After casting of the slabs, next operation involved in the manufacture of aluminum circle is re-rolling. Rolling is an operation by which various metallic structural shapes, section, sheet, etc. are made from ingots and caste slabs. This may be regarded as analogous to remolding of thermoplastics. The tools employed are rolls which control the reduction and shaping of ingots and slabs. For sheet rolling plain rolls are used and for sructural section rolling plain rolls are used and for structural section rolling groove rolls are used. In the case of aluminum, however, extrusion presses are used for manufacturing structural section. In sheet rolling the aluminum slab is made to pass through sets of two or more cylindrical rolls the center to center distance between which is adjustable. At any time the gap between the rolls is kept slightly smaller than the thickness of the sheet/slab being fed into them for reduction of the thickness. The sheet/slab after passing through these rolls undergo a reduction in their thickness and increase in their width. In any one pass only small reduction in thickness is achieved, therefore, the operation is repeated several times after reducing the gap between the rolls each time.
When the slab is reduced to a thickness of about 2.8 mm the slabs are subjected to intermediate annealing at a temperature of 350oC to 400oC for about one hour. This helps to relieve the stresses in the material brought about by work-hardening. If this operation is not carried out further rolling may cause cracking of the sheet and other surface reduction in thickness. The annealed slabs are again fed into the rolling mill to roll it down to 18 to 22 SWG thickness. Coconut oil is applied to the surface of rolls to facilitate smooth rolling and avoid undue heating.