The present invention relates to the low pressure casting of metal, more particularly but not exclusively light metal alloys such as aluminium-based and magnesium-based alloys, with a casting weight up to about 15 Kg. The metal may be cast in expendible sand moulds, permanent metal moulds or dies, or combination moulds.

In a typical low pressure metal casting process, metal from a massive e.g. 200-1000 Kg pool of molten metal in a sealed reservoir is forced to flow up a riser tube by gas pressure applied to the surface of the metal pool, the molten metal thus being forced up the riser tube into a mould to fill the latter. The metal in the mould is allowed to cool and shrinkage losses are made up by further metal forced up the riser tube by gas pressure, given appropriate temperature gradients between the casting and feed points.

A low pressure casting method according to the present invention is characterised in that the assembly formed by the reservoir for the molten metal, the riser tube and the mould connected thereto is inverted after the mould has been filled and is maintained in this attitude during cooling of the metal casting in the mould.

As a result, gravity, assisted by residual gas pressure feeds additional metal into the mould to make up shrinkage losses. Furthermore, as the metal within the mould cools, its density increases to a value greater than that of the metal in the riser tube. Thus, the metal within the mould can solidify without the risk of generating convection currents between the metal in the mould and the metal in the riser tube, which would otherwise intrude at critical points in the solidification process.

Typically, the size of the reservoir will be such that it will hold sufficient metal for at most a few castings, to enable the assembly to be readily inverted. As an advantageous result, the apparatus may be quickly changed from casting one alloy composition to

another, without risk of contamination of the second alloy by remnants of the first.

The invention thus also provides a low pressure metal casting apparatus comprising a closed reservoir for containing a quantity of molten metal, a riser tube extending across the interior of the reservoir from an outlet in a wall of the reservoir to a mouth near the opposed wall surface of the reservoir, means for applying gas pressure within the reservoir and means for mounting a casting mould in communication with the outlet end of the riser tube, wherein the reservoir, riser tube and mould mounting means are mounted by means enabling them to be inverted as a unit during a casting operation.

Advantageously, the reservoir is separable into two parts for cleaning purposes and any other necessary treatment. The reservoir and riser tube are advantageously formed of an appropriate metal, such as cast iron for the casting of light alloys, a refractory wash being applied to the surfaces which come into contact with the molten metal. The riser tube can then be formed integrally with one half of the reservoir. To assist the application of the refractory wash to the internal surfaces of the riser tube, the latter preferably flares down from a relatively large mouth end to an appropriately small outlet end to feed the mould advantageously through a replaceable nozzle element. Appropriate ceramic materials could also be used.

The invention will now be further described by way of example with reference to the accompanying drawings, in which:

Figure 1 is a vertial section through casting apparatus in accordance with the invention in a first stage in a casting operation; and

Figure 2 is a vertical elevational view of the apparatus in a second stage of the casting operation.

The casting apparatus shown diagramatically in Figure 1 comprises a

spherical reservoir 1 formed by two hemispherical cast iron (or alloyed cast iron) shell halves 2 and 3 which fit together along a diametral dividing plane 4 in this case of a diameter approximately 300 mm.

Formed integrally with the reservoir shell 3 is a hollow neck 5 and a frusto-conical riser tube 6 which is coaxial with the neck 5 and terminates at its widest part in a mouth 7 adjacent the inner wall surface of the other shell half 2. Opposite the mouth 7, the shell half 2 is formed with a drain port 8, with suitable closure, to enable the reservoir to be drained at the end of a casting operation or series of casting operations.

The shell half 3 is formed with a charging port 9 for admitting further superheated molten metal, the charging port having a rapid acting stopper (not shown). Additionally, the shell half 3 is formed with a gas port 10 for the admission of a gas under pressure which is inert with respect to the metal to be cast. The inlet may be of porous ceramic or may be a mechanical self closing valve and may include a pressure release valve as well as a connection for a pressure gauge.

Each shell half 2,3 has mountings 11 for thermo-couples for monitoring the temperature of molten metal within the reservoir. Each shell half is surrounded by insulation 12 within respective sheet metal outer casings 13 and 14. The assembly formed by the shell half 2 and its casing 13, together with the insulation 12 therein, is detachably secured to the shell half 3 by a set erf tie- bolts 15 engaged in screw-threaded holes around the perimeter of the shell half 3.

