GB2409423A - Metal casting apparatus with low permeability gypsum mould - Google Patents

Abstract

A casting apparatus 1 for the casting of metal articles, comprises an impermeable disposable gypsum mould 8 having a mould cavity 12, a chill plate 9 having a surface abutting a surface of the mould and defining a wall of the mould cavity and a metal pump 4 connected to the mould cavity by a runner 14 and adapted to pump molten metal 5 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. A mould assembly for the casting of metal articles is also provided, comprising 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.

Description

2409423 e

METAL CASTING APPARATUS AND METHOD

This invention relates to a metal casting apparatus and method, and more particularly to a metal casting apparatus and method for the production of high volume, high surface finish, cast metal articles.

Metal casting is an ancient technology that has evolved down the centuries. Traditionally molten metal l0 has been poured into a would 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 WO99/28065 and :: :e.: ::: * 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 Carl 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.

rI'he irsl of these conc-etr-r, the presence of air bubbles in the gypsum molds. Ihesc rcdcc. the surface cItl-Ii r.y c,L Or- ,.astir'rY arlct i ll<r<?<i.- f.-ITIC>Lt(!. ').[ lic.rl finishing which must be applied to t} e castinc3 after mould '^err<,val. Some defects of course cannot be repaired and the article must then be discarded.

:: :. . .: ::: ::: I:: :: at: . . 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 moul.ds 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.

I-\ third problem is that the metal flow characteristics are not optimised and filling of the mould its 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.

1'. i: Or t- t y: t-ji! l carts c.>i'<-?r ins .c ?:?^?sr_cf? At r Is i diva 1 e.r?-..aied Gases ire 1-r.> Rolls- mr.l:al and, ill Lee case r'L al.unini..m allr,vs, nucleation of hyrirr-en. the low p..?ssures :>r t-> art.:.ial prf.-?..c;s.r-es users in the porr.'us gypsum rnoulrl casting process are often insurLi.cir.nt to eliminate the.,,.. ert.aineri drfer:rx fr:r,m the can' :arts, l.eadirlg lo low neGh,-ri.cal. strength air:. poor faticJue properties.

Fossi.ble ways of reducing the voids caused by gas entrainment include, for example, increasing the porosity of the mc;uld and/or increasing the pressure under which c 1 ' ::: c:: :: ac: ee. e he 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 y-,ermeabitit-y 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 cr low iermeatility 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.

eee 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 mound 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 could cavi.ti.es.

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

A broad range of metals can be cast using the apparatus and method of the invention. It is part.icularly suitable for the casting of light metals . cafe e: ace: 1 1 c t 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 alumini.um silicon alloys, fibre or particulate reinforced aluminium alloys, and semi-sol.id al.umi.nium alloys. Exarinp.e, Cot pr:eLerre,::l alloys rcl.:d<> .30() an<:.l 5()0 se rices.-9 lumini.m all<,ys.

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"

En this specification is meant a mould which has a

reduced tendency to permit molten metal ingress into the mo.:ld wall during pouring. The impermeable or low permeability gypsum mould, after drying and hardening, preferably has an air permeability of less than 10 x 10-3 m2, more preferably less than 5 x 10-3 m2, and most preferably less than 1. x 10-3 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 f C I I, / '. a ''s $ # , r opposition, and/or by the omission of the usual foaming agents, frothing agents, or other poreforming materials which are conventionally added to the moulding composition in order to introduce porosity into the could.

Preferred moulding compositions comprise fi.nely- 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 fables; and organic fillers, for example, wood particles and fibres.

Preferably the moulding composition comprises from -10 to 99.9 % by weight calcified gypsum. Preferably the moulding composition comprises from O to 30 % by weight wollastonite, () to 5 ino:anic lubricant, and 0 to 5 by weight cement. A particularly preferred moulding composition and method of producing a gypsum moul.d 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 :: :; te t: t r I C..

1, , 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:ir,vention, 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 would 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 23, 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 abuLLing 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 clcirr,->s, but the : l a' I:' :: :: I:e a'I: 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 would and the bottom surface of the chill plate.

