TECHNICAL FIELD The present invention relates to the casting of semi-solid slurries of molten metal, particularly aluminum and aluminum alloys. More particularly, it relates to apparatus for preparing semi-solid slurries and use of a "slurry on demand" method of preparing such slurries.

BACKGROUND ART The casting of semi-solid slurries of molten metal has been carried out for many years. A number of methods for casting such slurries are known in the art. These generally involve reheating of previously prepared and conditioned solid ingots to a semi-solid state then transferring the resulting slurry to a casting mould. However, in recent years a method of casting has been developed wherein a semi-solid slurry is prepared by controlled cooling of a liquid alloy in a crucible. The semi-solid slurry formed on cooling is then transferred from the crucible to a casting machine and cast without the need for using an intermediate solid ingot. This approach has been referred to as "slurry on demand". US Patent 6,595,266, issued on July 22, 2003 to Orii describes such a slurry on demand system where a molten alloy is passed over a cooling plate and is then collected into a crucible, where it is held in a semi-solid state for a short period of time, to allow a preferred particle morphology to develop, and then cast. EP Patent Publication No. 0 745 694 (Adachi et al), published April 12, 1996, describes a slurry on demand system where the alloy initial temperature, grain refiner content and rate of temperature drop within a crucible are controlled to develop the desired morphology at which time the slurry is die cast. US Patent 6,428,636, issued on August 6, 2002 to Doutre et al, describes a slurry on demand system where the molten alloy is cooled to a predetermined temperature in the semi-solid range in a crucible of specified thermal mass and temperature, then some of the excess molten alloy is drained prior to transferring the semi-solid mass to a casting mould. This latter step causes the semi-solid mass to become detached from the wall of the crucible making the transfer to the casting machine more efficient. In crucibles where such draining is not provided, there is often a problem of materials sticking to the crucible walls, requiring the use of specific cleaning steps between uses of the crucible. This results in longer cycle times required for casting and increases the complexity of the equipment needed for slurry preparation and casting. The ability to completely and reliably remove metal from crucibles used to produce the slurry in slurry on demand processes is an important requirement to achieving viability and efficiency. Improvements to metal removal are beneficial even in processes in which transfer of the slurry is relatively efficient, as for example, in the case of draining or otherwise reducing the adherence of metal to the crucible. In such case, it is important that the means for draining the liquid phase, also be cleaned and freed of any molten metal that may later solidify and cause blockage of the opening. There is a need therefore for an improved crucible design and method of use suitable for slurry on demand processing that permits extended repeated use between cleaning and/or refurbishment and maintains metal cleanliness throughout its use.

DISCLOSURE OF THE INVENTION According to one aspect of the present invention, there is thus provided a crucible for preparing semi-solid metallic slurry, having a liquid phase and a solids phase. The crucible comprises a side wall having a lower edge, a closing plate engagable against and separable from the lower edge of the side wall and clamping means for holding the closing plate against the lower edge of the side wall. According to another aspect of the present invention, there is provided a method of preparing semi-solids slurry, having a liquid phase and a solids phase. The method comprises providing a crucible having a side wall with a lower edge and a closing plate, holding the closing plate in contact with the lower edge of the side wall, filling the crucible with a molten metal, cooling the molten metal to produce a semi-solid slurry, releasing the lower edge of the side wall from contact with the closing plate and finally, transferring the semi-solid slurry from the side wall to a casting machine.

BRIEF DESCRIPTION OF THE DRAWINGS Preferred embodiments of the present invention are described below, in conjunction with the accompanying figures, wherein: Fig. 1 is a vertical cross-sectional view of an embodiment of the present invention, illustrating the side wall and closing plate of the crucible; Fig. 2 is a detail of Fig. 1; showing the hole and plug in a plugged arrangement; and Fig. 3 is a vertical cross-sectional view of the support plate and closing plate, showing a preferred means of cleaning the hole.

