先进制造技术 第十单元

10 Rapid Prototyping and Manufacturing10.1 Introduction10.1.1 What Is RP&MManufacturing community is facing two important challenging tasks:(1) Substantial reduction of product development time; and (2) improvement on the flexibility for manufacturing small batch and diversified products. Computer-aided design and manufacturing (CAD and CAM) have significantly improved the traditional product design and manufacturing. However, there are a number of obstacles in true integration of computer-aided design with computer-aided manufacturing for rapid development of new products. Although substantial research has been done in the past for computer-aided design and manufacturing integration, such as feature recognition, CNC programming and process planning, the gap between CAD and CAM remains unfilled in the following aspects:(1) Rapid creation of 3D models and prototypes.(2) Cost-effective production of patterns and moulds with complex surfaces.To substantially shorten the time for developing patterns, moulds, and prototypes, some manufacturing enterprises have started to use rapid prototyping (RP) methods for complex patterns making and component prototyping. Over the past few years, a variety of new rapid manufacturing technologies, generally called Rapid Prototyping and Manufacturing (RP&M), have emerged; the technologies developed include Stereolithography (SL), Selective Laser Sintering (SLS), Fused Deposition Modeling (FDM), Laminated Object Manufacturing (LOM), and Three Dimensional Printing (3D Printing). These technologies are capable of directly generating physical objects from CAD databases. They have a common important feature: the prototype part is produced by adding materials rather than removing materials, that is, a part is first modeled by a geometric modeler such as a solid modeler and then is mathematically sectioned (sliced) into a series of parallelcross-section pieces. For each piece, the curing or binding paths are generated. These curing or binding paths are directly used to instruct the machine for producing the part by solidifying or binding a line of material. After a layer is built, a new layer is built on the previous one in the same way. Thus, the model is built layer by layer from the bottom to top.10.1.2 The History of RP&MAs usual with invention, one individual's impatience was the propellant for the rapid prototyping industry, now barely two decades old. Its father, CharlesW Hull, founded 3D Systems of Valencia, Calif. in 1986. As an engineer, Hull had always been bothered by the long time it took to make prototype models of plastic. They had to be machined by hand, he recalls. If more than one was needed, generally the case in industry, molds for making plastic prototypes had to be individually machined.Hull had been working for a small company that used ultraviolet lamps toharden photosensitive plastic coatings on glass and other objects. One of his insights was to substitute a laser for an ultraviolet lamp. "But taking thatinsight to a practical machine came slowly," Hull recalls, and required several years of Edison-style inspiration. (In fact, a prototyping machine based on conventional UV light was developed in 1998 by The Institute of Advanced Manufacturing Technology, Xi'an Jiaotong University, China).The result was the first prototyping machine, introduced by 3D Systems in 1987. It could fabricate small, transparent plastic parts from CAD drawings in hours or at most days. The machine builds the model in layers, from the bottom up. A laser, which causes molecules of a photosensitive liquid resin to polymerize, scans above a vessel filled with the resin. The laser first traces the outline of a layer on the resin's surface. Next, like an artist shading a panel in a pencil drawing, the beam crisscrosses the whole outlined area to harden it. Then the platform holding the model sinks so the layer is barely awash inliquid resin, the laser goes to work solidifying another layer atop it, and so on. When the translucent object is fabricated, it is raised from the vat, dripping like a mermaid just emerged from the sea. Hull dubbed the process stereolithography, and it still dominates RP.Before long other inventors jumped in. Michael Feygin, an immigrant Russian engineer, hit on the idea of building prototypes from inexpensiveslices of paper. His company, Helisys of Torrance, Calif., makes remarkably sturdy objects by a process called laminated-object manufacturing (LOM). A CO2 laser traces each layer by burning, moving like a crazed ice dancer carving a turn here, a straight line there. Successive layers are bonded by adhesive. Heisys's machines have modeled auto steering wheels, bumpers and other shapes that feel like wood to the touchThe early RP machines could make a metal prototype only in an indirect way. First a plastic model had to be "invested", or clad in a heat-resistant material such as a ceramic. Then the model was "sacrificed" by melting, just as the ancient Egyptians melted a wax model inside a mold to clear the way for a bronze casting. This leaves a mold suitable for making a metal or plastic prototype.Extrude Hone, a company in Irwin, Pa., is getting ready to market a machine, based on MIT's ink-jet technology, that will make not only metal molds but also salable metal parts. In Extrude Hone's machine, powdered steelis hardened with a binder and infiltrated with bronze powder to create a material that is 100% metal.Powerful new lasers may also open doors to direct manufacturing. Such laser systems are being explored at national laboratories such as Sandia andLos Alamos, as well as at the University of Michigan, Penn State, and elsewhere. They may soon be available commercially. In the Sandia system, a 1000-watt neodymium YAG (yttrium-aluminum-gallium) laser melts powdered materials such as stainless and tool steels, magnetic alloys, nickel-based superalloys, titanium, and tungsten in layers to produce the final part.Some experts look to a manufacturing future extensively liberated from today's noisy, hot routines. Instead of molds and machine tools, these visionaries foresee rows of lasers building parts, 3-D printers fashioning convoluted shapes no CNC machine can carve, and silent ceramic part-makers replacing the traditional noisy factory din. Many products turned out in future factories could be designed to take advantage of rapid-manufacturing techniques. Implantable drug-release devices, with medicine sealed in, couldbe made in a single operation, since 3-D printers can make a sandwich-like product.10.2 Rapid Prototyping and Manufacturing TechnologiesAs mentioned earlier, there are several technologies available for model production based on the principle of "growing" or "additive" manufacturing. The major differences among these technologies are in two aspects: (1) materials used; (2) part building techniques. The following sections will explain in detail these rapid prototyping technologies with respect to the above two aspects. 