WO2006021082A1

WO2006021082A1 - Apparatus for metal foam casting and methods therefor

Info

Publication number: WO2006021082A1
Application number: PCT/CA2005/001278
Authority: WO
Inventors: James Killingbeck; Scott Nichol; Roger Seucharan; Richard Gordon Manley; Mark Morita
Original assignee: Cymat Corp
Current assignee: Cymat Corp
Priority date: 2004-08-24
Filing date: 2005-08-24
Publication date: 2006-03-02
Anticipated expiration: 2007-02-24
Also published as: WO2006021082B1

Abstract

An apparatus for casting a metal foam article comprises a crucible for containing the molten metal, a gas injection nozzle located above the level of the molten metal when at the neutral level, and a mould having a mould cavity of the shape of the article. The apparatus includes a means for raising the level of the molten metal above the gas injection nozzle for generating the foam. A method for casting the foam article comprises maintaining the level of the molten metal below the gas injection nozzle until foaming is desired, raising the level of the molten metal above the gas injection nozzle and injecting a gas into the molten metal for generating the metal foam. The method also comprises directing the foam into the mould and cooling the foam to form the foam article. The apparatus may be constructed of separable modules thereby allowing the process to be tailored to meet specific requirements.

Description

APPARATUS FOR METAL FOAM CASTING AND METHODS THEREFOR

FIELD OF THE INVENTION

[0001] This invention relates to a process and apparatus for casting of metal foam articles. More particularly, the invention relates to an improved foam casting apparatus and method wherein the foaming nozzle is maintained above the level of the molten metal. Further, the invention relates to an improved foam casting apparatus and method wherein the foaming chamber is separated from the molten metal supply.

DESCRIPTION OF THE PRIOR ART

[0002] Lightweight foamed metal articles are of considerable interest to various industries, such as the automotive industry, due to their high strength to weight ratio. In general, such articles are formed by generating a gas within a bath or furnace or crucible containing a molten metal and allowing the gas to foam the molten metal. The foam comprises a plurality of closed cells containing the gas with the walls of the cells formed of the metal. The foam can be collected and formed into shaped articles or slabs and cooled to solidify. The foaming gas can be generated by means of gas generating particles added to the molten metal (in a process commonly referred to as "in-situ foaming") or by introduction of the gas into the molten metal through a port or nozzle. It is also commonly known to include certain additives to the molten metal to reinforce the walls of the cells formed during the foaming process and prevent cells from coalescing or otherwise collapsing. These additives generally comprise particles of, for example, refractory materials, silicon carbide (SiC) and other such materials as known in the art.

[0003] Examples of known foam generation processes are provided, inter alia, in the following US patents: 5,221,324; 5,622,542; and 5,334,236. Further examples are provided in the following PCT publication numbers: WO 03074163; WO 03064711; and WO

03015960 (which share a common assignee with the present application). A further example is provided in German patent number 4326982. The entire contents of the foregoing patents and applications are incorporated herein by reference.

[0004] With the known methods of gas introduction into a metal melt, it is known to provide one or more nozzles each having one or more openings through which a gas (such as air, nitrogen, or any other gas) is passed. The nozzles are commonly positioned under the liquid level of the molten metal so that the gas is introduced directly into the molten metal to form bubbles. The bubbles can then be collected, as a foam, on the surface of the melt. As is known in the art, the foam can be formed into a slab or directed into a mould (having a mould cavity in the three dimensional shape of a desired article) where it is cooled and hardened to form the desired foam article.

[0005] With these types of processes, it is often found that since the gas nozzles are maintained submerged under the molten metal, they are prone to clogging. To prevent this, it is common to maintain at least a base level of gas flow through the nozzles to prevent molten metal from entering the gas outlets in the nozzles. This base level of gas flow is of a lower flow rate than that used when the actual foaming step is conducted. However, this base level gas flow rate still results in a small amount of foam generation during periods when formation of foam is not desired (such as when cooling a foam filled mould). [0006] Further, with the prior art nozzles being submerged in the molten metal, any replacement (for example, when the nozzles become clogged as mentioned above) or repairs thereto requires draining of the crucible to gain access to the nozzles. This results in considerable down-time for the apparatus and, therefore, increased production costs. [0007] One of the other problems associated with prior art foaming apparatuses lies in the fact that the various components are integrated, thereby rendering it difficult or expensive to interchange or service one or more of the components. In such cases, the entire apparatus must be shut down, drained, and cooled before the required maintenance can be performed. Further, since each melt chamber is normally associated with one mould, the process of casting foam articles becomes time intensive.

[0008] For these and other reasons, a demand exists for an improved metal foam generation process and/or apparatus.

SUMMARY OF THE INVENTION

[0009] In one aspect, the present invention provides an apparatus and method for generating a metal foam.

[0010] In another aspect, the invention provides an apparatus for generating a metal foam wherein a gas injection nozzle is provided above the liquid level of the molten metal and wherein the apparatus includes a means of raising the level of the molten metal above the nozzle when foaming is required. [0011] In another aspect, the invention provides an apparatus for generating a metal foam by introduction of a gas without an agitator.

[0012] In a further aspect, the invention provides an apparatus for generating a metal foam that is modular in design.

[0013] Thus, in one aspect, the invention provides an apparatus for casting a metal foam article from a foam of a molten metal, the apparatus comprising:

- a crucible for containing a molten metal;

- a mould provided above the crucible, wherein the mould includes at least one mould cavity having the shape of the article and an opening for fluid communication between the crucible and the cavity;

- a gas injection nozzle, connected to a gas supply, the gas injection nozzle being positioned below the mould cavity opening; and

- a flow generating means for causing the molten metal to flow from the crucible to the mould, whereby the molten metal level in the crucible is varied from a first level below the gas injection nozzle to a second level above the gas injection nozzle.

[0014] In another aspect, the invention provides a method of casting a metal foam article from a foam of a molten metal comprising: a) providing a crucible containing the molten metal; b) providing a mould having a mould cavity having the shape of the article, the mould being located above the crucible and the mould cavity having an opening into the crucible for fluid communication therewith; c) providing a gas injection nozzle below the mould cavity opening; d) maintaining the level of the molten metal below the gas injection nozzle; e) raising the level of the molten metal above the gas injection nozzle; f) injecting gas through the gas injection nozzle to generate a foam of the molten metal; g) directing the foam into the mould cavity; h) filling the mould cavity with the foam and allowing the foam to cool; i) returning the level of the molten metal level to below the gas injection nozzle; j) removing the formed article from the mould cavity. [0015] In a further aspect, the invention provides an apparatus for casting a metal foam article from a foam of a molten metal, the apparatus comprising:

- a crucible for containing a molten metal;

- a mould, wherein the mould includes at least one mould cavity having the shape of the article and an opening for fluid communication between the crucible and the cavity;

- a gas injection nozzle, connected to a gas supply; and

- a flow generating means for causing the molten metal to flow from the crucible to the mould; wherein the crucible and mould are releasably connected together.

