GB2204816A

GB2204816A - Countergravity casting of metal with air exclusion

Info

Publication number: GB2204816A
Application number: GB08810587A
Authority: GB
Inventors: George D Chandley
Original assignee: Metal Casting Technology Inc
Current assignee: Metal Casting Technology Inc
Priority date: 1987-05-07
Filing date: 1988-05-05
Publication date: 1988-11-23
Anticipated expiration: 2008-05-05
Also published as: MX169962B; SU1722218A3; IT8867421A0; GB2204816B; AU1538288A; BR8802221A; CN1015519B; IT1219274B; GB8810587D0; YU89588A; CN88102623A; FR2614812A1; CA1317437C; DE3815828A1; JPH0586312B2; FR2614812B1; IN170880B; JPS6440162A; AU610878B2; DE3815828C2

Description

r. --- A 1 B&S No C88120 APPARATUS AND PROCESS FOR COUNTERGRAVITY CASTING

OF METAL WITH AIR EXCLUSION This invention relates to methods and apparatus for countergravity casting of metals, particularly of reactive metals, such as certain alloys, containing elements which are reactive at casting temperatures with air to form oxides and/or nitrides, harmful to the cast product.

For good quality casting of such reactive metals, all casting operations, during which the metal is at a temperature at.which it is reactive with air, should be conducted with air excluded, either by means of a vacuum or by air replacement with an inert gas. A method and apparatus in general use for countergravity casting with such air exclusion is disclosed in U.S. patents 3,863,706 and 3,900,064. According to these patents, an air-tight, evacuable compartment is mounted above an air-tight enclosure of a melting crucible for the metal, the compartment having access to the crucible through an air-tight conduit extending from the compartment through the top of the crucible enclosure, the conduit being provided with an intermediate closure slide valve. A vertically partable chamber within the compartment receives and is sealed around a pervious mold with the lower end of a fill pipe of the mold protruding from the chamber bottom.

In use of this patented apparatus as described in the patents, a supply of molten metal to be cast has been provided in the crucible by melting ingots thereof in the crucible with the crucible enclosure evacuated to a high vacuum. In casting, a mold is placed in the mold chamber inside the compartment, both chamber and compartment are sealed and evacuated, and they and the crucible enclosure are backfilled with inert gas such as argon to the same low vacuum. With the conduit valve open, the chamber is lowered through the conduit until the protruding end of the fill pipe of the mold is below the surface of the molten metal in the crucible. The chamber is then evacuated to a sufficiently high vacuum to cause the molten metal to flow through the fill passage to fill the mold cavities. After sufficient hardening of the metal in the mold cavities, the pressure is raised in the chamber and compartment, and the chamber is withdrawn from the crucible enclosure, into the compartment for removal of the mold from the chamber and compartment.

While the method and apparatus of these patents have been very successful in producing fine quality castings of air-reactive metals, the provision of the outer compartment around the mold chamber and its valved conduit connected to the crucible enclosure are expensive apparatus which complicate the operation and impose some undesirable restrictions on use. For example, the outer compartment needs sealable doors to provide access to the chamber within for inserting and removing the mold, hampering these operations. The compartment is exposed conductively to the heat of the crucible enclosure by the connecting conduit, and when the valve is open, is exposed to the heat within it, making it difficult to cool the compartment adequately. Location of the mechanism for lowering or raising the chamber in part within the compartment involves complexity of sealing the compartment about movable members extending into it, as well as prohibiting access to the mechanism and the mold while the compartment is sealed.

t Other prior art, though avoiding the complex apparatus described above, fails to effectively contain the inert atmosphere within the crucible containing the molten metal.

The present invention makes it possible to eliminate the compartment for the mold chamber, and its valved conduit connection to the crucible enclosure of apparatus such as disclosed in the aforesaid patents while maintaining an effective inert atmosphere at the surface of the molten metal.

This substantial improvement was made possible in part by the conception of a novel apparatus in which means are provided for spacing the upper surface of the molten metal at a distance from a source of ambient atmosphere beyond the opening sufficient to prevent circulating air currents (called Brillion zones) from drawing air through the crucible enclosure opening down to the metal and maintaining the metal surface at this distance except during filling of the mold.

