Precision-Engineered Bending Solutions for Stamping Die Systems

This paper focuses on the design of precision stamping dies, specifically in relation to bending. It explains key concepts related to bending design, including process analysis and the arrangement of sequences. Additionally, it outlines the requirements for the bending process, such as bend radius, straight edge length, and bend direction. The paper also describes the standard structure of precision bending dies, along with various bending techniques. It emphasizes the importance of rational design in ensuring part quality and reducing costs.

 

1. Introduction

Molds are often referred to as the “mother of machines,” playing a crucial role in ensuring manufacturing precision. Countries like Germany and Japan place a high value on the importance and status of molds in both industry and the economy. As the global economy becomes more integrated and production processes become increasingly concentrated, mass production has gained significant importance, with molds serving as essential tools in this context.

In modern parts production, precision stamping is becoming increasingly prominent. Notably, non-planar parts constitute nearly 90% of all stamped metal products. Bent metal components are widely utilized across various sectors, including machinery manufacturing, vehicle production, aerospace, and electronics, underscoring their vital importance in multiple applications.

 

2. Bending Design Concept

(1) Process Analysis Before Design
Before starting the design process, it is essential to have a clear understanding of the following aspects regarding the bent parts: dimensional accuracy, geometric tolerance, surface quality requirements, structural shape, and material selection. If any of these requirements exceed practical limits or processing capabilities, we should promptly communicate with the customer to mitigate production risks and costs.

 

(2) The bending sequence must be reasonable
When bending, we should follow the approach of “outside first, then inside.” In this context, the position closer to the fixed point is referred to as the “inside,” while the position farther from the fixed point is called the “outside.” A smaller number in the figure indicates the outside, while a larger number indicates the inside.

By arranging the bending process with the outside first and then the inside, we can easily determine the starting position for each bend (at the original position). When designing the bending of the workpiece, it is essential to avoid using the shape position of the first bend for subsequent bends. This ensures that each bending process does not interfere with the others and helps improve any aspects of the bending process that do not meet the required standards, which, in turn, facilitates future maintenance.

 

If the inner part is bent first and then the outer part, the bending position may become uncertain due to various influencing factors. Additionally, the processes can affect each other, making it difficult to judge and maintain later on.

 

(3) Complete the production using the mold.

The shape and size of the bent part must be finalized with the mold; it should not depend on subsequent mechanical or manual adjustments. Otherwise, this will be time-consuming and labor-intensive, and the quality cannot be guaranteed. Since there are few processes that can effectively interchange workpieces made by the mold—especially small precision parts—the mold is the best way to ensure the stability and reliability of the process.

 

3. Bending process requirements

(1) Bending radius.

The bending section is illustrated in Figure 2. The bending radius refers to the inner diameter (r) of the bend. A common measurement indicator for the bending process is the relative bending radius, represented as r/t, which is the ratio of the inner diameter to the material thickness. Harder materials typically require a relative bending radius of r/t ≥ 1.

It’s important to note that a larger bending radius results in greater springback, making it more challenging to control the actual angle. Conversely, a smaller bending radius increases the difficulty of the process; the outer layer of the material can stretch severely and is at greater risk of cracking. The choice of bending radius is based on a comprehensive assessment of various factors along with customer requirements.

 

(2) Bending straight edge length

The straight edge length for metal bending (refer to point ‘a’ in Figure 2) must be at least 2.5 times the material thickness; specifically, the requirement is a ≥ 2.5t. If the bending straight edge length is insufficient, it can lead to significant springback and increased distortion, making it difficult to achieve the desired shape and dimensions. Conversely, if the bending straight edge length exceeds 2.5t, it has negligible impact on springback.

 

(3) Bending direction

When bending metal, it is essential to ensure that the bending is as perpendicular to the fiber direction of the material as possible. If the bending direction is parallel to the fiber direction, the outer surface of the bent area may develop cracks or even fractures. This can result in reduced strength and inadequate holding force.

 

4. Standard structure of precision bending mold

Metal bending can be categorized into two designs: upward and downward bending. Upward bending is a time-consuming process that has minimal springback and allows for better angle accuracy. The mold’s floating value is relatively small, resulting in stable production and improved efficiency. However, it requires the pre-setting of the pressing mechanism, which can complicate its structure.

In contrast, downward bending utilizes an unloading plate to directly press the material, making it simpler in design and easier to maintain. The following section focuses mainly on downward bending.

Figure 3a illustrates the standard structure of the mold used for downward bending, while Figure 3b provides an enlarged view of its working position. These figures contain various design details. When a wrap-around bend experiences significant springback, this issue can be mitigated by lowering the bottom dead center of the press or raising the punch to decrease the clearance between the punch and the die. This may even involve applying a slight overpressure. Alternatively, the punch can be adjusted leftward by altering the die to further reduce the clearance, thus minimizing springback and achieving the desired forming angle. In Figure 3, the parameters are defined as follows: a = 2.5t, R = t, where t represents the material thickness in millimeters (mm).

 

Before bending, the unloading insert pre-compresses the material before the punch makes contact. This pre-compression helps prevent force shifting during the bending process, ensuring that the bend occurs at the correct position. The width of the die is slightly larger than that of the punch, which is, in turn, slightly larger than the width of the CNC milling products. The working position of the die is designed to align with the inner surface of the workpiece, with a reserved angle (A) ranging from 3° to 8° for compensation.

