Can-making factories have strict requirements for aluminum can materials. Not only do they demand good intrinsic quality and optimized chemical composition, low gas and moisture content, but they also require excellent deep-drawing performance, low anisotropy (material’s directional dependence), small thickness tolerance, good flatness, and high surface quality.
The production of aluminum pull-tab cans involves over 40 processes. The key processes related to the properties of aluminum strips include blanking, cup drawing, thinning and deep drawing, trimming, washing, external printing, internal coating, drying, necking, and flanging. Aluminum strips must have appropriate strength and good deep-drawing formability to ensure smooth continuous drawing, thinning, and deep-drawing, as well as proper yield strength after baking. In the production of pull-tab cans, the first step is to blank the strip with a thickness of 0.25-0.30mm into round discs with a diameter of around 138mm. Then, two deep-drawing processes are carried out to form the cup, with a diameter reduction rate of over 50%. After that, three additional thinning and deep-drawing processes reduce the wall thickness to 0.08-0.10mm, achieving a stretching thinning rate exceeding 65%. Since the thinning and deep-drawing process puts the workpiece in an extremely low ductility state, even small inclusions can cause rupture or edge folding. Subsequently, it is necessary to ensure that there is no fracture during trimming, necking, and flanging, requiring the material to have good plasticity. After several baking processes, the can body must meet the axial pressure resistance of 1.35 kN and bottom pressure resistance of 630 kPa to ensure smooth canning and storage. Therefore, comprehensive requirements are imposed on the aluminum strip used for can bodies: tensile strength of 270-310 MPa, yield strength of 250-300 MPa, elongation greater than 3%, ear-making rate less than 2%, uniform and smooth surface without visible wrinkles, oxidation, inclusions, indentation, or other defects, and uniform strip thickness with a tolerance within 0.005mm.
Modern can lid production lines use coil material, with 20-24 workstations for cup making and a material width of 1500-1550mm. Block production lines use sheet material with a width of 850-970mm. The lid thickness is 0.27mm. Currently, all pre-coated lid materials used in China are imported. Lid production involves separately producing base lids and pull rings on two production lines and then combining them to form composite lids. Lid materials have various width specifications and even more thickness specifications, ranging from 0.22mm to 0.315mm, including 0.23mm, 0.25mm, 0.27mm, 0.28mm, and 0.30mm. There are approximately 50 lid manufacturing companies in China, with both coil and block production methods, and the total production capacity exceeds 40 billion lids per year.
The main production process of pressure cans involves the following steps: manufacturing aluminum billets (aluminum ingots), high-speed extrusion to form the can body, edge trimming, interior and exterior coating, printing, doming, necking, flanging, inspection, and packaging. To produce high-quality aluminum cans, high-quality aluminum billets are necessary. During can production on high-speed automated lines, the process from the entry of aluminum billets into the mold cavity to the exit of the can body is a closed production process that operates at a high speed. It requires special deformation characteristics of the extruded can body, such as the absence of defects like inclusions or air pockets, no wrinkling during necking, and no cracking during flanging. The material needs to undergo three stages of work hardening without being damaged. Additionally, the formed aluminum can should be able to withstand a certain deformation pressure (NL2MPa) and burst pressure (2L4MPa). Therefore, strict requirements are imposed on the quality of the material, including excellent plastic formability, low planar anisotropy and ear-making rate, appropriate hardness (HB19±2), and a fine grain structure (100-130 grains per mm²). It is evident that the production of aluminum billets for pressure cans is a highly technical process with significant manufacturing challenges.
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