US 3788382 A
A metal casting apparatus especially adapted for the rapid melting and casting of small charges of metal, utilizing a metal-heating crucible in communication with a mold contained in a heated enclosure. The metal charge within the crucible is rapidly heated to melt the same to bring the molten metal to optimum casting temperature, whereupon the molten metal in the crucible is subjected to sufficient pressure differential to force the molten metal through a small opening in the crucible into the mold.
Description (OCR text may contain errors)
United States Patent 1191 Daniel et al.
[ Jan. 29, 1974 FOREIGN PATENTS OR APPLICATIONS 1 1 VACUUM METAL CASTING APPARATUS Great Britain...... 164/65 Great Britain..1...,................, 164/80 ABSTRACT A metal casting apparatus especially adapted for the rapid melting and casting of small charges of metal, utilizing a metal Primary Examiner-R. Spencer Annear Attorney, Agent, or FirmDonnelly, Maky, Renner & Otto heating crucible in communication Inventors: Arnold Daniel, 2219 Devonshire Dr.; Joseph Burdette Richey, I], 2160 Demington Dr., both of Cleveland Heights, Ohio 44106  Filed: Oct. 26, 1972  Appl. No.: 301,158
Related U.S. Application Data  Division of Ser. No. 92,583, Nov. 25, 1970, Pat. No.
with a mold contained in a heated enclosure. The metal charge Within the crucible is rapidly heated to melt the same to bring the molten metal to optimum casting temperature, whereupon the molten metal in 56 6 fi5lw5 2 2 M w. M 1 2 2 4 5 M U 6 3/ .1. 64 s 6 0 "2 5 "6 n m 4 M 6 A 1 m6 Tn 1 c a .r n "a e m .L hf. C 10 u min U IF MM UN 5 55 22 Claims, 5 Drawing Figures the crucible is subjected to sufficient pressure differential to force the molten metal through a small opening in the crucible into the mold.
164/62 UX 164/62 UX  References Cited UNITED STATES PATENTS 1,070,500 8/1913 Mackenzie.....................
1,644,543 10/1927 Richmond......................
VACUUM METAL CASTING APPARATUS CROSS-REFERENCE TO RELATED APPLICATIONS This application is a division of our copending United States application Ser. No. 92,583, filed Nov. 25, 1970, now U.S. Pat. No. 3,712,364, granted Jan. 23, 1973.
BACKGROUND OF THE INVENTION Investment casting procedures have long been employed in the production of investment castings of pre cious metals for use in jewelry, dental inlays and bridges, and the like. The present system is particularly advantageous when very precise control of the casting operation is required in order that the casting may be of exact predetermined dimensions and alloy composition.
In the conventional production of dental alloy castings, an investment mold is prepared about a wax model or pattern and the mold is preheated to volatilize the wax and to bring the mold up to the desired casting temperature. The achievement and maintenance of such temperature during the casting operation is of considerable importance inasmuch as both the mold and the casting tend to shrink upon cooling and in order to obtain a finished casting of exact predetermined dimensions it is essential that both the molten metal and the mold be at proper temperatures when the molten metal is introduced into such mold. In a typical operation, the mold has thus been preheated and then transferred to a centrifugal casting machine. The gold alloy charge is placed in the machine and heated by means of an oxyacetylene torch to melt the charge, whereupon the machine is activated and the molten metal forced into the mold under the influence of centrifugal force. Under such circumstances, it is obviously not possible to ensure that the mold is at the precise optimum temperature at the moment the molten metal enters the same and it is also not possible to ensure that the molten charge itself is at an optimum temperature. Furthermore, the alloy composition may be somewhat modified during the heating operation and the charge may be contaminated by atmospheric gases and the gases produced by employment of such torch. An appreciable excess of the alloy must be provided requiring subsequent cutting and grinding away of the metal sprue produced, and the entire operation requires such a degree of skill and experience that many dental offices prefer to send such work to specialist laboratories rather than to perform the work themselves. This results in considerable delay and increased expense, particularly when several castings must be produced until one is achieved which is found to be entirely satisfactory for the intended purpose.
It is accordingly an object of the present invention to provide a self-contained unit wherein the mold need not be removed from the heat bath prior to or during performance of the casting operation.
It is also an object to provide a metal casting system and apparatus for performing the same of small size which may be operated from the ordinary 1 volt AC source available in all dental offices and which will incorporate a control system to ensure the automatic production of high quality castings without the requirement of any special skills on the part of the operator.
