Automotive Molds | A Brief Discussion on the Design of Drawing Processes

I. Die Design
The die design includes: selection of stamping direction, process supplementation, generation of blank holder surface, arrangement of draw beads, arrangement of process holes and process cuts, determination of the radius of the punch and die fillets, and determination of the positioning form. We will explain each of these aspects in detail below.

1. Determination of Stamping Die
The stamping direction refers to the direction of the machine tool pressure on the blank during the die drawing process. Selecting the correct stamping direction is the first problem encountered in determining the drawing scheme. It not only determines whether a satisfactory part can be drawn, but also affects the amount of process supplementation and the scheme of each process after drawing. For drawing dies, the selection of the stamping direction should meet the following principles:

(1) Ensure that the punch can smoothly enter the die, and there should be no parts that the punch cannot contact.

(2) At the beginning of drawing, the contact area between the punch and the blank should be as large as possible, and the contact part should be located at the center of the die.

(3) The force on each part of the blank holder surface should be uniform. (4) Minimize the drawing depth as much as possible, and ensure uniform depth across all parts to prevent cracking due to excessive depth.

Usually, we can set the center of gravity of the 3D model as the origin, establish a coordinate system on the Y-axis and X-Z planes, and rotate the coordinate system on the X-Z plane with the Y-axis as the center until all surfaces do not produce negative angles with this direction. This direction is the drawing direction.

Below, we take the left and right front wheel covers as an example to see how to determine the stamping direction of the drawing process. As shown in Figures 1 and 2, rotating the coordinate system on the X-Z plane by 5° with the Y-axis as the center ensures that the punch can smoothly enter the die, without any parts that the punch cannot reach, and all surfaces do not produce negative angles with this direction. This direction can be determined as the stamping direction of the drawing process.

2. Reasonably Add Process Supplements
Automotive stamping parts are diverse, and some large body panels have complex and irregular shapes and asymmetrical structures, making it difficult to meet the requirements of the drawing forming process. Reasonably adding process supplements is beneficial to improving the manufacturability of the drawn parts and improving their quality. Process supplementation is an indispensable part of drawn parts. After drawing, the process supplementation needs to be removed, so it is also a necessary material consumption in the process. The shape of the process supplementation surface is mostly a complex spatial curved surface. It is necessary not only to determine the direction and range of the surface supplementation, but also to describe its spatial geometry. It is a creative process involving boundary conditions to ensure the smooth realization of forming. At present, the solution to this problem mainly relies on personal experience, which is determined through qualitative analysis of the part geometry and completed with the help of surface modeling software. When designing the process supplementation of drawn parts, the following design principles should also be followed:

(1) Make the drawing depth as shallow as possible;

(2) Make it as conducive as possible to the vertical trimming of the cutting edge;

(3) The process supplementation part should be as small as possible to improve the material utilization rate.

3. Design of the blank holder

The blank holder surface refers to the part outside the radius of the die corner. When determining the shape of the blank holder surface, the drawing depth should be reduced as much as possible to make the surface smooth. Because the punch exerts a certain stretching force on the drawn blank, the unfolded length of the blank holder must be shorter than the unfolded length of the punch for the material to be stretched. If the unfolded length of the blank holder is longer than the punch, ripples or wrinkles may form during stretching. There are two types of blank holders. One type is where the blank holder surface is the flange surface of the workpiece itself. The shape of this type of blank holder surface is fixed. In this case, when the blank holder surface is the flange part of the workpiece itself, the fillet radius of the die needs to be determined according to the specific situation. Because the fillet radius of the workpiece is generally relatively small, directly using it as the fillet radius of the die is not conducive to stretching and must be increased to avoid wrinkling or cracking during stretching. The increased fillet radius can be achieved through subsequent shaping processes to meet the product requirements. The other type of blank holder surface is composed of process supplementary parts. For this type of blank holder surface, the blank holder ring presses the drawn blank tightly onto the die. The blank holder surface should not produce wrinkles or cracks, thereby ensuring that the punch can stretch the drawn blank. Otherwise, ripples and wrinkles may form during the stretching process, or even cracks may occur. Therefore, the shape of the blank holder surface is generally composed of developable surfaces such as planes, cylinders, and cones. Among these, a plane blank holder surface is not only beneficial for blank forming but also easy to process, and should be used whenever possible.

