Design of cylindrical drawing die
The design of the cylindrical drawing die is a key technology for forming a flat blank into a cylindrical part through multiple deep drawing. It is suitable for materials such as low-carbon steel and aluminum alloy with a thickness of 0.5-8mm. It can produce cylinders with a diameter of 10-500mm and a height of 50-1000mm. The diameter accuracy of the parts can reach IT10 level, and the surface roughness is below Ra1.6μm. The core is to reasonably distribute the deformation amount (drawing coefficient) of each deep drawing. The first drawing coefficient is usually 0.5-0.65, and the subsequent drawing coefficient increases by 0.05-0.1 to ensure that the material is gradually deformed to avoid defects such as wrinkling and cracking. The diameter of the blank needs to be calculated in the early stage of design (according to the principle of constant volume). For example, for a cylinder with a diameter of 100mm and a height of 200mm, the diameter of the blank is about 280mm, and a trimming margin of 5-10mm is reserved.
The die structure is categorized as either a single-stage drawing die or a multi-station progressive drawing die, depending on the number of draw cycles. The single-stage drawing die consists of a punch, a die, a blank holder, and a discharge device. The punch is constructed of Cr12MoV steel (HRC58-62), with a working surface polished to Ra0.4μm and a corner radius of 5-8 times the material thickness (10-16mm for a 2mm thickness). The die is constructed of the same material (HRC55-58), with a clearance of 1.1-1.2 times the material thickness (2.2-2.4mm for a 2mm thickness). The corner radius is 1-2mm larger than that of the punch to guide material flow. A nitrogen spring provides a blanking force (20%-30% of the drawing force) to prevent wrinkling on the cylinder’s sidewalls. For cylinders deeper than 200mm, a double-acting press is required to ensure stable blanking force.
The multi-station progressive drawing die features 4-8 drawing stations within a single die set. An automatic feed mechanism enables continuous production, achieving a production rate of 50-100 pieces per minute. Key stations utilize carbide (YG15) dies for improved wear resistance (a lifespan exceeding 500,000 cycles). Leveling devices are installed between stations to eliminate work hardening. For viscous materials like stainless steel, a lubrication groove is installed at the die entrance to automatically spray drawing oil (viscosity 30-50 cSt), creating an oil film thickness of 5-10 μm, reducing the coefficient of friction to below 0.1.
The optimization of the drawing process needs to be combined with the material properties. The first drawing coefficient of low carbon steel can be as low as 0.5, while that of stainless steel needs to be ≥0.55 to avoid cracking. The drawing speed is controlled at 100-300mm/min. A cooling system (oil temperature controlled at 30-50℃) is required for high-speed drawing to prevent the mold from overheating. For cylinders that need to be drawn multiple times, annealing treatment is required in the middle (600-650℃/1h for low carbon steel) to reduce the hardness of the material (HB≤180) to ensure smooth subsequent drawing. The cylinder after drawing needs to be trimmed to remove the irregular parts of the mouth, with a trimming margin of 3-5mm to ensure that the flatness of the mouth is less than 0.1mm.
During debugging and quality control, attention should be paid to the dimensional accuracy and surface quality of the cylinder. During the mold trial, the diameter and height after each deep drawing should be measured. When the deviation is out of tolerance, the gap between the punch and the die should be adjusted (±0.05mm each time); if wrinkles occur, the blank holder force should be increased (5% each time); if there are scratches on the side wall, the punch surface should be re-polished (Ra≤0.4μm). After every 1,000 pieces of work, check the wear of the die edge (grind when the radius increases by >0.2mm), and regularly clean the iron filings and oil stains on the mold surface. When stored for a long time, the punch and die should be stored separately, anti-rust oil should be applied, and the blank holder should be in a relaxed state to avoid spring fatigue failure.