A further object is to provide a metal casting system wherein both the molten metal and the mold are brought to the optimum temperatures and the casting operation is performed very rapidly in a manner to keep the molten metal charge uncontaminated and to maintain the proportions of any alloy elements unchanged.
Other objects of the invention will appear as the description proceeds.
To the accomplishment of the foregoing and related ends, the invention, then, comprises the features hereinafter fully described, the following description and the annexed drawings setting forth in detail certain i1- lustrative embodiments of the invention, these being indicative, however, of but a few of the various ways in which the principles of the invention may be employed.
DESCRIPTION OF THE DRAWING In said annexed drawing FIG. 1 is an isometric view of a novel metal casting apparatus, including associated control means, for practicing the method of the present invention;
FIG. 2 is an enlarged vertical longitudinal section taken on the line 2-2 of FIG. 2, but omitting the control panel;
FIG. 3 is a wiring diagram illustrating suitable control means for the operation of such apparatus;
FIG. 4 is an enlarged exploded isometric detail view of the melting crucible assembly of such apparatus; and
FIG. 5 is a wiring diagram of an optional modification of the metal heating means.
DESCRIPTION OF THE INVENTION In accordance with the invention, investment casting apparatus is provided comprising an insulated enclosure or muffle having heating means for heating the interior thereof to a high temperature. A metal heating crucible is mounted in the upper portion of such enclosure and means is provided to support a mold directly beneath such crucible in clamping engagement therewith so that molten metal produced in the crucible may be directly discharged into the mold therebeneath. Suction means is applied to the mold to assist in drawing the molten metal charge from the crucible into the mold cavity, and electrical control means ensure that the various operations are carried out at the proper times and at the proper temperatures. The melting crucible may be enclosed within an inert atmosphere to avoid any possibility of contamination of the molten charge.
Now referring more particularly to said drawing and especially FIGS. 1 and 2 thereof, the embodiment of the invention there illustrated comprises a furnace or oven 1 desirably supported on legs such as 2 and having an outer sheet aluminum shell or casing 3. Such casing is lined with a thick wall of refractory insulating material 4 which encloses a refractory ceramic inner liner 5. Nichrome heating elements 6 are interposed between such outer liner 4 and inner liner 5 in order to raise the interior of the furnace to a high temperature. The front of the furnace is closed by an insulated hinged gate or door 7 normally held in closed position by means of counterweight 8.
A control panel 9 is carried by brackets spacing the same from the end of the furnace, and a removable hood 10 is mounted on the top of the furnace. Such hood may be connected, if desired, to a source of inert gas such as cylinder 11 by means of flexible tubing 12,
admission of such gas (e.g. helium, nitrogen, or argon) to the hood being controlled by means of hand operated valve 13.
Hood is adapted to cover an opening in the upper side of the furnace within which a metal heating crucible is received and mounted. Such crucible is a vertically split graphite crucible comprising half portions 14 and 15 (see also FIG. 4) separated by a layer of inert refractory material such as asbestos 16 which may be provided with a centrally disposed gap in the lower edge thereof to serve as a sprue for the crucible; the two halves of the crucible are thus electrically insulated from each other. In the preferred embodiment illustrated in detail in FIG. 4 the sprue 17 is, however, provided in one crucible half only, with the inner surfaces of the other half and such asbestos divider being inclined theretoward. Such sprue may, for example, be approximately 0.04 mm. diameter. Accordingly, when the crucible blocks 14 and 15 are assembled with the interposed insulating layer 16 they define a crucible cavity the upper end of which projects above the upper surface of furnace 1 and the lower end of which projects into the interior of furnace l. The graphite blocks 14 and 15 are respectively backed by heavy sheet aliminum or copper contact plates 19 and 20 to the upper edges of which electric power cables 21 and 22 are connected. Layers of electrical insulation 23 and 24 laterally enclose the crucible and contact plates so that the two halves of the crucible are not only electrically isolated from each other but also from the supporting upper furnace wall.
An optional, but ordinarily highly preferred device is also provided within hood 10 to bear downwardly upon a metal charge to be melted within the crucible and thereby ensure better contact between the two half sections of such crucible and the charge which bridges therebetween. Such device may comprise a bracket 25 mounted on the upper surface of the furnace and carrying a vertically disposed plunger 26 directly axially reciprocable above the sprue of the graphite melting crucible.
