Indian Patents. 219455:"A MOLD DEVICE AND METHOD FOR PRESS-FORMING A THIN SHEET"

Title of Invention	"A MOLD DEVICE AND METHOD FOR PRESS-FORMING A THIN SHEET"
Abstract	A mold device for press-forming a thin sheet, comprising a punch 1, a die 2, a blank-holding die 3, frictional force measuring means 4 mounted between the die and the blank-holding die, and hydraulic cylinders 5.

Full Text	MOLD DEVICE FOR PRESS-FORMING A THIN SHEET AND PRESS-FORMING METHOD Technical Field The present invention relates to a mold device for press-forming a thin sheet and to a press-forming method. More particularly, the invention relates to a mold device and a forming method capable of adjusting the distribution of a blank-holding force during the press working. Background Art A variety of inventions have been disclosed concerning methods of forming while controlling the blank-holding force. For example, JP-A-7-266100 (patent document I) discloses a method of adjusting the air pressures in air cylinders so as to conduct a press work under a proper blank-holding force by finding, in advance, a relationship between a proper blank-holding force for obtaining a predetermined press quality and physical quantities such as shape and mechanical properties of a blank to be pressed, chemical properties, laminate properties such as plating and surface state such as the amount of oil, and by finding the suitable blank-holding force depending upon the real physical quantities based on the above relationship. JP-A-9-38728 (patent document 2) discloses a method of preventing the occurrence of cracks and the residence of wrinkles caused by an excess flow of the material by increasing the blank-holding force at the time of draw forming from the initial stage to the intermediate stage of formation, and decreasing the blank-holding force to a suitable value in the latter stage of formation. Further, JP-A-6-190464 (patent document 3) discloses an invention of a die-cushioning device equipped with hydraulic cylinders for making the pressure uniform, wherein the hydraulic pressures in the hydraulic cylinders are temporarily varied by controlling the opening of the flow-rate control valves to control the blank-holding force. Disclosure of the Invention Patent documents 1 to 3 disclose inventions for controlling the blank-holding force, which, however, are not capable of finding proper blank-holding force, in advance, under a variety of varying factors, such as varying blank properties, wear of the die and temperature of the die. In particular, the lubricating properties to the die vary at all times, and measuring the above properties each time greatly decreases the productivity. Controlling the blank-holding force using the die-cushioning device requires a drastic remodeling of the press device and, besides, it is difficult to estimate a proper blank-holding force. It is an object of the present invention to provide a device capable of setting a proper load by finding a blank-holding force on the site without the need of finding, in advance, proper blank-holding forces for the varying factors. According to the present invention, there is provided a mold device for press-forming a thin sheet, comprising a punch, a die, a blank-holding die, punch drive means for inserting said punch in said die, and blank-holding die drive means for applying a blank-holding force to said blank-holding die, thereby to work a thin-sheet work by pushing it into said die by said punch, said mold device for press-forming a thin sheet further comprising: blank-holding force adjusting means for adjusting the blank-holding force applied to said blank-holding die via blank-holding die drive means; at least either one of fractional force measuring means for measuring the frictional force acting on said work or press reaction measuring means for measuring the press reaction acting on said punch; and control means which so controls said blank-holding force adjusting means that a measured value of said frictional force measuring means or of said press reaction measuring means becomes a predetermined value. According to another feature of the present invention, there is provided a method of press-forming a thin sheet by pushing a thin sheet work into a die by a punch by using a mold device to work the thin-sheet work, comprising the steps of: measuring at least either one of the frictional force acting on said work or a press reaction acting on said punch; and adjusting the blank-holding force or the punching speed, so that a value measured in said step of measurement becomes a predetermined value. The present invention makes it possible to impart a proper frictional force irrespective of varying factors such as lubricating property and surface property between the die and the work and, hence, to obtain favorably formed articles at all times irrespective of variations in the characteristics of the blank or a variation in the environment. Brief Description of the Drawings Fig. 1 is a sectional view of a press mold device having frictional force measuring means on the surface of a blank-holding die; Fig, 2 is a sectional view of a press mold device having frictional force measuring means on the surface of the blank-holding die and on the shoulder portions of the die; Fig. 3 is a plan view of a blank-holding die constituted by a plurality of die members and of frictional force measuring means; Fig. 4 is a sectional view illustrating, on an enlarged scale, the one side of the die and of the blank- holding die of Fig. 1; Fig. 5 is a sectional view of a press mold device having temperature sensors on the surface of the blank-holding die and on the shoulder portions of the die; Fig, 6 is a plan view of a blank-holding die constituted by a plurality of die members and temperature sensors; Fig. 7 is a sectional view illustrating, on an enlarged scale, the one side of the die and the blank-holding die of Fig. 5; Fig. 8 is a sectional view of a press mold device having frictional force measuring means on the surface of the blank-holding die and on the shoulder portions of the die, and having press reaction measuring means on the punch; Fig. 9 is a flowchart of an embodiment of the present invention for controlling the frictional force; Fig. 10 is a graph illustrating a relationship between the blank-holding force or the frictional force and the stroke of when the control method of the flowchart of Fig. 9 is applied; Fig. 11 is a flowchart of another embodiment of the present invention for controlling the frictional forcer-Fig. 12 is a graph of the blank-holding force or the frictional force with the passage of time of when the control method of the flowchart of Fig. 11 is applied; Fig. 13 is a flowchart of an embodiment of the invention for controlling the temperature; Fig. 14 is a flowchart of another embodiment of the invention for controlling the temperature; Fig. 15 is a flowchart of an embodiment of the invention for controlling the press reaction; Fig. 16 is a flowchart of another embodiment of the invention for controlling the press reaction; and Fig. 17 is a sectional view illustrating, on an enlarged scale, blank-holding force adjusting means including a hydraulic chamber. Best Mode for Carrying Out the Invention The invention will now be described in detail with reference to the drawings. Fig. 1 is a sectional view of a press mold device according to a first embodiment of the present invention. A mold device incorporating frictional force measuring means 4 is mounted on the surface of the blank-holding die 3 to control the blank-holding force via blank-holding die drive means 5 depending upon the frictional force that is detected. Fig. 4 is a view illustrating, on an enlarged scale, the one side of the die 2 and of the blank-holding die 3 of Fig. 1, and is a sectional view of a mold device incorporating the frictional force measuring means 4, The mold device according to this embodiment includes a punch 1 and a die 2 arranged facing the punch 1. A work 6 which is a thin sheet is pushed by the punch 1 into the die 2 to form the work 6. Further, a blank-holding die 3 is arranged facing the die 2 to prevent the development of wrinkles on the work 6 while the work 6 is being formed. The work 6 is held between the die 2 and the blank-holding die 3. The mold device is further provided with, for example, hydraulic cylinders 14 and 5 as punch drive means and as blank-holding die drive means for driving the punch 1 and the blank-holding die 3 toward the die 2. The hydraulic cylinders 14 and 5 ar« supplied with hydraulic pressure from, for example, variable capacity hydraulic pumps 13 and 12 which are the hydraulic pressure sources serving as punching speed adjusting means and blank-holding force adjusting means. The variable capacity hydraulic pumps 13 and 12 are controlled by a control unit 11. As the punch 1 rises, the work 6 which is held along the periphery thereof by the blank-holding die 3 and by the die 2 is drawn into the cavity of the die 2 with its periphery being pulled by the frictional force, and is formed into a shape in conformity with the punch 1. Here, if the tensile force is too great, the blank may be broken. If the tensile force is too small, the blank will be defectively formed such as developing wrinkles or making it difficult to accomplish the forming along the lower die. To obtain the product in a favorable shape, therefore, it is necessary to set a proper blank-holding force. On the other hand, the tension acting on the material stems from the frictional force among the work 6, punch 1 and die 2. In order to vary the relationship between the surface pressure and the frictional force, i.e., to vary the coefficient of friction, it is a widely accepted practice to vary the properties of the lubricating oil, to vary the surface roughness of the punch and the die, and to impart the beads. However, the coefficient of friction varies, from time to time, by being affected by the temperature, surface pressure and surface properties, and it becomes necessary to adjust the blank-holding force each time. In the constitution shown in Fig. 1, on the other hand, the frictional force among the work 6, blank-holding die 3 and die 2 is directly measured by the frictional force measuring means 4, and the measured results are fed back to the control device 11 to so control the hydraulic pressures that are fed from the variable capacity hydraulic pumps 13, 12 to the hydraulic cylinders 14, 5 that the measured frictional force becomes a predetermined value. According to this embodiment as described above, the punching speed and the blank-holding force are adjustable. By adjusting at least either the punching speed or the blank-holding force, a proper tension can be imparted to the blank at all times irrespective of variation in the coefficient of friction. The hydraulic cylinders 14 and 5 serving as the punch drive means and the blank-holding di« drive means are mere examples, and there may be employed air cylinders and electric motors instead of the hydraulic cylinders. Fig. 2 is a sectional view of the press mold device according to a second embodiment of the present invention. In Fig. 2, the constituent elements which are the same as those of the embodiment of Fig. 1 are denoted by the same reference numerals but are not described here again. In this embodiment, the mold device incorporating the frictional force measuring means 4 is mounted on the shoulder of the die 2, and the blank-holding force is controlled depending upon the detected frictional force via the blank-holding die drive means 5. In Fig. 2, the frictional force measuring means 4 are incorporated not only in the shoulder portions of the die but also in the surface of the blank-holding die 3. The frictional force measuring means 4, however, may be installed on the shoulder portions only, of the die. Referring to Fig. 3, further, if the blank-holding die 3 is constituted by using a plurality of die members 3a, the frictional force can be measured for each of the die members 3a by using the frictional force measuring means 4. Further, the hydraulic cylinders 5 may be arranged as drive means for the die members 3a and may be independently controlled to suitably adjust the distribution of the blank-holding force. Fig. 3 illustrates the blank-holding die according to a third embodiment of the present invention. In Fig. 3, the same constituent elements as those of the embodiment of Fig. 1 are denoted by the same reference numerals but are not described here again. In this embodiment, the blank-holding die 3 is constituted by a plurality of die members 3a, and the plurality of the die members 3a are each provided with the frictional force measuring means 4. Next, a principle for directly measuring the fractional force will be described with reference to Fig. 4. A work 6 is held by a pair of dies, i.e., by the die 2 and a flat plate 7 which is fastened by, for example, bolts in a manner allowing them to undergo resilient deformation relative to the die 3 in the right-and-left direction in the drawing. Further, a distortion measuring element 4 that serves as frictional force measuring means is held between the flat plate 7 and the blank-holding die 3. The distortion measuring element 4 can be formed by using a piezo element (piezoelectric element) or a strain gauge. When the work 6 slides in the direction of an arrow (toward the left in the drawing), a shearing distortion is generated in the distortion measuring element 4. If a piezo element (piezoelectric element) or a strain gauge is used as the distortion measuring element 4, it is allowed to easily detect the distortion as a voltage to thereby measure the frictional force. Fig. 3 illustrates a case where the frictional force is measured on one surface only of the blank-holding die 3. When the properties are not the same on the front surface and on the back surface of the work 6 or on the surfaces of a pair of die 2 and blank-holding die 3, the frictional forces are measured on the upper and lower surfaces of the work 6 to further improve the precision of measurement. As the material of the flat plate 7, there can be used a carbon steel, for structures, or a tool steel. The press mold device according to a fourth embodiment of the invention will be described next. Fig. 5 is a sectional view of a press mold device having temperature sensors 10 as frictional force measuring means. In Fig. 5, the temperature sensors 10 are incorporated not only in the shoulder portions of the die but also in the surface of the blank-holding die 3. The die having temperature sensors is mounted on at least one place on the surface of the blank-holding die 3 or on the shoulder of the die 2, and the blank-holding force is adjusted via the hydraulic cylinders 5 depending upon the detected temperature, or the punching speed is adjusted, making it possible