Dokonalý průvodce hlubokým tažením válcových dílů děrováním

Předpokládaná doba čtení: 13 minut

Deep drawing refers to a stamping processing method that uses a die to punch a flat blank into an open hollow part or to further change the shape and size of the open hollow part. The deep drawing process is widely used in the production of various industrial sectors such as automobiles, tractors, instruments, electronics, aerospace, and daily necessities. It is one of the basic processes of cold stamping. It can not only process rotating parts but also process box shapes. Parts and other thin-walled parts with complex shapes are shown in Figure 1-1 and Figure 1-2.

The drawing process is divided according to the shape of the blank: the forming method from a flat blank into an open hollow part with a bottom is called flat (first) drawing; the forming from a large-diameter hollow part to a small-diameter hollow part The method is called each subsequent deep drawing. The drawing process is divided according to the change of wall thickness: the drawing process in which the wall thickness of the part after drawing does not change much compared with the thickness of the blank is called constant thinning drawing; the wall thickness of the part after drawing is the same as the thickness of the blank. The drawing process that is significantly thinner is called thinning drawing. The non-thinness drawing process is widely used in production. This project focuses on its process analysis and mold design.

This project takes the drawing die design of the cylindrical part as shown in Figure 1-3 as the carrier, and comprehensively trains the readers to determine the drawing process and the preliminary ability to design the drawing die.

Part name: cylindrical part.

Production batch: medium batch.

Material: 08F steel.

Thickness: 1.0 mm.

Parts drawing: as shown in Figure 1-3.

Výkres deformation process and characteristics

Figure 1-4 shows the drawing process of cylindrical parts. A round flat blank with a diameter of D and a thickness of t is deep-drawn by a drawing die to obtain an open straight-wall circular simple part with an inner diameter of d and a height of h, and h> (D-d) /2.

What kind of plastic flow does the round flat blank under the action of the mold produce an open hollow part? The material transfer of the flat blank during deep drawing is shown in Figure 1-5. If the mold is not used, just remove the triangle shaded part in Figure 1-5, then bend the remaining part of the narrow strip along the circumference of the diameter d, and weld it to get the diameter d and the height h=(D-d)/ 2. An open cylindrical part with a welded seam on the periphery and a wavy mouth. This shows that the “excess material” must be removed when the round flat blank becomes a cylindrical part. However, the round flat blank did not remove the excess material during the deep drawing process, and the height of the workpiece obtained by deep drawing was greater than h, and the wall thickness of the workpiece increased. Therefore, the material in the triangle shaded part can only be considered as redundant material. Under the action, flow and transfer occurred.

Analyzing the transfer of material during hluboká kresba through the grid test can further illustrate the flow of metal during deep drawing, as shown in Figure 1-6.

Before deep drawing, draw a grid of concentric circles with equal spacing and equal-division radial lines on the round flat blank. After hluboká kresba, you can see that the grids in different areas have changed to different degrees. The following will analyze the flow of metal during the drawing process through the changes of the grid.

• The bottom grid of the cylindrical part basically keeps its original shape, indicating that the metal at the bottom of the punch has no obvious flow.
• Concentric circles with unequal tangential diameters are transformed into parallel circles with the same circumference on the cylinder wall. The distance increases, and the closer the upper part of the cylinder increases, a1>a2>a3>…>a, indicating the metal radial Strain is tensile strain, and the radial flow of the metal closer to the outer circle is greater.
• Concentric radial lines of equal division in the radial direction are transformed into parallel vertical lines on the cylinder wall, and the vertical lines are equally spaced with b. It shows that tangential strain is compressive strain, and the closer the metal is to the outer circle, the greater the tangential flow.
• As shown in Figure 1-6 (b), if you take a unit from the grid, it will be a fan-shaped grid before deep drawing with an area of A1. After deep drawing, it will become a rectangular grid with an area of A2, which is equivalent to A wedge-shaped slot that pulls the sector grid through the same way. Under the action of tangential compressive stress and radial tensile stress, the metal produces radial elongation deformation and tangential compression deformation to form a rectangular grid.
• According to the measurement, the bottom thickness is slightly smaller (generally ignored), and the thickness of the cylinder wall gradually increases from the bottom to the mouth, as shown in Figure 1-7, which indicates that the mouth of the cylinder has a large degree of deformation and a large amount of transferred metal. But because the average thickness of the drawn part is almost equal to the thickness of the blank, ignoring the slight thickness change can be approximated as the area of the small unit remains unchanged before and after drawing, that is, A₁=A₂, indicating that the surface area of the blank and the workpiece is equal before and after drawing.

In addition, due to the different degrees of deformation and work hardening of the blanks, the hardness of each part of the cylinder wall along the height direction is also different, and the hardness of the part mouth is higher, as shown in Figure 1-7.

In summary, the deformation characteristics during deep drawing can be summarized as follows.

• The material under the punch is basically not deformed and becomes the bottom of the cylinder after deep drawing. The deformation is mainly concentrated in the flat flange area on the surface of the die ( the ring part of D-d), which is the main deformation area of drawing deformation.
• The deformation of the deformation zone is uneven. It is compressed and shortened in the tangential direction and stretched in the radial direction. The more it goes to the mouth, the more it compresses and stretches. The thickness of the sheet at the mouth is increased.

Stress and strain during deep drawing

By analyzing the stress and strain of the sheet during the drawing process, it will help to solve the process problems in the drawing work and ensure the product quality. In the deep drawing process, the material has different stress and strain states in different parts. Cylindrical parts are the simplest and most typical deep-drawn parts. Figure 1-8 shows the stress and strain of a cylindrical part in a certain stage of the first drawing with a blank holder. The meaning of each symbol in the figure is as follows.

