Drawing process and mold design and manufacturing
Drawing processes and mold design and manufacturing are the core technologies for achieving efficient, high-quality drawing production. Drawing process design involves determining the number of draws, process parameters, and process schedule. Mold design addresses mold structure, part dimensions, and auxiliary systems. Manufacturing ensures mold precision and surface quality. These three interrelated elements influence each other and require coordinated optimization to ensure a stable drawing process and qualified part quality.
The first task in drawing process design is determining the number of deep draws. The required number of deep draws is calculated based on the part’s total drawing coefficient and the material’s limit drawing coefficient. The total drawing coefficient, mtotal, = finished product diameter / blank diameter. If mtotal is less than the initial limit drawing coefficient, multiple deep draws are required. The coefficients m_i for each draw must be greater than the corresponding limit, with m_1 < m_2 < ... < m_n. For example, if a part has a total drawing coefficient of 0.3, the initial limit drawing coefficient for low-carbon steel is 0.55, and the subsequent limit drawing coefficients are 0.7-0.85, then three deep draws are required (0.55 × 0.7 × 0.8 ≈ 0.308). After the number of deep draws is determined, the diameters and heights for each draw must be assigned, following the principle of "large first, small later." This means that the deformation is greatest in the first draw, and gradually decreases in subsequent draws to ensure uniform material deformation.
Determining the drawing process parameters is crucial to forming quality, primarily including blank holder force, drawing speed, lubrication conditions, and die clearance. The blank holder force should be moderate. Too little will cause flange wrinkling, while too much will increase deformation resistance and lead to wall cracking. It is generally 10%-20% of the drawing force and can be increased for thicker or higher-strength materials. The drawing speed is determined by the material and equipment. For standard materials, the drawing speed is 10-30 m/min. For high-precision parts or parts with complex shapes, the speed should be reduced to 5-10 m/min to prevent uneven material flow. Lubrication conditions should be selected based on the material. For steel, use an emulsion, while for stainless steel, use extreme-pressure cutting oil. When lubricating, ensure that only the die and blank holder surfaces are lubricated, leaving the punch surface unlubricated to prevent slippage. The die clearance is typically 1.1-1.2 times the material thickness. Too much can easily lead to uneven part edges, while too little increases friction and worsens die wear.
The structural design of a drawing die is determined based on the part shape and production batch size. It is primarily categorized into single-step drawing dies, progressive drawing dies, and compound drawing dies. Single-step drawing dies have a simple structure and are suitable for small-batch production. They consist of a punch, die, blank holder, guide device, and die frame. The blank holder can be elastic (spring, rubber) or rigid (air cushion). Progressive drawing dies are suitable for large-scale production, integrating multiple drawing steps into a single die. Parts are connected via a carrier to achieve continuous drawing. Positioning devices (such as guide pins) are required to ensure step accuracy. Compound drawing dies can complete drawing and other steps (such as punching and blanking) in a single stamping stroke. They are suitable for complex parts and offer high productivity, but their complex structure and manufacturing challenges are significant. For example, a compound die is used to draw motor rotors, completing blanking and initial drawing in a single step , increasing production efficiency by 3-5 times compared to single-step dies.
The design of the working parts of the drawing die (punch and die) is crucial to ensuring forming quality. Their dimensions, corner radii, and surface quality must be strictly controlled. The punch diameter is equal to the part’s inner diameter (or minus the clearance between the two sides), and the die diameter is equal to the part’s outer diameter (or plus the clearance between the two sides). For multiple draws, the punch and die diameters must be calculated for each draw to ensure a reasonable amount of deformation. The corner radii of the punch and die significantly affect material flow. The die corner radius, rconcave, is typically 5-10 times the material thickness. A smaller radius increases material flow resistance, leading to wall thinning; a larger radius reduces the blank holder area and increases wrinkling. The punch corner radius, rconcave, is slightly smaller than the die corner radius, typically (0.7-0.8)rconcave. The bottom corner radius should be gradually reduced until the final draw is close to the part dimensions. The surface roughness of the working part must be below Ra0.8μm to reduce friction during material flow and avoid scratching the part surface.
The manufacturing of drawing dies requires high-precision machining equipment and advanced processes to ensure accuracy and longevity. The punch and die are typically made of alloy tool steels such as Cr12MoV and DC53. After quenching, they achieve a hardness of HRC 58-62, enhancing wear resistance. The machining process includes rough machining (milling and turning), heat treatment, finishing (grinding and EDM), and surface treatment (polishing and chrome plating). The die surface is machined using CNC grinding or wire-cut machining, with dimensional accuracy controlled to ±0.005mm and corner radius accuracy to ±0.01mm. The guides (guide pins and guide bushings) utilize precision rolling guides with a clearance of ≤0.005mm to ensure accurate centering of the punch and die. During die assembly, the coaxiality of the punch and die must be ≤0.01mm, and the parallelism of the blank holder must be ≤0.01mm/m. During die trials, the blank holder force and clearance must be adjusted to ensure a stable drawing process. Through strict design and manufacturing, the service life of the drawing die can reach 100,000 to 1 million times, meeting the needs of different batch production.