to impart a proper tension to the material at all times irrespective of a change in the coefficient of friction and to obtain the effect of the present invention. It is economically desirable to use thermocouples as the temperature sensors. The temperature sensor will now be described with reference to Fig. 7 which illustrates, on an enlarged scale, one side of the die 2 and of the blank-holding die 3 of Fig. 5. The temperature sensor 10 is held between the flat plate 7 and the blank-holding die 3. In conducting the press-forming, heat of working is generated in large amounts where the frictional force is large on the flat plate 7 and heat of working is generated in small amounts where the frictional force is small. Therefore, it becomes possible to estimate the force of friction from a change in the temperature that is measured by using the temperature sensor 10. Namely, the frictional force is great where the temperature is high on the flat plate 7 and the material is prevented from flowing in. Therefore, the material is often broken. Further, the frictional force is small where the temperature is low and the material is not suppressed from flowing in, arousing such problems as the occurrence of wrinkles and defective formation. Therefore, the effect of the present invention can be obtained by directly measuring the temperature on the flat plate 7 at the time of forming by using the temperature sensor 10. As shown in Fig. 6, further, if the blank-holding die 3 is constituted by using a plurality of die members 3a, it is possible to measure the temperature of each of the die members 3a by using the temperature sensors 10. Further, the hydraulic cylinders 5 may be provided for each of the die members 3a and may be independently controlled to suitably adjust the distribution of the blank-holding force. The constitution of Fig. 5 deals with a case of using the temperature sensors 10 as the frictional force measuring means 4 of Fig. 2. Here, however, the frictional force measuring means 4 may be a combination of the distortion measuring elements 4 and the temperature sensors 10. Referring, further, to Fig. 8, described below is the press mold device having press reaction measuring means according to a fifth embodiment of the present invention. In Fig. 8, the constituent elements same as those of the embodiment of Fig. 1 are denoted by the sam« reference numerals but are not described here again. In working the work 6 with the constitution shown in Fig. 8, the punch receives the resultant force of the above frictional force and the force required for deforming the work 6, i.e., the punch receives the press reaction. In conducting the working, if the press reaction is too great, the material may be broken down. If the press reaction is too small, there may occur such problems as the development of wrinkles and defective forming. To obtain the products in good shape, therefore, a proper press reaction must be set. However, the above-mentioned frictional force varies from time to time depending upon the temperature and the surface shape and, hence, the press reaction having a component of frictional force also varies from time to time. To obtain a proper press reaction, so far, it was widely attempted to vary the properties of th« lubricating oil, to vary th« surface roughness of the punch and the die, or to impart beads in order to vary a relationship between the surface pressure and the frictional force, i.e., to vary the coefficient of friction. Referring to Fig. 8, on the other hand, it is made possible to carry out a orooer workina at all tim«s irrespective of a change in the press reaction by directly measuring the press reaction acting on the punch by using the press reaction measuring means 9, and by adjusting the punching speed and the blank-holding force by using the hydraulic cylinders 14 and 5 which are the punch drive means and the blank-holding die drive means, so that the press reaction assumes a predetermined value. In this case, too, the blank-holding die 3 is constituted by a plurality of die members 3a as shown in Fig. 3, the hydraulic cylinders 5 which are the blank-holding die drive means are provided for each of the die members 3a and are independently controlled, to suitably adjust the distribution of the blank-holding force. In Fig. 8, the frictional force measuring means 4 are incorporated not only in the press reaction measuring means 9 but also in the surface of the blank-holding die 3 and in the shoulder of the die 2. Any one or more of the frictional force measuring means 4 in the surface of the blank-holding die 3 or in the shoulder of the die 2 may, as required, be used in combination with the press reaction measuring means 9. Further, the frictional force measuring means may be replaced by the temperature sensors. Next, a method of controlling the mold device shown in Fig. 1 or 2 will be described with reference to a flowchart of Fig. 9. In this embodiment, at least either the blank-holding force or the punching speed is controlled during the working so that the frictional force measured by the frictional force measuring means 4 lies within a predetermined range during the working. Step 101: Start of forming where i • 1. Step 102: The stroke of the punch is advanced by ASi [mm]. For example, So = 0 {mm] when i « 1 and, hence, Si - ASj [mm]. ASi [mm] is determined prior to the working. Step 103: The frictional force Fmi [N] is measured when the stroke is Si [mm]. Step 104: Here, the frictional force Fmi [N] measured at step 103 and a target frictional force control value FCi [N] (set in advance prior to the working) are compared for their magnitudes. Step 105: When Frtii > Fcx as a result of having compared the magnitudes at step 104, there is executed at least either a process for decreasing the blank-holding force BHFi+i (NJ or a process for decreasing the punch stroke increment ASi+i [mm] depending upon a difference (Fmi - Fci) between the measured value and the target frictional force as represented by the formulas at step 105 in the drawing. Step 106: When Fmi Step 107: The working is conducted while effecting the feedback control in each time of forming, and the working ends when the stroke S [mm] is not smaller than the stroke S^ax [ram] at the time when the working has finished, but the loop returns to step 102 when the stroke S [mm} is smaller than the stroke S^ (mm] at the time when the working has finished. The value i at this moment increases by 1. The concrete blank-holding force BHFi+i [N] or the punch stroke increment ASi+i [mm] is calculated from the relationships of the drawing using proportional constants a, p, y and 8. This loop is repeated until the punch stroke Si [mm] reaches a punch stroke Sen If the above control is executed at regular intervals At [sec], the punching speed Vpi [mm/s] is found as Asi/At and can be controlled relvina uoon the ounch stroke increment. Fig. 10 illustrates the punch stroke hysteresis of the measured fractional force Fm {N] and the blank-holding force BHF [N] of when the above control method is used. It will be seen that a target BHF control value varies by a value corresponding to a difference between the measured fractional force Fm and the target frictional force control value Fc [SI unit], and the measured value of BHF varies during the working to meet therewith. Another method of controlling the mold device shown in Fig. 1 will be described with reference to a flowchart of Fig. 11. Here, the subscript "j" represents the number of times of forming in the step of press working. Step 201: First time of forming, j = 1. Step 202: Measure the hysteresis Fmj [N] of the frictional force at the time t [sec] of forming of the j-th time. Step 203: The time t [sec] in the forming of the j-th time is arbitrarily divided, and a predetermined lower-limit value of the frictional force is denoted by Fcl(t)[N], Here, when Faij(t) > Fcl^t) at each micro time t [sec], either the blank-holding force BHFji is decreased or the punching speed Vp3+1(t) is lowered depending upon a difference (Fmj(t) - Fclj(t)) between the measured value and a predetermined lower-limit value of the frictional force for the BHFj+i(t)[N] or for the punching speed Vpj+i(t)[mm/s] in the micro time t in the forming of the (j + l)-th time as represented by the formulas in the drawing. Step 204: A predetermined upper-limit value of the frictional force is denoted by Fcu(t)[N]. Here, when Fm3(t) for the BHFjti(t)[N] or for the punching speed Vp3+i(t)[mm/s] in the micro time t in the forming of the (j + l)-th time as represented by the formulas in the drawing. Step 205: As described above, the forming condition in the forming of the (j + l)-th time is set in advance based on the forming condition of the forming of the j-th time. The forming ends if j is the total number of times jmax of forming. Otherwise, the loop returns to step 202. The concrete blank-holding force BHFi+i [N] or the punching speed Vp^i (t)(mm/s] is calculated from the relationships of the drawing using proportional constants a, p, y and 8. The forming of the (j + l)-th time is conducted by using the thus obtained blank-holding force BHFj+i(t) [N] or the punching speed Vpjn (t) [mm/s] . This control is repeated until the number of times j of forming reaches a maximum number of times jnax of forming. Fig. 12 illustrates the time hysteresis of the measured frictional force Fm {N] and the blank-holding force BHF [N] of when the above control method is used. The BHF control target value is varied from BHFj to BHFD+i in a range of t [sec] where the frictional force Fm3(t)IN] is greater than the upper-limit value Fcu3(t)[N] of the frictional force or the frictional force Fmj(t)[N] is smaller than the lower-limit value Fclj(t)[NJ of the frictional force, and the working of the (j -4- l)-th time is executed by using the BHF control target value BHFj+1 that is varied. Next, a method of controlling the mold device shown in Fig. 5 will be described with reference to a flowchart of Fig. 13. In this embodiment, at least either the blank-holding force or the punching speed is controlled during the working so that the temperatures measured by the temperature sensors lie within a predetermined range during the working. The subscript i represents the number of times of control during the forming. Step 301: Start of forming where i - 1. Step 302: The stroke of the punch is advanced by ASi [mm]. For example, S0 = 0 [mm] when i = 1 and, hence, Si = ASi [nan). ASi {mm] is determined prior to the working. Step 303: The temperature ?% [°C] is measured when the stroke is Si (mm). Step 304: The temperature Tmi [C] measured at step 303 and the temperature control target value TCi f°CJ(set in advance before the working) are compared for their magnitudes. Step 305: When Tm4 > TcA as a result of having compared the magnitudes at step 304, there is executed at least either a process for decreasing the blank-holding force BHFi! [N] or a process for decreasing the punch stroke increment AS^i [mm] depending upon a difference (Tmi - Tci) between the measured value and the target temperature as represented by the formulas at step 305 in the drawing. Step 306: When Tmi Step 307: The working is conducted while effecting the feedback control in each time of forming, and the working ends when the stroke S [mm] is not smaller than the stroke SMX [mm] at the time when the working has finished, but the loop returns to step 302 when the stroke S [mm] is smaller than the stroke SM [mm] at the time when the working has finished. The value i at this moment increases by 1. The concrete blank-holding force BHFm [N] or the punch stroke increment ASii [mm] is calculated from the relationships of the drawing using proportional constants a, 0, y and 5. This loop is repeated until the punch stroke Si [nan) reaches a punch stroke S«» If the above control is executed at regular intervals At [sec], the punching speed Vpi [nan/s] is found as Asi/At and can be controlled relying upon the punch stroke increment. A further method of controlling the mold device shown in Fig. 5 will be described with reference to a flowchart of Fig. 14. Here, the subscript "j" represents the number of times of forming in the step of press working. Step 401: First time of forming, j - 1. Step 402: Measure the hysteresis TiOj (t) [C] of the temperature at the time t [sec] of forming of the j-th time. Step 403: The time t [sec] in the forming of the j-th time is arbitrarily divided, and a predetermined lower-limit value of the temperature is denoted by Tcl(t)[°C]. Here, when T»j(t) > Tclj{t) at each micro time t [sec], either the blank-holding force BHFj+i is decreased or the punching speed Vpjn(t) is lowered depending upon a difference {Tmj(t) - Tclj (t)) between the measured value and a predetermined lower-limit value of the temperature for the BHFj+a (t) [NJ or for the punching speed Vpjti(t)[N] in the micro time t in the formating of the (j + l)-th time as represented by the formulas in the drawing. Step 404: A predetermined upper-limit value of the temperature is denoted by Tcu(t)[t]. Here, when Tnij(t) in the micro time t in the forming of the (j + l)-th time as represented by the formulas in the drawing. Step 405: As described above, the forming condition in the forming of the (j + l}-th time is set in advance based on the forming condition of the forming of the j-th time. The forming ends if j is the total number of times JMX of forming. Otherwise, the loop returns to step 402. The concrete blank-holding force BHFi+i [NJ or the punching speed Vpj+J (t) [mm/s] is calculated from the relationships of the drawing using proportional constants a, p, y and 6. The BHF control target value is varied from BHF-)(t)[N] to BHFj+i (t) [N] and the target punching speed control value is varied from Vpj(t)[mm/s] to Vp-,+,1 (t) [mm/s] in a range of t [sec] where the temperature Tm.,(t) measured in advance in the forming of the last time is higher than the upper-limit temperature Tcu3 (t) [°C] or the temperature TiUj(t)[pC] is lower than the lower-limit temperature Tcl-,[°C], and forming of the (j + l)-th time is conducted by using the thus varied BHF control target value BHF3n(t)[N] or the punching speed control target value Vpj+i (t) [mm/s] . This control is repeated until the number of times j of forming reaches a maximum number of times junx of forming. Next, a method of controlling the mold device shown in Fig. 9 will be described with reference to a flowchart of Fig. 15. In this embodiment, at least either the blank-holding force or the punching speed is controlled during the working so that the press reaction measured by the press reaction measuring means lies within a predetermined range during the working. The subscript i represents the number of times of control during the forming. Step 501: Start of forming where i » 1. Step 502: The stroke