METAL CASTING

Abstract: Metal casting apparatus (10) comprises a tungsten inert gas (TIG) hand torch (12), a graphite crucible (14) and a graphite mould (16). Energised by a high frequency alternating current by supply (30), the torch (12) produces a pulsating arc within an inert gas shield (22). The arc melts metal (28) in the crucible (14), and cleaning of the molten metal is achieved by superimposing a positive DC bias on the alternating current. Different metals may be alloyed, with homogeneity resulting from agitation and stirring the molten metal. The molten metal is poured into mould (16) through conduit (24) by opening tap rod (26).

Description: METAL CASTING The present invention relates to a method and apparatus for producing a metal casting, particularly but not exclusively for the jewellery industry. There are many different kinds of casting processes, but not all are suited to the high standards required by the jewellery industry. Investment casting and centrifugal casting are two procedures which have been widely used to produce jewellery because castings with precise dimensions and good surfaces are achievable. However, even with these procedures there are problems, for example porosity and other defects arising in castings due to surface tension phenomenon and the decomposition of the mould materials. Although advanced casting techniques such as computer controlled 'pressure over vacuum' castings have the potential to overcome certain problems, they are relatively expensive and in any event are not suitable for the more reactive jewellery compositions. An object of the present invention is to provide a method and apparatus which is capable of overcoming or at least ameliorating some of the difficulties encountered especially in the jewellery industry when casting using conventional processes and apparatus. In accordance with a first aspect of the present invention, there is provided a method for producing a metal casting, comprising, providing a metal in a crucible; melting the metal m the crucible under an inert atmospnere using an arc from an electrode; and releasing the molten metal into a mould. The arc may produce a plasma temperature of around 10,000°C, and is thus able to heat the metal very rapidly and at least to a degree sufficient to melt all metals. The metal m the crucible may comprise at least two parts of different compositions. For example, one part may comprise a gold-rich alloy and another part may comprise an aluminium-rich alloy. The two parts may be alloyed together m the crucible. Alloying zn si tu may require stirring the molten metal m the crucible to give a homogenous melt. Stirring may be achieved by establishing relative movement between the arc and the crucible, possibly by oscillating the electrode. Preferably, the electrode does not contact the molten metal. The molten metal may also be agitated m the crucible by supplying to the electrode a pulsating alternating current of varying frequency, e.g., 0-50 HZ Such current agitation encourages homogeneity in the molten metal. It may be advantageous to superimpose a direct current bias to the alternating current m order to shift the balance. By adding a positive direct current bias, the arc is predominantly positive which may clean the molten metal. Such electric cleaning (ion-bombardment) enables use of materials with inherent oxides, for example aluminium alloy. It could also be used to recycle contaminated eld jewellery. Alternatively, by introducing a negative direct current bias, the arc will predominantly be negative which may give rise to greater heating of the metal in the crucible. The method for producing a metal casting may further comprise varying the pressure of the inert atmosphere during melting. By exerting positive or negative gas pressures on the molten metal, it is possible to lower surface tensions or remove trapped gases. During use of negative gas pressures to remove trapped gases, it is desirable to remove evolving vapours possibly by maintaining a supply of inert gas to purge the inert atmosphere around the molten metal. In addition to exerting a positive pressure on the molten metal, a negative pressure (suction) may be applied to the mould during souring of the molten metal . Such a pressure differential may encourage molten metal flow from the crucible to the mould. According to a second aspect of the present invention, there is provided apparatus for producing a metal casting, comprising a crucible, means for establishing an inert atmosphere around metal in the crucible, an electrode, means for supplying electricity to the electrode to generate an arc for melting metal m the crucible, and a mould for receiving molten metal from the crucible. The inert atmosphere establishing means may simply comprise a flow of inert gas directed from the electrode towards metal in the crucible. The flow should be sufficient to establish an inert gas shield around metal m the crucible and preferably from the electrode to metal m the crucible. Alternatively, the inert atmosphere establishing means may include a pressure chamber in which the electrode and metal m the crucible are located. The pressure chamber enables the pressure of the inert atmosphere to be decreased for removing trapped gases in the molten metal, and subsequently increased to lower molten metal surface tension. The pressure chamber may have means for changing the inert atmosphere without altering gas pressure m the pressure chamber. For example, an outgoing flow of inert gas contaminated w th vapours