The most suitable metals for the manufacture of containers and closures are steel and aluminium based materials. The many and varied physical properties of these materials present the opportunity for metal packaging to be constructed in a number of different way to satisfy both technical and economic requirements.
The refining of iron to become steel, by removing its impurities is a complex process to produce what is a very complex material. This complexity allows steel to be made with an almost unlimited number of differing properties and the metal packaging industry is able to use a number of these to its benefit.
Tinmill products is the generic title given to all steel-based products used for packaging, whether or not the steel is coated with tin or another metallic element.
Blackplate is steel without any additional metallic coating.
Tinplate is steel with a coating of tin deposited on both sides.
Electrolytic chromium/chromium oxide coated steel (ECCS), also referred to as tin free steel (TFS), is steel with a coating of chromium metal and chromium oxide deposited on both sides.
Molten steel may be cast into ingots or more usually in a continuous slab which is cut into lengths as it emerges from the caster. The steel slab is hot rolled into a coil then further cold rolled to produce the thicknesses of metal used in metal packaging. The above-mentioned finishing is carried out with the metal at its final thickness. A passivation layer is finally applied to tinplate to aid the adhesion of organic coatings.
More information on the manufacture of steel can be found on the following websites:
APEAL - Association of European Producers of Steel for Packaging www.apeal.org
Tata Steel - www.tatasteelpackaging.com
Aluminium is an abundant element, the ore from which it is extracted is bauxite. To prepare aluminium for the manufacture of cold rolled sheet for containers and ends, ingots of new aluminium together with scrap aluminium are added to a furnace operating at about 700°C. Fluxes are added to aid the removal of impurities and alloying elements such as magnesium and manganese are added to give the correct chemical composition. Before casting the liquid aluminium is degassed by the addition of chemicals and then filtered to ensure that all the metal in the ingot is very clean. An ingot normally weighs around 13 tonnes. Aluminium may also be cast into a continuous slab, which is then cut into individual slabs for rolling.
Aluminium which is supplied for end manufacture and for drawn or drawn/redrawn (DRD) can bodies has a passivation layer applied by the supplier which helps adhesion of organic coatings applied at the canmaker. Aluminium which is supplied for drawn and wall ironed (DWI) can bodies is supplied unpassivated, passivation being applied by the canmaker during his own process before the application of any organic coatings.
More details on the manufacture of aluminium can be found on the website of the European Aluminium Association (EAA) at www.eaa.net