of the punch is advanced by ASX [mm]. For example, So m 0 [mm] when i = 1 and, hence, Si [mm] . ASi [ran] is determined prior to the working. Step 503: The punch reaction Pnii [N] is measured when the stroke is Si [nun] . Step 504: The punch reaction PII^ [N] measured at step 503 and the target punch reaction control value fNJ(set in advance before the working) are compared for their magnitudes. Step 505: When Pmj > P^ [N] as a result of having compared the magnitudes at step 504, there is executed at least either a process for decreasing the blank-holding force BHFin [N]or a process for decreasing the punch stroke increment Asiti [nun] depending upon a difference (Pmi - Pci) between the measured value and the target press reaction value as represented by the formulas at step 505 in the drawing. Step 506: When Pmi Step 507: The working is conducted while effecting the feedback control in each time of forming, and the working ends when the stroke S [mm] is not smaller than the stroke S»mx [nun] at the time when the working has finished, but the loop returns to step 502 when the stroke S [mm] is smaller than the stroke Smax [nun] at the time when the working has finished. The value i at this moment increases by 1. The concrete blank-holding force BHF^i [N] or the punch stroke increment ASi+i [mm] is calculated from the relationships of the drawing using proportional constants a, P, y and 8. This loop is repeated until the punch stroke Si [mm] reaches a punch stroke SBmi [mm] at the end of the forming. If the above control is executed at regular intervals At [sec], the punching speed vpj. [mm/s] is found as Asi/At and can be controlled by relying upon the punch stroke increment. A further method of controlling the mold device shown in Fig. 9 will be described with reference to a flowchart of Fig. 16. Here, the subscript "j" represents the number of times of forming in the step of press working. Step 601: First time of forming, j • 1. Step 602: Measure the hysteresis Pm$ [t] of the punch reaction at the time t [sec] of forming of the j-th time. Step 603; The time t [sec] in the formation of the j-th time is arbitrarily divided, and a predetermined lower-limit value of the press reaction is denoted by Pcl(t)[N]. Here, when Pioj(t) > PcljCt) at each micro time t [sec], either the blank-holding force BHF3i [N] is decreased or tha punching speed Vp^ (t) [nrn/s] is lowered depending upon a difference (Pmj(t) - Pclj (t)) between the measured value and a predetermined lower-limit value of the press reaction for the BHFj+1(t)[N] or for the punching spocd Vp^ft) fiwrn/s] in the micro time t in the forming of the (j + l)-th time as represented by the formulas in the drawing. Step 604: A predetermined upper-limit value of the press reaction is denoted by Pcu(t)[N]. Here, when Pmj(t) Step 605: As described above, the forming condition in the forming of the (j + l)-th time is set in advance based on the forming condition of the forming of the j-th time. The forming ends if j is the total number of times jm«x of forming. Otherwise, the loop returns to step 602. The concrete blank-holding force BHFin (t)IN] or the punching speed Vpj+i (t) [mm/sj is calculated from the relationships of the drawing using proportional constants a, p, y and 8. The target BHF control value is varied from BHFj(t)[N] to BHFj+i (t) [N] and the target punching speed control value is variad from Vp3 (t) [mm/s] to Vp3+1{t) [mm/s] in a range of t [sec] where the press reaction Pmj(t)[N] measured in advance in the forming of the last time is greater than the upper-limit press reaction value PcUj(t) [N] or the press reaction Pmj(t)[N] is smaller than the lower-limit value of press reaction Pclj[N], and forming of the (j + l)-th time is conducted by using the thus varied target BHF control value BHFj+i(t)[NJ or the target punching speed control value Vpj+i (t) [mm/s] . This control is repeated until the number of times j of forming reaches a maximum number of times jMX of forming. The punch 1 may be constructed in a split structure like the blank-holding die 3, and each divided punch may be pressed using hydraulic cylinders resulting, however, in a complex mold device and in an expensive facility. Therefore, the punch 1 is constituted as a unitary structure and is uniformly pressed by an ordinary outer cylinder. Hydraulic chambers 8 are embedded as shown in Fig. 17 in the blank-holding die 3 divided and fastened (fixed) to the surface of the punch 1 by the method described above, and pressures thereto are separately adjusted to control the blank-holding force for each of the divided blank-holding die in an inexpensive manner. Example 1 Relying upon the above-mentioned invention, a mold device shown in Fig. 1 was fabricated as an embodiment of the invention, and a thin steel sheet was press-formed. Piezo elements were used as the frictional force measuring means 4, and the flat plate 7 was the one made of S4SC of which the surface has been hardened. Table 1 shows properties of the steel sheets that were used. There were used two kinds of steel sheets having a thickness of 1.2 mm and plated with alloyed molten zinc of different degrees of alloying. Table 1 (Table Remove) The forming test was conducted by continuously deep-draw-forming a square cylinder measuring 50 mm x 50 mm to investigate the forming load, and breakage and development of wrinkles of the formed articles. A square blank, measuring 100 mm x 100 mm was subjected to the forming experiment by using a blank-holding die constituted by eight die members 3a as shown in Fig, 2, Tabl« 2 shows the results of testing after continuous forming 100 times. As Comparative Examples/ Table 3 shows the results of a case where the blank-holding pressures were maintained constant by using a mold device without means for adjusting the blank-holding force. (Table Remove) (Table Remove) In the invention 1 so executing the forming that the frictions! force was constant (0.25 [JcN/die]} for all die members, variation in the forming load was very small and a generally favorable forming was accomplished as compared to Comparative Example 1 in which the blank-holding force was set to be 20 (kN] constant (the sum of the fractional forces of 2 [kN] when the coefficient of friction was presumed to be 0.1) and Comparative Example 2 in which the blank-holding force was set to be 40 [kN] constant (the sum of the fractional forces of 4 [kN] when the coefficient of friction was presumed to be 0.1). When the blank B having a low degree of alloying was used, however, zinc adhered to the die with an increase in the number of times of forming, the friction became nonuniform, and wrinkles developed to a slight degree at the corners. In the invention 2 so executing the forming that the frictional force was lowered down to 0.2 [kN/die] in the parallel portions where the material flows in at a large rate and that the frictional force was increased to 0.3 [kN/die] at the corner portions, a favorable forming was accomplished with either material irrespective of the number of times of forming. Example 2 Relying upon the above-mentioned invention, a mold device shown in Fig. 5 was fabricated as an embodiment of the invention, and a thin steel sheet was press-formed. Thermocouples were used as the temperature sensors 10, and the flat plate 1 was the one made of S45C of which the surface has been hardened. The steel sheet used for the experiment was the same as that used in Example 1. The forming test was conducted by continuously deep-draw-forming a square cylinder measuring 50 mm x 50 mm to investigate the forming load, and breakage and development of wrinkles of the formed articles. A square blank measuring 100 mm x 100 mm was subjected to the forming experiment by using a blank-holding die constituted by eight die members 3a as shown in Fig. 6. Table 4 shows the results of testing after a continuous forming of 100 times. Comparative Examples were the same as those conducted in connection with Example 1. (Table Remove) In the invention 3 executing the forming so that the temperature was a constant 180°C for all die members, variation in the forming load was very small and a generally favorable forming was accomplished as compared to Comparative Example 1 in which the blank-holding force was set to be 20 [kN] constant (the sum of the frictional forces of 2 [kN] when the coefficient of friction was presumed to be 0.1) and Comparative Example 2 in which the blank-holding force was set to be 40 [kN] constant (the sum of the frictional forces of 4 [kN] when the coefficient of friction was presumed to be 0.1). When the blank B having a low degree of alloying was used, however, zinc adhered to the die with an increase in the number of times of forming/ the temperature became nonuniform, and wrinkles developed to a slight degree at the corners. In the invention 4 so executing the forming that the temperature was lowered down to 150 [C] in the parallel portions where the material flows in at a large rate and that the temperature was increased to 200 £0C] at the corner portions/ a favorable forming was accomplished with either material irrespective of the number of times of forming. Example 3 Relying upon the above-mentioned invention, a mold device shown in Fig. 8 was fabricated as an embodiment of the invention, and a thin steel sheet was press-formed. Strain gauges were used as the press reaction measuring means 9, and the flat plate 7 was made of S45C of which the surfaces had been hardened. The steel sheet used for the experiment was the same as that used in Example 1. The forming testing was conducted by continuously deep-draw-forming a square cylinder measuring 50 mm x 50 mm to investigate the forming load, and breakage and development of wrinkles of the formed articles. A square blank measuring 100 mm x. 100 mm was subjected to the forming experiment by using a blank-holding die constituted by eight die members 3a as shown in Fig. 3. Table 5 shows the results of testing after a continuous forming of 100 times. Comparative Examples were the same as those conducted in connection with Example 1. (Table Remove) In the invention 5 executing the forming while controlling the blank-holding force so that the press reaction was constant {65 [kN]), variation in the forming load was very small and a generally favorable forming was accomplished as compared to Comparative Example 1 in which the blank-holding force was set to be 20 [kN] constant (the sum of the frictional forces of 2 [kN] when th« coefficient of friction was presumed to be 0.1} and Comparative Example 2 in which the blank-holding force was set to be 40 [kN] constant (the sum of the frictional forces of 4 [kN] when the coefficient of friction was presumed to be 0,1). When the blank B having a low degree of alloying was used, however, zinc adhered to the die with an increase in the number of times of forming, the press reaction became nonuniform, and wrinkles developed to a slight degree at the corners. In the invention 6 executing the forming so that the press reaction was lowered down to 20 kN in the initial stage of working where the material flows in at a large rate and that the press reaction was increased to 70 kN in the latter stage of working, a favorable forming was accomplished with either material irrespective of the number of times of forming. We claim: 1. A mold device for press-forming a thin sheet, comprising a punch, a die, a blank-holding die, punch drive means for inserting said punch in said die, and blank-holding die drive means for applying a blank- holding force to said blank-holding die, thereby to work a thin-sheet work by pushing it into said die by said punch, said mold device for press- forming a thin sheet comprising: blank-holding force adjusting means for adjusting the blank-holding force applied to said blank- holding die via blank-holding die drive means; at least either one of frictional force measuring means for measuring the frictional force acting on said work or press reaction measuring means for measuring the press reaction acting on said punch; and control means which so controls said blank-holding force adjusting means that a measured value of said frictional force measuring means or of said press reaction measuring means becomes a predetermined value. 2. A mold device for press-forming a thin sheet as claimed in claim 1, wherein said frictional force measuring means are disposed on the shoulder portions of the die to measure frictional forces between said work and said die. 3. A mold device for press-forming a thin sheet as claimed in claim 1, wherein said frictional force measuring means are disposed on said blank-holding die to measure frictional forces between said work and said blank-holding die. 4. A mold device for press-forming a thin sheet as claimed in claim 1, wherein the blank-holding die has a plurality of die members, and said die members are each provided with said blank-holding die drive means so as to be independently controlled. 5. A mold device for press-forming a thin sheet as claimed in claim 4, wherein said frictional force measuring means are disposed on the shoulder portions of the die so as to be corresponded to the die members and to measure frictional forces between said work and said die. 6. A mold device for press-forming a thin sheet as claimed in claim 4, wherein said frictional force measuring means are disposed on die members to independently measure frictional forces between said work and said die members. 7. A mold device for press-forming a thin sheet as claimed in any one of claims 1 to 6, wherein said frictional force measuring means has a piezo element or a strain gauge. 8. A mold device for press-forming a thin sheet as claimed in any one of claims 1, 2, 4, 5 and 6, wherein said frictional force measuring means has a temperature sensor. 9. A mold device for press-forming a thin sheet as claimed in claim 8. wherein said temperature sensor is a thermocouple. 10. A method of press-forming a thin sheet by pushing a thin sheet work into a die by a punch to work the thin sheet work, comprising the steps of: measuring at least either one of the frictional force acting on said work or a press reaction acting on said punch: and adjusting the blank-holding force or the punching speed, so that a value measured in said step of measurement becomes a predetermined value. 11. A method of press-forming a thin sheet as claimed in claim 10, further comprising a step of finding, by using said measured value, a relationship between the frictional force or the press reaction measured in the preceding forming process and the blank-holding force or the punching speed, and either the blank-holding force or the punching speed is adjusted based on said relationship that is found, so that said frictional force or said press reaction, that is measured, lies within a predetermined range.