In preferred embodiments, the distance between the upper metal surface and ambient atmosphere is at least 8 inches, more preferably at least 18 inches. One preferred spacing means is a flexible, reversibly compressible shield positioned between the crucible enclosure opening and the opening in the evacuable chamber through which the free end of the fill pipe projects. As the fill pipe is inserted into the crucible enclosure below the molten metal surface, the shield compresses. During the-insertion.process, the 3G chamber is evacuated to create a lower pressure in the mold interior than the pressure of inert gas in the crucible enclosure. This draws inert gas and molten metal from the crucible enclosure into the mold. Once 1 1 the mold cavities have been filled, the fill pipe is withdrawn through the crucible enclosure opening, whereupon the shield reve rts to its original shape.

Another preferred spacing means is a movable member that forms part of the crucible enclosure which also compresses when the fill pipe free end is projected into the crucible enclosure; preferably, compression occurs when the top wall of the crucible enclosure contacts the bottom wall of the evacuable chamber containing the mold. Once casting is complete and the fill pipe withdrawn, this member returns to its original position.

one advantage of this apparatus and process is that casting can be conducted without the complex sealed enclosures thereof of the prior art. For this purpose, the fill pipe has a gas impervious surface, and a sufficient pressure of inert gas is maintained in the crucible enclosure to prevent access of air to the interior of the crucible enclosure through the opening therein from a source of air therebeyond. Preferably, the inert gas pressure is slightly above atmospheric pressure. Consequent losses of the inert gas to atmosphere through the opening are minimal if the inert gas is argon (density greater than air) or nitrogen (density slightly less than air) or other gas having a density at least approximately as great as that of nitrogen under the same conditions.

Not only is complexity of prior art apparatus eliminated in this way, but also it becomes possible, with a fill pipe of suitable length, to maintain the mold chamber spaced from the crucible enclosure and thermally insulated therefrom by atmospheric air or -inert gas, enabling more effective and less expensive cooling of the chamber. In addition, the fill pipe is 1 1 1 1 accessible during casting operations for operations thereon, such as crimping it closed in accordance with U.S. patent 4,589,966 for example.

A further advantage, useful in molding large parts, is that the amount of time necessary to pump down the system and fill it with inert gas is decreased.

In the preferred process and apparatus a removable cover is provided for the opening in the crucible housing which is sealable to the enclosure about the opening. This cover, which is removed before, and replaced after, the casting operations, can closely fit the opening and be relatively light and easily hand manipulated, since it is not required to withstand evacuation of the enclosure. For melting a new supply of ingot of the metal under high vacuum, another larger, heavier imperforate cover is provided, also sealable to the enclosure about the opening and which may be manipulated by mechanical hoist. After the metal is melted under vacuum and the enclosure is backfilled with inert gas, the larger cover is replaced by the smaller cover.

In the process and apparatus additional inert gas to that from the. crucible enclosure is supplied to the mold chamber to aid the flushing operations and/or to raise the pressure in the chamber thereafter. The advantages of an evacuating connection to the mold in accordance with the aforesaid patent application are also utilized.

In the drawings:

Figures 1 and 2 are schematic sectional side views of one preferred apparatus of the invention, illustrating successive steps in the operation of the methods of the invention.

1 "--,\ 1 - 6 Figures 3 and 4 are schematic sectional side views of another preferred apparatus of the invention, also illustrating successive operational steps.

Figure 5 is a similar view of the apparatus 5 illustrating the addition of metal to the crucible.

Referring to Figure 1, a crucible 62 for molten molding metal having an upper surface 64 is enclosed in a generally box-like structure, designated generally 60. Enclosure 60 may rest on the floor and may have its top wall 66 removably seated on the tops of the side walls by an O-ring seal 68 (for complete access to the interior). The inside of the walls of enclosure 60 may be provided with a coil or coils (not shown) for the circulation of cooling fluid such as water or double walls may be provided, spaced apart to permit circulation of coolant between them. Crucible 62 is embedded in a block of refractory electrical insulation 70, containing induction heating coil 72 surrounding the crucible.

2Q An opening 74 is provided in the top wall 66 of enclosure 60, centrally of molten metal surface 64 of crucible 62, this opening being of a size to receive freely therethrough the lower end 44 of fill pipe 40 of evacuable molding apparatus 10. A like opening 76 is provided in the usual heat shield 78 of insulating material which is supported above the crucible on the top of block 70. A removable cover 80 for opening 74 is sealed to top wall 66 of the enclosure about opening 74 by O-ring seal 82. A small opening 04 may be provided centrally of cover So through which a thermocouple may be inserted in the melt to measure its temperature. Enclosure 60 has a connection 86 to a differential pressure apparatus (not shown) capable of evacuating - 7 crucible enclosure 60 to a high vacuum when the enclosure is sealed. A connection 88 is also provided to a source of inert gas (not shown).

