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Basics of stamping processing and concept law of sheet metal plasticity

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Estimated reading time: 15 minutes

In our daily life, we often encounter various parts as shown in Figure 1-1, which are closely related to our lives.

Figure 1-1 Various common products

What processing methods are used to produce the above parts, what materials are used to produce them, what tools or molds are needed to produce these parts, and what materials are used to manufacture these tools or molds. This is what we need to learn in this course.

The concept of stamping and stamping die

Stamping is one of the advanced and efficient processing methods in the modern machinery manufacturing industry. It uses the mold installed on the press to apply force to the material at room temperature, causing it to separate or plastically deform, so as to obtain one part of the required parts. Kind of pressure processing method. Stamping processing is a major form of cutting processing. Since stamping is usually carried out at room temperature, it is often called cold stamping. And because its processing material is mainly sheet material, it is also called sheet material processing. Stamping can not only process metal materials, but also non-metal materials.

In stamping processing, a piece of special process equipment for processing materials into stamping parts (or semi-finished products) is called stamping die or cold stamping die. Stamping dies are indispensable in the realization of stamping processing. Without stamping dies that meet the requirements, stamping processing cannot be carried out; without advanced stamping dies, advanced stamping processes cannot be achieved. Die design is the key to cold stamping processing. A stamping part often needs several sets of dies to be processed into shape. In the production of stamping parts, reasonable stamping forming technology, advanced molds, and efficient stamping equipment are indispensable three elements, as shown in Figure 1-2.


Figure 1-2 Influencing factors of the quality of stamping parts

Features of stamping processing

Compared with other processing methods (such as machining), stamping has the following characteristics.

  • It is possible to obtain parts with complex shapes that other processing methods cannot or are difficult to process, such as automobile covers, doors, etc.
  • Since the dimensional accuracy is mainly guaranteed by the mold, the processed parts have stable quality, good consistency, and have the characteristics of “identity.”
  • Stamping processing is a kind of processing without cutting. Some parts are stamped directly without any reprocessing, and the material utilization rate is high.
  • Plastic deformation of metal materials can be used to improve the strength and rigidity of the workpiece.
  • High productivity, easy to realize automation.
  • The mold has a long service life and relatively low production cost.
  • The stamping process is easy to operate, but it has a certain degree of danger, so safety should be paid attention to in production.

Application of stamping processing

Due to the many advantages of stamping processing, the application of stamping processing is very extensive. It occupies a very important position in the production of automobiles, tractors, motors, electrical appliances, instrument toys and daily necessities. Many parts manufactured by casting, forging, and cutting methods in the past are now replaced by stamping parts with good rigidity and light weight.

According to statistics in recent years, in the production of electromechanical and instrumentation, 60% to 70% of the parts are completed by stamping technology. About 60%~70% of parts in automobile production are made by stamping process, and the labor of stamping production is 25%~30% of the labor of the entire automobile industry. In electronic products, the proportion of stamping parts is also quite large. The metal products used in people’s daily life, such as aluminum pots, stainless steel tableware, etc., account for a larger proportion of stamping parts. Therefore, stamping technology is widely used, and learning, researching and developing stamping technology is of great significance to the development of my country’s national economy and the acceleration of modern industrial construction.

The development of stamping technology is of great significance to the development of my country’s national economy and the acceleration of modern industrial construction.

The basic process of stamping

Due to the different shapes, sizes, and precision of stamping parts, the types of processes used in stamping are different. According to its deformation characteristics, it can be divided into the following two categories.

  • Separation process. The process of separating the sheet material along a certain contour line to obtain a stamping part (commonly known as a blanking part) of a certain shape, size, and cross-sectional quality. The separation process mainly includes punching, blanking, trimming, and other processes.
  • Forming process. The process of making the material plastically deform without breaking to obtain a stamped part with a certain shape, size, and accuracy. The forming process mainly includes bending, deep drawing, flagging, bulging, braiding, etc.

The commonly used stamping processes are shown in Table 1-1

Also, to improve labor productivity, two or more basic processes are often combined into one process, such as blanking and stretching, cutting and bending, punching and flanging, etc., which are called composite processes. In actual production, most of the parts produced in batches are completed by composite processes.

Plastic deformation of sheet metal and its basic laws

The stamping forming process of stamping parts is essentially the plastic deformation process of the sheet metal. Regarding the basic theory of plastic deformation, there have been detailed and systematic expositions in the works on plastic processing mechanics, and only a brief description of the relevant theory is given here.

The basic concept of metal plastic deformation

  • Plasticity

Plasticity is the ability of a metal to stably undergo permanent deformation without damaging its integrity under the action of external force. It reflects the deformability of metal and is an important processing property of the metal. The size of plasticity can be evaluated by the plasticity index. For example, the plasticity index in the tensile test can be expressed by the elongation δ and the reduction of area ψ. The plasticity of metal is not fixed, it is affected by factors such as metal structure, deformation temperature, deformation speed, and workpiece size.

