Drawing is a plastic processing that involves passing the material, in the form of bars of different sections, wire rods or bars, through a die or spinneret [fig. 1], consisting of a steel plate provided with a series of calibrated flared holes gradually decreasing in size, so modifying its section until it reaches that desired. The section reduction that is obtained depends on the material and characteristics of the product to be produced.
This process can be performed hot, but more frequently it is performed cold. In the latter case, as in the cold rolling, a material hardening occurs which involves the need for subsequent annealing treatments. Cold rolling is particularly used to derive rods and wires.
The machines that carry out the drawing are called drawing machines and can be of various types: for wires, for bars, multiple. For special uses, sintered hard metal spinneret can also be used and, for very fine threads, diamond dies.
The metal thus stretched leaves the drawing machine with a smooth surface and with a strictly constant section equal to that of the spinneret holes.
Extrusion means a plastic transformation of metals, plastics, elastomers and pastes consisting in passing the material through a shaped hole of a matrix or die by applying high pressures [fig. 3] and it can be performed hot or cold. Extrusion processing however is not very common for steel because it requires very high pressures and material heating at 1100 °C.
There are two types of extrusion processes, direct and indirect. Direct extrusion is a process in which the die head is held stationary and a moving ram forces the metal through it. Indirect extrusion is a process in which the billet remains stationary while the die assembly located on the end of the ram, moves against the billet creating pressure needed for metal to flow through the die.
This process gives the product a defined shape and can change the properties of the material due to the temperatures to which it is subjected. By extrusion we produce tubes, bars and profiles of various sizes and shapes, even complex, with good surface finish and dimensional accuracy.