Formed within the neck 5 is a chamber 16 filled with ceramic wool insulation surrounding a replaceable pouring nozzle assembly 17 which extends between the narrow end of the riser tube 6 and an outer end face of the neck 5 where is registers with an expendible porous ceramic filter 18 through which molten metal can be forced to flow into a feeding space 19 for a casting mould 20 in this case

formed by a set of metal dies which are detachably held together by appropriate clamps 21.

A ceramic gasket 22 with a central apperature is preferably interposed between the top surface of the casing 14 and the underside of the mould assembly 20, as seen in Figure 1.

The entire assembly of the reservoir 1 and its enclosure and the mould 20 is pivotly mounted on stub-shafts 23 journalled in bearings in a support frame (not shown) on either side of the assembly. The entire assembly can thus be swung into an inverted position around the axis of the stub-shafts 23, for example by means of a lever arm 24 attached to the casing 14 and operated by a suitable mechanism (not shown).

To prepare the apparatus from cold for a casting operation, the tie- bolts 15 are removed, followed by the casing 13 and the shell half 2. The entire interior surface of the reservoir 1 is then readily accessible for any necessary cleaning, followed by the application of a layer of refractory material in the form of a wash. The tapering shape of the tube 6 greatly assists access to the interior of the tube for this purpose. A new nozzle assembly 17 is fitted and the reservoir reassembled and secured together by the tie-bolts 15. With a new filter 18 installed, the required mould assembly 20 is secured together and securly fastened to the casing 14 through a framework 24 secured to the latter.

The reservoir is then preheated by means of portable gas burners inserted into the reservoir with all its ports open. A gas burner ring 25 placed beneath the shell 2, counteracts heat losses, the central portion of the shell being left uninsulated for this purpose. Further heat can be allowed into the portion of the insulation adjacent the shell 2 through openings 26 in the casing 13.

The reservoir is then charged with superheated molten alloy (i.e. at a temperature above its melting point) through the inlet 9 up to the

level 26, the inlet then being closed. Inert gas is then introduced under pressure through the inlet 10 into the annular space surrounding the smaller diameter end of the riser tube 6 thereby forcing molten metal upwards within the riser tube and hence through the nozzle assembly 17 and the filter 18 into the interior of the mould which in this example is constructed to form an.air-cooled, finned cylinder head. In addition to the normal vents formed between the mould sections, air within the mould 20 can be extracted through a pipe 27 assisted by external vacuum source. The molten metal thus rises smoothly to fill the mould.

The gas burner 25 is then extinguished, and withdrawn if necessary, and the entire assembly formed by the reservoir 1, its casing 13 and 14 and the mould assembly 20 is swung around the pivots 23 to an inverted position as shown in Figure 2 for cooling of the metal in the mould. As the metal cools and solidifies, its density increases and thus, under gravity, tends to remain within the now-lowermost mould. Shrinkage losses due to this contraction on cooling are readily made up within the mould by further metal supplied through the nozzle assembly 17 under the head of molten metal within the riser tube 6 augmented, if desired, by gas pressure within the reservoir. Thus, the metal within the mould can cool and solidify progressively without risk of disturbance caused by convection currents within the metal as would have tended to have happened if the entire assembly had not been inverted.

The rate of cooling within the mould can be made very rapid for example by directing water jets for a predetermined period onto the exterior of the metal dies forming the mould (in particular, from below) and, if appropriate, flowing water through passages within the dies themselves. This very rapid cooling may itself have beneficial effects on the casting being formed, for example as a result of crystalisation of silicon to provide a hard surface in the case of a cylinder bore.

When the casting has solidified, the assembly can be inverted once more to the position shown in Figure 1, the mould 20 opened, the

casting removed, the mould closed again, an appropriate charge of molten metal added through the inlet 9 and the process then repeated.

The volume of metal held in the reservoir will be sufficient at most for only a few castings. For longer production runs, the reservoir can be recharged as necessary by syphoning or pumping from a bulk holder of superheated molten metal.

Where however single, different castings or small batches of castings of different alloys are to be made, the reservoir can be emptied by removal of the drain plug 8, the reservoir cooled or allowed to cool and then separated for cleaning and recoating prior to restarting with the new alloy.

After each new change of metal is introduced into the reservoir, it can be readily and effectively degassed, if required, for example with argon or nitrogen. It can be innoculated with any required additives.

For larger, especially longer castings, two or more apparatus of the kind shown in the drawings may be "ganged" together to fill a common mould.

The frusto-conical construction of the riser tube 6 may also be applied with advantage in low pressure casting machines of more conventional non-inverting type.