An advantage of the apparatus so far described in that it is suitable for air-free casting in conjunction with the simplified casting apparatus of U.S. patent 4,589,466. However, it is preferred to use an improvement thereof which is shown in the drawings hereof.

Molding apparatus 10 has a partable sealable loading chamber 12 mounted on a vertically movable support 14. Chamber 12 is made in two parts sealed together when closed as by an O-ring (not shown), and is opened and closed in the vertical direction by suitable is apparatus (not shown). Chamber 12 has, in its lower mold supporting wall, a central opening 18 for supporting a gas permeable mold, generally designated 20 and shown here as a shell-type mold, having a vertical fill passage 22 with a lower end 24 for introducing molten metal into mold cavities 26 therein. In its upper wall, chamber 12 has a connection 16 to differential pressure apparatus (not shown).

There is provided an impervious fill pipe 40 having an upper flared portion providing a radially outwardly extending upper flange 42 with a lower portion 44 extending vertically down from it. Upper flange 42 is sealingly interposed between the lower mold supporting wall of chamber 12 and the lower end of mold 20 around its opening 24. Lower portion 44 extends through central opening 18 of chamber 12 and vertically downwardly therefrom. A hydraulic power cylinder 34 connected to movable support 14 is provided for relatively moving crucible 62 and chamber 12 with mold 20 toward and away from one another by selectively raising or lowering chamber 12 with mold 20. Although it is not shown in the drawings, each of the partable halves of the mold chamber 12 may have means for cooling the interior, such as a surrounding coil, or circulation space between double walls connected to a source of coolant which may be water.

The differential pressure apparatus can be selectively operated to apply through a connection 16a to differential pressure apparatus (not shown) a lower pressure to the interior of mold 20 than is simultaneously provided in chamber 12 through connection 16. Preferably, porous plug, 47, highly permeable to gas but not to metal, covers an opening at the top of mold 20 over fill passage 22 and the mouth of connection 16a is sealed to the mold top over plug 47 by sealing means 46, permitting porous plug 47 and the mold interior beneath it to be selectively exposed to a lower pressure through connection 16a than is provided in chamber 12. Sealing means 48 prevents leakage between duct 45 and chamber 12. Connections 16 and 16a may be to different vacuum pumping systems or to a single such system equipped with suitable valve controls for providing different pressures in the two connections.

A flexible, reversibly compressible, conically shaped shield 89 is positioned between top wall 66 and molding apparatus 10. The shield is woven from fibers of a refractory material in order to withstand the heat. Preferred materials for shield 89 include a refractory glass fiber sold under the name Refresil'", which is commercially available from Niantic Rubber Co., Cranston, Rhode Island, U.S.A. The distance between molten metal surface 64 and the top of shield 89 located at central opening 18 of molding apparatus 10 is at least about 18 inches. With this arrangement, the 1 9 nearest source of air that could contaminate the molten metal when cover 80 is removed is opening 18, located relatively far away from the melt. Thus, cover 80 can be removed using a wire or rod (not shown), while purging enclosure 60 with argon. The argon purge is at higher than atmospheric pressure, causing the argon to fill shield 89 and to pass through the pores thereof to exclude air from passing through the shield. Therefore, the spacing of the melt surface from the only potential source of air (18) prevents circulating currents (Brillion zones) from drawing air into the melt.

In casting with the method and apparatus of the invention, a supply of molten metal is provided in crucible 62 under a substantially air-free atmosphere of inert gas (in a manner hereinafter described). Chamber 12 is positioned with fill pipe end 44 centered over crucible enclosure opening 74, as shown in Fig. 1, and cover 80 is removed before the casting operation commences. By sealing upper flange 42 of fill pipe 40 2G between the lower wall of chamber 12 and the lower end of mold 20 around opening 24 and using shield 89 coupled with maintaining the inert gas pressure in enclosure 60 above atmospheric, entrance of air into the crucible enclosure through opening 74 is prevented. With an inert gas such as argon, several times as dense as air, or nitrogen, with a density only fractionally lower than air, the gas flow may be easily controlled to preclude air entry into enclosure 60 through opening 74.

