Patent Publication Number: US-2003222121-A1

Title: Die casting sprue system

Description:
[0001] This invention relates to a sprue system for use in pressure casting machinery, such as hot chamber die casting machines or machines for producing castings from thixotropic alloy, and some forms of cold chamber casting machines. The invention also relates to a die assembly for a machine of either of these types, which system has a sprue system of the invention, and a casting machine having that assembly. The invention further relates to an improved process for producing die castings by use of a casting machine of the invention and improved castings produced by that process.  
       [0002] In the well known hot chamber die casting machine, a die cavity is defined by a multi-part die which is mounted adjacent to a vessel in which molten alloy is maintained at a suitable temperature for casting. Above the vessel, there is a shot cylinder which has a plunger extending into a component shaped like and referred to as a goose-neck within the molten metal. The plunger, when actuated by the cylinder, forces molten alloy through a nozzle of, or associated with, the gooseneck. From the nozzle, the alloy passes through a sprue region and, via a runner and gate system, into the die cavity. Upon completion of filling of the cavity and sufficient solidification of the cast alloy, the plunger movement is reversed to cause still molten alloy in the flow path to the sprue region to be drawn back towards the vessel.  
       [0003] The conventional sprue region for such apparatus has two functions. The first of these functions is to transport and distribute molten metal from the nozzle to the runners feeding the die cavity or die cavities. The second function is to provide a solidification zone from which the molten metal is to be drawn back to leave in the sprue region solidified alloy, referred to as a sprue, following solidification of the alloy from the die cavity, back along the sprue region to the solidification zone. Thus, alloy at the solidification zone in the sprue region is to be the last to solidify. T he sprue region can have a simple frusto-conical shape which is solid or, by use of a sprue post (sometimes referred to as a sprue pin or sprue spreader), which is hollow. Alternatively, the runners can be machined into the sprue post.  
       [0004] Difficulty in achieving each of the functions of the sprue region necessitates adoption of a number of compromises. For optimum transport of molten alloy from the nozzle to the gates, it is best that the alloy is kept molten at a substantially uniform temperature. However, for the alloy subsequently to be solidified at a solidification zone through the interface between nozzle and the sprue, it is necessary to actively cool the sprue post, as well as a bush defining the outer periphery of the sprue region, while the nozzle needs to be heated.  
       [0005] The conventional sprue region is designed so that the solidified sprue metal can be removed from the die with the casting. For this, the sprue region is designed as a cone shape with the larger end nearer to the casting and the smaller end at the nozzle. However, this presents a problem in satisfying thermal requirements, and the problem is most severe if a solid sprue is to be produced (that is, the sprue is formed without use of a sprue post). The largest volume of sprue metal is near the base of the sprue and the smallest volume of sprue metal is adjacent to the nozzle. Hence, if no cooling is supplied to the sprue bush, the sprue region would solidify initially near the nozzle and solidification then would proceed towards the die cavity. This would be unacceptable as solidification shrinkage would occur adjacent to the base of the sprue, with the likely consequence of localised fracture of nearly solidifying or newly solidified metal. There then would be a risk of sprue metal being left in the sprue region, rather than removed with the casting, necessitating its manual removal prior to the next casting cycle. Thus, solidification must be forced to occur in the reverse direction, that is, towards the nozzle, by the use of aggressive cooling. It is common for the sprue post and sprue bush to be maintained at close to 100° C. whereas the rest of the die is maintained at about 120° C. to 200° C. for optimum casting quality for each of zinc, lead and magnesium alloys. This drastically chills the molten metal as it passes through the sprue region, thus reducing the possible quality of the casting.  
       [0006] The present invention arises from our work in relation to die casting of magnesium alloys, as detailed in International patent application PCT/AU98/00987. That application discloses that some casting characteristics of magnesium alloys distinguish those alloys from other die casting alloys. Among other benefits, the differing casting characteristics of magnesium alloys enable a very substantial improvement in the casting yield; that is, the percentage ratio of casting weight to total shot weight. Thus, the weight of metal which needs to be recycled and reprocessed is able to be substantially reduced, with a resultant reduction in production costs.  
       [0007] The object of the invention is to provide a sprue system which better utilises the casting characteristics of magnesium alloys and thereby enables further enhancement of casting yield. However, it is found that the sprue system of the invention also has utility in the casting of other alloys. Thus, while it was not an object per se of the invention to overcome the problems of the prior art in satisfying thermal requirements, those problems can be alleviated with at least some embodiments of the invention for casting other alloys.  
       [0008] According to the invention there is provided a sprue system for use in a pressure casting machine, such as a machine for hot chamber die casting, or for producing castings from thixotropic alloy, or for some forms of cold chamber die casting machines, wherein the system includes a plurality of sprue dies which form a sprue housing, or bush, through which a sprue region extends longitudinally between inlet and outlet ends thereof; and wherein the sprue dies are relatively movable laterally with respect to the longitudinal extent of the sprue region, between an advanced position in which the sprue dies form the housing or bush and a retracted relative position.  
       [0009] With the sprue dies in their advanced position, the housing of the sprue system is adapted to be held under clamping pressure prevailing during a casting cycle, such as a hot chamber die casting cycle. When so clamped, the sprue dies are held against relative movement, that is between the gooseneck nozzle and the die cavity where casting is by use of a hot chamber die casting machine. Thus, molten metal is able to be forced through the sprue region during a casting cycle. On completion of a casting cycle, and release of the clamping pressure, the sprue dies are able to be moved to their retracted relative position.  
       [0010] The housing preferably is formed from two sprue dies. In that case, one sprue die preferably is substantially the mirror image of the other, while each defines substantially one half of the sprue region. More than two sprue dies can be used, although any benefit resulting from using at least three sprue dies generally is offset by added complexity in providing for relative movement of the sprue dies.  
