Deep Drawing Forming of Sheet Metal
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Deep Drawing Forming of Sheet Metal

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Deep Drawing Forming of Sheet Metal


Deep drawing is a stamping process in which a flat sheet material is passed through a concave die under the pressure of a punch to form an open hollow part. Among various types of sheet metal components, deep drawing is often used to process various round simple parts, hemispherical and parabolic heads composed of larger or thicker materials.

Deep Drawing Process and Requirements

Generally speaking, deep drawing processing must be completed by using a drawing die through the pressure of a press. Under normal circumstances, cold processing is used, and hot processing is only used for deep drawing forming of thicker sheets with larger outer dimensions or larger deformations.

Deep Drawing Forming of Sheet Metal

1. Drawing Process

The figure shows the drawing process of placing a circular flat plate blank with a diameter D and a thickness t in the positioning hole of the die and drawing it into a cylindrical part.


During the deep drawing process, due to the bending moment formed by the drawing force F and the gap Z between the convex and concave dies, the convex mold moves downward to contact the sheet and then exerts downward pressure, causing the sheet to bend and concave, and is guided at the rounded corners of the convex and concave dies. When the punch is pulled down into the hole of the die, the sheet material slowly evolves into three parts: the bottom, the wall, and the flange; as the punch continues to descend, the bottom basically does not move, and the annular flange continues to shrink toward the hole and is pulled into the cavity. 


The mold opening transforms into a cylinder wall, so the cylinder wall gradually increases and the flange gradually shrinks. Finally, the flange is completely pulled into the die opening and transforms into a cylinder wall, and the drawing process ends. The circular sheet becomes an open hollow cylinder with diameter d1 and height h.


2. Deep Drawing Deformation Analysis

According to the drawing deformation process, it can be known that the drawing process is the process in which the annular flange gradually shrinks and flows toward the die hole to become the cylinder wall. The deep drawing process is a relatively complex plastic deformation process, and each damaged part of the hair can be divided into several areas according to its deformation conditions.


2.1 Bottom of the cylinder: The circular part where the bottom of the punch presses down and contacts the center area of the sheet is the bottom. During the drawing process, this area always maintains a flat shape and is subject to uniform radial tension around it. It can be considered There is no plastic deformation or a small plastic deformation area, and the bottom material transfers the punch force to the cylinder wall, causing it to generate axial tensile stress.


2.2 Flange part: The annular area on the die is the flange, which is the main deformation area during deep drawing. During deep drawing, the flange material generates radial tensile stress due to the drawing force. , when the materials shrink and flow toward the die hole, the materials squeeze each other to generate tangential compressive stress 3. Its function is similar to that of pulling a sector-shaped part of the blank F through an imaginary wedge-shaped groove to become the deformation of F, as shown in the figure.

Deep Drawing Forming of Sheet Metal

When the flange is large and the sheet material is thin, the flange part will lose stability due to the tangential compressive stress during drawing, forming the so-called "wrinkling phenomenon". Therefore, a blank holder is often used to press the flange Carry out edge pressing.


2.3 Simple wall: This is the deformed area, which is formed by the flow transfer of the flange part material through tangential compression, radial stretching and shrinkage, and basically no longer undergoes major deformation. When the drawing continues, it plays the role of transmitting the drawing force of the punch to the flange. The simple wall material itself bears the unidirectional tensile stress during the process of transmitting the drawing force, and is slightly elongated longitudinally and slightly thicker. There is thinning.


2.4 The rounded corner part of the concave die: the transitional part where the flange and the cylinder wall meet. The deformation of the material here is more complicated. In addition to having the same characteristics as the flange part, that is, it is subject to radial tensile stress and tangential compressive stress. It also bears concave Thick compressive stress formed by the extrusion and bending of the die fillet.



2.5 The rounded part of the punch: the transitional part where the simple wall and the bottom of the cylinder meet, it is subjected to tensile stress in the radial and tangential directions, and the thick direction is subjected to the extrusion and bending of the rounded part of the punch to produce compressive stress. During the drawing process, The radial direction is elongated and the thickness is reduced. The most serious thinning occurs at the joint between the punch round corner and the barrel wall. 


When drawing starts, it is between the convex and concave dies, so less material needs to be transferred. The degree of deformation is small, the degree of cold work hardening is low, and there is no beneficial friction at the rounded corners of the punch. The area that needs to transmit the drawing force is small. Therefore, this part has become the "dangerous section" that is most likely to break during deep drawing.


3. Wall Thickness Changes of Deep-drawn Parts

The uneven wall thickness of the deep-drawn parts can be seen from the picture. The picture shows the change of the wall thickness of the carbon steel elliptical head during drawing, and the picture shows the change of the wall thickness of the flanged cylinder using the blank holder.


