模具外文翻译--可重构的冲压模具和冲压工艺.pdf

J. Shanghai Jiaotong Univ. (Sci.), 2010, 15(3): 313-318 DOI: 10.1007/s12204-010-1009-z A Reconfi gurable Stamping Die and Its Stamping Process SONG Ai-ping(宋爱平),WU Wei-wei (吴伟伟),ZHANG Jun (张军) (College of Mechanical Engineering, Yangzhou University, Yangzhou 225009, Jiangsu, China) Shanghai Jiaotong University and Springer-Verlag Berlin Heidelberg 2010 Abstract: A reconfi gurable fl exible poles die was developed. The die can be used to implement the process of “the multi-point pressing and forming sheet”. Sheet metal is restricted by the elastic pressing forces putting on the upper and lower surfaces of sheet in stamping process. The method is an eff ective way to enhance the buckling critical stresses and reduce wrinkling of sheet. The results of tests indicate that the die can achieve “one die brings multi-purpose” and suppress the wrinkle of sheet. The process of the multi-point pressing and forming sheet provides a practical and eff ective way for the curved sheet metal forming. Key words: stamping process, wrinkle, reconfi gurable, die CLC number: TG 386Document code: A 1Introduction Wrinkling of sheet metal seriously infl uences the forming quality and the life of die. It may make the forming process unable to carry on. So eff ectively sup- pressing wrinkle seems to be highly important1.In recent years, in order to meet the demand of light and tough sheet metal, thin sheet metal parts are widely ap- plied in various fi elds. The study of Wrinkling becomes one of the hottest research topics2. Since the near century, many scholars have brought massive studies and discussions into the instability of sheet. These researches covered from the experimental study and the theoretical analysis to the numerical sim- ulation, from the sheet drawing and the bend forming to the hydroforming, from the wrinkle forecast to the buckling control and so on1-3. Recently, numerical simulation of a forming process is applied to study on wrinkle. Depending on the mod- ern advanced research methods, numerical simulation based on the fi nite element method has become the main tool of study wrinkling in decades4. However, majority studies encircle wrinkling bifurca- tion of sheet metal and the situations of wrinkling at the present time. But these cannot bring any feasible method for controlling the wrinkles in stamping. Currently, the representative research and applica- tion of the reconfi gurable die technology is the multi- point forming technology. The basic principle is using a Received date: 2009-04-02 Foundation item: the National Natural Science Foundation of China (No. 50975249) and the Natural ScienceRe- search Fund of Yangzhou University (No. YZ2009093) E-mail: apsong series of regular and adjustable the basic bodies (poles) to construct the structure of a die. Thus, the recon- struction of die can be achieved, but the wrinkling in the sheet metal has not been suppressed5. The paper emphasizes how to control wrinkle of sheet forming. Main ideas in the paper are two: One idea is to improve the supporting condition of sheet metal and increase its buckling critical stresses. Thus, “the multi-point pressing and forming sheet” technique will be proposed. Another idea is to explore a reconfi gurable die which is combining the technique of “the multi-point pressing and forming sheet”. 2Poles Flexible Die In order to reduce the wrinkling of sheet, a method of using elastic pressing forces putting on surfaces of sheet in stamping process was presented, it can improve the stability of sheet metal. A new reconfi gurable die was developed. The die has high effi ciency in reconfi guration so that it can achieve multi-purpose use. Simultaneously the die can be car- ried out the process of “the multi-point pressing and forming sheet”. Curved sheet metal parts are used in many products such as aerospace products, military products, ships and engineering machines, the batch of parts is small. Diff erent shape requires diff erent die for forming sheet metal. It leads to high cost and long cycle6-7. To meet this situation, the reconfi gurable stamping die has been explored. The die can be applied to forming a variety of sheet parts by reconfi guring. The novel die is named as “poles fl exible die”. Poles fl exible die is a kind of reconfi gurable stamping 314J. Shanghai Jiaotong Univ. (Sci.), 2010, 15(3): 313-318 die.Figure 1 shows the structure.The die is com- bined with some discrete adjustable poles which are assembled in an array. The length of pole can be ad- justed. After analyzing 3D models data of sheet part, the length of each pole can be adjusted accurately8. 