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Aluminium rolling
Aluminium packaging is made from aluminium which has been rolled from large ingots weighing up to 20 tonnes down to thicknesses as low as 0.006mm or 6 microns.
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The ingots can be rolled: |
> to plate gauge (a thickness of 6mm to
250mm)
and further down .. |
| > through sheet gauges (0.2mm to 6mm) to ... |
> foil gauge (as thin as 0.006mm or 6 microns,
up to 0.25mm or 250 microns). |
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How is it done?
The original rectangular cast ingots, or slabs, of aluminium are heated to around 500 ºC, depending on the final properties required, and then passed repeatedly backwards and forwards through high pressure rollers, getting longer and longer and thinner and thinner. The width of the metal is maintained, with additional length cut to size at plate gauge, or coiled onto a spool at about 3mm thick (intermediate gauge), ready for further treatment to produce finished sheet and foil gauges.
The intermediate gauge aluminium coil is then cold-rolled - usually reel to reel through a series of pressurised rollers - to final gauge specification. At this stage, any specified coatings or lacquers are applied, before the coil is edge-trimmed ready for shipment to the customer.
The rolling of aluminium changes its metallic structure and the metal takes on new characteristics and properties. The brittleness of the coarse cast structure is replaced by a stronger and more ductile material, with degrees of strength and ductility controlled by rolling pressure and number of passes, as well as alloy composition. A thermal annealing process may also be employed after hot and cold rolling, to ensure the correct balance between flexibility and strength for different applications.
Careful attention to the mechanics and chemistry of the rolling process has enabled fine surface finishes and, most importantly, gauge uniformity, to be achieved - crucial benefits for the production of drinks cans, foil packaging and auto-body parts.
Drinks cans
Aluminium sheet for drinks cans is alloyed with manganese and magnesium for strength and ductility, with slightly different alloys and thicknesses for can bodies and lids. The precision of the gauge achievable with aluminium has led to its use for the easy-open tab end of both steel and aluminium cans: when the metal is scored to break under pressure from the tab, a consistent thickness of metal beneath the score line is crucial to reliable performance, and aluminium provides this consistency.
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Aluminium foil
Different alloys and gauges of foil are used for different packaging applications, with most alloys up to around a 3% mix of iron, silicon and manganese, with tiny amounts of copper occasionally added for extra strength. The thinnest foil used for wrapping chocolates may be only 6 microns, with household wrapping and cooking foil between 11 and 18 microns, lidding foil between about 30 and 40 microns and foil for foil trays generally between 40 and 90 microns.
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