σ₁, ε₁—the radial stress and strain;

σ₂, ε₂—the stress and strain in the thickness direction;

σ₃, ε₃—stress and strain in the tangential direction.

According to the different stress and strain states, the drawn blank can be divided into 5 areas: Zone I is the flange part, which is the main deformation zone of the drawing process; Zone II is the corner of the die, which is a transition zone; Ⅲ Zone is the wall part of the cylinder, which plays the role of transmitting force; Zone IV is the rounded part of the punch, which is also a transition zone; Zone V is the bottom of the cylinder, which can be considered as having no plastic deformation.

At the place slightly above the corner of the cylinder wall and the bottom, the tensile stress σ₁ generated is relatively large because the cross-sectional area for transmitting the drawing force is small. At the same time, there is less material that needs to be transferred in this place, so the degree of deformation of the material is very small, the work hardening is lower, and the strength of the material is also lower. Compared with the rounded corners of the punch, there is no greater frictional resistance like the rounded corners of the punch. Therefore, during the deep drawing process, the thinning is the most serious at the corners of the cylinder wall and the bottom, which becomes the weakest part of the entire part. This section is usually called the “dangerous section”. If the stress σ₁ on the dangerous section exceeds the strength limit of the material, the drawn part will be cracked there. Even if there is no crack, the material becomes too thin at the place due to the excessive stress, so that the workpiece is scrapped due to the over-tolerance.

From the above analysis, it can be seen that the main quality problems during deep drawing are wrinkles in the plane flange area and cracks in the “dangerous section”.

Problems in the deep drawing process

Vráska

During deep drawing, due to the tangential compressive stress σ₃ of the flange material, when this compressive stress reaches a certain level, the tangential direction of the sheet material will be arched due to instability, which produces waves in the tangential direction around the flange. The continuous bending of the shape is called wrinkling, as shown in Figure 1-9 (a). When the drawn part is wrinkled, the material in the flange deformation zone of the lighter one can still be pulled into the die, but it will cause ripples at the mouth of the workpiece, as shown in Figure 1-9 (b), which will affect the quality of the workpiece. When the wrinkling is severe, the drawn part will be broken because the flange material after the wrinkling cannot pass through the gap between the convex and concave dies, as shown in Figure 1-9 (c). Wrinkles are one of the main causes of waste in deep drawing.

Whether wrinkles during drawing are related to the size of σ₃, and also to the relative thickness of the blank t/D, and σ₃ is related to the degree of deformation of the drawing. When the deformation degree of each drawing is large and the relative thickness t/D of the blank is small, it will wrinkle. The most effective measure to prevent wrinkling (and the most commonly used in production) is to use a crimp ring. Reducing the degree of drawing deformation and increasing the thickness of the blank can also reduce the tendency to wrinkle.

Rupture

Wrinkling does not mean that the deformation of the sheet material has reached the limit because the degree of deformation can still be improved after measures such as pressurizing the edge ring. As the degree of deformation increases, the deformation force increases accordingly. When the deformation force is greater than the carrying capacity of the dangerous section, the drawn part will be broken, as shown in Figure 1-10. Therefore, the carrying capacity of the dangerous section is the key to determining whether the deep drawing can proceed smoothly.

Whether the dangerous section is broken during hluboká kresba depends on the properties of the material, the degree of deformation, the fillet radius of the mold, and the lubrication conditions. In production practice, materials with a large hardening index and a small yield ratio are usually used for deep drawing, and measures such as appropriately increasing the drawing convex and concave die fillet radius, increasing the number of drawing times, and improving lubrication to avoid the occurrence of cracks.

Hardening

The drawing process is a process in which the blank undergoes plastic deformation, which must be accompanied by work hardening. Therefore, compared with the blank, the hardness and strength of the workpiece obtained after drawing have increased, and the plasticity and toughness have decreased. Through the grid test, it can be seen that the deformation of the blank in each area during the drawing process is uneven, from the small deformation area at the bottom to the main deformation area of the flange of the cylinder mouth, so the properties of the deformed material after drawing are also uneven. The distribution of the hardness of the drawn part gradually increases from the bottom to the mouth, as shown in Figure 1-7, and a dangerous section with the most insufficient work hardening appears near the round corner of the punch. This is just the opposite of the process requirements. From a process point of view, in order to prevent cracking during the punching process, the hardening of the bottom of the drawing part should be large, and the hardening of the mouth should be small.

Due to the work hardening of the deep-drawn parts, its strength and rigidity are higher than that of the blank material, which is beneficial to improve the service life of the deep-drawn parts. However, when the drawing is designed multiple times, the plasticity of the drawn part is reduced and the semi-finished blank is difficult to deform when it is further drawn. Therefore, the degree of deformation of each time should be selected correctly, and whether the semi-finished part needs to be annealed to restore its plasticity. Especially for some metals with strong hardening ability (stainless steel, heat-resistant steel, etc.), you should pay more attention. For example, stainless steel 1Cr18Ni9Ti is very sensitive to cold work hardening in plastic deformation. A small degree of deformation will cause its hardness and strength. The increase is obvious, so it is often impossible to choose this type of blank for multiple deep drawings.

Lugs

When the cylindrical part is drawn, the regular unevenness at the mouth end of the drawn part is called a lug. The reason for the lugs is the anisotropy of the sheet. In the direction where the plate thickness directivity coefficient is low, the plate becomes thicker and the barrel wall height is lower; in the direction where the plate thickness directivity coefficient is high, the plate thickness changes little, and the barrel wall height is higher. During deep drawing, the larger the plate plane directivity coefficient Δr, the more serious the protrusion phenomenon.