Patent Publication Number: US-2019176222-A1

Title: Core assembly for casting, and casting process

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This specification is based upon and claims the benefit of priority from UK Patent Application Number GB 1720485.0 filed on Dec. 8, 2017, the entire contents of which are incorporated herein by reference. 
     BACKGROUND 
     Field of the Disclosure 
     The present disclosure relates to a method and apparatus for assembling cores in a fixed positional relationship in a shell mould and maintaining this fixed positional relationship in the subsequent casting process for production of a metal casting. 
     Description of the Related Art 
     The investment casting process is used to create metal components, e.g. turbine blades, by introducing molten metal into a ceramic shell of the desired final shape and subsequently removing the ceramic shell. 
     The process is an evolution of the lost-wax process whereby a component of the size and shape required in metal is manufactured using a wax pattern die into which molten wax is injected and allowed to solidify. The wax pattern is then dipped in ceramic slurry to create a shell on the wax pattern. The wax is removed and the shell fired. The resulting ceramic shell has an open cavity of the size and shape of the final component. Molten metal is introduced into the shell in order to form the component having near net-shape. The ceramic shell is subsequently removed, either or both of physically and chemically. 
     In order to make a component e.g. an aerofoil blade, with internal cavities e.g. internal cooling channels, a ceramic core is required. This is manufactured separately and is placed inside the wax pattern die prior to wax injection. After casting the metal in the ceramic shell and around the ceramic core, the ceramic core is removed. This can be done by leaching the ceramic core away using alkaline solution, for example, to leave the hollow metal component. 
     It is important to locate and support the ceramic core in a fixed positional relationship within the ceramic shell in order to accurately control the shape of the hollow metal component after casting. 
     Ceramic cores may be manufactured via particle injection moulding (PIM). A ceramic material, such as silica, is suspended in an organic binder (vehicle) to create a feedstock. This feedstock is then injected into a die cavity of the required size and shape and allowed to harden to create a “green” component comprising the ceramic and binder components. The binder is subsequently thermally or chemically removed and the ceramic is consolidated by sintering at elevated temperatures; this gives the final ceramic core. 
     New cooling concepts often require a complex configuration of core passages to give the most efficient level of cooling on the final component. To allow increased complexity of internal cooling passages whilst maintaining manufacturability of the ceramic core, the core can be manufactured in two pieces and assembled together. 
     In the case where the core is assembled from multiple components, then not only must the positional relationship between the core and the shell be controlled, but also the positional relationship between the component parts of the core must be controlled. 
     U.S. Pat. No. 5,295,530 discloses the manufacture of a single cast thin wall structure formed using multiple cores. As shown in  FIG. 5  of U.S. Pat. No. 5,295,530 (not reproduced here), a first core component is coated with a pattern wax and a second core is placed on top of the pattern wax coating. Pockets are drilled through the second core component into the first core component and rods used to secure the position of the second core component with respect to the first core component. A further pattern wax coating is formed on the second core component and further rods placed in the second core component and protruding from the further pattern wax coating. The casting shell is formed to cover the further pattern wax coating and the protruding rods. When the wax is removed, there remains the second core component suspended between the first core component and the casting shell by the rods. U.S. Pat. No. 5,394,932 discloses a composite core formed from first and second core components which join together via a tongue and groove arrangement. 
     U.S. Pat. No. 6,186,217 discloses a multi-piece core assembly for creating multi-wall components. The core components fit together by an arrangement of protrusions and recesses forming joints, the joints having an entry hole permitting the introduction of ceramic adhesive through the entry hole into the joint. 
     U.S. Pat. No. 6,557,621 discloses the assembly of core components by locating protruding members from one component into pockets of another component and using adhesive to hold the components together. 
     SUMMARY 
     The inventor has realised that the prior art approaches to the assembly of core components can be improved. The approaches disclosed in U.S. Pat. No. 6,186,217 and U.S. Pat. No. 6,557,621 are that the joints formed during assembly are likely to be weak and therefore that the cores are at risk of peeling away from each other during the casting process. The tongue and groove approach disclosed in U.S. Pat. No. 5,394,932 is restricted in that this approach does not allow for the assembly of complex components with multiple pedestals. Another difficulty with the approach of U.S. Pat. No. 6,557,621 is that it requires administering a precise dosage of adhesive for gluing the two components together. The approach of U.S. Pat. No. 5,295,530 is extremely time-consuming and therefore expensive, in view of the need to drill holes and place rods through the core components. 