       [0011] Each sprue die may be in the form of a slide which is mounted for reversible lateral movement. The lateral movement may be substantially at right angles to the longitudinal extent of the sprue region. However, the sprue dies may move at an angle to the longitudinal extent of the sprue region such that there is a sufficient lateral component of the movement which is substantially at right angles to the longitudinal extent of the sprue region.  
       [0012] It is preferred that each sprue die comprises a slide. However, other arrangements are possible even if more complex. Thus, for example, each sprue die may be mounted for movement on a spiral path such that they move between their advanced and retracted positions in a similar manner to elements of an iris. Alternatively, each sprue die may comprise a finger element of a collet form of device.  
       [0013] The tapered form of the sprue region used in conventional practice is necessary, in part, in order to enable a casting to be removed. That is, the resultant sprue metal is correspondingly tapered, enabling it to be extracted when the die is opened and the casting is ejected. A somewhat similar taper can characterise the sprue region of the sprue system of the present invention, in that the sprue dies can define the sprue region as a tapered sprue region which increases in cross-section in a direction towards the inlet end. However, in the invention, the sprue region is freed of a need to be of such form as a necessity. Thus, the sprue region can be designed with other objectives in mind, such as to achieve cooling through the sprue region to a solidification zone without the need for aggressive cooling, or to minimise the volume of the sprue region and hence, of sprue metal.  
       [0014] In a simple form of the sprue system, the sprue dies when in their advanced position may define a sprue region of substantially uniform cross-section of an area suitable for the metal to be cast. Thus each sprue die, where formed for providing a part of the sprue region, may simply be machined to define a groove which, for a sprue system of two dies, is of semi circular cross-section. However, with such simple form of sprue system it is preferred that the housing has a counter-sink at the end of the sprue region which is to accommodate the nozzle of the gooseneck.  
       [0015] The sprue region defined by the sprue dies may be of any suitable cross-section, whether circular or non-circular. Also, the sprue region may vary in cross-section between its inlet or upstream end to its outlet or downstream end, with respect to metal flow from a pressurising source of alloy to the die cavity. In a first form, the region may taper, such as frusto-conically, with its larger end nearer to the die cavity, such as at or adjacent to the outlet end of the sprue region. This form of taper is similar to that of the prior art discussed above but, in the present invention, that form can be particularly beneficial when making a magnesium alloy casting by direct injection. In accordance with the disclosure of PCT/AU98/00987, the sprue region may function as a direct injection runner with the smaller outlet end being of a cross-section resulting in a magnesium alloy flow velocity therethrough of from about 140 to about 165 m·s −1 , preferably about 150 m·s −1 . The flow velocity then decreases along the length of the sprue region and within the die cavity.  
       [0016] In a second form, the sprue region may be frusto-conical but with its larger end nearer to the source of alloy, such as at or adjacent to the inlet end of the sprue region. That is, in terms of taper, the arrangement may be the opposite of that used in prior art practice for hot chamber die casting. This arrangement also is suitable for making a magnesium alloy casting by direct injection. In accordance with the disclosure of PCT/AU98/00987, the sprue region again functions as a direct injection runner, with the smaller outlet end being of a cross-section resulting in a magnesium alloy flow velocity therethrough of from about 140 to about 165 m·s −1 , again preferably about 150 m·s −1 , and with the flow velocity decreasing further downstream on entering the die cavity.  
       [0017] In the second form, it is preferred that the smallest cross-section of the frusto-conical sprue region is spaced a short distance from the end of that region which is nearer to the die cavity. Between that end and the die cavity, there may be a short end length of the sprue region which does not decrease further, but which most preferably increases slightly, in cross-section. This results in a waist or channel in solidified sprue metal which, as illustrated in more detail later herein, facilitates breaking of the sprue metal to leave a casting with which it is associated with a smaller effective sprue.  
       [0018] In a third form, the sprue region may have a larger cross-section intermediate of its ends and taper frusto-conically from that larger cross-section towards each end. In that form, the taper towards the gooseneck nozzle may continue to the end of the sprue region in an arrangement providing for a solidification zone at the junction between the nozzle and the sprue region. The frusto-conical taper towards the end of the sprue region nearer to the die cavity may continue to a minimum cross-section adjacent to that end, with there being a short end length of the sprue region as described from the second form. Sprue metal solidified through to the solidification zone_enables its separation from the nozzle, with the casting, following retraction of the plunger to draw back still molten alloy. Also, the sprue metal is able to be broken at the waist or channel, resulting in the casting having only a small effective sprue thereon, with a major part of the sprue metal forming a separated plug or pellet.  
       [0019] In a fourth form, the sprue region is as in any of the first, second and third forms. However, it comprises only a downstream part of a passage defined by the sprue dies. A solidification zone is defined at the junction between the downstream and upstream parts of the passage and, to achieve this, the upstream part of the passage is of larger cross-section than the adjacent end of the sprue region. Thus, the upstream part of the passage provides a continuation, beyond the gooseneck nozzle, of the section of the overall molten alloy flow path in which the alloy is kept molten for withdrawal.  
       [0020] As indicated at the outset, the sprue dies which form the housing of the sprue system of the invention are relatively movable laterally with respect to the longitudinal extent of the sprue region. This is necessary where the form of the sprue region is such that solidified sprue metal otherwise would preclude extraction of the sprue metal from the sprue region. However, the ability of the sprue dies to move can be used to advantage where the sprue region is of a form such that solidified sprue metal therein is able to be broken. That is, at least one sprue die can be used to apply a force to the sprue metal, causing it to break or shear. Thus, with the sprue dies released for retraction from their advanced position to their retracted position, and their retraction at least initiated, and with a casting still partially constrained by other tool parts defining the die cavity, one of the sprue dies can be moved back to, and preferably slightly beyond, its normal advanced position so as to impact with the sprue metal and cause the sprue metal to break or shear at the casting or at the waist or channel. Alternatively, the sprue dies may move in union in a given lateral direction to shear or break the sprue metal at a waist or channel. The movement in unison can be prior to retraction of the sprue dies being initiated, or after a small initial retraction.  
