A. Teacher Background

ow that you are familiar with the history of the can making process, we turn to the technical side of exactly how aluminum cans are made. Keep in mind the distinction between food cans, which generally are made of steel, and beverage cans, which are almost always made of aluminum. For purposes of this resource kit, we are focusing on aluminum cans, even though some of the lessons and activities will help students explore the differences between the two types of cans.


Aluminum Manufacturing


Aluminum is a plentiful material found in the earth's crust. It occurs naturally in a mineral called bauxite. Most bauxite is surface mined overseas and shipped to the United States for processing. Bauxite, when refined to remove impurities, produces a fine, white powder called alumina or aluminum oxide. A continuous electric current is applied to the alumina powder, separating the aluminum from the oxygen. Next, small amounts of other metals are added to the molten aluminum to add strength and corrosion resistance to the final product. The molten metal is cast into ingots, which are then rolled into long sheets, coiled and sent to the can or end manufacturing plant.


Can Body Manufacturing

The aluminum beverage can consists of two pieces — the can body and the can end (or lid). The manufacturing process begins with coils of aluminum. Each coil typically weighs about 25,000 pounds and, when rolled out flat, can be anywhere from 20,000 feet to 30,000 feet long and five to six feet wide.

The aluminum coils are loaded onto an "uncoiler" at the beginning of the can making line. The uncoiler unrolls the strip of aluminum and feeds it to the lubricator. The lubricator deposits a thin film of water-soluble lubricant on both sides of the aluminum sheet. Lubrication allows the metal to flow smoothly over the tooling surfaces during the forming processes that follow.

step3 Metal forming begins in a large machine called a cupping press. The press cuts circular discs from the aluminum sheet and forms them into cups. The cups drop from the press onto the cup conveyor. These two metal-forming operations are called "blank and draw" and are performed at speeds ranging from 2,500 to 3,750 cups per minute. The scrap (or skeleton) aluminum left over from these operations is removed from the production line for recycling.

The cup conveyer moves the cups to one of a series of bodymakers where the can is made. Each bodymaker contains a tool called a punch, which forms the shape of the can by forcing the cup through a series of progressively smaller circular ironing rings. This action literally draws the metal up the sides of the punch, ironing it into a can body. As the cup is forced through the rings, its diameter is reduced, its walls are thinned and its height is increased. At the end of the punch stroke, the bottom is formed into a dome shape that strengthens the bottom of the can. During this process, referred to as wall ironing, the can must be lubricated to reduce frictional heat. This is a high-speed process producing 1,500 to 2,700 can bodies per minute. The lubricant is constantly recirculated through a filter and reused.

Process-7 Once released from the bodymaker, the top of the can is trimmed mechanically to a uniform height, and the trimmed-off scrap is recycled. The can bodies are conveyed through a washer that removes the forming lubricants before the application of the outside decoration (or label) and inside protective coating. Can washers are large, typically 12 feet wide and 100 feet long. The washed can is discharged through a dryer where it is dried with forced hot air and then moved to the decorator or printer.

The decorator applies the label to the outside of the can. The cans are conveyed through the decorator on a mandrel, which rotates the can in contact with a printing blanket. The can, now coated with wet ink, moves to a rotating varnish application roll that applies a clear coating over the entire sidewall. The clear coating protects the label from scratching and contains lubricants that facilitate can conveying. The can is transferred from the decorator onto a pin (so that only the inside surface is contacted) and is conveyed through a decorator, or "pin," oven where the label is dried with forced hot air.

Process-13 Following application and curing of the exterior label (lithography), the can is conveyed to a bank of spray machines that spray the inside with an epoxy-based organic protective coating. The inside coating is also cured by forced hot air. The coating prevents the beverage from contacting or reacting with the metal in the can body even though aluminum is a relatively nonreactive material. Both the exterior overvarnish and the interior protective coating are made with water-based materials to prevent harm to the environment.

Process-10 After the can leaves the inside bake oven, it passes through a lubricator that applies a thin film of lubricant to the exterior of the top of the can where the neck and flange will be formed. Most soft drink cans in the United States today are designated 202/211 diameter. This means the top of the can has been reduced — necked down to 2 2/16 inches in diameter — while the body is 2 11/16 inches. This necking process creates the unique curved shape of the top of the can. A flange (or lip) is formed at the top where the end will eventually be sealed into place after the can is filled.

Process-10 All finished cans are evaluated for leakage with a light tester. The can flange is clamped against a sealing surface and, as the machine rotates, the outside surface is exposed to a bank of extremely bright lights. A photocell inside the can detects any entering light, triggering a reject mechanism. Rejected cans are recycled. The photocell detects leaks as small as .001 inch in diameter, which is smaller than the diameter of a human hair. Many tests and measurements are conducted during the can manufacturing process to ensure that can dimensions and performance standards meet industry and customer criteria. After testing, the finished cans are placed on pallets for shipment to the customer filling operations.


Can End Manufacturing


Like can body manufacturing, modern can end manufacturing starts with a coil of aluminum being placed on an uncoiler. The aluminum used for end manufacture is precoated on both sides with organic protective coatings containing lubricants. There are no plate lubrication, washing, coating or baking operations in the modern end manufacturing plant.

Process-10 The uncoiler feeds the plate directly into a shell press, which is similar to the cupping press in the can body manufacturing process. The shell press blanks a circular disc and forms it into a shell. This is a high-speed operation — the latest design shell presses produce more than 5,500 shells per minute. The shell is discharged through a curler that forms the precise shape required for double seam formation (the operation that seals the can end to the flanged top of the can body after the can is filled).

After curling, a liquid sealing compound is applied to the end. The shell is moved onto a conversion press where the score (or opening area) is formed and the tab is attached. Conversion presses produce up to 1,800 ends per minute. Modern beverage can ends have retained (or stay-on) tab openings to prevent litter. Again, quality assurance testing and measurement takes place throughout the end manufacturing process to ensure that end dimensions and performance standards meet industry and customer criteria. Finally, the ends are wrapped in paper sleeves and stacked on pallets for shipment to the customer's filling operation.

This section has been adapted from William J. Forsythe's "Two Piece Aluminum Can and End Manufacture," 1996.