[]sually 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 moul.d/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 actrAa.l physical contact. llr-c<.ssary, vent plus Ca!] al; be pr<,virled In the cil-l pl.,;. sumac., and where..,r-esent terse are prc.ferably l<;ec] <.:,vc" =,rr,<....l;t c. it.y.

In a preferred embodiment, the molten metal is purnESed 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 disE,i.-..a Amen, pump, since these can deliver hi.f.3her pressures. The pump can be wholly or partl.y 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 r I a ce ëc /.5 psi, and most preferably at a pressure of from 10 to psi. Very good results have been achieved using pressures of: from 20 to 40 psi.

I'hough 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 pimp.

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 mou]d 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 would 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:t, more preferably at least l'):-]. 'the reservoir is also preferably large in cross sectional area in comparison to the cross sectional area of the molten e C 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 tiers aspect of the invention, the reservoir is substantially dish-shaped, with As 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 discshaped reservoir. The mould cavity flli.ng 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 molter, metal from the molten metal inlet by allowing the molten metal. to flow latera'ly as it enters the reservoir The molten metal in the reservoir will generally sc.lidlfy 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.

L'referab].y the reservoir extends to at least 80%, more preferably at least 90%, of the area of the bottom surface of the mound, but is relatively shallow.

Preferably the reservoir has a depth of less than 2.0 cm, more preferably less than '.'., cm. Most preferably the reservoir has a depth of from 1.. () to ().5 cm.

Preferably the filling holes or gates of the Would cavities are protected by a fairer that prevents impurities such as slag or inclusions in the metal from entering the could cavities. The filter can comprise, ë ate ': ace: a.

for example, a filter c cth, which can be disposed or stretched across the bottom sur ace Of the mould to remove any such unwanted impure ies.

As the molten metal i s pumped o' drawn into the mould caviti es from the reservoir, air and entrained gases are ate] e to escape along the interface between the top surface of the gypsum molly and the bottom surface of the chi l l plate.

Si ace the pressure at each mould cavi ty filling hole or gate is substantially the same, the would cavities fill simultaneously and at substanti ally the same rate.

By "simultaneously" in t his specification its meant that the could cavities are filled a' substantially the same time, although it does not exclude she possibility that the would cavities may be fi l.'.ed seg, uentially provi deaf that the time interval oe, weed r e is 2 ling of adjacent cavities i s relatively small, consist en' with obtaining at east one of the benef its of the invention recited herein.

It will be apprec ai.-ed that with the new mould assembly of the i nventi on, a single carrier plate and runner system can be used for moot ds raving different sizes and numbers of mould cavities, thereby reducing changeover time and laborer costs.

One embodiment of a casting apparatus, would 3G assembly and method according to the invention will now be described by way of example only with reference to the accompanying Drawings i n whi Oh: i, he 1 c. , ., C Figure l 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 l with the chill plate removed; and Figure 3 shows a sectional side elevation of the mould assembly of Eigure 1 along the line X-X of Figure 2.

lO Referring firstly to Figure l, the casting apparatus illustrated generally at l 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 its partly submerged in the molten metal 5. In order to avo d oxidation the molten metal 5 is protected by on inert atmosphere (not shown).

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

The mould assembly 3 comprises gypsum mould 8 surmounted by a chill plate 9 and supported on a carrier plate lO having a ceramic insert 4; defining a reservoir runner l4.

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 rr;oud 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 moul.d cavities 12 extend throughout the 'thickness of: "-e mould 8, as can be seen in Figure l and are open at: the top and the bottom. c

t. a. . . . . , . . . . . . . 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 fil.llng holes 13 open into a reservoir runner 14 which is bounded by the bottom surface 15 of the mould and the di.sh-shaped ceramic insert 11. The circular shape of the reservoir is indicated by the broken tine 16 in Figure 2.

A perforated filter clot-in 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 imp,rities from entering the mould cavities.

lI..ez:rli-ivc2.y, c.r in addiction, c, per-'rc?.t-ed filt:er- (not st;, wn) coulc3 lee' <range<3 ketwee->n the -.t:let of the metal p-urr:r1 he veservo.i..- iz--:et. pipe 6.