       [0021] In a conventional sprue region, the molten metal flow velocity into the melt end of the region usually is about 10 to 30 m·s −1 . Such a velocity level is still appropriate for use of the present invention for all zinc, lead and magnesium die casting alloys unless, in the case of magnesium alloys, use is to be made of the disclosure of PCT/AU98/00987. Where that disclosure is to be used in the present invention, the flow velocity for the magnesium alloy is to obtain a level of from about 140 to about 165 m·s −1 , preferably about 150 m·s −1 , followed by a decrease in flow velocity. These velocities, where achieved in a sprue region, necessitate a sprue region cross-sectional area which is two orders of magnitude smaller than in a sprue region of conventional cross-sectional area. Thus the sprue region, and the volume of sprue metal solidified in it, is able to be very small relative to conventional practice. This is one factor able to contribute to the very high casting yield obtainable with the invention of PCT/AU98/00987.  
       [0022] The present invention further provides a die assembly, for a machine for hot chamber die casting, or for producing castings from thixotropic alloy, or a cold chamber die casting machine, wherein the die assembly includes a die tool which at least partially defines at least one die cavity, and a sprue system which includes a plurality of sprue dies which form a sprue housing, or bush, through which a sprue region extends longitudinally between inlet and outlet ends thereof to define part of a path for receiving alloy from a source of supply for flow into the at least one die cavity; and wherein the sprue dies are relatively movable laterally with respect to the longitudinal extent of the sprue region, between an advanced position in which the sprue dies form the housing or bush and a retracted relative position.  
       [0023] In a die assembly according to the invention, the arrangement can vary. One important factor influencing this is whether the sprue region is to provide for direct injection or is to feed the die cavity via at least one runner and gate.  
       [0024] Where the sprue region is to provide for direct injection feed to the die cavity, an arrangement well suited to producing magnesium alloys by the invention of PCT/AU98/00987, each sprue die preferably has a surface at which the outlet end of the sprue region opens and which in part defines the die cavity. That is, the sprue dies co-operate with other die components to form and define the die cavity. Thus, the sprue dies may form part of or comprise a cover die half, or they may form part of or comprise a die cavity insert of a cover die half, with the cover die half in each case being co-operable with an ejection die half. Conversely, the sprue dies may form part of or comprise the ejection die half or a die cavity insert of the ejection die half.  
       [0025] For a die assembly in which the sprue region is to feed the die cavity via a runner and gate, the arrangement is more amenable to the die assembly being a multiple cavity die. However this simply results in alloy flow through the sprue region feeding to each of two or more die cavities via at least one respective runner and gate, and it is sufficient to consider the arrangement as it applies to only one of the die cavities. In such arrangement, the sprue dies are separated from the die cavity by a section of the assembly which defines the gate and at least part of the runner. The runner may be defined fully by that section or it may be defined in part by one or each sprue die. Thus the sprue dies may form part of a cover die half, with the cover die half being co-operable with the ejection die half to define the die cavity. Conversely, the sprue dies may form part of the ejection die half.  
       [0026] The present invention further provides a machine pressure casting, such as a machine for hot chamber casting, or for producing castings from thixotropic alloy, or a cold chamber die casting machine, wherein the machine includes a die assembly, clamping means associated with the die assembly, and pressurised supply means for feeding molten alloy to at least one die cavity defined by the die assembly; wherein the die assembly includes a die tool which at least partially defines the at least one die cavity, and a sprue system which includes a plurality of sprue dies which form a sprue housing, or bush, through which a sprue region extends longitudinally between inlet and outlet ends thereof to define part of a path for receiving alloy from the source of supply for flow into the at least one die cavity; and wherein the sprue dies are relatively movable laterally with respect to the lon 7 gitudinal extent of the sprue region, between an advanced position in which the sprue dies form the housing or bush and a retracted relative position; and wherein, with the sprue dies in their advanced position, the clamping means secures the sprue dies in relation to the die tool whereby alloy is able to flow from the supply means, through the sprue region and then to the at least one die cavity.  
       [0027] In one form of the casting machine according to the invention, each die half is mounted on a respective platen, with the die halves held together by means of a toggle clamp powered by a pneumatic or hydraulic ram. In the case of a hot chamber die casting machine, the supply means may be a shot cylinder extending into a vessel in which molten alloy is maintained, with the shot cylinder operable to force pressurised alloy through a gooseneck for filling the die cavity.  
       [0028] A hot chamber die casting machine according to the invention preferably is of conventional in-line form. That is, the gooseneck nozzle and sprue region preferably feed the alloy to the die cavity along a line substantially parallel to forces clamping the die halves together. However, the sprue dies are movable between their advanced and retracted positions in directions substantially at right angles to the clamping forces. Also, when the sprue dies are in their advanced position in preparation for and during casting, the clamping forces most preferably act to secure the sprue dies in that position. Thus, prior to the sprue dies moving to their retracted position, it is necessary to release the clamping force in order to enable that movement. The release of the clamping force initially may be such that the die halves still are held together, but with the sprue dies sufficiently freed for movement in a manner minimising the risk of damage to surfaces in sliding contact.  