Full Text

MOLD DEVICE FOR PRESS-FORMING A THIN SHEET AND PRESS-FORMING METHOD
Technical Field
The present invention relates to a mold device for press-forming a thin sheet and to a press-forming method. More particularly, the invention relates to a mold device and a forming method capable of adjusting the distribution of a blank-holding force during the press working.
Background Art
A variety of inventions have been disclosed concerning methods of forming while controlling the blank-holding force. For example, JP-A-7-266100 (patent document I) discloses a method of adjusting the air pressures in air cylinders so as to conduct a press work under a proper blank-holding force by finding, in advance, a relationship between a proper blank-holding force for obtaining a predetermined press quality and physical quantities such as shape and mechanical properties of a blank to be pressed, chemical properties, laminate properties such as plating and surface state such as the amount of oil, and by finding the suitable blank-holding force depending upon the real physical quantities based on the above relationship.
JP-A-9-38728 (patent document 2) discloses a method of preventing the occurrence of cracks and the residence of wrinkles caused by an excess flow of the material by increasing the blank-holding force at the time of draw forming from the initial stage to the intermediate stage of formation, and decreasing the blank-holding force to a suitable value in the latter stage of formation. Further, JP-A-6-190464 (patent document 3) discloses an invention of a die-cushioning device equipped with hydraulic cylinders for making the pressure uniform, wherein the hydraulic pressures in the hydraulic

