APPARATUS FOR SUPPORTING A CORE IN A MOULD

Abstract: This invention relates to an improved method of locating and supporting a ceramic core in fixed space relationship in a ceramic shell mould and maintaining this fixed space relationship in the subsequent casting process for production of a hollow metal casting. In the investment or "lost-wax" casting process the ceramic core is located within the ceramic mould by the insertion of recrystalised alumina pins through the wax encasing the core prior to encasing the whole in a ceramic slurry. On the subsequent removal of the wax, the molten metal i.e. superalloy, is injected into the resulting space. The recrystalised alumina pins remain intact during the casting process hence substantially increasing the success rate of achieving accurate core location during casting solidification.

Description: This invention relates to an improved apparatus for locating and supporting a ceramic core in fixed space relationship in a ceramic shell mould, according to the preamble of claim 1. The apparatus maintains the ceramic core in fixed space relationship with the shell mould in the subsequent casting process for production of a hollow metal casting. In the investment, lost-wax, casting process for the production of hollow metal castings, it is known to encase a core in wax and insert supports through the wax until they are in contact with the core. The wax encased core is then coated with a ceramic slurry which forms a shell mould. On hardening the ceramic slurry and thereafter removing the wax, the supports maintain the core in fixed space relationship with the shell mould. To support cores having a length of the order of 12.5 cms or more, a process known as 'core printing' is used. The core is extended to provide flattened ends which are held in a wax pattern die prior to encasing the core with wax. The core is extended sufficiently so that the ends protrude through the ceramic shell mould. The core printing method has the disadvantage that on subsequently removal of the core from the casting manufacturing steps have to be added to blank off an aperture which the extended core produces at the blade tip. For core lengths of the order of 12.5 cms or less it is sufficient to support the core at positions intermediate its extremities. These supports are called chaplets. A conventional form of chaplet consists of a pin or wire which is manufactured from an inert material, such as platinum. Platinum pins are expensive and so to reduce expenditure expanded pins may be used to support the core, EP-A-0084234, upon which the preamble of claim 1 is based. The pin is expanded by producing a plurality of slits in the pins and then stretching the pin to form a mesh. The expanded pin forms a more rigid support enabling less material to be used for a given support requirement. The expanded metal pins may be bent to form C, U or V shapes which support the core over a greater area. A disadvantage of these known core supports are that the platinum melts on casting the metal and dissipates into the casting during solidification. The core is therefore unsupported on casting the metal and may move from its precise location within the shell mould. The present invention seeks to provide an improved apparatus for supporting a core within a shell mould during the investment casting process. According to the present invention an apparatus for locating and maintaining a wax encased core in fixed space relationship with the interior of a ceramic shell mould, comprises a plurality of pins inserted through the wax and abutting the core, the wax being encased in a hardened ceramic slurry to fix the pins, the fixed pins maintaining support of the core on the removal of the wax and in the casting process, the pins being formed from a material which remains intact during the casting and subsequent solidification processes for production of hollow metal components. Preferably the pins are of recrystalised alumina. The invention will now be described by way of example and with reference to the accompanying drawing. Referring to the drawing a ceramic core (15) is encased with wax (16). Recrystalised alumina pins (18) are then inserted through the wax encasing the core until they abut said core (15) prior to encasing the whole in a ceramic slurry. The ceramic shell (17) is then hardened whereafter the wax (16) is melted and runs out, leaving the ceramic core (15) supported in space of relationship to the interior of the ceramic shell (17) by the recrystalised alumina pins (18). A molten metal e.g. a superalloy such as nickel/chrome, is then introduced into the shell to replace the lost wax. The recrystalised alumina pins remain intact during the casting process and thus maintain the accurate locations of the core during solidification of the metal. On completion of the casting process the ceramic core and outer shell are removed chemically. Mechanical machining processes such as friction polishing then remove any surface defects caused by the recrystalised alumina joins and any other defects which may have been introduced at any of the various stages of the casting process. Articles produced by the method of the present invention include nozzle guide vane and turbine blades for use in a gas turbine aeroengine. During operation of the turbine blades so produced it has been found that those portions of the recrystalised alumina pins which are embedded therein, tend to exit the blade under centrifugal forces and leave small apertures through the blade. This however does not adversely affect the cooling flow efficiency of the air flowing through the blade.

Claims: Apparatus for the investment casting process for locating and maintaining a core (15) encased in wax (16) in fixed spaced relationship with the interior of a ceramic shell mould (17), comprising a plurality of pins (18) inserted through the wax (26) and abutting the core (15), the wax being encased in a hardened ceramic slurry (17) to fix the pins (18), the fixed pins (18) maintaining support of the core (15) on the removal of the wax (16) and in the subsequent casting and solidification processes for the production of hollow metal components, the apparatus characterised by, the pins (18) being formed from a material which remains intact during the casting and solidification processes, thereby maintaining support and accurate location of the core throughout the investment casting process. Apparatus as claimed in claim 1, characterised in that the pins (18) are of recrystalised alumina. Apparatus as claimed in claim 1 characterised in that the component to be cast in the investment casting process is a superalloy turbine blade which has a passageway therein. Apparatus as claimed in claim 1 characterised in that component to be cast in the investment casting process is a superalloy nozzle guide vane which has a passageway therein.

Inventors: GARTLAND, FREDERICK HODGSON
Applicants: ROLLS-ROYCE PLC
Application year: 1988
Main classification: B22C9
IPC codes: B22C9, B22C21