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Principle, process flow and technical characteristics of rapid prototyping technology;
Rapid prototyping belongs to discrete/cumulative molding. Based on the forming principle, a new thinking mode size model is proposed, that is, the three-dimensional model of parts made on the computer is stored in grids and layered to obtain the two-dimensional contour information of each layer section. According to these contour information, the machining path is automatically generated. Under the control of the control system, the molding head selectively solidifies or cuts the molding material layer by layer to form profile sheets of various sections, and sequentially adds three-dimensional blanks layer by layer. Then, the blank is post-processed to form parts.
The process of rapid prototyping is as follows:
L) Establish a three-dimensional model of the product. Because the RP system is directly driven by the 3D CAD model, the 3D CAD model of the machined workpiece should be constructed first. Three-dimensional CAD model can be directly constructed by computer-aided design software (such as Pro/E, I-DEAS, Solid Works, UG, etc.). ), or you can convert the two-dimensional drawings of existing products into three-dimensional models, or you can scan product entities by laser and ct to obtain point cloud data, and then build three-dimensional models by reverse engineering.
2) Approximate processing of 3D model. Because products often have some irregular free-form surfaces, it is necessary to approximate the model before machining to facilitate the subsequent data processing. STL format file has become a quasi-standard interface file in the field of rapid prototyping because of its simple and practical format. It uses a series of small triangular planes to approximate the original model. Each small triangle is described by three vertex coordinates and a normal vector, and the size of the triangle can be selected according to the accuracy requirements. STL file has two output forms: binary code and ASCll code. The output form of binary code takes up much less space than the file output form of ASCII code, but the output form of ASCII code can be read and checked. Typical CAD software has the function of converting and outputting STL format files.
3) Slice the 3D model. According to the characteristics of the processed model, the appropriate processing direction is selected, and the approximate model is cut with a series of planes at certain intervals in the molding height direction, so as to extract the profile information of the section. The interval is generally 0.05mm~0.5mm, and 0. 1mm is commonly used. The shorter the interval, the higher the molding accuracy, but the longer the molding time, the lower the efficiency. Conversely, the lower the accuracy, the higher the efficiency.
4) forming and processing. According to the cross-sectional profile of the slice, under the control of the computer, the corresponding forming head (laser head or spray head) does scanning motion according to the cross-sectional profile information, accumulates materials layer by layer on the workbench, and then bonds the layers together to finally get the prototype product.
5) Post-treatment of molded parts. Take the molded part out of the molding system, grind, polish and coat it, or put it in a high-temperature furnace for post-sintering, so as to further improve the strength.
Rapid prototyping technology has the following important characteristics:
L) Any complex three-dimensional geometric entity can be manufactured. Due to the principle of discrete/stacked molding, a very complex three-dimensional manufacturing process is simplified as the superposition of two-dimensional processes, and the processing of parts with arbitrary complex shapes can be realized. The more complex the parts, the more obvious the advantages of RP technology. In addition, RP technology is especially suitable for parts with complex cavities and complex profiles, which are difficult or even impossible to manufacture by traditional methods.
2) rapidity. By modifying or reorganizing the CAD model, the design and machining information of new parts can be obtained. Parts can be manufactured in a few hours to dozens of hours, which has the outstanding characteristics of rapid manufacturing.
3) High flexibility. The complex manufacturing process can be completed without any special fixture or tool, and tools, prototypes or parts can be manufactured quickly.
4) Rapid prototyping technology has achieved two advanced goals pursued by mechanical engineering for many years, namely, the integration of material extraction (gas, liquid and solid) process and manufacturing process, and the integration of design (CAD) and manufacturing (CAM).
5) Combining reverse engineering, CAD technology, network technology and virtual reality, it has become a powerful tool for rapid product development.
Therefore, rapid prototyping technology is playing an increasingly important role in the manufacturing field and will have an important impact on the manufacturing industry.
Classification of rapid prototyping technology;
Rapid prototyping technology can be divided into two categories according to the molding methods: laser technology based on laser and other light sources, such as three-dimensional mask aligner (SLA), layered solid manufacturing (LOM), selective laser powder sintering (SLS), shape deposition molding (SDM) and so on. Spray technology, such as fused deposition molding (FDM), three-dimensional printing (3DP) and multiphase spray deposition (MJD). Here is a brief introduction to the more mature technology.