       [0029] However, the invention also extends to casting machines other than hot chamber die casting machines. Specifically, the machine according to the invention may be one for producing castings from a thixotropic alloy. Thus, the machine may be of the type in which a semi-solid charge of alloy of suitable microstructure is positioned in a shot chamber and then injected by a piston into a die cavity. The machine may be one which uses a semi-solid alloy charge produced by cooling liquid alloy, or it may be one using a billet of alloy heated so as to achieve the semi-solid condition. Alternatively it may be a machine which uses a charge of preformed pellets or chips, of appropriate alloy, which is fed into a heating chamber, and which then is driven by screw, or other means, through a nozzle and into the die cavity. Also, the invention extends to some forms of cold chamber die casting machines, specifically those able to operate with use of the sprue system of the invention or the die assembly of the invention.  
       [0030] In a die casting or other pressure casting process according to the invention the sprue dies are moved to and clamped in their advanced position prior to the commencement of casting. The sprue dies are kept in that position until the casting is complete and the cast alloy has solidified sufficiently in the die cavity and from that cavity back along the sprue region to a solidification zone. The sprue dies then are freed for movement to their retracted position, with such movement then initiated to an extent enabling relative movement between the sprue dies and the casting in a direction enabling solidified sprue metal to be withdrawn from the sprue region.  
       [0031] In one preferred form of the process, movement of one of the sprue dies towards its retracted position is reversed to enable a part, preferably a major part, of the sprue metal to be broken away or sheared from the casting at a designed breaking zone. For this, the one sprue die is moved back to and preferably beyond its advanced position so as to impact against and break the sprue metal at that zone. The casting then is removed from the casting machine, prior to the machine being made ready for a next casting cycle.  
       [0032] In another preferred form of the process, the sprue dies are movable in unison in a given lateral direction so as to shear or break the sprue metal therebetween from a casting. The movement in unison may be before movement of the sprue dies to their retracted position has commenced, or after a small initial part of that movement. The casting may be removed from the die prior to the movement of the sprue dies to their retracted position starting or restarting, with that movement allowing removal of the sheared or broken off sprue metal.  
       [0033] The sprue dies may be movable by actuators operable to move those dies between their advanced and retracted positions. The sprue dies preferably interfit with a guideway defined by an adjacent component of the machine. Conversely, such component may interfit with a respective guideway defined by each sprue die. Each actuator may be a pneumatic or hydraulic press which, preferably, extends outwardly at a respective side of the machine, substantially at right angles to the line of action of the clamping means of the machine. However other forms of actuators can be used such as, for example, actuators providing movement by a rack and pinion arrangement. 
     
    
    
     [0034] Reference now is made to the accompanying drawings, in which:  
     [0035]FIG. 1 is a side elevational view, partly in section, of a hot chamber die casting machine according to the present invention;  
     [0036]FIG. 2 is a sectional view taken on line II to II of FIG. 1;  
     [0037]FIG. 3 is a perspective view of castings as produced by a conventional die casting machine;  
     [0038]FIG. 4 shows components for defining a sprue region for castings in FIG. 3;  
     [0039]FIG. 5 is a sectional view, taken on line V-V of FIG. 4;  
     [0040]FIG. 6 is a perspective representation of a casting corresponding to that of FIG. 3, but as produced by the present invention;  
     [0041]FIG. 7 is a perspective representation of another form of casting as produced by the present invention;  
     [0042]FIG. 8 shows a variant on the casting of FIG. 7;  
     [0043]FIG. 9 is a sectional representation of one form of sprue region according to the invention;  
     [0044]FIG. 10 corresponds to FIG. 9, but shows another form of sprue region;  
     [0045]FIG. 11 also corresponds to FIG. 9, but shows a still further form of sprue region;  
     [0046]FIG. 12 is a perspective representation of the sprue region defining end of a die of the arrangement of FIG. 11;  
     [0047]FIG. 13 is a sectional view of a die assembly according to an embodiment of the invention;  
     [0048]FIG. 14 is a sectional view of an alternative form of sprue die for use in the assembly of FIG. 13; and  
     [0049] FIGS.  15  to  18  each shows an end elevation of a sprue die according to a respective further alternative form. 
    
    
     [0050] With reference to FIGS. 1 and 2, there is shown a high pressure die casting machine  10 . This has an in-line arrangement of a molten metal supply station  12 , a casting station  14 , a locking mechanism  16  and a closing mechanism  18 . The general detail of and mode of operation with machine  10  will be readily understood by those skilled in the art, and description largely will be limited to a broad overview.  
     [0051] Casting station  14  includes a die assembly  20  which has a fixed, cover die half  22  and a movable ejection die half  24 . The fixed die half  22  is secured to fixed platen  26  secured on support base  28 . The die half  24 , for which the ejection mechanism is not shown, is mounted on movable platen  30 . The die half  24  is able to be clamped against die half  22  to define a die cavity  32 , or moved away from die half  22 , under the action of the toggle clamp  34  of mechanism  16  and the pneumatic actuator  36  of mechanism  18 .  
     [0052] Station  12  includes a furnace  38  in which a suitable alloy  40  is kept molten at an appropriate casting temperature. A shot cylinder  42  is mounted above furnace  38  and has a plunger  44  which extends into a gooseneck shaped component  46  positioned in the molten alloy  40 . A nozzle  48 , which projects through the fixed platen  26 , provides communication between the outer end of gooseneck  46  and a sprue region  50  of die half  22 . Region  50  communicates with die cavity  32 . Thus, actuation of cylinder  42 , to drive plunger  44  further into gooseneck  46 , causes molten alloy to be forced under pressure through the gooseneck  46 , the nozzle  48  and the sprue region  50 , thereby filling die cavity  32 . This, of course, assumes that die halves  22 ,  24  are clamped together to close cavity  32  for filling. However, on completion of filling and solidification of metal in cavity  32  and back along the flow-path to a solidification zone through the interface between sprue region  50  and nozzle  48 , cylinder  42  is activated to raise plunger  44  and thereby withdraw molten alloy upstream from that plane.  
