The RP system can make a cross-section with a certain tiny thickness and a specific shape according to the shape of the part each time, and then bond them layer by layer to get the three-dimensional part that needs to be manufactured. Of course, the whole process is done automatically by the rapid prototyping system under the control of a computer. The forming materials used in RP systems manufactured by different companies are different, and the working principle of the system is also different, but the basic principle is the same, that is, manufacturing in layers and stacking layer by layer. This process can be visualized as the growth or addition method.
Each cross-section data is equivalent to a CT image in medicine; the whole manufacturing process can be compared to a process of integration.
The basic principle of RP technology is: the computer will be within the three-dimensional data model for layered slicing to get the contour data of the cross-section of each layer, the computer according to this information to control the laser (or nozzle) selectively sintering layer after layer of powder material (or curing layer after layer of liquid photosensitive resins, or cutting layer after layer of flake material, or spraying layer after layer of hot-melt material or adhesive) to form a series of a tiny thickness of a series of materials (or materials). ) to form a series of sheet-like entities with a small thickness, and then use fusion, polymerization, bonding and other means to make it stacked layer by layer into one, you can create the design of the new product samples, models or molds. Since the U.S. 3D company launched the first commercial SLA rapid prototyping machine in 1988, there have been more than a dozen different forming systems, which are more mature UV, SLA, SLS, LOM and FDM methods. Their forming principles are described as follows: Stereo lithography Appearance abbreviation, that is, three-dimensional light-curing molding method.
With a specific wavelength and intensity of the laser focused on the surface of the light-curing material, so that it is from point to line, from line to surface sequential solidification, the completion of a level of mapping operations, and then lifting the table in the vertical direction to move the height of a layer of the film, and then curing another level. In this way, the layers are stacked to form a three-dimensional solid.
SLA is the earliest practical rapid prototyping technology, the use of liquid photosensitive resin raw materials, the principle of the process shown in Figure. The process is, first of all, through the CAD design of three-dimensional solid model, the use of discrete program will be sliced model processing, the design of the scanning path, the resulting data will accurately control the movement of the laser scanner and lifting platform; laser beam through the numerical control device to control the scanner, according to the design of the scanning path irradiated to the surface of the liquid photosensitive resin, so that a layer of the surface of a specific region of the resin curing, when a layer of processing is completed, it will generate a part of a part. After the completion of a layer of processing, to generate a cross-section of the part; and then lift the platform down a certain distance, curing layer covered with another layer of liquid resin, and then the second layer of scanning, the second curing layer firmly bonded to the previous curing layer, so that a layer of superimposed three-dimensional prototype of the workpiece. After the prototype is removed from the resin, final curing is performed, and then the required product is obtained by polishing, plating, painting or coloring.
SLA technology is mainly used in the manufacture of a variety of molds, models, etc.; can also be added to the raw material by adding other components, SLA prototype mold instead of wax mold in the investment casting.SLA technology faster forming speed, higher precision, but due to the shrinkage of the curing process of the resin will inevitably produce stress or cause deformation. Therefore, the development of small shrinkage, fast curing, high strength photosensitive materials is its development trend.
3D Systems launched the Viper Pro SLA system
SLA advantages
⒈ ⒈ light-curing molding method is the first rapid prototyping process, high maturity, after the test of time.
Peake by the CAD digital model directly into the prototype, processing speed, short product production cycle, without cutting tools and molds.
3Prototypes and molds with complex structures and shapes, or those that are difficult to shape by traditional means, can be sung.
Sung to visualize the CAD model and reduce the cost of repairing errors.
Careful to provide specimens for experiments, and to verify and validate the results of computer simulations.
Special care should be taken to provide specimens for experiments to validate and verify the results of computer simulation calculations.
SLA shortcomings
⒈ SLA system is expensive, the use and maintenance costs are too high.
Peake SLA system is to operate the liquid precision equipment, the requirements of the working environment is harsh.
3 The molded parts are mostly resins with limited strength, rigidity and heat resistance, which are not good for long time storage.
Sung pre-processing software and drive software are too heavy and too correlated with the processing results.
