by the lost wax process is easily done with PureGold. However, there
are significant differences between casting a 14 K alloy and casting
a 24K microalloy. Since the microalloy melts at 1073.9 degrees Celsius,
it is best to use an induction heated melting device although this
is not essential. Casting can be carried out with a torch if care
is taken not to overheat the metal.
A casting temperature of 1175 degrees Celsius is a recommended
starting point for most patterns and a recommended flask temperature
is 600 degrees Celsius. These can be fine tuned depending on the
models being cast and their degree of fine versus heavy metal sections.
This casting temperature also indicates that the investment should
be phosphate bonded as the metal casting temperature equals the
temperature where gypsum bonded investment begins to decompose.
A gypsum bonded investment can be used if the wax patterns are first
coated with a ceramic emulsion before investing. The ceramic shell
prevents contact of the molten metal with the gypsum. It also provides
a very fine surface to the cast pieces.
The microalloy has a very narrow freezing range and this
indicates that gating is critical for successful casting. There
should be no transitions from thick - to thin – to thick metal
sections without a supply of molten metal to the thick section.
Following casting and a cool down period (at least thirty minutes
is recommended, the hot flask should not be quenched), investment
can be easily removed with a high pressure water stream. If there
is a slight skin of oxidation on the surface of the casting from
residual oxygen in the flask, it can be easily removed by a quick
dip in pickle.
Following removal of the gates, and before final finishing,
the age hardening step is completed. First the castings are brought
to a uniform state by heating at 800 degrees Celsius for one hour
followed by quenching. The casting is then hardened at 250 degrees
Celsius for three hours followed by air cooling. Both of these steps
can be carried out in air as the microalloy does not oxidize. Cast,
age hardened pieces will have a hardness of approximately 110 Vickers.
This is slightly greater than most platinum castings indicating
that designs made for platinum casting work well in PureGold.