4. Draw Bead Placement The main problems encountered during the drawing process of stamped parts are wrinkling and cracking. To solve this problem, draw beads must be placed to increase feed resistance and adjust the material flow rate and feed amount. The number and position of draw beads are mainly determined by the shape of the stamped part and the drawing depth. For parts with large drawing depths, draw beads are generally placed in straight sections, while they are not placed in curved sections. When the drawing depths of different parts of the same part vary significantly, draw beads are not placed in deeper sections, but are required in shallower sections. Draw beads are placed in areas with low feed resistance, in areas requiring less feed, and in areas prone to wrinkling. The direction of the draw beads must be perpendicular to the material flow direction of the drawn blank.

5. Arrangement of Process Holes and Cutouts When sheet metal is stretched beyond its limit, it will fracture. Therefore, process holes or cutouts are designed to facilitate material flow. Process holes and cutouts are primarily used for workpieces with severe localized deformation or reverse drawing. They must be distributed in the process supplements so that they can be removed in subsequent trimming and punching processes. Process holes and cutouts are often placed at corners where tensile stress is greatest and are adapted to the shape of localized protruding edges to ensure proper material flow. There are generally two methods for generating process holes and cutouts: one is to punch them out during blanking, which is generally used for situations where the local forming depth is shallow; the other is to cut them out during the drawing process, which is the most common method. During drawing, the material is fully deformed, and then the process cutout is made, utilizing the tangential extension of the material to achieve a deeper forming. Chips from the cutouts on outer sheet metal will form spots on the surface of the workpiece. Therefore, process holes and cutouts should ideally be cut in the blanking die, avoiding cutting them in the drawing die.

6. Determining the Corner Radius of the Punch and Die The size of the corner radius of the punch and die plays a significant role in obtaining ideal drawn parts. Common defects in the drawing process of large sheet metal parts include tearing and wrinkling. When the punch corner radius is too small, the bending deformation of the transition zone between the straight wall and the bottom of the drawn blank increases, weakening the strength of the critical section. Conversely, when the die corner radius is small, the tensile stress in the force transmission zone of the blank sidewall increases accordingly. Both situations increase the drawing coefficient, increase the deformation resistance of the sheet metal, and thus increase the total drawing force and reduce the die life. If the corner radii of the punch and die are too large, the deformation resistance of the sheet metal is small, and the metal flow is good, but it also reduces the effective area of ​​the blank holder, making the part prone to wrinkling. Therefore, the determination of the punch and die radii must comprehensively consider factors such as the deformation characteristics of the part, the draw bead, and the size of the punch and die corner radii.

7. Determination of Positioning Method

When formulating the drawing process for stamped parts, subsequent processes must be considered. A good positioning method is essential to ensure that the surface of the part is not damaged and that dimensional accuracy is not affected. There are three common positioning methods:

(1) Surface positioning: This method utilizes the shape of the inner and outer surfaces of the workpiece for positioning. This method is commonly used for outer covering parts.

(2) Hole positioning: This method utilizes holes or process holes on the part for positioning. It requires the holes to be spaced as far apart as possible. This method is commonly used for internal structural parts.

(3) Hole-surface combination positioning: This method combines the surface shape of the part with process holes. Many parts with complex shapes use this positioning method.

Summary: The quality of the drawing die surface design directly affects whether the product can be successfully formed, the quality of the forming, and the length of the product debugging cycle. Currently, with the development of computer technology, software such as Catia, UG, and AUTOform are widely used in mold design. The application of these software programs allows designers to directly design product profiles on computers based on their experience, and then use AUTOform to simulate the stretching process of the designed profiles. By analyzing the simulation results, they can optimize the product profile design to obtain the optimal product profile. This article provides a detailed analysis of the issues that need attention in the design of the drawing process, offering valuable guidance for the design of drawing dies.