Design of blanking and drawing compound die
The design of a blanking-drawing composite die integrates the blanking and drawing processes within a single die. This technology is suitable for the production of cup-shaped and cylindrical parts, such as small containers and covers. It can reduce process turnaround time and improve production efficiency by over 30%. Part coaxiality errors are less than 0.1mm, making it suitable for mass production. Initially, the blank size is determined based on the part’s desired draw height, diameter, and material properties. The blank diameter is calculated using the drawing coefficient. A typical initial drawing coefficient is 0.55-0.65. For a part with a diameter of 40mm and a height of 25mm, the blank diameter is approximately 60-70mm. Material plasticity must also be considered. For materials with less plasticity (such as high-carbon steel), the blank size should be appropriately increased to avoid cracking during drawing.
The die structure adopts either an inverted or upright layout. The inverted layout is suitable for thin parts, while the upright layout is suitable for thicker parts. It primarily consists of a blanking die, a blanking punch (which also serves as a drawing die), a drawing punch, a blank holder, a discharge device, and a guide mechanism. The blanking die and punch are made of Cr12MoV steel, hardened to a hardness of HRC 58-62. The blanking clearance is set at 10% of the material thickness, or 0.1mm for a 1mm thick material. The working surface of the drawing punch is polished to a Ra of less than 0.4μm, and the corner radius is 6-10 times the material thickness to ensure smooth material flow during the drawing process. The clearance between the blank holder and the drawing die is 1.05-1.1 times the material thickness to prevent wrinkling.
A proper match between the blanking force and the drawing force is a key design consideration. Too little blanking force can cause wrinkling in the material, while too much increases the drawing force and may even cause cracking. The blanking force is generally 25%-35% of the drawing force. A spring or nitrogen gas spring is used to provide the blanking force. The spring stiffness should be uniform to ensure a stable blanking force. The stroke of the drawing punch must be sufficient to ensure both the blanking process is completed and the required drawing depth is met. The stroke is the sum of the blanking die thickness and the drawing height, plus a safety margin of 5-10mm. For example, if the drawing height is 25mm and the blanking die thickness is 20mm, the stroke should be set to 50-55mm.
The guide system uses precision ball guide pins and guide sleeves. The diameter of the guide pins is 16-25mm, and the matching clearance is 0.005-0.01mm to ensure the coaxiality of the blanking and drawing processes. The positioning device uses a stop pin and a guide plate. The position accuracy of the stop pin is ±0.1mm. The distance between the guide plates is 0.1-0.2mm larger than the width of the blank to ensure accurate feeding of the blank. For coil production, it can be equipped with an automatic feeding mechanism with a feeding accuracy of ±0.05mm to achieve continuous automated production. The unloading device consists of a unloading plate and an ejector. The unloading plate removes the sheet after blanking, and the ejector ejects the part from the drawing die after the drawing is completed.
The following key points should be noted during commissioning and production. During mold trials, first conduct a single-stroke test to check whether the blanking dimensions and drawn shape meet requirements. The diameter deviation of the blanked part must be less than 0.2mm, and the height deviation of the drawn part must be less than 0.15mm. During continuous punching tests, observe the surface quality of the parts. No defects such as cracks or wrinkles should be present. After every 2,000 parts, inspect the wear of the blanking cutting edge and the drawing punch. Regrind any burrs on the cutting edge and polish any scratches on the drawing punch surface. Regularly check the spring’s elasticity and replace any deterioration to ensure stable blanking force and guarantee the production quality and life of the mold.
