Metal casting is an ancient technology that has evolved down the centuries. Traditionally molten metal has been poured into a mould and allowed to cool, whereupon the mould is broken to reveal the cast metal article.

Disposable sand moulds, wherein the sand is held together by a binder, are porous, permitting air and gases to escape from the mould during pouring of the metal, and thereby producing good, sound castings.

However sand moulds cannot easily produce a cast metal article with undercut regions and the surface finish is generally poor.

In high-pressure die casting molten metal is pumped under pressure into a re-usable metal mould. This process is used for high volume production of high surface finish, cast metal articles, but where the cast metal article has undercut regions the moulds become complex, expensive to manufacture and difficult to operate. High- pressure die-casting is also relatively inflexible and cannot easily accommodate the production of large numbers of cast metal articles where the articles have a variety of shapes and sizes. Examples of high-pressure die- casting methods are described in W099/28065 and W002/16062.

Porous disposable gypsum moulds, produced by pouring a slurry of a gypsum plaster around a pattern, allowing the plaster to harden and then removing the pattern, have also been employed in metal casting. By employing a wax pattern in the so-called"lost-wax"or investment casting method, or by using a flexible rubber pattern, moulds can be produced with severe undercut regions. The porous gypsum mould can be filled with molten metal from the top by gravity pouring from a ladle, with or without vacuum assistance, from the bottom by low pressure counter- gravity pouring, with or without vacuum assistance, or by partial pressure counter-gravity pouring with or without vacuum assistance. Such methods can use a vacuum or a low pressure pump, operating at around 2-3 psi. Air and gases escape through the porous mould during mould filling and after the metal has solidified the mould is broken to reveal the cast metal article. The process relies on the porosity of the mould to fill thin sections. Porous gypsum moulds can be used in high volume production, and can produce articles with high surface finish and tight dimensional tolerances.

Although the use of porous disposable gypsum moulds has found application in certain markets, particularly in the manufacture of aluminium alloy articles such as impeller blades for automotive turbochargers, pumps and fans, some problems still remain.

The first of these concerns the presence of air bubbles in the gypsum moulds. These reduce the surface quality of the casting and increase the amount of hand finishing which must be applied to the casting after mould removal. Some defects of course cannot be repaired and the article must then be discarded.

A second problem is that in a multi-cavity mould casting process, the mould cavities of the gypsum mould need to be filled with molten metal independently, through individual ceramic nozzles or feeder heads. Each mould has its own feeder head arrangement, and frequent changing of the carrier plate is required to accommodate moulds having different sizes and numbers of mould cavities. In addition to the extra labour required, the ceramic nozzles can introduce contaminants and are very fragile and frequently broken during carrier plate changes. Expensive single-shot metal filters are also required in large quantities.

A third problem is that the metal flow characteristics are not optimised and filling of the mould is poorly controlled, leading to turbulence and associated casting defects. Structural integrity is low because feeding of the mould cavity during solidification is difficult. For these reasons the choice of potential alloys is restricted and generally limited to high fluidity hypoeutectic aluminium-silicon casting alloys.

A fourth problem concerns the presence of residual entrained gases in the molten metal and, in the case of aluminium alloys, nucleation of hydrogen. The low pressures or partial pressures used in the porous gypsum mould casting process are often insufficient to eliminate these entrained defects from the cast parts, leading to low mechanical strength and poor fatigue properties.

Possible ways of reducing the voids caused by gas entrainment include, for example, increasing the porosity of the mould and/or increasing the pressure under which

the molten metal is pumped into the mould. However both these solutions increase the amount of metal ingress into the porous mould wall, thereby substantially reducing surface quality.

According to one aspect of the present invention, there is provided an improved casting apparatus and method wherein molten metal is pumped under pressure into an impermeable or low permeability gypsum mould.

In a first aspect, the invention provides a casting apparatus for the casting of metal articles, which comprises an impermeable disposable gypsum mould having a mould cavity, a chill plate having a surface abutting a surface of the mould and defining a wall of the mould cavity and a metal pump connected to the mould cavity by a runner and adapted to pump molten metal into the metal cavity through the runner, the arrangement being such that when molten metal is pumped into the mould cavity under pressure, air and entrained gases can escape from the mould cavity by passing between the abutting surfaces of the chill plate and the mould.

In a second aspect, the invention provides a method for the casting of metal articles, which comprises pumping molten metal under pressure into a mould cavity in an impermeable or low permeability disposable gypsum mould, the mould having a surface abutting a surface of a chill plate and the chill plate defining a wall of the mould cavity, the arrangement being such that when molten metal is pumped into the mould cavity under pressure, air and entrained gases can escape from the mould cavity by passing between the abutting surfaces of the chill plate and the mould.

