EP3590620B1

EP3590620B1 - Die set and method for forming a closed recess

Info

Publication number: EP3590620B1
Application number: EP18190706.4A
Authority: EP
Inventors: Yufeng Zheng; Tao Zhao; Shouzin WEI; Fuchuan Jiao; Songqiong ZHANG; Steve Arnold; Jianping Liu; Haoran Wang
Original assignee: BMW Brilliance Automotive Ltd
Current assignee: BMW Brilliance Automotive Ltd
Priority date: 2018-07-02
Filing date: 2018-08-24
Publication date: 2021-11-03
Anticipated expiration: 2038-08-24
Also published as: EP3590620A1; CN110666038A; CN110666038B

Description

TECHNICAL FIELD OF THE INVENTION

The present invention relates to a die set for forming a closed recess in a metal sheet material. Furthermore, the present invention relates to a method for forming a closed recess in a metal sheet material. The die set and method according to the present invention are especially suitable for forming a closed recess such as a recess of handle area on an exterior door panel of an automobile, a recess of badge area on an exterior panel of an engine cover and a fuel filling area on the exterior panel of side frame.

BACKGROUND OF THE INVENTION

It is known from the prior art that, there are several apparent recesses in the outer cover of an automobile, such as a recess of handle area on an exterior door panel, a recess of badge area on an exterior panel of an engine cover and a fuel filling area on the exterior panel of side frame, and such kinds of recesses are typically implemented in the prior art by draw forming or shaping methods. However, in the case of draw forming, due to the limitation of the drawing or shaping method itself, it is extremely likely to produce defects around such kinds of recesses, thereby affecting the aesthetic feeling of the entire vehicle. In addition, in the process of adjusting a die set for manufacturing a recess, there is a need for spending a large amount of labor and materials resources. Such method for manufacturing a closed recess will be illustrated by exemplifying a recess of handle area on the exterior door panel below. The forming of the recess of the handle area on an exterior door panel is an important forming feature on an entire vehicle, and such part of area is also an important concerning item in the surface evaluation of an entire vehicle. The shape at the recess of the handle area is normally formed by a drawing die. The shape of the recess of the handle area is mainly formed by an upper die of the die set. The upper die of the drawing die set has a punch made of a casting, and the lower die has a punch including a groove. In the drawing process, firstly, the sheet material is pressed tightly by the material pressing pad on the upper die and the lower die, and as the press slider of the upper die continues to run downward, the portion of punch of the upper die starts to contact with the sheet material and gradually press the sheet material into the groove of the punch of the lower die to form a recess of the handle area. In this process, the sheet material at the rounding corner of the recess of the handle area is in a free state, and the pressing force is only to tension such part of sheet material. In the area of the recess of the handle area, the force of the punch of the upper die acting on the sheet material is far greater than a tensile force transferred by the material pressing force to such part of area, so that the sheet material outside the rounding corner may flow over the rounding corner of the recess of the handle area into the recess of the handle area. In addition, the rounding corner of the recess of the handle area sometimes has an irregular contour, thereby resulting in uneven inflow amount of the material, so that an uneven tangential compressive stress is produced in such area of rounding corner of the recess of the handle area, which accordingly results in the production of a product defect, for example the production of a depression. For a recess of the handle area formed by drawing, restricted by the method itself, the product defect is inevitable. Even if a strong pressure is applied within the range of 20 mm to 40 mm in the peripheral area of the recess of the handle area, such kind of product defect cannot be completely overcome as well. At present, there are product defects in the handle recess area of basically all products on the market. Although a large amount of labor and material resources are spent in the adjusting process, the problem of the product defect still cannot be radically solved.
The method of stamping a sheet material by an elastic material to form a sheet material is also known in the prior arts. However, the principles thereof are still the same as the draw forming. The stamp forming of the groove is realized by rapidly deforming the elastic material so as to rapidly form the sheet material, so that it is still likely to cause the product defect.
Also know from SU602272A1 is a stamp for stamping a sheet material containing a rigid matrix and an elastic punch.

