What is stamping

What is stamping?

Stamping is one of the basic methods of plastic processing. Stamping is a pressure processing method that uses a die mounted on a press to apply force to a material at room temperature to produce separation or plastic deformation to obtain the desired part.

Since stamping is usually processed at room temperature, it is often referred to as cold stamping; and since the material it processes is mainly sheet material, it is also known as sheet processing. Stamping can process not only metal materials, but also non-metal materials.
In stamping processing, a special process equipment for processing materials into stamping parts (or semi-finished products) is called stamping die or cold stamping die. The stamping die is an essential process equipment in the actual stamping process. Without the stamping die that meets the requirements, the stamping process cannot be realized; without the advanced stamping die, the advanced stamping process cannot be realized. In the production of stamping parts, reasonable stamping and forming process, advanced die and mold, and efficient stamping equipment are the three essential elements.

Advantages of stamping production

Compared with other processing methods, stamping production has its advantages in both technical and economic aspects.

• (1) Stamped parts have stable quality and high dimensional accuracy. Since stamping production uses moulds for forming, the moulds are manufactured with high precision, so the quality of stamping parts is stable, the interchangeability of parts is good, and the dimensional accuracy is high.
• (2) High productivity and low cost. Stamping production is the use of stamping dies and stamping equipment to complete processing, its high productivity, easy to operate, easy to achieve mechanization, automation. For ordinary presses, it can produce a few pieces to dozens of pieces per minute, and high-speed presses can produce hundreds or even thousands of pieces per minute. The stamping parts are light in quality, rigid and strong, and the stamping process consumes less energy, so the cost is low in medium to large volume production.
• (3) High material utilization rate. Stamping production is one of the methods of less, no cutting and chipping processing, stamping production can achieve less scrap or even scrap-free production, in some cases, the utilization rate of the edge residue is high, generally 70% to 85%.
• (4) Easy to obtain complex parts. Because of the use of die processing, it is possible to obtain parts with complex shapes that cannot be manufactured by other processing methods or are difficult to manufacture.

Disadvantages of stamping production

• (1) Mold manufacturing cycle is long, high manufacturing costs, not suitable for small batch production. Because of the fast reciprocating movement of the ram and manual operation, the labor intensity is high and accidents are prone to occur, so attention must be paid to safe production, safety management and necessary safety technical measures.
• (2) Stamping process produces two public hazards, vibration and noise. These problems are not entirely caused by the stamping process itself, but mainly due to the backwardness of traditional stamping equipment, and with the progress of science and technology, these two public hazards will be solved to a certain extent.

Because of the above-mentioned advantages of stamping production, stamping processing is widely used and occupies a very important position in the production of automobiles, tractors, electromechanics, electrical appliances, instrument toys and daily necessities. Many parts made by casting, forging and cutting processing methods in the past are now replaced by stamping parts with good rigidity and light quality.
According to the statistics of this year, in the production of electromechanics, instruments and meters, 60% to 70% of the parts are made by stamping process. In the production of automobiles, about 60% to 70% of the parts are made by stamping process, and the labor volume occupied by stamping production is 25% to 30% of the labor volume of the whole automobile industry. For metal products in people’s daily life, the proportion of stamping parts is even greater, such as: aluminum pots and pans, immortal just tableware, etc. Stamping products can be seen everywhere, therefore, stamping technology is very widely used. Studying, researching and developing stamping technology is of great significance to the development of our national economy and the acceleration of modern industrial construction.

Method of stamping process

Because of the different shapes, sizes, precision requirements, production lots, raw materials, etc. of the parts produced by stamping, there are many different stamping process methods used in production.
(1) Separation process: The process of separating the sheet along a certain contour line to obtain stamping parts of certain shape, size and interface quality. The separation process mainly includes: punching, dropping, cutting and other processes.

(2) Forming process: The process of obtaining a certain shape, size and precision of stamped parts by plastic deformation of the material without breaking. Forming process mainly includes: bending, deep drawing, flanging, expansion, shrinkage, etc.

