This paper combines the experience of automobile mold design and manufacturing of Dongguan Changdong Mould Co., Ltd. for many years, analyzes the design points and main debugging experience of automobile panel moulds, and wrinkles which are easy to appear in the process of stretching of automobile panel metal moulds. The cracking phenomenon was analyzed. The importance of design and debugging of the tensile die was explained in detail from several aspects such as process analysis, automobile mold structure design and debugging.

Keywords: automobile cover, stamping die, automobile mould

The automobile cover is a thin plate stamping part that constitutes the automobile body, and has the characteristics of thin material, complicated shape, large structural size and high surface quality requirements. The stamping parts of the automobile with more complicated shapes can be completed through the stamping of multiple processes, but the quality of the covering parts is largely controlled by the quality of the drawing die, and the drawing die is the key to punching out the high-quality appearance parts. Therefore, the design and manufacturing debugging of large-size automotive molds is a difficult problem that automobile manufacturers and automobile stamping mold manufacturers must overcome.

1 Design of stamping die for large cover parts

1.1 Determination of the stamping process plan for tensile parts

The processability of the tensile part is the first problem to be considered in the preparation of the stamping process of the cover. Only by designing a reasonable and technically-stretched tensile part can it ensure that it does not wrinkle, crack, and wrinkle during the stretching process. Less cracking. The processing of the cover stretching process not only establishes good deformation conditions for the stretching process, but also provides convenience for the subsequent press working process. Therefore, in designing the tensile member, not only the direction of the pressing, the shape of the pressing surface, the shape and position of the drawbead, the rationality of the process replenishing portion, and the relationship with the next stamping process are considered.

(1) Determination of the punching direction.

Correctly determining the direction of stretching is not only a guarantee for obtaining an ideal stretched member, but also has a large influence on the arrangement of subsequent processes, and thus the direction of stretching is the first element for determining the stretched member. The main consideration when determining the direction of stretching is to ensure that the punch can smoothly enter the die and enter all corners of the tensile member; the punch and the blank should be as large as possible when starting the stretching, and the contact surface should be located in the center of the die as much as possible. The feed resistance of the binder surface should be uniform.

(2) Reasonably increase the process supplement.

There are many kinds of automobile cover parts, some cover parts are complex in shape, asymmetrical in structure, and direct forming is difficult. It is beneficial to improve the processability of the stretched parts and improve the quality of the stretched parts by providing necessary process supplements. The process replenishment part is an indispensable part of the tensile part, and it needs to be cut off after the stretching is completed. Therefore, it is determined that the replenishing part of the stretching part should follow the following principles: make the stretching depth as shallow as possible; The process supplement should be as small as possible.

(3) Determination of the shape of the binder surface.

The pressing surface is part of the metal mold process supplement. The shape of the pressing surface must be correctly determined when adding the process, so that the feeding resistance of each part of the pressing surface is uniform. In order to do this, it is necessary to ensure uniform stretching depth in all directions, because only the drawing blank is pressed against the die pressing surface in the blanking ring, and no wrinkles or creases are formed, so that the stretching member is not wrinkled. crack. When determining the shape of the pressing surface, the stretching depth should be reduced as much as possible to make the surface smooth. Since the convex mold has a certain stretching effect on the tensile hair, it is necessary to ensure that the unfolding length of the pressing surface is shorter than the protruding length of the convex mold. The material can be stretched. If the unrolled surface is longer than the punch, it may form ripples or wrinkles during stretching. Even if some of the tensile members do this, there is no guarantee that no corrugations or wrinkles will be produced at the end, because the pressing material is pressed at each moment in the whole process from the contact of the punch to the blank and the end of the stretching. The unfolding length is longer and shorter than the unfolding length of the surface of the punch. The short punch has a tensile force on the blank and a long time to form a corrugation or wrinkle. When determining the shape of the pressing surface, special attention must be paid to the angle between the pressing surface and the angle of the surface of the punch to avoid ripples or wrinkles. At the same time, it should be noted that the angle must be less than 180°. If the pressing surface is the flange part of the cover itself, the radius R of the die should be determined according to the specific conditions, because the radius of the fillet of the cover is generally small, directly As the die radius of the die is not conducive to stretching, it must be increased to not cause wrinkles or cracks during stretching. The enlarged rounded corners can be used to meet the product requirements through subsequent shaping operations.

