of unused resin. The wax is hardened by moving the object to the cooling station (G), where it is pressed against a cold plate. Both the wax and photo polymer are milled to a uniform thickness under the milling head (H). This complete cycle is repeated until the object is completed. The wax is removed by melting [5]. [pic] Figure 1.6 Solid Ground Curing 1.4.6 Holographic Interference Solidification (HIS) After obtaining the data from the CAD model not in the form of slices, a holographic image is projected into the resin, causing the entire surface to solidify. This system is being researched [3]. 1.4.7 Electrostatics (ES) Electrodes are printed onto a conductive material like aluminum. After printing all the layers, these are stacked and dipped into a bath of Electrostatics fluid and are energized. The part is formed by the solidification of the fluid in between the electrodes. After the composite has been removed and drained, the unwanted aluminum is trimmed from the part. Materials used to fabricate parts are silicon rubber, polyester, or epoxy. The system software is still being researched [3]. 8. Rapid Freeze Prototyping In three-dimensional parts are build as per the CAD model by solidification/freezing of water droplets in a layer-by-layer manner. It can build ice parts faster than other solid free form fabrication processes [6]. 9. Ballistic Particle Manufacture A continuous jet or a drop-on-demand of molten material is ejected from a nozzle (Figure 1.7). It is separated into droplets, which after hitting the substrate, cold-weld to form the part. The controlling mechanism to produce droplets form the nozzle is piezoelectric transducer. Regular streams, few small droplets and large droplets are possible to obtain. Temperature, velocity and the electrostatic charge carried by the droplets are the parameters, which affect the quality of the part being build [3]. Figure 1.7 Ballistic Particle Manufacture (Adapted from [3]) 10. Multi Jet Modeling Specially developed thermos-polymer material is deposited where necessary by individual jets in a print head, where a total of 352 jets are enclosed. A model is build by successive layers. utilities a technology similar to the ink jet. The print head shuttles back and forth along the X-axis like a line printer. Its resolution is 300( 400( 600 ). Support structures are removed from the build part after completion. Figure 1.8 shows a typical fabrication [7-8]. [pic] [pic]