The process is semi-continuous. One billet is extruded within a couple of minutes after which the extrusion is
paused to introduce a new billet hence known as the billet on billet extrusion. Before the new billet is introduced
into the container the outer skin of the billet accumulates in the back of the billet due to high friction between the
container and billet. This is known as the back end defect, and is sheared off because the outer layer contains coarse
iron-rich intermetallic and Mg2Si precipitations that are not suitable for extrusion[2]. The aluminium from the new
billet welds on to the material left in the die at the next press stroke, causing a continuous product to exit the die.
The region where the new billet welds onto the old billet is known as transverse weld; which is clearly visible on the
extruded product. The transverse weld is typically scrapped due to its reduced mechanical properties and surface
finish.