Moore, Christopher James (3 Longclose Avenue, Allesley Village Coventry, Warwickshire CV5 9GH, GB)
Holme, David (Phoenix House, 27 High Street Crick, Northamptonshire NN6 7TS, GB)
Pearce, Richard Kenneth (14 Wilson Close, Thorpe Astley, Leicestershire LE3 3RI, GB)
Moore, Christopher James (3 Longclose Avenue, Allesley Village Coventry, Warwickshire CV5 9GH, GB)
Holme, David (Phoenix House, 27 High Street Crick, Northamptonshire NN6 7TS, GB)
| 1. | A moulding composition for the production of a mould for metal casting from a pattern, wherein the composition comprises an effective amount of a particulate, waterinsoluble lubricant, the lubricant being selected such that it does not evolve deleterious quantities of gases or vapours at the casting temperature of the metal. |
| 2. | A moulding composition according to claim 1, which comprises finelydivided particulate inorganic water hardenable materials. |
| 3. | A moulding composition according to claim 1 or 2, which comprises gypsum, calcined gypsum, plaster of Paris, anhydrous calcium sulphate, wollastonite, cement, lime, other similar plasterforming materials, or a mixture thereof. |
| 4. | A moulding composition according to any one of the preceding claims, which comprises from 70 to 99. 9 % by weight calcined gypsum. |
| 5. | A moulding composition according to any one of the preceding claims, which comprises from 0 to 30 % by weight wollastonite. |
| 6. | A moulding composition according to any one of the preceding claims, wherein the composition is in the form of a slurry comprising a mixture of from 30 to 70 % by volume of the composition in water. |
| 7. | A moulding composition according to any one of the preceding claims, wherein the waterinsoluble lubricant is an inorganic lubricant. |
| 8. | A moulding composition according to claim 7, wherein the lubricant is bentonite, a clay, graphite, boron nitride, molybdenum disulphide or a mixture thereof. |
| 9. | A moulding composition according to claim 8, wherein the lubricant is bentonite. |
| 10. | A moulding composition according to any one of the preceding claims, which comprises calcined gypsum, cement, wollastonite, inorganic inert filler and bentonite. |
| 11. | A moulding composition according to any one of the preceding claims, which comprises from 0.5 to 4 % by weight of the lubricant, based on the total weight of the composition. |
| 12. | A method of manufacturing a mould for metal casting, wherein a slurry of a moulding composition is disposed around a pattern and allowed to harden to produce a mould, which method comprises including in the moulding composition an effective amount of a solid, waterinsoluble lubricant such that separation of the pattern from the mould is facilitated. |
| 13. | A method according to claim 12, wherein there is used a moulding composition according to any one of claims 1 to 11. |
| 14. | A method according to claim 12 or 13, wherein the pattern is a flexible rubber pattern. |
| 15. | A method of metal casting using a mould produced from a moulding composition according to any one of claims 1 to 11. |
| 16. | A method of metal casting using a mould produced by a method according to any one of claims 12 to 15. |
| 17. | A cast metal article produced by a method according to claim 15 or 16. |
| 18. | A disposable mould produced from a moulding composition according to any one of claims 1 to 11. |
| 19. | A disposable mould produced by a method according to any one of claims 12 to 15. |
| 20. | A disposable mould according to claim 18 or 19 that is relatively gas impermeable. |
The use of gypsum or plaster moulds for the manufacture of intricate, high surface quality metal castings is well-known. For example, aluminium alloy impeller blades for automotive turbochargers, pumps and fans, which often have severe undercut portions, can be cast successfully in disposable plaster moulds which are broken after the metal has solidified to extract the casting. Investment casting, wherein the mould is manufactured using the lost-wax process, can be employed for casting metal articles in small quantities, but is unsuitable for high volume production of aluminium alloy castings. In the latter case, the pattern for the mould is usually a re-usable, flexible shaped rubber member. A slurry of the plaster in water is poured around the rubber pattern and allowed to dry and harden. Subsequently the rubber pattern is extracted from the hardened plaster mould to leave a mould cavity of the required shape. With intricate designs, such as impeller blades, it is often difficult to remove the rubber pattern without damaging either the pattern or the mould. Frequently the fine detail of the internal cavity is disturbed, leaving defects that are not discovered until the metal casting is revealed.