In addition to the length of the boss, the groove should be designed to provide downward clearance (x) between the punch and the original length of the material. The height (h1) should be flush with the die’s mounting plate. The design of the compensating arc edge, with a straight edge dimension (C) of 0.1 mm, is critical for maintaining a stable bending angle. The working position of the punch aligns with the outer corner of the workpiece, with a corner length (b) of 0.1 to 0.2 mm.

It’s important to ensure sufficient strength in all other directional lengths. The effective height (h2) is calculated as follows: punch mounting plate height + unloading pad height + unloading plate height – material thickness + 0.05 mm. Additionally, the rounded corners of the punch and die, along with the R corners of the punch, must be polished to prevent damage to the surface of the workpiece during bending. This precaution helps maintain the surface quality of the bent workpiece and preserves the mechanical properties of the bent portion.

 

5. Introduction to bending techniques

(1) Double bending

To ensure that the bending angle effectively meets the requirements for mold production, a common technique involves completing two bending processes. This means that during the design phase, the angle is bent in two stages. For example, when aiming for a 90° bend, the first step is to bend the material 45° and then perform a second bend to reach the final 90°. At this point, the total bending angle is 45° + 90°. The two bends overlap significantly, with the exposed portion D of the first bend helping to compensate for any rebound angle.

By modifying the mold, the center distance ‘d’ between the two bends can be adjusted to change the size of D to meet different rebound requirements. As ‘d’ increases, D becomes larger, and the angle bends inward; conversely, as ‘d’ decreases, D becomes smaller, causing the angle to bend outward. Since the position of the second bend is fixed according to the blueprint and cannot be altered, only the position of the first bend can be adjusted through mold modifications.

This dual-bending technique addresses two challenges: first, it facilitates bending angles greater than 90°, which are difficult to achieve in a single pass; second, it is suitable for materials with poor plasticity that may crack if bent all at once. While this approach promotes stable production, it is more complex to maintain as it requires removing the mold from the machine, disassembling it, and extracting the core.

 

(2) Bend and adjust

First, bend the material to an angle slightly larger than specified in the blueprint (including the rebound angle), and then adjust the angle in the opposite direction. This method is known as “positive bend and reverse adjustment.” Both the bending and adjustment processes induce internal stress and a tendency for rebound. The purpose of “positive bend and reverse adjustment” is to partially offset the rebound that occurs during bending and adjustment, thereby reducing the risk of significant rebound.

In more extreme cases, it may be necessary to use “positive bend and positive adjustment” for structures exceeding 90°. The design for the adjustment structure is illustrated in Figure 5. The design of the auto spare parts of the adjustment die follows the same principles as previous die designs, providing a compensation angle. However, there is no need for a large groove component in this case.

The height of the slider should be slightly lower than the die fixing plate to ensure smooth operation. When the mold opens, a spring resets the slider. The position where the slider makes contact with the workpiece is crucial. If the contact point is too high, it can be overly sensitive and may damage the workpiece; if it is too low, the workpiece can become curved, resulting in significant rebound and adjustment.

In general, the design allows for a distance adjustment of approximately f = 0.3 mm. The contact angle between the slider and the punch is set at B = 45°, which simplifies calculations for the moving distance. The contact angle C between the adjusting rod and the punch is maintained between 2° to 5°, ensuring the required bending accuracy.

A bracket is installed outside the adjusting rod and is connected to the bracket with bolts. These bolts allow for adjustments to the position of the adjusting rod and secure it in place. When the mold is closed, the slider remains open due to the spring, allowing the workpiece to enter. As the upper mold descends, the unloading insert compresses the workpiece’s surface while the punch presses the slider against it. When the mold opens, the spring reopens the slider, enabling the workpiece to move with the material belt.

 

(3) Improve the structure of the bent workpiece.

Bending introduces different forces that affect the material in distinct ways. The outer material experiences tension, which can lead to cracks forming in the length direction, while simultaneously shrinking inward in the width direction. In contrast, the inner material is compressed, leading to potential wrinkles in the length direction and expansion outward in the width direction (see Figure 6a).

For workpieces that are robust and have significant width and thickness, it may be necessary to create a hole along the bending line or to form an inner groove at the edge of the bending line (see Figure 6b). This design effectively reduces the width of the material, providing the squeezed inner material enough space to recede during bending and minimizing the risk of cracking in the outer material.

Alternatively, a circular arc or a V-shaped or trapezoidal groove can be added to the inner layer of the bending line (see Figure 6c). The groove depth should be approximately one-third of the thickness of the material. This approach effectively reduces the material’s thickness, thereby lowering the risk of cracks occurring during bending.

However, both design modifications can reduce the mechanical properties of the bending section of the workpiece. Therefore, it is essential to carefully analyze performance requirements and discuss them with the customer before implementing any changes. Modifications should only be made if they do not compromise the workpiece’s functionality.

 

6. Conclusion

Precision stamping and bending are widely utilized in industrial production due to their numerous advantages, such as high production efficiency, low costs, dimensional stability, and good interchangeability. However, the quality of precision stamping and bending is affected by various factors, including mold design, material properties, and operating techniques. Therefore, it is crucial to conduct in-depth research on process characteristics and optimization methods.

Before designing, a proficient mold engineer must move beyond a workshop mentality and develop a solid philosophy for stamping mold design. They need to be well-acquainted with the structural characteristics of bending molds, master different bending techniques, and carry out a comprehensive process analysis. It is essential to minimize the use of unstable mechanisms during bending to ensure the dimensional accuracy and surface quality of the parts, as well as to facilitate production, assembly, and maintenance.

Accurate, rational, and flexible bending designs that are tailored to actual conditions contribute to smooth production and enable timely improvements.

 

 

 

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