Precision mold design and manufacturing technology

Precision forging is a processing method that is both ancient and plays an important role in modern industry, which dates back to the forming of natural metals such as gold and silver in the 40th century BC. In today's modern industry, what is displayed in front of us is a fully automatic high-speed forging production line. The most typical ones include hot forging production lines for crankshafts and connecting rods, cold forging production lines for gears and crossheads, and high-speed cold forging machines represented by fasteners.
　　From "strike while the iron is hot", it is natural to think that forging is carried out at high temperatures. Since the invention of phosphating saponification technology in Germany in 1938, cold forging technology has been widely used. In Germany during World War II, in order to use steel instead of copper to produce military products such as bullet casings, a phosphating saponified film with both adhesion and high ductility was invented. After the end of World War II, this technology first spread to the United States, and then quickly spread to the world. After the 1960s, the mass production of automobiles created a vast world for the development of cold forging technology.
　　Die forging uses a die with the same shape as the formed part to directly form by deforming the metal, so it is suitable for mass production. It can be predicted that with my country's potential requirements for automobiles, the spring of precision forging will come. This paper mainly introduces the current situation of precision forging technology, especially Japanese forging technology, and some work related to forging technology that the author has engaged in in recent years.
　　2 Status Quo of Precision Forging As far as today's precision forging is concerned, Japan is second to none in terms of popularity and technical level. The reason: Japan's automobile industry is after the United States and Germany. To compete with it, it must require low cost. In terms of mass production, cold forging has strong competitiveness. In addition, in the early stage of industrialization, the cooperation between industries, especially in Outstanding achievements have been made in the development of cold forging materials. In Japan, cold forging technology was first applied to bicycle parts in 1955. With the vigorous development of the automobile industry after the 1960s, cold forging has also developed rapidly as one of the pillar industries. In a Japanese car, in addition to fasteners, the use of cold forgings has reached more than 50kg.
　　2.1 Cold forging materials are just like everything in the world with dual nature. Cold forging parts can be processed with less chips and no chips, which reduces the cost, but it has very strict requirements on raw materials, requiring high dimensional accuracy, good ductility, low hardness, internal and No tiny cracks are allowed on the surface. Therefore, high requirements are put forward for steelmaking and rolling. In order to improve the ductility of the material, in addition to controlling the content of sulfur, phosphorus and oxides in the steel, some even peel the rolled material so that the material can withstand strong deformation without cracking at room temperature . Generally speaking, cold forging is formed in a closed die cavity. For this reason, a good blanking accuracy is required, generally more than 0.8%, so the rods and wires used for cold forging are usually cold drawn to obtain consistent Diameter size. Usually, the cold drawing of cold forging coil is carried out after phosphating and saponification, while the bar is carried out before phosphating and saponification and blanking.
　　2.2 Precision equipment There are two main types of cold forging equipment: one is a high-speed cold forging machine, and the other is a press. High-speed cold forging machine is a kind of multi-station automatic forging equipment, which generally uses coil material and is equipped with a blanking device. The fastest stroke speed of the cold forging machine has reached 1000 times per minute, the maximum number of stations has reached 7, the maximum tonnage has reached 800t, and the weight of the forgings produced is 8kg. Because the cold forging machine has multiple forming stations and adopts a horizontal type , so it is suitable for the production of shafts, cylinders and other parts (). The forming station of the mechanical press is generally not more than 3. Due to the use of the mold base structure in the production, the guiding accuracy is increased, and compound actions can also be realized.
　　Therefore, it is suitable for closed forming. Precision forged gears, crossheads, etc. are all produced on this type of machine (). The speed of the mechanical press is generally less than 30 times per minute, and the tonnage has reached more than 2000t. The structure of the cold forging press is different from that of the general hot forging or punch press, which mainly adopts the toggle type or multi-link type. One of its purposes is to prolong the poor precision caused by the elastic recovery of the material as much as possible. There are two types of presses, manual operation and automatic type, which are mainly automatic in developed countries. The automatic press is equipped with a feeding device and a clamping conveyor. The press uses the best blanks. The cutting methods mainly include shearing and sawing. Sometimes, the sheared and sawed blanks are end-faced as needed to achieve higher weight accuracy requirements.
　　3 The design of the cold forging process has a very high forming difficulty due to the wide variety of cold forgings, complex shapes, and extremely high forming pressure. Cold forging is a process of directly deforming a simple-shaped billet to form parts. Generally, several deformation stations are required to achieve the required shape through successive forming. The quality of the final formed forging will be affected by factors such as machine, material, die and lubrication, and it is very difficult to fully grasp and control these factors at one time. In production, the process design is mainly based on the experience of the designers and through repeated mold trials.
　　The prediction of the forging force is very important in the formulation of the cold forging process and the selection of the equipment, and it is often a key factor affecting the forging effect. In order to predict the forging force more accurately, the FE simulation technology of the material flow state has been widely used, but a certain knowledge of plastic mechanics must be mastered in order to successfully apply the technology.
　　More recently, researchers have attempted to develop a forging CAE system that fuses rules of thumb with plasticity theory and FE simulations to solve problems at different levels.