Be careful of the complexity of the software system, it is difficult to start, and the file format is not familiar to the majority of designers.
Select the stereo light-curing molding technology is monopolized by a single company.
The development trend and prospects of SLA
The development trend of stereolithography is high-speed, energy-saving and environmental protection, and miniaturization.
The ever-increasing precision of the process makes it the first of its kind to be used in the fields of biology, medicine, and microelectronics. Selective laser sintering (hereinafter referred to as SLS) technology was originally proposed by Carl ckard of the University of Texas at Austin in 1989 in his master's thesis. In 1992, DTM launched the commercialized production equipment Sinter Sation for this process. over the past decades, Austin and DTM have done a lot of research work in the field of SLS, and achieved fruitful results in the development of equipment and process and materials. Germany's EOS has also done a lot of research in this area, and developed a corresponding series of molding equipment.
There are also a number of units in China to carry out SLS related research work, such as Xi'an Jiaotong University School of Mechanical Engineering, National Engineering Research Center for Rapid Prototyping, Ministry of Education, Rapid Prototyping Engineering Research Center, Huazhong University of Science and Technology, Nanjing University of Aeronautics and Astronautics, Northwestern Polytechnical University, North Central University and Beijing Longyuan Automatic Forming Company Limited, etc., and has achieved many significant results, such as Nanjing University of Aeronautics and Astronautics Ltd. have also achieved many significant results, such as Nanjing University of Aeronautics and Astronautics RAP-I laser sintering rapid prototyping system developed by Beijing Longyuan Automatic Forming Company Limited developed AFS-300 laser rapid prototyping commercialization equipment.
Selective laser sintering is the use of laser selective layered sintering of solid powders, and sintering molding of solidified layers superimposed to generate the desired shape of the part. The whole process includes CAD model building and data processing, powder spreading, sintering and post-processing, etc. The working principle of the rapid prototyping system of SLS technology is shown in Fig. 1.
The whole process device consists of a powder cylinder and a molding cylinder, the piston of the powder cylinder (the piston of powder delivery) rises during the working time, and the powder spreading rollers spread a layer of powder on the piston of the molding cylinder (the piston of the working piston), and the computer controls the slicing of the laser beam according to the prototype. The computer controls the two-dimensional scanning trajectory of the laser beam according to the slicing model of the prototype, and selectively sinter the solid powder material to form a layer of the part. After a layer of powder has been applied, the working piston is lowered by one layer and a new layer of powder is applied by the powder spreading system. The controlled laser beam then scans and sinter the new layer. This cycle is repeated, layer by layer, until the three-dimensional part is formed. Finally, the unsintered powder is recycled into the powder cylinder and the molded part is removed. For laser sintering of metal powders, the entire table is heated to a certain temperature prior to sintering, which reduces thermal deformation in the molding and facilitates bonding between layers.
The most striking advantage of SLS over other rapid prototyping (RP) methods is the wide range of molding materials used. Theoretically, any powder material that can form interatomic bonds when heated can be used as a molding material for SLS. Materials that can be successfully molded by SLS include paraffin, polymer, metal, ceramic powders and their composite powder materials. Because of the variety of SLS molding materials, material savings, molded parts with a wide range of performance distribution, suitable for a variety of purposes, and SLS does not require the design and manufacture of complex support systems, so the application of SLS is becoming more and more widespread.
SLS technology metal powder sintering method
3.1 metal powder and binder mixing sintering
First of all, the metal powder and a certain binder according to a certain proportion of the mix, with a laser beam on the mixed powder selective scanning, the laser's role in the mixed powder binder melting and will be the metal powder bonded together, the formation of the metal parts of the blank. Then the metal part blank for appropriate post-treatment, such as the second sintering to further improve the strength and other mechanical properties of the metal parts. This process is more mature, has been able to manufacture metal parts, and used in practice. Nanjing University of Aeronautics and Astronautics with metal powder as the base material (iron powder), add the appropriate amount of withering agent, sintering molding to get the prototype, and then subsequent processing, including the burning loss of binder, high temperature roasting, metal fusion infiltration (eg, copper infiltration) and other processes, and ultimately create an EDM electrode (see Figure 2). And with this electrode on the EDM machine tool to process the three-dimensional mold cavity (see Figure 3).