SUMMARY OF THE INVENTION

It is an object of the present invention to overcome the product defects caused by forming the closed recess in the prior art and to improve the quality of metal sheet material having the closed recess.
In order to achieve the aforementioned object, according to the present invention, there is provided a die set according to the appended claim 1. According to the present invention, the closed recess means that the circumferential contour of the recess formed in the sheet material is closed.
In the forming process by means of the die set according to the present invention, firstly, the edge portion of the first punch contacts with the sheet material, and presses the sheet material against the second punch, thereby producing a material pressing effect. Along with the continued downward run of the upper die, since the first punch presents certain deformability, as the pressure increases, the deformation firstly begins at the portion of the first punch in contact with the sheet material, and the force of pressing the sheet material may be gradually enlarged. When the force is gradually enlarged to deform the first punch, the first punch may begin to be slowly compressed inwards from an edge of its first groove, and the edge region of the first groove may contact with the sheet material earlier than the inside of the first groove. When the first punch is gradually deformed to press the sheet material tightly, and gradually reaches the edge portion of the second groove of the second punch, since the sheet material above the second groove of the second punch has already lacked of support on the second punch, if there is a force applied on the sheet material, the sheet material may begin to be deformed along the edge of the second groove so as to form a rounding corner of the recess of handle area, and along with the gradual deformation of the first punch, the sheet material may be slowly attached against the inner wall of the second groove along the edge of the second groove under the action of the first punch. In the end, the first groove of the first punch disappears along with deformation, and the material protruding from a material pressing surface of the first punch completely attaches the sheet material against the inner wall of the second groove of the second punch, and thus finally forms the shape of a closed recess on the sheet material. At the beginning of the forming, the sheet material is only deformed along the edge of the second groove of the second punch. Similar to flanging, the force transferred by the first punch always presses the sheet material in the thickness direction of the material, without a radial tensile stress present. Thus, the material may not flow into the closed recess to be formed, so as to avoid the problem of producing a tangential compressive stress and a material flow around the closed recess. In the later forming process, the sheet material is also slowly attached against the second groove of the second punch along with the deformation of the first punch. The sheet material that is not deformed is always in a stress state with a bidirectional tensile stress in a plane, and the forming of the closed recess is gradually realized in minor local deformations. Therefore, the die set according to the present invention may also be referred to as a forming die set with "multiple flanging". The area after the deformation of the first punch becomes a material pressing area, in order to produce the material pressing action. Along with the continuous progress of the forming, the material pressing area is enlarged continuously. According to the present invention, it is also very difficult for the sheet material outside the rounding corner of the closed recess to be formed to flow into the closed recess under the action of a material pressing force and a resistance force of the rounding corner. Accordingly, the present invention solves the problem of producing a tangential compressive stress and a material flow outside the rounding corner of the closed recess, so that a defect may not be produced.
Preferably, according to the present invention, the first punch is made of polyurethane, which preferably has a Shore hardness of 80 to 95, and particularly preferably has a Shore hardness of about 90. Polyurethane has a favorable deformability and restorability after deformation, and can withstand certain pressure, so that it has a proper hardness to stamp the sheet material. In addition, polyurethane has a large friction coefficient, for example, about 0.4, while the metal sheet material usually has a friction coefficient of about 0.1. The present invention exactly utilizes such large friction coefficient of polyurethane to effectuate reducing the flow of the material and slowly stamp forming the metal sheet material, thereby uniformly performing the deformation, so that it is not likely to produce the product defect.
According to a preferred embodiment of the present invention, the first punch has a thickness of at least 80 mm. In order to effectuate punch forming of the sheet material, the first punch needs to have certain thickness. Based on such thickness selection, the first punch can stamp form the sheet material with sufficient strength.
According to a further preferred embodiment of the present invention, the first groove has a radial dimension greater than that of the second groove by at least 10 mm, and gradually deepens in a circular arc shape. The stamp forming time is favorably controlled by this technical feature. Further, since the first groove in the first punch gradually deepens, the edge region may contact with the sheet material earlier than the inside, which effectuates that the stamp forming is slowly and uniformly performed.
According to a further preferred embodiment of the present invention, the first punch has a radial dimension greater than that of the second groove by at least 40mm. Accordingly, the closed recess of the sheet material can be effectively stamped out.
According to a further preferred embodiment of the present invention, the second groove of the rigid second punch on the lower die has an air vent hole at its bottom, wherein the air vent hole preferably has a diameter of 4 mm to 6 mm. As a result, exhausting is realized in the forming process, thereby reducing the product defects caused by the gas.
