17
MELTING AND CASTING PROCESS
MELTING AND CASTING OF PRECIOUS ALLOYS WITH CRUCIBLE COMPOSED OF
AN INTERNAL GRAPHITE LINER A100.059 CAPACITY 80 gr Au
♦ The crucible is composed of an internal graphite liner of high quality. It is suitable for all precious alloys and
for ceramic alloys with a casting point of max 1450°C, excluding the precious alloys which have over 30%
palladium contents.
♦ For remelting old sprues, buttons, shanks, it is necessary to cut them into small pieces so that they perfectly
adhere to the crucible walls for an efficient heat transfer from crucible to metal. Generally it is recommended
to use 70% of new metal and 30% of old metal. The new metal usually in shape of small plates must be put first
in the crucible and when melting starts the old metal is added.
♦ During melting the graphite crucible produces a reducing atmosphere which will almost completely deoxidize
the precious alloy. Avoid the use of flux of whatever kind. Should the alloy be not completely clean, use pure
boric acid (H3B03) or deoxidizers supplied by the precious alloys manufacturers.
♦ It is always recommended a slow and gradual heating which can be easily reached since the machine OKAY
PLUS has a stepless heating power regulator: Recommended position from 20 to 40. Remember that the metal
is close to the melting point when the colour of the metal is approximatly the same as the graphite liner colour.
♦ All alloys have a “melting interval” expressed in temperature degrees, indicating the start of the melt and the
liquidus point ex. 1150-1200 °C. After the alloy has reached its liquidus point ( a complete melt) it is necessary
to attain the ideal casting temperature. This period of “overheating” the alloys is referred to as the “heatsoaking
interval”. The ideal casting temperature is generally 50-200 °C higher than the liquidus point of the alloy and
it must be left at this temperature from 5 to 30 seconds according to its quantity.
⇒ EXAMPLE = Precious alloy with melting interval 1150-1200 °C and casting temperature 1350 °C.
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Set the SET-POINT on 1350 °C and when this temperature has been reached, place the flask on the centrifugal
arm (when the machine cover is opened, power is interrupted and it will be automatically reset when cover is
closed). The operator will decide when it is the correct moment to cast. The heatsoaking interval from 5 to 30
seconds, depending on the metal quantity, is generally indicated by the alloy manufacturer or by personal
experience.
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If necessary, collect the unmolten pieces on the crucible bottom using the steel rod which will be introduced
through the lid on the crucible.
A100.059
MODULAR 6
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