BRIEF DESCRIPTION OF THE DRAWINGS

[0016] These and other features of the invention will become more apparent in the following detailed description in which reference is made to the appended drawings wherein:

[0017] Figure 1 is a cross sectional elevation of a foam casting apparatus according to an embodiment of the invention.

[0018] Figures 2 to 5 illustrate various stages during the process of using the apparatus of

Figure 1.

[0019] Figure 6 is cross sectional elevation of another embodiment of the apparatus of

Figure 1.

[0020] Figure 7 is a cross sectional elevation of a foam casting apparatus according to another embodiment of the invention.

[0021] Figures 8 to 11 illustrate various stages during the process of using the apparatus of Figure 7.

[0022] Figure 12 is a cross sectional elevation of an apparatus having modular components.

[0023] Figure 13 is a cross sectional elevation of a variation of the apparatus of Figure

12.

[0024] Figures 14 to 16 are cross sectional elevations of another aspect of the apparatus of the invention.

[0025] Figure 17 is a plan view of an apparatus incorporating multiple gas discharge nozzles. [0026] Figure 18 is a schematic cross sectional elevation of the apparatus of the invention incorporating pumps to cause flow of molten metal.

[0027] Figure 19 is a schematic cross sectional elevation of the apparatus of the invention incorporating a transfer channel for the generated foam

DETAILED DESCRIPTION OF THE INVENTION

a^ Apparatus Having Gas Injection Nozzle Above Liquid Level

[0028] Figure 1 illustrates an apparatus according to one embodiment of the invention. In this embodiment, the apparatus includes a crucible 10 containing a molten metal 12. The molten metal may, for example, comprise molten aluminum, although other metals or combinations of metals will be apparent to persons skilled in the art. As discussed above, it is also common to add stabilizing particles to the molten metal to thereby create a metal matrix composite (MMC). Such stabilizing particles may comprise, for example, refractory particles, SiC particles or any other material as known in the art. It will be understood that the term "molten metal" as used herein will include a molten MMC. [0029] In one aspect, the crucible 10 is divided into two chambers or partitions, 14 and 16, by means of a divider 18. The purpose for the division of crucible will be discussed below. The chambers 14 and 16 are preferably maintained in fluid communication with each other by means of an opening or channel 19, thereby permitting molten metal 12 to freely flow between both chambers.

[0030] As known to persons skilled in the art, the crucible 10 comprises a heat resistant vessel with walls that are capable of withstanding the temperatures required to contain a molten metal. The crucible may also be referred to in the art as a furnace. The crucible 10 is heated by means known in the art to maintain the molten metal 12 in the liquid state. For example, the crucible 10 may be provided with heating elements or be contained within a furnace in order to maintain the interior of the crucible at the desired temperature. It will be understood that the crucible and all associated components are made from materials that are capable of withstanding the temperatures required to maintain the molten metal in the liquid state.

[0031] Optionally, the crucible 10 may be attached to a reservoir 20 containing additional molten metal to supplement the crucible. The reservoir 20 and the crucible 10 may be in fluid communication through a conduit 22, which may be provided with a valve so that molten metal can be provided only upon demand, as it is depleted from the crucible. As discussed farther below, the flow of molten metal such as from the reservoir 20 to the crucible 10 may be controlled by a variety of means. For example, the flow may be controlled by establishing a pressure differential between the reservoir and the crucible by applying pressure and/or a vacuum to one or both components. As shown, the reservoir 20 may also be provided with a flared opening 21 to facilitate filling of the reservoir with the molten metal.

[0032] Further, the crucible 10 may be provided with optional mixers or impellers in one or both chambers or partitions 14 and 16, as shown as 24 and 25, respectively. By way of illustration only, Figures 1 to 6 show mixers in both chambers. The mixers 24 and 25 may be used to ensure adequate mixing of the molten metal and to ensure that any stabilizing particles added to the molten metal are evenly distributed. The mixers 24 and 25 are driven, respectively, by motors 26 and 27 as known in the art.

[0033] In general, the chambers 14 and 16 serve as pressurizing and foaming chambers, respectively. For this purpose, the pressurizing chamber 14 is provided with a gas introduction port 28 connected to a source of pressurized gas (not shown). The gas may comprise air, nitrogen or any other gas as will be apparent to persons skilled in the art. The port 28 is also provided with a valve 30 to control the direction of gas passing through the port 28. In operation, the crucible 10 is pressurized by injecting a gas into the pressurizing chamber 14. As is illustrated in Figure 1 and as known in the art, the port 28 is provided generally at the top of the chamber so that the gas is introduced above the liquid level of the molten metal 12. In this manner, the pressure within the chamber 14 is increased, thereby forcing the molten metal contained therein into the neighboring foaming chamber 16, via the channel 19.

[0034] The foaming chamber 16 is provided on its upper end with an opening or channel 32 that opens into a mould cavity 34 of a mould 36. The mould cavity 34 is designed to have the three dimensional shape of an article to be formed. It will be appreciated that the method and apparatus of the present invention can be used to form various articles as known in the art. The mould 36 is, in one aspect, provided in two halves 38 and 40. Various other configurations of the mould will be apparent to persons skilled in the art. The sections 38 and 40 of the mould may optionally be connected to pistons 42 and 44, which assist in opening and closing the mould. Actuators 46 and 48 may optionally be provided to drive the pistons 42 and 44 respectively to facilitate the opening and closing of the mould 40. [0035] In one embodiment, the entrance 33 to the mould cavity 34 may include an upwardly and outwardly flared inner wall 35 such that the entrance 33 has a lower cross sectional area than the remainder of the cavity. The present inventors have found that such tapered opening to the mould cavity prevents a thick "skin" from forming at the bottom of the foamed article. It will be understood that the tapered wall 33 is optionally provided on the apparatus.

[0036] The foaming chamber 16 may optionally be provided with a generally conical hood 50 for tunneling or directing the molten metal or metal foam into the mould cavity 34 (as will be discussed further).