According to another preferred embodiment of the present invention, the upper die has a material pressing pad having an installation space for the first punch to be embedded into, which installation space is constructed as a through hole having a stop shoulder, wherein the first punch rests against the stop shoulder after being embedded into the installation space. As a result, the space requirement is reduced, so that the entire press can be compactly constructed.
According to a preferred embodiment of the present invention, the upper die has an upper shoe, in which the material pressing pad is held by a retainer screw, and the retainer screw is provided with a washer preferably made of polyurethane, which washer is disposed between a lower side of the material pressing pad and a head of the retainer screw, for absorbing an impact. Based on the washer for absorbing an impact force, the vibration in the stamping process is alleviated, thereby prolonging the service life of the entire press.
According to a further preferred embodiment of the present invention, the upper shoe is provided with a fixing member preferably made of metal.
According to a preferred embodiment of the present invention, the first punch is fixed onto the fixing member by a screw bolt.
Further, according to the present invention, the first punch is fixed onto the fixing member by adhesion, or the first punch is directly fixed onto the material pressing pad by a lock in a form closed manner.
Further, according to the present invention, the second groove is a rounded groove without edges.
Further, according to the present invention, the contour of the first groove of the first punch and the overall contour of the first punch are determined according to the closed recess to be formed.
Further, according to the present invention, the first groove of the first punch has a depth that is greater than that of the second groove of the second punch.
Further, according to the present invention, the contact surface between the fixing member and the first punch may be planar or curved.
Furthermore, the present invention relates to a method for forming a closed recess in a metal sheet material, according to the appended claim 14.
In the forming process by means of the method according to the present invention, firstly, the edge portion of the first punch contacts with the sheet material, and presses the sheet material on the second punch, thereby producing a material pressing effect. Along with the continued downward run of the upper slider of the upper die, since the first punch presents certain deformability, as the pressure increases, the deformation firstly begins at the portion of the first punch in contact with the sheet material, and the force of pressing the sheet material may be gradually enlarged. When the pressing force is so large so as to deform the first punch, the first punch may begin to be slowly compressed inwards from an edge of its first groove, and the edge region of the first punch may contact with the sheet material earlier than the inside. When the first punch is gradually deformed to press the sheet material tightly, and gradually reaches the edge portion of the groove of the second punch, since the sheet material above the second groove of the second punch has already lacked of support on the second punch, if there is a force applied on the sheet material, the sheet material may begin to be deformed along the edge of the second groove so as to form a rounding corner of the recess, and along with the gradual deformation of the first punch, the sheet material may be slowly attached against the inner wall of the second groove along the edge of the second groove under the effect of the first punch. In the end, the first groove of the first punch disappears along with deformation, and the material protruding from a material pressing surface of the first punch completely attaches the sheet material against the inner wall of the second groove of the second punch, and thus finally forms the shape of a closed recess on the sheet material. At the beginning of the forming, the sheet material is only deformed along the edge of the second groove of the second punch. Similar to flanging, the force transferred by the first punch presses the sheet material in the thickness direction of the material, without a radial tensile force present. Thus, the material may not flow into the closed recess to be formed, so as to avoid the problem of producing a tangential compressive stress and a material flow around the closed recess. In the later forming process, the sheet material is also slowly attached against the second groove of the second punch along with the deformation of the first punch. The sheet material that is not deformed is always in a stress state with a bidirectional tensile stress in a plane, and the forming of the closed recess is gradually realized in minor local deformations. Therefore, the method according to the present invention may also be referred to as a forming method with "multiple flanging".
By the method according to the present invention, it is possible to improve the forming of a closed recess such as a recess of handle area on an exterior panel of a vehicle door, which radically solves the problem of the product defects around the recess of handle area, reduces the grinding fit workload in producing a die set, and reduce the repair rate. Moreover, there are such advantages as simple structure, convenient manufacture, stable material pressing and convenient maintenance.
According to the method of the present invention, the forming time when the closed recess on the sheet material is stamp formed is controlled by design of a first groove in the first punch.
According to the present invention, the forming of the closed recess can be realized in one process. Further, restricted by the shape of the recess of the product and the property of the material of the first punch, the forming of the closed recess may be realized in two processes, which may reduce the amount of deformation of the first punch and improve the service life of the material of the first punch.

