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Introduction of SLS technology
Selective laser sintering (SLS) technology is a rapid prototyping (RP) technology in which infrared laser is used as the heat source to sinter powder materials and form three-dimensional parts by stacking them layer by layer. SLS technology is characterized by a wide range of materials, which can not only make plastic parts, but also make parts made of ceramics, metal powder, paraffin and other materials, especially metal molds.
Selective laser sintering (SLS) process was successfully developed by C.R. Dechard of the University of Texas at Austin in 1989. At present, Germany EOS Company has launched its own SLS molding machine EOSINT, which is divided into three types: metal, polymer and sand mold. China's Beijing Longyuan Automatic Forming System Co., Ltd. and Huazhong University of Science and Technology have also developed commercial equipment.
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RP technology has been able to successfully manufacture prototypes including metals, ceramics, plastics, paraffin wax, resins, etc. Due to the limitation of materials used, the molded parts of RP can only replace real metal or other functional parts for functional experiments in limited occasions. With the increase of demand and the continuous development of technology, RP is developing towards RMT. RMT has its unique advantages, such as simple manufacturing process, especially the close combination with computer technology, rapid product manufacturing, and great potential in shortening manufacturing cycle, saving resources, exerting material properties, improving accuracy and reducing costs, etc. However, it is difficult to control the precision and performance of the mold, such as special post-processing equipment and technology, which increases the manufacturing cost and greatly limits the forming size. The key to the research and application of RMT lies in how to improve the surface accuracy and manufacturing efficiency of molds and ensure their comprehensive performance and quality, so as to directly and quickly manufacture durable, high-precision and surface quality molds that meet the conditions of industrial batch production. RMT technology based on RP can be divided into direct method and indirect method.
◆ Direct manufacturing method of metal mold
Direct manufacturing method is direct molding with RP prototype. The direct method based on SLS has attracted much attention because it does not need process conversion and has great application potential in the mold manufacturing cycle. Powder stacking forming is the most commonly used method to directly manufacture metal molds in SLS. SLS direct molding process includes high-temperature studio and high-power laser for melting metal. There is usually a reducing atmosphere, inert gas or vacuum in the high temperature working room to prevent the oxidation of materials. Before laser sintering, the material has been heated to near the melting point. The research shows that it can be sintered more effectively and the material warpage is less. The directly manufactured parts have high strength and can be directly used as molds. The direct steel 20-V 1 method of EOS Company in Germany is mainly composed of steel powder with a diameter of 20mm, a layer thickness of 0.02mm and a sintering density of 95% ~ 99% of the steel density, which successfully solves the problem of solidification shrinkage of metal powder. The mold directly manufactured by Lohner A. et al. with Ni-Cu powder has a density of 80% of the theoretical value, a strength of 100 ~ 200mpa, a precision of 0. 1mm and an average roughness of10 ~/5um, and has been used to produce hundreds of injection molded parts.
◆ Indirect manufacturing method of metal mold
Indirect manufacturing method is indirect molding with RP prototype. SLS prototype is mainly nonmetal (such as ABS, wax, resin, etc. ). In most cases, non-metallic prototypes cannot be directly used as molds, and metal molds need to be made by technological transformation based on prototypes. Indirect manufacturing is characterized by combining RP technology with traditional forming technology and making full use of their respective technical advantages, which has become a hot spot in application research and development. The indirect manufacturing process based on SLS includes powder metallurgy, rapid precision casting of SLS prototype, SLS ceramic shell casting, resin sand rapid casting and so on.
(1) powder metallurgy forming
Powder metallurgy is similar to metal powder stacking, but composite powder materials usually contain low-temperature fusible components or binders. The fusible component is sintered by low-power laser at low temperature, and the three-dimensional solid is obtained by SLS process, which is called green body. This kind of green body has low strength, but its shape is accurate, and it needs post-treatment to get high-strength metal or ceramic parts. Post-treatment usually includes two steps: debonding and sintering. The green body is put into a sintering furnace for debonding and re-sintering, and the fusible low-strength components are removed, and the remaining metal or ceramic powder is sintered into metal or ceramic parts. The finished metal parts are usually brown, which is called brown parts. In order to reduce porosity or relative density, brown parts can also be infiltrated with metals, such as epoxy resin or copper.
American DTM Company uses laser to sinter steel powder coated with binder, and the scanning path of laser beam is controlled by computer. The melted binder is heated to bond the metal powders together (non-metallurgical bonding), thereby producing a part with a porosity of about 45%. After drying and dehumidification, it is put into a high-temperature furnace for sintering and copper infiltration to produce parts with dense surfaces. At this time, the material components in the parts are 65% steel and 35% copper. After polishing and other post-treatment procedures,
The composite powder used for SLS usually has two mixing forms: one is to mechanically mix the binder powder with metal or ceramic powder in a certain proportion, and the other is to put the metal or ceramic powder into the diluent of the binder to make the matrix powder fully contact with the binder to prepare the metal or ceramic powder coated with the binder. The experiment shows that the preparation of this binder coated powder is complicated, but the sintering effect is better than that of mechanical mixed powder.