[0037] The foaming chamber 16 is also provided with a gas discharge nozzle 52. The nozzle 52 is, in one aspect, connected to and driven by a motor 54, which rotates the nozzle. A drive shaft 51 connects the nozzle 52 to the motor 54. The nozzle 52 is also connected to a gas source 56, which supplies the nozzle 52 with a gas for generating a metal foam from the molten metal as will be described below. In one embodiment, the drive shaft 51 may be hollow and serve as a conduit for the gas supplied to the nozzle 52. As known in the art, the gas may comprise air or nitrogen or any other gas as will be apparent to persons skilled in the art. It will be appreciated that the gas supply for the nozzle 52 may be the same as the source for the port 28. The nozzle 52 may, as known in the art, comprise a porous plug that allows passage of the gas but inhibits entry of the molten metal, or may comprise any other form of nozzle as will be apparent to persons skilled in the art. Further, as described above, in one aspect, the nozzle of the invention is rotated so as to serve as a mixer. In other aspects, rotation of the nozzle may not be necessary. A seal 53 is preferably provided around the shaft of the nozzle 52 to prevent leakage of molten metal. A similar seal would preferably be provided for each of the mixers 24 and 25. Seals of this type are known in the art. [0038] In another aspect, the nozzle 52 may simply serve to inject the foaming gas and need not be rotated.

[0039] The method of operation of the apparatus of Figure 1 will now be described. Firstly, Figure 1 illustrates the starting position of the process wherein no pressure is applied to the pressurizing chamber 14 thereby resulting in the levels of the molten metal 12 to equilibrate. With the present invention, the equilibrium level of the molten metal in the foaming chamber 16 is preferably retained below the gas discharge nozzle 52. As such, the nozzle is not wetted by the molten metal 12 when no foaming is required. In the result, the nozzle is prevented from being clogged with the molten metal thereby removing the need to continuously expel gas through the nozzle as discussed above.

[0040] Figure 2 illustrates the second step in the process of the invention wherein the chamber 14 is pressurized by forcing the pressurizing gas to enter through the gas port 28. Under such pressure, the molten metal 12 in the pressurizing chamber 14 is forced, as shown by the arrows, into the foaming chamber 16 thereby raising the liquid level in chamber 14 as shown in Figure 2. As also shown in Figure 2, the level of the molten metal is raised until the molten metal fills the mould cavity 34. As discussed above, the hood 50, when provided in the foaming chamber 16 (as with one embodiment of the invention), serves to guide the molten metal into the mould cavity 34. It will be understood that a suitable vent, although not shown, is provided on the mould to allow any air or other gas in the empty mould cavity to escape in order to allow the cavity to be filled by the molten metal. Such vents are generally known in casting equipment. In one aspect the vent on the mould may simply comprise a hole that is plugged once the molten metal 12 fills the mould cavity. The plugging occurs when the molten metal fills the vent hole and freezes. As will be understood, the vent hole (or holes) will be provided at the apex of the mould cavity so that plugging of the hole occurs after the air in the mould cavity is evacuated.

[0041] Figure 3 illustrates the next step in the process wherein, once the mould cavity is filled with the molten metal, the gas supply is to the gas discharge nozzle 52 in the foaming chamber 16 is switched on. The supply of gas to the nozzle 52 initiates foaming of the molten metal as is known in the art, wherein bubbles of the gas surrounded by a film of the molten metal are created. As the bubbles of the foam are generated, they rise through the molten metal due to the relatively lower density as compared to the molten metal. As the bubbles rise, they are guided by the hood 50 into the mould cavity 34. As is shown in the figures, in a preferred embodiment, the gas discharge nozzle is positioned directly beneath the channel 32 to the mould cavity 34. The rising foam displaces the molten metal within the mould cavity and occupies the space vacated by same, as shown in Figure 3. During this process, the pressure within the pressurizing chamber 14 is maintained. [0042] It will be appreciated that the time span between filling of the mould cavity 34 with the molten metal and the initiation of the gas discharge through nozzle 52 is relatively short so as to prevent the nozzle from becoming clogged and to prevent the molten metal from freezing (i.e. solidifying) in the mould. The specific time span will depend, inter alia, on the pressure supplied to the pressurization chamber 14, the viscosity of the molten metal, the temperature of the molten metal, the insulative capacity of the mould, and the volume of the mould cavity.

[0043] As shown in Figure 4, the foaming operation is continued until the mould cavity 34 is completely filled with the metal foam 58. At this point, the mould is allowed to cool, thereby solidifying the foam within the mould cavity 34. It will be understood by persons skilled in the art that foam will solidify faster than the liquid molten metal due to the lower heat capacity of the foam. As such, solidification of the foam within the mould cavity can be completed without any significant solidification of the liquid molten metal within the crucible 10 or the channel 32. The mould can optionally be provided with a cooling means (not shown) such as a cooling coil or the like as known in the art. It will be appreciated by persons skilled in the art that the time required for cooling the foam in the mould will depend on a number of factors such as the volume of the mould cavity (i.e. the size of the article being formed), the ambient temperature, and the thickness of the mould walls. [0044] Cooling of the foam 58 in the mould cavity 34 is continued until the foam is sufficiently hardened and does not leak or drip back into the crucible 10. At this point, or shortly before, the foaming gas supply can be turned off and the pressure in the pressurizing chamber released. Depressurizing of the chamber 14 is achieved by opening the valve 30 thereby releasing the pressurizing gas through the port 28. As the pressure within the chamber 14 (and, in the result, chamber 16 as well) is reduced, the liquid levels of the molten metal 12 in the two chambers 14 and 16 are again equilibrated, as shown in Figure 5, and the liquid level in the foaming chamber 16 is returned to a level below the nozzle 52. As mentioned above, by separating the molten metal from the nozzle when foaming is not required serves to prevent clogging of the nozzle with the molten metal and, therefore, avoids the need for anti-clogging measures such as continuously purging gas through the nozzle. Further, the service life of the nozzle may also be extended by reducing the time that the nozzle is exposed to the molten metal. As will be understood, by extending the life-span of the nozzle, the operating expenses associate with replacement nozzles and production down- i time for replacing them are reduced. In addition, by lowering the level of the molten metal and separating it from the metal foam 58 in the mould allows more rapid cooling and hardening of the foam.

[0045] Another advantage of the present invention relates to the positioning of the foaming nozzle 52 close to the channel 32 of the mould cavity 34. With such an arrangement, the bubbles that are formed when the gas is introduced through the nozzle 52 have a short distance to travel to reach the mould cavity. This reduces the travel time of the bubbles through the molten metal, as required with prior art methods (where nozzles are positioned at the base of the crucible), and therefore reduces potential damage to the bubbles. Such damage includes collapse or coalescence of the bubbles, which may reduce the quality of the foam

[0046] Once the foam 58 in the mould cavity 34 is sufficiently hardened, the mould 36 may be opened by separating mould halves 38 and 40. As mentioned above, this may be achieved with pistons 42 and 44 connected to each of the mould halves 38 and 40, respectively. Once the mould is opened, the formed foam article is removed and the mould 36 is again closed, thereby returning the system to the position shown in Figure 1. The process can then be repeated.