In the casting method of the invention, molten metal (in this case al.uminium alloy) is supplied from the bath 2 by the high pressure pump vla the pipe 6 to the metal inlet 7 of the mould assembly. On ezliering 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 equaiise the fluid pressure at the filling holes 13 ensuring that each of the mould cavities is filled at approximate'..y the same rate (desirably at a flow ve'oc-ty of less than 0.5 metres per second). Despite the tight restrictions imposed by the dimensions of the noilo cavities, the fluid pressure of the metal (of the order of 10 to 50 psi) is sufficient to fill the would cavities completely, up to the surface of the chit' plate 9. The high fluid pressure drives any air or entrained gases to the top of each mould cavity, from whence the aid Or gas can escape t. a: e

C

along the interface 18 between the chill plate 9 and the mould S. After the metal has cooled the would its separated from the chill plate and the carrier plate and the solidified reservoir of the runner s cut off. The would is broken and the cast almini.urn 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 ail such papers and documents are incorporated herel.n by reference.

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

Each feature disclosed in this specification

(incl.uing 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.

i l e . u.

* 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.

Claims (47)

. . . . . . . . . e ece e CLAIMS

1. A casting apparatus for the casting of metal articles, which corDpri. ses an impermeable or low permeability disposable gypsum mould having a mould cavity, a chit'. 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 would cavity by passing between the abutting surfaces of the chill plate and the mould.

2. A casting apparatus according to claim 1, wherein the mould cavity has an undercut region.

3. A Casting apparatus according to claim 1 or 2, wherein the impermeable or low permeability gypsum mould, after drying and hardening, has an air permeability of less than lO X 10-13 m2

4. A casting apparatus according to claim 1 or 2, wherein the impermeable or low permeability gypsum mould, after drying and hardening, has an air permeability of less than i, x 1C-3 m2.

5. A casting apparatus according to any one of the preceding claims, wherein the impermeable gypsum would comprises from 70 to 99.3 by weight calcified gypsum, from O to 3C by weight woLlastonite, from :: :. . c: ::: C to ^.; inorganic lubricant, and from O to 5 % by weight cement.

6. A casting apparatus according to any one of the preceding claims, wherein the impermeable or low permeability disposable gypsum mould comprises a plurality of mould cavities.

7. A casting apparatus according to claim 6, wherein lO the number of cavities is from 5 to 25.

8. A casting apparatus according to any one of the preceding claims, wherein the mould is disc-shaped, with flat top and bottom surfaces.

9. A casting apparatus according to any one of the preceding claims, wherein the mould cavities are open-ended and extend throughout the thickness of the mould from the top surface to the bottom surface thereof.

10. A casting apparatus according to claim 9, wherein the mould cavities are distributed across the top surface of the mould and disposed side-byside with their filling holes or gates on the underside or bottom surface of the mould.

11. A casting apparatus according to claim 10, wherein the filling holes or gates of the mould cavities are protected by a fitter which prevents slag or inclusions in the metal from entering the mould cavities.

l , 1 1 8 8 C <

12. A casting apparatus according to any one of the preceding claims, wherein the chill plate is disposed in abutting relationship with the top surface of the mould and both the top surface of the mould and the abutting bottom surface of the chill plate are flat.

13. A casting apparatus according to any one of the preceding claims, wherein the abutting chill plate surface covers the open mould cavity or cavities and defines a top wall of the mould cavity or cavities.

14. A casting apparatus according to any one of the preceding claims, wherein the chill plate has a ceramic coating that permits air and entrained gases to escape from the mould cavity or cavities along the interface between the mould and the chill plate durTrlc: filling of fife mould.

29

15. A casting apparatus according to any one of the preceding claims, wherein the pump is a high pressure dis,La.ernent pump.

16. A casting apparatus according to any one of the preceding claims, wherein the pump provides molten metal under a pressure of from 10 to 50 psi.

17. A casting apparatus according to any one of the preceding claims, substantially as hereinbefore described with reference to and as illustrated in the accompanying Drawings.

