Design of the drawing die for the cover
The design of the cover drawing die is a special technology for deep drawing of various cover parts (such as automobile engine hoods and electrical appliance housings). It is suitable for cold-rolled steel plates, aluminum alloys and other materials with a thickness of 0.8-4mm. It can process flat blanks into open parts with complex curved surfaces and a certain depth (10-200mm). The surface roughness after forming is Ra1.6μm or less, and the profile error is less than 0.5mm. The core is to control the flow rate and deformation uniformity of the material by reasonably designing the shape and fillet of the punch and die to avoid defects such as wrinkling and cracking. In the early stage of design, it is necessary to conduct process analysis based on the three-dimensional model of the cover to determine the number of deep drawing (usually 1-5 times). For covers with larger depth or complex shape, multiple deep drawing is required. The first drawing coefficient is generally 0.55-0.7, and the subsequent times increase by 0.05-0.1.
The mold structure consists of a punch, die, blank holder, ejector, and guide mechanism. The punch is made of Cr12MoV steel, hardened to HRC 58-62, and the working surface is polished (Ra 0.4μm or less). The corner radius is determined by the material thickness: 5-8mm for 0.8mm thick material and 15-20mm for 4mm thick material to reduce material flow resistance. The die is made of the same material, hardened to HRC 55-58, with a clearance of 1.1-1.2 times the material thickness. The corner radius at the die entrance is 2-3mm larger than that of the punch to guide the material smoothly into the mold cavity. The blank holder is hydraulically driven, providing uniform blank holding force (25%-40% of the drawing force) to prevent wrinkling at the cover edge. For large covers (>1000mm), the blank holder is designed in zones, with the blank holding force in each zone independently adjustable.
The drawing process parameters need to be optimized according to the material properties and the shape of the cover. The drawing speed is controlled at 50-200mm/min, with a higher speed for aluminum alloy materials and a lower speed for high-strength steel to avoid material rupture due to excessive speed. In terms of lubrication, emulsions (concentration 5%-8%) can be used for cold-rolled steel plates, and special aluminum alloy drawing oil (viscosity 30-50cSt) is required for aluminum alloys to form a uniform oil film (thickness 5-10μm) on the surface of the die, reducing the friction coefficient to below 0.1. For covers that require multiple deep drawing, annealing treatment is required in the middle (600-650℃ for steel plates, insulation for 1 hour; 300-350℃ for aluminum alloys, insulation for 1.5 hours) to eliminate work hardening and restore material plasticity.
The positioning and guiding system ensures drawing accuracy, utilizing a combination of locating pins and retaining plates. The clearance between the locating pins and the pre-punched holes in the blank is 0.05-0.1mm, and the retaining plates control the blank’s lateral position (accuracy ±0.1mm). The guiding mechanism utilizes guide pins and sleeves (30-60mm diameter) with a clearance of 0.01-0.03mm, ensuring alignment error between the punch and die is less than 0.03mm. For asymmetrical housings, balancing blocks are required to balance the forces applied to the die to prevent damage or part deformation due to unbalanced loading.
Debugging and maintenance require attention to detail control. During mold testing, measure the key dimensions (such as depth, opening size) and surface quality of the cover, sample 5 pieces per batch, and adjust the gap between the punch and the die when the size deviation exceeds the tolerance (±0.05mm each time); if wrinkles occur, increase the blanking force in the corresponding area (increase by 5% each time); if there are scratches on the surface, re-polish the surface of the punch and die. 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 in the mold. When stored for a long time, anti-rust oil needs to be applied to the surface of the punch and die, and the blank holder is in a relaxed state to avoid long-term stress damage to the spring or hydraulic components.