     [0053] The preceding description of FIGS. 1 and 2 relates to detail of prior art machines and their operation. However machine  10  is in accordance with the present invention.  
     [0054] The fixed die half  22  has a backing structure  52  on which there is slidably mounted a laterally opposed pair of sprue dies  54  and  56 . Only sprue die  54  is visible in FIG. 1. This is because in the condition shown, sprue dies  54 ,  56  sealing abut at faces above and below sprue region  50  on a vertical plane through the longitudinal centre line of machine  10 . However, the dies  54 ,  56  are laterally movable in opposite directions from the advanced position of the condition shown, to a retracted position.  
     [0055] Each of sprue dies  54 ,  56  is mounted in relation to structure  52  by structure  52  defining a laterally extending guideway or track with which the dies  54 ,  56  interfit (for example, in the manner described later herein with reference to FIG. 13). In order to move dies  54 ,  56  laterally along the guideway or track, each of dies  54 ,  56  is connected to a respective actuator  58 ,  59 , each shown as comprising a pneumatic or hydraulic actuator or piston and cylinder device.  
     [0056] Sprue region  50  is of frusto-conical form and, in the arrangement shown, it has its smaller end nearer to the die cavity  32  while, at its larger end, it is in communication with the bore of nozzle  48 . Each half of region  50  is defined by a respective groove  54   a ,  56   a  formed in each die  54 ,  56 . Thus, for casting, it is necessary that actuators  58 ,  59  are extended so as to hold the abutting faces of dies  54 ,  56  firmly in sealing relationship. After this relationship is achieved by operation of actuators  58 ,  59 , it is made even more secure by the die halves  22 ,  24  being forced and locked together by operation of actuators  36  and clamps  34 .  
     [0057] For ejection of a casting produced in die cavity  32 , it is necessary for clamp  34  to be unlocked and for die half  24  to be withdrawn sufficiently from die half  22  by operation of actuator  36 . However, after an initial slight separation of halves  22 ,  24  which is necessary to free the sprue dies  54 ,  56  for lateral movement, and commencement of action of the ejection mechanism of half  24 , movement of dies  54 ,  56  to their retracted position needs to commence. This is because of the re-entrant form of sprue metal solidified in sprue region  50 , since the sprue will be held in region  50  until there is sufficient retraction of dies  54 ,  56 .  
     [0058]FIG. 3 shows castings  60  as produced by a conventional hot chamber die casting machine having a multiple die cavity. The form of the components comprising castings  60  is incidental. However, each casting has a rectangular box-like form and as ejected from the casting machine, the castings  60  are held together by runner and sprue metal  64 . This metal  64  includes sprue metal  68  which solidified in a frusto-conical sprue region of opposite taper to that shown in FIGS. 1 and 2; a short strip  70  to each plate  64  which solidified in a respective main runner; a tapered strip  72  along the side of each plate  64  which solidified in a respective tapered tangential runner; and a disc  74  at the end of each strip  72  which solidified in a respective “shock absorber” at the end of each tangential runner. As will be appreciated, the castings  60  need to be separated from the runner metal strips  72  and from any flash. However, the weight of metal comprising the sprue metal  68 , the strips  70  and  72  and the discs  74  is substantial relative to the weight of the metal comprising the four castings  60 . Thus, the casting yield is relatively low, in this instance about 50%.  
     [0059]FIGS. 4 and 5 illustrate part of a sprue region  76  suitable for producing castings  60  as in FIG. 3. Thus region  76  is one applicable to a conventional hot chamber die casting machine rather than a machine according to the present invention. Also, in region  76 , the components giving rise to strips  70  and  72  and discs  74  are not shown. Rather, there is shown the components for producing sprue metal  68 .  
     [0060] Components for defining the sprue region  76  of FIGS. 4 and 5 comprise a sprue bush  80  mountable in relation to a fixed die half, and a sprue post  82  mountable in relation to a movable die half. The bush  80  defines a bore  84  which, from a short intermediate portion  84   a , tapers outwardly at one end to provide a seat  84   b  engaged by the bevelled outlet end of a nozzle  86 . The bore  84  also tapers outwardly from portion  84   a  to define a main frusto-conical surface  84   c . Post  82  has a tapered external surface  82   a  which is substantially complementary to and provides a seal with surface  84   c , except at grooves,  82   b  formed in surface  82   a . Each groove  82   b  is covered by surface  84   c  of bush  80  to define a respective sprue runner  86 .  
     [0061] In the arrangement of FIGS. 4 and 5, a thermal energy balance is achieved by strong cooling of sprue bush  80  and sprue post  82 , and by heating of nozzle  86 . For the cooling bush  80  and post  82 , each is provided with a respective channel  88  and  89  through which cooling water is able to be circulated, while heating of nozzle  86  can be by suitable use of gas burners or of an electric heating element. The thermal energy balance is directed to achieving solidification, on completion of filling of the die cavities, which proceeds from those cavities and back along the sprue runners to a solidification zone which is transverse to the axis of bush  80  and is at the interface between bush  80  and the outlet end of nozzle  86 . The zone may be perpendicular to the drawing of FIG. 4 and is represented by the line X-X.  
     [0062] The specific arrangement shown in FIGS. 4 and 5 results in sprue metal  68  of FIG. 3 including a short stub  77  of metal which solidified in portion  84   a  of bore  84  of sprue bush  80 . Also, even though the surface  82   a  of post  82  is substantially complementary to surface  84   c  of bush  80 . allowance has to be made for thermal expansion and the need to avoid post  82  becoming locked in bush  80 . Thus, sprue metal  68  comprises a thin-walled frusto-conical shell  79  and, within shell  79 , sprue runner metal strip  81  which forms in each sprue runner  86 . While not shown in FIGS. 4 and 5, region  76  includes means which defines runners in which each strip  70  forms as a continuation of a respective runner strip  81 .  
