SPECIFICATIONInvestment castingThis invention relates to investment casting and in particular to the manufacture of a mould for use in the investment casting of an object.
In the invention casting process, a wax pattern of the object which is to be cast is covered with a refractory material to define a mould whereupon the wax is melted and run out and the resultant cavity filled with molten metal. It is frequently necessary when casting objects using this process to provide one or more disposable cores within the wax pattern which, when casting has been carried out, serve to define holes or cavities in the object. The wax is usually injection moulded around the core or cores prior to the application of the refractory material. There is a tendency however for the wax to lift off certain of the core surfaces as it cools after the injection moulding step. Concave core surfaces are particularly prone to this effect. Such lifting results in the distortion of the shape of the wax pattern and hence the shape of the cast object.
One type of cast object which is often troubled by wax lifting during its manufacture is a hollow gas turbine engine aerofoil blade. It is frequently found that the wax of the pattern which abuts the concave surfaces of the core defining the hollow interior of the eventually cast blade tends to lift off those surfaces. One way in which this problem has been tackled in the past has been to provide a hook or similar feature on the core surface which serves to retain the wax pattern in position. However such hooks, since they are on the core, subsequently serve to define possibly undesirable holes in the eventually cast blade.
It is an object of the present invention to provide a method of manufacturing a mould for use in investment casting in which wax lifting can be substantially eliminated and which does not result in undesirable holes in the cast object.
According to the present invention, a method of manufacturing a mould for use in investment casting an object having spaced apart portions comprises forming a wax pattern of the object which wax pattern includes a core member configured to separate the spaced apart portions thereof and has at least one communicating passage between said spaced apart portions which passage contains a tie member, said tie member protruding from each side of said core member and being so configured as to key into each of said spaced apart portions of said wax pattern and maintain them in engagement with said core member, coating said wax pattern with a refractory material to define a mould and fix the position of said core member in relation to said mould, and melting said wax and pouring it out of said mould to leave said core member within said mould, said tie member being formed from a material which is sufficiently rigid at room temperature to maintain said spaced apart wax pattern portions in engagement with said core member but which is sufficiently mobile at the temperature at which said wax is melted to be poured out of said mould with said molten wax.
The invention will now be described, by way of example, with reference to the accompanying drawings in which:Figure 1 is a sectioned plan view of an aerofoil shape cross-section wax pattern within a mould manufactured in accordance with the method of the present invention.
Figure 2 is a view on section line A-A of Figure 1.
In the manufacture of an aerofoil cross-section turbine blade for a gas turbine engine by investment casting, an injection moulded wax pattern 10 (as can be seen in Figure 1) is coated with a conventional refractory material 11 in order to define a mould. The turbine blade to be manufactured is hollow in order to facilitate its cooling by the passage of cooling air therethrough. The hollow interior is defined within the wax pattern 10 by a lengthwise extending ceramic core 12. The core 12 extends beyond the wax pattern 10 in a lengthwise direction to engage and be retained in position by the refractory material 11. Such a method of mould construction is well known in the manufacture of components by casting and will not therefore be described in detail.
The core 12 is provided with a lengthwise extending slit 13 which links the concave and convex portions of the aerofoil shape of the wax pattern 10. The slit 13 accommodates a polystyrene sheet tie member 14 which can be seen more easily inFigure 2. The edges 15 and 16 of the tie member 14 protrude from each side of the core 12 and are adjacent the concave and convex portions of the wax pattern 10. They are provided with a number of projections 17 which extend into the wax pattern 10 and are so configured as to key into the wax pattern 10.
The tie member 14 serves to link the concave and convex portions of the wax pattern 10. This ensures that after the injection moulding of the wax pattern 10 and before the application thereto of the refractory material 11, the tie member 14 ties the wax pattern 10 portions together, thereby maintaining the wax pattern 10 in engagement with the ceramic core 12. Thus any tendency for the injection moulded wax pattern 10 to lift off the concave surface of the ceramic core 12 as it cools after moulding is substantially eliminated.
After the refractory material 11 has been applied to the wax pattern 10, to define a mould, the whole assembly is heated in order to melt the wax 10 whereupon the molten wax is poured out of the mould. At the temperature at which the wax melts viz approximately 70"C the polystyrene tie member 14 becomes sufficiently mobile to be poured out of the mould along with the molten wax. It will be appreciated that although the tie member 14 has been described as being formed from polystyrene, it could in fact be formed from an alternative material. Essentially the chosen material should be sufficiently rigid at room temperature to maintain the wax pattern 10 in engagement with the core 12 but sufficiently mobile at the temperature at which the wax is melted to be poured out of the mould with the molten wax.
When the wax pattern 10 and tie member 14 have been removed from the mould defined by the refractory material 11, the resultant enclosure is filled with molten metal in the usual manner. It will be appreciated however that since the tie member 14 originally occupied a slit 13 in the core 12, then upon casting that slit 13 will be filled with molten metal, thereby defining an interconnecting web between the concave and convex surfaces of the resultant aerofoil blade. However, such webs are commonly incorporated into aerofoil blades for strength purposes and may therefore be a desirable feature of the blade.
Although the present invention has been described with reference to the manufacture of a turbine aerofoil blade, it will be appreciated that it is in fact applicable to the investment casting of other objects which are so configured as to necessitate the use of a wax pattern having spaced apart portions.