cylinders are temporarily varied by controlling the opening of the flow-rate control valves to control the blank-holding force.
Disclosure of the Invention
Patent documents 1 to 3 disclose inventions for controlling the blank-holding force, which, however, are not capable of finding proper blank-holding force, in advance, under a variety of varying factors, such as varying blank properties, wear of the die and temperature of the die. In particular, the lubricating properties to the die vary at all times, and measuring the above properties each time greatly decreases the productivity.
Controlling the blank-holding force using the die-cushioning device requires a drastic remodeling of the press device and, besides, it is difficult to estimate a proper blank-holding force.
It is an object of the present invention to provide a device capable of setting a proper load by finding a blank-holding force on the site without the need of finding, in advance, proper blank-holding forces for the varying factors.
According to the present invention, there is provided a mold device for press-forming a thin sheet, comprising a punch, a die, a blank-holding die, punch drive means for inserting said punch in said die, and blank-holding die drive means for applying a blank-holding force to said blank-holding die, thereby to work a thin-sheet work by pushing it into said die by said punch, said mold device for press-forming a thin sheet further comprising:
blank-holding force adjusting means for adjusting the blank-holding force applied to said blank-holding die via blank-holding die drive means;
at least either one of fractional force measuring means for measuring the frictional force acting on said work or press reaction measuring means for measuring the

press reaction acting on said punch; and
control means which so controls said blank-holding force adjusting means that a measured value of said frictional force measuring means or of said press reaction measuring means becomes a predetermined value.
According to another feature of the present invention, there is provided a method of press-forming a thin sheet by pushing a thin sheet work into a die by a punch by using a mold device to work the thin-sheet work, comprising the steps of:
measuring at least either one of the frictional force acting on said work or a press reaction acting on said punch; and
adjusting the blank-holding force or the punching speed, so that a value measured in said step of measurement becomes a predetermined value.
The present invention makes it possible to impart a proper frictional force irrespective of varying factors such as lubricating property and surface property between the die and the work and, hence, to obtain favorably formed articles at all times irrespective of variations in the characteristics of the blank or a variation in the environment.
Brief Description of the Drawings
Fig. 1 is a sectional view of a press mold device having frictional force measuring means on the surface of a blank-holding die;
Fig, 2 is a sectional view of a press mold device having frictional force measuring means on the surface of the blank-holding die and on the shoulder portions of the die;
Fig. 3 is a plan view of a blank-holding die constituted by a plurality of die members and of frictional force measuring means;
Fig. 4 is a sectional view illustrating, on an enlarged scale, the one side of the die and of the blank-

holding die of Fig. 1;
Fig. 5 is a sectional view of a press mold device having temperature sensors on the surface of the blank-holding die and on the shoulder portions of the die;
Fig, 6 is a plan view of a blank-holding die constituted by a plurality of die members and temperature sensors;
Fig. 7 is a sectional view illustrating, on an enlarged scale, the one side of the die and the blank-holding die of Fig. 5;
Fig. 8 is a sectional view of a press mold device having frictional force measuring means on the surface of the blank-holding die and on the shoulder portions of the die, and having press reaction measuring means on the punch;
Fig. 9 is a flowchart of an embodiment of the present invention for controlling the frictional force;
Fig. 10 is a graph illustrating a relationship between the blank-holding force or the frictional force and the stroke of when the control method of the flowchart of Fig. 9 is applied;
Fig. 11 is a flowchart of another embodiment of the present invention for controlling the frictional forcer-Fig. 12 is a graph of the blank-holding force or the frictional force with the passage of time of when the control method of the flowchart of Fig. 11 is applied;
Fig. 13 is a flowchart of an embodiment of the invention for controlling the temperature;
Fig. 14 is a flowchart of another embodiment of the invention for controlling the temperature;
Fig. 15 is a flowchart of an embodiment of the invention for controlling the press reaction;
Fig. 16 is a flowchart of another embodiment of the invention for controlling the press reaction; and
Fig. 17 is a sectional view illustrating, on an enlarged scale, blank-holding force adjusting means including a hydraulic chamber.

Best Mode for Carrying Out the Invention
The invention will now be described in detail with reference to the drawings.
Fig. 1 is a sectional view of a press mold device according to a first embodiment of the present invention. A mold device incorporating frictional force measuring means 4 is mounted on the surface of the blank-holding die 3 to control the blank-holding force via blank-holding die drive means 5 depending upon the frictional force that is detected. Fig. 4 is a view illustrating, on an enlarged scale, the one side of the die 2 and of the blank-holding die 3 of Fig. 1, and is a sectional view of a mold device incorporating the frictional force measuring means 4,
The mold device according to this embodiment includes a punch 1 and a die 2 arranged facing the punch 1. A work 6 which is a thin sheet is pushed by the punch 1 into the die 2 to form the work 6. Further, a blank-holding die 3 is arranged facing the die 2 to prevent the development of wrinkles on the work 6 while the work 6 is being formed. The work 6 is held between the die 2 and the blank-holding die 3.
The mold device is further provided with, for example, hydraulic cylinders 14 and 5 as punch drive means and as blank-holding die drive means for driving the punch 1 and the blank-holding die 3 toward the die 2. The hydraulic cylinders 14 and 5 ar« supplied with hydraulic pressure from, for example, variable capacity hydraulic pumps 13 and 12 which are the hydraulic pressure sources serving as punching speed adjusting means and blank-holding force adjusting means. The variable capacity hydraulic pumps 13 and 12 are controlled by a control unit 11.
As the punch 1 rises, the work 6 which is held along the periphery thereof by the blank-holding die 3 and by the die 2 is drawn into the cavity of the die 2 with its

periphery being pulled by the frictional force, and is formed into a shape in conformity with the punch 1. Here, if the tensile force is too great, the blank may be broken. If the tensile force is too small, the blank will be defectively formed such as developing wrinkles or making it difficult to accomplish the forming along the lower die. To obtain the product in a favorable shape, therefore, it is necessary to set a proper blank-holding force. On the other hand, the tension acting on the material stems from the frictional force among the work 6, punch 1 and die 2. In order to vary the relationship between the surface pressure and the frictional force, i.e., to vary the coefficient of friction, it is a widely accepted practice to vary the properties of the lubricating oil, to vary the surface roughness of the punch and the die, and to impart the beads. However, the coefficient of friction varies, from time to time, by being affected by the temperature, surface pressure and surface properties, and it becomes necessary to adjust the blank-holding force each time.
In the constitution shown in Fig. 1, on the other hand, the frictional force among the work 6, blank-holding die 3 and die 2 is directly measured by the frictional force measuring means 4, and the measured results are fed back to the control device 11 to so control the hydraulic pressures that are fed from the variable capacity hydraulic pumps 13, 12 to the hydraulic cylinders 14, 5 that the measured frictional force becomes a predetermined value. According to this embodiment as described above, the punching speed and the blank-holding force are adjustable. By adjusting at least either the punching speed or the blank-holding force, a proper tension can be imparted to the blank at all times irrespective of variation in the coefficient of friction.
The hydraulic cylinders 14 and 5 serving as the punch drive means and the blank-holding di« drive means

are mere examples, and there may be employed air cylinders and electric motors instead of the hydraulic cylinders.
Fig. 2 is a sectional view of the press mold device according to a second embodiment of the present invention. In Fig. 2, the constituent elements which are the same as those of the embodiment of Fig. 1 are denoted by the same reference numerals but are not described here again.
In this embodiment, the mold device incorporating the frictional force measuring means 4 is mounted on the shoulder of the die 2, and the blank-holding force is controlled depending upon the detected frictional force via the blank-holding die drive means 5. In Fig. 2, the frictional force measuring means 4 are incorporated not only in the shoulder portions of the die but also in the surface of the blank-holding die 3. The frictional force measuring means 4, however, may be installed on the shoulder portions only, of the die.
Referring to Fig. 3, further, if the blank-holding die 3 is constituted by using a plurality of die members 3a, the frictional force can be measured for each of the die members 3a by using the frictional force measuring means 4.
Further, the hydraulic cylinders 5 may be arranged as drive means for the die members 3a and may be independently controlled to suitably adjust the distribution of the blank-holding force.
Fig. 3 illustrates the blank-holding die according to a third embodiment of the present invention. In Fig. 3, the same constituent elements as those of the embodiment of Fig. 1 are denoted by the same reference numerals but are not described here again.
In this embodiment, the blank-holding die 3 is constituted by a plurality of die members 3a, and the plurality of the die members 3a are each provided with the frictional force measuring means 4.