[0047] As indicated above, a molten metal reservoir 20 may optionally be provided to replenish molten metal in the crucible 10 as it is used. The provision of such a reservoir serves to reduce the time required in shutting down arid re-filling the crucible 10 as the volume of the molten metal is reduce during production of the foam articles. As mentioned, the reservoir 20 may be connected to the crucible 10 by means of a port or conduit 22. It will be understood that the operation of the apparatus can be automated so that, for example, replenishing of the molten metal from the reservoir (if provided) can be conducted while the formed article is being removed from the mould.

[0048] In another embodiment, as illustrated in Figure 6, the port between the reservoir 20 and the pressurizing chamber 14 may be replaced with an open channel 23, which permits molten metal to freely flow between the reservoir 20 and the chamber 14. In such case, the reservoir will be of such a size as to allow the molten metal level there-within to rise to at least the height of the top of the mould cavity 34. Such an arrangement permits the liquid levels in both the reservoir 20 and the foaming chamber 16 to rise simultaneously as the pressure within the pressurizing chamber 14 is increased (as discussed above). This is required since, in the embodiment illustrated in Figure 6, the molten metal in both the reservoir 20 and the foaming chamber 16 is permitted to freely flow in and out of the pressurizing chamber 14. In the result, the level of the molten metal will rise equally in both the reservoir 20 and the foaming chamber and mould. Thus, the height of the reservoir 20 should be at least as high as the top of the mould cavity (that is, the height to which the molten metal rises in the mould) in order to prevent the molten metal from spilling out of the reservoir. As a further feature, this arrangement of the reservoir allows for direct¹ monitoring of the filling of the mould cavity by molten metal. In other words, by monitoring the level of the molten metal in the reservoir 20, it is possible to determine when the molten metal rises to fill the mould.

[0049] In another aspect, the invention permits the use of monitoring equipment (not shown) to monitor the height of the molten metal in the reservoir and to signal when the mould is filled. In a further aspect, such monitoring equipment can be electronically connected to the pumping apparatus used to provide gas to the nozzle 52 thereby allowing automation of the foaming process. That is, the gas supply to the nozzle 52 can be automatically commenced when the monitoring equipment senses that the height of the molten metal in the reservoir 20 has risen to the height of the top of the mould cavity, signaling that the mould has been filled with molten metal. As mentioned above, the time between filling of the mould with molten metal and commencement of the foaming step should preferably be minimized. It will be understood that the aforementioned automation permits rapid sensing of the filled state of the mould cavity and initiation of foaming. The sensing and signaling equipment discussed above will be known to persons skilled in the art and any commercially available equipment can be used with the invention. For example, the reservoir may be provided with a sensor (not shown) for monitoring and/or measuring the liquid level of the molten metal with such sensor being connected to a processor (not shown). The processor may be included in a computer system. The processor may also be programmed with the necessary instructions to be executed depending upon the level of molten metal. In the above example, the processor may be programmed to switch the gas supply to the gas injection nozzle on or off depending upon the sensed level of the molten metal.

[0050] In another embodiment, the pressurizing chamber 14 and/or the foaming chamber 16 may be provided with a filling port, instead of a reservoir 20, whereby molten metal can simply be poured into the chamber 14 or 16 as needed. It will be understood that if such a port (not shown) is provided, a suitable seal would also be provided so as to allow the chambers to be pressurized as discussed above.

[0051] In the above discussion, pressurization of the crucible has been effected by means of injection of a pressurizing gas. However, it will be appreciated that any other pressurizing means can be used to achieve the same purpose. For example, a mechanical plunger may be used to force the molten metal in the pressurizing chamber 14 to enter the foaming chamber 16 and, thereby, fill the mould cavity. Other such pressurizing means will be apparent to persons skilled in the art.

[0052] In the above discussion, the apparatus and method have focussed on a single mould. However, it will be appreciated that the crucible can be provided with multiple moulds and associated gas injection nozzles whereby multiple foam articles can be manufactured concurrently.

[0053] Another embodiment of the invention is illustrated in Figures 7 to 11 where elements of the apparatus common to the above described embodiment are indicated with like reference numerals but with the letter "a" added for clarity.

[0054] In the embodiment of Figure 7, the crucible 10a comprises a single crucible or furnace with only one chamber 60. The chamber preferably includes a concave bottom as will be described below. The crucible 10a is provided with an upwardly extending neck or foaming chamber 62 above which is positioned the mould 36a. Preferably, the mould 36a is provided with a supporting stage 64 (which may comprise a table or other such surface) to facilitate the opening and closing of the mould. It will be understood that although this type of supporting stage or table can also be provided (although not shown) in the embodiment of Figures 1 to 5.

[0055] As before, the mould 36a is preferably provided in two halves 38a and 40a, although various other means of sectioning the mould 36a would be apparent to persons skilled in the art. Each of the mould halves 38a and 40a may also be provided with pistons 42a and 44a, respectively, to facilitate movement of the mould halves to and from each other as shown by the arrows. As shown in Figure 7, the mould halves can be slidably supported on the stage 64. The mould 36a also includes a mould cavity 34a having the shape of the desired article to be formed. As indicated above, the mould 36a will generally be provided with one or more vents (not shown) to allow air to escape from the mould cavity 34a when being filled as described below.

[0056] The foaming chamber 62 includes an opening through which the drive shaft 51a for the nozzle 52a is extended. As discussed above, the nozzle 52a is connected to a motor 54a and a pressurized gas source 56a. A seal 53 a is provided to prevent any molten metal from leaking through the opening for the drive shaft 51a. As shown in Figure 7, the gas discharge nozzle 52a is positioned above the starting liquid level of the molten metal 12a. [0057] The crucible 10a is provided with a gas filling port 28a with an associated valve 30a as discussed above. The gas filling port 28a serves to inject a pressurizing gas into the crucible 10a.

[0058] The apparatus of this embodiment further includes a riser tube 66 extending vertically from the foaming chamber 62 to a position close to the concave bottom of the crucible 10a. In one aspect of the invention, the tube 66 may be provided with an outwardly flared bottom end 68. The tube 66 provides a conduit for the molten metal in the crucible 10a to travel to and from the mould cavity 34a as will be described further below. [0059] Figure 7 illustrates the starting position of the apparatus when the foaming process is to begin. At this point, the liquid molten metal level in the crucible 10a and the tube 66 is equilibrated and the level in the tube 66 is below the nozzle 52a. As with the previous embodiment, the first step in the process involves pressurization of the crucible 10a. This is accomplished by injecting a pressurizing gas through the gas port 28a. The gas supplied to the port 28a may be from any source as known in the art and, as mentioned above, may comprise the same source as that for the nozzle 52a.

[0060] As a result of pressurizing the crucible 10a, the molten metal in the crucible is forced into the tube 66, through the opening or channel 32a, and into the mould cavity 34a as shown in Figure 8. As discussed above, although not shown, it will be understood that the mould will include a suitable vent to allow venting of any gases during filling of the mould cavity by the molten metal.

