Design of compound die for blanking, positive and negative drawing
The blanking and forward and reverse drawing composite die design is a specialized forming technology developed for cup-shaped parts requiring a reverse raised feature on the bottom, such as the bearing seat in a motor end cap. This technology completes the blanking, forward drawing, and reverse drawing processes in a single press, achieving simultaneous forming of the part’s internal and external shapes. The coaxiality error between the boss and the barrel wall is less than 0.05mm, and production efficiency can reach 80-150 pieces per minute, making it suitable for high-volume production in the automotive and motor industries. The core of the design is to resolve the material allocation conflict between forward and reverse drawing. Forward drawing requires a reserve of material for reverse drawing. For a φ80mm × 30mm cylindrical part (with a φ30mm × 10mm boss), the forward drawing depth is reduced by 5mm to ensure sufficient material to fill the boss during reverse drawing.
The die structure utilizes a dual-punch layout with opposing dies, consisting of a blanking die, a forward drawing punch, a reverse drawing punch, a composite die, and a bidirectional pressure system. The blanking punch is made of Cr12MoV steel, hardened to HRC 58-62, with a cutting edge clearance of 10% of the material thickness (0.2mm for 2mm thick material). The forward drawing punch is an annular structure with an inner diameter 0.5mm larger than the reverse boss and a corner radius of 8-10mm. The reverse drawing punch is a solid structure with a head radius of 5-6mm. Both are made of W18Cr4V high-speed steel and titanium-plated (0.005mm thick) to reduce the friction coefficient to 0.08. The composite die features a bidirectional forming cavity, with a forward cavity depth of 25mm and a reverse cavity depth of 10mm. It utilizes carbide (WC-Co) inserts with a compressive strength of ≥2000MPa.
The pressure coordination system is key to synchronized forward and reverse drawing. It utilizes a dual-cylinder hydraulic drive: the forward-drawing cylinder provides 80-100kN of main pressure, while the reverse-drawing cylinder provides 30-40kN of reverse force. A pressure sensor monitors the force ratio between the two in real time (required to maintain a stable range of 2.5:1-3:1). When the ratio fluctuates by more than 5%, a servo valve automatically adjusts the flow rate to prevent excessive stretching or folding of the material. The blank holder utilizes a separate inner and outer ring design. The outer ring controls the blank holder force (15-20kN) in the forward drawing phase, while the inner ring controls the blank holder force (8-12kN) in the reverse drawing phase. Independent adjustments ensure balanced material flow in both deformation zones.
The positioning and guiding system must meet bidirectional precision requirements. It utilizes a four-pin guide mechanism (25mm diameter) with a clearance of 0.005-0.01mm to ensure centering of the front and back punches. Blank positioning utilizes a combination of a central locating pin and an outer guide plate. The pin diameter is 0.1mm smaller than the part’s inner hole, and the clearance between the guide plate and the blank is 0.1-0.2mm. Feeding step accuracy is ±0.03mm. A material flow sensor is installed at the reverse drawing station, and a laser displacement meter monitors material accumulation during the boss forming process. If the thickness deviation exceeds 0.1mm, the reverse drawing force (±2kN) is automatically adjusted.
Debugging and process optimization require the establishment of a parameter database. During mold trials, orthogonal testing is performed to determine the optimal force ratio (e.g., 2.8:1). Forming parameters for different materials (e.g., 50 steel and 6061 aluminum alloy) are recorded. An ultrasonic thickness gauge is used to inspect the wall thickness distribution of parts. The positive barrel wall thinning ratio must be less than 15%, and the reverse boss thickening must be less than 8%. During routine maintenance, the reverse drawing punch undergoes three-dimensional coordinate measurement every 10,000 units, and the head is reground if wear exceeds 0.03mm. The hydraulic system filter element (5μm accuracy) must be replaced weekly to ensure a pressure control accuracy of ±1%. The mold must be equipped with an automatic lubrication system, applying 0.1ml of extreme-pressure grease to the guide pins and sliders every cycle to reduce wear.