     [0063]FIG. 6 shows the form of castings  90 , similar to castings  60  shown in FIG. 3, but as produced by the present invention, using a die casting machine as in FIGS. 1 and 2. The castings  90  are held together by a runner and sprue metal  91  which includes a small sprue  92  and, from sprue  92 , a respective small runner  93  extending to a convenient location on the edge of each casting  90 . As shown, the sprue  92  extends in the opposite direction to sprue  68  in the arrangement of FIG. 3, but this is simply to allow for the lateral movement required for the sprue dies of machine  10  of FIGS. 1 and 2. As will be evident from a comparison with FIG. 3, the quantity of runner and sprue metal is very small, enabling a very substantially enhanced casting yield. At least with the casting of magnesium by the invention of PCT/AU98/00987, the casting yield can be higher than about 95%.  
     [0064]FIG. 7 shows a casting of a dish  100  similar to that shown in FIGS. 10 and 11 of the above mentioned International patent application PCT/AU98/00987. As in that application, dish  100  is well suited to high pressure die casting from a magnesium alloy, using a hot chamber machine as in FIGS. 1 and 2 and a die assembly providing for direct injection.  
     [0065] The dish  100 , as cast, has associated therewith only sprue metal  102 . The dish is of a substantially uniform, thin-walled construction throughout, with a wall thickness of about 2 mm. As is evident from the sprue  102 , the dish  100  was cast by flow of magnesium alloy into the die cavity direct from a sprue region communicating with the die cavity at a single point located centrally with respect to what was to become the basal wall of the dish. However, the sprue projects from the lower surface of the basal wall of the dish rather than the upper surface of that wall as in FIGS. 10 and 11 of PCT/AU98/00987. This difference is to facilitate lateral movement, after solidification of cast alloy, of sprue dies which defined therebetween the sprue region for sprue  102  and which provided respective parts of a surface of the die cavity against which the lower surface of the basal wall was to form.  
     [0066] The sprue  102  tapers in the opposite direction to sprue region  50  of the die assembly of FIGS. 1 and 2. However, if required, the sprue  102  need not taper at all, or it may taper to provide its smaller end at the lower surface of the basal wall of dish  100 , or it may be of more complex form. However, regardless of its form, the sprue is able to be such as to substantially enhance the casting yield. For some other castings, this applies regardless of the alloy used, but significantly greater enhancement of that yield is possible with magnesium alloys. For the casting comprising dish  100 , use of an alloy other than a magnesium alloy is not likely to be possible with the direct injection alloy feed arrangement shown.  
     [0067]FIG. 8 is a sectional view of a dish  101  as in FIG. 7, but shown in relation to sprue dies for its production using a different die system to that used to produce dish  100  of FIG. 7. In the case of dish  100 , the sprue dies used define a surface of the die cavity against which the lower surface of the basal wall of dish  100  is formed. Thus, sprue  102  of dish  100  projects down from that lower surface. However, in dish  101 , sprue  103  projects up from the upper surface of the basal wall of the dish; that is, sprue  103  is within the dish as cast. Thus, the sprue dies D( 1 ) and D( 2 ), which define the sprue region  104  in which sprue  103  solidifies, also define internal surfaces of the die cavity in which dish  101  is cast. Thus, the dies D( 1 ) and D( 2 ) comprise angled slides which are movable in the direction of respective arrows Y-Y. That is, dies D( 1 ) and D( 2 ) simultaneously move both laterally and longitudinally with respect to sprue region  104  and with respect to alloy flow through region  104 . In the specific example illustrated, arrows Y-Y are substantially parallel to the flared side walls of dish  101 , while the frusto-conical sections of sprue region  104  are angled such that sprue  103  does not impede retraction of dies D( 1 ) and D( 2 ).  
     [0068] FIGS.  9  to  11  show alternative forms of sprue region able to be used with the present invention. In each case, there is a partial representation of sprue dies D( 1 ) and D( 2 ), with line L in each case representing a plane on which the dies abut to each side of the sprue region (most conveniently above and below that region) when the dies are in the advanced position for casting. Also in each case, the upper end of the sprue region as shown is flared outwardly for sealing engagement with the outlet end of a gooseneck nozzle. Thus, it is at the lower end of each sprue region that it communicates with a die cavity, either directly or via a runner/gate system.  
     [0069] The sprue region  106  of FIG. 9 has an enlarged, somewhat spherical mid-portion  107  below the flared end  108  at which it is engageable with a gooseneck nozzle. From portion  107  to its other end, region  106  has a taper frusto-conically outwardly tapered portion  109 . Solidification of alloy back from the die cavity is able to proceed along portion  109  to a solidification zone through the junction of portions  109  and  107 . If the solidification stops at or just short of that zone, sprue metal in portion  109  would be able to be withdrawn without moving sprue dies D( 1 ) and D( 2 ) to their retracted position. However, such movement of dies D( 1 ), D( 2 ) would assist withdrawal of the sprue metal, while it would be necessary if solidification extended into region  107 .  
     [0070] Similar considerations apply to the arrangement of FIG. 10 in which parts corresponding to those of FIG. 9 have the same reference plus “a”. In this case, the mid-portion  107   a  of sprue region  106   a  is frusto-conical, rather than spherical, and tapers in the same direction as its portion  109   a . Again the solidification zone is intended to be at the junction of portions  109   a  and  107   a.    
     [0071] In FIG. 11, parts corresponding to those of FIG. 9 have the same reference numeral plus “b”. In this instance the mid-portion  107   b  of sprue region  106   b  is of overall cylindrical form but has a hemi-spherical end through which it communicates with portion  109   b . Also, the solidification zone is intended to be beyond portion  109   b , intermediate the ends of portion  107   b . As a consequence, movement of dies D( 1 ) and D( 2 ) towards their retracted position is necessary for withdrawal of the solidified sprue metal from sprue region  106   b , due to the waist formed in that metal as a consequence of the constricted junction between portions  109   b  and  107   b.    