     Accordingly, there is a need for a method and apparatus for assembling core components that provides an efficient and secure approach to fixing the positional relationship between the core components, ameliorating the problems associated with the prior art approaches discussed above. 
     In a first aspect, the present disclosure provides a method for manufacturing an assembly of core components for investment casting, the method comprising the steps: 
     providing a first core component and a second core component, wherein the first core component has an arrangement of either or both of pedestals and holes and the second core component has an arrangement of either or both of holes and pedestals; 
     assembling the first and second core components to mate in a required positional relationship, wherein the pedestals and holes of the first and second core components correspond to each other to allow the first and second core components to mate in the required positional relationship, the holes extending from a hole entry side to a hole exit side of the respective core component, and wherein the pedestals extend through the holes from the hole entry side to the hole exit side so that a protruding portion of each pedestal protrudes from the hole exit side; and
         applying a moulding material to encapsulate the protruding portions of the pedestals extending from the hole exit side with the moulding material to secure the pedestals with respect to the holes and thereby to secure the first core component with respect to the second core component.       

     In a second aspect, the present disclosure provides an assembly of core components for investment casting, the assembly comprising a first core component and a second core component, wherein the first core component has an arrangement of either or both of pedestals and holes and the second core component has an arrangement of either or both of holes and pedestals, the first and second core components being assembled to mate in a required positional relationship, wherein the pedestals and holes of the first and second core components correspond to each other to allow the first and second core components to mate in the required positional relationship, the holes extending from a hole entry side to a hole exit side of the respective core component, and wherein the pedestals extend through the holes from the hole entry side to the hole exit side so that a protruding portion of each pedestal protrudes from the hole exit side, the assembly further comprising a moulding material applied to encapsulate the protruding portions of the pedestals extending from the hole exit side to secure the pedestals with respect to the holes and thereby to secure the first core component with respect to the second core component. 
     In a third aspect, there is provided an investment casting process for manufacturing a cast metal component, the process comprising the steps: 
     providing a shell mould containing an assembly of core components, the assembly of core components comprising a first core component and a second core component, wherein the first core component has an arrangement of either or both of pedestals and holes and the second core component has an arrangement of either or both of holes and pedestals, the first and second core components being assembled to mate in a required positional relationship, wherein the pedestals and holes of the first and second core components correspond to each other to allow the first and second core components to mate in the required positional relationship, the holes extending from a hole entry side to a hole exit side of the respective core component, and wherein the pedestals extend through the holes from the hole entry side to the hole exit side so that a protruding portion of each pedestal protrudes from the hole exit side, the assembly further comprising a moulding material applied to encapsulate the protruding portions of the pedestals extending from the hole exit side to secure the pedestals with respect to the holes and thereby to secure the first core component with respect to the second core component; 
     introducing a molten metal into the shell mould to fill space between the shell mould and the assembly of core components; 
     allowing the molten metal to solidify; and 
     removing the shell mould and the core components. 
     In a fourth aspect, the present disclosure provides a cast component e.g. a turbine blade or guide vane having an arrangement of either or both of cavities and channels formed by the process of the third aspect. 
     In a fifth aspect, the present disclosure provides a gas turbine engine having a cast component according to the fourth aspect. 
     Accordingly, the present disclosure allows core components of complex shape to be assembled efficiently without necessarily requiring precise dosage of adhesive but yet allowing the assembly to have substantial strength to withstand the investment casting process. 
     Optional features of the present disclosure will now be set out. These are applicable singly or in any combination with any aspect of the present disclosure. 
     In some embodiments, the investment casting process provides a multi-cavity cast component, such as a gas turbine component. The cavities may be used for cooling during gas turbine operation, e.g. in a ducted fan turbine engine. The pedestals of the core components provide holes in the cast component, these holes linking cavities formed by the core components to allow flow communication of coolant in use, to enhance the cooling efficiency. 
