GB1605341A

GB1605341A - Improvements in investment casings of moulds

Info

Publication number: GB1605341A
Application number: GB2355977A
Authority: GB
Inventors: J B Oliver; D Mills
Original assignee: Rolls Royce PLC
Current assignee: Rolls Royce PLC
Priority date: 1977-06-03
Filing date: 1977-06-03
Publication date: 1992-01-02
Anticipated expiration: 1993-06-03

Description

(54) IMPROVEMENTS IN INVESTMENT CASTING OF MOULDS We, RoLLs-Rouce LIMITED, a British Company, of 65 Buckingham Gate, London, SWIPE 6AT. do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to investment casting and has particular reference to a method of investment casting a mould in which an article is produced having a first passage therein and at least one branch passage communicating with the first passage.

An example of such an article is a gas turbine rotor blade which is provided with a first longitudinal cooling passage and branch passages passing from this cooling passage to the surface of the blade.

Such branch passages have previously been produced by drilling from the surface of the blade into the longitudinal passage which is formed by casting. This technique is however costly and very difficult to execute because of the very small size and awkward inclinations necessary for the branch passages.

It would be very convenient to produce the passages using small silica rods or tubes connected to the core for the longitudinal passage in the blade but the fragility of the rods or tubes and the difficulty of accurately locating and supporting them from the core has proved exceptionally difficult.

Analogous problems can be found with other anicles in which a first passage and a branch passage are necessary.

The present invention seeks to provide a method of investment casting a mould for producing an article having a first passage and a branch passage communicating therewith without the need first to fabricate a core having projections corresponding to the branch passages.

According to the present invention there is provided a method of investment casting a mould for producing an article having a first passage therein and a branch passage communicating with the first passage, the method comprising the steps of: forming a first core to define in the finished article the first passage, the first core being provided with a recess at a location corresponding in the finished article to the entry of the branch passage.

forming a cladding around the first core, the cladding having an aperture therein at the recess, inserting into the aperture a second core corresponding in the finished article to the branch passage, providing cladding around the second core to fill the aperture in the cladding that surrounds the first core, the second core passing through the cladding into the recess, investing the cladding to retain the location of the second core, and subsequently removing the cladding.

In one embodiment the method comprises the following steps: forming a first core to define in the finished article the first passage, the first core being provided with a recess at a location corresponding in the finished article to an entry of the branch passage, inserting into each recess a putty, forming a cladding around the first core. the cladding having an aperture therein at the recess, inserting into the aperture a second core corresponding in the finished article to the branch passage, providing cladding around the second core to fill the aperture in the cladding that surrounds the first core, the second core passing through the cladding into the putty, investing the cladding to retain the location of the second core.

and subsequently removing the cladding.

The method is particularly suited to investment casting of a mould in which the first core is of ceramic, the cladding comprises a wax, the second core is a tube of fused silica. and the putty is a ceramic putty heat hardenable on firing the mould.

Preferably the method is used to produce a plurality of relatively small branch passages from a main passage and the second cores defining the branch passages are assembled together into a cassette from which their ends protrude prior to insertion through the aperture into the putty.

The invention funher comprises a mould when made by any of the ahove methods.

Embodiments of the Invention will now be described by way of example only and with reference to the drawings filed with the provisional specificalion in which: Fig. 1 is a perspective view of a gas turbine rotor blade.

Fig. 2 is a perspective view of a section of a first core for use in producing the blade of Fig. I bul to an enlarged scale, Fig. 3 illustrates the core of Fig. 7 clad with a wax, Fig. 4 shows a cassette of second cores for insertion into apertures in the wax cladding of Fig. 3, Fig. 5 shows a die suitable for producing the cassette of Fig. 4, Fig. 6 illustrates the final assembled wax pattern for the blade of Fig. 1, and, Figs. 7, 8 and 9 illustrate alternative embodiments.

Turning now to Fig. 1 there will be seen a gas turbine rotor blade 10 having an aerofoil 11, a platform 12 and a root fixing 13 with the aerofoil partly cut away to illustrate details of the cooling passages.

The cooling passages comprise a first longitudinal passage 14 with branch passages 15, 16, 17 communicating with the first passage 14 and also with the surface 19 of the blade. The blade of Fig. 1 is produced by investment casting using a wax pattern and cores to define the first passage 14 and the branch passages 15, 16. 17.

A section of the first core 20 which in the finished blade defines the first passage is shown in Fig. 2 and will be seen to comprise a ceramic moulded in a die by any one of the known processes for consolidating a ceramic slunk. The core 20 has recesses in the form of longitudinal grooves 21, 22, 23 at locations corresponding in the finished anicle to the entries of the respective branch passages 15, 16, 17.