In casting, chamber 12 is moved by operating power cylinder 34 providing the relative motion of the fill pipe 40 and the crucible enclosure 60 to move the fill pipe through opening 74 in the enclosure. Referring to Figure 2, the fill pipe end 44 is inserted through opening 74 and immersed in the molten metal f below its surface. The relative motion of fill pipe end 44 and crucible enclosure 60 causes shield 89 to compress to a small, relatively flat shape. In accordance with the method of the aforesaid related U.S.

patent application, as fill pipe end 44 and crucible enclosure 60 are moved towards each other, reduced differential pressure is applied to the interior of mold 20 through vertical passage 22 by operating connection 16a to cause argon and then molten metal to flow up fill pipe 40 and fill vertical passage 22, the metallostatic head in vertical passage 22 also causing lateral flow into mold cavities 20. Simultaneously, a second pressure equal to or higher than the pressure applied through connection 16a, but lower than the pressure on the molten metal in crucible 30, is applied to chamber 12, and so to the-exterior of mold 20, through connection 16, to insure that the mold cavities 26 fill with molten metal. The magnitude of this second pressure is just adequate to cause the mold cavities 26 to fill. Once filling is complete, the pressure surrounding mold 20 may be increased, while maintaining the low pressure in the mold interior, to improve part quality and reduce tensile stresses in the mold. The pressure will be raised in chamber 12 by admission of inert gas through connection 50 so long as the casting metal remains at reactIve temperature.

Once molding is complete, the aforementioned pressures are restored to atmospheric pressure. Chamber 12 is then opened and the filled mold 20 and fill pipe 40 are removed in preparation for another molding cycle. Shield 89, because it is reversibly compressible, returns to its original shape. Cover 80 may be replaced over opening 74 in enclosure 60 unless another mold is ready for casting.

X I- -,/ 11 - The use of connection 16a and the method of its operation just described are preferred but not essential. With connection 16a omitted,.the method as described is changed only in that the low pressure to cause the filling of the mold cavities is produced entirely in chamber 12 through connection 16, the external low pressures on the mold being transmitted to its interior through its porous body. The results are generally not as satisfactory as with the two vacuum connections operated at differential. Impurities are not removed from the mold system as effectively during filling of the mold cavities, and the external pressure on the mold to obtain adequate filling needs to be lower, with resulting greater load of the metal weight on the mold. However, the method and apparatus are markedly beneficial, as compared with the prior art, without connection 16a.

Referring to Figure 3, there is shown another way of protecting the molten metal from air contamination. Crucible enclosure 60 is provided with a movable portion 67 supported by hydraulic cylinders (not shown) that spaces metal surface 64 from opening 74 (the nearest source of air) and maintains that spacing at about 18 inches or more. At this distance, contamination due to circulating air currents is prevented when cover 80 is removed under flowing argon. As shown in Figure 4, during casting when the bottom portion of apparatus 10 through which fill pipe 40 projects contacts the top wall 66 of.crucible enclosure 60 as fill pipe 40 and crucible enclosure 60 are moved relatively towards each other, movable portion 67 is engaged and compressed. Once casting is complete and fill pipe 40 withdrawn, movable portion 67 is returned to its original position. The engagement of the bottom of apparatus 10 and wall 66 seals opening 74 to prevent any significant air intrusion as movable portion 67 is compressed and later returned to its original position after mold filling.

Referring to Figure 5, for initially supplying metal to the crucible under an air-free inert gas atmosphere, which needs to be done only occasionally, a repla - cement cover 90 is provided which is impervious, larger and stronger than the cover 80, and capable of withstanding the evacuation of crucible enclosure 60 to high vacuum. With cover 80 removed, and the mold chamber 12 and its fill pipe 40 of Fig. 1 (not shown) moved out of the way, an ingot 92 of the metal to be melted is placed in the crucible through openings 74 and 76, and replacement cover 90 is removably sealed to top wall 66 of the enclosure by an O-ring 94 and covers opening 74. The enclosure is evacuated through connection 86 to a substantially air-free condition, and induction coil 72 is operated to melt the ingot. When the melt has reached the desired temperature, the inert gas to the desired pressure is admitted to the enclosure through connection 88, cover 80 is reapplied using a manipulating rod (not shown), and the replacement cover 90 is removed. Since cover 90 seats on the periphery of, and covers, removable top wall 66 of enclosure 60, wall 66 is not exposed to the pressure differential of atmosphere on one surface and high vacuum on the other and, like cover 80, may be made lighter and easier to manipulate than the cover 90.

