What are the common mistakes in the production of stamping dies?

2021/07/20




In the process of stamping processing more or less all kinds of problems will appear, and a large part of these problems are caused by very low-level errors. It reflects that our basic knowledge in stamping processing is not good enough. The following summarizes some common basic mistakes in the stamping process:


1. The depth of the upper die into the lower die is too large when stamping
When stamping, the depth of the upper die into the lower die should not be too large, generally it is appropriate to just punch off the plate material, this depth should be 0.5-1mm, if the depth of the upper die into the lower die is too large, it will aggravate the wear of the upper die and the lower die, in case of poor die guidance and press movement accuracy, it will also gnaw the die edge, especially when punching thick materials, punching small holes and high-speed stamping, the depth of the upper die into the lower die should not be too deep. In order to prevent the upper die from entering the lower die too deeply, a limit post can be installed on both sides of the die to limit the depth of the upper die entering the lower die. When resharpening the upper die, also grind the limit sleeve to a same amount of resharpening.

2、The center of stamping pressure and the center of press pressure appear eccentric
The point of action of the combined punching force is called the center of punching pressure. Take a small punching machine as an example, if the center of punching pressure is not on the same axis with the center of press pressure (generally located on the axis of die shank hole), the punching slider will be subjected to eccentric load, which will lead to abnormal wear of the slider guide and the guiding part of the die, damage the movement accuracy of the punching machine, reduce the die life and even damage the die. Therefore, determining the center of punching pressure is an important task in the design of the die. For workpieces with simple and symmetrical shapes, the point of action of the stamping force is at its geometric center, and the center of pressure does not need to be calculated. For workpiece with complex shape and continuous stamping die with multiple processes, the center of stamping pressure should be determined by the method of finding the point of action of the combined force of parallel force system.


3、The punching pressure exceeds the nominal pressure of the punching machine
The selection of punching press is mainly based on the punching pressure. The principle is that the punching pressure should not exceed the nominal pressure of the punching machine. The main factors affecting the punching pressure are the material thickness and mechanical properties, the perimeter length of the stamped part, the size of the die clearance and the sharpness of the cutting edge. When stamping high-strength materials or workpieces with large thicknesses and contours (e.g. thick plate stamping), the required punching pressure is often close to or exceeds the nominal pressure of the punching machine, and the available punching machines in the factory are limited, then it is necessary to consider reducing the punching pressure from the die structure. The main methods to reduce the punching pressure are: beveled edge stamping, step-up die stamping, step-by-step part stamping, heating stamping, etc. The beveled edge punching method is to make the edge of the upper die (when punching) or the lower die (when dropping) inclined at an angle to its axis, the angle is less than 150 degrees, generally 80~100 degrees similar to the beveled edge shearing, the whole edge is not in contact at the same time, but gradually punching and cutting the material, so the punching force is significantly reduced, and the vibration and noise when punching can be reduced. Heated stamping is stamping with the material in a heated state (or red stamping). Since the shear strength of metal materials generally decreases significantly in a heated state, it can effectively reduce the stamping pressure. However, the disadvantage of this method is that the material produces oxide skin after heating, which affects the surface quality of the parts, so it is generally used for thick plate stamping or stamping of parts with low requirements for size and surface quality. In addition, a dull, chipped or unsharp die edge will also significantly increase the punching force, therefore, maintaining a sharp edge is one of the conditions for the normal operation of the stamping die. In order to keep the edge of the die sharp, the edge should be resharpened after stamping for a period of time.

4、Fine blanking die has a large cavity under the die holder when the upper die is fixed
The structure type of fine blanking die can be divided into fixed upper mode fine blanking die and movable upper mode fine blanking die. Different die structure forms require the press table structure to match with them. For the movable top mode fine blanking die, the press table is required to be a floating hydraulic table with a ring cylinder and plunger around the center part fixed. For the fixed top mode fine blanking die, the press table is required to have a plunger cylinder in the middle of the table. The characteristics of this die structure are: the upper and lower dies are fixed on the lower die holder, and the crimp ring keeps relative movement with the upper and lower dies through the transfer bar and die holder. This is because: when the upper die is pressed down, the hydraulic cylinder moves downward under the action of the transfer bar, so a big hole appears under the die holder, and all the punching pressure acts on the top of the hole, which makes the upper and lower die bend, which is very unfavorable, and the day. Under the action of increasing punching pressure, the lower part of the upper and lower die will be bent and there is a danger of pulling and cracking. In order to avoid this situation, when the punching pressure is large, special jointing rings are needed to improve the support conditions of the lower die holder to avoid the bending of the upper and lower dies due to the large shabu. As the fine blanking technology develops to large size and compound process, it needs to punch multiple holes or large inner shape contour, the punching pressure is very large, and the required crimping force and counter pressure are large, therefore, the middle of the table of the punching machine is required.


5、The movable upper mode fine punching die punches multiple holes or parts with large internal contours.
The upper and lower dies of the movable upper pattern fine blanking die are fixed directly in the center of the worktable, and the support conditions are good. The characteristics of this die structure are: the upper and lower dies are movable relative to the die holder, and the upper and lower dies are guided by the die holder and the inner hole of the crimp ring. The lower die and the crimp ring are fixed on the upper and lower die holders respectively, and the upper and lower dies are kept in relative positions through the crimp ring and the lower die, so the clearance between the upper and lower dies and the lower die is required to be smaller, and the alignment can only be guaranteed if the upper and lower dies have longer guidance and correct positioning. Therefore, the movable upper mode fine punching die cannot punch multiple holes or parts with large inner contour, because the die assembly is difficult to align and the clearance is difficult to guarantee, so it is mainly suitable for fine punching of medium and small parts.

6、The heat treatment hardness of upper and lower die of stamping die is lower than 55HRC
The upper and lower dies of stamping dies are in contact with the stamping materials, which are subject to greater force and faster wear. Therefore, the upper and lower dies of stamping dies must be heat treated, and the hardness should not be lower than 55HRC, because the higher the hardness, the higher the strength of the die and the more wear-resistant. Different die steel materials, heat treatment process and hardness are different. Cold work die steel Cr12MoV, high-speed steel W18Cr4V2, high heat treatment hardness, good hardenability, quenching deformation is small, not cracked, suitable for complex shape of the stamping parts, while T8A hardenability is good, but poor hardenability, quenching deformation is easy to crack, commonly used for punching simple shape and softer parts. As the processing of the lower die is difficult compared with the upper die, the hardness of the lower die is higher than the upper die, generally 2-3 Rockwell hardnesses higher, that is, the heat treatment hardness of the upper die is generally 58~60HRC, and the heat treatment hardness of the lower die is 60~62HRC.



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