Each of the recesses is almost completely filled with a ceramic putty produced by mixing together a fused silica powder of 200 mesh (British fine series) size with a liquid silicone resin, such as Dow Coming resin QR4-3 157, to the consistency of plasticine. The core is then introduced into a die not shown but of a type well known in the art, and is provided with cladding of wax by injecting the wax into the die. The particular configuration of the cladding chosen is illustrated in Fig. 3 where it will be seen to encompass the core 20 except at apertures 27. 28, 29 provided at the recesses 21, 22.23 respectively.Each of the apenures penetrates the cladding so that the ceramic putty is accessible through the apertures, the width of each aperture is greater than the width of the recess and the outer envelope of the cladding corresponds to the surface of the as cast blade. The second cores for forming the branch passages can be seen by referring to Fig. 4 and comprise small fused silica tubes 30 held in generally rectangular wax cassettes such as 34. The ends 35. 36 of the silica tubes project from sides 37, 38 of the wax cassettes and each cassette has outer dimensions corresponding to the shape of its respective aperture and to the wax envelope and ceramic core so that it fits snugly into the aperture provided.

The cassettes are formed using the apparatus of Fig. 5 which will be seen to show a wax injection die having two parts 39, 41 together defining the envelope of each wax cassette and provided with grooves 42 for supporting the silica tubes during injection of the wax.

Tuming now to Fig. 6 the cassettes 34 are shown assembled into the apenures in the wax cladding into which they are secured either by glueing or by a smear of molten wax It will be noted that the ends 35 of the silica tubes project into the ceramic putty and that the ceramic putty is displaced by the ends of the silica tubes to fill the space by which the recess was underfilled with ceramic putty.

The wax pattern is then invested with a ceramic shell in the usual way and autoclaved to remove the wax prior to pouring of the metal into the mould.

During the step of firing the mould the ceramic putty hardens thus retaining the location of the second cores, which in this embodiment are supponed at their other ends by the investment shell and the melting point of the wax is chosen to be sufficiently above the hardening temperature of the putty so that the putty hardens before the wax suppon for the second cores disappears.

Turning now to Figs. 7, 8, 9 alternative configurations are shown for the second cores. In Fig. 7 the second core comprises a plurality of silica tubes 30 the ends 35 of which are embedded in a generally rectangular block of ceramic 50. The block of ceramic 50 is introduced into the recess which is smeared with ceramic putty so as to retain the ceramic block 50 on firing the mould. The space in the aperture between the silica tubes and the cladding is either filled with wax as a separate injection operation or by pre forming an appropriately shaped wax block around the tubes. The outer ends of the silica tubes are retained in the investment shell 51 as previously described.In Fig. 8 the silica tubes are shown supported also at their outer ends by a generally rectangular block of ceramic 52 and this block of ceramic is embedded in the investment shell on investment of the mould. In the embodiment of Fig. 9 the silica tubes are extended in length and provided with a block of ceramic 53 which remains outside the investment shell following investment of the pattern.

It will be appreciated by those skilled in the an that many modifications are possible to the method outlined ahove, for example. the ceramic putty may be hardened by other means such as a chemical reaction or may be replaced with a putty of a material other than ceramic, or indeed may be omitted entirely if suitable alternative location features are provided in the recess.

It may prove advantageous to use cladding of materials other than wax, for example.

polystyrene or a bamed material. For very small branch passages silica or other rods may be substituted for the fused silica tubes and the method Is of course equally applicable to producing only a single branch passage which may be blind if desired.

WHAT WE CLAIM 1S: 1. The method of investment casting a mould for producing an article having a first passage therein and a branch passage communicating