The mold selected to illustrate the invention in the drawings is of the high temperature bonded, ceramic type for multiple parts. It will be understood,' however, tha-L this is merely illustrative and that other C, types of mold may be used, such as low temperature bonded sand molds, for single or multiple parts and of various sizes and shapes, the choices being increased when the low pressure connection 16a is provided and used as described.

k1 1)

Claims

1. A method of counter gravity casting of molten metal with the exclusion of air in a gas-pervious mold sealed in an evacuable chamber with a fill pipe for the mold cavities having a free end projecting therefrom, comprising the steps of:

providing a supply of the molten metal to be cast in a crucible in an enclosure under a substantially air free atmosphere of inert gas, said enclosure having an opening therein adapted to receive the free end of said fill pipe therethrough and said enclosure being exposed to a source of ambient atmosphere between said opening and said chamber; characterized in that the method further comprises spacing the upper surface of said metal at a distance from the source of ambient atmosphere sufficient to prevent circulating air currents from drawing air through said opening down to said metal and maintaining said surface at said distance except during the moving steps and during the filling of said mold.

relatively moving said fill pipe and said crucible enclosure to project the free end of said fill pipe through said crucible enclosure to a position below the surface of molten metal in said crucible, and evacuating said chamber to provide in the mold interior a pressure sufficiently lower than the pressure of said inert gas atmosphere in said crucible enclosure to cause molten metal to rise through said fill pipe to fill the cavities in said mold; and thereafter reversely relatively moving said mold fill pipe and said crucible enclosure to withdraw said fill pipe through said opening.

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2. The method according to claim I further characterized in that said chamber is evacuated while said fill pipe and said crucible enclosure are being relatively moved towards each other to draw inert gas 5 from said enclosure into said mold.

3. The method according to either of the preceding claims further characterized in that the upper portion of said fill pipe is sealed between said chamber and said mold.

4. The method according to any of the preceding claims further characterized in that a flow of gas is provided into the chamber externally of said mold while said fill pipe free end is below the surface of molten metal in said crucible.

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5. The method according to any of the preceding claims further characterized in that separate evacuating connectors are provided to said chamber and to an upper part of a fill passage of said mold and thereby maintaining in said mold passage upper part a lower pr essure than the pressure in the chamber externally of said mold during at least part of the time said fill pipe is below the surface of molten metal in said crucible.

6. The method according to any of the preceding claims further characterized in that said distance maintained between said upper surface of said metal and said source of ambient atmosphere is greater than 8 inches.

7. The method according to.claim 6 further characterized in that said distance maintained between said upper surface of said metal and said source of ambient atmosphere is at least about 18 inches.

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S. The method according to any of the preceding claims further characterized in that said distance between said upper surface of said metal and said source of ambient atmosphere is maintained by providing a flexible, reversibly compressible shield between said crucible enclosure opening and the opening in said evacuable chamber through which said fill pipe projects which shield compresses and seals to said chamber during the first of said relatively moving steps and regains its original shape following the second of said relatively moving steps.

9. The method according to any of claims 1-7 further characterized in that said distance between said upper surface of said metal and said source of ambient atmosphere is maintained by providing said crucible enclosure with a movable membet that comDresses and seals to said chamber when said crucible enclosure and said fill pipe are relatively moved towards each other and returns to its original position when said crucible enclosure and said fill pipe are moved away from each other.

10. The method according to claim 9 further characterized in that said movable member compresses when the top wall of said crucible enclosure contacts the bottom wall of said evacuable chamber through which said fill pipe projects.

11. The method according to any of claims 1 to 10 further characterized in that said fill pipe is impervious and which includes: maintaining a sufficient pressure of the inert gas in said crucible enclosure to prevent access of atmospheric air to the interior of said enclosure through said opening.

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12. The method according to claim 11 further characterized in that the pressure of inert gas maintained in said crucible enclosure is above atmospheric pressure.

13. The method according to claim 12 further characterized in that said inert gas is at least approximately as dense as nitrogen.

14. The method according to claim 13 further characterized by thermally insulating said chamber from said crucible enclosure by atmospheric air or inert gas except for thermal communication by and through said fill pipe.

15. The method according to any of claims 11-14 further characterized by providing a first removable cover for said opening in said crucible enclosure, removing said first cover from said opening before projecting the lower end of said fill pipe through the opening and replacing said first cover over said opening after said fill pipe is withdrawn from said opening.

