The Lost Wax Process
Part 4: Casting the Metal
Once the flask has matured in the burnout oven, it is time for the fun to begin: injecting it with glowing hot, molten metal!
The centrifugal casting machine has a spring driven arm that holds the hot flask and a crucible that contains the molten metal to be injected into the flask. The arm of the machine “breaks” near the crucible end to help keep the metal in the crucible as the machine accelerates up to speed. The other end has adjustable weights to help balance the machine. The heavy spring inside the base provides the centrifugal force that forces the molten metal into the flask.
It is important that the flask be just the right temperature for casting. If it is too hot, the metal will not cool and harden and will just flow back out. If it is too cool, the metal will solidify too quickly and form a clog inside the mold.
With the burned out flask simmering in the oven at the correct casting temperature for whatever metal is to be used, the centrifugal casting machine is prepared for launch. It is important to bring the flask to just the right temperature for casting. If it is too hot, the metal will not cool and harden and will just flow back out. If it is too cool, the metal will solidify too quickly and form a clog inside the mold.
- A centrifugal casting machine with crucible and flask in place ready to cast sterling silver Thor’s Hammer pendants.
The metal to be cast is weighed out and placed into the centrifuge’s crucible. The right amount of metal is determined by weighing the wax model multiplied by 10 for silver, or 13 for gold. Then add 20% for the “button” – a reservoir of metal for the casting to draw from as it shrinks.
The grain is placed into the crucible and then melted using an oxy/acetylene torch. A small amount of flux is sprinkled onto the metal to help clean it. Its heated until it turns glassy and rolls like mercury and then the arm of the centrifuge is released, spinning the metal into the flask.
After the machine comes to a rest, the hot flask is removed from the cradle of the casting machine and plunged into a bucket of water. The thermal shock causes the “investment” to explode, releasing the metal casting inside.
The casting is done, and all that’s left is finishing and polishing.