Static machines

All modern, good quality static casting machines are “vacuum assist”, i.e. are equipped with a suction system, acting through the flask, which facilitates mould filling, Figure 4.6.11. The best machines are equipped with separate crucible and flask chambers. In this way, process time can be shortened further.

Nearly all static casting machines operate under an inert atmosphere, usually nitrogen or argon although some use a hydrogen/nitrogen reducing atmosphere.

Presently, argon is frequently preferred, even if it is more costly than nitrogen. The machines can also be equipped with a pressure system, acting (after pouring) only in the flask chamber on the sprue button to facilitate better mould fill and surface detail. In some very recent machines, pouring is also carried out under pressure.

Figure 4.6.7 Centrifugal casting machine operating in an inert atmosphere, with suction through the flask bottom

Figure 4.6.8 Detail of the crucible zone, without the flask

Figure 4.6.9 Detail of the crucible zone, with the flask and the thermocouple for liquid metal temperature measurement

Figure 4.6.10 Detail of the programming system of a high performance centrifugal casting machine

Figure 4.6.11 Modern basic level static casting machine

In many machines, pouring takes place through the crucible bottom; this minimises heat loss during pouring, allowing a lower degree of superheat and also reduces the risk of oxide entrapment in the casting, since any oxide on the surface of the melt will tend to fill the sprue button.

Many casting machines can be equipped with a grain-making accessory, for making casting grain.

Heating and temperature measurement

Many static machines are induction heated, although more basic small machines can be electrical resistance heated. In general, the better quality machines have medium – low frequency induction. Heating depth and hence melting speed as well as electromagnetic stirring forces increase with decreasing frequency.

Temperature measurement can be by optical pyrometer or, better and preferable, by a sheathed thermocouple dipped into the melt, often via the central stopper in bottom pouring crucibles.

Figure 4.6.12 Computer assisted static casting machine:

a – General view b – Detail of the crucible chamber c – Detail of the control panel d – Display with operation parameters

a b c d

Figure 4.6.13 Computer controlled static casting machine:

a – General view

b – Only the essential process parameters are displayed, because the machine is computer operated

c – Flask temperature measuring attachment equipped with optical pyrometer

d – Operation scheme of the machine e – Graining attachment for making alloy grains

f – Connection with the computer for recording process parameters g – Process evolution can be observed on the computer screen

a b c

f g

d

e

Trends in process control

For all the leading machine manufacturers, the trend is towards an even more complete automation of the machines. In some cases, artificial intelligence software is being utilised. These control systems remove the risk of operator error. He only feeds the metal charge into the crucible and sets the casting temperature. Then the control system takes all other technical decisions on the subsequent steps of the melting and casting process.

There are two trends in the technical development of static machines:

programmable machines or self-programming machines. We could say “computer assisted” machines, Figure 4.6.12, or “computer controlled” machines, Figure 4.6.13.

In the first group, the operating cycle is planned by the operator, who inputs a set of instructions. Generally, with these machines, data collection must be done by the operator, who should write down all data recorded by the machine. In contrast, computer controlled machines are self-programming. They can automatically evaluate the weight of the charged metal and correct the thermocouple temperature readings in real time. This correction is needed, because thermocouples are always enclosed in a refractory sheath and temperature readings always lag slightly behind in comparison with the true metal temperature (they are lower in the heating phase and higher in the cooling phase). Data collection is carried out automatically: the data are recorded in the computer and can be retrieved for subsequent processing.

The most recent developments involve the use of pressure assistance in casting, as shown in, Figures 4.6.14, 4.6.15, 4.6.16 and 4.6.17.

Figure 4.6.15

a – Detail of the crucible of the machine of Figure 4.6.14. The thermocouple is off-centre to facilitate crucible filling

b – Detail of the control panel

4 – The crucible chamber is filled with inert atmosphere with controlled pressure 5 – (Optional) the flask chamber can be put

under dynamic vacuum

6 – Pouring is started: the crucible chamber is pressurized, while the flask chamber is vacuumed

7 – Pouring ends. The flask chamber is pressurized, to facilitate mould filling and prevent shrinkage porosity

8 – The cast tree is solidified. The pressure in the flask chamber is lowered

9 – The crucible chamber is filled with inert gas, to protect the heating assembly. In the meanwhile the flask chamber is opened, to recover the cast flask Figure 4.6.14 Operation of a pressure casting machine:

1 – Preparation for melting

2 – Crucible and flask chambers are exhausted (vacuumed)

3 – Inert gas is introduced in the crucible chamber and slowly, also in the flask chamber; induction heating of the crucible is started

a b

Cost

The price range is even wider than for centrifugal casting machines. Electric resistance heated, non programmable machines may cost a few thousand Euros/US$, and resistance heated, programmable machines can cost up 7,000-8,000 Euros/US$.

But induction heated machines with a good level of programming capability can cost up to 20,000 Euros/US$ and more sophisticated, fully automatic machines that can be interfaced with a computer for data collecting and processing may cost even more than 60,000-70,000 Euros/US$. A typical range of machines produced by one manufacturer is shown in Table 21.

Model Heating Max. Crucible Flask size, Typical Casting Optional Temperature capacity*, maximum, mm cycle time rate, features

Grammes (dia. x height) Mins flasks/hour

J-2 Resistance 1204°C 900 102 x 229 6-8 8-10

J-z Induction 1513°C 1440 127 x 229 4 12-15 Yes

J-5 Induction, 5kW 1513°C 1440 152 x 254 4 12-15 Yes

J-10 Induction, 10kW 1513°C 1960 152 x 254 <3 20-25 Yes

J-15 Induction, 15kW 1513°C 1960 152 x 254 <3 20-25 Yes

* 14 carat gold

Table 21 Range of static casting machines from a US manufacturer

Figure 4.6.16 A machine for static casting under pressure (made in U.S.A.)

Figure 4.6.17 Another machine for static casting under pressure (made in Germany)

1 FOV Srl

Via del Progresso 45 Z.I.

I-36100 Vicenza Italy

Tel: +39 0444 566211 Fax: +39 0444 566830 E-mail: [email protected] Web: www.fovsrl.com

2 Gesswein & Co Inc 255 Hancock Ave.

3 Gold International Machinery Corp PO Box 998

It is impossible to give a comprehensive list of manufacturers and suppliers. Good advice is to visit an International (or your local) Jewellery Trade Fair and visit the material and equipment stands to see who supplies in your region.

Note: The following lists of suppliers do not imply endorsement of the company, their products or technical service by the authors or by World Gold Council.

In the list there are companies normally present at two or more international fairs.

The companies are listed in alphabetic order and are subdivided according to the process steps, starting from general suppliers. The latter are mainly commercial companies that sell a wide range of machines, consumables and tools, necessary for the investment casting process.

5.1 GENERAL SUPPLIERS