Design of multi-layer concave drawing die
The design of multi-layer concave die drawing die is an efficient deep drawing forming technology, which is suitable for complex cylindrical or special-shaped parts that need multiple drawing, such as automobile fuel tanks, pressure vessels, etc. By arranging multiple concave dies in a stacked manner, multiple drawing processes can be completed in a single stroke, greatly improving production efficiency. Compared with the traditional single-die multiple drawing, the multi-layer concave die drawing die can reduce the process flow time and positioning error, and is especially suitable for mass production. In the early stage of design, the number of concave die layers should be determined according to the final size of the part, the material thickness (usually 1-5mm) and the drawing coefficient. Each layer of concave die corresponds to one drawing process, and the drawing coefficient increases by 0.05-0.1 in sequence to ensure uniform plastic deformation of the material. For example, a low-carbon steel part that needs four drawing can be designed with a four-layer concave die, with a drawing coefficient of 0.6 for the first time, 0.65 for the second time, 0.7 for the third time, and 0.75 for the fourth time, so as to gradually draw the blank to the target size.
The die structure consists of a multi-layered die, a punch assembly, a blank holder, a guide mechanism, and a discharge device. The die is made of Cr12MoV alloy tool steel. Each die layer is quenched to a hardness of HRC 58-62, and the working surface is polished to Ra 0.4μm or less. The interlayer clearance is controlled to 0.01-0.03mm to prevent material jamming during the drawing process. The corner radius of each die layer is gradually reduced. The initial die radius is 8-10 times the material thickness, and then decreases by 1-2 times with each subsequent layer. For a 2mm thick material, the first layer has a 16mm corner radius, the second layer 14mm, and so on, guiding the material’s gradual deformation. The punch assembly corresponds to the number of die layers. The diameter of each punch decreases according to the drawing coefficient, and the length is determined based on the number of die layers and the drawing stroke, ensuring that each punch layer can complete the corresponding drawing process simultaneously.
The design of the blank holder system is key to multi-layer die drawing dies. Each layer of the die must be equipped with an independent blank holder. Springs or nitrogen springs are usually used to provide the blank holder force, and the blank holder force gradually increases from the first layer to the last layer, with an increase of 10% to 15% to prevent wrinkling in the material. The gap between the blank holder and the die is 1.05 to 1.1 times the material thickness to ensure smooth passage of the material while suppressing wrinkles. For the deep drawing of special-shaped parts, the blank holder must match the part shape and adopt a segmented structure to ensure uniform blank holder force in all parts. In addition, the surface of the blank holder needs to have lubricating oil grooves with a width of 2 to 3 mm and a spacing of 10 to 15 mm to reduce friction with the material and lower the drawing force.
Guiding and positioning accuracy directly impact the quality of multi-layer deep drawing. The mold utilizes a composite guiding structure, with guide pins and bushings installed on each die and punch layer. The guide pins have diameters ranging from 15 to 25 mm, with a clearance of 0.005 to 0.01 mm, ensuring precise alignment of each die layer. Blank positioning utilizes a pneumatic feeder combined with locating pins, with a feeding accuracy of ±0.05 mm to prevent uneven wall thickness in the drawn part due to positioning deviations. For large parts, guide cones with angles of 30° to 45° can be installed at the entrance of each die layer to guide the blank smoothly into the next die layer and reduce resistance to material deformation. The mold base is constructed of HT300 cast iron, with a thickness of at least 200 mm. It is rigidly connected to the press table via anchor bolts to minimize vibration during the drawing process.
Debugging and maintenance must follow strict specifications. When testing the mold, first run it without load to check the coordination of each layer of the die and the punch, then use scrap material for trial drawing, and measure the size of each layer after drawing. If the wall thickness deviation exceeds 10%, the corresponding layer of the die radius or the clamping force needs to be adjusted. After every 500 operations, the iron filings between the layers need to be cleaned and the die wear needs to be checked. When the fillet radius increases by more than 0.5mm, it needs to be repaired and restored. The guide device is filled with high-temperature grease once a week to ensure smooth movement. The surface of the guide column is polished once every 1,000 strokes to maintain a roughness of Ra0.2μm. For materials that need annealing, a heating device can be added between the multi-layer dies to control the material temperature at 200-300℃ (for low-carbon steel), reduce the yield strength, and reduce the drawing force.
