sls process direct production of metal mold process

SLS technology based on the rapid manufacturing of metal molds

Favorite this information Print this information Add: Unknown Source: Unknown

Introduction of SLS technology

Selective Laser Sintering (Selective Laser Sintering, referred to as SLS) technology is the use of infrared lasers as a source of heat to sinter the powdered material, to layer by layer additive SLS technology is the use of infrared laser as a heat source to sinter powder material to add layer by layer to form three-dimensional parts of a rapid prototyping (also known as rapid prototyping, Rapid Prototyping, referred to as RP) technology. SLS process is characterized by a wide range of materials to adapt to the surface, not only to manufacture plastic parts, but also to manufacture ceramics, metal powder, paraffin wax and other materials parts, in particular, can be made of metal molds.

Laser selective sintering (SLS) process by the University of Texas at Austin, the United States, C.R. Dechard in 1989 developed successfully. At present, the German EOS company launched its own SLS process molding machine EOSINT, divided into three models suitable for metal, polymer and sand. Ltd. and Huazhong University of Science and Technology have also developed commercialized equipment.

Rapid metal mold manufacturing (Rapid bbbbl Tooling, referred to as RMT)

RP technology has been able to successfully manufacture prototypes, including metal, ceramics, plastics, paraffin, resin and so on. Due to the limitations of the materials used, RP development of the initial molding parts are often only in a limited number of occasions to replace the real metal or other types of functional parts to do functional experiments, with the increase in demand and the continuous development of technology, RP is to the direction of the development of RMT. RMT has its own unique advantages, such as manufacturing links are simple, in particular, with the close integration of computer technology, rapid completion of the product manufacturing, in order to shorten the manufacturing cycle, save resources, play the role of material performance, and the development of RMT. RMT has its unique advantages, such as simple manufacturing process, especially with the close integration of computer technology, rapid completion of product manufacturing, in shortening the manufacturing cycle, saving resources, play the material performance, improve accuracy, reduce costs, etc. has great potential, but in the mold accuracy and performance control is more difficult, such as special post-processing equipment and processes to make the manufacturing cost, forming size is also subjected to greater limitations. the key to the research and application of RMT in how to improve the surface accuracy of the mold and the efficiency of the manufacturing, as well as to ensure the overall performance of the quality, so as to The key to the research and application of RMT is how to improve the surface accuracy and manufacturing efficiency, as well as to ensure its overall performance quality, so as to directly and rapidly manufacture durable, high-precision and surface quality to meet the conditions of industrial mass production of molds. RP-based RMT technology can be divided into direct and indirect methods.

◆ Metal mold direct manufacturing method

The direct manufacturing method is to use the RP prototype direct mold making. SLS-based direct method because it does not require process conversion, in the mold manufacturing cycle and other aspects of the application has great potential, and thus received a high degree of attention. Powder stack forming is the most common method of direct metal mold making in SLS.The SLS direct mold making process consists of a high temperature chamber and a high power laser used to melt the metal. The high-temperature chamber usually has a reducing atmosphere, inert gas, or a vacuum to prevent material oxidation. Prior to laser sintering, the material is heated close to the melting point, which has been shown to result in more efficient sintering and less material warpage. Directly manufactured parts have high strength and can be used directly as molds. Germany EOS company Direct Steel 20-V1 method based on steel powder, its diameter of 20mm, layer thickness of 0.02mm, sintering density of steel density of 95% to 99%, successfully solved the problem of solidification of metal powder shrinkage.Lohner A. et al. using Ni-Cu powder directly manufactured molds, density of 80% of the theoretical value of the strength of 100 ~ 200MPa, accuracy of 0.5mm. 200MPa, precision of 0.1mm, the average roughness Ra for 10 ~ 15um, has been used to produce hundreds of injection molding parts.

◆ Metal mold indirect manufacturing method

Indirect manufacturing method, namely, indirect mold making with RP prototype. SLS prototype is mainly non-metallic type (such as ABS, wax, resin, etc.), in most cases non-metallic prototype can not be directly used as a mold, the need for prototype as a model, after the process of conversion, the manufacture of metal molds. Indirect manufacturing is characterized by the combination of RP technology and traditional forming technology, making full use of their respective technical advantages, has now become a hot spot for applied research and development. SLS-based indirect manufacturing processes include powder metallurgy molding, SLS prototype rapid precision casting, SLS ceramic shell casting, resin sand rapid casting.

(1) Powder metallurgical molding

Powder metallurgical molding is more similar to the metal powder accumulation molding, but the composite powder material usually contains low-temperature fusible components or adhesive. A low-power laser is used to sinter the fusible components at lower temperatures, and a three-dimensional solid, called a billet, is obtained after the SLS process. These blanks have low strength but accurate shapes and require post-processing to obtain high-strength metal or ceramic parts. Post-processing usually consists of two steps: debonding and re-sintering. The billet is placed in a sintering furnace for debonding and re-sintering to remove the fusible, low-strength components, and the remaining metal or ceramic powder is sintered to form the metal or ceramic part. The finished metal parts are usually brown in color and are called brown parts. In order to reduce the void ratio or relative density, brown parts can also be infiltrated with epoxy resin or metals such as copper.

The U.S. DTM company used laser sintering coated with a binder of steel powder, computer-controlled scanning path of the laser beam, heated after the melting of the binder will be bonded to the metal powder (non-metallurgical bonding), generating about 45% of the porosity of the parts, drying and dehumidification, put into a high-temperature furnace for sintering, seepage of copper to generate the surface of the parts of the dense, the parts at this time in the composition of the material is 65% steel and 35% copper, after grinding and other post-treatment processes, to get the final mold.

