Making Cans
How cans are made

Teacher Briefing

Within this unit, children study the processes involved in the manufacture of two and three-piece welded cans. Children are very much aware of the wide range of canned foods available through visiting supermarkets and shopping with their family. However, few children appreciate the complex processes that go into the making of cans, the preparation of food and the canning process.

In this unit children learn about the two different can making processes, two-piece and three-piece cans, through information text sources in order to complete sequential text activities.

This unit also provides an opportunity to further develop an understanding of capacity and volume in relation to cylinders (cans). Children measure the circumference and height of a range of cans, predicting and estimating capacities.

Investigating the volume and weight of a range of different containers used to preserve or contain food could extend this unit.

Two and Three Piece Can Processes

The basic material used for most food cans is steel. The first step in the steel making process is to produce molten iron. This is done by mixing iron ore with coke and limestone and heating them at very high temperatures in a blast furnace. This molten iron is fed into a vessel along with a percentage of scrap steel (recycled steel) and other chemicals and oxygen is blown into the vessel, again at high temperatures, to produce steel. The steel is poured out of the vessel into moulds and allowed to cool (these are called slabs). The slabs are hot rolled into huge 25 tonne coils. The coils are finely coated with either tin or chromium oxide (for tin free steel) which protects the steel from oxidation.

There are two ways to produce cans, either using two or three pieces of metal.

The traditional way is three-piece can making. Here 10 tonne coils of steel are cut into 1 metre square sheets and, in most cases, are then coated on the inside with a coating of lacquer to extra protect the food when inside the cans. A special ‘feeder’ machine picks up individual coated sheets with suction cups and carried them to a ‘slitter’ machine to cut into smaller rectangular ‘body blanks’ depending upon the size of can to make.

The ‘body blanks’ are formed into a cylindrical shape and electronically welded together. The top and bottom of the cylinder are curled or ‘flanged’ into a lip to attach the bottom end to the can. The cans are also ‘beaded’ with concentric rings around the middle of the can for strength to allow stacking when empty or filled. It also prevents collapse of the cans while the foods are cooked inside to produce a vacuum on cooling and to give the cans long shelf life. All cans are tested to make sure they will not leak and are then palletised ready to ship to the food-filling factory that fills and closes the cans with the other lid.

The latest and more modern way to make food cans is two-piece can making. Here the 10 tonne coils are fed through an automatic ‘cupping’ machine where small steel cups are formed and fed into a machine which draws and irons out the walls of the cup into a taller can body with the bottom end all in one piece. The result is a can body with thinner sidewalls and the original steel thickness. This is called the ‘draw and wall iron process.

The tops of cans are then trimmed, flanged, beaded and spray coated in the same way as three-piece cans ready to ship to the food filling factory.