A compression spring 27 serves normally to maintain such plunger in depressed condition so that the graphite rod 28 coaxially secured thereto may thus bear firmly downwardly upon the charge in the crucible. Plunger 26 and rod 28 may be temporarily manually elevated when it is desired to place a new charge within the crucible by depressing lever handle 29.
A circular metal, e.g. stainless steel, jig or platform 30 is supported within the furnace chamber or muffle 31 on the upper end of vertically reciprocable metal tube 32. The upper end of such tube opens to a centrally disposed orifice in the. upper surface of platform 30 and the lower end of such tube connects through an elbow 33 of stainless steel tubing to vacuum hose 34. A manually rotatable nut 35 is threaded on the lower projecting end portion of metal tube 32 and held against axial movement by upper washer 36 interposed between such nut and the underside of the furnace and by lower washer 37 interposed between such nut and the centrally apertured spider 38 affixed to the underside of the furnace and projecting downwardly therefrom. Accordingly, when nut 35 is thus manually rotated, jig or platform 30 will be correspondingly elevated or lowered within the furnace chamber.
The remaining elements of the assembly may best be understood from the description of the operation of the metal casting system set forth below.
OPERATION The operation of this new metal casting system will first be generally described, and then more specifically with reference to the electric control system.
The usual wax pattern or model is prepared and embedded in a suitable porous plaster mold 39 enclosed in the usual tubular metal casting ring 40. Such mold may desirably initially be placed within furnace chamber 31 laterally of platform 30 as shown at 40' for preheating prior to performance of the casting operation. Such preheating serves to volatilize and drive off the wax pattern, the fumes being vented from chamber 31 through vents V, and to heat the mold 39 to proper casting temperature. The door to the furnace will then be opened and the operator will lift the mold onto platform 30 where it is supported on annular asbestos sealing ring 41 which serves thus to space the underside of the mold slightly from the upper surface of platform 30, thereby permitting suction subsequently to be applied to the entire undersurface of the mold through metal tube 32. In some cases, if the mold is not sufficiently porous, it may be desirable to provide a very small vent passage 42 leading from the mold cavity 43 to a layer of porous asbestos 44 extending to the lower surface of mold 39 and thereby permitting suction (e.g. 28-29 inches of mercury) to be applied more directly to such mold cavity. The upper surface of the mold is cratered at 45 to provide a funnel shaped cavity terminating in a small sprue 46 leading to the mold cavity 43. A centrally apertured sheet metal cover 47 fits over the top of the mold and asbestos sealing ring 48, and a smaller asbestos sealing ring 49 is interposed between such cover and the underside of the crucible so that the sprue 17 of the crucible communicates directly with a closed vacuum system.
A crucible assembly is clamped firmly in place by means of large set screw 50 which may be insulated from contact plate 19, or which may merely be grounded by grounding of outer aluminum casing 3 of the furnace). The hand nut 35 will now be rotated to elevate platform 30 to raise the mold and clamp cover 47 firmly against sealing ring 49 interposed between the same and the underside of the split crucible.
Cover 10 is now lifted, handle 29 is depressed to raise rod 28, and the measured solid metal charge 51 is placed in the crucible. Handle 29 is thereupon released so that rod 28 may firmly press such charge downwardly against the respective crucible half sections. Heating current may now be directed to the crucible sections through leads 21, 22 for a sufficient period of time (e.g. 10 seconds) to melt the metal charge. The plunger assembly 26, 27, 28 will ordinarily be so dimensioned and proportioned that, even when the crucible is completely empty, the lower end of rod 28 will not extend entirely to the bottom of the crucible, it being desired only that the lower end of such rod should bear upon the solid metal charge with sufficient force to ensure adequate electrical contact between such charge and the respective crucible sections. When the charge has melted and been brought to optimum temperature (as determined by thermocouple or infrared detector sensing means described in more detail in connection with the discussion of the wiring diagram,
below) suction is applied to tubing 34 and therefore through elbow 33, tubular support 32, porous mold 39, mold cavity 43, and sprue 46 to the crucible sprue to overcome the surface tension of the molten metal and rapidly draw the latter downwardly into the mold cavity. Platform 30 may thereupon be lowered, door 7 opened and the mold removed from chamber 31. Alternatively, if it is desired to cool the mold more slowly in order better to control the solidification phenomena which affect the grain structure of the casting, the mold may be left within such chamber for an extended period of time while the chamber is very gradually cooled.