• Bending: The plastic forming method of bending metal sheets, fittings and profiles into certain angles, curvatures and shapes. Bending is one of the main processes widely used in the production of stamped parts. Bending of metal materials is essentially an elasto-plastic deformation process, after unloading, the workpiece will produce the direction of elastic recovery deformation, called rebound. Rebound affects the accuracy of the workpiece, is the bending process must consider the technical key.
• Deep drawing: deep drawing, also known as stretching or calendering, is a stamping process that uses a die to make the flat blank obtained after punching into an open hollow part. With the deep drawing process can be made into barrel-shaped, stepped, conical, spherical, box-shaped and other irregularly shaped thin-walled parts. If combined with other stamping and forming processes, it can also produce parts of extremely complex shapes. In stamping production, there are many types of drawn parts. Due to their different geometric characteristics, the location of deformation zone, the nature of deformation, the distribution of deformation and the stress state and distribution law of each part of the billet have considerable, even essential differences. So the process parameters, the number of processes and sequence of determining the method and mold design principles and methods are not the same. Various drawing parts can be divided into four types according to the characteristics of deformation mechanics, such as straight-walled rotary body (cylindrical parts), straight-walled non-rotary body (box-shaped body), curved rotary body (curved shape parts) and curved non-rotary body.
• Pulling: Pulling is to apply tensile force to the sheet through the pulling die, so that the sheet produces uneven tensile stress and tensile strain, and then the sheet gradually expands with the pulling die until it completely fits with the pulling model surface. The applicable object of drawing is mainly the manufacture of double curvature skins with certain plasticity, large surface area, gentle and smooth curvature change, and high quality requirements (accurate shape, smooth streamline, stable quality). Drawing shape is relatively simple due to the process equipment and equipment used, so the cost is low and flexible; however, the material utilization rate and productivity is low.
• Spinning: Spinning is a metal rotary machining process. In the process, the billet with the spinning die active rotation or spinning head around the billet and spinning die active rotation, spinning head relative to the core die and the billet for feeding motion, so that the billet produces continuous local deformation and get the required hollow rotary parts.
• Shaping: Shaping is the use of the established shape of the abrasive tool to the shape of the product for secondary trimming. Mainly embodied in the pressure plane, spring feet, etc. For some materials exist elasticity, can not guarantee the quality of primary molding, the use of re-processing.
• Expansion: expansion is the use of the mold to stretch the sheet thinning local surface area to increase the processing method to obtain parts. Commonly used are undulation forming, cylindrical (or tube-shaped) blank expansion and flat blank tension forming. Expansion can be achieved using different methods, such as rigid die expansion, rubber expansion and hydraulic expansion.
• Flanging: Flanging is a plastic processing method that bends the material along the curve or straight line from the edge of a thin slab blank or the narrow strip area at the edge of a prefabricated hole in the blank into a vertical edge. Flanging is mainly used to strengthen the edge of the part, remove the cut edge and made on the part with other parts assembly, connection parts or three-dimensional parts with complex special shape, reasonable space, while improving the stiffness of the part. In large sheet metal forming, it can also be used as a means of controlling rupture or creasing. Therefore, it is very widely used in industrial sectors such as automobile, aviation, aerospace, electronics and household appliances.
• Indentation: Indentation is a stamping method to reduce the diameter of the open end of a hollow part or billet that has been stretched without flanges. The diameter of the end of the workpiece should not change too much before and after shrinkage, otherwise the end material will be wrinkled due to severe compression deformation. Therefore, shrinking from a larger diameter into a very small diameter neck opening often requires multiple shrinkage.

Nowadays, with the continuous progress of science and technology and the development of industrialized production, the stamping process technology is also constantly innovating and developing. These innovations and developments are mainly manifested in the following aspects.

• (1) Modernization of process analysis and calculation methods. For example, for the stamping process of producing automobile coverings, the traditional method is to conduct comparative analysis according to the available design information and the experience of the designer, determine the process scheme and relevant parameters, then design the mold and carry out test punching, and after repeated tests and modifications, it can be transferred to mass production. In recent years, some foreign companies have started to adopt the elastic-plastic finite element method with finite deformation to calculate and simulate the forming process of overlay parts, analyze the stress-strain relationship, so as to predict the feasibility of a certain process scheme and the problems that may arise, and display the results on the graphic terminal for designers to make choices and modifications. In this way, it can not only save the expensive mold trial cost and shorten the product trial cycle, but also establish the advanced design method that meets the production reality; it not only promotes the development of cold stamping process, but also can play the role of plastic forming theory to know the production reality.
• (2) The modernization of mold design and manufacturing technology. In order to speed up the renewal of products and shorten the design and manufacturing cycle of tooling, the research and application of computer-aided design and manufacturing (CAD/CAM) technology for molds is being vigorously carried out. The use of this technology, generally can improve the efficiency of mold design and manufacturing 2 to 3 times, with the production cycle can be shortened by 1/2 to 1/3. The ultimate goal of the development of this technology, is to achieve the integration of mold CAD / CAM, and mold drawings will only be used as a test mold. The use of mold CAD / CAM technology, can also improve the quality of the mold, greatly reducing the design and manufacturing personnel duplication of efforts, so that the designer may be used in innovation and development.
• (3) Mechanization of stamping production and automatic Hua. In order to meet the needs of mass production, stamping equipment has developed from single-station low-speed presses to multi-station high-speed automatic presses. Generally, small and medium-sized stamping parts can be produced on multi-station presses, and multi-station continuous die processing can also be adopted on high-speed presses, so that stamping production can be highly automated. Large stamping parts (such as automobile coverings) can be produced on multi-station presses with automatic feeding and picking devices for mechanized assembly line production, thus reducing labor intensity and increasing productivity.
• (4) In order to meet the development trend of accelerated product renewal and reduced production volume, some new forming processes, simple dies, general combination dies, CNC stamping equipment and flexible stamping manufacturing system (FMS) have been developed. In this way, stamping production can be suitable for both mass production and small batch production.
• (5) Continuously improve the performance of sheet materials to enhance their forming ability and use effect. For example, high strength steel plates are developed for the production of automobile coverings; new material plates are developed for the production of aviation components, etc.

