GB2053047A

GB2053047A - Cores for lost wax casting

Info

Publication number: GB2053047A
Application number: GB7923795A
Authority: GB
Original assignee: Rolls Royce PLC
Current assignee: Rolls Royce PLC
Priority date: 1979-07-07
Filing date: 1979-07-07
Publication date: 1981-02-04
Also published as: GB2053047B

Abstract

A wax pattern used in the lost wax process is restrained from movement with respect of an adjacent ceramic core by an aperture in the core which becomes filled with wax during the wax injection; the aperture being shaped such as to prevent removal of the wax therefrom after solidification.

Description

SPECIFICATION Lost wax casting This invention relates to making castings including disposable cores by the lost wax casting process.

It is at present well known to manufacture castings including cores to extremely close tolerances by the lost wax casting process.

The process essentially consists of producing the disposable core, placing the core within a mould conforming to the external shape of the finished casting; and injecting wax into the mould to make a pattern. The injected wax pattern is subsequently removed from the mould and coated with a layer or layers of ceramic material, after which the ceramic is fired and wax melted out or burned off to define a casting cavity.

Such a method of manufacturing castings usually results in the production of nearly identical castings to close tolerances, and for this reason the process has been used extensively to manufacture blades for gas turbine engines.

It has been found however that when such blades have a relatively small diameter concave radius on their pressure face, the wax pattern tends to move away from the concave surface of the disposable core. This leaves a space between the wax pattern and the disposable core such that the resulting casting is incorrectly sized and many fall outside the manufacturing tolerances.

It is believed that one of the main factors causing this problem is the different coefficients of expansion of the wax and the disposable core. In an attempt to alleviate this problem the completed wax patterns are often stored in insulated containers which are then stored at carefully maintained temperatures.

However this method is rather expensive and has not really reduced the problem by a significant factor.

An object of the present invention is to provide means whereby the aforementioned problem is substantially eliminated.

According to the present invention means for attaching a wax pattern to a disposable core comprises providing the disposable core with at least one aperture which becomes filled with wax during the wax injection, the aperture being shaped such as to restrain the wax pattern from movement with respect of the disposable core when the wax is in the solid state.

Preferably the aperture is a parallel sided or tapered hole located at an angle to the surface of the disposable core.

Alternatively the aperture is a re-entrant hole provided within the disposable core.

Furthermore the aperture may comprise an elongated dovetail or re-entrant slot.

Preferably the disposable core consists of a ceramic member used to define the hollow interior of the blade of a gas turbine engine.

For better understanding of the invention an embodiment thereof will now be more particularly described by way of example only and with reference to the accompanying drawings in which: Figure 1 shows a pictorial view of a blade for a gas turbine engine in the as-cast condition.

Figure 2 shows a cross-section through a disposable core and wax pattern of the type used for making gas turbine blades and shows the problem which occurs when using such a core and wax pattern.

Figure 3 shows a cross-section through a disposable core and a wax pattern as made in accordance with an embodiment of the present invention.

Referring to Fig. 2 of the drawings shows a cross-section through a disposable core 1 2 and wax pattern 1 3. Such disposable cores are usually manufactured from a ceramic material which is subsequently leached from the finished casting. The wax portion 1 3 is injected around the ceramic core 1 2 in a mould not shown.

As can be seen from the drawings due to contraction of the wax pattern 1 3 a space 14 has appeared between the concave face of the disposable core and its adjacent portion of the wax pattern. Obviously if a casting was made from such a wax pattern it would be incorrectly sized on its concave side this could perhaps be subsequently rectified by machining however this could result in an extremely thin blade wall thickness.

Fig. 3 of the drawings shows a disposable core 1 2 in which there is drilled angled hole 1 5 which during injection becomes filled with a portion of the wax which is used to form the pattern 1 3. After the wax has become solidified the portion of it within the angled hole prevents the remainder of the wax pattern 1 3 from being displaced so that it must stay within close proximity to the core such that no space can be formed between the core 1 2 and the pattern 13.

It will be appreciated by those skilled in the art that whilst the described and illustrated embodiment show an angled hole 15, the invention is in no way restricted to such a shaped aperture. The angled hole 1 5 could easily be substituted for any type of re-entrant hole or alternatively a dovetail slot or similar shaped re-entrant slot.

