BACKGROUND TO THE INVENTION Spin casting is a technique which has been used for the production of articles in low melting point metals such as zinc. The mould used is made of hard rubber and comprises an upper part and a lower part each of which is in the form of a disc. The mould cavities are formed by recesses in the top surface of the lower part which register with recesses in the bottom surface of the upper part when the upper part is placed on the lower part. Once in register the mould parts are clamped between upper and lower plates.

The upper mould part and the upper plate have registering central openings into which molten metal is poured. The lower mould part does not have a central opening but is unapertured. Thus molten metal entering via said central openings drops onto the lower mould part. Runners cut in the lower mould part extend outwardly from the central opening in the upper mould part to the mould cavities which are arranged in an array around the common vertical axis of the two parts.

The registering mould parts and the two plates are then spun about the common vertical axis of the mould parts so that the molten metal flows outwardly from the central opening along the runners to the mould cavities.

After spinning, the upper and lower plates are separated and the registering mould parts removed from between the plates. The upper mould part is lifted-off the lower mould part leaving the casting exposed on the lower mould part. The casting is then lifted off the lower mould part, the lower mould part being distorted if necessary to free the casting. The cycle described is then repeated.

The present invention seeks to provide improved spin casting apparatus and an improved spin casting method which can be used particularly, but not exclusively, with high melting point metals such as aluminium.

BRIEF DESCRIPTION OF THE INVENTION According to one aspect of the present invention there is provided spin casting apparatus which comprises an upper mould plate mounted for rotation about a vertical axis, a lower mould plate mounted for rotation about said axis, means for displacing said plates towards and away from one another between a mould closed position and a mould open position, ejector pins for ejecting a casting, and means for retracting said ejector pins to inoperative positions as the mould plates are moved towards one another and for advancing said ejector pins to operative positions as the

mould plates are moved apart.

The spin casting apparatus can include means for lifting and lowering said lower mould plate, means on which said ejector pins are mounted, an operating element for displacing the ejector pin mounting means downwardly with respect to said lower mould plate as the lower mould plate is lifted, and a further operating element for lifting said mounting means with respect to said lower mould plate as the lower mould plate is lowered. Preferably each ejector pin comprises a shank and a head and the mounting means therefor comprises a pair of plates with said heads between them, the shanks of the ejector pins passing through the upper plate of said pair of plates.

In a specific form said operating element for displacing the mounting means downwardly comprises an element mounted on said upper plate of said pair of plates and extending upwardly toward said upper mould plate so that it is forced down with respect to the lower mould plate when its upper end encounters the upper mould plate as said lower mould plate is lifted. In this form said operating element for lifting said mounting is carried by a base plate which is itself below said pair of plates, the operating element which lifts said mounting passing through the base plate and being displaceable vertically with respect thereto to displace the mounting means upwardly with respect to the lower mounting plate as the lower mounting plate is lowered.

The displacing means can comprise a pneumatic cylinder having a rod, a

thrust bearing to one race of which said rod is connected, and a shaft connected to the other race of said thrust bearing, said lower mould plate being mounted on said shaft and being displaceable vertically with said shaft. The apparatus can further comprise a hollow column, bushes within said column, said shaft sliding axially within said bushes, and means for rotating said column, said upper mould plate being connected to said column for rotation thereby.

According to a further aspect of the present invention there is provided a spin casting method which comprises feeding molten material to a central space of a mould comprising upper and lower mould parts, spinning the mould so that the material flows outwardly from said space to a mould cavity to fill the cavity, separating said mould parts, and advancing ejector pins from retracted inoperative positions to advanced operative positions as the mould parts are separated thereby to eject the casting from the mould cavity.

BRIEF DESCRIPTION OF THE DRAWINGS For a better understanding of the present invention, and to show how the same may be carried into effect, reference will now be made, by way of example, to the accompanying drawings in which:- Figure 1 is a diagrammatic side elevation of spin casting apparatus in accordance with the present invention; and

Figure 2 illustrates a detail of the apparatus of Figure 1 to a larger scale.

DETAILED DESCRIPTION OF THE DRAWINGS The spin casting apparatus 10 illustrated in Figure 1 comprises a framework only some parts of which have been shown. More specifically, the illustrated parts of the framework comprise a lower channel 14, an upper channel 16 and an intermediate channel 18.

A vertically positioned pneumatic cylinder 18 is mounted on the channel 14. The body of the cylinder is designated 20 and its piston rod 22. The lower race 24 of a thrust bearing 26 is connected to the upper end of the rod 22.

A rotatable structure 28 comprises a hollow column 30, a base plate 32 welded to the upper end of the column 30, a plurality of posts 34 arranged in a circular array around the base plate 32, and a top plate 36 secured to the tops of the posts 34.

