Stamping is a forming process that relies on presses and dies to apply external forces to plates, strips, pipes and profiles to make them plastically deform or separate to obtain workpieces (stamping parts) of the required shape and size. Stamping and forging are both plastic processing (or pressure processing), collectively referred to as forging. The blanks used for stamping are mainly hot-rolled and cold-rolled steel plates and strips. 60-70% of the world's steel is plate material, and most of it is made into finished products through stamping. The body, chassis, fuel tank, radiator fins of automobiles, the drum of boilers, the shell of containers, the iron core silicon steel sheets of motors and electrical appliances, etc. are all stamped. There are also a large number of stamping parts in products such as instruments and meters, household appliances, bicycles, office machinery, and daily necessities.
Stamping is a production process that deforms parts to obtain product parts of a certain shape, size and performance. Sheet material directly uses the power of traditional or special stamping equipment in the mold. Sheet material, mold and equipment are the three elements of stamping. According to the processing temperature of stamping, there are hot stamping and cold stamping. The former is suitable for processing plates with strong deformation resistance and low plasticity; the latter is carried out at room temperature and is a commonly used stamping method for thin plates. It is one of the main methods of metal plastic processing (or pressure processing) and also belongs to material forming engineering technology.
The mold used for stamping is called a stamping mold, or simply a stamping die. The mold is a special tool for batch processing of materials (metal or non-metal) into the required stamping parts. Without a mold that meets the requirements, it is difficult to carry out batch stamping production; without advanced molds, advanced stamping technology cannot be achieved. The stamping process, mold, stamping equipment, and stamping materials together constitute the three elements of stamping processing. Only when they are combined with each other can stamping parts be produced.
Compared with forgings and stampings, it is thin, uniform, light and strong. Stamping can make the workpiece have reinforcing ribs, reinforcing ribs, undulations or flanges that are difficult to manufacture by other methods to improve its rigidity. Due to the use of precision molds, the workpiece accuracy can reach micron level, with high repetition accuracy and consistent specifications, and holes and tabs can be punched out. Cold stamping parts generally no longer use cutting processing, or only require a small amount of cutting processing. The accuracy and surface state of hot stamping parts are lower than those of cold stamping parts, but they are still better than castings and forgings, and there are fewer cutting processes.
Stamping is an efficient production method. Using compound dies, especially multi-station progressive dies, multiple stamping processes can be completed on one press (single station or multi-station), realizing fully automated production from unwinding, leveling, blanking, forming, and finishing. High production efficiency, good labor conditions, low production costs, generally hundreds of pieces can be produced per minute. Compared with other mechanical processing and plastic processing methods, stamping has many unique advantages in technology and economy. The main performance is as follows:
• (1) Stamping has high production efficiency, convenient operation, and is easy to mechanize and automate. This is because stamping relies on molds and stamping equipment to complete the processing. The stroke frequency of ordinary presses can reach dozens of times per minute, and high-speed presses can reach hundreds or even thousands of times per minute. Each stamping stroke may produce a stamped part.
• (2) Since the mold guarantees the size and shape accuracy of the stamped part, it generally does not damage the surface quality of the stamped part. In addition, the life of the mold is generally long, the quality of the stamped part is stable, interchangeable, and has the characteristics of "one mold".
• (3) Stamping can process parts with a wide range of sizes and complex shapes, such as clock second hands, automobile longitudinal beams, and covers. In addition, the cold deformation and quenching of the stamped parts during stamping make the strength and rigidity of the stamped parts higher.
• (4) Stamping does not produce chips and waste, consumes less material, and does not require other heating equipment. It is a material-saving and energy-saving processing method.
Due to the advantages of stamping, stamping is widely used in various fields of the national economy. For example, stamping parts are used in aerospace, aviation, military industry, machinery, agricultural machinery, electronics, information, railways, post and telecommunications, transportation, chemical industry, medical equipment, daily electrical appliances, light industry and other industries. Not only is the entire industry using it, but everyone is directly connected with stamping products. There are many large, medium and small stamping parts on airplanes, trains, cars and tractors. The body, frame and rim of small cars are all stamping parts. According to survey statistics, 80% of bicycles, sewing machines and watches are stamping parts; 90% of televisions, video recorders and cameras are stamping parts; there are also food metal can shells, steel pots and pans, enamel pots and pans, stainless steel tableware, etc., which are all products stamped by molds. Even computer hardware lacks stamping parts.
1. Mold problems
The molds used for stamping are generally special. Sometimes a complex part requires several sets of molds for processing and molding. The mold manufacturing has high precision and high technical requirements. It is a technology-intensive product. Therefore, the production volume of stamping parts is large, so as to fully reflect the advantages of stamping processing and obtain better economic benefits.