4. Process Requirements for Deep Drawing Processing

The deep drawing process can be used to process parts with complex shapes, and obtain thin-walled parts with cylindrical, stepped, conical, square, spherical and various irregular shapes. However, the accuracy of deep-drawn parts processing is related to many factors, such as the mechanical properties and material thickness of the material, the mold structure and mold accuracy, the number of processes and the sequence of the processes, etc. The manufacturing accuracy of deep-drawn parts is generally not high, and the appropriate accuracy is below IT11 level. 


At the same time, due to the influence of the deep-drawing deformation performance, the processability of the deep-drawn parts directly affects whether the part can be used in the most economical and simple way. It is processed by the deep drawing method, and even affects whether the part can be processed by the deep drawing method. The process requirements for deep-drawn parts are as follows.

Deep Drawing Forming of Sheet Metal

Structural and mold accuracy, number of processes and sequence of processes, etc. The manufacturing accuracy of deep-drawn parts is generally not high, and the appropriate accuracy is below IT11 level. At the same time, due to the influence of the deep-drawing deformation performance, the processability of the deep-drawn parts directly affects whether the part can be used in the most economical and simple way. It is processed by the deep drawing method, and even affects whether the part can be processed by the deep drawing method. The process requirements for deep-drawn parts are as follows.


4.1 The shape of the deep-drawn parts should be as simple and symmetrical as possible. When designing deep-drawn parts, the processing technology of the deep-drawn parts should be taken into consideration, and a form that is easier to form and can meet the use requirements should be adopted as much as possible. The picture shows the classification according to the difficulty of deep drawing forming. In the figure, the forming difficulty of various types of deep-drawn parts increases from top to bottom. 


The difficulty of similar deep-drawn parts increases from left to right. Among them: e represents the minimum straight edge length, f represents the maximum size of the deep-drawn part, a represents the short axis length, and 6 represents the long axis length.


4.2 For cylindrical drawn parts with flanges, when drawing with a blank holder, the most suitable flange is in the following range:

Deep Drawing Forming of Sheet Metal

4.3 The drawing depth should not be too large (that is, H should not be greater than 2d). When it can be drawn at one time, its height should preferably be:

Deep Drawing Forming of Sheet Metal

4.4 For cylindrical drawn parts, the fillet radius r between the bottom and the wall should satisfy ra>t, and the fillet radius between the flange and the wall r>2t. From the perspective of conditions conducive to deformation, it is best to take r ≈(3~5)t, r≈(4~8)t. If r (or r)>(0.1~0.3)t, shaping can be added.


Structural form of Drawing Die and Its Selection

Although the shapes of deep-drawing parts are diverse, the structure of the drawing dies is relatively standardized. Depending on the drawing work conditions and the equipment used, the structures of the drawing dies are also different. The adoption of the drawing die structure generally requires necessary process calculations, and then the drawing process plan can be selected accordingly.


Deep drawing processing can be performed on a general single-action press, or on a double-action or three-action press. The drawing dies that work on single-action presses can be divided into two types: first-time drawing dies and first-time and subsequent drawing dies. Depending on whether a blank holder is used, it can be divided into two types: with a blank holder and without a blank holder. According to the type of press, it can be divided into drawing dies used on single-action presses, drawing dies used on double-action presses, etc.


1. First Drawing Die

The picture shows the first deep drawing die without edge holder. When drawing, first place the flat blank into the positioning plate on the die, and the punch moves downwards driven by the slider of the press, pressing the bad material into the die until all the bad material is pulled into the die and made. The upper end of the deep-drawn workpiece exceeds the scraper ring. When the press slide drives the punch to move upward, the scraper ring scrapes the workpiece from the punch to complete a deep drawing process.

Deep Drawing Forming of Sheet Metal

The first drawing die without edge holder is generally used for shallow drawing parts with a small drawing depth that can be pressed in one go. When the punch is small, the overall structure can be adopted and fixed by the punch fixing plate. In order to prevent the workpiece from sticking tightly to the punch, ventilation holes should be designed on the punch.

Deep Drawing Forming of Sheet Metal

Figure a shows an ordinary flat-end concave die with arcs, which is mainly suitable for processing large parts. Picture b shows a tapered die opening, and picture c shows a concave die opening with an involute shape. They are suitable for processing small parts. Since the die structure of picture bc shows a curved transition shape of the blank during drawing, the size increases. 


Anti-instability ability, the force of the die mouth on the deformation zone of the blank also helps it produce tangential compression deformation, reducing friction resistance and resistance to bending deformation, which is beneficial to deep drawing deformation and can improve the quality of the part, but the processing is longer difficulty.