1Frame, 2Screw rod, 3Adjustable sleeve, 4Baffl e 5Jointer, 6Punch head, 7Blank-holder, 8Sheet metal Fig. 1 The structure of poles fl exible die Punch head located in the end of pole can be replaced with relevant shape head. All heads are constituted the surface of die so that it is conformed to the shape of sheet part. The die is composed with fl exible poles.In order to realize the multi-point pressing and forming sheet process, the adjustable pole is designed as a fl exible pole with the function of “pressing sheet” and “forming sheet”. Figure 2 shows the scheme of the fl exible pole. 12 3456 8 7 A A AABB B B 1Stepping motor, 2Frame, 3Screw rod 4Pressure spring, 5Adjustable nut, 6Adjustable sleeve 7Punch head, 8Pressing distance Fig. 2 The schematic of the fl exible pole It can be seen from Fig. 2 that position of sleeve is restricted by the adjustable nut in fl exible pole. Rotat- ing screw rod can control the position of sleeve. Sleeve is tightly held the adjusted nut under the eff ort of pres- sure spring. Figure 3 shows the materialization of the fl exible pole. 1Screw rod, 2Pressure spring, 3Adjustable nut 4Pressing-distance slot, 5Adjustable sleeve 6Punching head Fig. 3 The fl exible pole In the process of sheet metal forming, the correspond- ing fl exible poles of the upper and lower dies fi rstly pro- ceed to “press sheet” by the pressure spring. With the descending of the upper die, punching heads and sleeves are retracted inward. Pressing force of punching head is gradually increased under the eff ort of spring. After all punching heads and sleeves are retracted a defi nite distance (pressing distance), the adjusted nuts will re- sist sleeves and locate the sleeves. Consequently, the heads and sleeves cannot retract inward and all punch- ing heads will be combined to form the die surface, so that a sheet metal can be forced to forming. The length of pressing-distance slot has various sizes. The long pressing-distance slot may be used for construct- ing the blank-holder. Pressing force on sheet metal is produced by pressure spring inside die. Changing the corresponding coeffi cient of elasticity of spring can gen- erate desired pressing force.Pressing force on sheet metal is far smaller than forming force, and the press- ing force applied on a sheet metal is from 50 to 600 N. Figure 4 shows the upper die fi xed on the upper worktable of the press machine.The surface of the die is spherical face composed by some punching heads of poles.Poles in the periphery of die are blank- holder poles. 1Blank-holder, 2Surface of die Fig. 4The upper die with spherical surface 3The Stamping Process of the Multi- point Pressing and Forming Sheet Poles fl exible die can be realized the stamping pro- cess of “the multi-point pressing and forming sheet”. The stamping process with discrete fl exible poles can be seen in Fig. 5. It must be gone through four steps that are “holding blank”, “pressing sheet”, “pressing and forming sheet” and “shaping sheet”. At the fi rst stage of the forming, blank holders of the die achieve “holding blank” as shown in Fig. 5(a). Then both upper and lower punching heads of dies are pressing sheet, and the process is called “multi-point pressing sheet”, as shown in Fig. 5(b). In the process of stamping, some sleeves reach the designated point fi rstly and other sleeves and punching heads are press- ing sheet surfaces, as shown in Fig. 5(c). Finally, all J. Shanghai Jiaotong Univ. (Sci.), 2010, 15(3): 313-318315 sleeves and punching heads of poles reach the desig- nated point, surfaces of punching heads are combined the surface of die, the sheet metal is shaped by the die, as shown in Fig. 5(d). During the process, the sheet metal is always confi ned by punching heads. In the process of forming sheet metal, poles fl exible die composed by “fl exible poles” proceeds to “pressing sheet” and “forming sheet” at the same time. The pro- cess of “the multi-point pressing and forming sheet” can prevent buckling and wrinkling of sheet. In the stamp- ing process the force of pressing sheet is far smaller than the force of forming sheet, and the force of pressing sheet is produced by springs and the force of forming sheet is produced by press machine. Experiment results indicate that poles fl exible die has good forming characteristics and can suppress wrinkle of sheet. The upper and lower surfaces of sheet metal in the forming process are pressed by elastic poles, in order to improve the stability of sheet metal. It can be seen from Fig. 6(a) that sheet metal is pressed by loads Px, the elastic pressing forces Feare located in the upper and lower surfaces of sheet metal. Pressing forces are produced by springs in fl exible poles. The upper elastic pressing force equals to the lower cor- responding force and the condition of sheet metal main- tains plane, as shown in Fig. 6(a). 