BRIEF DESCRIPTION OF THE DRAWINGS

Next, the present invention will be illustrated in more detail by way of the accompanying drawings. Wherein:

FIG. 1 shows a sectional view of the die set according to the present invention;
FIGS. 2a) to 2c) show a comparison of stress states of a closed recess in a drawing method, and a bulging forming method, as well as a forming method with multiple flanging according to the present invention for manufacturing a closed recess;
FIGS. 3a) to 3e) show deformed states of the sheet material B in the case of different closed heights of the die set before the final forming is completed, wherein the sheet material A which has completed the deformation is used for comparison;
FIGS. 4a) and 4b) show the thinning rates of the closed recess of the sheet material formed by the draw forming method and the multiple flanging method according to the present invention at different positions;
FIG. 5 shows a curve of the thinning rates of the closed recess of the sheet material formed by the draw forming method and the multiple flanging method according to

DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention will be described below with reference to the drawings, in which one embodiment of the present invention is shown. It should be understood, however, that the present invention may be presented in multiple different ways, and not limited to the embodiment described below. In fact, the embodiment described hereinafter is intended to make a more complete disclosure of the present invention and to adequately explain the protection scope of the present invention to a person skilled in the art. It should also be understood that, the embodiments invented herein can be combined in various ways to provide more additional embodiments.
It should be understood that, in the drawings, for the sake of clarity, the sizes of certain features may be deformed.
It should be understood that, the wording in the specification is only used for describing particular embodiments and is not intended to define the present invention. All the terms used in the specification (including the technical terms and scientific terms), have the meanings as normally understood by a person skilled in the art, unless otherwise defined. For the sake of conciseness and/or clarity, the well-known functions or constructions may not be described in detail any longer.
The wordings "comprising", "containing" and "including" used in the specification indicate the presence of the claimed features, but do not repel the presence of one or more other features.
In the specification, when one element is referred to as being "on" another element or "connected to" another element, the element may be directly located on another element or connected to another element, or there may be present with an intermediate element.
Hereinafter, the die set and method for forming a closed recess of the present invention will be introduced in detail by taking the forming of the recess of handle area on the exterior door panel as an example. In the manufacture of the exterior door panel of a motor vehicle, the shape of the product is firstly changed to form the shape of the exterior door panel of the automobile, which pertains to a conventional draw forming. Then, the shape of the recess of handle area is pressed out by the upper die and lower die, the present invention is mainly embodied in the die set and method used in this forming step.
The present invention is described below in conjunction with the accompanying drawings. FIG. 1 shows a sectional view of a die set for forming a closed recess in accordance with the present invention. The die set according to the present invention has an upper die and a lower die. The upper die includes an upper shoe 2 which is made of metal. The upper shoe 2 is provided with a safety side pin 1 which is embedded into the upper shoe in a form closed manner and serves to prevent the material pressing pad 4 from falling off. The number of the safety side pins may be one or more. The upper shoe is provided with a material pressing pad 4 which is made of metal. The material pressing pad 4 is connected with the upper shoe 2 by means of a retainer screw 9 on which a washer 8 made of polyurethane is mounted. The washer is disposed below the material pressing pad 4 and supports the material pressing pad, so that the washer produces the effect of absorbing an impact force when the material pressing pad presses a blank material. The upper side of the material pressing pad 4 is fixed with an upper pin-like member 5 by means of a screw bolt, which upper pin-like member 5 is fixed on the material pressing pad by its lower end and passes through a through hole in the upper shoe by its upper end. The upper pin-like member 5 during die set closing is in contact with the upper air cushion of the stamping device by its upper end, for providing a material pressing force. Wherein, the material pressing pad 4 has a opening at a middle portion thereof, which opening forms an installation space for embedding the first punch. A stop shoulder is formed in the opening, the first punch 7 is inserted into the installation space and is stopped against the stop shoulder. The first punch 7 which is made of polyurethane, is attached against the fixing member 6 on the upper side and fastened onto the fixing member 6 by a screw bolt. The fixing member 6 in turn is fixed onto the upper shoe 2 by a screw bolt. The fixing member is made of metal. The fixing member 6 functions to fix the first punch 7, and transfers the pressure of the device onto the first punch 7 in the closing process of the die set.
