Total drawing process force
Total drawing force refers to the total force required to complete the drawing process, including drawing force, blank holder force, and unloading force. It is an important basis for selecting press tonnage and evaluating mold strength. Accurately calculating total drawing force ensures sufficient press power output, preventing equipment damage or part defects caused by overload. It also provides a force basis for mold structural design, ensuring sufficient mold strength and rigidity. The magnitude of total drawing force is closely related to material properties, part size, drawing times, and process parameters, and must be determined through a combination of theoretical calculation and practical verification.
Drawing force is a core component of total process force. It refers to the force required by the punch to pull the material into the die to form the cylindrical shape. Its calculation formula is: F pull = K × π × d × t × σb, where K is a correction factor (K = 0.5-0.8 for initial drawing, K = 0.8-1.0 for subsequent drawing), d is the diameter of the drawn part, t is the material thickness, and σb is the material’s tensile strength. The value of the correction factor K must take into account the drawing factor and material plasticity. The smaller the drawing factor (greater deformation), the larger the K value; the greater the plasticity of the material, the smaller the K value. For example, for low-carbon steel (σb = 450 MPa), with an initial drawing depth of d = 100 mm, t = 2 mm, and K = 0.6, then F pull = 0.6 × 3.14 × 100 × 2 × 450 ≈ 169,560 N ≈ 170 kN. For non-circular parts, its circumference needs to be converted into an equivalent diameter for calculation. For example, the equivalent diameter of a rectangular part is d = 2 × (length + width) / π, and then substituted into the formula.
The blank holder force is the force required to prevent wrinkling on the flange edge, accounting for 10%-30% of the total process force. It is calculated as: F press = A × q, where A is the area of the flange (A = π × (D² – d²)/4, and D is the blank diameter), and q is the unit blank holder force (MPa). The value of the unit blank holder force q depends on the material type and thickness: for mild steel, q = 2-3 MPa, for stainless steel, q = 4-6 MPa, and for aluminum alloy, q = 1.5-2.5 MPa. The thicker and stronger the material, the larger the q value. For example, if the diameter of a low-carbon steel blank is D = 200mm, the drawn diameter is d = 100mm, and t = 2mm, then A = 3.14 × (200² – 100²) / 4 ≈ 23550mm² = 0.0235 5m² , and F pressure = 0.02355 × 2.5 × 10⁶ ≈ 58875N ≈ 59kN . The blank holder force must be evenly distributed. For large molds, multi-cylinder air cushions or nitrogen cylinders are used for segmented control to ensure consistent pressure across all areas.
The unloading force is the force required to remove the drawn part from the punch, typically 3%-5% of the drawing force. It is calculated as: Funload = Kunload × Fpull, where Kunload is the unloading coefficient (0.03-0.05). For parts with flanges or complex shapes, Kunload should be higher (0.05-0.08); for shallowly drawn parts, Kunload should be lower (0.02-0.03). For example, for a cylindrical part with a drawing force of 170 kN, Funload = 0.04 × 170 = 6.8 kN. The unloading force is related to the surface roughness of the punch and lubrication conditions: the smoother the surface and the better the lubrication, the lower the unloading force. When using an elastic unloading device, ensure that the total spring or rubber force exceeds the unloading force, and allow a 20%-30% margin to prevent unloading problems.
The calculation of total drawing process force requires considering the superposition of various forces. For single-action presses, the total process force, Ftotal, = Fdraw + Fpress + Fun, because the blank holder and drawing forces act simultaneously. For double-action presses, the blank holder force is provided by the outer slide, and the total process force, Ftotal, = Fdraw + Fun. In this case, the press’s inner slide tonnage must satisfy Fdraw + Fun, and the outer slide tonnage must satisfy Fpress. For example, in the above example, Fdraw = 170kN, Fpress = 59kN, and Fun = 6.8kN. For a single-action press, Ftotal = 170 + 59 + 6.8 = 235.8kN. A press with a nominal force of 250kN or higher should be selected (allow at least 10% margin). For a double-action press, the inner slide must meet 176.8kN or higher, and the outer slide must meet 59kN or higher. When multiple deep drawing is performed, the total process force is calculated based on the force of the largest single deep drawing, because the drawing forces do not occur at the same time.
Factors affecting the total drawing process force and the corresponding adjustment measures must be flexibly applied in actual production. Material mechanical properties are the primary factor. High-strength steel has a drawing force 50%-100% greater than low-carbon steel, necessitating the use of a larger tonnage press. Increasing drawing speed increases process force. When speeds exceed 100 strokes/min, process force must increase by 10%-15%. Therefore, high-speed drawing requires appropriate reduction in deformation. Excessively small die clearance can increase drawing force by 20%-30%, requiring adjustment through trial punching to a reasonable range. For large, complex parts, numerical simulation software (such as ABAQUS) can be used to calculate the process force distribution, identify critical stress areas, and strengthen the die structure. During trial punching, the process force should be measured using a pressure sensor and compared to the calculated value. If the deviation exceeds 10%, the material parameters or calculation method should be reviewed to ensure safe and reliable press selection. Through precise calculation and dynamic adjustment, optimal control of the total drawing process force can be achieved, ensuring production safety and efficiency.