(2)SLS lost foam precision casting
SLS prototype is made of plastic, paraffin, resin and other powders by sintering, and then coated, polished and dried repeatedly until a mold shell with the required thickness is formed on the prototype. Then heating to melt and vaporize the model to form a mold shell, and then baking. Finally, the molten metal is poured into the mold shell, and the required mold casting can be obtained after cooling. This process can control the precision, surface quality, mechanical properties and service life of the die, and can meet the economic requirements. Metal mold parts manufactured by this method usually have good machinability, can be processed locally, and can obtain high precision. They can be embedded in inserts, cooling parts and runners, and are often used to manufacture plastic molds, die-casting molds, injection molds, etc.
SLS-based EPC metal mold for tractor gearbox shell. The prototype is manufactured by SLS process, and then the parts are precision cast. It only takes 15 days from CAD design to obtaining parts, while the traditional process takes at least 45 days, and the manufacturing cost is reduced by 60%. The prototype is manufactured by SLS process. Compared with the traditional process, the manufacturing cycle of the mould for EPC is shortened by 40%.
(3)SLS ceramic shell casting
Ceramic powder coated with reactive resin is sintered by laser. After sintering, the powder is poured out and then solidified to obtain the ceramic shell for casting. After pouring, metal mold parts can be manufactured. This method saves many technological processes of traditional precision casting and is a great change of traditional precision casting. Under the control of computer, the mixed powder material of resin sand and epoxy resin was selectively sintered by laser beam according to the scanning line in the layer to obtain a layer of casting shell. After that, the workbench dropped by one layer, and the feeding platform rose by one layer, and new materials were laid. In this way, the materials are spread layer by layer and sintered layer by layer, and finally the reverse of the required die-casting shell is obtained. After selective laser sintering, there is still uncured powder binder in the casting shell, and the binder distribution is uneven and the strength is low, so it is necessary to bake and harden the casting shell. After baking and hardening, the casting shell eliminates the accumulation of binder in the prototype, making the prototype uniform and hard. The water and evaporable substances in the prototype volatilize, which improves the permeability of the casting shell and reduces the gas generation during casting, thus making the shell have good casting performance. Combined with the traditional sand casting process, a gating system was established outside the casting shell to pour the mold castings.
Its biggest advantage is that it is fast, and it doesn't need any molds or even drawings. The design engineer can complete the design and manufacture of the shell by sending the data to the foundry system through the computer network. In CAD environment, the pattern of die parts is directly converted into shell type, and then the gating system is equipped. The thickness of the mold shell can be 5 ~ 10 mm, but the disadvantage of this process is that the surface roughness of the parts is high, and the key technologies are the thickness of the mold shell, the surface roughness of the mold shell and the curing treatment process.
(4)SLS resin sand casting
Taking casting resin sand as SLS sintering material, the three-dimensional CAD model of die parts is designed according to drawings or requirements, and the casting process is analyzed, mainly including designing gating system and riser, determining solidification shrinkage allowance of castings, and designing the CAD model of die according to shrinkage allowance and part size. Under the control of the computer, SLS scans the cross-sectional shape on the powder according to the information of the cross-sectional profile, and the power of the laser should be large enough to make the powder at the boundary of the profile completely carbonize and lose the curing effect, and scan layer by layer until the parting surface of the three-dimensional curved surface structure of the part accumulates. With the gradual descent of the workbench, resin sand powder is spread on the workbench layer by layer, and then the powder is compacted by leveling roller, and the thickness of each layer of powder corresponds to the slice thickness of CAD model. Each layer of powder heated by laser scanning is destroyed, losing its curing effect, and the powder not scanned by laser remains in place until the whole part is scanned. The whole workbench is heated, and the heating temperature is between 200 ~ 280℃ according to the difference of resin sand. Because the resin sand on the surface of the part loses its curing effect after laser carbonization, the surface of the part is equivalent to a parting surface, which separates the part from the surrounding waste blocks, removes the waste blocks and finally gets the mold. The surface of the inner cavity of the mold is polished or coated properly to reduce the roughness of the inner cavity surface, and then the metal part or mold is obtained by pouring.
Resin sand casting has the following advantages over rapid investment casting: (a) Good collapsibility. Compared with investment casting, resin sand casting can cast more complex parts, and SLS is mainly used to process water jacket gas passages and sand cores that cannot be processed by numerical control. (b) Short modeling time. The combination of SLS and numerical control technology has greatly accelerated the speed of sand mold manufacturing. Investment casting requires a fixed shell-making cycle, while rapid sand casting can flexibly control the casting time. (c) lar mold parts can be cast. On the one hand, CNC milling technology can improve machining efficiency, on the other hand, it can process larger sand molds, so it can produce larger mold parts.