1. Means for attaching a wax pattern to a disposable core comprises providing the disposable core with at least one aperture which becomes filled with wax during the wax injection, the aperture being shaped such as to restrain the wax pattern from movement with respect to the disposable core when the wax

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. SPECIFICATION Lost wax casting This invention relates to making castings including disposable cores by the lost wax casting process. It is at present well known to manufacture castings including cores to extremely close tolerances by the lost wax casting process. The process essentially consists of producing the disposable core, placing the core within a mould conforming to the external shape of the finished casting; and injecting wax into the mould to make a pattern. The injected wax pattern is subsequently removed from the mould and coated with a layer or layers of ceramic material, after which the ceramic is fired and wax melted out or burned off to define a casting cavity. Such a method of manufacturing castings usually results in the production of nearly identical castings to close tolerances, and for this reason the process has been used extensively to manufacture blades for gas turbine engines. It has been found however that when such blades have a relatively small diameter concave radius on their pressure face, the wax pattern tends to move away from the concave surface of the disposable core. This leaves a space between the wax pattern and the disposable core such that the resulting casting is incorrectly sized and many fall outside the manufacturing tolerances. It is believed that one of the main factors causing this problem is the different coefficients of expansion of the wax and the disposable core. In an attempt to alleviate this problem the completed wax patterns are often stored in insulated containers which are then stored at carefully maintained temperatures. However this method is rather expensive and has not really reduced the problem by a significant factor. An object of the present invention is to provide means whereby the aforementioned problem is substantially eliminated. According to the present invention means for attaching a wax pattern to a disposable core comprises providing the disposable core with at least one aperture which becomes filled with wax during the wax injection, the aperture being shaped such as to restrain the wax pattern from movement with respect of the disposable core when the wax is in the solid state. Preferably the aperture is a parallel sided or tapered hole located at an angle to the surface of the disposable core. Alternatively the aperture is a re-entrant hole provided within the disposable core. Furthermore the aperture may comprise an elongated dovetail or re-entrant slot. Preferably the disposable core consists of a ceramic member used to define the hollow interior of the blade of a gas turbine engine. For better understanding of the invention an embodiment thereof will now be more particularly described by way of example only and with reference to the accompanying drawings in which: Figure 1 shows a pictorial view of a blade for a gas turbine engine in the as-cast condition. Figure 2 shows a cross-section through a disposable core and wax pattern of the type used for making gas turbine blades and shows the problem which occurs when using such a core and wax pattern. Figure 3 shows a cross-section through a disposable core and a wax pattern as made in accordance with an embodiment of the present invention. Referring to Fig. 2 of the drawings shows a cross-section through a disposable core 1 2 and wax pattern 1 3. Such disposable cores are usually manufactured from a ceramic material which is subsequently leached from the finished casting. The wax portion 1 3 is injected around the ceramic core 1 2 in a mould not shown. As can be seen from the drawings due to contraction of the wax pattern 1 3 a space 14 has appeared between the concave face of the disposable core and its adjacent portion of the wax pattern. Obviously if a casting was made from such a wax pattern it would be incorrectly sized on its concave side this could perhaps be subsequently rectified by machining however this could result in an extremely thin blade wall thickness. Fig. 3 of the drawings shows a disposable core 1 2 in which there is drilled angled hole 1 5 which during injection becomes filled with a portion of the wax which is used to form the pattern 1 3. After the wax has become solidified the portion of it within the angled hole prevents the remainder of the wax pattern 1 3 from being displaced so that it must stay within close proximity to the core such that no space can be formed between the core 1 2 and the pattern 13. It will be appreciated by those skilled in the art that whilst the described and illustrated embodiment show an angled hole 15, the invention is in no way restricted to such a shaped aperture. The angled hole 1 5 could easily be substituted for any type of re-entrant hole or alternatively a dovetail slot or similar shaped re-entrant slot. CLAIMS

1. Means for attaching a wax pattern to a disposable core comprises providing the disposable core with at least one aperture which becomes filled with wax during the wax injection, the aperture being shaped such as to restrain the wax pattern from movement with respect to the disposable core when the wax is in the solid state.

2. Means as claimed in claim 1 in which the aperture is a re-entrant hole provided within the disposable core.

3. Means as claimed in claim 1 in which the aperture-is a parallel sided or tapered hole located at an angle to the surface of the disposable core.

4. Means as claimed in claim 1 in which the aperture comprises an elongate dovetail or re-entrant slot located within the disposable core.

5. A disposable core as claimed in claims 1, 2, 3 or 4 consisting of a ceramic member used to define the hollow interior of the blade of a gas turbine engine.

6. A disposable core substantially as here it before described by way of example only and with reference to the accompanying drawings.