10.2.1 StereolithographyStereolithography apparatus (SLA) was invented by Charle Hull of 3D Systems Inc. It is the first commercially available rapid prototyper and is considered as the most widely used prototyping machine. The material used is liquid photo-curable resin, acrylate. Under the initiation of photons, small molecules (monomers) are polymerized into large molecules.111 Based on this principle, the part is built in a vat of liquid resin as shown in Fig.10.1. The SLA machine creates the prototype by tracing layer cross-sections on the surface of the liquid photopo-lymer pool with a laser beam. Unlike the contouring or zigzag cutter movement used in CNC machining, the beam traces in parallel lines, or vectorizing first in one direction and then in the orthogonal direction. An elevator table in the resin vat rests just below the liquid surface whose depth is the light absorption limit. The laser beam is deflected horizontally in X and Y axes by galvanometer-driven mirrors so that it moves across the surface of the resin to produce a solid pattern. After a layer is built, the elevator drops a user-specified distance and a new coating of liquid resin covers the solidified layer. A wiper helps spread the viscous polymer over for building the next Layer.The laser draws a new layer on the top of the previous one. In this way, the model is built layer by layer from bottom to top. When all layers are completed, the prototype is about 95% cured. Post-curing is needed to completely solidify the prototype. This is done in a fluorescent oven where ultraviolet light floods the object (prototype).10.2.2 Selective Laser SinteringDTM Corp. (Austin TX) offers an alternative to liquid-curing systems with its Selective laser sintering (SLS) systems which were developed by Carl Deckard and Joseph Beaman at the Mechanical Engineering Department of University of Texas at Austin. SLS uses a carbon dioxide laser to sinter successive layers of powder instead of liquid. In SLS processes, a thin layer ofpowder is applied by a counter-rotating roller mechanism onto the work place. The powder material is preheated to a temperature slightly below its melting point. The laser beam traces the cross-section on the powder surface to heat up the powder to the sintering temperature so that the powder scanned by the laser is bonded. The powder that is not scanned by the laser will remain in place to serve as the support to the next layer of powder, which aids in reducing distortion. When a layer of the cross-section is completed, the roller levels another layer of powder over the sintered one for the next pass. Fig. 10.2 shows the working principle of SLS. The product may suffer shrinkage and warpage due to sintering and cooling. These two factors can be partly eliminated by choosing powder particles which have a small size, and a high aspect ratio and air flow temperature above the softening point of the powder, but below the sintering point.10.2.3 Fused Deposition ModellingAnother rapid prototyping system, developed by Stratasys Inc., constructs parts based on deposition of extruded thermoplastic materials, is called FDM process.In an FDM process as shown in Fig. 10.3, a spool of thermoplastic filament feeds into a heated FDM extrusion head. The movement of the FDM head is controlled by computer. Inside the flying extrusion head, the filament is melted into liquid (10 above the melting temperature) by a resistant heater. The head traces an exact outline of each cross-section layer of the part. As the head moves horizontally in X and Y axes, the thermoplastic material is extruded outof a nozzle by a precision pump. The material solidifies in 1/10 second as it is directed onto the workplace. After one layer is finished, the extrusion head moves up a programmed distance in Z direction for building the next layer. Each layer is bonded to the previous layer through thermal heating.10.2.4 Laminated Object Manufacturing (LOM)The LOM processes produce parts from bonded paper plastic, metal or composite sheet stock. LOM machines bond a layer of sheet material to a stack of previously formed laminations, and then a laser beam follows the contour of the part cross-section generated by CAD to cut it to the required shape. The layers can be glued or welded together and the excess material of every sheet is either removed by vacuum suction or remains as next layer's support. Fig. 10.4 shows the working principle of LOM.10.2.5 Three-Dimensional PrintingThree-dimensional printing (3D Printing) was developed at Massachusetts Institute of Technology. In the 3D Printing process, a 3D model is sliced into2D cross-section layers in computer. A layer of powder is spread on the top of the piston, the powder bed, in a cylinder, and then an inkjet printing head projects droplets of binder material onto the powder at the place where the solidification is required according to the information from the computer model. After one layer is completed, the piston drops a predefined distance and a new layer of powder is spread out and selectively glued. When the whole partis completed, heat treatment is required to enhance the bonding of the glued powder, and then the unbonded powder is removed. Fig.l0.5 shows the working process of 3D Printing10.5 Problems and ResearchRP&M is still in its infancy. The physical models made by most of these systems can not be used as working parts, mostly due to material and economic constraints. The major problems in the current RP&M systems include: part accuracy, limited material variety and mechanical performance.10.5.1 The Problem About Part AccuracyA large number of factors limit the ability of rapid prototyping systems to create parts as accurate as the CAD designs on which they are based. The most common sources of error among the RP&M systems could be categorized as mathematical, process-related or material-related.