Modified Beverage Cans

Raw material

Beverage can consist mostly of aluminum, but it contains small amounts of other metals as well. These are typically 1% magnesium, 1% manganese, 0.4% iron, 0.2% silicon, and 0.15% copper. Before understanding the manufacturing process for beverage cans, we will have to understand the processes of drawing of sheet and plate.

Drawing of Sheet and Plate

In drawing process, a combination of a punch and a die is used which draw a circular blank of metal sheet into a 3-D cylindrical cup. Basically, the punch descends, pushing metal through die, converting circular blank to a cylindrical cup. Height of cup walls is determined by difference between the diameter of original blank and diameter of punch. The drawing operation is done in multiple stages and is not a one stage process. Wrinkles can appear in cup walls as circumference is reduced, or punch can act as a piercing tool. If gap between punch and die is less than thickness of incoming material, cup wall is thinned and elongated. This process is often called ironing or wall ironing. The punch and die must have corner radii, given by Rp and Rd. The sides of the punch and die are separated by a clearance c. For Drawing, the clearance is greater than the stock thickness as follows:

As the punch first begins to push into the work, the metal is subjected to a bending operation. As the punch moves further down, a straightening action occurs in the metal that was previously bent over the die radius. Holding force is critical for a successful drawing operation. If too small, wrinkling occurs and if too large, it prevents the metal from flowing properly toward the die cavity. This results in stretching and possible tearing of the sheet metal.

Drawing Ratio

For a cylindrical shape the drawing ratio is the ratio of blank diameter Db to punch diameter Dp. The greater the ratio, the more severe the operation. An approximate upper limit of drawing ratio=2.0

Reduction

For a given drawing operation, the reduction ‘r’ is also used as: Thickness to diameter ratio A third measure in deep drawing is the thickness-to-diameter ratio, which gives the tendency for wrinkling It is desirable for the t/Db ratio >1%. As t/Db decreases, tendency for wrinkling increases. In cases where these limits on drawing ratio, reduction, and t/Db ratio are exceeded by the design of the drawn part, the blank must be drawn in two or more steps, sometimes with annealing between the steps.

Drawing Force

Force equation estimates the maximum force in the operation Where, F=drawing force, N, t=original blank thickness, mm, TS=tensile strength, MPa, and Db and Dp are the starting blank diameter and punch diameter, mm. The constant 0.7 is a correction factor to account for friction.

Holding Force

As a rough approximation, the holding pressure can be set at a value=0.015 of the yield strength of the sheet metal. This value is then multiplied by that portion of the starting area of the blank that is to be held by the blank holder. In eq. form Holding force is usually about one-third the drawing force.

Ironing of sheet metal

If gap between punch and die is less than thickness of incoming material, cup wall is thinned and elongated. This process is often called ironing or wall ironing Ironing of sheet metal is a manufacturing process that is mostly used to achieve a uniform wall thickness in deep drawings. Ironing of sheet metal can be incorporated into a deep drawing process or can be performed separately. A punch and die pushes the part through a clearance that will act to reduce the entire wall thickness to a certain value. While reducing the entire wall thickness, ironing will cause the part to lengthen. The percentage reduction in thickness for an ironing operation is usually 40% to 60%. Percent reduction can be measured (ti - tf)/ti X 100%. With ti being initial thickness and ti being final thickness. Many products undergo two or more ironing operations. Beverage cans are a common product of sheet metal ironing operations.

Cleaning and Decorating

The above-mentioned process leaves the outer wall of the can with a smooth and shiny surface, thus it will not require any finishing such as polishing. Then the next procedure is the trimming of the ears and finally the can is cleaned and a label is imposed or printed over it.

Formation of a Neck at the Top

The can is squeezed at the top to make a neck and from the very top portion of it, outward flanges are made which will be folded over the lid at the time of application of the lid.

The Lid

The alloy used for the formation of lid is slightly different than that used for making the base and walls of the can. At the bottom of the can an inward bulge was provided so that the it can sustain the pressure of the liquid filled inside but as the upper lid is flat it must be stronger to sustain the pressure so it is made with aluminum with higher percentage of magnesium and less percentage of manganese. Diameter of the lid is kept smaller than the walls and after that with the help of a rivet a pull tab is attached on lid and the lid is scored so that a proper opening can be made when the pull tab is opened.

Filling and seaming

After the formation of the neck, the can is filled and thus it is filled and then the lid is added and the upper flanges made during the neck formation are now bent around the lid and closed. After these processes the can is ready to be saled.