The aluminum extrusion process
Aluminium Extrusion is a plastic deformation process of a pre heated aluminium billet which is forced to flow by compression through a steel or ceramic die with an aperture of a smaller cross-sectional area than the original billet.
The extrusion process can be used to produce solid sections, bars, rods from solid billets through flat dies. Tubes and hollow sections can be produced by extrusion through porthole or bridge type dies (for certain alloys).
Seamless tubes and hollows sections must be produced using either hollow billets or solid billets using a piercing press and mandrel system.
There are two main methods of aluminium extrusion.
In direct extrusion the preheated billet is placed in a heated part of the press called the container. Here, the billet is pushed through he die by ram pressure. The direction of the metal flow is in the same direction as the ram travel. During this process, the billet slides relative to the walls of the container, resulting in a necessary increase in ram pressure to overcome the friction between the billet surface and container liner.
Not all the aluminium billet is extruded. A percentage of the compressed billet, called the discard or butt is left at the end of the extrusion cycle. This is ejected from the press during its dead cycle operation, and ‘recycled’ at a later time. The extruded profile is then transferred on to the press’s handling system for further process.
With indirect extrusion, the die located at the front end of a hollow stem moves relative to the container. The die is either pushed through the container or the container is pushed over the die. There is no relative displacement between the billet and the container liner. Therefore one advantage of the indirect extrusion process is that there is no friction, during the process, between the billet and the container liner. The metal flow is more uniform during indirect extrusion than direct. This usually relates to better section uniformity along its length.
A discard/butt is produced as with the direct process, which is handled in the same way. The extruded profile is again transferred on to the press’s handling system.