1, SLA(stereolithography Apparatus) process SLA process, also known as light modeling or stereolithography, was patented by Charles Hull of the United States in 1984. 1988 American 3D systems company launched commercial prototype SLA-I, which is the first rapid prototyping machine in the world. SLA molding machine occupies a large share of RP equipment market.
SLA technology is based on the photopolymerization principle of liquid photosensitive resin. This liquid material can quickly undergo photopolymerization under the irradiation of ultraviolet light with a certain wavelength and intensity, and its molecular weight increases sharply, and the material changes from liquid to solid.
Working principle of SLA: The laser beam filled with liquid photocurable resin in the liquid tank can be scanned on the liquid surface under the action of deflection mirror, and the scanning trajectory and the presence or absence of light are controlled by computer. Where the light spot hits, the liquid will solidify. At the beginning of molding, the working platform is at a certain depth below the liquid level. The focused light spot is scanned point by point on the liquid surface according to the instruction of the computer, that is, solidified point by point. When a layer of scanning is completed, the unirradiated area is still liquid resin. Then the lifting platform drives the platform to descend one layer, and the molded layer is covered with a layer of resin. Scraper will scrape the resin with high viscosity, and then scan the next layer, and the newly circulated layer will be firmly bonded with the upper layer, and so on until the whole part is manufactured, and a three-dimensional solid model will be obtained.
SLA method is the most researched method in the field of rapid prototyping technology at present, and it is also the most mature method in technology. The parts formed by SLA process have high precision, the machining precision can generally reach 0. 1 mm, and the utilization rate of raw materials is close to 100%. However, this method also has some limitations, such as the need for support, the decline of precision caused by resin shrinkage, and the toxicity of light-cured resin.
2.LOM (Laminated Object Manufacturing) process The LOM process is called laminated entity manufacturing or layered entity manufacturing, which was successfully developed by Michael Feygin of Helisys Company in the United States in 1986. The LOM process uses thin materials such as paper and plastic film. The surface of the sheet is pre-coated with a layer of hot melt adhesive. In the process of processing, the hot-pressing roller carries out hot pressing on the plate to make it adhere to the workpiece formed below. The CO2 laser is used to cut the cross-sectional profile of the part and the outer frame of the workpiece on the newly bonded layer, and the grid aligned up and down is cut in the redundant area between the cross-sectional profile and the outer frame. After the laser cutting is completed, the workbench drives the formed workpiece to descend and separate from the strip plate. The feeding mechanism rotates the receiving shaft and the feeding shaft to drive the material belt to move and move the new layer to the processing area. The workpiece joint rises to the machining plane, and the hot roller is hot pressed, so that the number of layers of the workpiece is increased by one layer and the height is increased by one material thickness. Then cut the outline on the new layer. Repeat this process until all parts of the part are bonded and cut. Finally, the chopped redundant parts are removed, and the solid parts manufactured in layers are obtained.
LOM process only needs to cut the outline of the section of the part on the plate, and does not need to scan the whole section. Therefore, the forming speed of thick-walled parts is faster, and it is easy to manufacture large parts. There is no material phase change in the process, so it is not easy to cause warping deformation. The surplus material between the outer frame and the cross-sectional profile of the workpiece plays a supporting role in the machining, so the LOM process does not need support. The disadvantages are serious waste of materials and poor surface quality.
3.SLS (selective laser sintering) process SLS process is called selective laser sintering, which was successfully developed by C.R.Dechard of the University of Texas at Austin in 1989. SLS process is made of powder material. Spread the material powder on the upper surface of the molded part and scrape it flat. Scanning the section of the parts on the newly laid new layer with high-intensity CO2 laser. The material powder is sintered together under the irradiation of high-intensity laser to obtain the cross section of the part, which is connected with the part formed below. When sintering a section, a new layer of material powder is spread out and the lower section is selectively sintered.
After sintering, excess powder is removed, and then the parts are polished and dried.
