Aluminium encyclopaedia

Deep drawing

Packaging such as trays, beverage cans and bottle closures are usually produced by deep drawing. This is actually a forming process that has nothing to do with "drawing". A blank produced from strip or sheet (a so-called "circle" or "slug") is forced through a draw ring by a punch to produce a hollow shape in the form of the punch. The final shape can be obtained in one or more operations. As the process takes place at room temperature, the aluminium is strengthened as a result of cold working; for certain products, therefore, it may be necessary to heat treat the aluminium between stages (intermediate annealing) in order to nullify the strengthening. In order to be suitable for deep drawing, aluminium should have a fine grain structure. In addition, it is beneficial to have aluminium whose strength is between "hard" and "soft" (because of the reduced tendency for cracking) and a composition that is less pure than unalloyed aluminium.

Cold stretch forming is a variation on deep drawing. Strip, foil or composite packaging materials can be formed into dish shapes without the rigidly clamped material springing back (for example trays for ready meals or pet food, see Packaging).

Deep drawing of superplastic aluminium alloys, such as an alloy with six per cent copper and 0.5 per cent zirconium, is of interest for isolated applications. These alloys can be stretched to more than ten times their original length if the grain structure is very fine (grain size less than a ten-thousandth of a millimetre), the deformation temperature is very high and the rate of deformation is very low. The long duration of the forming process contrasts with high dimensional accuracy, even for the most difficult shapes, and low tool costs.