3.2 metal powder laser sintering
Laser direct sintering of metal powder manufacturing parts process is not very mature, more research is a mixture of two metal powder sintering, one of the lower melting point, the other higher. Laser sintering will be low melting point of the powder melting, molten metal will be high melting point metal powder bonded together. Since the sintered parts are lower in strength, they need to be post-treated to achieve higher strength. The University of Texas at Austin conducted an SLS molding study of metal powders without polymer binder such as CuSn NiSn bronze-nickel powder composite powder and successfully fabricated metal parts. They have conducted research on laser sintering molding of single metal powders and have successfully fabricated metal parts of Worker NCONEL 625 superalloy and Ti6A 14 alloy for F1 fighter jets and AIM9 missiles. American Aerospace Materials has successfully researched and developed advanced laser rapid prototyping technology for Chin alloy components. Chinese Academy of Sciences Institute of Metals and Xi'an Jiaotong University and other units are working on high melting point metal laser rapid prototyping research, Nanjing University of Aeronautics and Astronautics also conducted research in this area, with Ni-based alloy mixed with copper powder for sintering and forming of the test, the successful manufacture of metal parts with a large angle of the shape of the inverted cone (see Figure 4).
3.3 metal powder billet sintering
Metal powder billet sintering is the high and low melting point of the two kinds of metal powder pre-pressing into a thin sheet of billet, with the appropriate process parameters for laser sintering, the low melting point of the metal melting, flow to the high melting point of the particles between the pores, so that the high melting point of the powder particles rearranged, to get the high densities of the specimen. Jilin University, Guo Zuoxing, etc. with this method of FeCu, Fe C and other alloys for experimental research, found that the billet laser sintering with conventional sintering is completely different from the phenomenon of densification, the organization of laser sintering with the cooling method varies, air-cooled to get a fine pearlitic, quenched to get the martensite and granular.
4 Problems of metal powder molding by SLS technology
SLS technology is a very young manufacturing field, which is still not perfect in many aspects, such as the manufacturing of three-dimensional parts generally have low strength, low precision and poor surface quality, etc. The SLS process involves a lot of parameters (e.g., the physical and chemical properties of the material, laser parameters and sintering process The SLS process involves many parameters (such as physical and chemical properties of the material, laser parameters and sintering process parameters, etc.), which affect the sintering process, molding accuracy and quality. Parts in the molding process, due to a variety of material factors, process factors, etc., will make the sintered parts produce a variety of metallurgical defects (such as cracks, deformation, porosity, uneven organization, etc.).
4.1 The influence of powder materials
The physical properties of powder materials, such as powder particle size, density, coefficient of thermal expansion, and mobility have an important influence on the formation of defects in the parts. Powder particle size and density not only affect the formation of defects in molded parts, but also has a significant impact on the precision and roughness of molded parts. The effect of the expansion and solidification mechanisms of powders on the sintering process can lead to increased porosity and reduced tensile strength of molded parts.
4.2 Influence of process parameters
Laser and sintering process parameters, such as laser power, scanning speed and direction and spacing, sintering temperature, sintering time, and layer thickness, etc., have an influence on the adhesion between layers, shrinkage and deformation of the sintered body, warping deformation and even cracking. The above parameters tend to interact with each other during the molding process. For example, Yong Ak Song et al. showed that decreasing the scanning speed and scanning spacing or increasing the laser power can reduce the surface roughness, but decreasing the scanning spacing will lead to the warpage tend to increase.
Therefore, when performing optimal design it is necessary to consider the optimization of each parameter in general to obtain the most effective parameter set for the improvement of the quality of molded parts. The manufactured parts generally have some problems such as low densities, strengths and precision, and mechanical and thermal properties that do not meet the requirements for use. These molded parts can not be used directly as functional parts, and need to be post-treatment (such as hot isostatic pressing HIP, liquid phase sintering LPS, high temperature sintering and melting and immersion) before they can be put into practical use. In addition, it should be noted that due to the high SLS temperature of the metal powder, in order to prevent the oxidation of the metal powder, the sintering must be closed in a container filled with protective gas.