SLS technology is characterized by a wide range of materials, which can not only make plastic parts, but also make parts made of ceramics, wax and other materials, especially metal parts. This makes SLS process attractive. SLS process does not need support, because there is no sintered powder to support it.
4.3DP (three-dimensional printing) process The three-dimensional printing process was developed by E-manual Sachs of Massachusetts Institute of Technology. It has been commercialized by Soligen Company in the United States, named DSPC (direct shell production casting), and is used to manufacture ceramic shells and cores for casting.
3DP process is similar to SLS process, using powder materials, such as ceramic powder, metal powder, etc. The difference is that the material powder is not connected by sintering, but the cross section of the part is "printed" on the material powder by the nozzle with adhesive (such as silica gel).
Parts bonded with adhesive have low strength and need post-treatment. Firstly, the binder is burned, and then the metal is infiltrated at high temperature, so that the parts are densified and the strength is improved.
5.FDM (Fused Deposition Forming) Process Fused Deposition Manufacturing (FDM) was successfully developed by American scholar Scott Crump on 1988. The materials of FDM are generally thermoplastic materials, such as wax, ABS, nylon and so on. Eat in a filamentous form. The material is heated and melted in the nozzle. The nozzle moves along the cross-sectional profile and filling trajectory of the part, and at the same time, the molten material is extruded, rapidly solidified and condensed with the surrounding materials.
Application fields of rapid prototyping technology;
At present, the development level of RP technology is mainly used in the design verification of new product (including product upgrade) development and the trial production of simulated samples in China, that is, the development process from product conceptual design (or modified design)-modeling design-structural design-basic function evaluation-trial production of simulated samples is completed. Some products mainly made of plastic structures can also be trial-produced in small batches, or some physical function tests, assembly verification and actual appearance effect inspection can be carried out, and even products can be assembled in small batches and put on the market first to achieve the purpose of asking for directions.
The application of rapid prototyping is mainly reflected in the following aspects:
(1) Design verification and function verification in the process of new product development. RP technology can quickly transform the CAD model of product design into a physical model, and can easily verify the rationality, assemblability and aesthetics of the designer's design ideas and product structure, and find problems in design that can be corrected in time. If the traditional method is adopted, it needs to complete many links such as drawings, process design, tooling, mold manufacturing, etc., which has a long cycle and high cost. If it is put into production directly without design verification, once there is a design error, it will cause great losses
(2) manufacturability and assemblability inspection, supply inquiry and marketing. Using RP method to test and design the manufacturability and assemblability of complex systems with limited space, such as automobiles, satellites and missiles, will greatly reduce the difficulty of designing and manufacturing such systems. For complex parts that are difficult to determine, RP and technology can be used for trial production to determine the best and reasonable process. In addition, RP prototype is also an effective means of communication between product design and commercialization. For example, providing customers with product samples and marketing, rapid prototyping technology has become a technical way of concurrent engineering and agile manufacturing.
(3) Direct production of single piece, small batch and special complex parts. For polymer parts, high-strength engineering plastics can be used for direct rapid prototyping to meet the use requirements; For complex metal parts, it can be obtained by rapid casting or direct metal forming. This application is of special significance to aviation, aerospace and national defense industries.
(4) Rapid mold manufacturing. Through various conversion technologies, the RP prototype is transformed into various rapid molds, such as low melting point alloy molds, silica gel molds, metal cold spraying molds, ceramic molds, etc. , the production of small and medium batch parts, in line with the development trend of fast product upgrading and smaller batch. The application field of rapid prototyping technology includes almost all industries in the manufacturing field, and it has also been widely used in medical treatment, human engineering, cultural relics protection and other industries.
The main applications of rapid prototyping technology are as follows:
Automobile and motorcycle: design, modification and assembly test of exterior and interior parts, trial production of engine and cylinder head.
Household appliances: structural design, assembly test, functional verification, marketing and mold manufacturing of various household appliances.
◆ Communication products: product shape and structure design, assembly test, function verification and mold manufacturing.
Aerospace: direct manufacturing of special parts, trial production of impellers, turbines and blades, trial production and assembly test of engines.
◆ Light industry: design, verification, assembly and marketing of various products, and rapid manufacturing of toys and footwear molds.