Next, a principle for directly measuring the fractional force will be described with reference to Fig. 4. A work 6 is held by a pair of dies, i.e., by the die 2 and a flat plate 7 which is fastened by, for example, bolts in a manner allowing them to undergo resilient deformation relative to the die 3 in the right-and-left direction in the drawing. Further, a distortion measuring element 4 that serves as frictional force measuring means is held between the flat plate 7 and the blank-holding die 3. The distortion measuring element 4 can be formed by using a piezo element (piezoelectric element) or a strain gauge. When the work 6 slides in the direction of an arrow (toward the left in the drawing), a shearing distortion is generated in the distortion measuring element 4. If a piezo element (piezoelectric element) or a strain gauge is used as the distortion measuring element 4, it is allowed to easily detect the distortion as a voltage to thereby measure the frictional force.
Fig. 3 illustrates a case where the frictional force is measured on one surface only of the blank-holding die 3. When the properties are not the same on the front surface and on the back surface of the work 6 or on the surfaces of a pair of die 2 and blank-holding die 3, the frictional forces are measured on the upper and lower surfaces of the work 6 to further improve the precision of measurement.
As the material of the flat plate 7, there can be used a carbon steel, for structures, or a tool steel.
The press mold device according to a fourth embodiment of the invention will be described next. Fig. 5 is a sectional view of a press mold device having temperature sensors 10 as frictional force measuring means. In Fig. 5, the temperature sensors 10 are incorporated not only in the shoulder portions of the die but also in the surface of the blank-holding die 3. The die having temperature sensors is mounted on at least one

place on the surface of the blank-holding die 3 or on the shoulder of the die 2, and the blank-holding force is adjusted via the hydraulic cylinders 5 depending upon the detected temperature, or the punching speed is adjusted, making it possible to impart a proper tension to the material at all times irrespective of a change in the coefficient of friction and to obtain the effect of the present invention.
It is economically desirable to use thermocouples as the temperature sensors.
The temperature sensor will now be described with reference to Fig. 7 which illustrates, on an enlarged scale, one side of the die 2 and of the blank-holding die 3 of Fig. 5. The temperature sensor 10 is held between the flat plate 7 and the blank-holding die 3. In conducting the press-forming, heat of working is generated in large amounts where the frictional force is large on the flat plate 7 and heat of working is generated in small amounts where the frictional force is small. Therefore, it becomes possible to estimate the force of friction from a change in the temperature that is measured by using the temperature sensor 10. Namely, the frictional force is great where the temperature is high on the flat plate 7 and the material is prevented from flowing in. Therefore, the material is often broken. Further, the frictional force is small where the temperature is low and the material is not suppressed from flowing in, arousing such problems as the occurrence of wrinkles and defective formation. Therefore, the effect of the present invention can be obtained by directly measuring the temperature on the flat plate 7 at the time of forming by using the temperature sensor 10.
As shown in Fig. 6, further, if the blank-holding die 3 is constituted by using a plurality of die members 3a, it is possible to measure the temperature of each of the die members 3a by using the temperature sensors 10. Further, the hydraulic cylinders 5 may be provided for

each of the die members 3a and may be independently controlled to suitably adjust the distribution of the blank-holding force.
The constitution of Fig. 5 deals with a case of using the temperature sensors 10 as the frictional force measuring means 4 of Fig. 2. Here, however, the frictional force measuring means 4 may be a combination of the distortion measuring elements 4 and the temperature sensors 10.
Referring, further, to Fig. 8, described below is the press mold device having press reaction measuring means according to a fifth embodiment of the present invention. In Fig. 8, the constituent elements same as those of the embodiment of Fig. 1 are denoted by the sam« reference numerals but are not described here again.
In working the work 6 with the constitution shown in Fig. 8, the punch receives the resultant force of the above frictional force and the force required for deforming the work 6, i.e., the punch receives the press reaction. In conducting the working, if the press reaction is too great, the material may be broken down. If the press reaction is too small, there may occur such problems as the development of wrinkles and defective forming. To obtain the products in good shape, therefore, a proper press reaction must be set. However, the above-mentioned frictional force varies from time to time depending upon the temperature and the surface shape and, hence, the press reaction having a component of frictional force also varies from time to time. To obtain a proper press reaction, so far, it was widely attempted to vary the properties of th« lubricating oil, to vary th« surface roughness of the punch and the die, or to impart beads in order to vary a relationship between the surface pressure and the frictional force, i.e., to vary the coefficient of friction.
Referring to Fig. 8, on the other hand, it is made possible to carry out a orooer workina at all tim«s

irrespective of a change in the press reaction by directly measuring the press reaction acting on the punch by using the press reaction measuring means 9, and by adjusting the punching speed and the blank-holding force by using the hydraulic cylinders 14 and 5 which are the punch drive means and the blank-holding die drive means, so that the press reaction assumes a predetermined value.
In this case, too, the blank-holding die 3 is constituted by a plurality of die members 3a as shown in Fig. 3, the hydraulic cylinders 5 which are the blank-holding die drive means are provided for each of the die members 3a and are independently controlled, to suitably adjust the distribution of the blank-holding force.
In Fig. 8, the frictional force measuring means 4 are incorporated not only in the press reaction measuring means 9 but also in the surface of the blank-holding die 3 and in the shoulder of the die 2. Any one or more of the frictional force measuring means 4 in the surface of the blank-holding die 3 or in the shoulder of the die 2 may, as required, be used in combination with the press reaction measuring means 9. Further, the frictional force measuring means may be replaced by the temperature sensors.
Next, a method of controlling the mold device shown in Fig. 1 or 2 will be described with reference to a flowchart of Fig. 9. In this embodiment, at least either the blank-holding force or the punching speed is controlled during the working so that the frictional force measured by the frictional force measuring means 4 lies within a predetermined range during the working.
Step 101: Start of forming where i • 1.
Step 102: The stroke of the punch is advanced by ASi [mm]. For example, So = 0 {mm] when i « 1 and, hence, Si - ASj [mm]. ASi [mm] is determined prior to the working.
Step 103: The frictional force Fmi [N] is measured when the stroke is Si [mm].
Step 104: Here, the frictional force Fmi [N]

measured at step 103 and a target frictional force control value FCi [N] (set in advance prior to the working) are compared for their magnitudes.
Step 105: When Frtii > Fcx as a result of having compared the magnitudes at step 104, there is executed at least either a process for decreasing the blank-holding force BHFi+i (NJ or a process for decreasing the punch stroke increment ASi+i [mm] depending upon a difference (Fmi - Fci) between the measured value and the target frictional force as represented by the formulas at step 105 in the drawing.
Step 106: When Fmi Step 107: The working is conducted while effecting the feedback control in each time of forming, and the working ends when the stroke S [mm] is not smaller than the stroke S^ax [ram] at the time when the working has finished, but the loop returns to step 102 when the stroke S [mm} is smaller than the stroke S^ (mm] at the time when the working has finished. The value i at this moment increases by 1.
The concrete blank-holding force BHFi+i [N] or the
punch stroke increment ASi+i [mm] is calculated from the relationships of the drawing using proportional constants a, p, y and 8. This loop is repeated until the punch stroke Si [mm] reaches a punch stroke Sen If the above control is executed at regular intervals At [sec], the punching speed Vpi [mm/s] is found as Asi/At and can be controlled relvina uoon the ounch

stroke increment.
Fig. 10 illustrates the punch stroke hysteresis of the measured fractional force Fm {N] and the blank-holding force BHF [N] of when the above control method is used. It will be seen that a target BHF control value varies by a value corresponding to a difference between the measured fractional force Fm and the target frictional force control value Fc [SI unit], and the measured value of BHF varies during the working to meet therewith.
Another method of controlling the mold device shown in Fig. 1 will be described with reference to a flowchart of Fig. 11. Here, the subscript "j" represents the number of times of forming in the step of press working.
Step 201: First time of forming, j = 1.
Step 202: Measure the hysteresis Fmj [N] of the frictional force at the time t [sec] of forming of the j-th time.
Step 203: The time t [sec] in the forming of the j-th time is arbitrarily divided, and a predetermined lower-limit value of the frictional force is denoted by Fcl(t)[N], Here, when Faij(t) > Fcl^t) at each micro time t [sec], either the blank-holding force BHFj*i is decreased or the punching speed Vp3+1(t) is lowered depending upon a difference (Fmj(t) - Fclj(t)) between the measured value and a predetermined lower-limit value of the frictional force for the BHFj+i(t)[N] or for the punching speed Vpj+i(t)[mm/s] in the micro time t in the forming of the (j + l)-th time as represented by the formulas in the drawing.
Step 204: A predetermined upper-limit value of the frictional force is denoted by Fcu(t)[N]. Here, when Fm3(t)
for the BHFjti(t)[N] or for the punching speed Vp3+i(t)[mm/s] in the micro time t in the forming of the (j + l)-th time as represented by the formulas in the drawing.
Step 205: As described above, the forming condition in the forming of the (j + l)-th time is set in advance based on the forming condition of the forming of the j-th time. The forming ends if j is the total number of times jmax of forming. Otherwise, the loop returns to step 202.
The concrete blank-holding force BHFi+i [N] or the punching speed Vp^i (t)(mm/s] is calculated from the relationships of the drawing using proportional constants a, p, y and 8. The forming of the (j + l)-th time is conducted by using the thus obtained blank-holding force BHFj+i(t) [N] or the punching speed Vpjn (t) [mm/s] . This control is repeated until the number of times j of forming reaches a maximum number of times jnax of forming.
Fig. 12 illustrates the time hysteresis of the measured frictional force Fm {N] and the blank-holding force BHF [N] of when the above control method is used. The BHF control target value is varied from BHFj to BHFD+i in a range of t [sec] where the frictional force Fm3(t)IN] is greater than the upper-limit value Fcu3(t)[N] of the frictional force or the frictional force Fmj(t)[N] is smaller than the lower-limit value Fclj(t)[NJ of the frictional force, and the working of the (j -4- l)-th time is executed by using the BHF control target value BHFj+1 that is varied.
Next, a method of controlling the mold device shown in Fig. 5 will be described with reference to a flowchart of Fig. 13. In this embodiment, at least either the blank-holding force or the punching speed is controlled during the working so that the temperatures measured by the temperature sensors lie within a predetermined range during the working. The subscript i represents the number of times of control during the forming.
Step 301: Start of forming where i - 1.