The lower die includes a lower shoe 10 which is made of metal. The second punch 11 is supported on the upper side of the lower shoe 10. The second punch 11 is rigid and made of metal. The upper side of the lower shoe is formed with a flange 19 at an edge side, and a positioning key 14 is provided between the flange 19 and the second punch 11. The second punch 11 is fixed on the lower shoe 10 after being positioned by the positioning key 14. The lower side of the material pressing pad is provided with an upper balance member which is preferably an upper conical balance block 5 having a conical recess. The lower balance member which is preferably a lower conical balance block 15 includes an upper conical section and a lower cylindrical section. The upper side of a flange of the lower shoe is provided with a cylindrical concave 20, and the lower conical balance block is embedded into the concave by its lower cylindrical section and forms a connection with the flange 19 in a form closed manner. The lower conical balance block is embedded into a conical recess of the upper conical balance block by its upper conical section, such as to be adapted to the conical recess of the upper conical balance block. The upper conical balance block 3 and the lower conical balance block 15 produce the guiding and pressure balancing effect during the closing of the die set. Further, the lateral surface of the flange 19 of the lower shoe 10 is provided with a projecting step 21, which is supported with a guiding member that is preferably a guiding plate 16. The upper shoe 2, the lower shoe 10 and the guiding plate 16 form a die frame of the entire die set, and the guiding plate produces the guiding function for the upper die 2 during the closing of the die.
The first punch 7 has a first groove 17 on the side facing the sheet material, for guiding the first punch 7 to be deformed during the material pressing. The second punch 11 has a second groove 18 on its side facing the first punch 7. The first groove 17 has a radial dimension at least 10 mm greater than that of the second groove 18, and the first groove gradually deepens in a circular arc shape. The first punch 7 has a radial dimension at least 40 mm greater than that of the second groove. The upper end surface of the first punch away from the first groove 17 is planar, and the fixing member 6 is attached against the upper end surface. In the present embodiment, the first punch has a Shore hardness of about 90, and the first punch 7 has a thickness of at least 80 mm. The first punch is pressed against the sheet material 13 by its lower side having a first groove, and the sheet material 13 is made by draw forming in the previous step. Wherein, the contact area between the sheet material 13 and the first punch 7 is a forming surface after draw forming in the first step. The first punch 7 is made of polyurethane by injection forming, wherein a fastening screw bolt is built-in when polyurethane is injected, and afterwards the deformable first punch is fastened onto the fixing member made of metal by the fastening screw bolt 6. The bottom of the second groove 18 of the second punch 11 is provided with an air vent hole 12, which has a diameter of 4 mm to 6 mm, and produces the exhausting effect in the forming process.
When the closed recess is formed, the sheet material 13 is firstly placed on the second punch 11. Afterwards, the upper die moves downward, and the upper pin-like member 5 is gradually exposed out of the upper shoe 2 and comes in contact with the upper air cushion of the press, thereby starting to transfer a material pressing force. The material pressing force is applied to the sheet material 13 by the material pressing pad 4, and the sheet material 13 is pressed against the second punch 11. The upper die continues to move downward, and the pressure of the press device is transferred onto the first punch 7 through the upper shoe 2 and the fixing member 6. The first punch begins to press the sheet material 13 tightly and begins to be deformed under the effect of a gradually increased pressure. When the first punch after deformation gradually reaches the edge portion of the second groove 18 of the second punch, the material of the sheet material 13 on the edge of the second groove 18 begins to be deformed and gradually attached against the groove inner wall of the second groove 18 from the outside to the inside until being finally completely attached to the groove inner wall of the second groove 18 tightly. Afterwards, the upper die moves upward, and the first punch 7 slowly restores an initial state under the driving of the screw bolt, and starts to move upward along with the fixing member 6. At the same time, the material pressing pad 4 leaves the sheet material 13 under the pulling of the washer 8 and the retainer screw 9, so that the entire forming process is finished.
In order to more explicitly explain the principles of the present invention, FIGS. 2a) to 2e) show a comparison of stress states of a closed recess in a drawing method, and a bulging forming method, as well as a multiple flanging method of the present invention for manufacturing a closed recess. In the following comparison process, a point is taken from different parts of the closed recess of the sheet material to be formed respectively, and on such points stress analysis is performed in cells.
FIG. 2a) is a drawing method, in which the first punch is formed by a casting in FIG. 2a); FIG. 2b) is a bulging forming method, in which the first punch is formed from a liquid; FIG. 2c) is a multiple flanging method, in which the first punch is formed of a deformable material, such as polyurethane. In each forming process, in order to explain the stress during the forming process, four points 1, 2, 3, 4; 1', 2', 3 ', 4'; 1", 2", 3", 4" are selected on the closed recess to be formed according to a sequence from the outside to the inside. The stress states at such four points are compared respectively.