[0061] The combination of a concave bottom for the crucible 1 Oa and the positioning of the bottom of the tube 66 proximal to the bottom of the crucible ensures that any stabilizing particles, or other materials, added to the molten metal and which may have settled to the bottom of the crucible are entrained when the tube is filled. This type of arrangement avoids the need for any stirring mechanism and the associated costs and related equipment associated therewith. Of course, if desired, mixers as described above can still be provided as an option.

[0062] Figure 9 illustrates the next step in the process wherein gas is injected into the molten metal contained in the foaming chamber 62 of the apparatus forming bubbles of the gas within the liquid. The bubbles rise into the mould cavity 34a and, as described above in relation to another embodiment, displace the molten metal contained therein. The bubbles accumulate in the mould cavity 34a as a metal foam. As mentioned above, the time required to fill the mould cavity with the molten metal and the delay between the filling of the cavity and the initiation of the foaming gas is generally minimised and will vary based on a number of factors. During this step, the pressure within the crucible 10a is maintained so as to keep the tube 66 filled with molten metal.

[0063] Figure 10 illustrates the mould cavity 34a after it is filled with a foam of the metal. The foam in the mould cavity 34a is provided some time to cool and harden to a point where the foam is retained within the cavity. The gas supply to the nozzle 52a is then switched off and the valve 30a is opened to release the pressure accumulated in the crucible 10a.

[0064] As shown in Figure 11, once the pressure is released from the crucible 10a, the liquid level of the molten metal is again equilibrated to a level below the gas discharge nozzle 52a. The foam within the mould cavity 34a is allowed more time to cool and harden, after which the mould halves 38a and 40a are separated (i.e. the mould is opened) and finished metal foam article is removed. The mould is then closed to return the system to the starting position as shown in Figure 7.

[0065] Although not illustrated in Figures 7 to 11, the crucible 10a will include either a molten metal filling port, a molten metal reservoir, or any other means to supply molten metal, as described above for replenishing molten metal consumed during the process. [0066] As indicated above, the crucible illustrated in Figures 7 to 11 can be provided with multiple moulds whereby multiple foam articles can be manufactured concurrently. [0067] Similarly, although the apparatus of Figures 7 to 11 is shown with one gas discharge impeller, the apparatus may include several such impellers so as to decrease the filling time of the mould.

[0068] The following description relates to other embodiments of the invention. The description of the various embodiments herein merely serves to illustrate the various aspects of the invention and is not meant to limit same in any way.

b) Apparatus Having Modular Design

[0069] In another aspect of the invention, as illustrated in Figures 12 to 19, the apparatus of, for example, Figures 7 to 11 may be modular in design by having one or more of the mould 36a, the foaming chamber 62, and the melt chamber 60 releasably secured to one another. In such arrangement, any combination of these components can be separated and interchanged without a significant disruption to the process. As will be understood by persons skilled in the art, this would allow, for example, the foaming chamber to be replaced with a new chamber, including an impeller, in cases where the original foaming chamber is malfunctioning or otherwise requires servicing. This may occur, for example, when an impeller becomes obstructed (or clogged). The foaming and casting processes used with the apparatus of Figures 12 to 19 are essentially the same as that for the previously described figures.

[0070] In a similar way, such an interchangeable apparatus allows the mould and/or casting chamber, to be easily replaced as well. Such change of moulds may be required where, for example, a mould becomes fouled or where a different shaped mould is required in situations where differently shaped articles are required to be formed. Thus, one mould can be interchanged for another by simply disconnecting the mould 36a from the foaming chamber 62 and replacing it with a new mould. As will be appreciated, in situations where the mould (i.e. casting chamber) and/or foaming chamber is being replaced, the modular nature of the apparatus allows such replacement to take place without emptying or otherwise disturbing the crucible 10a.

[0071] As discussed above, a modular design of the apparatus offers various advantages. Included in these advantages is the ability of a mould and foaming chamber combination to be mounted on known low pressure casting crucibles. As will be understood, this allows avoids the need for any specially designed crucibles. This aspect of the apparatus is illustrated more clearly in Figure 12 where elements common with previously described figures are identified with common reference numerals but with the letter "b" added for clarity. As can be seen, in one embodiment, a crucible 10b is provided within a furnace 70. The furnace 70 is provided with heating elements 72 to provide the necessary heat to maintain the metal contained in the crucible 10b in a molten state. As known in the art, the walls of the furnace 70 are made with materials that are capable of withstanding the heat required for the crucible and also with any required insulation. The crucible 10b and furnace 70 may, as described above, be those used for commonly known casting operations. As shown in Figure 12, a cover plate 74 is provided over the opening 76 of the crucible 10b. Generally, the opening 76 will be circular. In the result, the cover plate 74 is provided in the form of an annular disk, the outer diameter of which is larger than the diameter of the opening 76 thereby allowing the cover plate 74 to rest on the upper end of the crucible 10b or furnace 70. A riser tube 66b is secured to the cover plate 74 and extends generally vertically through the inner diameter of thereof. As shown, the upper end of the riser tube 66b is provided with a flange 78 having a diameter greater than the inner diameter of the annular cover plate 74. In such manner, the riser tube is suspended by the flange 78 contacting the upper surface of the cover plate 74. The riser tube 66b is then secured to the cover plate 74 with bolts 80 that extend through apertures (not shown) provided in both the flange 78 and cover plate 74. Thus, once the cover plate 74 is provided over the crucible 10b, the riser tube 66b extends into the molten metal 12b contained there-within. The crucible may also be provided with a pressurizing means such as described herein to cause the molten metal to rise through the riser tube 66b. Figure 12 shows the riser tube 66b filled with molten metal 12b. [0072] A guide 82 is provided above the riser tube 66b and may also be secured to the cover plate 74. The guide 82 includes a lower opening 84 that generally corresponds to the upper opening of the riser tube 66b and, optionally, a tapered upper end that terminates in an upper opening 86. The upper opening 86 serves to feed molten metal 12b into a foaming chamber 62b. The foaming chamber 62b is supported on a support plate 88 which, in turn, may rest on a stage 90. As shown, the base of the foaming chamber 62b is provided with an opening that allows fluid communication with the upper opening 86 of the riser tube 66b. [0073] As shown in Figure 12 and as described previously, the foaming chamber 62b is provided with one or more gas discharge nozzles 52b. Figure 12 illustrates for convenience one nozzle 52b but it will be appreciated by persons skilled in the art that additional nozzles may also be provided in the foaming chamber 62b. The nozzle 52b serves to introduce a foaming gas into the molten metal contained in the foaming chamber 62b thereby forming the metal foam. As described previously, the level of the molten metal is raised above that of the nozzle 52b when foaming is to commence. However, as will be understood, Figure 12 illustrates a stage in the process where foaming is not commenced, wherein the level of the molten metal 12b is below the nozzle 52b.