2. Safety issues
Stamping also has some problems and shortcomings. The main manifestations are the two major public hazards of noise and vibration generated during the stamping process, and safety accidents of operators occur from time to time. However, these problems are not entirely caused by the stamping process and the mold itself, but mainly caused by traditional stamping equipment and backward manual operation. With the advancement of science and technology, especially the development of computer technology and the advancement of mechatronics technology, these problems will be solved as soon as possible and perfectly.
3. High-strength steel stamping
Today's high-strength steel and ultra-high-strength steel have well achieved the lightweight of automobiles, improved the collision strength and safety performance of automobiles, and therefore become an important development direction for automobile steel. However, with the increase in the strength of the sheet, the traditional cold stamping process is prone to fracture during the forming process and cannot meet the processing requirements of high-strength steel plates. In the case of not meeting the forming conditions, the hot stamping forming technology of ultra-high-strength steel plates is gradually being studied internationally. This technology is a new process that integrates heat transfer and organizational phase transformation. It mainly uses the high-temperature austenite state to increase the plasticity of the sheet through the mold and reduce the yield strength. However, hot forming requires in-depth research in terms of process conditions, metal phase change, CAE analysis technology, etc. At present, this technology is monopolized by foreign manufacturers and has developed slowly in China.
Stamping is mainly classified by process, which can be divided into two categories: separation process and forming process. The separation process is also called blanking. Its purpose is to separate the stamping parts from the sheet along a certain contour line and ensure the quality of the separated section. The purpose of the forming process is to make the sheet material plastically deform without breaking the blank to make a workpiece of the required shape and size. In practice, multiple processes are often applied to one workpiece. Stamping, bending, shearing, stretching, expansion, spinning and straightening are some of the main stamping processes.
Separation process (blanking)
It is a basic stamping process that uses a die to separate materials. It can directly make flat parts or prepare blanks for other stamping processes such as bending, drawing, forming, etc., and it can also trim the formed stamping parts. Stamping parts are widely used in industries such as automobiles, home appliances, electronics, instruments, machinery, railways, communications, chemicals, light industry, textiles and aerospace. The stamping process accounts for about 50% to 60% of the entire stamping process.
Forming process
Bending: A plastic molding method that bends metal sheets, pipes and profiles into a certain angle, curvature and shape. Bending is one of the main processes widely used in stamping parts production. The bending of metal materials is essentially an elastic-plastic deformation process. After unloading, the workpiece will produce elastic recovery deformation in a certain direction, which is called springback. Springback affects the accuracy of the workpiece and is a technical key that must be considered in the bending process.
Drawing: Drawing, also known as stretching or calendering, is a stamping process that uses a die to make an open hollow part from a flat blank obtained after stamping. The drawing process can be used to make thin-walled parts with irregular shapes such as barrels, steps, cones, spheres, and boxes. If combined with other stamping forming processes, parts with extremely complex shapes can also be produced. In stamping production, there are many types of deep-drawn parts. Due to their different geometric characteristics, the location of the deformation zone, the nature of the deformation, the distribution of the deformation, and the stress state and distribution law of each part of the billet are quite different, even essentially different. Therefore, the determination method of process parameters, number of processes and sequence is different from the principles and methods of mold design. According to the characteristics of deformation mechanics, various drawn parts can be divided into four types: straight-walled rotating bodies (cylindrical parts), straight-walled non-rotating bodies (box-shaped parts), curved rotating bodies (curved parts) and curved non-rotating bodies.
Drawing is to apply tensile force to the sheet through the drawing die, so that the sheet produces uneven tensile stress and tensile strain, and then the sheet gradually expands with the fitting surface of the drawing die until it is completely fitted with the drawing model surface. The applicable objects of wire drawing are mainly to manufacture double-curvature skins with certain plasticity, large surface area, smooth curvature change, and high quality requirements (accurate shape, smooth streamline, and stable quality). Due to the process equipment and equipment used, the drawing shape is relatively simple, so the cost is low and flexible; but the material utilization rate and productivity are low.
Spinning is a metal spinning processing technology. During this process, the blank rotates actively with the spinning die or the spinning head rotates actively around the blank and the spinning die, and the spinning head feeds relative to the core die and the blank, so that the blank undergoes continuous local deformation and obtains the desired hollow rotating part.
Trimming is the secondary trimming of the product shape using a mold of a predetermined shape. It is mainly reflected in the aspects of pressing the surface and bursting the feet, and is used for secondary processing when some materials are elastic and the quality of the first molding cannot be guaranteed.