18. A casting apparatus substantially as hereinbefore described.

te.:. A: c.d e. :e ë:

19. 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 chil' 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.

20. A method according to claim]9, wherein the metal article is an impeller blade.

21. A method according to claim IS or 20, wherein the metal. is an al. uminium alloy.

22. A method according to any one of claims 19 to 21, wherein the pump provides molten metal under a pressure of from 10 to 50 psi.

23. A method according to any one of claims 19 to 22, wherein the molten metal flow velocity into the could cavity is less than 0.5 mist

24. A method according to any one of claims 19 to 23, wherein there is used a casting apparatus according 3() to any one of claims 1 to 17.

25. A method according to any one of claims 19 to 24 substantially as hereinbefore described.

ë c I : :. c:: :: :: B. c. .:. c. e:

26. 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.

27. A mould assembly according to claim 26, wherein the mound is an impermeable or low permeability disposable gypsum mould.

28. A could assembly according to claim 26 or 27, wherein each mould cavity is provided with an undercut region.

29. A mould assembly according to any one of claims 26 to 28, wherein the metal inlet is connected to a pollen metal pump.

3(). A mould assembly according to any one of claims 26 to 29, wherein the runner provides a reservoir which comprises a region of cross sectional area that is large in comparison to the cross sectional area of the mould cavity filling holes or gates.

31. A mould assembly according to any one of claims 26 to 30, wherein the ratio of the cross sectional area of the reservoir to the cross sectional area of any one of the mould cavity filling holes or gates is at least 5:l.

32. A mould assembly according to any one of claims 26 to 31, wherein the reservoir is large in cross 1 1 2 2 ' ' ' ' ë 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.

33. A mould assembly according to any of claims 26 to 32, wherein the runner comprises a reservoir which is substantially dish- shaped, with its upper surface bounded by the lower surface of the mould and its extent defined by a support member whose upper surface comprises a dish-shaped reservoir cavity.

34. A mould assembly according to claim 33, wherein the support member comprises a carrier plate with a ceramic insert that defines the dimensions of the disc-shaped reservoir.

35. A mould assembly according to any one of claims 26 to 34, wherein the mould cavity filling holes or gates are open to, or depend Pinto, the reservoir.

36. A oul.d assembly according to any one of claims 26 to 35, wherein the reservoir extends to at least 80% of the area ol: the bottom surface of the mould.

37. A mould assembly according to any one of claims 26 to 36, wherein the reservoir has a depth of from 0.5 T_ 0 2.() c.n:.

38. A mould assembly substantially as hereinbefore described with reference to and as illustrated in the accompanying Drawings.

c c t e a C

39. A mould assembly substantially as hereinbefore described.

40. A method for the casting of metals, 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.

41. A method according to claim 40, wherein the reservoir acts to equalise the pressure and hence the flow rate at each filling hole or gate.

42. A method according to claim 40 or 41, wherein the reservoir also functions to reduce the flow velocity of molten metal from the molten metal inlet by allowing the molten metal to flow 'aterally as it enters the reservoir.

43. A method according to any one of claims 40 to 42, wherein the pressure at each mould cavity filling hole or gate is substantially the same, and the mould cavities fill simultaneously and at substantially the same rate.

44. A method according to any one of claims 40 to 43, wherein there is used a mould assembly according to any one of claims 26 to 39. :

45. A method according to any one of claims 40 to 44 substantially as herei nbefore described.

4 6. A casti ng apparatus act ording to any one of claims 1 to 18, in which there is used a would assembly according to any one claim 26 to 39.

47. A could assembly for the casting of metal articles, which comprises a carrier plate, a runner system and a metal inlet, the runner system being adapted to connect the inlet lo a plural ity of cavities in an int exchangeable could and so arranged that the carri er plate and runner system can accommodate moulds having different numbers and sizes of mould cavities.

4 8. A cast metal article produced using an apparatus according to any one of claims 1 to 18 and 46, a method according to any one of claims 19 to 25, a mould assembly according to any one of claims 26 to 39 and 47, or a method according to any one of claims 40 to 45.