SLS with the composite powder usually have two mixing forms: one is the adhesive powder and metal or ceramic powder according to a certain proportion of mechanical admixture, the other is the metal or ceramic powder into the adhesive dilution, so that the substrate powder and adhesive full contact, the preparation of adhesive-coated metal or ceramic powder. Experiments have shown that the preparation of this adhesive-coated powder is complicated, but the sintering effect is better than that of the mechanically adhered powder.

(2) SLS disappearing mold precision casting

With plastic, paraffin, resin and other powder sintering SLS prototype, and then paint, sand, drying and other treatments and repeated many times until the prototype on the formation of a desired thickness of the shell, and then heated so that the model melting vaporization disappears, the formation of shell, and then baked shell, and finally the melted metal into the shell, cooling can get the required mold. After cooling, the required mold castings can be obtained. The process can better control the accuracy of the mold, surface quality, mechanical properties and service life, can meet the economic requirements. With this method of metal mold parts usually have good machinability, can be local cutting, can get higher precision, and can be embedded in the block, cooling components and sprue, etc., commonly used in the manufacture of plastic molds, die-casting molds, injection molds and so on.

SLS-based vanishing mold precision casting process made of tractor gearbox shell metal mold. Using the SLS process to manufacture prototypes, and then precision casting parts, from CAD design to obtain parts in just 15 days, the traditional process requires at least 45 days, the manufacturing cost is reduced by 60%. Using the SLS process to manufacture prototypes, using the process to manufacture molds for disappearance mold casting, 40% shorter than the traditional process manufacturing cycle.

(3) SLS ceramic shell casting

The use of laser to reactive resin-coated ceramic powder sintering, sintering is completed after the powder is poured out, and then by the curing process to obtain the casting of ceramic shells, pouring can be made after the metal mold parts. This method eliminates the traditional precision casting a variety of processes, is a major change in the traditional precision casting. Laser beam under the control of the computer, according to the level of the scanning line, selective sintering of resin sand and epoxy resin mixture of powder materials, a level of casting shells after the table down a layer of thickness, the feeder table rises a layer of height, and began to lay new material. In this way, layer by layer, layer by layer sintering, and finally the desired mold casting shell of the reverse type. Selective laser sintering after the casting shell, the residual uncured powder binder and binder distribution is not uniform, low strength, the casting shell must be baked hardening treatment. Baking hardened casting shells eliminates the binder aggregation phenomenon in the prototype and makes the prototype uniformly hard. Volatilization of the prototype in the water and can be gasification material, improve the casting shell permeability, reduce the casting of the amount of gas, so that the shell has a good casting performance. Combined with the traditional sand casting process, the casting shell outside the construction of the pouring system, pouring mold castings.

Its biggest advantage is the speed, do not need any molds, and even do not need to draw, design engineers through the computer network will be sent to the foundry system can complete the design and production of the shell. In the CAD environment, the mold parts directly converted to shell type, and then with the pouring system, the thickness of the shell can be taken 5 ~ 10 mm. the shortcomings of the process is mainly the parts surface roughness value is high, the key technology is the shell thickness, shell surface roughness and curing treatment process.

(4) SLS resin sand casting

The casting resin sand as SLS sintering material, according to the drawings or requirements to design the mold parts of the three-dimensional CAD model, casting process analysis, mainly the design of the pouring system, risers and to determine the casting of the solidification shrinkage allowance, according to the shrinkage allowance and the dimensions of the parts to design the casting of the CAD model. SLS in the computer control, in accordance with the cross-section, under the control of the SLS, in accordance with the cross-section of the parts, the surface roughness and curing process. SLS under computer control, in accordance with the cross-section profile information, scanning the cross-section shape on the powder, the laser power should be large enough to make the outline of the boundary of the powder at the boundary of the complete carbonization and loss of curing effect, scanning layer by layer until the accumulation of parts of the three-dimensional surface structure of the parting surface. As the table descends step by step, the resin sand powder is spread on the table layer by layer, and then the powder is rolled and compacted with a leveling roller, and the thickness of each layer of powder corresponds to the thickness of the slice of the CAD model. Each layer of powder heated by laser scanning was destroyed and lost its curing effect, the powder not scanned by the laser remained in place to play a supporting role until the scanning of the whole part. The whole part of the table is heated at a temperature of 200 to 280°C, depending on the resin sand. As the parts surface of the resin sand by the laser carbonization and loss of solidification, and thus the surface of the parts is equivalent to a parting surface, the parts and the surrounding waste pieces separated from the waste pieces, remove the waste pieces, and ultimately get the casting. The casting cavity surface for appropriate grinding and finishing or paint to reduce the surface roughness of the cavity, and then pouring to get the metal parts or molds.

Resin sand casting has the following advantages over rapid investment casting: (a) good collapse. Compared with investment casting, resin sand casting can cast more complex parts, SLS is mainly used for processing can not be processed by CNC airway, water jacket part of the sand core, etc.. (b) molding time is short, SLS and CNC technology significantly accelerates the speed of manufacturing sand mold. Investment casting need to have a fixed shell-making cycle, and fast sand casting can be flexible to control the casting time. (c) can cast larger mold parts. The use of CNC milling technology to process the cavity part on the one hand improves the processing efficiency, on the other hand, can process a larger sand, so you can produce larger mold parts.