Due to the different shapes, sizes, precision requirements, batch sizes and raw material properties of the stamped parts, there are various methods of stamping processes used in the production. However, in summary, they can be divided into two categories: separation processes and forming processes. The purpose of the separation process is to separate the stamped parts from the sheet along a certain contour line during the stamping process, and at the same time, the quality of the separated section of the stamped parts must meet certain requirements. The purpose of forming process is to make plastic deformation of stamping blank without destroying it, and transform it into the required shape of finished product, and also to meet the requirements of dimensional accuracy.

Processing characteristics of stamping

Compared with castings and forgings, stamped parts are thin, uniform, light and strong. Stamping can make workpieces with reinforcement, ribs, undulations or flanges that are difficult to manufacture by other methods to improve their rigidity. Due to the use of precision dies, the accuracy of workpieces can reach micron level, and with high repetition accuracy and consistent specifications, holes nests and tabs can be stamped out. Cold stamped parts are generally no longer processed by cutting, or only a small amount of cutting is required. The precision and surface condition of hot stamped parts are lower than those of cold stamped parts, but they are still better than castings and forgings, and the cutting process is less.
Stamping is a highly efficient production method. By using compound die, especially multi-station progressive die, multiple stamping processes can be completed on one press (single-station or multi-station) to realize fully automatic production from uncoiling, leveling, punching and cutting to forming and finishing. It has high production efficiency, good labor conditions and low production cost, and can generally produce hundreds of pieces per minute.　Compared with other methods of machining and plasticity processing, stamping has many unique advantages in terms of technology and economy. The main performance is as follows.

• (1) Stamping has high production efficiency and is easy to operate, and it is easy to realize mechanization and automation. This is because stamping relies on the die and stamping equipment to complete the processing. The number of strokes of an ordinary press can reach dozens of times per minute, and the high-speed press can reach hundreds or even thousands of times per minute, and it is possible to get a punching part with each stamping stroke.
• (2) Since the die guarantees the size and shape accuracy of the stamped parts and generally does not damage the surface quality of the stamped parts, and the life of the die is generally long, the quality of the stamped parts is stable and interchangeable, and it has the characteristic of “one mold”.
• (3) Stamping can process parts with a wide range of sizes and complex shapes, such as the second hand of a clock, the longitudinal beam of a car, and the covering parts, etc. In addition, the cold deformation and hardening effect of materials during stamping makes the strength and rigidity of stamping higher.
• (4) Stamping is a material-saving and energy-saving processing method because there are no chips and scraps generated and the material consumption is less, and no other heating equipment is needed.

Because of such superiority of stamping, stamping is widely used in various fields of national economy. For example, stamping is used in aerospace, aviation, military, machinery, agricultural machinery, electronics, information, railway, post and telecommunications, transportation, chemical industry, medical apparatus, daily-use electrical appliances and light industry. Not only the whole industry uses it, but also everyone is directly connected with stamping products. There are many large, medium and small stamping parts in airplanes, trains, automobiles and tractors. The body, frame and rim of a small car are all stamping parts. According to the survey statistics, 80% of bicycles, sewing machines and watches are stamping parts; 90% of TV sets, recorders and cameras are stamping parts; and metal cans and shells of foodstuffs, steel pots and pans, enamel pots and bowls and stainless steel tableware are all stamping products using molds; even the hardware of computers is missing stamping parts.

Problems of stamping

Die problem

Sometimes a complex part needs several sets of molds to be processed and shaped, and the high precision and high technical requirements of mold manufacturing make it a technology-intensive product. Therefore, the advantages of stamping processing can be fully reflected only when the production volume of stamping parts is large, so as to obtain better economic benefits.

Safety problems

Press processing also has some problems and shortcomings. They are mainly manifested in two public hazards of noise and vibration generated during stamping processing, and safety accidents of operators occur from time to time. However, these problems are not entirely brought about by the stamping process and the die itself, but mainly by the traditional stamping equipment and the backward manual operation. With the progress of science and technology, especially the development of computer technology, and the progress of mechatronics technology, these problems will be solved as soon as possible and perfectly.

