Body Wing Stamping Process Design Research

Current situation of wing panel mold development

The product structure of the body wing panel is complex and has all the quality characteristics of both inner and outer covering parts. Japan, Korea, and other mainstream automobile factories have specialized mold factories to undertake the design of the wing panel stamping process and mold manufacturing. These professional mold factories have studied wing panel technology for decades, with rich experience and mature technology, and they only develop key complex coverings such as wing panels. Other door coverings are generally not undertaken. The wing panel and the body-in-white assembly are fastened with screws, and its exposed surface and contour match the outer door panel, hood panel, light trim, etc. The gap and surface difference depends on its precision and the precision of the body-in-white welding assembly. The wing has many qualities, such as surface smoothness, rebound, the geometric accuracy of mounting lugs and mounting holes, etc. These elements of concern to OEMs are studied more by experts in the industry. There is another characteristic, called invisible characteristic, which is most perceived and experienced by stamping factories, such as poor or bad process structure design, resulting in unstable mass production, increased defect rate, narrow safety margin, etc., which are problems of dies and presses, but are caused by process modeling. In this paper, we focus on the invisible characteristics of the airfoil, i.e., the quality stability of the stamping process in mass production.

Stamping production process quality stability ability has a strong hidden nature because the host plant receives qualified products and generally can not see any piece of defective products and scrap. The defects such as rupture and neck-sucking that appear in the wing panel’s debugging stage and stamping process are only the concern of the technicians and clamp mechanics who manufacture and use the dies. They tirelessly explore and improve, paying a lot but with little success. Because stamping process parts’ wrinkling, neck absorption, or breakage rate exceeds the standard defects, the essence belongs to the product structure, and stamping process modeling design stage defects or deficiencies only be in the product design and process design stage can effectively solved.

The stamping process has to solve two problems, product structure, and process modeling problems. The process engineer drives the product design structure change based on the analysis. Once the product is defined, the process design will solve the stamping process stability and the passing rate of each process piece. Zero scrap rate of stamping parts and maintenance-free tooling are key quality characteristics of the tooling, which must be given in the process design stage. Otherwise, once the mold is finalized and delivered, once the scrap rate exceeds the standard and frequent shutdown and debugging occurs, the space to reduce the scrap rate and maintenance-free is very narrow and cannot even be changed. Unless remanufactured, even if the waste is amazing, the project has no time to start from scratch.

Wing panel product characteristics

The wing product structure is complex and requires high precision. Figure 1 shows the wing product structure, in which the front upper part matches with the light trim, the front part matches with the bumper, the upper part matches with the hair cover outer panel assembly, and the rear part matches with the outer door panel.

Fig. 1 Product structure of wing panel

Figure 2 shows the complex curved surface of the wing head mounting structure; four small mounting planes are parallel to each other; to make the mounting screws in an easy-to-operate assembly direction, the surface transition between each small piece of the plane, see Figure 2(a); wing head product structure to adapt to the bumper matching and cross member installation, the design drop is large, the characteristic plane from the outer surface of the wing, the general Y-direction distance 45 ~ 68mm, see Figure 2(b). 68mm; see Figure 2(b).

Figure 2 Wing head mounting structure

Airfoil stamping process characteristics

The depth of the head mounting surface of the wing panel is deep and cannot be stretched in one step, as shown in Figure 2(b); the depth is 48.9~69.38mm. If stretched in place, the top prism and side elevation will break and suck the neck. The depth of the early wing panel is shallow and free shaping after trimming. The shaping will cause the mounting surface to wrinkle, which is obscured by other parts and give way to the reception. In recent years, the host manufacturers have had high requirements for their products, and the mounting surface must conform to the number type and not be wrinkled even if it is obscured. To meet the high standard, the trimming edge is not trimmed at once, and the shaping process surface is reserved and trimmed off later. This surface consists of the clamping surface and process surface, shaping, shaping press first clamping surface, shaping knife down to force the process surface material movement, and finally transforming into the installation surface, that is, the product surface. The control of the clamping surface can effectively reduce or eliminate the mounting surface wrinkles. Still, not all clamping surfaces can make shaping very safe, such as the shaping process surface shown in Figure 3.