It will be apparent from the foregoing that the molten metal may thus be very rapidly drawn into the mold cavity immediately upon attainment of optimum temperature and with minimal opportunity to absorb any impurities. As above explained, an inert gas may be admitted to cover or hood 10 from cylinder 11 prior to performance of the melting operation and, if desired, such inert gas may even be admitted under sufficient pressure substantially to purge the hood and crucible prior to elevation of the mold into clamping relationship with the underside of the crucible. The inert gas pressure within the hood may thereupon assist in ejecting the molten metal from the crucible into the evacuated mold cavity if the hood is tightly clamped down and the crucible is adequately sealed at the top of the furnace.
Now referring more particularly to FIG. 3 of the drawing, the operation of the metal casting system of this invention will be described in greater detail, with particular reference to the electrical control system. After the investment rings 40 and 40' with their enclosed molds have been placed within furnace chamber 31, and door 7 closed, line switch 52 will be manually closed to admit 100 volt AC heating current from main line 53, 54 to lines 55, 56 connected with the nichrome heating elements 6 in furnace 1. Line 56 is connected through a standard temperature control unit 57 which may be preset to disconnect such line when a desired temperature has been attained within the furnace. Temperature sensing means in the form of the thermocouple 58 may be provided in the furnace and connected with such temperature control unit 57 to activate the latter. A neon pilot light 59 signals that power is on.
Hand screw 35 isrotated to elevate platform 30 to clamp the mold in place beneath the split crucible, and the metal charge 51 is placed within such crucible and downwardly biased rod 28 is caused to press thereon. Hood 10 is closed, clamped, and inert gas is admitted thereto if desired.
When thermocouple 58 signals that the predetermined temperature has been attained in furnace chamber 31, the temperature control unit 57 automatically cuts off or limits further power to the nichrome heating elements 6 to maintain the constant preset temperature. After sufficient time is allowed for the mold to reach optimum temperature, the operator may thereupon initiate the melting cycle by depressing the reset switch 60 on crucible temperature controller 61. This energizes the coil of contactor 62 which in turn applies line voltage to the primary of transformer 63, causing very high current to flow through the secondary of such transformer, the two graphite mold sections 14 and 15, and the metal charge 51 bridging the latter. When the predetermined melting optimum temperature of the molten metal produced has been sensed and signaled by thermocouple 64 embedded in the crucible but electrically insulated therefrom or by infrared detector D located above the crucible, the temperature control unit 61 deenergizes the coil of contactor 62 and energizes the coil of time delay relay 65 which in turn energizes the coil of normally closed solenoid valve 66 to open the latter and connect suction line 34 to vacuum for a length of time determined by the time delay relay. This immediately operates as above described to draw the molten metal through the crucible sprue into the evacuated mold cavity 43. After a preset time determined by time-delay relay 65, valve 66 automatically recloses, cutting off the vacuum connection to the mold.
During application of the vacuum, the pressure differential is effective to overcome the surface tension of the molten metal and force the latter through the small crucible sprue.
It will be seen from the foregoing that a method of discharging molten metal from the crucible has been provided wherein the outlet or sprue leading therefrom is of sufficiently small size that the surface tension of the molten metal normally serves to prevent discharge therethrough by gravity, the suction which is then applied at the proper moment affording a pressure differential sufficient to overcome such effect of surface tension and cause the molten metal to flow through the sprue. The crucible should, of course, be of material not wettable by such molten metal.
The investment may be prepared in generally wellknown manner for the production of cast dental inlays, onlays, crowns, bridges, and frameworks for removable appliances, utilizing dental casting gold alloys in accordance with American Dental Association specification No. 5 (Federation Dentaire Internationale Specification No. 7 for Dental Casting Gold Alloy) Approved April, 1965; Effective April 1, 1966.
The wax pattern is prepared and a 12 gauge (Brown & Sharpe) sprue pin is inserted in the bulkiest part of the pattern and a conical sprue former is affixed to the other end of such pin.
Normally, the extent of such pin from the pattern to the sprue former should not exceed three-eighth inch. The assembly is carefully washed and allowed to dry.
