The present invention relates to the cooling of molten metal in a mold with a body of a molten metal which is at a lower temperature than the molten metal in the mold.
It has previously been suggested that a casting furnace may employ a body of molten metal as a bath to promote directional solidification of an article in a mold. One apparatus for doing this is disclosed in U.S. Pat. No. 6,308,767. It has also been suggested that a plurality of articles may be floated on a body of molten metal forming a bath so as to form an insulating layer which extends across an upper surface of the bath. One apparatus for doing this is disclosed in U.S. Pat. No. 6,446,700.
The present invention relates to a new and improved method of casting metal articles. The method includes providing a support structure having a plurality of support sections. Article mold sections are positioned on the support sections. Article mold cavities in the article mold sections are filled with a first molten metal which is at a first temperature.
The support sections and a body of a second molten metal are moved relative to each other. The body of a second molten metal is at a temperature which is less than the first temperature of the first molten metal in the article mold cavities. Spaces extending between side surfaces of the support sections and between side portions of the article mold sections are filled with the second molten metal.
If desired, a layer of insulating material may be provided above an upper side of the body of a second molten metal. Portions of the layer of insulating material are aligned with spaces extending between side surfaces of the support sections. These portions of the layer of insulating material extend across space between the support sections and extend across space between portions of the article mold sections during at least a portion of the relative movement between the support sections and body of molten metal. A baffle may be provided between a container holding the body of a second molten metal and a furnace assembly.
The present invention has a plurality of different features which are advantageously utilized together in the manner described herein. However, it is contemplated that the features may be utilized separately and/or in combination with features from the prior art.
The foregoing and other features of the invention will become more apparent upon a consideration of the following description taken in connection with the accompanying drawings wherein:
A casting apparatus 10 is illustrated schematically in
The housing enables an evacuated atmosphere to be maintained around the furnace assembly 16 and container 20 holding the bath or body 22 of molten metal. The housing may have any one of many known constructions, including the construction disclosed in U.S. Pat. No. 3,841,384 and/or the construction shown in U.S. Pat. No. 6,308,767. Of course, the housing may have a construction which is different than the known constructions illustrated in the aforementioned patents.
An improved framework 26 (
The support rods 28 are connected with an upper drive assembly 34 and with the mold support structure 32. The upper drive assembly 34 is operable to raise and lower the framework 26 relative to the furnace assembly 16 and container 20 holding the body 22 of molten metal. If desired, the support rods 28 may be disposed outside the furnace assembly 16.
A lower drive assembly 38 is connected with the container 20 which holds the body 22 of molten metal. The lower drive assembly 38 is operable to raise and lower the container 20 relative to the furnace assembly 16. The upper and lower drive assemblies 34 and 38 may be operated simultaneously and/or sequentially to raise and/or lower the framework 26 and/or container 20 holding the body 22 of molten metal.
During operation of the casting apparatus 10, the one piece ceramic mold structure 12 is supported in the furnace assembly 16 by the framework 26. The mold structure 12 is disposed on the support structure 32 forming the base of the framework 26. The mold structure 12 may be connected to the support structure 32 by suitable clamps and/or fasteners.
Heat is transmitted from the mold structure 12 to the metal support structure 32 which functions as a chill plate. The mold structure 12 is raised and lowered relative to the furnace assembly 16 by operation of the upper drive assembly 34 which is connected to the support structure 32 by the support rods 28. If desired, a flow of cooling liquid may be conducted through the support structure 32. It is contemplated that the support structure 32 may be constructed so as to be located outside the furnace assembly 16.
While the mold structure 12 is supported in the furnace assembly 16 on the framework 26, in the manner illustrated schematically in
The mold structure 12 has a construction which is generally similar to the construction disclosed in U.S. Pat. Nos. 5,048,591; 5,062,468; and/or 5,072,771. The mold structure 12 is utilized to cast turbine engine components. However, it should be understood that the mold structure 12 may have a construction which is different than the construction which is disclosed in the aforementioned patents and/or may be used to cast articles other than turbine engine components.
The mold structure 12 is filled with molten liquid metal while the mold structure is in the furnace assembly 16. The molten metal with which the mold structure 12 is filled is a molten nickel-chrome super alloy which melts at a temperature which is greater than 3,000 degrees Fahrenheit. Of course, the mold structure 12 may be filled with a different molten metal which melts at a different temperature. For example, the mold structure 12 may be filled with molten titanium or a titanium alloy.
