Viewed from the side of the new Bora sedan, the two “shoulder lines” running across the side elevation are simple and three-dimensional, making the body more slender. The four-door outer panel is an important part of the stamping parts on the outer surface of the car body. The quality of the shape and quality of the parts directly affects the sensory impression. The appearance of the new Bora sedan is shown in Figure 1.
1.1 Four-door product features
The main feature of the four-door outer panel surface of the new Bora body is the waistline, which has a strong visual effect. This is the Volkswagen model with the sharpest angle and the deepest model (Figure 2). In order to highlight the effect of the main waistline, a short and shallow sharp ridgeline is set about 50 mm below the main waistline, and this sharp ridgeline runs through the door handle. In the past, the waistline and ridgeline of Volkswagen models avoided the door handle as much as possible, because the molding shape of the door handle was complex and the material was deformed greatly; the surface quality after forming was prone to dark pits and wave defects. The sharp edge of the new Bora model passes through the handle, and the R angle of the sharp edge is 0.5 mm. The new Bora model also has a sharp edge at the bottom of the door connecting the front and rear wheel covers. When the part is formed, the sheet material cannot slide greatly, otherwise it will cause slip line defects, and in severe cases, the part will be cracked and scrapped (Figure 3). The stamping process, stamping direction and stamping angle of the door are formulated after repeated simulation and improvement. It requires different solutions from the design of the mold, the molding process of the part, and the structure of the mold. This time, in order to improve the product quality of the parts and reduce the slip line, it is realized by changing the stamping angle and increasing the shape of the process convex hull, and reducing the production efficiency to ensure the best body quality.
1.2 Manufacturing characteristics of four-door sharp ridges
When machining the waistline and sharp ridgeline of the four-door outer panel, ordinary profile processing methods cannot be used, and special processing techniques must be used. Especially the sharp ridges put forward higher requirements on the quality of mold casting. First, the processing surface of the casting should not have defects such as blisters and cracks. Second, the alloy ductile iron material is used. This material has good wear resistance due to its self-stress hardening characteristics. performance. Its quenching performance is also superior, suitable for air-cooled flame quenching. The flame quenching hardness can reach HRC50~57, which is an ideal material for drawing punch, drawing die and blank holder casting.
The main waistline punch fillet of the four-door outer panel is generally processed with a reduction of 1 mm according to the product size. When processing the punch at the position of the sharp edge line, it is necessary to strictly follow the processing technology of the sharp edge, and the fillet of the sharp edge of the punch is 0. (Figures 4 and 5). After the mold is processed, the ridge line of the punch can only be polished with a whetstone. During the training of the concave mold, the 15 mm strong pressure coloring on both sides of the ridge line should be uniform, and the coloring rate should be more than 90%. If the strong pressure coloring is not uniform, the position of the ridge line on the surface of the workpiece will have obvious waves, which will seriously affect the surface quality.
1.3 Four-door matching relationship
There are not many parts in the car body that match the four-door outer panel, but the requirements for matching clearance and flat dimensions determine the quality level and quality of the whole vehicle. The front door at the A-pillar matches the fender, the clearance is 3.5+0.5 mm, and the flatness is 0 (-1 0); the front door at the B-pillar matches the rear door, the clearance is 4.2+0.5 mm, and the flatness is 0 (0 1) ; The C-pillar rear door matches the side panel, the clearance is 3.7+0.5 mm and the flatness is 0 (0+1) (Fig. 6).
2 Drawing die features
2.1 Process characteristics of door outer panel
The forming depth of the outer panel of the door is shallow, and the drawbead is generally square. Extend the sheet to the desired plastic deformation. In the simulation, there is a phenomenon of large local springback of the workpiece. Therefore, during the process design of the mold, focus on the springback problem, do a good job in the early springback compensation, uniformly increase the height of the convex model surface at the appropriate position, and reduce the amount of springback (Figure 7).
According to the previous experience of door outer panels, local profile compensation settings are performed in the parts where the molding changes of the parts are large, and process convex hulls are added outside the forming areas of the punch and die parts to improve the surface quality of the drawn parts. Do the basics for the next step.
During the simulation, it was found that the outer panel of the new Bora door has three ridges, and the main waistline will have a slip line when it is formed. During the design, through actuarial simulation and many debugging experiments, continuous optimization and improvement, the top drawbead was changed to light feed (Fig. 8) to meet the quality requirements and ensure that the two sharp edges have no slip line defects.
It can be seen from the simulation diagram (Fig. 9) that the door buckle of the drawing die is deeply formed, and there will be quality defects such as necking, waves and dark pits on the surface. In order to alleviate such problems in debugging, the split movable door buckle hand punch and die are generally used in the mold structure. During debugging, by changing the height of the punch and die, the punch and die are rounded. optimization to obtain better surface quality. In order to better solve the problem of waves and dark pits around the door handle, the coloring rate of the punch and die at the door handle position can only reach more than 90% during the training of the drawing die. Under the condition of uniform coloring and strong pressure, the surface quality of the door handle position of the workpiece is obviously improved, and the quality is stable in mass production.