1Coupled pole, 2Blank, 3Blank-holder, 4Variant heads Fig. 5The stamping processes of the multi-point pressing and forming sheet Px PxPx Px Px z Px x x PxPx b y a x x O O b FeFeFe Fe1Fe2Fe3Fe4 Fe Fe K O L Fe FeFeFeFe z O O a y (a) Pressing forces on surfaces of sheet (c) Pressure difference by pressure springs (b) Pressure differences of surfaces Fig. 6The relationship of elastic pressing forces on sheet and buckling critical stress K is the coeffi cient of elasticity of the spring inside the fl exible pole, L is the compressed length of the spring, as shown in Fig. 6(c). When sheet metal is gen- erated defl ection , spring in the convex side of sheet can be further compressed and elastic force made by spring is increasing.Springs on the concave side of sheet will relax a bit. The elastic pressing force diff er- ence Femade by the two side springs can be obtained as Fe= 2K. When sheet metal occurs bending made by the pres- sure load Pxas shown in Fig. 6(b), it will produce force diff erences Fe1, Fe2 at the pressing points. These pressure diff erences may inhibit further bending of sheet and can suppress wrinkling of sheet as well. The upper and lower surfaces of sheet metal are ap- plied loads Fe. When sheet metal occurs buckling, it must overcome the work produced by pressure diff er- ence Feof load. The work is W = mFe/2. There m is the number of pressing points, is max- imum value of defl ection after buckling and /2 is the 316J. Shanghai Jiaotong Univ. (Sci.), 2010, 15(3): 313-318 average defl ection of m pressing points approximately. When the surfaces of sheet arent applied with pres- sure forces Fe, the sheet metal is pressed with face- parallel load Px. Critical compressive stress is calculated with energy approach, the formula of buckling critical stresses cr of sheet9-10: cr= 2D a2d ? 1 + a2 b2 2 .(1) When the surfaces of sheet are applied with pressure forces Fe, the buckling critical stress crqcan be got. crq= 2D a2d ? 1 + a2 b2 2 + 4mFe 2d , according to Fe= 2K, crq= 2D a2d ? 1 + a2 b2 2 + 2mK 2d ,(2) where D = ETd3 12(1 2), d is the sheet metal thickness, ETis the tangent modulus, is Poissons ratio. It is noted from Eqs. (1) and (2) that crqis greater than crobviously. Comparing Eq. (1) with Eq. (2), it can be discovered from Eq. (2) that the buckling critical stress of sheet metal increases along with the increasing of elastic coeffi cient of springs. Increasing the elastic coeffi cient of springs will lead to enlarge the pressure diff erence Feand improve the buckling critical stress. It indicates that the elastic pressing force applied to the surfaces of sheet can improve the buckling critical stresses of sheet metal. The process of the multi-point pressing and forming sheet can suppress buckling and wrinkling of sheet eff ectively. 4Tests The process of “the multi-point pressing and form- ing sheet” is carried out by the poles fl exible die. Poles fl exible die has the ability of quick reconfi guration. The die can be constructed by adjusting the length of poles and choosing sleeves and fi tting the suitable spring in poles and changing corresponding heads. Poles fl exi- ble die has many advantages to surpass traditional die. It can eff ectively suppress wrinkle of sheet, also it can bring new method into the sheet metal forming. Follow- ing experiments will show the characteristics about the process of “the multi-point pressing and forming sheet”. 