     [0072] Typically, with the arrangement of FIG. 11, the solidification zone will be sufficiently beyond portion  109   b  of the sprue region  106   b  for the proportion of sprue metal solidified in portion  109   b  to be small. In such case, it is preferred that, after initial movement of sprue dies D( 1 ) and D( 2 ) towards their retracted position, one of the dies is returned to and most preferably moved beyond the advanced position. The arrangement preferably is such that the one die impacts against the sprue metal, to cause the larger part to break off and to leave on the casting only the sprue metal that solidified in relatively small end portion  109   b . Alternatively, the sprue dies D( 1 ), D( 2 ) may move in unison to shear off the larger part of the sprue metal.  
     [0073]FIG. 12 shows a perspective view of the inner end of the sprue die D( 1 ) of FIG. 11. This highlights that the solidification zone does not have to be the smallest cross-section for sprue region  106   b  (of which only one half is shown). Since the dies D( 1 ), D( 2 ) move oppositely in directions shown by arrows Y-Y, release of the sprue metal in the direction of arrow X is able to be achieved. Conventional extraction in the direction of arrow X requires that the sprue metal has a continuous taper increasing in that direction. Also, the required control for cooling can be substantially reduced since the solidification can proceed to a zone of larger cross-sectional area (a more natural place to stop solidification) and over a reasonably large distance. Thus, design constraints for the sprue region are able to be reduced, enabling more flexibility. Such form of sprue region is particularly suited for the casting of magnesium alloys by the method disclosed in the above-mentioned International patent application PCT/AU98/00987.  
     [0074] As indicated, instead of the sprue dies, such as dies D( 1 ), D( 2 ) in each of FIGS.  9  to  11 , separating to enable extraction of the casting and the sprue as one section, the sprue dies can be moved in unison in the same direction, prior to their movement for die opening. This would shear the sprue off the casting and enable the casting only to be ejected. Subsequently in the process the sprue dies would open and eject the sprue metal. Thus, a trimming press could be done away with and this would reduce the cost as well as the required area for a casting machine and trim press.  
     [0075] In each of FIGS.  9  to  11 , the sprue region is shown as being of circular transverse cross-section. However this is not necessary, and they can be of other suitable cross-sections, for example elliptical, square or hexagonal.  
     [0076] The sprue dies of the invention allow for a much shorter sprue region, thus improving the overall casting yield. One way this is able to be achieved is by increasing the diameter of the sprue region to within a short distance from the casting. The relatively large volume of molten alloy in the larger diameter section of the sprue region is able to be left uncooled and hence naturally remain in the flowable condition, thus being easily drawn back into the gooseneck through the nozzle when the shot cylinder plunger is retracted.  
     [0077] Also one of the major problems with hot chamber sprue regions is that, in order for the molten alloy metal to flow back through the nozzle and gooseneck during retraction of the shot cylinder plunger, a vent for air must be produced. Otherwise, a vacuum is produced and the molten alloy stays in the nozzle. The alloy which stays in the nozzle can solidify and either be sufficient to block the nozzle or, on subsequent shots, build up until the nozzle is blocked. With the present invention the sprue dies can be open slightly during retraction of the plunger and thus provide an easily controlled venting position.  
     [0078]FIG. 13 is a sectional view of part of a die assembly  120  according to the present invention for use in producing a complex casting, such as of the form of casting shown in FIG. 15 of PCT/AU98/00987. As shown, the assembly  120  has a fixed die half  122  in engagement with a gooseneck nozzle  124 , and a movable die half  126 . During casing, molten alloy received from nozzle  124  is able to be injected, via a sprue region  130 , into a die cavity  128  defined by die halves  122 ,  126 .  
     [0079] The fixed die half  122  has a stationary backing plate  132  which is connected to a fixed platen (not shown). Die half  122  also has sprue dies  134  and  136  which define sprue region  130 , and a front plate  138  which is spaced from plate  132  by dies  134 ,  136 . As shown, the nozzle  124  is located within a sleeve assembly  140  by which it is mounted in an opening  142  through plate  132 . Within sleeve assembly  140 , an electric heating element  144  is provided around nozzle  124  to enable molten alloy with nozzle  124  to be maintained at a suitable temperature.  
     [0080] Each of sprue dies  134  and  136  has a respective carrier plate  134   a  and  136   a  and, secured at an inner edge of its plate, a respective sprue region defining insert  134   b  and  136   b . The arrangement enables use of tool steel for the inserts  134   b  and  136   b  and a less expensive steel for the carrier plates  134   a  and  136   a . Also, it enables replacement of inserts  134   b ,  136   b.    
     [0081]FIG. 13 shows the die assembly  120  in a view corresponding to that of FIG. 2. Each sprue die  134  and  136  is movable in the directions of the arrows Y from their advanced position illustrated to the retracted position. The dies  134 ,  136  are guided in this movement by each having a spline coupling  146  with plate  138 , which extends in the direction of movement. Each coupling  136  is defined by at least one elongate key  134   c ,  136   c  on each die and a complementary groove or keyway  138   a  formed in plate  138 .  
     [0082] The sprue dies  134 ,  136  when in their advanced position define a circular recess  148  in which the end of sleeve assembly  140  is received with slight clearance. The arrangement is such that, with the die halves  122 ,  126  clamped together, the sprue dies  134 ,  136  are clamped securely between plates  132 ,  138 , while inserts  134   b ,  136   b  are clamped securely against the end of nozzle  124 . Thus, dies  134 ,  136  are securely held in their advanced position, with sprue region  130  in line with the bore of nozzle  124 .  