Figure 3 shaping process surface

The role of the shaping process surface

The reserved shaping process surface is designed when drawing and shaping. Generally, drawing and shaping will be no problem, only that the shaping is prone to wrinkles and breakage, sometimes extremely sensitive. If the shaping process surface is prone to wrinkles, it can be appropriately increased resistance to increase the clamping surface pressure. If the shaping process has wrinkles and cracks, prove that the size of the clamping force has nothing to do with the process modeling problems. Once the mold debugging stage, we want to optimize the solution through debugging, little improvement, or can not be improved; figure 4 shows the process surface after shaping the wing panel.

Figure 4: Process surface after shaping the wing panel

 

The wrinkles generated in the process surface outside the trim line, the trim can be trimmed off. On the surface, it does not seem to affect the quality of the product, but it is very risky. After mass production, the wrinkles may extend to the product part because the wrinkles are generated uncontrollably. As shown in Figure 4(b), the shaping insert starts to receive material at the process cross-section line and presses down to the mounting surface cross-section line to stop; this process is free to shape the process surface shift, deformation and wrinkles, theoretically, uncontrolled process, wrinkles are inevitable. Shaping process surface wrinkle generation mechanism wing panel reserved process surface is part of the design of the drawing process; its structural design should consider both the drawing plane without defects and also take into account the shaping sequence without cracking or wrinkling. The shape of the reserved process surface is spreadable or close to the spreadable surface, and the area is slightly smaller than the area of the product after shaping.

Fig. 5 Design of wing head extension

Conversely, wrinkling or rupture occurs, as CAE analysis can demonstrate, as shown in Figure 4(a), tangential pile-up thickening, instability, and wrinkling. As shown in Figure 5, the wing panel head is pulling extension shape design plastic wrinkling for two reasons: the product structure is deeper. It can not pull extension in place; the second is the process surface for the non-expandable surface. The reserve area is large.

Fig. 6 Design of the wing head drawing process

Spreadable surface for easy flow

The head mounting surface of the airfoil has a complex structure and is generally pre-stretched and shaped into place. To prevent the mounting surface from wrinkling, edge shaping is used. The shaping clamping surface is reserved and trimmed off after shaping. The shaping process surface consists of three parts: clamping surface, supplementary surface, and transition surface. Clamping surface, that is, a part of the drawing crimping surface; supplemental surface, that is, the trimming line outside to the area of the drawing parting line; this part allows shaping to move (flow) to supplement the product surface but shall not be wrinkled. Transition surface, that is, the bumper mounting surface of the process surface, shaping inserts down, upsetting, forming the product surface; figure 7 shows the wing shaping process surface.

Figure 7 wing shaping process surface

The “supplementary surface” part of the shaping process surface is a compound displacement plus rheology along the direction perpendicular to the parting line and the stamping direction under the action of the shaping insert. The clamping surface provides the appropriate tension to slow the material flow too fast and prevent wrinkles on the vertical and mounting surfaces. Figure 7(a) shows the supplementary surface between the trim line and the clamping surface designed as a spreadable surface. The parting line of the drawing process is taken straight and parallel to the matching prism of the wing and bumper. If it is designed as a curve, as shown in Figure 5(a), the “supplementary surface” is a non-expandable surface, which not only stores a larger area, but mainly destabilizes and wrinkles the three-dimensional surface under the pulling force of the clamping surface; the material flow resistance increases after wrinkling, which is transferred to the matching prism of the bumper and the vertical surface, increasing the risk of neck absorption and rupture. The wing bumper matching prongs form two sharp corners at both ends, making trimming more difficult and seemingly requiring side trimming. However, the structure is too small, and the strength of the lower die edge for side trimming is very poor, so it is not suitable for side trimming.

