Process measures to reduce blanking force
Blanking force is the shearing force exerted by the punch on the material during the stamping process. Excessive blanking force can lead to press overload, increased die wear, and part deformation. Process measures to reduce blanking force include optimizing the blanking method, adjusting process parameters, or improving die structure. While ensuring blanking quality, these measures can reduce the required shear force, thereby protecting equipment, extending die life, and reducing energy consumption. These measures are applicable when blanking thick or high-strength materials, or when equipment tonnage is insufficient, and are a key component of stamping process optimization.
Beveled blade blanking is a common method for reducing blanking force. The principle is to create an inclined bevel on the punch or die edge, allowing the material separation process to gradually expand from a single point to the entire contour, thus avoiding instantaneous maximum blanking force. The bevel blade’s inclination angle α is generally 5°-15°. A larger angle can easily cause part bending and deformation. For blanking, the bevel blade is typically located on the die, while the punch has a flat edge. For punching, the bevel blade is located on the punch, while the die has a flat edge to ensure part flatness. Beveled blade blanking can reduce blanking force by 30%-50%. The calculation formula is F oblique = F flat × (1-cos α), where F flat is the flat blade blanking force. For example, for a part with a flat blade blanking force of 1000 kN, using a 10° bevel blade, the blanking force can be reduced to 1000 × (1-cos 10°) ≈ 1000 × 0.015 ≈ 150 kN, significantly reducing equipment load.
Step blanking utilizes multiple punches at different heights, allowing each punch to perform its punching action sequentially, preventing all punches from shearing the material simultaneously and thus distributing the peak punching force. The step height difference is typically 3-5 times the material thickness, ensuring that the next punch begins contacting the material after the previous punch has completed its punching. Step blanking can reduce blanking force by 40%-60%, making it particularly suitable for punching multiple rows of holes or complex shapes. For example, a part requiring the simultaneous punching of ten 10mm diameter holes would require a total punching force of 800kN with a flat blade. Using a three-step step blanking method, the maximum punching force is reduced to 300kN, making it feasible for production on a smaller press. The step design emphasizes the importance of properly arranging the punch heights, positioning larger punches or those with greater punching force lower down so they can perform the punching first and reduce the resistance of subsequent punches.
Heating blanking (also known as hot blanking) reduces the shear strength of the material by heating it to a certain temperature (usually below the recrystallization temperature, such as heating low-carbon steel to 600-800°C), thereby reducing the blanking force. The shear strength of the material decreases with increasing temperature. For example, the shear strength of low-carbon steel is approximately 300 MPa at room temperature, but it drops to 150 MPa when heated to 700°C, reducing the blanking force by approximately 50%. Hot blanking is suitable for blanking hard and brittle materials such as high-strength steel and high-carbon steel. However, it should be noted that heating changes the mechanical properties of the material, and caution should be exercised for parts requiring subsequent heat treatment. The mold for heated blanking must be made of heat-resistant material (such as heat-resistant steel 3Cr2W8V) and equipped with a cooling system to prevent damage from overheating. Furthermore, the increased investment and energy consumption of heating equipment are disadvantages, making it suitable for production scenarios with large batches and hard materials.
Reducing the perimeter of the blanking part or employing segmented blanking are also effective measures to reduce blanking force. Blanking force is proportional to the perimeter of a part’s contour. By optimizing the part shape and shortening unnecessary contour length, blanking force can be directly reduced. For example, by rounding a part’s sharp corners and shortening its perimeter by 10% without affecting its usability, blanking force can be reduced by 10%. Segmented blanking divides the blanking of large parts into multiple passes, each time punching a portion of the contour, keeping the single-pass blanking force within the permitted range of the equipment. For example, the blanking of a large flange can be divided into 3-4 passes, each punching 1/3-1/4 of the circumference, reducing the single-pass blanking force to approximately 1/4 of the original, making it suitable for production on small and medium-sized presses. Segmented blanking requires the design of a dedicated positioning device to ensure the connection accuracy of each segment.
Optimizing the die gap and lubrication conditions can also indirectly reduce the blanking force. A reasonable gap can minimize the shear deformation when the material is separated, and the blanking force is 10%-15% lower than when the gap is too large or too small. For example, the reasonable gap for blanking low-carbon steel is 8%-12% of the material thickness. Within this range, the blanking force is the smallest and the cross-section quality is the best. Good lubrication can reduce the friction between the punch, die and material, reduce additional resistance, and the blanking force can be reduced by 5%-10%. The lubrication method is selected according to the material. Emulsion is used for steel parts, and extreme pressure cutting oil is used for stainless steel. When lubricating, it is necessary to ensure that the cutting edge area is fully lubricated, but avoid grease contamination of the parts. By combining the above measures, the blanking force can be significantly reduced, and efficient, low-consumption and safe stamping production can be achieved.