　　The design of the cold forging process starts from the forging diagram and imagines several possible forging deformation processes. After analyzing the material flow and required tonnage of each deformation process, it is finally applied to production through mold design, mold making, mold trial, mold repair and other processes, which is the flow of this process. Although the shapes of the forgings are varied, it is not difficult to find that the final shape is obtained by combining some regular deformation styles. These deformation styles can be classified as face straightening (for shear blanking), forward extrusion, reverse extrusion, upsetting, closed forging, perforation, various finishing operations, and combinations of the above simple processing methods.
　　A process design scheme for cold forging machine is given. Five forming steps were used to obtain the final part shape. The cold forging process of the gear is shown in . It is forged on a press in just 3 processes.
　　4. Process Design Auxiliary System On the basis of many years of forging process research and design, the author developed a forging process design software system. The design idea of ​​this software comes from industrial production, and its purpose is to focus on applicability and simplicity. "Basic parameters" include the material's yield stress, coefficient of friction, and mold material properties. In "Approximate Analysis", calculations are performed in a large number of windows based on plasticity theory.
　　Here, according to the forming characteristics in forging, the forging styles are divided into 6 categories, and are divided into subcategories under the categories, and each subcategory contains different geometric shapes. Geometries that are not specified can be estimated in the Arbitrary Geometry project. The system has been tested in more than 30 companies in Japan, and it has been confirmed that it has a very high prediction accuracy and has a good effect in helping engineers and technicians in process design.
　　5. The manufacture of precision forging die is designed according to the deformation diagram of the forging process and the relevant dimensions of the forging machine. The general manufacturing process of the mold is shown. 0 shows some typical latest precision forging dies. For forging, especially cold forging, due to the large pressure of the forming surface, the mold generally adopts a prestressed structure (ie, multi-sleeve type), that is, a mold consists of a mold core and one or more prestressed sleeves. The core is generally made of superhard alloy material (WC), and the sleeve is made of mold steel (H13) with better toughness. In the multi-layer structure, the intermediate sleeve is also made of cemented carbide. The core material of super hard alloy is generally made into a shape close to the mold cavity in the powder metallurgy stage because of its high price and difficult to process, and only with EDM. But with the advancement of cutting tools, cutting processing has also been applied in recent years. In EDM, the processing method and precision of the electrodes used are constantly improving, and the precision has reached below 2Mm. Electrodes of two-dimensional shape are processed by NC lathes, while three-dimensional shapes are to be completed on machining centers or some special NC machines. 1 is a photo of a new type of electrode machining center. In recent years, electrical discharge machining technology has also been developed, mainly reflected in high speed, discharge accuracy and discharge surface roughness. The surface roughness after discharge has reached 10%! In the following, the grinding and polishing time of the mold surface is greatly reduced.
　　In precision forging, the surface roughness of the die will be directly transferred to the forging, so the roughness is generally required to be below 0.4%m. In addition to the roughness requirements, since it is the final process of mold manufacturing, it is also required that the dimensional accuracy cannot be destroyed during the grinding and polishing process, so it is an important process in mold manufacturing. Grinding and polishing mainly rely on manual operations. In recent years, automatic grinding and polishing machines have also been applied. Heat treatment plays an extremely important role in mold manufacturing, especially for high-speed tool steel (JIS, SKH series), temperature control and heat treatment process are very demanding. Almost all tool steel heat treatments in Japan are carried out in vacuum furnaces. But for high-speed tool steel, the use of salt bath treatment is also quite common. Improving the uniformity of the heating temperature and preventing heat treatment deformation are also key factors in the heat treatment of the mold. Even many of the technical factors (such as the placement of the mold in the furnace, the skills to promote soaking, etc.) should be strictly required. .
　　6 Conclusion The forging industry is undergoing global changes. From the perspective of reducing production costs, the forging industry is shifting from developed countries to developing countries. Another trend is to increase the added value of forgings and improve the productivity of forging. Such as the combination of hot, warm and cold forging technology to achieve precision forging of complex parts, the combination of forging and stamping technology, the combination of forging and welding, and riveting technology. Forging technologies of non-ferrous alloys such as ferrous alloys, aluminum alloys and magnesium alloys have also received much attention.
　　In the design of forging process, the development and use of CAE technology has been the focus of research in recent years and has made considerable progress, but there is still a long way to go. Two-dimensional FE simulation can be said to have reached the stage of industrial application. Although there are many three-dimensional commercial programs, it can be said that it is still a long way from industrial application due to the limitations of mesh subdivision technology and computer speed. Integrated CAE systems combining FE simulations with approximate analytical and empirical data have been developed and applied in recent years. But there is still a lot of work to be done in the establishment of basic data and the collection of examples. In the development of new forging technology, such as occluded forming and back pressure technology, it has been widely used to solve the precision forging problem of bevel gears, crosshead universal joint inner rings, screw pumps and other parts. Precision forging technology, etc. have yet to be researched and developed. The accuracy of precision forgings is also moving towards the micron level, and the development of comprehensive forging technology that affects the forging accuracy of factors such as temperature, elastic deformation, and blanking accuracy has been put on the agenda