Diameter expansion is a processing method that uses a die to stretch the sheet to make the local surface area thinner to obtain a part. Commonly used methods include corrugation forming, cylindrical (or tubular) blank expansion, and flat blank stretching. Expansion can be achieved using different methods, such as rigid die expansion, rubber expansion, and hydraulic expansion.
Flanging is a plastic processing method that bends the material from the edge of a thin sheet blank or a narrow band area at the edge of a prefabricated hole on the blank along a curve or straight line into a vertical edge. Flanging is mainly used to strengthen the edge of parts, remove trimmings, assemble parts with other parts, connect parts or three-dimensional parts with complex special shapes and reasonable spaces, and improve the rigidity of parts. It can also be used as a means to control cracking or creases in large sheet metal forming. Therefore, it is widely used in industrial sectors such as automobiles, aviation, aerospace, electronics and household appliances.
Creasing is a stamping method used to reduce the diameter of the open end of stretched flangeless hollow parts or tube blanks. The diameter of the end of the workpiece should not change too much before and after shrinking, otherwise the end material will wrinkle due to severe compression deformation. Therefore, shrinking from a larger diameter to a very small diameter neck opening usually requires multiple shrinking.
The surface and internal properties of the sheet used for stamping have a great impact on the quality of the stamped product. The requirements for stamping materials are
• ① Precise and uniform thickness. The thickness of the sheet will increase the deformation force, causing the die to get stuck or even crack; too thin sheet will affect the quality of the finished product, and even crack during drawing.
• ② Surface finish, no spots, no scars, no scratches, no surface cracks, etc. All surface defects will remain on the surface of the finished workpiece. In the bending, drawing, forming and other processes, crack defects may extend to deep and wide, causing scrapping.
• ③ Uniform yield strength, no obvious directionality. Anisotropy (see the plastic deformation of the sheet during the stamping process such as drawing, flanging, and expansion. Due to the appearance of continuous yielding, the plastic deformation is inconsistent, which will cause uneven deformation, inaccurate forming, and defective or waste.
• ④ High uniform elongation. In the tensile test, the elongation of the specimen before the necking phenomenon begins is called uniform elongation. During drawing, the deformation of any area of the sheet cannot exceed the uniform elongation range of the material, otherwise uneven deformation will occur.
• ⑤ Low yield-to-strength ratio. The ratio of the yield limit to the strength limit of the material is called the yield-to-strength ratio. A low yield-to-strength ratio can not only reduce the deformation resistance, but also reduce the tendency of wrinkling during drawing, reduce the amount of springback after bending, and improve the accuracy of the bent parts.
• ⑥ Low work hardening. Cold deformation after work hardening will increase the deformation resistance of the material and make it difficult to continue to deform, so sheets with a low hardening index are generally used. However, materials with high hardening index have good plastic deformation stability (i.e., plastic deformation is more uniform), and local cracking is not easy to occur.
In actual production, processes similar to stamping processes are often used to test the stamping performance of materials, such as deep drawing performance tests, expansion performance tests, etc., to ensure the quality of finished products and a high pass rate.
Stamping die
The precision and structure of the stamping die directly affect the forming and precision of stamping parts. The manufacturing cost and service life of the die are important factors affecting the cost and quality of stamping parts. Mold design and manufacturing require more time, which prolongs the production preparation time of new stamping parts. Standardization of die frames, die frames, and guides, development of simple dies (small batch production), compound dies, and multi-station progressive dies (large batch production), and development of rapid die change devices reduce the workload of preparation work, shorten the preparation time for stamping production, and make advanced stamping technology suitable for large-scale production reasonably applied to small batch and multi-variety production.
1. Obtain necessary information
Analyze the stamping processability of common structures according to relevant data, and conduct process review and standardization review of workpieces.
• 1) Obtain product part drawings indicating specific technical requirements. Understand the shape, size and precision requirements of the workpiece. The size (size and position) of the keyhole and key surface, analyze and determine the benchmark of the workpiece.
In fact, the various processing performance requirements of stamping parts are not absolute. Especially in the current situation of rapid development of stamping technology, it is necessary to comprehensively apply various stamping technologies, reasonably select stamping methods, and correctly formulate stamping processes and select stamping materials according to the actual needs and possibilities of production. Mold structure to meet the technical requirements and stamping process conditions of the product.
• 2) Collect workpiece processing process cards. From this, we can study the relationship between the previous and subsequent processes, as well as the processing process requirements and assembly relationships that must be guaranteed between each process.