According to the present invention, it has surprisingly been found that the provision of a lubricant can facilitate the removal of the pattern from the metal casting mould.
In a first aspect, the present invention provides a moulding composition for the production of a mould for metal casting from a pattern, wherein the composition comprises an effective amount of a particulate, water- insoluble lubricant, the lubricant being selected such that it does not evolve deleterious quantities of gases or vapours at the casting temperature of the metal.
In another aspect, the invention provides a method of manufacture of a mould for metal casting, wherein a slurry of a moulding composition is disposed around a flexible pattern and allowed to harden to produce a mould, which method comprises including in the moulding composition an effective amount of a solid, water-insoluble lubricant such that separation of the pattern from the mould is facilitated.
In still further aspects, the invention also provides a method of metal casting using a mould produced from a moulding composition and/or by a method of manufacture according to the invention and a cast metal article produced therefrom.
The moulding composition is preferably an inorganic composition, although moulding compositions derived from hardenable organic polymeric compounds are not excluded.
Such hardenable organic compounds can include, for example,
polyacrylates and glass ionomers, comprising, for example, metal oxides and polyacrylic acid. Preferably, however, the moulding composition comprises one or more finely- divided particulate inorganic water-hardenable materials, for example, gypsum, calcined gypsum, plaster of Paris, anhydrous calcium sulphate, 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, and 0 to 5 % by weight cement.
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.
If desired, the moulding composition can comprise foaming agents or other pore-forming materials for introducing porosity into the mould. Such materials are, however, not always advantageous, and, for example, an improved casting apparatus and method using a relatively impermeable plaster mould is described in our co-pending UK
patent application No. 039805.6. In a preferred moulding composition according to the invention, therefore, pore- forming additives are omitted and the resultant mould is relatively impermeable to gases.
The water-insoluble lubricant is preferably an inorganic lubricant, since these are less likely to evolve deleterious gases at the pouring temperature of the molten metal. Suitable lubricants include, for example, bentonite, various clays, graphite, boron nitride, molybdenum disulphide and mixtures thereof. Organic lubricants which could also be used include metal stearates, paraffin or microcrystalline waxes, and higher molecular weight polyethylene glycols. Very good results have been obtained using bentonite, which is the preferred lubricant for the present invention.
A particularly preferred moulding composition in accordance with the invention comprises calcined gypsum, cement, inorganic inert fillers and bentonite.
Preferably the moulding composition comprises from 0.1 to 5% of the lubricant, by weight of solids content, more preferably from 0.5 to 4% by weight, and most preferably from 1 to 3% by weight.
In the method of the invention, the pattern is preferably a shaped, flexible member, more especially a flexible natural or synthetic rubber member. The flexibility of the pattern, together with the presence of the lubricant enables the pattern to be deformed and
removed from the hardened mould, even when there are severe undercut regions in the mould cavity.
The invention is illustrated by the following non- limitative Example: EXAMPLE A plaster moulding composition was prepared by mixing 87 % by weight calcined gypsum plaster with 12 % by weight wollastonite and 1 % by weight cement. All percentages are based on the total weight of the composition (without water addition). Bentonite was then added to the composition as a lubricant, in amounts of 0. 5%, 1 %, 2 % and 5 % by weight respectively, based on the total weight of the composition.
After thorough mixing of the constituents, in each case, 56 % by volume of the moulding composition was mixed with 44 % by volume of water to form a slurry.
In manufacturing the metal casting mould, a flexible rubber pattern in the shape of an impeller blade was placed in an open vessel and the vessel filled with the slurry.
After allowing the slurry to dry and harden the rubber pattern was removed to expose the mould cavity. The mould was filled with aluminium alloy in a standard casting method and after quenching a high quality impeller blade was produced.
It was found that the presence of the bentonite lubricant made separation of the rubber pattern from the mould much easier than when the lubricant was omitted. In
addition, far less finishing work was required on the aluminium casting.
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.