Design of composite die for deep drawing and edge extrusion
The design of a composite die for deep drawing and edge extrusion is a highly efficient forming technology that integrates deep drawing and edge extrusion into the same die. This technology is suitable for applications where flat edges are required on the ends or bottoms of cylindrical parts, such as thermos cups and pressure vessels. Deep drawing and edge extrusion can be completed in a single stamping stroke, achieving an edge flatness error of less than 0.05mm and a verticality error of less than 0.1mm/m. This improves efficiency by over 50% compared to separate processes. The key lies in coordinating the forces between the deep drawing and edge extrusion steps to avoid interference between the two processes. Initially, the ratio of the deep drawing force to the edge extrusion force must be calculated. Typically, the edge extrusion force is 30%-50% of the deep drawing force. For a mild steel cylindrical part (Ø80mm diameter, 2mm thickness), the deep drawing force is approximately 80kN, while the edge extrusion force should be controlled between 24-40kN to ensure adequate edge extrusion without cracking.
The die structure consists of a drawing punch, an extrusion punch, a composite die, a blank holder, an ejector, and a guide mechanism. The drawing punch is made of Cr12MoV steel, hardened to HRC 58-62, and features a chrome-plated working surface (0.01-0.03mm thick) to reduce friction. The extrusion punch is coaxially arranged with the drawing punch, with a diameter 5-10mm larger than the drawing punch, forming an annular extrusion surface. The material used is W18Cr4V high-speed steel, hardened to HRC 62-65, with an extrusion surface roughness of Ra 0.2μm or less. The composite die consists of a drawing section and an extrusion section. The drawing section has a corner radius 5-8 times the material thickness (10-16mm for 2mm thick material). The extrusion section has a taper of 0.5°-1° to guide material flow toward the edge. The die is made of cemented carbide (WC-Co) for improved wear resistance.
The force coordination system dynamically distributes the drawing and extrusion forces through elastic elements. The blank holder is driven by a nitrogen spring, providing the blank holder force (15-25kN) required for drawing. During the extrusion phase, the extrusion punch receives its extrusion force from another set of nitrogen springs. The compression strokes of the two sets of springs are designed according to the process sequence. After the drawing stroke is completed, the extrusion stroke begins. A stroke switch controls the timing of the transition to ensure seamless process transitions. For thick-walled parts (thickness > 3mm), a cooling water channel (5-8mm diameter) must be installed inside the extrusion punch, supplying cooling water at 30-40°C to keep the mold temperature below 60°C to avoid extrusion deformation caused by overheating of the material.
The positioning and guiding system must ensure precise coaxiality. Dual guide pins and guide sleeves are used for guidance, with guide pin diameters of 30-40mm and clearances of 0.01-0.03mm. This ensures coaxiality errors of less than 0.02mm between the drawing punch, extrusion punch, and composite die. Blank positioning utilizes a combination of locating pins and retaining rings. The locating pins control radial position (accuracy ±0.1mm), while the retaining rings limit axial dimension (error <0.2mm). For coil production, a servo feeder with a feeding accuracy of ±0.05mm can be used, with secondary positioning using guide pins.
During commissioning and quality control, attention should be paid to edge forming. During die trials, measure the edge thickness (thickness increase must be less than 5%) and flatness after extrusion. If the edge is too thick, increase the extrusion force or extend the extrusion stroke (adjustment range ±0.1mm). If flatness exceeds tolerance, adjust the parallelism of the extrusion punch and die (tolerance <0.03mm/m). Check the wear of the extrusion punch every 2,000 pieces. Use a profilometer to check the flatness of the extrusion surface. Regrind if wear exceeds 0.02mm. Polish the die in the drawing section every 5,000 pieces to maintain a surface roughness of Ra 0.4μm to ensure consistent drawing quality.
