Aluminium encyclopaedia

Drawing

In metal and aluminium processing drawing describes forming in the following manner: a blank pointed at one end is fed into a die, which is a hole that tapers in the direction of pull in a carbide, ceramic or steel tool. The pointed end is gripped by a pair of pincers on the end of a chain, which is wound around a revolving drum, and the blank is pulled through the die. For tube drawing, a mandrel is positioned in the die aperture (and fixed using a rod that passes through the hollow blank) and the blank is drawn between the die and the mandrel. Extruded profiles, tube, and rod and bar with a shape similar to the final shape are used as blanks; for wire, strands produced by extrusion or casting and subsequent rolling are used.

Drawing can be repeated with ever smaller die apertures or carried out with two dies positioned one behind the other until the desired cross-section is obtained. The higher dimensional accuracy compared with extrusion is one of the benefits of the process (and often the reason for using it); other merits are the smooth surfaces (which eliminate the need for surface treatment) and the possibility to manufacture thin-walled tube or extremely thin wire.

Drawing is usually carried out "cold" at room temperature, which leads to hardening of the material as a result of the deformation. Where there is a large degree of deformation, such as in wire drawing, the material has to be annealed between passes (see heat treatment).

Profiling or roll forming is a special form of drawing and rolling: strip is formed into a profiled shape (such as trapezoidal sheet used in roofing) by passing it between several sets of rolls arranged one after the other.