  • Plastic deformation

The object deforms under the action of external force. After the external force is removed, the object can return to its original shape and size. Such deformation is called plastic deformation.

  • Deformation resistance

Deformation resistance refers to the ability of a metal to resist shape changes and residual deformation. Deformation resistance reflects the difficulty of plastic deformation of the material. Generally speaking, good plasticity and low deformation resistance are beneficial to stamping deformation, but it cannot be said that certain material has good plasticity and deformation resistance must below. When the material is cold extruded, it exhibits good plasticity under the action of three-way compressive stress, but the cold extrusion force is also very large.

  • Stress

Under the action of external force, the interaction force between the various particles in the object is called internal force. The internal force per unit area is called stress. There are normal stress and shear stress. Normal stress is expressed by σ and shear stress is expressed by τ. The unit of stress is generally MPa.

  • Strain

When an object is subjected to external and internal forces, it will deform. The physical quantity that represents the magnitude of the deformation of an object is called strain. Like stress, the strain also has the normal strain and shear strain. Normal strain is represented by ε, and shear strain is represented by γ.

  • Point of stress state

The force of each point in the material is usually called the stress state of the point. The stress state of a point is represented by the stress on each surface perpendicular to each other on the unit body taken at the point, as shown in Figure 1-3(a). Generally, these forces can be decomposed into 9 stress components along the coordinate direction, including 3 normal stresses and 6 shear stresses, as shown in Figure 1-3(b).

  • Principal stress

For any kind of stress state, there is always such a set of coordinate systems, so that only normal stress appears on each surface of the unit body, and there is no shear stress, as shown in Figure 1-3(c). These three normal stresses are called principal stresses and are represented by σ1, σ2, and σ3 respectively. When the stress σ1>0 is called tensile stress, when the stress σ1<0 is called compressive stress.


Figure 1-3 stress state

Experiments have proved that the stress state has a great influence on the plasticity of metals. The more the number of compressive stresses, the greater the value, the better the plasticity of the metal; the more the number of tensile stresses, the greater the value, the worse the plasticity of the metal.

  • Principal strain and principal strain diagram.

Figure 1-4 Main strain state diagram

The stress in the deformed body must be accompanied by strain, and the strain state of the point is also represented by the element body. Similar to the stress state, a strain state diagram can also be used to indicate the strain state of a point. A set of coordinate systems can be found so that only the principal strain components ε1, ε2, ε3 and no shear strain components appear on each surface of the unit body, as shown in Figure 1-4(a). A strain state has only the primary principal strain. There are only three possible strain states, as shown in Figure 14(b).

The strain state has a great influence on the plasticity of the metal. It can be known from practice that the degree of deformation obtained by unidirectional compression is much greater than that of uniaxial stretching, and the extrusion in the state of three-directional compressive stress can exert greater plasticity than the drawing with two-directional compression and one-way stretching. In the stress state, the number of compressive stresses is large, the compressive stress is large, the plasticity is good; on the contrary, the number of compressive stresses is small, the compressive stress is small, and even tensile stress exists, and the plasticity is poor. This is because material cracks and defects are easy to be exposed and developed in the direction of tensile strain, but not easy to be exposed and developed in the direction of compressive strain.

Stress-strain curve

Figure 1-5 shows the stress-strain curve of low carbon steel under tensile test. It can be seen from the figure that the material begins to plastically deform when the stress reaches the initial yield limit σ0. At this time, a large deformation can occur when the stress is not increased, and a platform appears on the figure. This phenomenon is called yielding. After a period of yielding plateau, the stress begins to rise with the increase of strain (as shown in the cGb curve). If it is unloaded in the middle of the deformation (G in the figure), the stress and strain will return along the GH straight line to restore the elastic deformation (HJ) and retain its plastic deformation (OH). If the test piece is reloaded, the curve will start from H and rise along the HG straight line for elastic deformation until point G does not begin to yield, and the subsequent stress and strain will still change according to the Gb curve. It can be seen that the stress at point G is the yield stress when the specimen is reloaded. If you repeat the above unloading and loading process, you will find that the yield stress during reloading is continuously increasing along the Gb curve due to the successive increase in deformation, which indicates that the material is gradually hardening. The work hardening of the material has a great influence on the forming of the sheet metal, which not only increases the deformation force, but also limits the further deformation of the wool. For example, when a deep-drawn part is drawn multiple times, it is generally annealed before the subsequent drawing to eliminate the work hardening caused by the previous drawing. But hardening is sometimes beneficial. For example, in the elongation forming process, it can reduce excessive local deformation and make the deformation more uniform.