Once the mold structure 12 has been filled with the molten nickel-chrome super alloy or other metal, the upper drive assembly 34 is operated to lower the framework 26 and mold structure 12 into the body 22 of a second molten metal in the container 20. While the upper drive assembly 34 is operated to lower the mold structure 12, the lower drive assembly 38 may be operated to raise the body 22 of liquid metal. It should be understood that the mold structure 12 may be immersed in the body 22 of molten metal by lowering the support structure 32 without raising the body 22 of molten metal. Alternatively, the furnace assembly 16 may be raised relative to the mold structure 12 and the body 22 of molten metal raised relative to the mold structure to immerse the mold structure in the body of molten metal. Although either one of the mold structure 12 and body 22 of molten metal may be moved relative to the other to effect immersion of the mold structure 12 in the body 22 of molten metal, it may be desired to both raise the body 22 of molten metal and lower the mold structure 12.
The molten super alloy in the mold structure 12 is at a temperature above 3,000 degrees Fahrenheit. The body 22 of molten metal is at a temperature below 1,000 degrees Fahrenheit. The resulting temperature differential between the molten metal in the mold structure 12 and the molten metal in the body 22 of molten metal results in directional solidification of the molten metal in the mold structure 12 as the mold structure is immersed in the body 22 of molten metal. The molten metal in the mold structure 12 may solidify with either a columnar grain crystallographic structure or with a single crystal crystallographic structure.
In the illustrated embodiment of the invention, the body 22 of molten metal is formed of tin and is at a temperature of approximately 500 degrees Fahrenheit. However, the body 22 of molten metal may be formed of lead or aluminum if desired. The molten metal in the mold structure 12 is a nickel-chrome super alloy with a melting temperature which may be approximately 3,700 degrees Fahrenheit. Of course, a different molten metal may be poured into the mold structure 12.
It should be understood that the specific temperatures for the body 22 of molten metal and the molten metal in the mold structure 12 will vary depending upon the composition of the metal. For example, the body 22 of molten metal may be any one of many metals which is liquid (molten) at a temperature below 1,500 degrees Fahrenheit. The molten metal in the mold structure 12 may be any one of many different metals which melt at a temperature above 2,000 degrees Fahrenheit.
The greater the temperature differential between the temperature of the molten metal in the mold structure 12 and the body 22 of molten metal, the greater will be the rate at which heat is withdrawn from the molten metal in the mold structure as the mold structure is immersed into the body of molten metal. Of course, the rate of transfer of heat from the molten metal in the mold structure 12 to the body 22 of molten metal will also vary as a function of the rate at which the mold structure and body of molten metal are moved relative to each other by the upper and/or lower drive assemblies 34 and 38.
The support structure 32 is constructed so as to minimize agitation of the body 22 of molten metal as the support structure and mold structure 12 are immersed in the body 22 of molten metal. In addition, the support structure 32 is constructed so as to enable the mold structure 12 to be readily withdrawn from the body 22 of molten metal with minimal adherence of liquid metal to the support structure 32 and/or mold structure 12.
The support structure 32 is constructed so as to have open space, indicated by arrows 52 in
In the illustrated embodiment of the invention, the support structure 32 has a generally X-shaped configuration. Thus, the support structure 32 includes support sections 56, 58, 60, and 62 (
The support sections 56-62 are interconnected at a central portion 66 (
Although the illustrated embodiment of the support structure 32 includes four support sections 56-62, it should be understood that the support structure 32 may have a greater or lesser number of support sections if desired. It should also be understood that although the support sections 56-62 extend at right angles to adjacent support sections, a different angle may be provided between the support sections if desired. The support sections 56-62 may have a different configuration than the illustrated configuration. For example, the support sections may flare outwardly from the central portion 66 of the support sections to outer end surfaces 70, 72, 74 and 76. Although the illustrated outer end surfaces 70-76 have the same rectangular configuration, the end surfaces 70-76 may have a different configuration if desired.
