4. Principles of extrusion
Extrusion starts with ingots of aluminium alloy. These are cut into billets, which are then heated in an induction – or, gas – furnace to the correct extrusion temperature of 450 – 500°C.
The heated billet is then forced through a die under great pressure, and the finished profile is squeezed out of the die rather like toothpaste from a tube.
The profile emerges at a speed of 5 to 50 meters per minute and is normally between 25 and 45 meters long. The profile is cooled using air or water as it leaves the die.
After cooling, the profile is stretched to relieve any stress and to give it the desired straightness. The quality of the surface and of any dimensions that are important to the function of the profile are checked at the same time. The profile is then cut to a suitable length or to the length requested by the customer.
The final strength of the material is controlled by natural or artificial ageing.