Medical treatment: design, trial production and trial use of medical devices, materialization of CT scanning information, surgical simulation, preparation of human bones and joints.
National defense: design, assembly and trial production of various weapon parts, direct production of special parts, modeling of remote sensing information.
In a word, the development of rapid prototyping technology is a breakthrough in the manufacturing field in recent 20 years. It is not only completely different from traditional methods in manufacturing principle, but more importantly, under the current situation that market response speed is the first in industrial strategy, RP technology can shorten product development cycle, reduce development cost and improve enterprise competitiveness. The following examples illustrate the role of this technology in product development.
1. Design verification: used to verify the appearance design and structural design of new products, find design defects and improve product design. In modern product design, design methods are more and more advanced, and computer-aided design makes product design quick and intuitive. However, due to the limitation of software and hardware, designers still can't intuitively evaluate the effectiveness of the designed products, the rationality of the structure and the feasibility of the production process. Rapid prototyping technology provides advanced technical means for designers to obtain product samples quickly and judge products intuitively. Our company made a model for a new 250 motorcycle in a motorcycle factory, including fuel tank, front and rear baffles, seats and side covers. *** 13 pieces. Using AFS molding technology, the whole production was completed in only 12 days. The designer installed the sample on the car body, and after careful evaluation and repeated comparison, the appearance of the product was modified again, reaching an ideal state. This verification process makes the design more perfect and avoids the waste caused by blind production.
2. Assembly verification: make samples and carry out assembly experiments. A company in Tianjin commissioned us to process the fax machine shell and telephone. Users should not only evaluate the appearance, but also put the internal parts of the fax machine into the sample for assembly experiment and structural evaluation. The company first chose the traditional processing methods, block processing and manual bonding. It took 4000 yuan and 20 days to process only one set of telephone handsets. It is estimated that it will take 2 months to make a fax machine sample, and the cost is 2? 50 thousand yuan. Our company adopted rapid prototyping technology, and delivered a set of 6 pieces of this product to the entrusting party in only 15 days. In the assembly experiment, users found seven assembly interference points and unreasonable structures. Compared with the two methods, the traditional processing method of plastic assembly samples of fax machine BABS has many procedures, and manual splicing is time-consuming, laborious, wasting materials and long processing cycle. For complex structures and surfaces, the machining is rough and the dimensional accuracy is low, so it is difficult to establish a one-to-one correspondence between physical models and design models, so it is difficult to detect design errors in assembly experiments. The automatic molding method has the advantages of high automation, one-time molding, short cycle and high precision, which corresponds to the design model one by one and is more suitable for the production of sample components.
3. Functional verification: Our company manufactures 250 cylinder heads for a motorcycle factory. This is a newly designed engine, and users need 10 samples for engine simulation experiment. The internal structure of the part is complex, which can not be processed by traditional machining and can only be cast. The whole process needs to go through mold opening, core making, mold assembly, casting, sandblasting and machining, just like the actual production process. Among them, it takes three months to open the mold only. This is unacceptable for making small batches of samples in time and wood. We use selective laser sintering technology, using investment casting materials as molding materials, and process 10 investment castings on the rapid prototyping machine in only 5 days, and then use investment casting technology to get the casting blank after 10 days. After necessary processing, the trial production of this engine was completed within 30 days.
4. Rapid casting: In manufacturing industry, especially in key industries such as aviation, aerospace, national defense, automobile, etc., the core components of * * * foundation are generally metal parts, and a considerable number of metal parts are asymmetric, with irregular surface or complex structure and fine internal structure. These parts are usually produced by casting or disassembling. In casting production, the manufacture of templates, core boxes, wax molds and die-casting molds is often completed by mechanical processing, and sometimes it needs fitters to trim them, which is not only a long period and high cost, but also a complicated process from mold design to processing and manufacturing. If something goes wrong, it will lead to all rework. Especially some castings with complex shapes, such as blades, impellers, engine blocks, cylinder heads, etc. Mold manufacturing is a difficult problem. Even if expensive equipment such as CNC machining center is used, there are still great difficulties in machining technology and process feasibility. It is conceivable that the manufacturing cycle, cost and risk of such parts will be considerable if they are trial-produced or produced in small batches.