Points 1, 1', 1": at the rounding corner of the recess. For the drawing method, the cell at the point 1 is subjected to radial tensile stress, and such part of force is produced when the bottom of the punch contacts with the sheet material; the cell is subjected to compressive stress in the tangential direction, because the sheet material is to enter the groove under the driving of the punch. Compressive stress is produced upon shrinkage of the sheet material; the cell at the point 1 is close to the rounding corner of the recess, and thus is subjected to compressive stress in the material thickness direction. For the bulging forming method, the cell at the point 1' is also subjected to stress in three directions, but is subjected to tensile stress in the tangential direction, because the sheet material is subjected to compressive stress in the material thickness direction, thereby forcing the sheet material to be close to the second groove. For the multiple flanging method, the cell at the point 1" is only subjected to compressive stress in the material thickness direction, because the deformation of the sheet material in such area has ended, and the sheet material is completely attached to the groove, so that it is not subjected to stress both in the radial direction and the tangential direction.
Points 2, 2', 2": for the drawing method, the cell at the point 2 does not contact with the die, and thus is not subjected to a stress in the material thickness direction, but is subjected to a tensile stress in the radial direction and still subjected to a compressive stress in the tangential direction. For the bulging forming method, since the sheet material contacts with the punch, the point 2' is still subjected to a stress effect in three directions, and coincides with the state at the position of the point 1'. For the multiple flanging method, the sheet material has been deformed at the point 2", so that there is no stress in the tangential direction. However, due to the that that it is in the vicinity of the rounding corner of the recess to be formed, the sheet material is still subjected to a slight tensile stress in the radial direction, and subjected to a compressive stress in the material thickness direction.
Points 3, 3', 3": for the drawing method, the stress state is consistent with the point 2, but is changed from compressive stress to tensile stress in the tangential direction; for the bulging forming method, the state at the point 3' is the same as the previous point 2'; for the multiple flanging method, the cell at the point 3" is at the moment of deformation, and the sheet material is gradually being attached against the second groove of the second punch under the effect of the first punch, so that the cell is subjected to a tensile stress both in the tangential direction and the radial direction, and subjected to a compressive stress in the material thickness direction.
Points 4, 4', 4": for the drawing method, the bottom of the first punch contacts with the sheet material, and the cell is subjected to a tensile stress both in the radial direction and the tangential direction; for the bulging forming method, the stress state at the point 4' is the same as the previous point 3'; for the multiple flanging method, since the second groove exists on the second punch, the cell at the point 4" does not contact with the die, so that it is not subjected to stress in the material thickness direction, but subjected to a tensile stress both in the tangential direction and the radial direction.
In summary, the multiple flanging method according to the present invention is substantively different from the conventional drawing and bulging forming methods in the forming process, and there is a very great radial tensile stress at the rounding corner of the recess to be formed in the drawing method. According to the bulging forming method, during the forming process, the sheet material is always uniformly subjected to a compressive stress in the material thickness direction, and there is a very great material pressing force so that the material may not flow into the rounding corner. However, the bulging forming method is relatively hard to be realized in highspeed automatic production. According to the multiple flanging method, at the time of forming, the first punch has a contour greater than the recess to be formed, and thus the first punch completely covers the second groove of the second punch. At the beginning of the forming, the first punch produces the material pressing effect. As the upper die continuously runs downward, the first punch is deformed, thereby gradually pressing the sheet material on the edge of the second groove of the second punch, so as to form the rounding corner of the closed recess. The forming at the rounding corner is similar to flanging based on simple material pressing. At this time, there may be probably flow of a small amount of materials, but the flow has a very small distance and is uniform. The deformation is realized by continuous accumulation of minor local deformations rather than sudden change, and the area after deformation becomes a material pressing area at once. The sheet material that is not deformed at a middle part of the recess to be formed is always subjected to bi-directional tensile stresses in the plane, and the entire forming process is similar to a flanging process having the material pressing function. Since the middle part of the recess is always subjected to bi-directional tensile stresses in the plane, the deformation of the material is more adequate. As the forming progresses continuously, the material is continuously attached against the groove from the outside to the inside. At the same time, the material pressing area of the first punch is continuously enlarged. The sheet material beyond the rounding corner of the closed recess also stops flowing soon under the effect of the material pressing force and the rounding corner resistance. That is, the material that accomplishes the deformation does not take part in the following deformation, so that the deformation is more stable and thus a defect may not be produced. It is particularly to be noted that, restricted by the shape of the closed recess to be formed and the property of the material of the first punch, the forming of the closed recess probably needs to be realized in two processes, so that it is possible to reduce the amount of deformation of the first punch and improve the service life of the material of the first punch.