High-strength steel stamping

Today’s high-strength steel, ultra-high-strength steel is very good to achieve the light weight of the vehicle, improve the crash strength and safety performance of the vehicle, and therefore become an important development direction of automotive steel. However, as the strength of the plate increases, the traditional cold stamping process is prone to rupture during the forming process, which cannot meet the processing process requirements of high-strength steel plates. In the case that the forming conditions cannot be met, the hot stamping and forming technology for ultra-high strength steel plates is being gradually studied internationally. This technology is a new process that integrates forming, heat transfer and tissue phase change. It is mainly a process that uses the high temperature austenite state to increase the plasticity of the plate and reduce the yield strength through the die. However, thermoforming requires in-depth research on process conditions, metal phase transformation, and CAE analysis technology, and the technology is currently monopolized by foreign manufacturers, with slow development in China.

Stamping materials

The surface and intrinsic properties of the sheet material used for stamping have a great impact on the quality of the finished stamped product. The requirements for stamping materials are:

• ① Precise and uniform thickness. The thickness of the sheet will increase the deformation force and cause jamming and even cracking of the concave die; too thin sheet will affect the quality of the finished product and even cracking when drawing.
• ② Surface finish, no spots, no scars, no abrasions, no surface cracks, etc. All surface defects will remain on the surface of the finished workpiece, cracked defects in the bending, deep drawing, forming and other processes may expand to the deep and wide, resulting in scrap.
• ③ Yield strength uniformity, no obvious directionality. Anisotropy (see plastic deformation of the plate in the deep drawing, flanging, expansion and other stamping process, because of the emergence of successive yield, plastic deformation is not consistent, will cause uneven deformation, so that the forming is not accurate and cause defective or scrap.
• ④ High uniform elongation. In the tensile test, the elongation of the specimen before the start of the thin neck phenomenon is called uniform elongation. In deep 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-intensity ratio. The ratio of the yield limit of the material to the strength limit is called the yield strength ratio. Low yield strength ratio can not only reduce the deformation resistance, but also reduce the tendency to wrinkle when deep drawing, reduce the amount of rebound after bending, improve the accuracy of bending parts.
• ⑥ Low process hardening. Cold deformation after the emergence of process hardening will increase the deformation resistance of the material, so that continue to deformation difficulties, so the general use of low hardening index of the plate. But the high hardening index of the material plastic deformation stability (i.e., plastic deformation is more uniform), not easy to appear localized pulling crack.

In actual production, it is often used to test the stamping performance of the material with process tests similar to stamping process, such as deep drawing performance test and expansion performance test, in order to ensure the quality of finished products and high qualification rate.

Moulds

The precision and structure of the mold directly affect the forming and precision of the stamped parts. The cost and life of the mold manufacturing is an important factor affecting the cost and quality of the stamped parts. Tooling design and manufacturing takes more time, which extends the production preparation time for new stampings. The standardization of mold base, mold frame and guiding parts, the development of simple mold (for small batch production), compound mold and multi-station progressive mold (for mass production), and the development of quick mold change device can reduce the preparation workload and shorten the preparation time for stamping production, and enable the advanced stamping technology suitable for mass production to be reasonably applied to small batch and multi-species production.

Specialized equipment

Mechanical presses are generally used, except for thick plates which are formed by hydraulic presses. With a modern high-speed multi-station mechanical press as the center, equipped with machinery for uncoiling, leveling, finished product collection and conveying, as well as die storage and quick die change devices, and controlled by computer programs, a high-productivity automatic stamping production line can be formed.

Lubrication

In general, during the stamping process, the workpiece must be lubricated with lubricating oil because the temperature rises quickly during the stamping process, especially during the cold forging stamping process. If the workpiece is directly stamped without using lubrication, in addition to the impact on the finish of the workpiece, the die life will be shortened and the accuracy will be reduced, for which a lot of money will be invested in the improvement of the die. It is for this reason that stamping lubrication is necessary in cold forging stamping.

Safe production

In the case of producing tens or hundreds of stamping pieces per minute, personal, equipment and quality accidents often occur when the processes of feeding, stamping, discharging and scrap discharge are completed in a short period of time. Therefore, safety production in stamping is a very important issue. The safety measures for stamping are.

• ① Realize mechanized and automatic feeding and discharging of materials.
• ② Set up mechanical guards to prevent injuries to hands. Apply die guards, automatic rewind devices and hand tools to enter and exit the material.
• ③ Setting electrical protection and power-off device. Set photoelectric or air curtain protection switch, two-hand or multi-hand tandem start switch, anti-misoperation device, etc.
• ④ Improve the clutch and brake structure so that the crankshaft, connecting rod and punch of the press can stop in the original position immediately after the danger signal is issued.

Source: China Thin Wall Parts Manufacturer – Yaang Pipe Industry (www.epowermetals.com)

(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)

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