The rectangular cross sectional configuration of the support section 62 is illustrated in
The upper surface 84 on the support section 62 is disposed in a coplanar relationship with and has the same configuration as the upper surfaces on the support sections 56, 58, and 60. Similarly, the lower surface 86 of the support section 62 is coplanar with and has the same configuration as the lower surfaces on the support sections 56, 58 and 60. Although the upper surfaces 84 on the support sections 56-62 are disposed in a coplanar relationship, one or more of the upper surfaces may be offset from one or more of the other upper surfaces. Similarly, one or more of the lower surfaces may be offset from one or more of the other lower surfaces.
In one specific embodiment of the support structure 32, the distance from the end surface 70 (
It should be understood that the aforementioned specific dimensions for the support structure 32 have been set forth herein merely for purposes of clarity of description and not for purposes of limiting the invention. It is contemplated that the support structure 32 may and probably will be constructed with different dimensions. It is believed that the dimensions of the support structure may vary with variations in the size of an article to be cast in the mold structure 12. Thus, the larger the mold structure 12, the larger is the support structure 32.
The support sections 56-62 may have different lengths. Thus, the support section 56 may be longer than the support section 58. The support sections 56 and 60 may have a combined length which is greater than the combined length of the support sections 58 and 62.
The support structure 32 may be considered as being inscribed within a spatial envelope formed by a polygon. In the specific embodiment of the support structure 32 having the configuration illustrated in
In the specific embodiment of the invention illustrated in
It should be understood that the support structure 32 may have a construction which is different than the illustrated construction. For example, the support structure 32 may have a greater or lesser number of support sections. As another example, the support sections 56-62 may have arcuately curving lower surfaces and/or side surfaces rather than the flat lower surfaces 86 and flat side surfaces 80 and 82. If the support sections 56-62 are provided with arcuately curving lower surfaces, the side surfaces 80 and 82 may be eliminated or substantially reduced in vertical (as viewed in
The support structure 32 functions as a chill plate. Accordingly, heat is transmitted from the mold structure 12 to the support structure 32. If desired, passages may be provided in the support structure 32 to conduct a flow of cooling fluid through the support structure. If cooling fluid passages are provided in the support structure, suitable cooling fluid conduits may be formed in or connected with the support rods 28. As was previously mentioned, the support rods 28 may be disposed outside the furnace assembly 16.
The mold structure 12 (
The illustrated mold structure 12 has a single article mold section 44 disposed on the portion of the base plate 90 which overlies one of the support sections 56-62 of the support structure 32. However, the mold structure 12 may be formed with a greater number of article mold sections 44 on each of the support sections 56-62 of the support structure 32 if desired. An article mold section 44 may extend upward from the portion of the base plate 90 which overlies the central portion 66 of the support structure 32.
The mold structure 12 is integrally formed as one piece by repetitively dipping a wax pattern in a slurry of ceramic mold material in the manner disclosed in U.S. Pat. No. 4,955,423. However, it should be understood that the mold structure 12 may be formed in many different ways and could be utilized to form many different types of cast metal articles. In the illustrated embodiment of the mold structure 12, the article mold sections 44 are configured so as to cast blades or vanes for use in a turbine engine. However, the article mold sections 44 may be configured so as to cast any desired metal article.
A layer 100 (
The illustrated layer 100 of insulating material floats on the upper surface 102 of the body 22 of molten metal. The layer 100 of insulating material shields the body 22 of molten metal from the relatively hot environment of the furnace assembly 16. Thus, the layer 100 of insulating material retards heat transfer from the furnace assembly 16 and mold structure 12 to the body 22 of molten metal. This enables the body 22 of molten metal to be maintained a relatively low temperature during preheating of the mold structure 12 and during the pouring of molten metal into the mold structure.
The layer 100 of insulating material may be formed of many different materials. In the illustrated embodiment of the invention, the layer 100 of insulating material is formed of refractory particles which float on the body 22 of molten metal. However, it is contemplated that the layer 100 of insulating material may be formed in a different manner if desired. For example, the layer 100 of insulating material may be formed by hollow members which have a construction similar to any one of the constructions disclosed in U.S. Pat. Nos. 6,446,700 and 6,035,924.
If desired, the layer 100 of insulating material may be disposed above and spaced from the body 22 of molten metal. At least a portion of the layer 100 of insulating material may have a relatively rigid construction and have one or more openings through which the mold structure 12 and support structure 32 move. If this is done, the layer 100 of insulating material may be connected with the upper end portion of the container 20.