(4) Process cut and process hole settings.

Process cuts and process holes are mainly used for some parts with severe local deformation or reversed stretched workpieces. They must be distributed on the process replenishment and can be removed in the subsequent trimming and punching process. The process cuts and process holes are permanently located at the corners where the tensile stress is greatest, and are adapted to the shape of the local raised edges for proper material flow. The position, size, number and shape need to be determined during the adjustment of the tensile die. The general process incision is punched out during the working process of the automobile mold, and sometimes it is also punched out on the blanking or blanking material to change the stress state during forming, so that the local deformation can be alleviated.

2 Cover stamping die structure design

2.1 Determination of the radius of the convex and concave corners

The size of the fillet radius of the convex and concave molds plays a large role in obtaining the ideal tensile member. Common defects in the stretching of the cover are cracking and wrinkling. When the radius of the punch fillet is too small, the bending deformation of the transition portion between the straight wall portion and the bottom portion of the stretched blank is increased, so that the strength of the dangerous section is weakened. When the fillet radius of the die is small, the tensile stress of the sidewall of the blank increases correspondingly. In both cases, the tensile coefficient increases, and the deformation resistance of the sheet increases, thereby causing an increase in the total tensile force. And the life of the mold is reduced. If the radius of the fillet of the punch and the die is too large, the deformation resistance of the sheet is small, and the fluidity of the metal is good, but the effective area of the flange is also reduced, so that the workpiece is easily wrinkled. Therefore, the convex and concave die radius must be considered in combination with the deformation characteristics of the workpiece, the drawbead and the radius of the convex and concave die fillet.

2.2 Determination of the position and quantity of the drawbead

In the design of the cover automotive mold, the direction of stretching, the replenishment part of the process and the shape of the pressing surface are the prerequisites for determining whether or not the satisfactory part can be stretched, and the setting of the drawbead is a necessary condition for the drawbead. The main function is to increase the deformation resistance of the material around the whole week or part to control the inflow of the material and improve the rigidity of the workpiece, and it is also the most effective method for preventing wrinkling and cracking of the cover.

The number and position of the drawbeads are mainly determined according to the shape of the cover and the depth of the stretch. For the parts with large tensile depth, 1 to 3 drawbeads are generally arranged in the straight part, and no pull is provided in the arc. Yanjin; when the tensile depth of each part of the same part is large, there is no drawbead in the deep part, and the drawbead is needed in the shallow part; the drawbead is set in the part with small feed resistance; The drawbead is set in a small part; the drawbead is set in the part which is easy to wrinkle. The direction of the drawbead must be perpendicular to the direction of flow of the material of the drawn blank. The drawbead is installed on the pressing surface of the presser ring, or it is installed on the die pressing surface, which does not affect the action of the drawbead. However, when the die is adjusted on the press, the drawbead is generally not polished, so the drawbead is required. It is mounted on the pressing surface of the upper pressing ring, and the drawing rib groove is arranged on the lower die pressing surface for sanding and grinding. If the pressing surface is the flange of the cover itself, the drawing rib groove on the concave mold is often polished, and the loss of the pressing surface of the concave mold is larger, which may affect the stretching depth. When the loss reaches a certain level, it needs to be repaired. At this time, the layout of the drawbead should take into consideration the convenience of maintenance. If the maintenance is convenient, the drawbead can be placed on the pressing surface of the upper crimping ring. If the maintenance is difficult, Then, the drawbead should be mounted on the lower die pressing surface, so that the loss of the die blank surface can be reduced.