A cylindrical brass or stainless steel casting ring is lined with a circularized strip of asbestos, the ends of which overlap slightly. Such ring may be 1 /2 inch in length and 1 A; inch in diameter, for example, and the asbestos liner should be about one-eighth inch shorter than the ring, at each end.
The investment plaster (usually a gypsum composition) such as Kerr Luster Cast or Cristobolite is mixed with water (e.g. 30 to 34 parts of water to grams of powder), the plaster being sifted into the water, until a creamy consistency is obtained. The pattern is painted with the mixture, usually after first lightly painting it with a wetting agent, and inverted and placed within the ring which is then filled with the same mix.
After being allowed to set for about one hour, the burnout operation may be commenced. The sprue former and sprue pin are removed and the casting ring and investment may now be placed in the furnace with the sprue hole in the down position. When the mold has gradually been brought to approximately 500F it is turned with the sprue upward and brought to casting temperature (l,292 F in reference to Cristobolite). It should desirably be allowed to heat soak for about 20 minutes at this temperature prior to casting.
The crucible is charged with ingots of gold alloy (about 2 to 4 pennyweight for inlays and crowns, and about 20 to 30 pennyweight for large partial framework castings) and brought to fusion temperature, this ordinarily being nearer the upper (liquidus) than the lower (solidus) limit of the melting range.
The mold is clamped and the vacuum applied as described above. The vacuum should be high, e.g. 28-29 inches of mercury, if possible, (l2-l3 psi) and the diameter of the crucible sprue opening may be 16 gauge (Brown & Sharpe) or less to prevent premature dribbling.
Now referring more specifically to FIG. 5 of the drawing, the optional form of hold-down there illustrated may comprise a graphite electrode 67 corresponding to rod 28 in function but also being electrically connected to transformer 63. It must accordingly be electrically insulated as at 68 in its support, and a portion of the heating current will flow between the electrode and the crucible sections.
The temperature sensing thermocouple 64 (FIGS. 3
and 4) may have its leads extending through an elongated ceramic insulating member 69 inserted in one of the graphite crucible blocks with the thermocouple itself embedded in refractory ceramic insulating cement adjacent the metal fusing Zonev Insulating pads 70 and 71 may be provided on the crucible projections such as 72 and 73 to ensure that the crucible is insulated from the furnace under the upward pressure of threaded tube 32 and also to provide a gas tight seal around the crucible.
The infrared radiation detector D referred to above as a preferred alternative to thermocouple 64 is a commercially available unit semi-diagrammatically shown in FIG.. 2 as mounted in an extension of bracket 25. Such detector (preferably the silicon type) is clamped at an angle to focus upon the charge 51 in the bottom of the crucible and thereby is enabled to sense the temperature of such charge by response to the infrared radiation of the metal (infrared radiation pyrometry). The leads from detector D thus connect with temperature controller 61 instead'of the leads shown coming from thermocouple 64 in FIG. 3.
The apparatus and method of the present invention accordingly provide a metal casting system adapted to ensure the integrity of the resulting casting as to composition, physicals, and dimensions to a degree not heretofore readily attainable. Such apparatus is not unduly expensive, and the mode of operation is such that it may be carried out without the benefit of exceptional skill and training.
We, therefore, particularly point out and distinctly claim as our invention:
1. Metal casting apparatus comprising an insulated enclosure, a mold within said enclosure, heating means for heating the interior of said enclosure to heat said mold to optimum casting temperature, a metal-heating crucible above said mold having an opening in communication with said mold within said enclosure, additional heating means for rapidly heating a metal charge within said crucible to melt the same and bring the molten metal to optimum casting temperature within said crucible, said opening in said crucible being too small to permit ready passage of such molten metal therethrough, means for sensing the temperature of the molten metal in said crucible, and means for subjecting such molten metal in said crucible to sufficient pressure differential to force such molten metal through said opening into said mold after the molten metal has reached such optimum casting temperature.
2. The apparatus of claim 1 wherein said additional heating means for rapidly heating the metal charge in said crucible comprises electrical contact plates connected to said crucible for passage of electric current through said crucible and through the metal charge to cause resistance heating of the latter.
3. The apparatus of claim 1 wherein said crucible is vertically split to provide split portions of electrically conductive material, with insulating material separating said split portions, and said additional heating means for rapidly heating the metal charge in said crucible comprises means for passing an electric heating current between said split portions through the metal charge contained therein to melt such metal charge.
