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Aluminium Foil

Manufacturing, properties & Uses

History of Aluminium

➢ Aluminium is the third most abundant element in the earth’s crust. Aluminium is made from

its ore called bauxite, a reddish-brown rock discovered in Les Baux, France, in 1821. ➢ A French geologist Pierre Berthier discovered bauxite near the village of Les Baux - de- Provence, in southern France.

Bauxite is an important ore which is used for

making aluminium. It is an oxide of aluminium

(name derived after Les Baux in France).

Clay-like block of bauxite

The aluminium & Aluminium foil production process

➢ Alumina (Al 2 O 3 ) is refined from the ore bauxite in a caustic soda/high temperature process. ➢ Aluminium is obtained by an electrolytic smelting process, which breaks down the alumina into molten aluminium and oxygen. The silver-coloured molten aluminium is syphoned off into furnaces where other elements are added to produce particular alloys, before being poured into moulds to form ingots. ➢In round figures it takes 4 tonnes of bauxite to make 2 tonnes of alumina, which makes 1 tonne of pure aluminium. ➢The ingots of aluminium are heated to make them more malleable. Heated ingots are then rolled & passed backwards and forwards through large rollers. In this process the slab gets thinner and thinner, longer and longer. ➢This metal strip is hot rolled to a thickness of 2 to 6 mm and then coiled, before being cold rolled to metal thicknesses of between 6 and 400 microns. ➢The thinnest foil used for wrapping chocolates may be only 6-7 microns thick, with household wrapping foil between 11 and 18 microns. Pharmaceutical blister & strip foil thickness varies from 20 to 40 microns, ➢The foil is then annealed by a thermal process to make it pliable, great care is taken to ensure the correct balance between flexibility and strength for different applications. ➢The most common foil alloys - the 1000, 3000 and 8000 series - contain between 0.5% to 1.5% iron, 0.1% to.7% silicon and 0.02% to 1.5% manganese. Up to 0.2% copper may be added for additional strength.

**Fe Si Mn Ti Al Cu Mg Zn Impurity 0.6 - 1 0.5 - 0.9 max 0.2 max 0. 97.57 -

max 0.1 max 0.05 max 0. other, each 0.05; Chemical composition in % for grade 8011 ( 8011 )- Mostly used in packaging applications**

➢Traditionally the molten metal is cast after refining and alloying processes into 10 to 15-

ton slabs in semi-continuous casters. The slabs are then pre-annealed & hot-rolled in

single-stand or tandem hot rolling mills. When the strip thickness becomes 6 mm or less

it is coiled at a temperature of approx at 300 deg Celcius

➢After the hot strip production the cold strip rolling starts. Even though this deformation

process is called “cold” rolling, the strip is heated up to approx. 100 °C during each pass

and large quantities of coolant are poured over the rolls to keep a thermal equilibrium.

➢After three to four passes, the coils have to be cooled down to room temperature for

several hours. During each cold rolling pass, a material hardening is effected due to

deformation process of the strip.

Aluminium Foil Manufacturing

The Rolling processes

Rolling is the process of reducing the thickness

or changing the cross section of a long

workpiece by compressive load applied

through a set of rolls, as shown in figure

Aluminium foil is usually produced by rolling down strip in a number of subsequent cold-

rolling steps, which reduce the material thickness by almost a half in each step.

Cold rolling of foil

For a final foil thickness of 60 μm and below, two layers of foil are wound together before the last rolling step. Rolling oil is sprayed between the two layers as a release agent and the “twin foil” is rolled down to the desired final thickness. After the final rolling step, the foils are separated in a “de - doubling“ machine, slit and wound in the form of a coil. Most of the foil in this so- called “hard“ state is subsequently annealed as a coil, where the grain structure is recrystallized and the rolling oil is removed.

➢After casting, the ingots are scalped, i.e. the casting skin is removed by machining and the

two ends are sawn off.

➢This is a necessary step, because in their as-cast condition, these particular parts of the

ingot are not metallurgical homogeneous and so are unsuitable for thin-gauge foil

production.

➢ All process scrap is conducted back to the melting furnaces for immediate recycling.

❖ The DC(Direct Chill semi-continuous cast) ingot is usually cooled after casting to room

temperature and then re-heated to around 500 °C prior to successive passes through a

hot rolling mill where it is reduced to a thickness suitable for cold rolling.

❖ This continuous casting process bypasses the ingot stage and converts molten metal

directly into a thick strip which is immediately rolled into the coil from which the

thinner gauge foil is produced by cold rolling process.

➢ During hot rolling, the ingot is first reduced in thickness down to 6 mm or less by passing it several times forwards and backwards through a rolling mill. ➢ The temperature of the metal has to be maintained above the re-crystallisation temperature (350 - 400 deg.C) so that the material's metallurgical behaviour is suitable for subsequent processing steps. ➢ On completion of the hot rolling process, the strip is coiled for the first time. Cold rolling and slitting then take place. The coils are delivered as "reroll" or "foil stock“ downstream foil mills.

SLITTING

HOT

ROLLING

COLD ROLLING

After rolling, a "reverse doubling" operation takes place. In this, the doubled foil is simultaneously separated into single layers and slit to the required width and length according to customers' requirements. As aluminium, similar to most other metals , becomes more and more inelastic with each rolling pass. Apart from some end products which use foil in the "hard” temper or "as - rolled" condition, such as lacquered and printed "push-through" lids for blister packaging of solid dosage pharmaceutical products, aluminium foil is usually required in the “hard" temper form To obtain "soft" foil, the material has to be annealed at a temperature of around 400 to 500 ºC for Several hours. The time and temperature are carefully set to allow the metallic grain structure to recrystallise and, at the same time, to remove the rolling oil from the surface of the aluminium foil by evaporation. The improper removal of this rolling oil will result in poor wetting properties and in the low adherence of coatings or adhesives during the converting stage