The invention also provides, in another aspect, a mould assembly having a novel runner system that provides a reservoir of molten metal extending beneath the mould from which molten metal can be pumped or drawn into a multi-cavity mould.

In a third aspect, therefore, the invention provides a mould assembly for the casting of metal articles which comprises a mould comprising a plurality of discrete cavities, a metal inlet and a runner, the runner connecting the inlet to the plurality of cavities and providing a reservoir extending in a plane beneath the mould from which in use metal can be pumped or drawn into the cavities simultaneously.

In a fourth aspect, the invention provides a method for the casting of metal articles, which comprises pumping or drawing molten metal into a mould comprising a plurality of discrete mould cavities, the metal being pumped or drawn through an inlet into a runner connecting the inlet to the plurality of mould cavities, the runner providing a reservoir extending in a plane beneath the mould such that the metal is distributed evenly between the mould cavities.

In a still further aspect, the invention also provides a cast metal article produced using an apparatus or a method according to any one of the first, second, third and fourth aspects of the invention.

A broad range of metals can be cast using the apparatus and method of the invention. It is particularly suitable for the casting of light metals

such as, for example, aluminium, zinc and magnesium, and for the casting of light metal alloys. The invention is particularly suitable for the casting of aluminium and aluminium alloys, including aluminium silicon alloys, fibre or particulate reinforced aluminium alloys, and semi-solid aluminium alloys. Examples of preferred alloys include 300 and 500 series aluminium alloys.

The invention is particularly applicable to the production of aluminium alloy articles such as impeller blades for automotive turbochargers, pumps and fans and will henceforth be more particularly described with reference thereto. It is to be understood, however, that the invention is not limited to the casting of aluminium alloys, or to the production of impeller blades, but is generally applicable to the casting of molten metals and the production of a wide range of cast metal articles.

The invention is particularly applicable to the production of high quality cast metal articles having severe undercut regions.

By an"impermeable or low permeability gypsum mould" in this specification is meant a mould which has a reduced tendency to permit molten metal ingress into the mould wall during pouring. The impermeable or low permeability gypsum mould, after drying and hardening, preferably has an air permeability of less than 10 x 10-13 m2, more preferably less than 5 x 10 13 m2 and most preferably less than 1 x 10-13 m2. In preferred embodiments of the invention, it has been found that the surface finish of the casting can be greatly improved by using a substantially impermeable gypsum mould. The gypsum mould can be rendered impermeable or of low permeability by an appropriate choice of moulding

composition, and/or by the omission of the usual foaming agents, frothing agents, or other pore-forming materials which are conventionally added to the moulding composition in order to introduce porosity into the mould.

Preferred moulding compositions comprise finely- divided particulate inorganic water-hardenable mould forming gypsum-based materials, for example, gypsum, calcined gypsum, plaster of Paris and anhydrous calcium sulphate. In addition, the moulding composition can comprise wollastonite, cement, lime, other similar plaster-forming materials, and mixtures thereof. The moulding composition can also contain inorganic aggregates and inert fillers, for example, sand, perlite, magnesium oxide and vermiculite; inorganic fibres, for example glass, alumina or boron carbide fibres; and organic fillers, for example, wood particles and fibres.

Preferably the moulding composition comprises from 70 to 99.9 % by weight calcined gypsum. Preferably the moulding composition comprises from 0 to 30 % by weight wollastonite, 0 to 5% inorganic lubricant, and 0 to 5 % by weight cement. A particularly preferred moulding composition and method of producing a gypsum mould is described and claimed in co-pending UK Patent Application No. 0326999.0 (Agents reference no. P104232GB).

The moulding composition is preferable one that hardens on contact with water, although other liquids, for example, organic liquids, are not excluded. The moulding composition is preferably used in the form of a slurry, which can comprise, for example, a mixture of

from 30 to 70 % by weight of particulates, and is poured around a pattern and allowed to harden to form the mould.

Whilst for many applications impermeable disposable gypsum moulds are preferred, in certain embodiments of the invention, and notably in the third and fourth aspects of the invention, other types of mould and mould composition could be used and the mould need not necessarily be disposable.

The mould can comprise a single cavity, but preferably comprises a plurality of mould cavities, for volume production. The cavities can number, for example, from 3 to 30, preferably from 4 to 28, more preferably from 5 to 25, most preferably from 7 to 22-The mould can be of any suitable shape, but is conveniently disc- shaped or rectangular slab-shaped, with flat top and bottom surfaces. The mould cavities are preferably open- ended and preferably extend throughout the thickness of the mould from the top surface to the bottom surface thereof. Preferably the mould cavities are of equal size. In a particularly preferred embodiment, the mould cavities are distributed across the top surface of the mould and disposed side-by-side with their filling holes or gates on the underside or bottom surface of the mould.