Picture b shows the first deep drawing with an elastic edge ring. The elastic edge ring is installed on the upper mold. When the punch moves downward, the bad material is pressed tightly under the action of the spring force, so that the bad material is close to the concave part during the drawing process.


Due to the limitation of the upper mold space, thick springs cannot be installed, so this type of mold is only suitable for drawing parts with low pressure. It is usually used for drawing workpieces with thin materials, small depth and easy to wrinkle.


When drawing a workpiece with a large depth, a larger spring (or rubber) is required and it is difficult to install if the spring is still placed on the upper part of the mold. Therefore, a structure mounted on the lower part can be used to facilitate the adjustment of the blank holder force.


2. Deep Drawing Mold for Each Time After the First Time

Figure a shows the first and subsequent deep drawings without a blank holder. It can draw semi-finished products that have been deep-drawn to a certain size and then deep-draw them again. Generally, it can be used for applications where the degree of deformation is not large and the wall thickness of the drawn parts is required to be uniform. 


And ensure the diameter and dimensional accuracy of workpieces with slight thinning. For this kind of mold, usually in order to prevent friction loss, the length of the straight wall working part of the concave mold needs to be reduced as much as possible.


Figure b shows the structure of the drawing die for the first and subsequent times for cylindrical parts with edge rings. The positioner 11 adopts a sleeve-type structure and plays the role of edge pressing and positioning at the same time. The pressing force is provided by the cylinder force transmitted by the ejector pin 13. 


In order to prevent wrinkles when the material is deep drawn, the position of the limit ejector pin 3 can be adjusted to adjust the pressure. The size of the edge force can keep the edge holder force balanced while preventing the bad material from being clamped too tightly.


The working process of the mold is: the punch slide moves upward, the mold is opened, and the ejector pin 13 lifts the positioner 11 to the punch 1 through the positioner fixing plate 12 under the action of the press cylinder.

Deep Drawing Forming of Sheet Metal


Deep Drawing Forming of Sheet Metal

The end faces are flush. At this time, the drawn blank is inserted into the outer ring of the positioner 11. The press slide begins to move downward. The limit ejector pin 3 begins to contact the upper end face of the positioner fixing plate 12. At the same time, the die 2 It also begins to contact the upper end surface of the positioner 11, and as the press


As the slider gradually moves downward, the limit ejector pin 3 gradually presses down on the positioner fixing plate 12, and the die 2 and the positioner 11 work together to gradually draw the semi-finished product into a finished product. When the drawing is completed, the ejector pin 13 pushes the positioner 11 to be flush with the upper end surface of the punch 1 under the action of the press cylinder. At the same time, the hammer 7 ejects the drawn parts from the cavity of the female mold 2.

Deep Drawing Forming of Sheet Metal

For deep-drawn parts with a diameter d ≤ 100 and deep-drawn parts with flanges or complex shapes, in order to facilitate deep drawing forming, attention should be paid to the correct relationship between the shape and size of the punching dies in the previous and subsequent processes, so that the shapes and sizes of the punches made in the previous processes are correct. The shape of the intermediate blank is conducive to forming in subsequent processes. The relationship between the dimensions of each drawing process and its fillet radius is shown in Figure a, where t is the material thickness.


For large and medium-sized cylindrical deep-drawing parts with a diameter of d>100, for the first few drawings and the deep drawing before the final forming, the cylinder corners often use a 45 bevel angle connection structure to avoid excessive material at the rounded corners. It is thinner and is conducive to deep drawing. This structure not only makes it easier to locate the hair in the next process, but also reduces the repeated bending and positioning of the hair, improves the conditions for material deformation during deep drawing, and reduces the thinning of the material. 


It is helpful to improve the quality of the side walls of stamped parts. However, it should be noted that the bottom diameter should be equal to the outer diameter of the punch during the next drawing process. The relationship between the fillet radius of the punch and the concave die and the fillet radius of the edge ring in the front and rear processes is as shown in Figure b. Show.


3. Drawing Die for Double-action Press

When using a double-action press for deep drawing, the outer slider presses the edge and the inner slider draws deep. The deep-drawn parts shown in Figure a are directly cut and drawn from strips, and processed by a double-action drawing press.

Deep Drawing Forming of Sheet Metal

Figure b is a schematic diagram of the mold structure of the above parts. After the strip is positioned by the positioning pin 2, the blank holder 7 and the lower die base 1 work together to implement blanking. The drawing convex 4 and the drawing concave die 3 are ejected. The blocks 6 work together to draw and shape the bad material after blanking. Finally, the ejector pin 5 drives the ejector block 6 to push the drawn parts out of the cavity of the drawing concave die 3.

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