BEST MODES FOR CARRYING OUT THE INVENTION With reference to Fig. 1, a crucible 1 is shown that is designed to receive molten metal to be cooled into a semi-solid metallic slurry. The crucible 1 is formed from a side wall in the form of an open ended tube 2, which rests on a closing plate 3. Any convenient cross-sectional shape may be used, but a generally cylindrical shape is particularly preferred. The tube 2 is held in contact against the closing plate 3 to form a seal between a lower edge 4 of the tube 2 and a surface of the closing plate 3. The tube 2 is preferably slightly tapered, more preferably typically with less than about 5 degrees of taper and most preferably about 1 degree of taper. The taper may be in either direction but most preferably having a larger diameter at the lower edge 4, which contacts the closing plate 3. The closing plate 3 in turn can be mounted on a larger support plate 5 that forms part of a support for the crucible 1. Pressure between the tube 2 and the closing plate 3 can be maintained by a number of means well known in the art, including the use of a clamping device 14. Preferably, the clamping device 14 is in the form of a horseshoe style clamp that is connected to the support plate 5 and which can be remotely actuated to rotate onto and clamp down on a flange 6 that runs around the tube 2, above the lower edge 4. Actuation for the clamp can be pneumatic or hydraulic. The support plate 5 can be remotely controlled to move in a swirling motion to thereby agitate the crucible and enhance cooling of the molten metal in forming the semi-solid slurry. The closing plate 3 may be manufactured from refractory materials, and particularly from alumina-silicate refractory boards such as N-17™ refractory board produced by Pyrotek Inc., or can be a metal, such as steel, stainless steel or titanium. The metal tube 2 may be manufactured from a refractory material or metal as well, and is preferably made from steel or stainless steel. In a preferred embodiment, material for the tube 2 and tube thickness and the material and dimensions of the closing plate 3 are chosen based on predetermined heat capacity and mass requirements for cooling the molten metal into the desired semi-solid slurry consistency. However, it is also possible to use a tube and closing plate with less precisely determined properties, together with an external heating or cooling system. Any metallic surfaces of either the closing plate 3 or the tube 2 are preferably coated with suitable metal resistant coatings. Such coatings are well known in the art and, in the case of aluminum alloys, can include mica wash and boron nitride. Any other operable refractory based coatings and coating methods may be used, including coatings applied as wash coats, spray coats or coatings applied by plasma spraying. With reference to Fig. 2, the closing plate 3 and the support plate 5 preferably contain at least one hole 10 running through them. The hole 10 is preferably tapered, having a smaller diameter adjacent the lower edge 4 of the tube 2 and widening therefrom. The hole 10 has a preferred diameter of from about 3 to about 5 millimeters. In cases where the hole is tapered, the smaller diameter is from about 3 to about 5 millimeters. The particular diameter used is dependent on the alloy cast and size of slurry charge to be prepared. The closing plate 3 can preferably have an upwards conical profile, to enhance draining of liquid metal from the hole 10. A plug 11 fits within the hole 10 from an underside of the supporting plate 5. The plug 11 is optionally mounted on a pivoting cantilever arm 12 that allows for manipulation of the plug 11 in and out of the hole 10. The pivoting cantilever arm 12 can be remotely actuated to plug or unplug the hole 10, and can be either pneumatically or hydraulically actuated. The plug 11 can be made from any suitable material known in the art to be resistant to the molten metal, and is preferably made of boron nitride. In a preferred mode of operation, the tube 2 is first cooled to a predetermined temperature to aid in cooling the molten metal into a semi-solid slurry. The tube 2 can be cooled to a temperature ranging from room temperature up to 100 - 15O0C. The plug 11 is inserted in the hole 10 in the closing plate 3 and the tube 2 is clamped to the closing plate 3 by the clamping device 14. Molten metal, typically having a temperature greater than 7000C, is poured into the crucible 1 , where it is agitated by movement of the support plate 5 and cools to form a semi-solid slurry. The semi-solid slurry comprises a liquid phase and a solids phase. A cooling process such as that taught in US Patent no. 6,428,636 is particularly preferred for this purpose. After a pre-determined period of time, which can be while agitation is still in progress, the plug 11 is removed from the hole 10 by pivoting the pivoting cantilever arm 12 and some of the liquid phase is allowed to flow out through the hole 10. In draining off some of the liquid phase, the fraction of solids phase in the slurry is increased, typically, from about 35 - 40wt% to about 45 - 55wt%. Although a single relatively large hole 10 is preferably used to drain a portion of the liquid phase, the semi-solid structure and thixotropic behaviour of the slurry within the crucible 1 prevents the solid phase from escaping and only liquid phase is removed. As mentioned before, it is desirable to drain off some of the liquid phase so that the semi-solid slurry in the crucible 1 becomes detached from the tube 2 of the crucible 1 , thereby easing transfer of the slurry to a casting machine. Once some of the liquid phase is drained off, the clamping device 14 is released from the tube 2 and the tube 2 containing the semi-solid mass is transferred to a shot sleeve of a conventional die casting machine for forming and solidifying the slurry. The semi-solid slurry leaves the tube 2 through its lower edge 4. The tube can be transferred using any known means in the art, for example, using a remotely controlled robot (not shown). Once the tube 2 has been un-clamped and removed from the support plate 5, the hole 10 in the closing plate 3 can be cleaned of any molten metal that would otherwise build up and obstruct the hole 10. Because there is no filter element of similar device required in the hole 10, there is little surface area for molten metal to accumulate and the removal of any small residue is quickly accomplished. With reference to Fig. 3, the hole 10 can preferably cleaned by a simple burst of compressed from an air jet 8. The hole 10 and the closing plate 3 are then ready to be clamped to the tube 2 for formation a next batch of semi¬ solid slurry. The air jet 8 can optionally be remotely moved into position under the hole 10 and can be remotely actuated to deliver a burst of compressed air. Actuation of the air jet 8 can be pneumatic or hydraulic. A rotating air jet (not shown) may also preferably be used to blow away any molten metal droplets that may still adhere to the tube after the semi-solid slurry has been transferred to the casting machine. In this manner, each of the steps for clamping the support plate 5 to the tube 2, agitating the support plate 5, unclamping the tube 2 from the support plate 5, transferring the slurry to the casting mould, cleaning the hole 10 in the closing plate 3 and re-clamping the tube 2 to the support plate 5 can be timed, automated and remotely controlled. Because of the particular design features, the crucible 1 , in particular when used in the preferred manner, remains free of significant metal residues and can be continuously reused without stopping on each casting cycle for mechanical cleaning and recoating. This detailed description of the devices and methods of the present invention is used to illustrate the prime embodiments of the present invention. It will be apparent to those skilled in the art that various modifications can be made in the present devices and methods and that various alternative embodiments can be utilized. Therefore, it will be recognized that modifications can be made in the present invention without departing from the scope of the invention, which is limited only by the appended claims.