[0074] The upper end of the foaming chamber is provided with an opening 32b through which the generated foam is passed into the mould or casting chamber 36b. As with the previously described embodiment, the mould 36b may be preferably provided above the foaming chamber 62b to facilitate entry of foam.

[0075] Figure 12 also illustrates a preferred structure of the foaming chamber 62b wherein the chamber is provided in two sections - a top section or foaming box 92 and a bottom section or feeder box 94. As shown, the feeder box 94 receives molten metal from the riser tube 66b. The nozzle 52b is connected to the foaming box and, when foaming is required, the level of the molten metal in the foaming chamber 62b is raised (as described herein) so that molten metal is caused to enter the foaming box 92, thereby submerging the nozzle 52b. It will be understood that dividing the foaming chamber 62b into sections allows for ease of access for servicing or maintenance functions.

[0076] It is noted in the embodiment of Figure 12 that the foaming chamber 62b and the mould 36b are separate modules that are combined with the crucible 10b to form the apparatus of the invention. As discussed above, such modularity allows either of the components to be easily removed and replaced with minimal interruption to the process. It will be appreciated by persons skilled in the art that another feature of the modular design of the foaming apparatus described herein lies in the fact that various components of the apparatus may be removed and serviced or changed without moving the crucible. The advantage in this case being that heavy and/or hot components need not be manipulated. [0077] Figure 13 illustrates another aspect of the apparatus of Figure 12 and serves to illustrate the interchangeability of the modules of the apparatus. Elements of the apparatus in Figure 13 that are identical to those of Figure 12 are identified with the same numerals. However, elements that are similar but different in construction or function are identified with the same reference numerals but with the letter "c" added for clarity. Specifically^ Figure 13 illustrates an embodiment of the apparatus of Figure 12 wherein the single cavity mould 36b is replaced with a multiple cavity mould 36c. As shown, the crucible 10b and riser tube 66b are the same as that of Figure 12. However, the foaming chamber 62c illustrated in Figure 13 is provided with multiple (i.e. more than one) gas discharge nozzles 52c, which are preferably positioned below multiple (i.e. more than one) upper openings 32c provided on the foaming chamber 62c. The mould 36c of this embodiment also includes multiple cavities each having an entry port in fluid communication with the openings 32c of the foaming chamber 62c. Although the embodiment of Figure 13 is illustrated with a mould having three similarly shaped cavities, it will be appreciated that this is only by way of example and that any number or shape of cavities may equally be provided. [0078] Further, it will be appreciated that it is not necessary for the number of nozzles to be commensurate with the number of cavities and that any number of nozzles may be used. For example, in one embodiment, one or more nozzles may be used with a suitable manifold or diverter to direct molten metal and/or foam into the appropriate cavities of the mould. [0079] An example of multiple nozzles being provided in a foaming chamber is illustrated in Figure 17, where like reference numerals are used but with the letter "f ' added for clarity. As shown, multiple nozzles 52f are provided within a foaming chamber wherein each nozzle is arranged below an opening into the mould. It will be appreciated that such arrangement serves to increase the speed of foam generation and, therefore, the casting process. Each of nozzles 52f may be associated with one or more gas sources 56f. As discussed above, the nozzles 52f may also be connected to one or more drive motors to cause rotation of same.

[0080] With the modularity discussed above, it will be understood that any number of foaming chambers and/or casting chambers or moulds may be used with the invention. Thus, it is possible for multiple foaming chambers to be associated with a single pressurizing chamber. Each of the foaming chamber can, in turn, be associated with multiple casting chambers. Thus, it can be understood that the apparatus of the invention can be optimized or tailored as needed in order to simultaneously form multiple articles or various forms and at various speeds. For example, with various foaming and casting chambers, it would be possible for various articles (of the same or different shape) to be formed with one pressurization cycle. As mentioned above , the process followed by the apparatus of Figures 12 and 13 is essentially the same as that described herein with respect to other figures.

c) Apparatus Having Assisted Filling System

[0081] Figures 14 to 16 illustrate a further aspect of the invention wherein a vacuum source is used to supplement the pressure feed system for the molten metal. Specifically, Figures 14 to 16 are similar to the embodiment of the invention illustrated in Figures 1 to 6. As such, the reference numerals used in Figures 1 to 6 are also used in Figures 14 to 16 but with the letter "d" added for convenience.

[0082] As shown in Figure 14, the apparatus according to this aspect of the invention includes a pressurizing chamber 14d and a foaming chamber 16d, which are in fluid communication by means of a channel 19d. The apparatus also includes a mould 36d having a mould cavity 34d. As illustrated, the pressurization and foaming chambers, the channel and the mould are preferably formed with thick insulative walls so as to conserve heat within the apparatus and thereby prevent or minimize the molten metal from solidifying. [0083] As shown, the pressurizing chamber 14d contains a volume of molten metal 12d. The channel 19d allows the molten metal 12d to freely flow between the pressurization and foaming chambers 14d and 16d, respectively. The foaming chamber 16d includes at least one gas discharge nozzle 52d connected to a gas source (not shown).

[0084] The pressurizing chamber 14d includes a gas introduction port 28d connected to a source of pressurized gas (not shown). However, according to the embodiment illustrated, the pressurizing chamber is also provided with a vacuum port 96 connected to a vacuum source (not shown). Further, the mould 36d of this embodiment also includes a vacuum port 98 connected to a vacuum source (not shown).

[0085] In operation, the apparatus of Figures 14 to 16 functions similarly to that of Figures 1 to 6. Generally, the level of molten metal in the foaming chamber 16d is raised until the gas discharge nozzle 52d is submerged at which point gas is discharged through the impeller 52d thereby causing the molten metal to foam. However, the method of raising the molten metal level is slightly different from the previous embodiment. Specifically, the starting position of the apparatus is illustrated in Figure 14 wherein the molten metal level is maintained below the level of the nozzle 52d. This is achieved by avoiding pressurization of the chamber 14d but, instead, engaging the vacuum source and causing a vacuum (or negative pressure) to be developed in chamber 14d through the vacuum port 96. At this point, the vacuum port 98 provided on the mould is not engaged. As will be understood, this arrangement causes the molten metal 12d to be drawn into the pressurizing chamber 14d. It will also be understood that the level of the molten metal 12d in the foaming chamber 16d can be adjusted by choosing a suitable pressure within the pressurizing chamber 14d. [0086] Figure 15 illustrates the next step in the process wherein the vacuum source for the pressurizing chamber 14d is released while the vacuum source, connected to the vacuum port 98 of the mould 36d is engaged. In addition, the pressure within the pressurizing chamber 14d is increased by injecting the pressurizing gas through the port 28d. As shown in Figure 15 and as will be understood, this arrangement causes the molten metal 12dto flow from the pressurizing chamber 14d into the foaming chamber 16d. By appropriately choosing the pressure differential between the chambers 14d and 16d, the level of the molten metal 12d can be adjusted so as to rise above the gas discharge nozzle 52d. Figure 16 illustrates the apparatus wherein both vacuum ports 96 and 98 are in the "off position and where the pressurized gas port 28d is used to maintain a constant pressure within the pressurizing chamber 14d.