For the draw forming, the side wall portion produces the effect of transferring and supporting a force of the first punch in the deformation process, and the side wall portion is a force conduction area of the sheet material that is draw formed. The bearing capacity of the sheet material at the position of the side wall determines a maximum deformation degree of the drawing member. In the drawing process, the place where the sheet material becomes the thinnest is at the side wall portion, which is a vulnerable portion for fracture, so that most of the fracture by drawing the sheet material occurs here. For the bulging forming method, various positions of the sheet material are always subjected to stresses in consistent states, so that the thickness of the part of the sheet material to be formed may always be uniform and consistent. For the multiple flanging method, the deformation of the sheet material at the closed recess to be formed is performed from the outside to the inside, and the material beyond the recess basically does not take part in the deformation. Moreover, the sheet material at the middle position of the recess is always subjected to bi-directional tensile stresses in the plane with the same magnitude in the deformation process, and the middle position of the recess is the final forming area, at which a position with a maximum thinning rate also occurs. In the forming process, the sheet material that takes part in the deformation is adequately deformed, and the thinning rate is gradually reduced from the inside to the outside, thereby improving the forming property of the material. Therefore, under the same conditions, the sheet material is less prone to fracture when the forming method with multiple flanging is used.
FIGS. 3a) to 3e) present the deformation conditions of the sheet material when the die set has different closed heights, before the final forming is accomplished. Wherein, FIG. 3a) shows a deformation condition of the sheet material when there is a play of 20mm between the upper die and the lower die before the die set is completely closed; FIG. 3b) shows a deformation condition of the sheet material when there is a play of 14mm between the upper die and the lower die before the die set is completely closed; FIG. 3c) shows a deformation condition of the sheet material when there is a play of 8mm between the upper die and the lower die before the die set is completely closed; FIG. 3d) shows a deformation condition of the sheet material when there is a play of 3mm between the upper die and the lower die before the die set is completely closed; FIG. 3e) shows a deformation condition of the sheet material when the die set is completely closed. In order to more clearly illustrate the deformation process, the sheet material A that has been formed already is stacked together with the sheet material B which is in the forming process. It can be seen in the figures that the formation of the closed recess is also realized by gradual deepening from the outside to the inside.
FIG. 4a) shows the thinning rates of the closed recess of the sheet material formed by the draw forming method at different positions. FIG. 4b) shows the thinning rates of the closed recess of the sheet material formed by the multiple flanging method according to the present invention at different positions. The thinning rate which is obtained by dividing the difference between the thickness before deformation and the thickness after deformation by the thickness before deformation, reflects the degree of deformation.
FIG. 5 is a comparison of the thinning rates of the sheet material at the closed recess to be formed when it is formed by the drawing method and the multiple flanging method. The curve a and the curve b are measurement results of two measurements performed at symmetrical positions on both sides of the closed recess formed by the draw forming method. It can be seen that the peak of the thinning rate is in the middle position, that is, the maximum thinning rate of the sheet material at the closed recess to be formed appears on the side wall. The curve c and the curve d are measurement results of two measurements performed at symmetrical positions on both sides of the closed recess formed by the multiple flanging forming method. It can be seen that in the multiple flanging forming, the thinning rate of the sheet material at the closed recess to be formed is gradually reduced from the inside to the outside, and the maximum thinning rate appears at the center point. In the drawing method, the point with a minimum thinning rate is in the vicinity of the point 6 or 7, and thus the fracture of the side wall is likely to occur. At the same time, it can be seen that, in the draw forming, there is still material deformation at the points 10, 11, 12 of the sheet material located outside the closed recess. Thus, it is possible to obtain that there is still material flow at these points, so that a product defect is likely to be produced. However, in the multiple flanging forming, there is basically a minor material deformation at the corresponding points 10, 11, 12. Thus it can be known that there is less material flow at such points, so that a product defect is less likely to be produced on the sheet material to be formed. In addition, the multiple flanging forming is more favorable for adequate deformation of the material.

