Metal

Overview
The substrate metal represents the major proportion of the weight, and the raw material cost of the finished can. It also imparts the strength. For reason of economics, it is desirable to use as thin a gauge of metal as possible consistent with achieving and maintaining the performance requirements of the can during its manufacture, transit, filling and distribution to the ultimate consumer. This also satisfies the Essential Requirements of the European packaging waste regulations which call, amongst other things, for minimisation of material usage. Different types of finished steel – tinmill products – and different alloys of aluminium are used in can and component manufacture. As a general rule across all light metal packaging types (excluding foil), steel-based products take in Europe more than 85% of the total metal tonnage used, but bearing in mind the fact that steel is nearly three times the density of aluminium. More detail on the specific types of substrate metal can be obtained via the appropriate links. Descriptions of the different means by which the metal can be converted into cans, end components, caps or closures are also given in the Design and Manufacture section.

Tinmill Products
For the light metal packaging sector, these mainly comprise tinplate and so-called tin free steel (TFS), more accurately known as electrolytically chromium/chromium oxide coated steel (ECCS). The steel basis, low carbon mild steel, is further processed to increase its corrosion resistance and adhesion of any organic coatings subsequently applied.

Tinplate, as the name suggests, is coated on both sides with pure tin at various coating weights between 1 and 12 grams per square meter. Decades ago this was achieved by literally dipping the base steel sheet-by-sheet into molten tin – the ‘hot dip’ process resulting in plate coated in expensive tin at about 15 grams per square meter on each side. Nowadays the process is electrolytic, leading to very close control of tin coating weights and much lower weights can be achieved. It is also possible to apply different coating weights to the two sides of the steel. A typical tin weight where an organic protective coating is subsequently to be applied is 2.8 g/m2 although weights of less than 1 g/m2 are possible. External tin coating weight where no additional coating is added would typically be 5.6 g/m2. Whereas the hot dip process was carried out on pre-cut sheets, the current process is carried out at much higher speeds with the steel still in the coil form in which it leaves the rolling mill. The resultant, matt product may optionally be ‘flow brightened’ by using heat to melt and reflow the tin. This enhances the protection that the tin affords to the base steel, and is normally carried out for all end uses other than drawing and wall ironing (DWI) for which a matt finish better carries the lubricant for the wall ironing process.

The tinning process is followed by passage through a passivating bath whereby a thin passivation layer is applied either chemically or electrolytically dependent on final requirement. This inhibits the formation of tin oxide, particularly during the elevated temperature curing of any organic coatings applied later, and thereby enhances the adhesion of these coatings.

ECCS uses the same specifications of base steel as the tinplate, but there is no tinning process, simply an electrolytic process resulting in the deposition of thin, protective layers of chromium and chromium oxide equally on both sides of the coil. In finished form, ECCS is less intrinsically corrosion resistant than tinplate and is always subsequently organically coated in practice on both sides. One factor inhibiting the wider use of ECCS in can making is the fact that it cannot be welded into a cylinder (three-piece can – see elsewhere in Design and Manufacture) because of the presence of the chromium/chromium oxide. Edge cleaning prior to welding is an option, but this has not yet developed into widespread practice. Three-piece welded can bodies therefore are generally in tinplate only. ECCS can be used, pre-coated or laminated, for ends and for two-piece cans made by the draw/redraw process (see elsewhere in Design and Manufacture).

Blackplate is a third tinmill product, not generally used in light metal packaging but which finds application for example in the manufacture of heavy steel drums. It is the same base low carbon mild steel, minimally protected with oils, and is prone to ambient corrosion. It too, therefore, is most normally used fully coated.

Base Steel Properties
Following raw separation of the iron from its ore in the blast furnace, the molten iron is fed to a basic oxygen furnace, already containing about 25% scrap steel, where it is purified via oxygen injection at about 2000?C. The molten steel is then cast into slabs which are subsequently thinned and elongated, firstly by hot rolling down to about 2mm thickness, then by cold rolling into finished coils of up to 1.25 metres wide and a fraction of a mm thick, after edge slitting to remove uneven edges. Each cold rolling regime is followed by annealing at high temperature to relieve the stresses from the rolling process and consolidate the properties. There is an extra ‘skin pass’ after the final anneal to accurately achieve the required gauge. ‘Single reduced’ tinmill steel undergoes one cold rolling regime and ‘double reduced’ two regimes. The latter gives higher hardness for a given thickness allowing the use of comparatively thin gauges for many applications. Minimum thicknesses currently in use are 0.18mm for single reduced and 0.12mm for double reduced plate.

Aluminium
The extraction of aluminium from bauxite ore is a comparatively energy intensive process resulting in a substrate which is fundamentally more expensive than tinmill products. After extraction, however, aluminium can easily and cheaply be remelted making it very suitable for recycling. It is easy to work therefore finds particular application where extensive metal deformation is needed. Also it presents a good external finish for printed work. Examples of use are in the manufacture of easy opening ends, cans which are drawn or drawn and wall-ironed, bottle closures and impact extruded collapsible tubes and aerosols (see elsewhere in Design and Manufacture). Aluminium is less prone to both ambient corrosion and reaction with packed product therefore savings can be made in the organic protection of the metal. The manufacture of aluminium normally involves the semi-continuous casting of slabs that are then hot rolled down to about 2.5mm thickness. Several stages of cold rolling reduce this down to finished thicknesses of between 0.18 and 0.45mm with edge-trimmed coil widths up to 1.782 metres for packaging applications. Softer tempers for drawing or extrusion may involve intermediate annealing, but for DWI beverage cans and easy opening ends the metal is made fully hard by cold rolling. The ‘formability’ of the aluminium is tailored both through the rolling and annealing themselves and by the introduction of small quantities of alloying elements such as manganese and magnesium. Pure aluminium is very soft and has very limited application for light metal packaging, impact extruded tubes and aerosols being the exception. DWI cans and bottle caps require harder alloyed grades and, particularly in the case of the DWI can, the formed product becomes further strengthened through ‘work hardening’. Grades for easy opening ends and drawn cans are harder alloys still.

Coils for DWI can making are normally supplied and converted ‘as rolled’, while most of the metal that is used for ends and drawn cans is supplied with an organic coating on each side applied by the metal supplier via his own coil coating facility.