Step 302: The stroke of the punch is advanced by ASi [mm]. For example, S0 = 0 [mm] when i = 1 and, hence, Si = ASi [nan). ASi {mm] is determined prior to the working.
Step 303: The temperature ?% [°C] is measured when the stroke is Si (mm).
Step 304: The temperature Tmi [*C] measured at step 303 and the temperature control target value TCi f°CJ(set in advance before the working) are compared for their magnitudes.
Step 305: When Tm4 > TcA as a result of having compared the magnitudes at step 304, there is executed at least either a process for decreasing the blank-holding force BHFi*! [N] or a process for decreasing the punch stroke increment AS^i [mm] depending upon a difference (Tmi - Tci) between the measured value and the target temperature as represented by the formulas at step 305 in the drawing.
Step 306: When Tmi Step 307: The working is conducted while effecting the feedback control in each time of forming, and the working ends when the stroke S [mm] is not smaller than the stroke SMX [mm] at the time when the working has finished, but the loop returns to step 302 when the stroke S [mm] is smaller than the stroke SM* [mm] at the time when the working has finished. The value i at this moment increases by 1.
The concrete blank-holding force BHFm [N] or the punch stroke increment ASi*i [mm] is calculated from the relationships of the drawing using proportional constants

a, 0, y and 5. This loop is repeated until the punch stroke Si [nan) reaches a punch stroke S«» If the above control is executed at regular intervals At [sec], the punching speed Vpi [nan/s] is found as Asi/At and can be controlled relying upon the punch stroke increment.
A further method of controlling the mold device shown in Fig. 5 will be described with reference to a flowchart of Fig. 14. Here, the subscript "j" represents the number of times of forming in the step of press working.
Step 401: First time of forming, j - 1.
Step 402: Measure the hysteresis TiOj (t) [*C] of the temperature at the time t [sec] of forming of the j-th time.
Step 403: The time t [sec] in the forming of the j-th time is arbitrarily divided, and a predetermined lower-limit value of the temperature is denoted by Tcl(t)[°C]. Here, when T»j(t) > Tclj{t) at each micro time t [sec], either the blank-holding force BHFj+i is decreased or the punching speed Vpjn(t) is lowered depending upon a difference {Tmj(t) - Tclj (t)) between the measured value and a predetermined lower-limit value of the temperature for the BHFj+a (t) [NJ or for the punching speed Vpjti(t)[N] in the micro time t in the formating of the (j + l)-th time as represented by the formulas in the drawing.
Step 404: A predetermined upper-limit value of the temperature is denoted by Tcu(t)[t]. Here, when Tnij(t)
in the micro time t in the forming of the (j + l)-th time as represented by the formulas in the drawing.
Step 405: As described above, the forming condition in the forming of the (j + l}-th time is set in advance based on the forming condition of the forming of the j-th time. The forming ends if j is the total number of times JMX of forming. Otherwise, the loop returns to step 402.
The concrete blank-holding force BHFi+i [NJ or the punching speed Vpj+J (t) [mm/s] is calculated from the relationships of the drawing using proportional constants a, p, y and 6. The BHF control target value is varied from BHF-)(t)[N] to BHFj+i (t) [N] and the target punching speed control value is varied from Vpj(t)[mm/s] to Vp-,+,1 (t) [mm/s] in a range of t [sec] where the temperature Tm.,(t) measured in advance in the forming of the last time is higher than the upper-limit temperature Tcu3 (t) [°C] or the temperature TiUj(t)[pC] is lower than the lower-limit temperature Tcl-,[°C], and forming of the (j + l)-th time is conducted by using the thus varied BHF control target value BHF3n(t)[N] or the punching speed control target value Vpj+i (t) [mm/s] . This control is repeated until the number of times j of forming reaches a maximum number of times junx of forming.
Next, a method of controlling the mold device shown in Fig. 9 will be described with reference to a flowchart of Fig. 15. In this embodiment, at least either the blank-holding force or the punching speed is controlled during the working so that the press reaction measured by the press reaction measuring means lies within a predetermined range during the working. The subscript i represents the number of times of control during the forming.
Step 501: Start of forming where i » 1.
Step 502: The stroke of the punch is advanced by ASX [mm]. For example, So m 0 [mm] when i = 1 and, hence, Si [mm] . ASi [ran] is determined prior to the working.

Step 503: The punch reaction Pnii [N] is measured when the stroke is Si [nun] .
Step 504: The punch reaction PII^ [N] measured at step 503 and the target punch reaction control value fNJ(set in advance before the working) are compared for their magnitudes.
Step 505: When Pmj > P^ [N] as a result of having compared the magnitudes at step 504, there is executed at least either a process for decreasing the blank-holding force BHFin [N]or a process for decreasing the punch stroke increment Asiti [nun] depending upon a difference (Pmi - Pci) between the measured value and the target press reaction value as represented by the formulas at step 505 in the drawing.
Step 506: When Pmi Step 507: The working is conducted while effecting the feedback control in each time of forming, and the working ends when the stroke S [mm] is not smaller than the stroke S»mx [nun] at the time when the working has finished, but the loop returns to step 502 when the stroke S [mm] is smaller than the stroke Smax [nun] at the time when the working has finished. The value i at this moment increases by 1.
The concrete blank-holding force BHF^i [N] or the punch stroke increment ASi+i [mm] is calculated from the relationships of the drawing using proportional constants a, P, y and 8. This loop is repeated until the punch stroke Si [mm] reaches a punch stroke SBmi [mm] at the end of the forming.

If the above control is executed at regular intervals At [sec], the punching speed vpj. [mm/s] is found as Asi/At and can be controlled by relying upon the punch stroke increment.
A further method of controlling the mold device shown in Fig. 9 will be described with reference to a flowchart of Fig. 16. Here, the subscript "j" represents the number of times of forming in the step of press working.
Step 601: First time of forming, j • 1.
Step 602: Measure the hysteresis Pm$ [t] of the punch reaction at the time t [sec] of forming of the j-th time.
Step 603; The time t [sec] in the formation of the j-th time is arbitrarily divided, and a predetermined lower-limit value of the press reaction is denoted by Pcl(t)[N]. Here, when Pioj(t) > PcljCt) at each micro time t [sec], either the blank-holding force BHF3*i [N] is decreased or tha punching speed Vp^ (t) [nrn/s] is lowered depending upon a difference (Pmj(t) - Pclj (t)) between the measured value and a predetermined lower-limit value of the press reaction for the BHFj+1(t)[N] or for the punching spocd Vp^ft) fiwrn/s] in the micro time t in the forming of the (j + l)-th time as represented by the formulas in the drawing.
Step 604: A predetermined upper-limit value of the press reaction is denoted by Pcu(t)[N]. Here, when Pmj(t) Step 605: As described above, the forming condition in the forming of the (j + l)-th time is set in advance

based on the forming condition of the forming of the j-th time. The forming ends if j is the total number of times jm«x of forming. Otherwise, the loop returns to step 602.
The concrete blank-holding force BHFin (t)IN] or the punching speed Vpj+i (t) [mm/sj is calculated from the relationships of the drawing using proportional constants a, p, y and 8.
The target BHF control value is varied from BHFj(t)[N] to BHFj+i (t) [N] and the target punching speed control value is variad from Vp3 (t) [mm/s] to Vp3+1{t) [mm/s] in a range of t [sec] where the press reaction Pmj(t)[N] measured in advance in the forming of the last time is greater than the upper-limit press reaction value PcUj(t) [N] or the press reaction Pmj(t)[N] is smaller than the lower-limit value of press reaction Pclj[N], and forming of the (j + l)-th time is conducted by using the thus varied target BHF control value BHFj+i(t)[NJ or the target punching speed control value Vpj+i (t) [mm/s] . This control is repeated until the number of times j of forming reaches a maximum number of times jMX of forming.
The punch 1 may be constructed in a split structure like the blank-holding die 3, and each divided punch may be pressed using hydraulic cylinders resulting, however, in a complex mold device and in an expensive facility. Therefore, the punch 1 is constituted as a unitary structure and is uniformly pressed by an ordinary outer cylinder. Hydraulic chambers 8 are embedded as shown in Fig. 17 in the blank-holding die 3 divided and fastened (fixed) to the surface of the punch 1 by the method described above, and pressures thereto are separately adjusted to control the blank-holding force for each of the divided blank-holding die in an inexpensive manner.
Example 1
Relying upon the above-mentioned invention, a mold device shown in Fig. 1 was fabricated as an embodiment of the invention, and a thin steel sheet was press-formed.