4. The apparatus of claim 3 further comprising means for pressing such metal charge against said crucible split portions to enhance passage of such electrical current through such metal charge rapidly to heat the same.
5. The apparatus of claim 3 further comprising a holddown member mounted to bear downwardly upon the metal charge in said crucible to insure good electrical contact of such metal charge with both of said crucible split portions to enhance passage of such electric current through such metal charge rapidly to heat the same.
6. The apparatus of claim 5 wherein said hold-down member is also electrically conductive and is connected to an electric current source for passage of current between said hold-down member and said crucible split portions through such metal charge.
7. The apparatus of claim 5 wherein said hold-down member is resiliently mounted for downward pressure upon such metal charge, and stop means are provided for limiting downward movement of said hold-down member to prevent direct contact of said hold-down member with said crucible split portions.
8. The apparatus of claim 1 wherein said mold is a suction mold, and said means for applying such pressure differential to force such molten metal through said opening in said crucible into said mold comprises a suction pump for applying suction to said mold to draw such molten metal from said crucible into said mold.
9. The apparatus of claim 8 further comprising clamping means for clamping said mold in sealing engagement with the underside of said crucible thereby to permit application of such suction to said crucible through said mold.
10. The apparatus of claim 1 wherein the surface tension of such molten metal is sufficient to prevent passage of such molten metal through said crucible opening under the influence of gravity, and such pressure differential is effective to overcome such surface tension when applied.
11. The apparatus of claim 1 further comprising separate temperature sensing means for sensing the temperature of said mold and the molten metal in said crucible, signal means connected with said mold temperature sensing means for signalling that a predetermined desired temperature has been achieved in said mold,
means for activating said additional heating means, and electrical control means responsive to a signal from said molten metal temperature sensing means that a predetermined desired temperature has been achieved in such molten metal automatically operative thereon to activate said means for subjecting such molten metal to sufficient pressure differential to force such molten metal through said crucible opening into said mold as aforesaid.
12. The apparatus of claim 1 wherein said insulated enclosure has an upper wall with an opening therein for receipt of said crucible, and means are provided for clamping said crucible in place within said opening.
13. The apparatus of claim 12 further comprising a removable cover enclosing said crucible, means for admitting an inert gas to said cover, and a vent for said enclosure.
14. The apparatus of claim 12 further comprising a vertically reciprocable support for said mold within said enclosure, and means for reciprocating said support to clamp said mold against the underside of said crucible with said opening in said crucible in juxtaposition to a sprue inlet in said mold.
15. The apparatus of claim 14 wherein said means for subjecting such molten metal to sufficient pressure differential to force such molten metal through said crucible opening into said mold comprises a vacuum pump, a suction passage through said support leading to said mold for applying suction to said mold to draw such molten metal from said cricible rapidly into said mold, and means for connecting said suction pump to said suction passage.
16. The apparatus of claim 15 wherein said support has a platform on the upper end thereof for supporting said mold thereon, and there is a sealing ring interposed between said platform and mold around the outer periphery of said mold which serves to space the underside of said mold slightly from the upper surface of said platform to permit suction to be applied to the entire undersurface of said mold.
17. The apparatus of claim 15 further comprising a cover over said mold, a sealing ring interposed between said cover and mold to provide an air-tight seal therebetween, said cover having a central aperture therein in alignment with said opening in said crucible and a sprue inlet to said mold, and a sealing ring interposed between said cover and the underside of said crucible providing an air-tight seal between said crucible and cover around said central aperture.
18. The apparatus of claim 1 wherein said temperature sensing means electrically senses the temperature of the molten metal charge in said crucible.
19. The apparatus of claim 1 wherein said temperature sensing means comprises an infrared radiation detector which is focused upon the metal charge in said crucible to sense the temperature of such metal charge by response to the infrared radiation of the metal.
20. The apparatus of claim 1 wherein said additional heating means comprises means for passing an electric heating current through the metal charge within said crucible to cause resistance heating of the latter to melt such metal charge.
21. The apparatus of claim 20 wherein said mold is a suction mold, and said means for applying such pressure differential to force such molten metal through said opening in said crucible into said mold comprises a suction pump for applying suction to said mold to draw such molten metal from said crucible into said mold.
22. The apparatus of claim 20 wherein the surface tension of such molten metal is sufficient to prevent passage of such molten metal through said crucible opening under the influence of gravity, and such pressure differential is effective to overcome such surface tension when applied.