2.2 Requirements for trimming angle
The maximum angle of die punching is 15. , if the hand punching and trimming angle of the door buckle is greater than 15. , it is necessary to make a process profile in advance at the punching angle position of the drawing punch, and make the part with a large punching angle into a reasonable angle, and then punch it. The cutting edge of the die cannot be a wave-type cutting edge. When trimming, it must be punched at the same time (Figure 10). Finally, the angle required by the product is achieved by the post-pressing plate shaping.
2.3 Mold structure setting of the four corners of the door outer panel and the door buckle hand
During the flanging process and pressing, the dimensions of the four corners of the door outer panel need to be changed frequently according to the quality of the pressed parts and the size of the gap. In the actual working process of other models, the four corners of the trimming punch are designed in the form of removable inserts to facilitate the modification of the size of the trimming edge of the punch (Figure 11).
The quality of the door handle directly affects the quality level of the door panel. Trimming is an important process in the production of door buttons. Generally, the die and the pressure plate are made of separate blocks, which is convenient for repair and adjustment of defects. When the door button is punched and trimmed, the parts are affected by the punching force, and the defects generated in the drawing will be aggravated again. The optimization can be achieved by applying strong pressure to the defect position of the hand button and local increase of the profile. The concave die and the pressure plate screw are fastened back to ensure that the profile around the handle is intact (Figure 12).
It should be noted that, first of all, it is necessary to ensure that the workpiece and the punch are docile, and the trimming positioning must be accurate and stable, so as to meet the needs of continuous and rapid production. Secondly, avoid the interference between the rounded corners formed at the handle of the door and the punch or the rounded corners of the pressure plate. It is also necessary to check the compliance rate and coloring rate between the punching convex model surface and the punching position of the workpiece, and check the angle of the punch cutting into the profile surface to eliminate the deformation force on the workpiece during punching.
2.4 Features of B-pillar Clamping and Flanging of Back Door
For the flanging of front door products, the quality of the flanging facade can be guaranteed by adjusting the flanging sequence of the flanging inserts and the flanging gap at the position of the main ridge line; the B-pillar flanging of the back door product is a two-layer desktop flanging. The first time is to flange into an L shape, and the second time is to flange again vertically at the 6 mm position on the table that was turned out for the first time (Figure 13).
Because of the small angle between the main ridges on both sides of the new Bora, the main ridges will produce serious aggregates during flanging, making the flanging façade defective. In order to solve the problem of the flanging defect of the B-pillar of the rear door, the new Bora mold adopts the method of clamping and flanging. That is, during the first flanging and shaping, a movable pressing device is added to the outside of the punch. During the flanging work, the flanged sheet has been formed in the state of clamping between the flanging insert and the movable pressing device. The flow of the sheet material is well controlled, and the facade quality of the parts is improved to meet the quality requirements. Comparing the clamping flanging at the B-pillar position of the rear door by punching CAE simulation and the conventional flanging state, it can be seen that the quality of the parts is improved (Fig. 14).
From the comparison diagram in Figure 14, it can be clearly seen that at the first contact, the clip flanging sheet is flat, while the conventional flanging sheet is slightly deformed. When it is 5 mm from the bottom dead center, the clamping and flanging sheets are good. The conventional flanging sheet has serious distortion. At the bottom dead center, the clip flanging sheet has no defects, while the conventional flanging sheet has obvious stacking.
Because it is a whole type of flanging, when the pressing surface of the clamping flanging is at the bottom dead center, it must be stationary. After the flanging insert returns, the workpiece is taken out, and the movable pressing surface of the clamping material can return. Otherwise, the parts will be deformed due to the lifting of the movable binder surface. Therefore, the power source of the movable binder surface is required to be controllable. Generally, in the design, the air cushion of the press is used to lock, or the controllable air cylinder and the controllable nitrogen cylinder, and the movable pressing surface is stationary at the bottom dead center, etc., to meet the requirements of clamping and flanging. The lower mold of the rear door and the upper mold of the rear door are shown in Figure 15.
Due to the limitation of the structural space of the mold and the influence of the angle of the ridge line, the movable binder surface can also be designed as a partial binder block, as shown in Figure 16.
a. The springback profile compensation setting is performed on the drawing punch and concave die surface, and the process convex hull is added in the forming area, which can improve the surface quality of the workpiece.
b. The pressing plate at the handle of the door outer panel adopts the form of insert, which is more convenient for the maintenance of the mold and the adjustment of the quality of the part.
c. When the main feature line angle of the workpiece is small and the depth is large, the conventional flanging method cannot obtain good quality. After the flanging with the clip material for the test, the problem of material stacking at the edge of the flanging is solved to ensure the surface quality of the workpiece.
Through the debugging of the new Bora door outer panel mold, experience has been accumulated for the debugging of new models in the future. It is necessary to conduct meticulous and in-depth research on the mold technology and structural design of the door outer plate, and absorb advanced technology and production experience to improve the surface quality and dimensional position accuracy of the parts, so as to achieve stable, efficient and continuous high-quality production.
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