evolved from the molten metal may be matched by an incoming flow of uncontammated inert gas. The apparatus may further comprise a conduit communicating between the crucible and the mould, and having a valve for regulating molten metal flow through the conduit. The apparatus may be arranged with the crucible above the mould so that molten metal flow through the conduit is aided by gravity, the molten metal flow through the conduit may further be encouraged by establishing a pressure differential across the "valve. For example, a vacuum pump may be used to lower gas pressure m the mould prior to opening the valve. The crucible or the mould may be of graphite. A graphite crucible would be able to carry a high current and at the same time additional heating and subsequently some cooling by thermal conduction would be possible. By the same token, a graphite mould would facilitate preheating of the mould before molten metal is introduced into it . The graphite mould may be heated by electric heating elements. Graphite is much less reactive than certain other mould materials, and thus is compatible with the more reactive jewellery compositions. Other features of both aspects of the present invention are set out in the appended dependent claims, to which reference should now be made. Embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a schematic showing manual apparatus embodying the present invention; and Figure 2 is a section of automated apparatus embodying the present invention. Figure 1 shows schematically manual apparatus 10 for producing cast jewellery, comprising a tungsten inert gas (TIG) hand torch 12, a graphite crucible 14, and a graphite mould 16. The TIG hand torch 12 has a tungsten electrode 20 which produces an arc (not shown) within an inert gas shield 22 when supplied with high frequency alternating current. The inert gas shield 22 is provided by a flow of inert gas directed through the TIG hand torch 12 and beyond the electrode 20. The graphite crucible 14 communicates with the mould 16 through conduit 24 when opened by graphite tap rod 26. A metal sample 28 to be cast, or a mixture of metal samples 28 to be alloyed and then cast, are placed in the crucible 14. The tap red 26 is positioned to seal off the conduit 24, blocking communication between the crucible 14 and the mould 16. The TIG hand torch 12 is energized by the high frequency alternating current supply 30. The arc thus generated strikes the sample (s) 28 and rapidly produces molten metal. The molten metal is agitated by pulsing of the arc caused by the alternating current. The balance of alternating current is adjusted by superimposing a direct current supply 32. The DC supply 32 may be switched between positive and negative, to make respectively the alternating current supplied to the electrode 20 either predominantly positive or predominantly negative. The molten metal is further agitated by a stirring action imparted by oscillating the TIG hand torch 12; the electrode 20 does not contact the molten metal. The graphite mould 16 is preheated by heating elements 34. The pressure of gas in the mould 16 is reduced by a vacuum unit 36 which withdraws gas through suction hole 38. When the molten metal is ready for casting, tap rod 26 is moved to allow molten metal to flow through conduit 24 into the mould 16 where it is allowed to cool. Figure 2 shows a section of an automated jewellery casting apparatus 50. Features of the apparatus 50 which are in common with the manual apparatus 10 of figure 1 have been given the same reference numerals. In the apparatus 50, the tungsten electrode 20 of a pulsating arc torch 52 and the crucible 14 are mounted in a pressure chamber 54 which is connected to vacuum pump 56 through coolant unit 58. The pressure chamber 54 is supplied with inert gas through supply hose 60. The pulsating arc torch 52 is connected to a mctcrised cam which in use causes the electrode 20 to oscillate in such a way that stirring of molten metal in the crucible 14 is achieved. The separation of the electrode 20 from the crucible 14 is varied by adjusting the length of support struts 64. The operating procedure of the automated jewellery casting apparatus 50 will now be described: 1) Alloying elements are placed in the graphite crucible 14. 2) The pressure chamber 54 is sealed. 3) The graphite mould 16 is preheated (the graphite crucible 14 may also be preheated) . 4) The pressure chamber 54 is purged with argon inert gas. 5) The pressure of the gas in the pressure chamber 54 is reduced. 6) The gas pressure in the pressure chamber 54 is balanced at between 10 and 10^ ' torr, with the graphite mould 16 at about 300°C. 7) The AC pulsed arc (argon-tungsten) is started using the high frequency supply. 8) The motorised cam 62 is started to oscillate the torch 52. 9) The alloying elements are melted in the crucible, cleaned using the predominantly positive (DC biased) arc using ion bombardment to break up intermetailic oxides and the like, and homogenized by stirring and agitating. 10) In the negative-pressure argon atmosphere of the chamber, impurities and oxides of the alloying elements are transformed into vapours and removed by continuous action of the unit 56. 11) The purified and homogenized molten alloy is then cast into the graphite mould 16 (pre-purged with inert argon) . To improve molten metal flow into the mould 16, the pressure in the chamber 54 is increased and at the same time, the pressure in the mould 16 is decreased by suction through hole 38. 12) The cast metal is allowed to cool.

