Drawing die and die structure design
The structural design of the punch and die directly impacts material flow, part quality, and die life during the drawing process. Their design must be tailored to the shape, size, material properties, and production batch of the part being drawn. The punch and die structure consists of a working portion (radii, cutting edges), a fixed portion, and auxiliary components (vents, positioning devices, etc.). The design of each component must be coordinated to ensure a stable and efficient drawing process.
The structural design of the punch requires key considerations regarding the corner radius of the working portion, the fixing method, and air venting performance. The punch’s corner radius, r , is a key parameter influencing material flow. A too small radius increases material flow resistance, leading to increased wall thinning. A too large radius reduces the contact area between the punch and the material, reducing drawing stability. Generally, the punch’s corner radius is 0.7-0.8 times the die’s corner radius. For the initial drawing, r is set at (5-10) t (where t is the material thickness). This value is gradually reduced with subsequent draws, reaching a value close to the part’s bottom corner radius for the final draw. For example, for an initial draw of 2mm material thickness, the punch’s corner radius, r , is 10-20mm. Punch fixing methods include step fixing, bolt fixing, and press-fit fixing. Press-fit fixing (fitting H7/m6) is commonly used for small punches, while large punches are secured with bolts and pins to ensure a secure fit and prevent loosening during stamping. In addition, an exhaust hole with a diameter of 3 to 5 mm must be set inside the punch, located at the center of the top of the punch or near the rounded corner. It is used to exhaust the air between the punch and the material during the drawing process to prevent the formation of an air cushion that may cause pits or cracks on the surface of the part.
The structural design of the die requires careful attention to the corner radius, inlet shape, and discharge channel of the working part. The die’s corner radius, r , is one of the most important parameters in a drawing die. It directly determines the resistance to material entering the die. Too small r , resulting in severe material bending and deformation, resulting in severe thinning and scratching; too large r , reducing the blank holder area and increasing the risk of wrinkling. The die’s corner radius, r , for the initial drawing is typically set between 5 and 15 mm, and gradually decreases with subsequent drawing. For example, for a 1mm material thickness, r is 8 to 15 mm for the initial drawing, and 5 to 8 mm for the secondary drawing. Die inlet shapes include cylindrical, conical, and curved. Cylindrical inlets are suitable for general drawing, while conical inlets (with a taper angle of 30° to 45°) are suitable for deep drawing, reducing material flow resistance. Curved inlets are suitable for high-precision parts and can reduce surface scratching. The discharge channel of the die must be smooth and unobstructed. For the die that is drawn after blanking, the channel diameter should be 5~10mm larger than the inner diameter of the die, and the slope should be 10°~15° to facilitate the smooth discharge of parts.
The design of the mating structure of the punch and die must ensure accurate guidance and uniform clearance. For small and medium-sized drawing dies, a one-piece die and stepped punch are typically used, offering simple construction and ease of manufacture. For large or complex drawing dies, a modular die and combined punch can be used, facilitating machining and maintenance, thereby reducing manufacturing costs. For example, the die of an automotive panel drawing die utilizes a modular structure, comprised of multiple modules joined together by bolts and pins. Each module can be machined and replaced individually. The punch and die are guided using a guide pin and sleeve structure, with four to six guide pins located at the four corners of the die. The clearance between the guide pins and sleeves is H7/H6, ensuring a centering accuracy of ≤0.01mm between the punch and die. For high-precision drawing dies, a guide plate or sleeve can be placed between the punch and die to further improve clearance uniformity.
The punch and die structure design for specially shaped drawn parts must adapt to their deformation characteristics. For spherical parts, the punch should be spherical, and the die corner radius should be sufficiently large (r concave = (10-20) t), with a large gap to prevent excessive material thinning. Furthermore, the punch surface should be polished to Ra 0.4μm or less to reduce friction. For conical parts, the taper angles of the punch and die should match the part, and a transition radius should be provided at the die entrance to prevent material tearing at the tapered surface. For rectangular parts, the corner radius of the punch and die should be larger than that of the straight-side portions. The corner radius of the die should be r concave = (1.5-2) × the straight-side die corner radius to mitigate severe deformation at the corners. For example, for a rectangular part with a straight-side die corner radius of 5mm, the corner radius should be 8-10mm to ensure smooth material flow.
The material selection and heat treatment of the punch and die are also crucial components of structural design. Common materials include alloy tool steels such as Cr12MoV, DC53, and SKD11. For high-volume production or deep drawing of high-strength materials, cemented carbides (such as WC-Co alloy) are required to improve wear resistance. The working surfaces of the punch and die must be hardened to a hardness of HRC 58-62, while maintaining a certain toughness in the core (HRC 30-40) to prevent brittle fracture. The working surface roughness should be controlled below Ra 0.8μm. Polishing or chrome plating (coating thickness 0.01-0.03mm) should be performed as necessary to reduce friction and adhesion. For example, chrome plating of the punch and die surfaces of stainless steel drawing dies can increase their service life by 2-3 times and significantly improve part surface quality. Through appropriate structural design, material selection, and heat treatment, the punch and die can meet the requirements of different drawing processes, ensuring part quality and production efficiency.