The layer 100 of insulating material may have any one of many known constructions. For example, it is contemplated that the layer 100 of insulating material may have a construction similar to any one of the constructions disclosed in U.S. Pat. No. 6,698,493. The disclosure in the aforementioned U.S. Pat. No. 6,698,493 is hereby incorporated herein in its entirety by this reference thereto.
Alternatively, the layer 100 of insulating material may have a relatively rigid base which extends around the mold structure 12. A plurality of flexible segments may extend from the rigid base into engagement with the mold structure 12. The flexible segments cooperate with the relatively rigid base of the layer 100 of insulating material to close space between irregular surfaces of the article mold sections 44 and the relatively rigid base. If this is done, the base of the layer 100 of insulating material may be supported on the support structure 32 during upward movement of the mold structure 12 into the furnace assembly 16. As the mold structure 12 moves downward from the furnace assembly 16, the relatively rigid base of the layer 100 of insulating material may be engaged by members (pins) connected with the furnace assembly 16 during downward movement of the mold structure 12 from the furnace assembly 16. These members (pins) would support the layer of insulating material. The layer 100 of insulating material may have the same construction and cooperate with the mold structure 12 in the same manner as disclosed in U.S. Pat. No. 6,827,124. The disclosure in the aforementioned U.S. Pat. No. 6,827,124 is hereby incorporated herein in its entirety by this reference thereto.
Assuming that the mold structure 12 is to be immersed in the body 22 of molten metal in the container 20 by lowering the support structure 32 through a layer of 100 (
As the support structure 32 moves into engagement with the layer 100 of insulating material, only the portion of the layer 100 of insulating material which is aligned with the narrow support sections 56-62 and the central portion 66 of the support structure is engaged by the support structure. The portion of the insulating layer 100 which is engaged by the bottom side of the support structure 32 is deflected downwardly and sidewardly. The relatively large portions of the layer 100 aligned with the spaces 52 between the support sections 56-62 of the support structure 62 are not engaged by the support structure and are substantially undisturbed. This results in a relatively tight heat seal being maintained between the insulating layer 100 and the support structure 32 as the insulating layer is initially penetrated by the support structure. Therefore, there is minimal heat transfer from the furnace assembly 16 to the body 22 of molten metal.
As the support structure 32 continues to move slowly downward, the layer 100 of insulating material engages the side surfaces 80 and 82 on the support sections 56-62 to maintain a baffle of heat insulating material between the body 22 of molten metal in the container 20 and the hot furnace assembly 16. As the support structure moves still further downward, the body 22 of molten metal and the layer 100 of insulating material move inwardly over the base plate 90 of the mold structure 12. As this occurs, the layer 100 of insulating material moves into engagement with side portions 108 (
As the support structure 32 and mold structure 12 continue to be lowered, the body 22 of insulating material remains substantially intact until the gating system 46 moves downward into engagement with the upper side surface of the layer 100 of insulating material. This enables the layer 100 of insulating material to continue its function of providing a baffle to block heat transfer between the relatively hot furnace assembly 16 and the body 22 of molten metal as the mold structure is moved almost completely into the body 22 of molten metal. Since the arms of the gating system 46 are aligned with the support sections 56-62 of the support structure 32, there is also relatively little disturbance of the layer 100 of insulating material as the gating system 46 moves downward to and, if desired, through the insulating layer 100.
As the article mold portions 44 of the mold structure 12 move into the body 22 of molten metal, heat is transferred from the relatively hot molten metal in the article mold cavities 112 (
The metal in the article mold section 44 is directionally solidified upwardly in the article mold cavity 112. The metal in the article mold cavity may solidify with either a columnar grain or single crystal crystallographic structure. As this mold structure 12 continues to move into the body 22 of molten metal, the molten metal 118 in the article mold cavity 112 will solidify upwardly with the same crystallographic structure as the metal 116 in the lower portion of the article mold cavity 112.
Once the molten metal has solidified in the article mold cavity 112, the mold structure 12 is withdrawn from the body 22 of molten metal. This may be accomplished by lowering the body 22 of molten metal with the lower drive assembly 38 (
As the mold structure 12 is withdrawn from, the molten metal in the body 22, the molten metal passes through the open spaces 52 (
In the foregoing description, it has been assumed that the support structure 32 and mold structure 12 are lowered into the body 22 of molten metal and are subsequently raised from the body 22 of molten metal. However, it should be understood that the mold structure 12 may be lowered from the furnace and then the body 22 of molten metal raised to immerse the mold structure 12 in the body 22 of molten metal. Similarly, the mold structure 12 may be removed from the body 22 of molten metal by lowering the body 22 of molten metal relative to the mold structure.