Piezo elements were used as the frictional force measuring means 4, and the flat plate 7 was the one made of S4SC of which the surface has been hardened.
Table 1 shows properties of the steel sheets that were used. There were used two kinds of steel sheets having a thickness of 1.2 mm and plated with alloyed molten zinc of different degrees of alloying. Table 1

(Table Remove) The forming test was conducted by continuously deep-draw-forming a square cylinder measuring 50 mm x 50 mm to investigate the forming load, and breakage and development of wrinkles of the formed articles. A square blank, measuring 100 mm x 100 mm was subjected to the forming experiment by using a blank-holding die constituted by eight die members 3a as shown in Fig, 2,
Tabl« 2 shows the results of testing after continuous forming 100 times.
As Comparative Examples/ Table 3 shows the results of a case where the blank-holding pressures were maintained constant by using a mold device without means for adjusting the blank-holding force.
(Table Remove)

(Table Remove)
In the invention 1 so executing the forming that the frictions! force was constant (0.25 [JcN/die]} for all die members, variation in the forming load was very small and a generally favorable forming was accomplished as compared to Comparative Example 1 in which the blank-holding force was set to be 20 (kN] constant (the sum of the fractional forces of 2 [kN] when the coefficient of friction was presumed to be 0.1) and Comparative Example 2 in which the blank-holding force was set to be 40 [kN] constant (the sum of the fractional forces of 4 [kN] when the coefficient of friction was presumed to be 0.1). When the blank B having a low degree of alloying was used, however, zinc adhered to the die with an increase in the number of times of forming, the friction became nonuniform, and wrinkles developed to a slight degree at the corners. In the invention 2 so executing the forming that the frictional force was lowered down to 0.2 [kN/die] in the parallel portions where the material flows in at a large rate and that the frictional force was increased to 0.3 [kN/die] at the corner portions, a favorable forming was accomplished with either material irrespective of the number of times of forming.
Example 2
Relying upon the above-mentioned invention, a mold device shown in Fig. 5 was fabricated as an embodiment of

the invention, and a thin steel sheet was press-formed. Thermocouples were used as the temperature sensors 10, and the flat plate 1 was the one made of S45C of which the surface has been hardened.
The steel sheet used for the experiment was the same as that used in Example 1.
The forming test was conducted by continuously deep-draw-forming a square cylinder measuring 50 mm x 50 mm to investigate the forming load, and breakage and development of wrinkles of the formed articles. A square blank measuring 100 mm x 100 mm was subjected to the forming experiment by using a blank-holding die constituted by eight die members 3a as shown in Fig. 6.
Table 4 shows the results of testing after a continuous forming of 100 times.
Comparative Examples were the same as those conducted in connection with Example 1.
(Table Remove)
In the invention 3 executing the forming so that the temperature was a constant 180°C for all die members, variation in the forming load was very small and a generally favorable forming was accomplished as compared to Comparative Example 1 in which the blank-holding force was set to be 20 [kN] constant (the sum of the frictional forces of 2 [kN] when the coefficient of friction was presumed to be 0.1) and Comparative Example 2 in which the blank-holding force was set to be 40 [kN] constant (the sum of the frictional forces of 4 [kN] when the

coefficient of friction was presumed to be 0.1). When the blank B having a low degree of alloying was used, however, zinc adhered to the die with an increase in the number of times of forming/ the temperature became nonuniform, and wrinkles developed to a slight degree at the corners. In the invention 4 so executing the forming that the temperature was lowered down to 150 [*C] in the parallel portions where the material flows in at a large rate and that the temperature was increased to 200 £0C] at the corner portions/ a favorable forming was accomplished with either material irrespective of the number of times of forming.
Example 3
Relying upon the above-mentioned invention, a mold device shown in Fig. 8 was fabricated as an embodiment of the invention, and a thin steel sheet was press-formed. Strain gauges were used as the press reaction measuring means 9, and the flat plate 7 was made of S45C of which the surfaces had been hardened.
The steel sheet used for the experiment was the same as that used in Example 1.
The forming testing was conducted by continuously deep-draw-forming a square cylinder measuring 50 mm x 50 mm to investigate the forming load, and breakage and development of wrinkles of the formed articles. A square blank measuring 100 mm x. 100 mm was subjected to the forming experiment by using a blank-holding die constituted by eight die members 3a as shown in Fig. 3.
Table 5 shows the results of testing after a continuous forming of 100 times.
Comparative Examples were the same as those conducted in connection with Example 1.

(Table Remove)
In the invention 5 executing the forming while controlling the blank-holding force so that the press reaction was constant {65 [kN]), variation in the forming load was very small and a generally favorable forming was accomplished as compared to Comparative Example 1 in which the blank-holding force was set to be 20 [kN] constant (the sum of the frictional forces of 2 [kN] when th« coefficient of friction was presumed to be 0.1} and Comparative Example 2 in which the blank-holding force was set to be 40 [kN] constant (the sum of the frictional forces of 4 [kN] when the coefficient of friction was presumed to be 0,1). When the blank B having a low degree of alloying was used, however, zinc adhered to the die with an increase in the number of times of forming, the press reaction became nonuniform, and wrinkles developed to a slight degree at the corners. In the invention 6 executing the forming so that the press reaction was lowered down to 20 kN in the initial stage of working where the material flows in at a large rate and that the press reaction was increased to 70 kN in the latter stage of working, a favorable forming was accomplished with either material irrespective of the number of times of forming.

We claim:
1. A mold device for press-forming a thin sheet, comprising a punch, a die,
a blank-holding die, punch drive means for inserting said punch in said
die, and blank-holding die drive means for applying a blank- holding
force to said blank-holding die, thereby to work a thin-sheet work by
pushing it into said die by said punch, said mold device for press-
forming a thin sheet comprising:
blank-holding force adjusting means for adjusting the blank-holding force applied to said blank- holding die via blank-holding die drive means;
at least either one of frictional force measuring means for measuring the frictional force acting on said work or press reaction measuring means for measuring the press reaction acting on said punch; and
control means which so controls said blank-holding force adjusting means that a measured value of said frictional force measuring means or of said press reaction measuring means becomes a predetermined value.
2. A mold device for press-forming a thin sheet as claimed in claim 1,
wherein said frictional force measuring means are disposed on the
shoulder portions of the die to measure frictional forces between said
work
and said die.
3. A mold device for press-forming a thin sheet as claimed in claim 1,
wherein said frictional force measuring means are disposed on said
blank-holding die to measure frictional forces between said work and
said blank-holding die.

4. A mold device for press-forming a thin sheet as claimed in claim 1,
wherein the blank-holding die has a plurality of die members, and said
die members are each provided with said blank-holding die drive means
so as to be independently controlled.
5. A mold device for press-forming a thin sheet as claimed in claim 4,
wherein said frictional force measuring means are disposed on the
shoulder portions of the die so as to be corresponded to the die members
and to measure frictional forces between said work and said die.
6. A mold device for press-forming a thin sheet as claimed in claim 4,
wherein said frictional force measuring means are disposed on die
members to independently measure frictional forces between said work
and said die members.
7. A mold device for press-forming a thin sheet as claimed in any one of
claims 1 to 6, wherein said frictional force measuring means has a piezo
element or a strain gauge.
8. A mold device for press-forming a thin sheet as claimed in any one of
claims 1, 2, 4, 5 and 6, wherein said frictional force measuring means
has a temperature sensor.
9. A mold device for press-forming a thin sheet as claimed in claim 8.
wherein said temperature sensor is a thermocouple.
10. A method of press-forming a thin sheet by pushing a thin sheet work into
a die by a punch to work the thin sheet work, comprising the steps of:
measuring at least either one of the frictional force acting on said work or a press reaction acting on said punch: and

adjusting the blank-holding force or the punching speed, so that a value measured in said step of measurement becomes a predetermined value.
11. A method of press-forming a thin sheet as claimed in claim 10, further comprising a step of finding, by using said measured value, a relationship between the frictional force or the press reaction measured in the preceding forming process and the blank-holding force or the punching
speed, and either the blank-holding force or the punching speed is adjusted based on said relationship that is found, so that said frictional force or said press reaction, that is measured, lies within a predetermined range.

Documents:

3371-DELNP-2005-Abstract (19-OCT-2007).pdf

3371-delnp-2005-abstract.pdf

3371-DELNP-2005-Claims (19-OCT-2007).pdf

3371-delnp-2005-claims-01-05-2008.pdf

3371-delnp-2005-claims.pdf

3371-DELNP-2005-Correspondence-Others (19-OCT-2007).pdf

3371-delnp-2005-correspondence-others-01-05-2008.pdf

3371-delnp-2005-correspondence-others.pdf

3371-delnp-2005-description (complete).pdf

3371-DELNP-2005-Drawings (19-OCT-2007).pdf

3371-delnp-2005-drawings.pdf

3371-DELNP-2005-Form-1 (19-OCT-2007).pdf

3371-delnp-2005-form-1.pdf

3371-delnp-2005-form-18.pdf

3371-DELNP-2005-Form-2 (19-OCT-2007).pdf

3371-delnp-2005-form-2.pdf

3371-delnp-2005-form-3.pdf

3371-delnp-2005-form-5.pdf

3371-DELNP-2005-GPA (19-OCT-2007).pdf

3371-delnp-2005-gpa.pdf

3371-delnp-2005-pct-308.pdf

3371-delnp-2005-pct-311.pdf

3371-delnp-2005-pct-notificatian.pdf

3371-delnp-2005-pct-search report.pdf

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Patent Number

219455

Indian Patent Application Number

3371/DELNP/2005

PG Journal Number

26/2008

Publication Date

27-Jun-2008

Grant Date

06-May-2008

Date of Filing

28-Jul-2005

Name of Patentee

NIPPON STEEL CORPORATION

Applicant Address

Inventors:

#	Inventor's Name	Inventor's Address
1	NORIYUKI SUZUKI
2	TAKUYA KUWAYAMA
3	MITSUHARU YAMAGATA

PCT International Classification Number

B21D 24/02

PCT International Application Number

PCT/JP2004/000917

PCT International Filing date

2004-01-30

PCT Conventions:

#	PCT Application Number	Date of Convention	Priority Country
1	2003-023216	2003-01-31	Japan
2	2003-325492	2003-09-18	Japan