     [0083] On completion of filling of die cavity  128 , solidification of alloy in that cavity is continued back along sprue region  130  to a solidification zone at the interface between inserts  134   b ,  136   b  and nozzle  124 . The shot cylinder (not shown) then is retracted to withdraw molten alloy in nozzle  124 . Action then is able to proceed for release of the casting from die cavity  128 . For this, the clamping pressure acting on the die halves  122  and  126  is released in an initial stage which enables sprue dies  134  and  136  to move with plate  138  away from backing plate  132 . The resultant spacing of dies  134 ,  136  from plate  132  need not be great, such as from a few millimeters up to about 15 mm, as it primarily is to free dies  134 ,  136  for movement to their retracted position thereby releasing sprue metal solidified in die region  130 .  
     [0084]FIG. 14 shows an alternative form of sprue region suitable for use in the arrangement of FIG. 13. Corresponding parts have the same reference numeral plus 100. However the principal difference warranting attention is the form of the sprue region  230 . As shown, region  230  is defined by sprue dies  234  and  236  each formed integrally of a suitable tool steel. The region  230  has a maximum cross-section intermediate its ends from which it tapers on each axial direction to define inlet portion  230   a  which tapers outwardly in the alloy flow direction for casting, and an outlet portion  230   b  with tapers inwardly in that direction. However, portion  230   b  ends short of the end at which region  230  communicates with the die cavity, and is followed by a terminal portion  230   c  which again tapers outwardly in the flow direction for casting. Portion  230   c  has a small volume relative to either of portions  230   a  and  230   b , while the waist between it and portion  230   b  is such that solidified sprue metal can be broken or sheared to leave only the metal solidified in portion remaining on the casting. The breaking may be achieved by one of the sprue dies  234 ,  236  returning to, and most preferably beyond, its advanced position to impact against the sprue metal or by dies  234 ,  236  before separating, moving in unison to shear the sprue metal from its casting. As with other embodiments, there may be some movement of the sprue metal axially of the sprue region  230  whereby a larger diameter section of the sprue metal is engaged by a part of one die at which a smaller diameter section of the region  230  is defined.  
     [0085]FIG. 15 shows one sprue die  310  of an opposed pair. The die  310  is illustrated in end elevation to show its end face  312  which, in use, is opposed to and abuts against the corresponding face of the other die of its pair. Thus, sprue die  310  has a longitudinal extent disposed at right angles its face  312  and it is movable between its advanced and retracted positions in a direction at right angles to the plane of FIG. 15.  
     [0086] Face  312  of sprue die  310  has been machined to provide a V-shaped groove system  314  which comprises one half of a sprue region defined by the opposed dies when in their advanced position. Groove system  314  has an enlarged inlet end  314   a  at which it is able to be in communication with a molten alloy supply nozzle depicted schematically at  316 . From end  314   a , system  314  has diverging arms  314   b , each for forming a sprue runner for a common or respective die cavity. With the opposed dies in their advanced position, a respective gate may be defined at the end of each arm  314   b , or each arm may lead to a respective main runner. Thus, the surface  318  of die may form a die cavity surface or it may be spaced from the die cavity by a further tool part.  
     [0087]FIG. 16 is similar to FIG. 15 and corresponding parts have the same reference numeral plus  10 . As shown, sprue die  320  differs in that the system  314  formed in its face  312  has only a single arm  314   b  diverging from the line of its inlet end  324   a . In the arrangement of FIG. 16, the surface  328  defines part of a die cavity to which a runner defined by arms  324   b  of the opposed dies opens for direct injection. As shown, the arm  324   b  is parallel to a surface  329  of an adjacent die tool part, such that injected alloy is able to flow across surface  329 .  
     [0088]FIG. 17 is similar to FIG. 16 and corresponding parts have the same reference numeral plus  10 . In this case, arm  334   b  of system  334  is arcuate and its end remote from inlet end  334   a , in the orientation shown, is at a top surface  337  rather than at a side surface  328 . It is surface  337  which defines part of a die cavity and the arcuate form of arm  334   b  corresponds substantially to a curved surface  339  defined by an adjacent tool part which also defines part of the die cavity. Thus, injected alloy is able to maintain a curved path in its flow across surface  339 .  
     [0089]FIG. 18 shows a variant on the arrangement of FIG. 15 and corresponding parts have the same reference numeral plus  30 . With sprue die  340 , the inlet end  344   a  of groove system  344  is somewhat longer. Also the diverging arms  344   b  are bent to define arcuate portions curving oppositely from end  344   a , and a respective linear portion extending parallel to surface  348 . The linear portion of each arm  344   b  communicates with surface  348  via a pair of gates  347  such that each arm can provide two inlets to a respective die cavity or to a common die cavity.  
     [0090] FIGS.  15  to  18  illustrate the design flexibility permitted by use of the sprue dies of the invention. The sprue region is able to be shaped and directed as required to meet the needs for a given casting. Also, the sprue dies are relatively inexpensive to produce, enable a significant reduction in capital costs, particularly for a short run production life.  
     [0091] The sprue system of the invention is particularly well suited to a direct injection form of casting. In this, the sprue region defined by the sprue system is able to communicate directly with a die cavity or with multiple die cavities. In this regard, the flexibility of design illustrated by FIGS.  15  to  18  is such that erosion of die tools and of sprue/runner systems is able to be minimised, overcoming a major disadvantage of previous attempts at producing quality castings by direct injection. In the latter regard, it is to be noted that the ASM Handbook, Chapter 15, entitled “Casting” indicates that, as recently as its April 1996 third printing, the process of direct injection was still under development.  
     [0092] Finally, it is to be understood that various other modifications and/or alterations may be made without departing from the spirit of the present invention as outlined herein.