Metal drawing can refer to either the method for producing wire or the process used to make deep, cup-shaped forms, such as stainless steel sinks and soft drink cans. This article will concentrate on the wire drawing process, although an overview of deep drawing will also be provided.
Wire drawing consists of pulling a metal wire through a small circular opening called a die. This results in a stretching or elongation of the material along with a reduction in cross sectional area. The pulling force is limited by the strength of the material: pull too hard and the metal will break. The force needed to pull the wire through the die is determined by the extent of the reduction in cross-sectional area: the larger the reduction, the greater the force needed. Thus it can be seen that the maximum achievable reduction in diameter is limited by the yield strength of the wire.
Yield strength depends on material composition but typically the reduction in area through a die is in the region of 20 to 40%. If a greater reduction is needed this must be done by drawing the wire through a series of dies, each one smaller than the one before.
However, the plastic deformation experienced by the metal as it is pulled through the die tends to increase hardness and reduce ductility. (Ductility refers to the ease with which metal can be deformed.) As this will make it harder to reduce the cross-section, it is often necessary to perform an annealing process between successive draws to improve the way the metal deforms. On the other hand, the increased tensile strength resulting from drawing is often seen as a very desirable material property.
Wire drawing is usually performed cold, although there are some cases where metal is drawn hot to improve ductility. Die lubrication is essential in cold drawing to achieve a good surface finish as well to maximize the life of the die.
In contrast to wire drawing, deep drawing is a sheet metal process where the material is stretched over a male form. This can be used to create complex three-dimensional shapes such as sinks or beverage cans. This is often carried out in progressive dies, where the metal workpiece is moved through a series of tools that gradually stretch the material to the required form. One design issue to be addressed in deep drawing is that as the metal stretches it also becomes thinner. Unless care is taken in engineering the way the metal deforms this can result in hole in the workpiece.
Both wire drawing and deep drawing involve stretching metal to the required shape, and as such are considered deformation rather than removal processes. A significant advantage of drawing is that there is very little material waste. However, this benefit has to be set against the high cost of the dies and the possible need to carry out annealing to counteract work hardening.