(1) Mathematical errors include facet approximation of the part surfaces in the standard input to RP&M systems, limited layer resolution along the Z axis, such as stairsteps and accuracy of vertical dimensions. Alternative data preparation methods based on CSG solid modeling which can input precise part surface to RP&M systems are under development. Parts are represented with CSG and a ray-tracing algorithm is used to generate the curing path. Carnegie-Mellon University (CMU) uses a 5-axis CNC mill to remove the excess material to shape the geometry of the layer in a newly developed RP&M system (shape deposition manufacturing) which can eliminate the stairsteps. This 5-axis mill could also be used to maintain the accuracy of vertical dimensions by milling away the excess thickness.(2) Process-related errors affect the shape of the layer in the X-Y plane and along the Z axis, the registration between different layers and the overall 3D shape. These errors are mainly dependent on the accuracy of the RP&M machines and operators' experiences.(3) Major material related errors are shrinkage and distortion. The shrinkage is a by-product of solidification-the cooling down of material with rapid prototyping processes. Predictable dimensional shrinkage can be compensated for by scaling the CAD model. Sometimes, the shrinkage is not identical along X, Y and Z axes. During the processes of building parts, stresses due to shrinkage may be locked into parts. Eventually, these stresses may cause the part to creep and distort. There may be several ways to minimize the effect of shrinkage: selection of appropriate manufacturing control parameters, development or exploration of materials with relatively small shrinkages or stressfree properties, and stress relief methods. All these approaches requirein-depth research on the materials and understanding of the processes. For the SLA system, 3D System's STAR(staggered alternative retract) Weave method can limit some of the distortion by curing more resin in the vat, and hatching the inside cross-section area on each layer. Many researchers have conducted serious study on the physical and chemical characteristics of the materials used in stereolithography systems. In Stratasys Inc. experimental research formanufacturing control parameters and material selection is carried out forFDM system. The in-depth research on FDM processes results in system refinements in accuracy, speed and surface finish. The physical mechanismsthat drive the SLS process are being studied in University of Texas. The physical and chemical properties of materials in 3D Printing are being studied for better product dimensions. Japanese Synthetic Rubber Co. Ltd. And DMEC Ltd. developed a new resin for stereolithography, which was effective in reducing the distortion. DuPont Imaging Systems also developed a low-shrinkage resin for its stereolithography systcm-SOMOS. CMU uses a method of striking metallic balls or shots under pressure on each solidifiedlayer to relieve large stresses generated during the processes of depositingmetal material for its rapid prototyping systems.10.5.2 The Problem About MaterialsThe current RP&M systems use very limited types of materials. Parts built by RP&M systems tend to be weak and fragile compared to those made conventionally from metals and engineering plastics. Some materials for theRP&M machines are expensive or toxic. Most of the research and development efforts have been focused on improving part materials, which are carried out in two different directions.One is that plastic companies with products already on the market, particularly those based on SL, SLS and FDM, have developed plastics with better physical properties-less brittle, lower shrink, lower viscous resin and similar to the end-user applications. Chemists developed a new less-brittle polymer that is used in 3D System's SLA-500. Another method for improving the mechanical properties of the material uses fibre-resin composites composed of glass, carbon, or graphite fibre bonded together by polymeric resin andcured into a solid part. Some researchers are trying to mix ceramics powder in the photopolymer resin to create ceramic green bodies such as structural ceramic parts and refractory ceramic parts for investment casting using stereolithography systems.The other direction is to focus on metals. The users of RP&M technologies tend to build models with materials whose properties are similar to the materials they might use in their end-use applications. Metal is most-commonly used in current industry. All rapid prototyping machines onthe market can be used to produce metal parts indirectly through various casting processes such as investment casting. The direct production of metal parts is still in development. CUM is developing a metal based RP&Msystem-Shape Deposition in which a part is built by successively depositing molten metal materials such as zinc, steel and copper in thin layers. WithMIT's 3D Printing technique, a ceramic shell or mould complete with corescan be produced with a direct monolithic casting shell fabrication process.Then an alloy is poured into the shell to cast a functional metal part. The University of Texas is pursuing the direct sintering of metal powder (SLS). Several metal mixtures are used in the SLS process to produce metallic parts,such as low melting-point binary metal powders: Cu-70Pb-3OSn solder, Cu-Sn and Ni-Sn; and high temperature materials: prealloyed 90Cu-IOSn bronzemixed with Ni powder.10快速原型制造10.1简介10.1.1什么是RP＆M制造社会面临两个重要的挑战性的任务：（1）大幅减少产品开发时间;和（2）的改善在灵活制造小批量，多样化的产品。