Casting - Gold & Silver
The casting of gold and silver alloys is now carried out in a Vacuum Pressure Casting machine. The metal is melted in a graphite crucible that is heated by an induction coil. This is like a highly concentrated microwave oven. The hot mould with a perforated steel sleeve is placed in the lower chamber. The advantage over the older open centrifugal system is that VPC machines have a totally enclosed and controlled casting environment. This alone has resulted in significant improvements in the metal quality, as oxygen is no longer present during the casting cycle.
When the metal has reached the desired casting temperature, the operator will activate the casting sequence.
The casting chamber is flushed with helium to remove any erroneous gases that may be present. Then a vacuum is applied to the lower chamber, the stopper is lifted in the upper chamber, and gravity allows the metal to flow into the mould. This sequence is entirely controlled by preset programmes as the valve opening and closing times are very quick and critical. When the cycle has completed, the vacuum is released and the chamber opens, then the cast flask is removed, and quenched in water to reveal the cast tree.