Preferably the chill plate is disposed in abutting relationship with the top surface of the mould.

Preferably both the top surface of the mould and the abutting bottom surface of the chill plate are flat. The abutting chill plate surface covers the open mould cavity or cavities and defines a wall, preferably a top wall, of the mould cavity or cavities. The chill plate is firmly secured to the mould, for example, using clamps, but the

abutting mould and chill plate surfaces are adapted such that they do not form a completely gas-tight seal. As the molten metal is pumped or drawn into the mould cavities from the reservoir, air and entrained gases are able to escape along the interface between the top surface of the gypsum mould and the bottom surface of the chill plate.

Usually there will be enough clearance between the abutting surfaces for this evacuation to take place, although if necessary the surfaces can be roughened to increase the permeability of the interface. In a preferred embodiment, the chill plate has a ceramic coating that permits air and entrained gases to escape from the mould cavity or cavities along the mould/chill plate interface during filling. If necessary a porous gasket could be provided between the mould surface and the chill plate surface. In this specification, the term "abutting"includes this possibility and is not to be construed as limited only to actual physical contact. If necessary, vent plugs can also be provided in the chill plate surface, and where present these are preferably located above each mould cavity.

In a preferred embodiment, the molten metal is pumped under high pressure into the mould cavity or cavities. The metal pump can be of any suitable type capable of delivering molten metal at high pressure, and can be, for example, mechanical or electromagnetic.

Preferably the pump is a high-pressure mechanical displacement pump, since these can deliver higher pressures. The pump can be wholly or partly immersed in a metal bath. If necessary the pump and the metal bath can be protected by an inert atmosphere to prevent metal oxidation. Preferably the pump delivers molten metal at a pressure of at least 5 psi, more preferably at least

7.5 psi, and most preferably at a pressure of from 10 to 50 psi. Very good results have been achieved using pressures of from 20 to 40 psi.

Though not normally preferred, it is also possible to draw the molten metal into the mould cavities using a vacuum system, or to use a combination of a vacuum and a low-pressure pump.

In a preferred embodiment, a molten metal pump delivers molten metal under pressure to a runner through a molten metal inlet. The runner can be a simple flow passage, especially where a single cavity mould is used, and is preferably configured to give a relatively slow mould filling rate. A metal flow velocity into the mould cavity or cavities of preferably less than 1.0 m/s, more preferably less than 0.5 m/s, is desirable. However, in a further and independent aspect of the invention, where a multi-cavity mould is used, the runner is preferably of a novel configuration providing a reservoir extending in a plane beneath the mould from which metal can be pumped or drawn into the mould cavities simultaneously.

In the mould assembly of the invention, the runner comprises a reservoir which can, for example, comprise a region of cross sectional area that is large in comparison to the cross sectional area of the mould cavity filling holes. Preferably the ratio of the cross sectional area of the reservoir to the cross sectional area of any one of the mould cavity filling holes is at least 5 : 1, more preferably at least 10: 1. The reservoir is also preferably large in cross sectional area in comparison to the cross sectional area of the molten

metal inlet so that the molten metal spreads laterally when entering the reservoir from the inlet.

In a preferred embodiment of the mould assembly of this aspect of the invention, the reservoir is substantially dish-shaped, with its upper surface bounded by the lower surface of the gypsum mould and its extent defined by a support member whose upper surface comprises a dish-shaped reservoir cavity. The support member can comprise, for example, a carrier plate with a ceramic insert that defines the dimensions of the disc-shaped reservoir. The mould cavity filling holes or gates are open to, or depend into, the reservoir which acts to equalise the pressure and hence the flow rate at each filling hole or gate. The reservoir also functions to reduce the flow velocity of molten metal from the molten metal inlet by allowing the molten metal to flow laterally as it enters the reservoir. The molten metal in the reservoir will generally solidify together with the metal in the mould cavities and will need to be cut off and discarded or re-cycled. Accordingly the volume of the reservoir is preferably kept as small as possible, consistent with the advantageous effects recited herein.

Preferably the reservoir extends to at least 80%, more preferably at least 90%, of the area of the bottom surface of the mould, but is relatively shallow.

Preferably the reservoir has a depth of less than 2.0 cm, more preferably less than 1.5 cm. Most preferably the reservoir has a depth of from 1. 0 to 0.5 cm.

Preferably the filling holes or gates of the mould cavities are protected by a filter that prevents impurities such as slag or inclusions in the metal from entering the mould cavities. The filter can comprise,

for example, a filter cloth, which can be disposed or stretched across the bottom surface of the mould to remove any such unwanted impurities.

