Punching pressure and process measures to reduce punching pressure
Punching force is the force required to separate the material during the blanking process. Its magnitude not only affects the load on the stamping equipment but also the service life of the die and the quality of the blanked parts. Excessive punching force can lead to equipment overload, premature die wear, or even damage, and can also cause deformation and reduced cross-sectional quality of the blanked parts. Therefore, while ensuring blanking quality, taking effective process measures to reduce punching force is of great engineering significance. This can not only extend the life of the equipment and die, but also reduce energy consumption and improve production economics.
The use of oblique-edged dies is a classic process measure to reduce punching pressure. The principle is to design the cutting edge of the punch or die to be inclined so that the shear separation of the material during the blanking process is gradual rather than instantaneous contact, thereby reducing the maximum punching pressure. The inclination angle α of the bevel edge is generally 5°-15°. Too large an angle will cause the blank to bend and deform. The oblique blade of the punch is suitable for the blanking process, and the oblique blade of the die is suitable for the punching process. The inclination direction of the oblique blade should be consistent with the feeding direction to avoid affecting the subsequent processes. Compared with flat-edge dies, bevel-edge dies can reduce the punching force by 30%-50%. For example, when punching a 5mm thick mild steel plate, the maximum punching force can be reduced from 500kN to below 300kN by using a 10° bevel edge, significantly reducing the requirements for equipment.
A stepped punch arrangement is an effective method for reducing punching forces in multi-station blanking dies. By designing multiple punches at different heights, the punching actions of each punch are staggered, preventing all punches from contacting the material simultaneously and thus distributing peak punching forces. The step height difference is generally 3-5 times the sheet thickness, ensuring that the next punch begins contacting the material after the previous punch completes its punching. The order of the stepped punches should follow the principle of “small first, large later” and “inside first, outside later,” meaning that the small-hole punch should be higher than the large-hole punch, and the inner punch higher than the outer punch, to minimize the mutual effects of material deformation. This method can reduce total punching force by 20%-40%, and is particularly suitable for blanking parts with multiple rows of holes or complex shapes, such as motor stators and rotors. Using stepped punches can reduce the required equipment tonnage from 1000kN to 600kN.
Heat blanking is an effective stress-reducing measure for high-strength, low-plasticity materials (such as stainless steel and titanium alloys). By heating the material or mold, it reduces the material’s shear strength, thereby reducing the blanking force. The heating temperature is determined based on the material’s properties and is generally between 200°C and 600°C. For example, when stainless steel is heated to 300°C, the shear strength can be reduced by approximately 40%, resulting in a corresponding decrease in blanking force. Heating methods include overall heating (furnace preheating) and localized heating (electrical heating of the mold or laser heating). Localized heating reduces heat loss and prevents changes in the overall material properties. Heat blanking requires consideration of mold thermal expansion; appropriate clearance compensation must be reserved. High-temperature-resistant mold materials (such as high-speed steel) and lubricants (such as graphite grease) should be used to prevent increased mold wear.
Precision blanking not only improves part quality but also reduces blanking forces to a certain extent. By reducing the blanking gap (typically 1%-3% of the sheet thickness), increasing back pressure, and using a toothed hold-down plate, precision blanking separates the material in a near-pure shear state, avoiding the bending and stretching deformation associated with conventional blanking and thus reducing the material’s resistance to plastic deformation. Compared to conventional blanking, precision blanking can reduce punching forces by 10%-20%, while also enabling a bright band width exceeding 80% of the sheet thickness, reducing subsequent processing steps. Precision blanking is suitable for demanding parts, such as the tooth profile of automotive transmission gears, ensuring dimensional accuracy and surface quality while reducing punching forces.
Reasonable selection of materials and optimization of the structure of blanking parts can also indirectly reduce the punching pressure. On the premise of meeting the use requirements, materials with lower strength and better plasticity are preferred. For example, low carbon steel is used instead of high carbon steel, which can significantly reduce the blanking force. The structural design of blanking parts should avoid excessive blanking circumference and sharp corners. For example, changing the right angles of rectangular parts to rounded corners can reduce stress concentration and lower local blanking force; breaking large parts into multiple small parts can reduce the load of a single blanking. In addition, reasonable control of the surface quality and thickness deviation of the sheet material to avoid fluctuations in the blanking force due to material defects can also help stabilize the punching force. By comprehensively applying the above-mentioned process measures, the punching force can be reduced to the maximum extent while ensuring production efficiency and product quality, thus achieving the goal of energy saving and consumption reduction.