The column 30 is supported by and rotatable in bearings 38 and 40 which are carried by the channels 16 and 18. A pulley 42 is secured to the column 30 and connected by a drive belt 44 to an electric motor and gear box designated 46.

It will be understood that the column 30, base plate 32, posts 34 and top plate 36 are all rotated by the motor and gear box 46.

Within the column 30 there are upper and lower bushes 48 and 50 through which a shaft 52 passes.

The shaft 52 has its lower end connected to the upper race 54 of the thrust bearing 26.

The upper end of the shaft 52 is fast with a lower mould structure designated 56 (see also Figure 2). The structure 56 comprises a bottom plate 58 and an array of columns 60 which join the bottom plate 58 to a lower mould plate 62. A mould 64 having runners 66 and recesses 68 cut therein is bolted to the plate 62. This facilitates changing of the mould 64 when other articles are to be manufactured.

Two further plates 70 and 72 are located between the upper and lower plates 58 and 62. The plates 70 and 72 are restrained from moving sideways by the columns 60 which pass through them.

At least one ejector pin 74 having a shank 76 and a head 78 is provided in conjunction with each recess 68. Preferably there are a number of ejector pins per recess. The shanks 76 of the pins 74 pass through the plate 62 and the plate 70, the heads 78 of the pins 74 being between the plates 70 and 72. The ends of the shanks 76 remote from the heads 78 enter the mould recess 68 from below.

Ejector pin operating elements 80 are carried by the plate 58, the elements 80 having heads 82 and shanks 84. The heads 82 are above the plate 58, the shanks 84 passing through the plate 58. The lower ends of the shanks 72 co- operate with the plate 32. Further ejector pin operating elements 86 are carried by the plate 70 and protrude upwardly above the mould 64. Only one of the elements 86 has been shown.

An upper mould 88 is bolted to the underside of an upper mould plate 90 which is itself bolted to the underside of the top plate 36. Recesses 92 are cut in the underside of the mould 88. The plates 36 and 90 and the mould 88 have central openings 94,96 and 98 which register with one another and form a passage 100 through which molten metal enters the casting apparatus. It will be noted that, when the moulds 64 and 88 are in contact, the mould 64 bounds the lower end of the passage 100.

In use of the apparatus, the parts are initially as shown in Figure 2 of the drawing. The cylinder 18 is used to push the shaft 52 upwardly. The entire lower mould structure designated 56 is thus lifted until the lower mould 64 abuts the upper mould 88. The recesses 68 in the lower mould thus register with the recesses 92 in the upper mould 88 to form the mould cavities. The runners 66 join the passage 100 into which molten metal is poured to the mould cavities. By pressing the moulds 66 and 88 together they are rendered fast in rotation with one another.

As the structure 56 lifts the upper ends of the elements 86 encounter the underside of the mould 88. Their movement is thus stopped and they slide with respect to the plate 62. In so doing they move the plates 70 and 72 downwardly with respect to the plate 62. This has the effect of retracting the ejector pins 74 from the mould cavities.

A charge of molten metal is poured into the passage 100. The structure 28 is then spun by means of drive transmitted to the column 30 from the motor and gear box 46 via the belt 44 and pulley 42. Because the moulds are in face-to-face contact, the lower mould structure 56 also rotates. The molten charge flows outwardly from the central passage 100, through the runners 66 and into the mould cavities. A ring of material remains in the passage 100 and joins the inner ends of the strips of material which remain in the runners 66.

After spinning, the motor is switched off and the rod 22 retracted to move the structure 56 downwards away from the upper mould 88. The lower ends of the shanks 84 of the elements 80 encounter the plate 32 before the structure 56 reaches its lowermost position. These elements thus stop moving but the remainder of the structure 56 continues downwardly. The stationary heads 82 are encountered by the underside of the plate 72 and as a consequence the plates 70,72 are in effect lifted with respect to the lower mould 64. As the lower mould continues to move down the shanks 76 of the pins 74 enter the mould cavities and lift the casting away from the

mould 64. The casting can be gripped using tongs and removed from its position on the mould 64. The cycle is then started again by raising the structure 56 by means of the cylinder 18.

To compensate for misalignment of the moulds 64 and 88, the shaft 52 is welded to a plate 102 which is itself connected to the plate 58 by rubber bushes 104.

Thus as the structure 56 encounters the mould 88, some tilting of the structure 56 is possible.

The plate 58 can include two lugs 106 which protrude outwardly therefrom and lie one on each side of a post 34. The lugs 106 thus form guides which ensure that the moulds 64 and 88 are always in approximate alignment. Final alignment can be by means of tapering pins (not shown) on the plate 62 which enter tapering bores (not shown) in the plate 90.