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

Claims

**WARNING** start of CLMS field may overlap end of DESC **. Fig. 4 shows a cassette of second cores for insertion into apertures in the wax cladding of Fig. 3, Fig. 5 shows a die suitable for producing the cassette of Fig. 4, Fig. 6 illustrates the final assembled wax pattern for the blade of Fig. 1, and, Figs. 7, 8 and 9 illustrate alternative embodiments. Turning now to Fig. 1 there will be seen a gas turbine rotor blade 10 having an aerofoil 11, a platform 12 and a root fixing 13 with the aerofoil partly cut away to illustrate details of the cooling passages. The cooling passages comprise a first longitudinal passage 14 with branch passages 15, 16, 17 communicating with the first passage 14 and also with the surface 19 of the blade. The blade of Fig. 1 is produced by investment casting using a wax pattern and cores to define the first passage 14 and the branch passages 15, 16. 17. A section of the first core 20 which in the finished blade defines the first passage is shown in Fig. 2 and will be seen to comprise a ceramic moulded in a die by any one of the known processes for consolidating a ceramic slunk. The core 20 has recesses in the form of longitudinal grooves 21, 22, 23 at locations corresponding in the finished anicle to the entries of the respective branch passages 15, 16, 17. Each of the recesses is almost completely filled with a ceramic putty produced by mixing together a fused silica powder of 200 mesh (British fine series) size with a liquid silicone resin, such as Dow Coming resin QR4-3 157, to the consistency of plasticine. The core is then introduced into a die not shown but of a type well known in the art, and is provided with cladding of wax by injecting the wax into the die. The particular configuration of the cladding chosen is illustrated in Fig. 3 where it will be seen to encompass the core 20 except at apertures 27. 28, 29 provided at the recesses 21, 22.23 respectively.Each of the apenures penetrates the cladding so that the ceramic putty is accessible through the apertures, the width of each aperture is greater than the width of the recess and the outer envelope of the cladding corresponds to the surface of the as cast blade. The second cores for forming the branch passages can be seen by referring to Fig. 4 and comprise small fused silica tubes 30 held in generally rectangular wax cassettes such as 34. The ends 35. 36 of the silica tubes project from sides 37, 38 of the wax cassettes and each cassette has outer dimensions corresponding to the shape of its respective aperture and to the wax envelope and ceramic core so that it fits snugly into the aperture provided. The cassettes are formed using the apparatus of Fig. 5 which will be seen to show a wax injection die having two parts 39, 41 together defining the envelope of each wax cassette and provided with grooves 42 for supporting the silica tubes during injection of the wax. Tuming now to Fig. 6 the cassettes 34 are shown assembled into the apenures in the wax cladding into which they are secured either by glueing or by a smear of molten wax It will be noted that the ends 35 of the silica tubes project into the ceramic putty and that the ceramic putty is displaced by the ends of the silica tubes to fill the space by which the recess was underfilled with ceramic putty. The wax pattern is then invested with a ceramic shell in the usual way and autoclaved to remove the wax prior to pouring of the metal into the mould. During the step of firing the mould the ceramic putty hardens thus retaining the location of the second cores, which in this embodiment are supponed at their other ends by the investment shell and the melting point of the wax is chosen to be sufficiently above the hardening temperature of the putty so that the putty hardens before the wax suppon for the second cores disappears. Turning now to Figs. 7, 8, 9 alternative configurations are shown for the second cores. In Fig. 7 the second core comprises a plurality of silica tubes 30 the ends 35 of which are embedded in a generally rectangular block of ceramic 50. The block of ceramic 50 is introduced into the recess which is smeared with ceramic putty so as to retain the ceramic block 50 on firing the mould. The space in the aperture between the silica tubes and the cladding is either filled with wax as a separate injection operation or by pre forming an appropriately shaped wax block around the tubes. The outer ends of the silica tubes are retained in the investment shell 51 as previously described.In Fig. 8 the silica tubes are shown supported also at their outer ends by a generally rectangular block of ceramic 52 and this block of ceramic is embedded in the investment shell on investment of the mould. In the embodiment of Fig. 9 the silica tubes are extended in length and provided with a block of ceramic 53 which remains outside the investment shell following investment of the pattern. It will be appreciated by those skilled in the an that many modifications are possible to the method outlined ahove, for example. the ceramic putty may be hardened by other means such as a chemical reaction or may be replaced with a putty of a material other than ceramic, or indeed may be omitted entirely if suitable alternative location features are provided in the recess. It may prove advantageous to use cladding of materials other than wax, for example. polystyrene or a bamed material. For very small branch passages silica or other rods may be substituted for the fused silica tubes and the method Is of course equally applicable to producing only a single branch passage which may be blind if desired. WHAT WE CLAIM 1S:

1. The method of investment casting a mould for producing an article having a first passage therein and a branch passage communicating

with the first passage, the method comprising the steps of: forming a first core to define in the finished article the first passage, the first core being provided with a recess at a location corresponding in the finished article to an entry of the branch passage, forming a cladding around the first core, the cladding having an aperture therein at the recess, inserting into the aperture a second core corresponding in the finished article to the branch passage, providing cladding around the second core to fill the aperture in the cladding that surrounds the first core, the second core passing through the cladding into the recess, investing the cladding to retain the location of the second core, and subsequently removing the cladding.

2. The method of investment casting a mould for producing an article having a first passage therein and a branch passage communicating with the first passage, the method comprising the steps of:- forming a first core to define in the finished article the first passage, the first core being provided with a recess at a location corresponding in the finished article to an entry of the branch passage, inserting into each recess a putty, forming a cladding around the first core, the cladding having an aperture therein at the recess, inserting into the aperture a second core corresponding in the finished article to the branch passage, providing cladding around the second core to fill the aperture in the cladding that surrounds the first core, the second core passing through the cladding into the putty, investing the cladding to retain the location of the second core, and subsequently removing the cladding.

3. The method of investment casting a mould substantially as described herein with reference to the drawings filed with the provisional specification.

4. A mould when made by the method according to any of the preceding claims.