Claims: CLAIMS 1. A method for producing a metal casting, comprising: providing a metal in a crucible; melting the metal in the crucible under an inert atmosphere using an arc from an electrode; and releasing the molten metal into a mould. 2. A method for producing a metal casting according to claim 1, in which the metal provided in the crucible comprises at least two parts of different compositions. 3. A method for producing a metal casting according to claim 1 or 2 , further comprising stirring the molten metal in the crucible. 4. A method for producing a metal casting according to claim 3, in which the molten metal is stirred by establishing relative movement between the arc and molten metal in the crucible. 5. A method for producing a metal casting, in which the relative movement is established by oscillating the electrode. 6. A method for producing a metal casting according to any one of the preceding claims, further comprising agitating the molten metal in the crucible by supplying a pulsating alternating current to the electrode. 7. A method for producing a metal casting according to claim 6, in which the pulsating alternating current is of varying frequency. 8. A method according to claim 6 or 7, further comprising superimposing a direct current to alter the balance of the alternating current. 9. A method according to claim 8, in which a positive direct current is superimposed for cleaning the molten 5 metal . 10. A method according to any one of the preceding claims, further comprising varying the pressure cf the inert atmosphere during melting. 11. A method according to any one of the preceding 10 claims, further comprising heating the mould prior to pouring the molten metal. 12. A method according to any one of the preceding claims, further comprising introducing a pressure differential between the crucible and the mould to 15 encourage molten metal flow from the crucible to the mould when pouring commences . 13. An item of jewellery cast in accordance with any one of claims 1 to 12. 14. Apparatus for producing a metal casting, comprising a 20 crucible, means for establishing an inert atmosphere around metal in the crucible, an electrode, means for supplying electricity to the electrode to generate an arc for melting metal in the crucible, and a mould for receiving molten metal from the crucible. 25 15. Apparatus according to claim 14 further comprising means for stirring molten metal in the crucible. 16. Apparatus according to claim 15, in which the stirring means establishes relative movement between the arc and molten metal in the crucible. 17. Apparatus according to claim 16, in which the stirring means comprises drive means for oscillating the position of the electrode. 5 18. Apparatus according to any one of claims 14 to 17, in which the electricity supply means supplies high frequency alternating current to the electrode. 19. Apparatus according to claim 18, further comprising means for superimposing a direct current to alter the 10 balance of the alternating current. 20. Apparatus according to any one of claims 14 to 19, further comprising means for varying the pressure cf the inert atmosphere established. 21. Apparatus according to any one of claims 14 to 20, 15 further comprising a conduit communicating between the crucible and the mould, and having a valve for regulating molten metal flow through the conduit. 22. Apparatus according to claim 21, further comprising means for establishing a pressure differential across the 20 valve for urging molten metal flow through the conduit when the valve is open. 23. Apparatus according to claim 22, .in which the pressure differential establishing means comprises suction means for reducing gas pressure in the mould. 25 24. Apparatus according to any one of claims 14 to 23, in which the electrode is a tungsten electrode. 25. Apparatus according to 24, in which the tungsten electrode is part of a tungsten arc torch.

Inventors: LOH PENG CHUM, LOH, PENG, CHUM
Applicants: LOH PENG CHUM, SINGAPORE POLYTECHNIC, LOH, PENG, CHUM
Application year: 2000
Main classification: B22D41
IPC codes: B22D23