4.1The Forming of Spherical Sheet Part A spherical sheet metal part is a typical kind of curved sheet metal part. If the sheet metal part was formed by traditional die, wrinkles often occur in the surrounding area as well as blank-holder area11.A common way for solving the wrinkles is increasing the blank-holder force. The way may decrease wrinkles of sheet. However, if the value of the blank-holder force is too large, cracking may occur in sheet metal12. How to control wrinkle of sheet is the key of the experiment. Simulation of the forming process is performed with the software DYNA-form. It is shown from Fig. 7 that the material is prone to wrinkle along the section M-N. Fig. 7Stresses status in the forming of sheet In the course of reconfi guration, the 3D model of sheet part should be analyzed, the extension elongation of each pole and the shape of each punching head are calculated fi rstly. Subsequently, it is to adjust the posi- tion of the adjustable nuts and to change the punching heads, and to construct the surface of the die. Next step is to constitute the blank-holder structure of the die. Some surrounding poles of die are used for blank- holder. The sheet metal part with the spherical shape has its spherical radius of 150 mm. Heads in the poles are customized. The upper die is installed in the upper table of the press machine, as shown in Fig.4. The lower die is in- stalled in the lower table.In the process of forming sheet, the blank sheet is placed on the working face of lower die, and then the upper die moves down. Fi- nally the upper and lower dies are matched to fi nish the forming of sheet metal. The shaped sheet metal part is shown in Fig. 8. No wrinkle exists in the sheet. This is due to the process of “the multi-point pressing and forming sheet”. Fig. 8Spherical sheet parts The material of the sheet is AISI1020, of elastic mod- ulus E = 2.06 GPa, of yield strength 0.35 GPa. When the surfaces of sheet arent applied with pressure forces, according to Eq. (1)，under the condition of the sheet metal without pressing force, the critical stresses cr can be obtained. cr 3.55 102ETd2= 34.3 MPa.(3) J. Shanghai Jiaotong Univ. (Sci.), 2010, 15(3): 313-318317 There, a, b = 80 mm, d = 1 mm, ET 0.3E. During the forming of the spherical sheet metal, the sheet can be confi ned by each fl exible pole. The surfaces of sheet are applied with pressure forces, according to Eq. (2), the critical stresses of sheet metal with pressing force can be got. crq= 359.3 MPa.(4) Comparing Eq. (3) with Eq. (4), it is easy to know that the value of buckling critical stress crqis obviously bigger than crunder the condition of no pressing forces on sheet metal. The sheet metal cannot be prone to wrinkle with setting bigger coeffi cient of elasticity of spring inside poles. Elastic pressing forces putting on surfaces of sheet can increase the buckling critical stress and decrease wrinkle of the sheet. 4.2The Forming of Hemispherical Sheet Part Hemispherical sheet metal part is an unsymmetrical sheet, as shown in Fig. 9.The opening curved face is inconvenient to set blank-holder. In the forming of the sheet metal, it is prone to wrinkle in the unsup- ported area because the sheet metal is applied imbal- ance stamping force. In order to ensure the trouble-free forming, it is necessary to increase the pressing force onto the sheet so as to prevent wrinkle of sheet. Fig. 93D model of hemispherical sheet metal By discharging a half of poles of spherical stamping die, it can be used to form the hemispherical sheet metal part. Now, it can be observed conveniently about the steps of the stamping process. The steps can be seen from Fig. 10. Figure 10(c) shows the step of pressing sheet and forming sheet.Figure 11 shows the hemi- spherical sheet with the thickness of 1.8 mm. Fig. 10The steps of the multi-point pressing and forming sheet process Fig. 11Hemispherical sheet metal part 4.3Forming of Saddle Sheet Part As shown in Fig. 12, saddle sheet metal is a kind of curved sheet metal part with double curvature. If the sheet is formed directly, wrinkle is often occurred3. The blank-holder is constructed uneasily because the peripheral boundary lines of saddle sheet part are distributed in the diff erent altitude. Currently, it is al- most impossible to form saddle sheet metal by directly using the conventional stamping die. The poles fl exible die can be used to form the sheet metal part directly withthemulti-pointpressingandformingsheet process. Figure 13 shows the fl exible die which is fi nished by re-constructing.During the forming sheet the peripheral poles are retracted corresponding dimension more than other poles.When the upper and lower dies are matched, surfaces of peripheral poles and that of middle poles are combined a continuous forming 318J. Shanghai Jiaotong Univ. (Sci.), 2010, 15(3): 313-318 surface. Then the sheet is shaped to the saddle sheet metal directly. Th