Cans for food and drinks may be constructed out of either two or three pieces of metal.
Three-piece cans, which were the first to be developed in the middle of the 19th century, consist of a cylindrical body rolled from a piece of flat metal with a longitudinal seam (usually formed by welding) together with two can ends, which are seamed onto each end of the body. Three-piece cans may be made in almost any practical combination of height and diameter. This process is particularly suitable for making cans of mixed specifications as it is relatively simple to change the specification of can being made.
Two-piece cans are made from a disc of metal which is reformed into a cylinder with an integral end. To this is seamed a loose end to finally close the can. The operation of reforming sheet metal without changing its thickness is called “drawing”. The operation of reforming a two-piece can into one of smaller diameter, and therefore greater height, also without changing its thickness is called “re-drawing”. The operation of thinning the walls of a two-piece can by passing through circular dies is called “ironing”. Drawn and ironed cans are referred to as “DWI” or “D&I” cans. The DWI process is used for making cans where the height is greater than the diameter and is particularly suited to making large volumes of cans of the same basic specification.
Lids for cans are called “can ends” or simply “ends”. For a ring-pull end, the main circular part of the end is called an “end shell” whilst the ring pull is called the “tab”.
Because of the large volumes in which cans are manufactured, statistical sampling techniques must be used for checking and controlling all quality aspects of the can and end making processes. However, video scanning, light or pressure testing may be applied to all finished components.
Food and drinks cans may be constructed from either steel or aluminium depending on the precise method of container manufacture. The raw materials for both these materials occur naturally in large quantities throughout the world and this allows the finished materials for can making to be available at relatively low cost. Steel for can making is supplied either as tin plate, which is steel with a very thin layer of tin electro-deposited onto both sides, or tin-free steel, where no tin is present. Both steel and aluminium are non-toxic materials and as such are ideal for packaging foodstuffs; they are both also very easy to recycle after use. For many food and drinks cans it is necessary to coat the metal with an organic material to prevent chemical actions occurring between the metal of the container and the product or the external environment. All metal is initially delivered to the can making factory in large coils.