3 Debugging of the cover tensile die

The mold debugging of the automobile cover tensile die plays a vital role in the quality of the parts. The debugging is good, not only can improve the quality of the stamping parts, but also shorten the manufacturing cycle of the molds. The purpose of hardware mold debugging is to improve the mold designer's intention and make up for the defects in mold design and manufacturing. Since the cracking and wrinkling are the main defects of the tensile mold, the key to the trial is to control the flow of the material during the stretching process. Because the material flows easily, it is easy to cause wrinkles, and the difficulty of material flow will cause the workpiece to crack, so it is necessary to find a balance between the two. The size of the die gap, the size of the blanking force, the number and position of the drawbeads, etc. also restrict the occurrence of cracking and wrinkling. The following is only a little experience accumulated in the process of automobile mold debugging.

(1) Adjustment of the clearance between the convex and concave molds.

In the debugging process of the tensile die, the reliability of the convex-cavity gap directly affects the quality of the tensile member. If improperly adjusted, the side wall of the tensile member is likely to wrinkle on the side with a large gap, and even a wavy defect may occur at the periphery; and the side with a small gap may cause partial sheeting due to excessive extrusion. Thin, increasing the tensile force, causing the workpiece to crack. In addition, uneven gaps may also cause streaks on the sidewalls of the tensile members.

For symmetrical or closed tensile dies, the correct method of avoiding the above phenomena during production is to install the dies on the press table.

In time, first fix the upper mold to the press slide with the fixing screws, and simply fix the lower mold to the press table (not tightening the lower mold)

Press the plate bolt), and adjust the bottom dead center position of the press slider up a bit to avoid the die end when the mold is closed, then start the press to let the slide

The block is aired several times. Finally, the slider is lowered to the bottom dead center to stop. Re-tighten the lower mold fixing bolts, and then start the test mode to ensure the metal mold convex and concave mold circumference.

The side gap is even.

(2) Adjustment of the blanking force.

The adjustment of the blank holder force mainly refers to the case where the double-action press is used for stretching. The larger blank holder force can prevent wrinkles and increase the rigidity of the tensile member, and the smaller the blank holder force causes the workpiece to wrinkle. Due to the asymmetry of some complicated tensile members, the deformation is not uniform everywhere. If the same blanking force is used, the peripheral resistance of the material is the same, which is bound to wrinkle in the small deformation of the workpiece, and pull in the severely deformed part. Crack phenomenon. The outer slide of the double-action stretching press is connected with the link mechanism at four suspension points, and each point can be mechanically adjusted to adjust the blanking force at each point to form a deformation condition favorable to the metal flow. If the working principle and structural principle of the double-action stretching press are mastered, the blanking force is adjusted according to the degree of deformation of the stretching member, so that the feeding resistance which is adapted to the deformation is maintained everywhere, and the wrinkling and pulling are effectively suppressed. The occurrence of cracking.

(3) Adjustment of the gap of the pressing surface.

In the case that the pressing force is difficult to control, the method of adjusting the stretching gap can eliminate the adverse effect of the change of the pressing force caused by the change of the material thickness on the deformation of the material. The adjustment of the gap of the pressing surface mainly has the following two methods. : As shown in Figure 1, the principle of tightening and loosening is adopted. At the die mouth, the linear bending deformation zone and the elongation deformation zone should allow the pressing surface to be tight and loose. The material thickness does not change during the deformation process. Thinning, which causes the press gap to become relatively large, which is actually equivalent to reducing the pressing force. When the sheet flows through the tight zone, the bead ring weakens the pressing action, while the tightly pressed outer pressing surface can balance the pressing force. As the material flows, the beading ring always maintains the pressing action to prevent defects such as wrinkles. As shown in Fig. 2, in the compression deformation zone, the material is in a radially stressed, tangentially stressed state of stress, and the blank undergoes compression deformation in the circumferential direction. As the material flows, the material thickness tends to increase, which causes the pressure surface gap to be relatively reduced.

Small and thus increase the feed resistance, the material is easy to break under the tensile stress. Therefore, when adjusting the gap of the die pressing surface, the inner loosening method should be adopted to eliminate the adverse effect of the increase of material thickness on the material deformation.