The upper drive assembly 34 is operable to both raise and lower the support structure 32 and mold structure 12 relative to the body 22 of molten metal. Similarly, the lower drive assembly 38 is operable to both raise and lower the container 20 holding the body 22 of molten metal relative to the support structure 32 and mold structure 12. The drive assemblies 34 and 38 may be simultaneously operated and/or sequentially operated.
In the embodiment of the invention illustrated in
A mold structure 12a is disposed on a support structure 32a. The support structure 32a can be raised and lowered relative to a bath or body 22a of molten (liquid) metal in the manner previously described in conjunction with the embodiment of the illustrated in
To facilitate minimizing disturbances in the layer 100a of insulating material as it is penetrated by the support structure 22a, the support structure 32a has a tapered lower or leading end portion 130. The tapered leading end portion 130 penetrates the continuous layer 100a of insulating material and deflects the insulating material sidewardly as the support structure 32a moves downward through the layer 100a of insulating material into the body 22a of liquid metal. The tapered leading end portion 130 of the support structure 32a includes leading side surfaces 134 and 136 which intersect at a point 138.
Although only the tapered leading end portion 130 on the support section 62a has been illustrated in
If the insulating layer 100a is eliminated or positioned above and is spaced apart from the body 22a of molten metal, the tapered leading end portion 130 on the support structure 32a minimizes disturbance of the body of molten metal. The tapered leading end portion 130 pushes the molten metal aside with a smooth penetrating action. This smooth penetrating action begins as the support structure 32a initially engages the body 22a of molten metal and continues as the support structure is lowered further into the body of molten metal.
In the embodiment of the invention illustrated in
A support structure 32b is utilized to support a mold structure 12b. The mold structure 12b includes a plurality of article mold sections 44b which are disposed on a base plate 90b. The base plate 90b is coextensive with and rests on the support structure 32b. The base plate 90b is held against movement relative to the support structure 32b by suitable clamps and/or fasteners (not shown).
The support structure 32b is connected to a pair of upright support rods 28b corresponding to the support rods 28 of
The support structure 32b and mold structure 12b (
If desired, a layer of insulating material, corresponding to the layer 100 of
The support structure 32b may have a tapered lower or leading end portion, corresponding to the tapered leading end portion 130 of
The illustrated support structure 32b has a polygonal configuration. Although the support structure has been illustrated herein as having six support sections, corresponding to the support sections 56-62 of
The support structure 32b may be considered as being inscribed within a spatial envelope formed by a hexagonal polygon having the same size as the outside of the support structure. The support structure 32b may have an upper surface area which is between approximately five percent (5%) and approximately twenty-five percent (25%) of the area of the hexagonal spatial envelope within which the support structure 32b is inscribed. The remainder of the area of the polygonal spatial envelope within which the support structure 32b is inscribed will be open space.
The illustrated support structure 32b has a hexagonal configuration. However, the support structure 32b may have a different configuration if desired. For example, the support structure 32b may have a rectangular configuration. Alternatively, the support structure 32b may have an octagonal configuration. As another example, the support structure 32b may have an annular configuration.
It is contemplated that the mold structure 12b and support structure 32b may be utilized with a layer of insulating material, corresponding to the layer 100 (
Assuming that the mold structure 12b is to be immersed in the body of molten metal, that is, the body 22 of
As the support structure 32b moves into engagement with the layer of insulating material, only the portion of the layer of insulating material which is aligned with the narrow sections of the support structure 32b is engaged by the support structure. The portion of the layer of insulating material aligned with the space 52b is not engaged by the support structure 32b and is substantially undisturbed. This results in a relatively tight heat seal being maintained between the insulating layer and the support structure 32b as the insulating layer is penetrated by the support structure.
As the support structure 32 continues to move slowly downward, the insulating layer engages side surfaces of the mold sections 44b. This results in a baffle of insulating material, corresponding to the insulating material 100 of
The layer of insulating material may be disposed above and spaced from the body of molten metal. At least a portion of the layer of insulating material may have a relatively rigid construction and have one or more openings through which the mold structure 12b and support structure 32b move. If this is done, the layer of insulating material may be connected with the upper end portion of a container which holds the body of molten metal in which the mold structure 12b is immersed.