• 3) Understand the production batch number of the workpiece. The production of parts plays a decisive role in the economy of the stamping process. To this end, it is necessary to determine the type, structure, material and other related matters of the mold according to the production batch of the parts and the quality requirements of the parts, so as to analyze the economy and production efficiency of the mold processing process. The rationality of public production and the outline of the stamping work steps are described.
• 4) Determine the specifications and rough material conditions of the workpiece raw materials (such as plates, strips, coils, scraps, etc.), understand the nature and thickness of the materials, determine whether to use less scrap-free materials, and make samples based on the processing performance of the parts to preliminarily determine the specifications and accuracy levels of the materials.
Under the premise of meeting the requirements of performance and stamping performance, materials with good quality and low price should be selected as much as possible.
• 5) Design and process requirements analysis of material fiber direction and burr direction.
• 6) Analyze the technical capabilities and equipment conditions of the tooling workshop for manufacturing molds, as well as the available mold standard parts.
• 7) Be familiar with the equipment information or conditions of the stamping workshop.
• 8) Study and digest the above information and preliminarily conceive the structural plan of the mold. If necessary, the established product design and process can be modified to better combine product design, process and mold design and manufacturing to achieve a more perfect effect.
2. Determine the process plan and mold structure type
The determination of the process plan is the top priority of the process analysis of stamping parts. This includes.
• 1) Process analysis: According to the shape characteristics, dimensional accuracy and surface quality requirements of the workpiece, its main attributes are judged and the basic process properties are determined. That is, the basic processes such as dropping, punching, bending, drawing, flanging and expanding are determined. List all the single processes required for stamping, which can generally be determined directly from the product part drawing requirements.
• 2) Determine the number of processes based on process calculation. For deep-drawn parts, the number of deep-drawn parts should also be calculated. Bending parts and stamping parts should also be determined based on their shape, size and accuracy requirements to determine whether they are processed once or multiple times.
• 3) Determine the order of process arrangement based on the deformation characteristics, size and accuracy requirements of the processing and the requirements of ease of operation. For example, use punching first and then bending, or punching first and then bending.
• 4) According to factors such as production volume, size, accuracy requirements, mold manufacturing level, equipment capacity, etc., preliminarily arrange the single process sequence into possible process combinations. Such as compound stamping process, continuous stamping process, etc. Generally, single-process production with simple molds is suitable for stamping parts with thick materials, low precision, small batches, and large sizes; progressive die continuous production is suitable for stamping parts with thin materials, small sizes, and large batches; compound molds are suitable for stamping parts with high printing position accuracy.
After determining the nature and sequence of the processes and the combination of processes, the process plan for stamping is determined. In other words, the structural type of each process tube core is determined.
3. Perform necessary process calculations
• 1) Design the material arrangement and calculate the blank size.
• 2) Calculate the punching force (including punching force, bending force, tensile force, flanging force, hole expansion force, unloading force, thrust, clamping force, etc.), and if necessary, calculate the punching work and power.
• 3) Calculate the pressure center of the mold.
• 4) Calculate or estimate the thickness of the main parts of the mold. Such as the thickness of the die, the free height of the punch fixing plate, the pad, and the unloading rubber or spring.
• 5) Determine the gap between the convex and concave dies and calculate the size of the working part of the convex and concave dies.
• 6) For the drawing process, it is necessary to determine the drawing method (whether crimping or not), calculate the number of drawing times and the size of the semi-finished product in the intermediate process.
For some processes, such as continuous wire drawing with material, special process calculations are required.
4. Overall design of the mold
Based on the above analysis and calculation, the overall design of the mold structure is carried out (generally only a sketch is needed at this time), the mold clamping height is preliminarily calculated, and the shape and size of the mold are roughly listed.
5. The main parts of the mold structure design
1) Parts of the workpiece. Such as the design of the punch, die, and the structure type of the punch and die, and the selection of the fixing form.
2) Positioning part. There are many commonly used positioning devices in the mold. Such as adjustable positioning plates, fixed stop pins, movable stop pins, and fixed distance sides, etc., which need to be selected and designed according to the specific situation.
In the continuous mold, it is also necessary to consider whether to use the initial stop pin.
3) Unloading and pushing device. Such as the choice of rigidity or elasticity, the selection and calculation of springs and rubber, etc.
4) Guide parts. Such as the selection of guide pins, guide sleeve guide rails or guide plate guide rails, the selection of intermediate guide pins, side and rear guide pins or inclined guide pins, whether to use sliding guide sleeves or ball guide sleeves with steel balls, etc. .
5) Support and clamping parts, fastening parts. Such as die handles, upper and lower die frame structure selection, etc.