As the molten metal is pumped or drawn into the mould cavities from the reservoir, air and entrained gases are able to escape along the interface between the top surface of the gypsum mould and the bottom surface of the chill plate.

Since the pressure at each mould cavity filling hole or gate is substantially the same, the mould cavities fill simultaneously and at substantially the same rate.

By"simultaneously"in this specification is meant that the mould cavities are filled at substantially the same time, although it does not exclude the possibility that the mould cavities may be filled sequentially provided that the time interval between the filling of adjacent cavities is relatively small, consistent with obtaining at least one of the benefits of the invention recited herein.

It will be appreciated that with the new mould assembly of the invention, a single carrier plate and runner system can be used for moulds having different sizes and numbers of mould cavities, thereby reducing changeover time and labour costs.

One embodiment of a casting apparatus, mould assembly and method according to the invention will now be described by way of example only with reference to the accompanying Drawings in which:

Figure 1 shows a diagrammatic representation of, a casting apparatus according to the invention in sectional side elevation ; Figure 2 shows a plan view of the mould assembly of Figure 1 with the chill plate removed ; and Figure 3 shows a sectional side elevation of the mould assembly of Figure 1 along the line X-X of Figure 2.

Referring firstly to Figure 1, the casting apparatus illustrated generally at 1 comprising a metal furnace bath 2 and a mould assembly illustrated generally at 3.

The metal bath 2 is provided with a high-pressure metal displacement pump 4 that is partly submerged in the molten metal 5. In order to avoid oxidation the molten metal 5 is protected by an inert atmosphere (not shown).

The metal pump 4 is connected via a pipe 6 to a metal inlet 7 of the mould assembly 3.

The mould assembly 3 comprises a gypsum mould 8 surmounted by a chill plate 9 and supported on a carrier plate 10 having a ceramic insert 11 defining a reservoir runner 14.

Turning now to Figure 2, the gypsum mould 8, which in this example is used for the manufacture of aluminium alloy impeller blades, has twelve mould cavities 12. The mould cavities have very severe undercut regions (not shown) and the individual elements of the impeller blades are of very thin section (not shown). The mould cavities 12 extend throughout the thickness of the mould 8, as can be seen in Figure 1 and are open at the top and the bottom.

As can be seen more clearly from Figure 3, the mould cavities 12 are surmounted by the chill plate 9, which forms the top wall of each of the mould cavities. The filling holes 13 open into a reservoir runner 14 which is bounded by the bottom surface 15 of the mould and the dish-shaped ceramic insert 11. The circular shape of the reservoir is indicated by the broken line 16 in Figure 2.

A perforated filter cloth 17 is arranged between the reservoir 14 and the filling holes 13 of the mould cavities 12 in order to prevent any slag or other impurities from entering the mould cavities.

Alternatively, or in addition, a perforated filter (not shown) could be arranged between the outlet of the metal pump and the reservoir inlet pipe 6.

In the casting method of the invention, molten metal (in this case aluminium alloy) is supplied from the bath 2 by the high pressure pump 4 via the pipe 6 to the metal inlet 7 of the mould assembly. On entering the reservoir runner 14 the molten metal spreads laterally and fills the reservoir before passing through the filter cloth 17 and entering the mould cavities 12 through the filling holes or gates 13. The reservoir acts to equalise the fluid pressure at the filling holes 13 ensuring that each of the mould cavities is filled at approximately the same rate (desirably at a flow velocity of less than 0.5 metres per second). Despite the tight restrictions imposed by the dimensions of the mould cavities, the fluid pressure of the metal (of the order of 10 to 50 psi) is sufficient to fill the mould cavities completely, up to the surface of the chill plate 9. The high fluid pressure drives any air or entrained gases to the top of each mould cavity, from whence the air or gas can escape

along the interface 18 between the chill plate 9 and the mould 8.

After the metal has cooled the mould is separated from the chill plate and the carrier plate and the solidified reservoir of the runner is cut off. The mould is broken and the cast aluminium alloy impeller blades recovered. The blades are found to be of excellent quality with no inclusions or defects. Minimal hand finishing is required before the cast impeller blades are ready for use.

The reader's attention is directed to all papers and documents which are filed concurrently with or previous to this specification in connection with this application and which are open to public inspection with this specification, and the contents of all such papers and documents are incorporated herein by reference.

All of the features disclosed in this specification (including any accompanying claims, abstract and drawings), and/or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and/or steps are mutually exclusive.

Each feature disclosed in this specification (including any accompanying claims, abstract and drawings), may be replaced by alternative features serving the same, equivalent or similar purpose, unless expressly stated otherwise. Thus, unless expressly stated otherwise, each feature disclosed is one example only of a generic series of equivalent or similar features.

The invention is not restricted to the details of any foregoing embodiments. The invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims, abstract and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed.