Before we begin with extrusion
Before the extrusion process can begin, the manufacturing company will select the appropriate aluminium or related alloy for the desired finished profile. This comes in the form of an aluminium log, cast in varying lengths of up to seven meters and a range of diameters. After these parameters are set and the metal has been cast, the logs are heat-treated or ‘homogenised’ and finally shipped off to the extrusion facility.
Pre-heating the billet
When the extrusion log arrives at the plant, it has to go through a series of processes before they can begin extrusion. The log is initially cut into shorter, more suitable lengths for extrusion. These new lengths are known as Billets and their size is normally governed by the press capacity of the plants machinery, and by the final desired length of the extruded profile. Before extrusion begins, the plant heats the Billet to around 450°C. The temperature of the pre-heating process is decided by the alloy, complexity of the dies shape and the press’ capacity.
The main event – Extrusion
Extrusion relies on the combination of both heat and pressure in copious amounts. The pre-heated Billet is subjected to pressures ranging from 1600 – 6500 tonnes dependent on the machines press capacity. The pressure forces the Billet through an opening in the steel die, the shape of which dictates the shape of the final profile. The speed of extrusion is governed by how aluminium-rich the metal is and the complexity of the die profile.
There are two types of extrusion, direct and indirect. The most commonly used method is direct, this involves a stationary die which the Billet is driven through by the press machine. Indirect extrusion is a mirrored process to this, where the die moves against the stationary billet. The desired press force is high enough to sufficiently push the aluminium smoothly through the die, if too much force is applied the metal will escape the confines of the die spilling over the sides. The extruded Billet can exit the die in lengths as long as fifty metres, in varying widths, in single and multiple strands.
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