For this reason, the product type surface was partially modified to be suitable for positive trimming, and the subtle features of the modification were changed by shaping. Although the R-angle of the bumper-matching prism has been designed to prevent or reduce necking and rupture, the safety of the bumper-matching side elevation has been enhanced. However, to have a safe and robust batch production, the angle of the side elevation needs to be increased in the stretching process to enhance the safety margin of the stretching process. The larger the angle between the side elevation and the stamping direction, the more it can reduce the sensitivity to rupture of the prismatic R angle. Generally, this angle should be taken as 35°~45°, Figure 8 for the wing bumper matching the elevation of the drawing shape designed to bevel.

Figure 8 wing mounting surface extension process surface

 

The R angle of the prism is certain, the angle between the two faces of the prism increases, the resistance to material flow decreases, the area involved in plastic deformation near the tangent line of the R angle expands, and it can withstand a larger tensile force; or the tensile force remains unchanged, which can reduce the thinning rate of the plate. The angle of the side elevation increases but also reduces the depth of pulling extension of the bumper mounting transition surface. Reduce the risk of shaping bumper mounting surface parts see Figure 9, wing bumper mounting surface drawing extension as close as possible to the product’s shape, reducing the uncertainty of shaping class mold-free forming.

Figure 9 Bumper mounting surface

The design of the transitional surface of the wing plate head stretching and bumper mounting surface as close as possible to the shape of the product surface will bring a problem, that is, the initial stretching, the upper mold (concave mold) surface undulation, the low point of contact with the material earlier, the initial stage will cause the plate material undulation is not smooth, the surface of the ripple, not conducive to the smoothness of the exposed surface of the product. However, with the extension of the wing panel, the parts of the product are closer to the parting contour of the convex and concave die, and the upper die is far away from the lower dead point before the process initially produces unevenness with sufficient opportunity to pull away from the final defect. Figure 10 shows the effect of oil stone polishing on the exposed surface of the wing panel, with continuous polishing marks and uniform lines.

Figure 10 shows the effect of oil stone polishing on the exposed surface of the wing panel.

The smoothness of the surface of the airfoil is an indication of the unevenness or wrinkles of the sheet material in the early stage of the extension of the outer plate, as long as the surface of the wrinkled part has sufficient opportunity to unfold before the extension is closed, it will not leave “uneven traces.” Therefore, the deeper the head of the wing is, the better, and the less free shaping, even if controlled, the better. Wing shaping clamping structure design general shaping mold, design clamping surface press for the integral type, that is, the clamping surface press and the main press design. Its advantage is that the structure design is simple; the disadvantage is that it is not easy to adjust the pressure. There are many successful cases of this structure. Figure 11 shows the integral press for wing shaping.

Figure 11: Integral press for wing panel

Although CAE analysis software and technology are becoming increasingly mature, the failure modes and their constraints collected in the software database are purely idealized, so there are unexpected differences between virtual verification and realistic modes. Some elements may not be the dominant factor by common sense. Still, back in reality, a secondary factor may be the decisive factor for design success or failure, just as this element is sometimes ignored by CAE analysis. Pre-shaping the wing mounting surface is a typical stamping process design. The industry has always regarded drawing extension as the key to success or failure, so the analysis is generally in place, such as neck absorption, rupture, rebound, etc., can be treated rigorously and carefully analyzed. However, the product structure of the wing panel is special, and the final success does not depend on the drawing; the shaping process is the key element to determine the success or failure of product development. For example, the product shaping surface rebound, distortion, suction neck rupture, etc… The design of the shaping clamping surface presser and the main presser has special characteristics. The success of an individual case is supported by many elements and should be treated differently. Suppose the design of a product shaping process surface is optimal and CAE analysis is adequate, pending mold commissioning. In that case, the clamping surface is not a critical element of success or failure, reducing the sensitivity of the clamping surface pressure. Nonetheless, it is recommended to use an independent press member to facilitate mold debugging, only with a slightly more complex structural design.

Conclusion From the beginning of the design of the wing panel, the process modeling optimization design solves the difficulties and pain points of the stamping operation process and also provides a new solution to the analysis of the instability of the complex body covering parts stamping, drawing and shaping for complex, high-risk covering parts, we should take into account and analyze carefully, not to be biased, that is, drawing is more stable, try to pull more, shaping is freer, must be less whole.