Once the molten metal in the article mold portions 44b has solidified, the mold structure 12b is withdrawn from the body of molten metal, corresponding to the body 22 of molten metal in
In the embodiment of the invention illustrated in
A casting apparatus 10c (
A framework 26c supports the mold 12c for movement to and from the furnace assembly 16c and for movement to and from the body 22c of molten metal. The framework 26c includes a plurality of support rods 28c and a mold support structure 32c. The support rods 28c may be disposed outside the furnace assembly 16c if desired.
As was previously mentioned, the support structure 32c functions as a chill plate. The support rods 28c are connected with an upper drive assembly 34c and with the mold support structure 32c. The upper drive assembly 34c is operable to raise and lower the framework 26c relative to the furnace assembly 16c and container 20c holding the body 22c of molten metal.
A lower drive assembly 38c is connected with the container 20c which holds the body 22c of molten metal. The lower drive assembly 38c is operable to raise and lower the container relative to the furnace assembly 16c. The upper and lower drive assemblies 34c and 38c may be operated simultaneously and/or sequentially to raise and/or lower the framework 26c and/or container 20c holding the body 22c of molten metal.
While the mold structure 12c is supported in a furnace 16c on the framework 26c, in the manner previously described in conjunction with the embodiments of the invention illustrated in
A layer 100c (
The illustrated layer 100c of insulating material floats on the upper surface 102c of the body 22c of molten metal. The layer 100c of insulating material shields the body 22c of molten metal from the relatively hot environment of the furnace assembly 16c. Thus, the layer 100c of insulating material retards heat transfer from the furnace assembly 16c and mold structure 12c to the body 22c of molten metal.
The layer 100c of insulating material may have any of the constructions previously described herein. Although the layer 100c of insulating material is supported by floating on the body 22c of molten metal, the layer of insulating material may be supported in a different manner if desired. For example, the layer of insulating material may be supported by the container 20c and be above and spaced apart from the body 22c of molten metal.
In accordance with a feature of the embodiment of the invention illustrated in
The rigid baffle 150 is fixedly connected to the upper end portion of the container 26c and extends radially inwardly from a cylindrical side wall of the container 20c toward the generally cylindrical furnace assembly 16c. The furnace assembly 16c is disposed in a circular central opening 152 in the baffle 150. The opening 152 has a slightly larger diameter than the exterior of the furnace assembly 16c to enable the container 20c to move vertically relative to the furnace assembly 16c. Although the illustrated baffle 150 has an annular configuration, the baffle may have a different configuration which is a function of the configuration of the container 20c and/or furnace assembly 16c.
The present invention relates to a new and improved method of casting metal articles. The method includes providing a support structure 32 having a plurality of support sections 56-62. Article mold sections 44 are positioned on the support sections 56-62. Article mold cavities 112 in the article mold sections 44 are filled with a first molten metal which is at a first temperature.
The support sections 56-62 and a body 22 of a second molten metal are moved relative to each other. The body 22 of a second molten metal is at a temperature which is less than the first temperature of the first molten metal in the article mold cavities 112. Spaces 52 extending between side surfaces 80, 82 of the support sections 56-62 and between side portions of the article mold sections 44 are filled with the second molten metal.
If desired, a layer 100 of insulating material may be provided above an upper side 102 of the body 22 of a second molten metal. Portions of the layer 100 of insulating material are aligned with spaces 52 extending between side surfaces 80, 82 of the support sections 56-62. These portions of the layer 100 of insulating material extend across spaces between the support sections 56-62 and extend across spaces between portions of the article mold sections 44 during at least a portion of the relative movement between the support sections 56-62 and body 22 of molten metal. A baffle 150 may be provided between the container 20 holding the body 22 of a second molten metal and the furnace assembly 16.
The present invention has a plurality of different features which are advantageously utilized together in the manner described herein. However, it is contemplated that the features may be utilized separately and/or in combination with features from the prior art. For example, the layer 100 of insulating material may be formed by hollow bodies which float on the body 22 of liquid metal. Alternatively, the layer 100 of insulating material may be spaced from the body 22 of liquid metal. If desired the layer 100 of insulating material may be omitted.