Figure 1-5 Stress-strain curve of low carbon steel under tensile test

For practical needs, the stress-strain curve must be expressed by a mathematical formula. However, because the hardening curves of various materials have different characteristics, it is impossible to accurately express them with the same mathematical formula. The mathematical expressions of several hardening curves commonly used at present are all approximate. For example, the linear expression of the stress-strain curve is σ=σ0+ Fε

In the formula, the approximate yield limit of σ0 is also the intercept of the hardening line on the ordinate axis;

F-The slope of the hardening straight line is called the hardening modulus, which shows the size of the hardening strength of the material.

The law of in variance of plastic deformation volume

Practice has proved that in the plastic deformation of an object, the volume before deformation is equal to the volume after deformation. This is the law of invariance of metal plastic deformation volume. It is the basis for us to calculate the blank size in the deformation process in the future. Expressed by formula

ε123=0

The law of least resistance to plastic deformation

Plastic deformation destroys the overall balance of the metal and forces the metal to flow. When the mass points of the deformable body may move in different directions, each mass point moves in the direction of least resistance, which is the law of least resistance. The blank is deformed in the mold, and its maximum deformation will be in the direction of least resistance. The law of least resistance has a very flexible and extensive application in the stamping process, which can correctly guide the stamping process and die design, and solve the quality problems in actual production.

Plastic conditions

The so-called plastic condition is that in a unidirectional stress state if the tensile or compressive stress reaches the yield point of the material, it can yield and enter the plastic state from the elastic state. However, for complex stress states, it is not only possible to judge whether a point has yielded based on one stress component, but also to consider the comprehensive effect of each stress component. In a complex stress state, when the stress components conform to a certain relationship, it can be equivalent to the yield point determined under the unidirectional stress state. So that the object enters the plastic state from the elastic state. At this time, the relationship between the stress components is called the plastic condition, or the yield criterion.

Plastic conditions must be verified by experiments. There are two types of plastic conditions that have been tested and recognized in practice: the H. Tresca yield criterion and the Von Mises yield criterion.

  • Kureisgar Yield Criterion

In 1864, the French engineer H. Tresca believed that the material began to yield when the maximum shear stress reached a certain value, that is, the Tresca yield criterion. Its mathematical expression is

In the formula, σs—the yield limit of the material.

  • Von Mises Service Guidelines

In 1913, the German scholar Von mises proposed that under certain deformation conditions, no matter what the stress state of the deformed object is, as long as its three principal stresses meet the following conditions, the material will begin to yield, that is, Missis Yield

Then, its mathematical expression is

12)2+(σ23)2+(σ31)2=2σ2s

The relationship between stress and strain

The body deforms under force, so there must be a certain relationship between stress and strain. When an object is elastically deformed, the relationship between stress and strain is linear, the deformation process is reversible, and its deformation can be restored regardless of the loading process of the object. The relationship between stress and strain can be determined by the generalized Hooke’s law. Said. After the object enters into plastic deformation, the relationship between its stress and strain is different. In unidirectional tension or compression, the relationship between stress and strain can be represented by a hardening curve. However, when subjected to two-way or three-way stress, the relationship between stress and strain in the deformation zone is quite complicated. Studies have shown that under simple loading (only loading and not unloading during the loading process, and the stress components increase in a certain proportion), at each moment of plastic deformation, there is the following relationship between the principal stress and the principal strain

In the formula, C—non-negative constant of proportionality;

σm—average stress. Under certain conditions, C is only related to the properties of the material and the degree of deformation, and has nothing to do with the stress state of the object, so the C value can also be obtained by uniaxial tensile experiments.

The above-mentioned physical equations are also called the full-quantity theory of plastic deformation.

Work hardening phenomenon

Commonly used metal materials increase in strength and hardness during plastic deformation, while the phenomenon of a decrease in plasticity and toughness is called work hardening or cold work hardening. Work hardening has a great impact on many stamping processes. For example, the reduction of plasticity limits the further deformation of the blank. It is often necessary to increase the annealing process before the subsequent process to eliminate work hardening. Work hardening also has a positive side, such as improving the ability to resist local instability and wrinkles.

Reload softening phenomenon

If the material is reversely loaded after the cold plastic deformation, the yield limit of the material will be reduced. That is, plastic deformation is more likely to occur under reverse loading, which is the so-called back-load softening phenomenon. The phenomenon of backloading softening is of practical significance for the analysis of certain stamping processes (such as stretch bending).

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Wendy Xu

Wendy has her own unique knowledge of punching machines. When she sees a customer’s drawing, Wendy can accurately recommend the best punching machine and the right tooling for the customer. Because Wendy knows enough about punching machines, she is often called upon by distributors to buy large quantities of punching machines.

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  1. Nina Smithy

    This article introduces the stamping foundation and material plasticity in detail, it is really professional! I like it so much!

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