We do not wish here to discuss in detail now-superceded methods such as that devised by Solbrigg and Taggart in 1907 where wet asbestos was placed on top of a molten charge of metal which in turn rested on the flask sprue entry of the flask, the steam generated forcing the metal into the mould. Neither do we wish
to detail the various torch-melting centrifugal systems or manually poured vacuum casting systems, which were certainly capable of producing castings, but of very variable quality due to lack of control over yhe variables.

We shall instead look at the systems in current use, namely:

• Mould occupation by centrifugation, melting by induction
• Mould occupation by vacuum-assisted gravity method, melting by induction

There are approximately 10 manufacturers of jewellery "lost wax" casting machines in the world. We intend to talk about machines of our design and manufacture here. We have recently carried out a series of detailed comparative tests on centrifugal, gravity and vacuum casting systems. These tests were brought with difficulties due to the very short time span of the process of metal injection and solidification (0.5-1.5 seconds) and also due to our pressure and temperature sensor frequently being blocked off by molten metal.

The conclusions drawn are described briefly as follows.

Centrifugal casting

During centrifugation, the pressure acting on the melt depends upon the varying acceleration from rest of the casting an-n. Metal flow commences at a low acceleration after 1.3 seconds, at a rotational speed of 30 RPM. The casting arm has during this interval, moved through about 120'. Pressure on the melt reaches its maximum (about 0.6 bar) after about 2.4 seconds. The occupation of the mould is then complete (figure 53).

Maximum gas/air pressure developed in the mould depends upon the acceleration of the arm; the higher the acceleration the higher the pressure, whose maximum is achieved in between 0.5 and 2.5 seconds from rest.

The final maximum speed of rotation has no bearing on the changes in gas pressure, since the pressure rise and subsequent fall have normally occurred before that speed is reached.

Without complicating the picture with mathematical formulae, figure 54 demonstrates the difference in compressive forces acting on molten metal entering the flask. Figure 55 shows a hinged casting arm such as we employ on our larger centrifugal machines, where both the counterweight and the flask/crucible are pivoted in such a manner that, starting from a "Z" configuration, the arm progressively straightens itself as it takes off from rest. Our smaller machines have rigid (non?hinged) arms (machines with a capacity of up to 3 Kg fine gold). In all cases, the arm geometry is determined by considerations of optimum flask length, correct acceleration and terminal rotation.

Crucible design and attitude are also important. Our crucibles rest at an angle of 7' to 10' towards the flask (figure 56) thus enabling the metal flow to commence immediately at the inception of rotation. The size of the molten charge of metal is also important here.

Upon entering the flask, the metal immediately encounters an obstacle consisting of air and gases within the mould cavity. Although the fired investment is permeable, these entrapped gases cannot always escape sufficiently quickly by this route and may, thus, be entrapped within the solidifying metal.

The effect is comparable to pouring a liquid into a closed container (figure 57) with no exit route for the air within.

The mould should, therefore, be evacuated prior to metal occupation and, on our casting arms, this is accomplished by means of a large vacuum pump acting on the rear of the flask immediately prior to and during rotation.

Figure 58 shows the removal of harmful gases during metal entry to the mould. The vacuum is maintained throughout the whole of the period of rotation.

Figure 59 depicts the mould being occupied by the fluid mass under vacuum.

Figure 60 illustrates one of our most convincing tests, where a nylon net of thickness 0.26 min was invested, burned out and metal injected into the cavity left. A sound casting 50 mm long with only one sprue to the base was obtained in 18 carat gold.

Figure 61 shows a centrifugal casting machine featuring automatic temperature control. Figure 62 shows the interior of the casting chamber of this machine with the flask positioned against the suction backplate of the arm. This machine is capable of performing the entire casting cycle (melting and centrifugation) in less than six minutes, with the melt temperature maintained automatically and featuring the aforementioned vacuum flask extraction system, permitting the casting of extremely fine sections.

This machine offers the possibility of using either graphite or ceramic crucibles - also a recently-introduced ceramic-lined graphite crucible to eliminate the possibility of graphite inclusions within the melt. Casting of platinum and steels is also feasible with this machine.

Figure 63 shows our latest "under vacuum" centrifugal unit, where the melting and casting cycle may be carried out entirely in an air-free environment.

Vacuum-assisted gravity casting

The general scheme of this casting system is depicted in figure 64.

In the preceding section on centrifugal casting, reference was made to the pressure applied to the stream of molten metal. In the case of vacuum casting, we have to consider mould occupation by means of a vacuum acting through the pores of the investment.

In practice, we have discovered that the maximum vacuum effect on the metal is achieved at the moment of metal entry - normally after about 0.5 seconds - and then progressively decreases as entrapped gases and air are drawn out.

Permeability of the investment mould is obviously an important factor in this process and this is in turn influenced by the water/powder ratio of the investment mix. Tests have demonstrated that a low water content (35%) tends to create mould cracking. A 37% content produces a mould which is insufficiently permeable - even with a high vacuum applied - and a 40% content has been shown to produce a highly permeable mould enabling a more effective vacuum effect to be achieved during the casting process. The rate of increase of this effect and the rate of mould occupation also depends on the external pressure.

The higher this rate, the more efficient the occupation or filling, although it has not proved possible to provide facts and figures experimentally due to the relatively slow response time of the sensing equipment currently available. The whole of this process is normally completed in 0.5-1.0 seconds. In practice, this process is generally unsuitable for very fine section and filigree objects, which tend not to fill completely. To assist in filling, we have devised a refinement to the system.

A positive pressure is applied to the surface of the melt during casting. This has to be applied at the moment of metal entry; a mere moment later, and this will be ineffective due to the metal having already partially solidified.

One major difference between castings produced by this method and those produced centrifugally is in respect of uniformity of density.

Castings produced centrifugally tend to vary slightly in density due to random forces acting on the metal during centrifugation; this is not the case with the vacuum-assisted gravity method. Also, the constant action of the vacuum on the cooling and solidifying metal tends to produce a gradual and uniform cooling effect, which can be beneficial.

Another plus point to this process is that it is not necessary to add additional metal for a sprue "button"; this, thus, reduces the amount of precious metal being recycled with a consequent reduction in "losses".

Figure 65 shows a vacuum-assisted gravity casting machine, where the cycle is practically fully automatic. The metal is rendered molten in an inert gas atmosphere - also pouring into the mould, which is evacuated during casting with a heavy-duty vacuum pump. Special systems control metal temperature and pouring (figures 66 and 67) and the machine uses the new ceramic-lined crucible mentioned in the previous system, thus eliminating the possibility of graphite inclusions. This machine will handle all metals apart from platinum and steels.

The equipment depicted in figure 69 is another approach to vacuum casting, featuring a "bottom pour" crucible with positive pressure above the melt, as previously mentioned. The machine also features a medium-frequency generator and has fully computerised surveillance and control/printout. It is capable of memorising several casting programmes in several languages.

Investment removal, sprue cutting and finishing operations

After casting and quenching residual investment has to be removed from the cast "trees" without affecting surface or mechanical properties and the most efficient way to do this is by means of a high pressure pump within an enclosure.

Figure I shows such an enclosure mounted onto a settlement tank for the powder which is thus prevented from finding its way into - and blocking - the drainage system.

Parting-off of the cast patterns from the "tree" may be performed using either hand or pneumatic cutters (figure 73).

The final cleaning-up and investment removal is performed in a suitable ultrasonic cleaner (figure 74). The frequency of such a unit (35 kHz) removes every tiny trace of investment in a remarkably short time.