5 Summary and Prospect
In rapid prototyping technology, metal powder SLS technology is a hot spot in people's research. The realization of the use of high melting point metal direct sintering molding parts, it is difficult to manufacture high-strength parts with traditional cutting and processing methods, the rapid prototyping technology for a wider range of applications has a particularly important significance. Looking ahead, SLS shape technology in the field of metal materials research direction should be a unit system of metal parts sintering molding, multi-alloy material parts sintering molding, advanced metal materials such as metal nanomaterials, amorphous metal alloys such as laser sintering molding, etc., especially suitable for cemented carbide materials, micro-component molding. In addition, according to the specific functional and economic requirements of the parts to sintering forming parts with functional gradient and structural gradient. We believe that with the mastery of the laser sintering metal powder forming mechanism, the optimal sintering parameters of various metal materials to obtain, as well as the emergence of specialized rapid prototyping materials, SLS technology research and references will certainly enter a new realm. Layered Entity Manufacturing (LOM - Laminated Object Manufacturing) method, LOM is also known as the laminated method of forming, it is a sheet (such as paper, plastic film or composite materials) as the raw material, and its molding principle as shown in the figure, the laser cutting system in accordance with the cross-section extracted by the computer Contour line data extracted by the computer, the paper coated with hot melt adhesive on the back side of the laser cut out the inner and outer contours of the workpiece. After cutting a layer, the feeding mechanism will be a new layer of paper superimposed on the use of thermal adhesive pressure device will have been cut layer bonded together, and then cut, so that layer by layer cutting, bonding, and ultimately become a three-dimensional workpiece.LOM commonly used materials are paper, metal foils, plastic film, ceramic film, etc., the method can be made in addition to the manufacture of molds, models, but also can be directly manufactured structural components or functional parts. The method is characterized by cheap raw materials and low cost.
Molding material: fiber paper coated with heat-sensitive adhesive;
Part-making properties: equivalent to high-grade wood;
Main application: rapid manufacturing of new product samples, models, or casting wood molds. Fused Deposition Molding (FDM - Fused Deposition Modeling) method, which uses filamentary materials (paraffin, metals, plastics, low-melting-point alloy wires) as raw materials, the use of electric heating will be heated to a slightly higher than the melting temperature of the filament (about 1 ℃ higher than the melting point), in the control of the computer, the Under the control of computer, the nozzle makes x-y plane movement, coats the molten material on the working table, forms a layer of cross-section of the workpiece after cooling, and after a layer is formed, the nozzle moves up a layer of height for the next layer of coating, so that the three-dimensional workpiece is formed by layer by layer stacking. This method is less polluting, the material can be recycled, and is used for the molding of small and medium-sized workpieces. The following figure shows the principle diagram of FDM molding.
Molding material: solid filamentary engineering plastics;
Part-making properties: equivalent to engineering plastics or wax molds;
Main applications: plastic parts, wax molds for casting, prototypes or models.
Features: 1. Advantages: (1) clean and safe operating environment, carried out in the office class; (2) clean, simple, easy to operate the process and does not produce waste; (3) high dimensional accuracy, good surface quality, easy to assemble, can quickly build bottles or hollow parts; (4) raw materials in the form of rolls of filament is provided in the form of easy to handle and amount of rapid replacement; (5) raw materials are cheap; (5) raw materials; (6) high material utilization; (6) raw material is cheap; (7) raw materials, raw materials, raw materials, raw materials, raw materials, raw materials, raw materials, and raw materials. (6) high material utilization; (7) more materials can be selected, such as dyed ABS, PLA and medical ABD, PC, PPSF, artificial rubber, casting wax.
2. Disadvantages: (1) lower precision, difficult to build structurally complex parts; (2) small strength in the direction perpendicular to the cross-section; (3) relatively slow molding speed, not suitable for building large parts.