[0087] Figure 15 illustrates an embodiment wherein the foaming operation is commenced as soon as the nozzle 52d is submerged by the rising molten metal level. At such point, the gas supply to the nozzle is turned on thereby resulting in the generation of the foam. The foam then rises into the mould cavity 34d and fills same. Gas supply to the nozzle 52d is turned off when a sufficient volume of foam is generated, specifically, when the mould cavity 34d is filled. In another embodiment, as described above with reference to Figures 1 to 6, the molten metal level may be increased until the mould cavity 34d is filled with molten metal prior to the foaming operation. In this case, after the mould cavity is filled, the gas supply to the nozzle 52d is engaged thereby resulting in the formation of foam, which rises to fill the mould cavity 34d and displace the molten metal contained therein. [0088] As discussed above with reference to Figures 14 to 16, in one embodiment, a vacuum source is used to assist the pressurizing system in adjusting the level of molten metal within the pressurization and foaming chambers. However, in another embodiment, such flow assistance can be provided by using an object (not shown) that can be raised and lowered within the molten metal contained in the pressurizing chamber 14d. It will be understood that such object will be made of a material capable of withstanding the heat of the molten metal. It will also be understood that lowering the object into the molten metal 12d contained in the pressurizing chamber 14d will displace the molten metal, forcing same into the foaming chamber 16d. In the latter case, the pressurizing chamber will be understood as having no other means of egress for the molten metal except into the foaming chamber or, alternatively, the route for the melt into the foaming chamber offers the least resistance to flow. In the latter case, it will be understood that molten metal, once displaced by the object, will preferentially flow into the foaming chamber.

[0089] In a further embodiment, as illustrated in Figure 18, the molten metal level can be adjusted by means of a pump connected to a separate molten metal tank. Elements of Figure 18 that are common to those discussed above are identified with the same reference numerals but with the letter "e" added for clarity. As shown in Figure 18, a furnace 100 serves as a source of molten metal 12e that is fed into a foaming chamber 16e. The furnace 100 will be provided with suitable heating elements (not shown) to maintain the metal in the molten state and, if necessary, to melt any solid metal provided therein. The furnace may optionally be provided with a mixer 24e to stir the molten metal. In the embodiment shown, the furnace is provided with at least one of each of entry and exit ports 104 and 106, respectively, through which the molten metal is allowed to flow. Similarly, the foaming chamber 16e is also provided with entry and exit ports 108 and 110, respectively. The exit port 106 of the furnace 100 is fluidly connected to the entry port 108 of the foaming chamber 16e. Similarly, the exit port 110 of the foaming chamber 16e is fluidly connected to the entry port 104 of the furnace 100. The fluid connections are provided by suitable conduits 112 and 114, respectively, as will be known in the art. Each of the conduits 112 and 114 include at least one pump 116 and 118, respectively, for pumping molten metal there-through. The pumps 116 and 118 are operable independently of one another and, therefore, by operating one pump at a higher speed (i.e. flow rate) than the other, the molten metal volume in each of the furnace 100 and the foaming chamber 16e can be adjusted. For example, the level of the molten metal 12e in the foaming chamber 16e can be increased by operating pumps 116 and 118 so as to increase the flow rate of molten metal into the foaming chamber 16e as compared to the flow rate entering the furnace 100. It will be understood that the speed of the pumps will vary depending on their capacities, the cross sectional area of the conduits, 112 and 114, and the dimension of the openings for ports 104, 106, 108 and 110.

[0090] In one embodiment, the pumps 116 and 118 can be used to circulate molten metal 12e and cause mixing thereof. Such an arrangement would avoid the need for mixers and the like to cause mixing of the molten metal.

[0091] As also illustrated in Figure 18, the foaming chamber can be provided with one or more nozzles 52e for introducing a gas into the molten metal 12e thereby generating the required foam. The foam can then be directed into one or more moulds (not shown). In one embodiment, a hood 5Oe or other such diverter can be used to direct the foam into the mould or moulds.

[0092] In another embodiment of the apparatus of Figure 18, one of the pumps 116, 118 can be removed and the flow of molten metal can be achieved with only one pump. In such case, it will be understood that a return flow of molten metal, where needed, can be established with a conduit alone and that such return flow could be powered by gravity. [0093] It will be appreciated that the circulating flow of molten metal flow as established in the apparatus of Figure 18 also serves to maintain a generally constant temperature of the molten metal in both the furnace and the foaming chamber. It will be understood that the furnace 100 of Figure 18 is functionally equivalent to the pressurizing chambers described above.

[0094] It will be understood that any one or a combination of pressure, vacuum, displacement objects, pumps, or any other similar means, can be used to adjust the molten metal level within the chambers of the apparatus of the invention and that any of these means can be used in any of the embodiments discussed herein. All these means serve to control the level of molten metal in the various chambers.

d) Apparatus Having A Transfer Channel Between Foaming and Casting Chambers

[0095] A further embodiment of the invention is illustrated in Figure 19 wherein like elements are indicated with like reference numerals but with the letter "g" added for clarity.

As shown, the apparatus includes a pressurizing chamber 14g, a foaming chamber 16g and a casting chamber or mould 36g. The pressurizing chamber 14g and foaming chamber 16g are connected by a channel 19g, which allows molten metal 12g to flow between the two chambers. As with the previously described embodiments, the foaming chamber 16g is provided with one or more gas discharge nozzles 52g, which are connected to a gas source 56g. The nozzles 52g may also be rotatable by means of a motor (not shown). The pressurizing chamber 14g is provided with a gas port 28g through which is injected a pressurizing gas as described above. Foaming in the foaming chamber 16g is accomplished in the manner described above. Namely, the level of molten metal in the foaming chamber 16g is increased until the gas discharge nozzles 52g are submerged. At such time, the gas supply to the nozzles 52g is actuated thereby causing foaming of the molten metal 12g. [0096] The foaming chamber 16g and casting chamber 36g are connected by means of a transfer channel 120, which serves to transfer foam generated in the foaming chamber 16g to the mould cavity 34g in the casting chamber or mould 36g. It will be understood that the components of the apparatus, including the transfer channel 120 will be provided with any required insulation or heating means to prevent undesired solidification of foam and/or molten metal in areas apart from the mould cavity.

