With the wide application of the gooseneck bending die in the field of stamping, the manufacturing cost of the curved formed parts is greatly reduced. At the same time, the mold damage problem occurring during the use of the gooseneck bending die has become a common inertia problem in the production workshop, and the damage is caused by insufficient mold strength design and unreasonable mold design structure.

1. Process analysis of parts

Taking the side column of railway freight car as an example, the design process and force analysis of the gooseneck bending die are described in detail. Figure 1 shows the cross-section of the side column of the export railway wagon. The thickness is 12mm. The material is Q450NQR1. The high-strength and corrosion-resistant steel for railway wagons has a length of 2530mm. The process flow is: shot blasting, painting → cutting → cutting → leveling → bending → storage. 

As shown in Fig. 2, the bending process is divided into 4 steps. During the bending process of step 4, the gooseneck bending mode plays a role. Therefore, in the design process of the gooseneck bending die, the parameter design of the gooseneck bending die is mainly carried out according to step 4.

2. Calculation of bending force

  P——total bending force, N

  B——bend width, mm

  δ——material thickness, mm

  σb——tensile strength, MPa

  R——inner bending radius, mm

  The bending force required for the calculation of the part is 5930 kN, which means that the bending die needs to withstand 5930 kN of pressure from the bending machine.

  3. Gooseneck Bending Die Mold design principle

  As shown in the bending step 4 in Fig. 2, if there is no gooseneck structure part, the workpiece will interfere with the bending mode during the bending process, thereby terminating the bending and making the workpiece unable to be formed. The design principle of the gooseneck mold is to use the gooseneck part of the mold to avoid the mold design method in which the workpiece interferes with the mold during the forming process.

  4. Gooseneck Bending Die parameters determination

  As shown in Fig. 3, a schematic diagram of a gooseneck bending die, in which the eccentric size L of the gooseneck and the width dimension t of the gooseneck are the key parameters affecting the strength of the die. In order to meet the needs of forming parts, the initial design of the gooseneck width is 50mm, and the gooseneck eccentricity L should be (t/2+2.5) mm, where t is the width dimension of the mold section farthest from the center of the pressure, ie t=50mm.

5. Intensity analysis

The strength analysis of the gooseneck portion of the mold is carried out. In addition to the pressure from the bending machine, the mold is subjected to the bending moment caused by the pressure in the gooseneck portion. Select the section A-A of the gooseneck for strength analysis, and perform the column equation calculation: the strength analysis of the gooseneck part of the mold, in addition to the pressure from the bending machine, the mold is also subjected to pressure in the gooseneck part. The bending moment. As shown in Fig. 4, the analysis of the A-A stress state of the gooseneck dangerous section shows that the section width is t, the vertical distance between the pressure center of the bending machine and the centroid of the A-A section is L, and the pressure provided by the bending machine to the bending die is F, The force F0 of the workpiece reaction to the bending die, the bending moment of the section is M, and there is a possibility of breakage at the B point of the section. After analysis, a simplified diagram of the force state of the section shown in Fig. 4 A-A is drawn.

  σ1——stress generated by external force F0

  σ2——the stress generated by the bending moment

  In equation (5), W is the bending section coefficient. Since section A-A is a rectangle of height t and length h, so, w = t2h/6.

  From formula (2), M=F0×L, and substitute W and M into the formula:

  t——the thickness of the A section, mm

  L——the vertical distance between the pressure center of the bending machine and the centroid of the A section, mm

  h——the length of the bending die, mm

  Substituting σ1 and σ2 values into equation (3) yields σ3 as:

  σ3—the sum of the bending moment and the stress generated by M and external force F0

  F1——Maximum stress that can be withstood by the dangerous section A-A of the mold

  δs——the yield strength of the bending mode material

  Substituting the result σ3 of the formula (7) into the formula (8) to obtain F1

  In formula (9), α is the safety factor, usually taking the value from 1.1 to 1.2. In this calculation, α=1.15 is taken, and α and F1 values are substituted into formula (9):

  δs=450MPa, h=2530mm, t=50mm, L=27.5mm, substituted into formula (10), the F2 value is 1553t, which means that the designed A-A section with large bending force can withstand 1553t stress. The value is much larger than the bending force of the forming of the part, which can meet the forming requirements of the part.

  6. Structural optimization

  According to the above calculation results, the stress of the dangerous section A-A is 15530kN, which is much larger than the bending force of the workpiece forming 5930kN, which can meet the molding requirements of the workpiece. 

  However, in order to further reduce the labor intensity of the operator and reduce the manufacturing cost of the mold, it is necessary to optimize the design of the mold so that it can satisfy the realization of the product, reduce the 

  labor intensity of the operator, and reduce the manufacturing cost of the mold.

  According to formula (10), the stress experienced by the dangerous section A-A is related to the yield strength σs of the mold material, the thickness t of the A section, the length h of the bending mode, and the vertical distance L between the pressure center of the bending machine and the centroid of the A section. Since the mold material is usually not changed, that is, σs is a fixed value; the length of the workpiece is 2530 mm, which is also a fixed value L = t/2+2.5; so the variable in the formula is only t, and the value of t is gradually optimized:

  Recalculate by changing the value of t from 50 to 30:

  Recalculate the value of t from 30 to 25 for recalculation:

  Recalculate by changing the value of t from 25 to 20:

  According to the above calculation results, it can be seen that F32 is smaller than the maximum bending force of the part forming, F12 and F22 are larger than the maximum bending force of the forming of the part, but the mold manufacturing cost is low, which is convenient for the operator to install and disassemble the mold, so It was finally determined that the dangerous section A-A of the mold had a width of 25 mm. The thickness of the working part of the remaining part of the mold is designed according to 25mm. The curve of the gooseneck is excessively curved to avoid local stress concentration. The interface size of the mold and equipment can be designed according to the clamping mechanism of the equipment.

  7. Effect verification

  Practice has proved that the mold can withstand the stress state of the bent part, and its rigidity and strength can meet the actual production needs. In order to adapt to the main melody of today’s high-efficiency, low-cost, fast-paced parts manufacturing workshop, mold design as the source cost input is an important component of the cost of the parts. The formula and calculation process can be promoted and applied in the design process of gooseneck mold.

  8. Conclusion

  The dangerous section of the gooseneck bending die is the farthest from the center of the pressure. Under the condition of certain mold material and mold structure, the strength of the dangerous section is proportional to the thickness dimension of the dangerous section.