As was previously stated, during the direct extrusion process, the energy expended to overcome the friction between the billet and the container causes the temperature at the back end of the billet to increase in many operations. This is compensated by slowing the press at the end of each billet cycle. If the ram speed is kept constant, the quality of the surface of the extrudate will deteriorate at the backend. Taper heating (or cooling) the billet will allow extrusion to proceed at a constant ram speed and the extrudate will emerge from the die at a constant temperature to achieve isothermal extrusion (Figure 5 and 6).All high performance aluminum extrusion presses employ some form of isothermal extrusion. This is essential if back end defects are to be minimized, and requires that the temperature of the back end of the billet when loaded into the press container is Tower than the front end. This compensates for the energy imparted to the back end of the billet during the extrusion cycle.The billet temperature at the end of the heating cycle must be uniform across the diameter and be controlled to a predetermined gradient along its length. The desired temperature gradient is achieved by controlling the energy input along the length of the billet in direct flame impingement or electric induction furnaces; by taking heat out in a controlled quench, or taper heating after first heating the billet to a uniform temperature. Precise control of temperature gradient is best achieved by controlled movement of the billet through the heating or quenching system to achieve the desired temperature gradient.
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