16. The method according to claim 15 further characterized by:

removing said first cover from said opening; inserting ingot of said metal through said opening in said crucible enclosure into said crucible; providing a second larger and imperforate removable cover for said opening, sealable pressure tight to said crucible enclosure about said opening, and of sufficient strength to withstand evacuation of said crucible enclosure to high vacuum; applying said second cover, evacuating said crucible enclosure and second cover to a substantially air-free low pressure and heating said ingot in said crucible to reduce said ingot to a molten condition suitable for casting; providing a flow of said inert gas into said crucible enclosure to produce said inert gas atmosphere therein, and removing said second cover and replacing it with said first cover.

17. Apparatus for casting molten metal with the exclusion of air in a gas pervious mold sealed in an evacuable chamber with a fill pipe for the mold cavities having a free end projecting therefrom having enclosed crucible means for providing a supply of molten casting metal in a crucible under a substantially air-free atmosphere of inert gas, said crucible enclosure having an opening for receiving said fill pipe free end therethrough; means for relatively moving said fill pipe and said crucible means to project said fill pipe free end into, and to withdraw it from, said crucible enclosure through said opening therein; and means for evacuating said chamber to provide in the mold interior a pressure sufficiently below the pressure of said inert gas in said enclosure to cause molten metal to rise through said fill pipe to fill the cavities of said mold, characterized in that:

in one position of said fill pipe and said crucible means the upper surface of said metal is spaced at a distance from a source of ambient atmosphere beyond said opening sufficient to prevent circulating air currents from drawing air through said opening down to said metal.

1 4r---\, U 1 - 19

18. Apparatus according to claim 17 further characterized in that the upper portion of said fill pipe is sealed between said chamber and said mold.

19. Apparatus according to either of claims 17 or 18 further characterized in that means are provided for supplying inert gas to said chamber externally of said mold.

20. Apparatus according to any of claims 17-19 further characterized in that said means for evacuating said chamber includes an evacuating connector to an upper part of a fill passage of a mold therein, and means for providing a lower pressure in said connector than in said chamber outside the mold.

21. Apparatus according to any of claims 17-20 further characterized in that said distance between said upper surface of said metal and said source of ambient atmosphere is greater than 8 inches.

22. Apparatus according to claim 21 further characterized in that said distance between said upper surface of said metal and said source of ambient atmosphere is at least about 18 inches.

23. Apparatus according to any of claims 1722 further characterized in that said spacing means comprises a flexible, reversibly compressible shield positioned between said crucible enclosure opening and the opening in said evacuable chamber through which said fill pipe free end projects that compresses and seals to said chamber when said fill pipe free end is projected into said crucible enclosure and regains its original shape after said fill pipe free end is withdrawn from said crucible enclosure.

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24. Apparatus according to any of claims 17-22 further characterized in that said spacing means comprises a movable member forming part of said crucible enclosure that compresses and seals to said chamber when said fill pipe free end is projected into said crucible enclosure and returns to its original position after said fill pipe free end is withdrawn from said crucible enclosure.

25. Apparatus according to claim 24 further characterized in that said movable member compresses when the top wall of said crucible enclosure contacts the bottom wall of said evacuable chamber through which said fill pipe projects.

26. Apparatus according to any of claims 17 to 25 further characterized in that said fill pipe is impervious and means is provided for supplying inert gas to said crucible enclosure at a rate to maintain a pressure of said gas in said enclosure sufficient to prevent access of atmospheric air or inert gas to the interior thereof through said opening therein.

27. Apparatus according to any of claims 17-26 further characterized in that said chamber is spaced sufficiently from said crucible enclosure as to be thermally insulated therefrom by atmospheric air or inert gas except for thermal communication by and through said fill pipe.

28. Apparatus according to any of claims 17-27 further characterized in that said apparatus includes a first removable cover for said crucible opening and a second removable cover to replace said first cover, said second cover being imperforate, larger and heavier than said first cover, sealable to said crucible enclosure about said opening and capable of withstanding evacuation of said enclosure to high vacuum.

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29. A method of counter gravity casting of molten metal substantially as hereinbefore described with reference to any one of the drawings.

30. Apparatus for casting molten metal substantially as hereinbefore described with reference to any one of the drawings.

Published 1988 at The Patent Office, State House, 66171 High Holborn, London WClR 4TP. Further copies may be obtained from The Patent Office, Sales Branch, St Mary Cray, Orpington, Kent BR5 3RD. Printed by Multiplex techniques It4d, St Mary Cray, Kent. Con. 1/87.