List of reference signs

1.: safety side pin
2.: upper shoe
3.: upper conical balance block
4.: material pressing pad
5.: upper pin-like member
6.: fixing member
7.: first punch
8.: washer
9.: retainer screw
10.: lower shoe
11.: second punch
12.: air vent hole
13.: sheet material
14.: positioning key
15.: lower conical balance block
16.: guiding plate
17.: first groove
18.: second groove
19.: flange
20.: concave
21.: step

Claims

A die set for forming a closed recess in a metal sheet material, wherein the die set has an upper die and a lower die, wherein the upper die is provided with a deformable first punch (7) which on the side facing the sheet material to be formed includes a first groove (17) for guiding deformation, and the lower die has a rigid second punch (11) which on the side facing the sheet material to be formed has a second groove (18) corresponding to the closed recess to be formed, and the first groove (17) has an outer contour greater than that of the second groove (18) so that the first groove (17) covers the second groove (18), characterized in that the upper die has a material pressing pad (4) for pressing a sheet material (13) against the second punch (11) when the first punch (7) presses the sheet material (13) tightly against the second punch and deforms, wherein the first groove has a depth that is greater than that of the second groove.
The die set according to claim 1, characterized in that, the first punch (7) is made of polyurethane, which preferably has a Shore hardness of 80 to 95, and especially a Shore hardness of 90.
The die set according to claim 1 or 2, characterized in that, the first punch (7) has a thickness of at least 80 mm.
The die set according to one of the preceding claims, characterized in that, the first groove (17) has a radial dimension greater than that of the second groove (18) by at least 10 mm, and the first groove (17) gradually deepens in a circular arc shape.
The die set according to one of the preceding claims, characterized in that, the first punch has a radial dimension greater than that of the second groove (18) by at least 40 mm.
The die set according to one of the preceding claims, characterized in that, the second groove (18) has an air vent hole (12) at its bottom, wherein the air vent hole preferably has a diameter of 4 mm to 6 mm.
The die set according to one of the preceding claims, characterized in that, the material pressing pad (4) has an installation space for the first punch (7) to be embedded into, which installation space is constructed as a through hole comprising a stop shoulder, wherein the first punch (7) rests against the stop shoulder after being embedded into the installation space.
The die set according to one of the preceding claims, characterized in that, the upper die has an upper shoe (2), in which the material pressing pad (4) is held by a retainer screw (9), and the retainer screw (9) is provided with a washer (8) preferably made of polyurethane, which washer is disposed between a lower side of the material pressing pad (4) and a head of the retainer screw, for absorbing an impact.
The die set according to claim 8, characterized in that, the upper shoe (2) is provided with a fixing member (6) preferably made of metal.
The die set according to claim 9, characterized in that, the first punch (7) is fixed on the fixing member (6) by a screw bolt.
The die set according to claim 9, characterized in that, the first punch (7) is fixed on the fixing member (6) by adhesion, or the first punch (7) is directly fixed on the material pressing pad (4) by a lock in a form closed manner.
The die set according to one of the preceding claims, characterized in that, the second groove (18) is a rounded groove without edges.
The die set according to claim 9, characterized in that, the contact surface between the fixing member (6) and the first punch (7) is planar or curved.
A method for forming a closed recess in a metal sheet material by means of the die set according to one of the preceding claims, characterized in that, the method is carried out as follows:
placing a sheet material formed by draw forming on a rigid second punch (11) of the lower die of the die set while the die set is in an opened state, wherein the first punch (7) disposed on the upper die rests against the sheet material and a first groove (17) of the first punch for guiding deformation covers a second groove (18) provided on the second punch (11) of the lower die, the second groove (18) corresponding to the closed recess to be formed, wherein the first groove (17) has a depth that is greater than that of the second groove, wherein the upper die has a material pressing pad (4);

moving the upper die downwards, so that the material pressing pad (4) presses a sheet material (13) against the second punch (11);

pressing the sheet material (13) against the second punch (11) tightly by the first punch (7), wherein in the process where the upper die continues to run downwards, as the pressure acting on the first punch increases, the material at the edge of the first groove of the first punch (7) is gradually deformed towards the inside of the first groove, so that the first punch along with deformation thereof gradually presses a sheet material located above the second groove (18) of the second punch towards an inner wall of the second groove;

wherein the first groove (17) of the first punch (7) disappears along with deformation, and the material gradually protruding from a material pressing surface of the first punch (7) gradually presses the sheet material against the inner wall of the second groove (18) of the second punch (11) from the outside to the inside, so as to form the closed recess.