6. Selection of stamping equipment
The selection of stamping equipment is an important part of process design and mold design. The reasonable selection of equipment has a significant impact on the quality assurance of workpieces, the improvement of productivity and the safety of operation, and also brings convenience to mold design.
The selection of stamping type mainly depends on process requirements and production batches.
The specifications of stamping equipment mainly depend on process parameters and mold structure dimensions. For crank presses, the following requirements must be met.
• 1) The nominal pressure of the press must be greater than the process force of stamping. That is
More precisely, the load curve of the stamping process must be below the allowable load curve of the press.
For deep drawing parts, the drawing time also needs to be calculated.
• 2) The height of the press must meet the requirements of the die height.
• 3) The stroke of the press should meet the requirements of workpiece forming. For the press used in the deep drawing process, its stroke must be greater than 2~2.5 times the height of the workpiece in the process, so as to put in the blank and take out the workpiece.
• 4) The size of the worktable of the press must be larger than the outer dimensions of the mold under the mold frame, and leave a position for fixing the mold. Generally, each side is greater than 50~70mm. The size of the air leakage hole on the press must be larger than the size of the workpiece (or waste).
7. Draw the general drawing of the mold
The general drawing of the mold (including the part workpiece drawing) is drawn strictly in accordance with the drawing standards (GB/T4457~GB/T4460 and GB/T131-1993). At the same time, in actual production, combined with the working characteristics of the mold and the needs of installation and adjustment, form a certain habit for the layout of its drawings.
The general drawing of the mold includes.
• 1) Main view. Draw a cross-sectional view of the mold in the working position. Generally, half of the drawing shows the situation of placing the blank before stamping starts (when the punch is in the upper stop position), and the other half shows the state when the punch is in the lower stop position after stamping ends and the workpiece is formed (or separated).
• 2) Top view. Usually the lower half of the top view and the other half of the top view are drawn. Depending on the needs, sometimes the lower half of the top view is also drawn completely.
• 3) Side view, front view and partial section view, etc. If necessary, a side view of the working position of the mold must be drawn. Sometimes the upper elevation view of the mold and the partial section view are also drawn in the upper right corner of the drawing.
• 4) Workpiece drawing. Generally, the workpiece drawing is in the upper right corner. For work completed by several sets of molds, in addition to drawing the workpiece drawing of this process, the workpiece drawing of the previous process also needs to be drawn.
• 5) Layout drawing. For progressive dies, it is necessary to draw the graphic arrangement, process arrangement order and stamping content completed in each process; the step spacing, overlap value and strip size should be indicated. The die layout drawing should show the layout, strip size and overlap value size.
• 6) List the parts list, indicating the material and quantity. All standard parts should be selected with specifications.
• 7) Technical requirements and instructions. Technical requirements include stamping pressure, type of equipment selected, overall line tolerance of mold and assembly, installation and debugging, mold closing height, mold clearance and other requirements.
8. Draw the parts drawings of each non-standard part
The parts drawings should indicate all dimensions, tolerances and fits, shape tolerances, surface roughness, materials used and their heat treatment requirements, as well as other technical requirements.
Fill in the mold record card and prepare the stamping process documents.
For small batch production, the process route list should be filled in; for large batch production, process cards and program cards should be made for each part.
Professional equipment
Except for the hydraulic press for thick plate forming, mechanical presses are generally used. With modern high-speed multi-station mechanical presses as the center, it is equipped with unwinding, leveling, finished product collection, conveying machinery, as well as mold storage and rapid mold change devices, controlled by computer programs, and high production efficiency can form an automatic stamping production line.
Lubrication
Generally speaking, during the stamping process, lubricants must be added because the temperature rises quickly during the stamping process, especially during cold forging. If the workpiece is directly stamped without lubricant, in addition to affecting the finish of the workpiece, it will also shorten the life of the mold and reduce the precision. For this reason, a lot of money needs to be invested to improve the mold. It is for this reason that stamping lubrication is needed in cold forging.
Safety in production
When dozens or hundreds of stamping parts are produced per minute, personal, equipment and quality accidents often occur when the feeding, stamping, discharging, and waste discharge processes are completed in a short time. Therefore, safety in stamping is a very important issue. The safety measures for stamping are.
• ① Realize mechanization and automatic feeding and discharging.
• ② Set up mechanical protection devices to prevent hand injuries. Use mold protection devices, automatic rewinding devices, and manual tools to feed and unload materials.
• ③ Set up electrical protection and power-off devices. Set up photoelectric or air curtain protection switches, two-hand or multi-hand series start switches, and anti-misoperation devices.
• ④ Improve the clutch and brake structure so that the press crankshaft, connecting rod and punch stop in place immediately after the danger signal is issued.
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