[0097] With the apparatus of Figure 19, it is possible to transfer foam to the mould cavity 34g in various ways. For example, in one aspect, a sufficient volume of foam can be generated and accumulated in the transfer channel. Subsequently, the pressure within the pressurizing chamber 14g can be further increased to raise the level of molten metal within the foaming chamber 16g. This will result in the more buoyant foam being pushed further into the transfer channel 120. Such flow of foam can be continued until the foam is pushed or injected into the mould cavity 34g. It will be understood that the amount of pressure used to move the foam within the transfer channel 120 will be minimal so as to prevent damage to the cells forming the foam.

[0098] In another aspect, foam can be pushed into the mould cavity 34g by continuing the foaming operation. In such case, the continuously generated foam fills the transfer channel 120 and subsequently enters the mould cavity 34g.

[0099] As illustrated in Figure 19, one of the advantages of an apparatus incorporating a transfer channel 120 lies in the fact that the casting chamber or mould 36g need not be located above the foaming chamber. Thus, as shown, a standard sand mould can be used wherein the mould sections 38g and 4Og are located above one another. As such, the sand incorporating the mould cavity 34g can be contained in the lower section of the mould. [00100] It will be understood that the pressurizing chamber 14g of Figure 18 can be associated with more than one foaming chamber in the manner described above. Further, the flow of molten metal between the pressurizing chamber and foaming chamber can be accomplished using any of the means discussed above.

[00101] Although the invention has been described with reference to certain specific embodiments, various modifications thereof will be apparent to those skilled in the art without departing from the spirit and scope of the invention as outlined in the claims appended hereto. The disclosures of all prior art recited herein are incorporated herein by reference in their entirety.

Claims

We Claim:

1. An apparatus for casting a metal foam article from a foam of a molten metal, the apparatus comprising:

- a crucible for containing a molten metal;

- a mould provided above the crucible, wherein the mould includes at least one mould cavity having the shape of said article and an opening for fluid communication between said crucible and said cavity;

- a gas injection nozzle, connected to a gas supply, the gas injection nozzle being positioned below said mould cavity opening; and

- a flow generating means for causing said molten metal to flow from the crucible to the mould, whereby the molten metal level in said crucible is varied from a first level below the gas injection nozzle to a second level above the gas injection nozzle.

2. The apparatus of claim 1 wherein said flow generating means comprises a pressurizing means, a vacuum means, a pump, a displacement means, gravity, or any combination thereof.

3. The apparatus of claim 2 wherein said crucible comprises a first chamber and a second chamber, wherein:

- said first and second chambers are in fluid communication with each other whereby molten metal flows between said chambers;

- said flow generating means is provided in the first chamber; and,

- said mould and said gas injection nozzle are provided on the second chamber.

4. The apparatus of claim 3 wherein said first chamber is provided with a filling port for filling the first chamber with the molten metal.

5. The apparatus of claim 3 further including a molten metal reservoir in fluid communication with said first chamber.

6. The apparatus of claim 5 wherein said reservoir extends vertically higher than the height of the mould cavity.

2U43839.1 24

7. The apparatus of claim 6 wherein said reservoir includes a monitoring means for monitoring the level of molten metal contained therein.

8. The apparatus of claim 7 wherein said monitoring means is connected to a processor and wherein said processor controls the gas supply for said gas injection nozzle.

9. The apparatus of claim 3 wherein said second chamber includes a guide for guiding molten metal or metal foam into said mould cavity.

10. The apparatus of claim 2 wherein said pressurizing means includes a pressurized gas supply.

11. The apparatus of claim 3 wherein at least one of said chambers is provided with a mixer for stirring the molten metal.

12. The apparatus of claim 1 further including a riser tube provided in said crucible and extending through the molten metal contained in the crucible, the riser tube having an upper end opening into said mould cavity and a lower end proximal to the bottom of the crucible.

13. The apparatus of claim 12 wherein the bottom of said crucible is concavely shaped.

14. The apparatus of claim 12 wherein said pressurizing means includes a pressurized gas supply.

15. The apparatus of claim 3 wherein one or more of said crucible, first chamber, second chamber, and mould are releasably connected to each other.

16. A method of casting a metal foam article from a foam of a molten metal comprising: a) providing a crucible containing said molten metal; b) providing a mould having a mould cavity having the shape of said article, the mould being located above said crucible and the mould cavity having an opening into said crucible for fluid communication therewith;

21443839.1 25 c) providing a gas injection nozzle below said mould cavity opening; d) maintaining the level of the molten metal below said gas injection nozzle; e) raising the level of the molten metal above the gas injection nozzle; f) injecting gas through said gas injection nozzle to generate a foam of said molten metal; g) directing said foam into said mould cavity; h) filling said mould cavity with said foam and allowing said foam to cool; i) returning the level of said molten metal level to below the gas injection nozzle; j) removing said formed article from the mould cavity.

17. The method of claim 16 wherein said gas injection is ceased when said molten metal level is below the gas injection nozzle.

18. The method of claim 16 wherein said step (e) includes filling said mould cavity with the molten metal.

19. The method of claim 16 wherein said step (e) comprises establishing a pressure within the crucible, said pressure being sufficient to cause the level of the molten metal to rise above the gas injection nozzle and flow into and fill said mould cavity.

20. An apparatus for casting a metal foam article from a foam of a molten metal, the apparatus comprising:

- a crucible for containing a molten metal;

- a mould, wherein the mould includes at least one mould cavity having the shape of said article and an opening for fluid communication between said crucible and said cavity;

- a gas injection nozzle, connected to a gas supply; and

- a flow generating means for causing said molten metal to flow from the crucible to the mould; wherein said crucible and mould are releasably connected together.

21. The apparatus of claim 20 wherein said flow generating means comprises a pressurizing means, a vacuum means, a pump, a displacement means, gravity, or any combination thereof.

21443839.1 26

22. The apparatus of claim 21 wherein said crucible comprises a first chamber and a second chamber, wherein:

- said flow generating means is provided in the first chamber; and,

- said mould and said gas injection nozzle are provided on the second chamber.

23. The apparatus of claim 22 wherein said mould is releasably connected to said second chamber.

24. The apparatus of claim 23 wherein said first and second chambers are releasably connected together.

25. The apparatus of claim 24 wherein said mould is fluidly connected to said second chamber through a conduit.

26. The apparatus of claim 24 wherein said first and second chambers are fluidly connected through a conduit.

27. The apparatus of claim 20 wherein said mould includes multiple cavities.

28. The apparatus of claim 22 wherein said apparatus includes multiple gas injection nozzles.

29. The apparatus of claim 20 wherein said flow generating means maintains the level of molten meal between a first level below the gas injection nozzle and a second level above the gas injection nozzle.

21443839.1 27