Laser rapid prototyping technology has been proved to be a very effective means to solve the problem of small batch complex parts manufacturing. Up to now, we have successfully produced more than 1000 scanning disk drilling parts including bell, blade, engine rotor, pump body, engine cylinder block, cylinder head and so on through laser rapid prototyping. We call the combination of rapid prototyping and casting process rapid casting process. Fig. 5 shows the comparison between the rapid casting process and the traditional casting process. Because there is no need to open the mold in the rapid casting process, the manufacturing cycle and cost are greatly saved. Fig. 6 shows the S-shaped cross section of the second gas Mover produced by the rapid casting method. The diameter of this part is 80Omm and the height is 4 10m. It is made by traditional metal casting method. The mold manufacturing cycle is about half a year, and the cost is several hundred thousand. Using rapid casting method, the investment in rapid prototyping casting is 7 days (divided into 6 sections), and the assembly, assembly and casting 10 days, and the cost of each piece does not exceed 20,000 (* * * 6 pieces). Using the rapid prototyping method to produce the casting investment model of the new tank supercharger, we completed the production of 37 wax models in five days, which made the whole trial production task three months ahead of schedule.
5. Flip molding: In practical application, many products can only be processed by molds. It is a time-saving and money-saving method to make a product sample first and then copy the mold with the molding machine. The prototype product of engine pump shell is difficult to be processed by traditional processing methods, so it must be molded by mold. It is estimated that the mold opening time will be 8 months and the cost will be at least 300,000. If the product design is wrong, the whole set of molds will be scrapped. We made a plastic sample for this product by rapid prototyping, and used it as the mother mold to copy the silicone mold. Fix the master mold in the aluminum standard mold frame, pour the prepared silicone rubber, and let it stand 12? After 20 hours, the silicone rubber is completely cured, the mold frame is opened, the silicone rubber is taken out, the master mold is taken out by cutting along the predetermined parting line with a knife, and the silicone rubber mold for casting the pump shell wax mold is successfully copied. After coating, baking, wax loss, die casting and sand blasting, qualified pump shell castings can be manufactured in only two months. After necessary processing, it can be installed and operated, which shortens the whole trial production period by two thirds and saves three quarters of the cost compared with the traditional method.
6. Sample making: making product substitutes for displaying new products and marketing, such as making communication, household appliances and building models.
7. Process and material verification: quickly make various wax models for the exploration and verification of new investment casting processes and materials, as well as the test of auxiliary tools and components needed for new product manufacturing. An experiment of precision casting blades with little margin. Firstly, a molding machine is used to make several wax models of blades at one time according to different shrinkage rates, and then wax coating, numbering and lost wax casting are carried out. By measuring the obtained blade castings, the shrinkage of different materials can be determined by repeating several times, which lays the foundation for mass production. If the experiment is carried out by opening the mold, its cost and cycle will be greatly increased. The high-speed turbine of the engine needs high materials and dense castings. Four wax molds for precision casting were made by laser rapid automatic molding machine, numbered and coated with paint, and were cast with different proportions of special alloys. Four samples were tested and compared, and the best formula of the material was determined. It only takes one month from modeling to getting the result.
8. Reverse Engineering and Rapid Prototyping: The motorcycle front panel sample molded by the molding machine contains a headlight and two side lampshades, which form a complete curved surface with the panel. This is a typical example of detailed design of parts by reverse engineering. The whole technological process is: firstly, the modeler makes a conceptual model with sludge according to the overall image requirements of the motorcycle, and after the evaluation is satisfactory, it is digitized with a three-coordinate measuring instrument. The measured data is collated by Scantools module of Pro/E software, converted into curve model, then converted into solid model, and "details" are calculated. Finally, the prototype model is made by the molding mechanism, polished and painted, and then installed on the motorcycle for appearance and assembly inspection. The whole process takes only one week from coordinate measurement to sample acquisition. At this time, the sample model obtained is different from the original sludge model, and it has become a part model with the same wall thickness and size as the actual part, and has a complete structure such as ribs and holes, which is undoubtedly a great progress compared with the sludge model. If you need to modify the model at this time, you can only do it on the CAD system. When the appearance and detailed structure of the model are determined, the final model data can be used for mold design and processing.