While in vacuum casting machines with an open furnace chamber one can afford to carefully pour the product during its melting, in the case of pressure-vacuum devices this kind of action would be associated with the unsealing of the hermetic furnace system.
So, what to pay attention to?
At the beginning, one should ask the question what the casting machine will be used for. If you plan to cast only gold alloys, make one-off castings (artistic jewellery) or the machine will be used occasionally, then we can afford to prepare the alloy before casting. Cutting large metal nibs will make it easier to fill the crucible.
NOTE: It is not allowed to "stuff" the metal in the crucible. Temperature expansion can break the walls of the crucible and result in the melt leakage.
If our goal is mass production, one-time casting of one or two full loads of the flask burning off furnace, in addition, the material on which we work is silver, then the time of performed operations plays a significant role. Especially that in most cases the shanks from old trees will be melted. The optimal solution is to load the crucible with the material without having to prepare the alloy. To make this feasible, the largest possible space between the wall of the crucible and the graphite stopper is important.
In practice, this space is obtained by using a crucible whose cross-section is drawn into a square.
Comparing two crucibles with the same volume and different heights, we can clearly see that with the increase in altitude we lose the capacity of the crucible. This effect is influenced by the stopper, whose diameter is identical in both cases and must be longer in the high crucible.
The ideal solution is a graphite crucible in which the stopper together with the drain hole is located off centre at the wall of the crucible. The charging space is then as large as possible, guaranteeing comfort during work.
The most popular way to measure temperature in the jewellery industry is to use a thermocouple. In most cases it is a K-type thermocouple, or NiCr-NiAl. This is due to the fact that the thermocouple works in the temperature range up to 1200 ° C and is relatively inexpensive.
For the temperature to be measured with the smallest possible error the device would have to be immersed in the liquid alloy whose temperature is measured. Of course, such a situation cannot be allowed, for example due to the physicochemical properties of the alloy and thermocouple.
In order for the thermocouple to be able to serve us as long as possible and to be sure that the measurement reflects the true measured temperature value, three methods of placing the thermocouple are used:
1. Measurement with the use of a thermocouple in a graphite shield immersed in the alloy is the best solution from the point of view of measurement accuracy. This method is most often used in bottom drain foundries, where the graphite cover of the thermocouple is at the same time a stopper in the crucible‘s drain hole.
2. Temperature measurement in the crucible wall is subject to an error resulting from the fact that the thermocouple is not in the alloy itself. In addition, the crucible wall is the place with the lowest temperature in the entire system, especially in the case of crucibles with a ceramic cover. In this case, you should take into consideration a possibility of such an error. The temperature of the crucible wall can be below the melt temperature by about 50°C.
3. The temperature measurement of the bottom of the crucible is burdened with the possibility of the biggest error among the previously mentioned systems. The sensor indication can be even worse when the crucible is in the ceramic cover. Ceramics is a heat insulator whose main goal is to limit the emission of high temperature towards the inductor, in this case also towards the thermocouple.
This solution is quite popular in induction furnaces manufactured by Italian producers.