Design of deep drawing convex and concave dies
The design of the punch and die is a core component of drawing die development. Its quality directly determines the forming accuracy, surface quality, and die life of the drawn part. The design must comprehensively consider part shape, material properties, production batch size, and equipment requirements. By optimizing the working part structure and selecting appropriate materials and heat treatment processes, a stable and efficient drawing process can be ensured. The punch and die must be designed in tandem to ensure uniform clearance and precise guidance to accommodate the complex stress variations during material plastic flow.
The core of punch design is ensuring strength, rigidity, and guiding accuracy. Depending on the complexity of the drawn part, the punch structure is categorized into either integral or modular types. Integral punches are suitable for simple cylindrical parts, offering a compact design and ease of manufacture. They utilize a stepped mounting system with a precision of H7/m6 to the mounting plate, ensuring secure installation. Modular punches are suitable for large or irregularly shaped parts. They consist of a working head, connecting rod, and mounting base, connected by bolts and pins, facilitating replacement of worn parts and reducing maintenance costs. For example, automotive panel drawing punches utilize a modular design. The working head is made of quenched Cr12MoV steel, while the connecting rod is quenched and tempered 45 steel, ensuring wear resistance while conserving precious materials. The punch working surface must be polished to a Ra of 0.4μm or less to minimize material flow resistance and avoid scratching the part surface.
Die design must prioritize addressing material entry resistance and smooth discharge. Based on structural form, dies can be categorized as monolithic, modular, and inlay-type. Monolithic dies are suitable for small and medium-sized parts. Made from forged steel, they offer high corner radius precision and excellent surface quality, but they also offer low material utilization. Inlay dies are constructed from multiple modular components secured by locating pins and bolts, with inter-module clearances of ≤ 0.01mm. These dies are suitable for large parts (such as washing machine drums), reducing machining complexity and equipment investment. Inlay-type dies utilize carbide inserts in areas prone to wear to extend service life. For example, WC-Co alloy inserts in the die mouth of a stainless steel drawing die can increase service life by 5-10 times. The die discharge channel should be tapered, with a taper of 5°-10° and a surface roughness of Ra 1.6μm or less, to ensure smooth part discharge and prevent buildup and jamming.
The design of the guide system for the punch and die is crucial for ensuring uniform clearances. Guide pins, guide bushings, and guide plates are commonly used. Small and medium-sized molds utilize rolling guide pins and guide bushings with a clearance of H7/h6 and a guiding accuracy of ≤0.005mm to ensure precise alignment between the punch and die. Large molds utilize guide plates, with wear-resistant plates (made of hardened T8 steel, HRC50-55) installed on both the upper and lower die holders. The clearance is 0.01-0.03mm, and the guide surface length is no less than 1.5 times the guide pin diameter. The guide system should be symmetrically positioned at the four corners of the mold, with at least four guide pins. For extra-long molds (>3m in length), additional intermediate guide pins are required to prevent bending and deformation. Furthermore, high-precision drawing dies require pre-guiding devices (such as pre-fitting the guide bushing to the punch) to establish guidance during the initial die closing phase to prevent skew collision between the punch and die.
The cooling and lubrication systems for both the punch and die are designed for high-speed deep drawing or forming high-strength materials. When punching speeds exceed 300 strokes/min, cooling channels must be installed within the die using circulating water. The inlet water temperature is controlled at 20-25°C and the outlet water temperature does not exceed 40°C to prevent die overheating and hardness loss. The lubrication system includes an automatic oil sprayer that evenly applies lubricant (an emulsion for steel sheets and a specialized aluminum drawing oil for aluminum sheets) to a film thickness of 5-10μm at the die inlet, reducing the coefficient of friction. For example, high-speed drawing dies for motor stators and rotors are equipped with a servo-controlled oil spray system that automatically adjusts the oil spray rate based on the punching speed, ensuring effective lubrication while minimizing oil waste. The cooling and lubrication system design must be integrated with the die structure to avoid compromising part formation and die strength.
Targeted design measures are required for punch and die designs of specialized drawn parts. For the deep drawing of conical parts, the die mouth requires a transition radius (rconcave = 10-15t), and the concentricity error between the punch and die tapers should be ≤0.02mm to prevent material deflection. For the deep drawing of spherical parts, the punch surface should be machined with annular oil reservoirs 0.5-1mm deep and 5-10mm apart to ensure uniform lubrication. The die should adopt a modular structure consisting of multiple sector-shaped blocks, with spring-adjusted pressure to prevent wrinkling. For the deep drawing of rectangular parts, the corner radius of the die should be 50%-100% larger than that of the straight edge, and a localized thickening design (20%-30% increase in thickness) should be used to improve wear resistance. Through personalized design, the forming challenges of specialized parts can be effectively addressed, expanding the application scope of the deep drawing process.