     In some embodiments, the pedestals are integral with the respective core component from which they extend. In this manner, the pedestals can be formed with the core component via moulding of the entire core component. This provides an efficient approach to the manufacture of accurate shape and positioning of the pedestals on the core component. In some embodiments, however, the pedestals may additionally be machined to shape. This ensures accuracy of shape and dimensions. 
     Similarly, the holes may be formed via moulding of the entire respective core component. In some embodiments, however, the holes may be machined, which may be additional to or alternative to forming the holes via moulding. Such machining also ensures accuracy of shape and dimensions. 
     Accuracy of shape and dimensions of the pedestals and holes assists in the reduction of leakage of the moulding material into the gap between the two cores. Furthermore, such accuracy assists in preventing the metal during casting entering a clearance gap between the pedestals and holes. 
     In some embodiments, one core component, e.g. the first core component, may be provided with the pedestals. Thus, in some embodiments, the other core component, e.g. the second core component, may be provided with the corresponding holes. However, alternatively, it is possible for each core component to be provided with pedestals and holes, for mating engagement with corresponding holes and pedestals of the other core component. 
     In some embodiments, the second core component may be provided with a cavity into which the holes extend. Thus, when assembled, the respective pedestals of the first core component may extend into the cavity. The cavity may be common to at least some of the holes. In some embodiments, the cavity may be filled with the moulding material. As will be understood, the moulding material ideally remains solid and stable during the investment casting process. 
     In some embodiments, the pedestals formed on the first component may abut with a surface of the second component in order to define a limit of travel of the pedestals. In this way, the spacing of a gap between the first and second components can be defined. Where the second component has a cavity, the surface of the second component against which the pedestals of the first component abut may be a surface of the cavity. 
     In some embodiments, the surface of the second component against which the pedestals of the first component abut may be partially or fully machined. This can further improve the accuracy of control of the gap between the first and second components. 
     In some embodiments, the protruding portions of the pedestals may have an interlock shape to promote engagement with the moulding material. The interlock shape may include at least one re-entrant feature. 
     In some embodiments, there may be provided additional pedestals on either or both of the first and second core component that do not engage with through holes on the other core component but rather engage with pockets formed on the other core component. These pedestals need not necessarily be secured. They may be provided to further fix the spacing between the assembled core components. 
     The pedestals may have any suitable cross sectional shape when view along their principal axis, such as circular, elliptical or racetrack cross sectional shape. 
     In some embodiments, the core components are fired prior to assembly together. In other embodiments, however, the core components may be assembled in an as-moulded condition or in a partially fired condition. 
     In some embodiments, a core component may be fired or de-binderized or partially fired, then dipped in an inorganic, ceramic-forming liquid. The core component may then be fired, if for example before dipping it was only de-binderized or partially fired. If for example before dipping the core component was fired before dipping, then a further firing process after dipping is optional. 
     The ceramic forming liquid dip may provide sufficient adhesion between the pedestals and holes to allow the assembled core components to be secured together. In such embodiments, the application of the moulding material to encapsulate the protruding portions of the pedestals may be carried out after assembly and optional firing of the first and second core components. The ceramic forming liquid dip may be, for example, ethyl silicate, colloidal silica, colloidal alumina, colloidal yttria, or any other suitable substance which penetrates the pores of a core component leaving behind a residue which forms a ceramic material during the core firing or casting process. 
     In some embodiments, the inorganic material of the ceramic-forming liquid comprises particles having an average particle size smaller than the average pore size of the material of the core component. In this case, the assembled core components can be fully immersed in the dip, and then extracted and excess dip from the surface drained. 
     In some embodiments, the ceramic-forming liquid can provide a coating. Where the inorganic material of the ceramic-forming liquid has an average particle size larger than the average pore size of the material of the core component, the inorganic material substantially does not penetrate into the pores. In this case, where the assembled core components provide an internal cavity, the internal cavity can be selectively coated. In this way, the ceramic-forming liquid can be considered to be an example of a suitable moulding material for encapsulating the protruding portions of the pedestals to secure the first core component to the second core component. The coating can be applied by spraying, painting, or pouring and draining the ceramic-forming liquid, for example. 
     As mentioned above, in some embodiments, the pedestals formed on the first component may abut with a surface of the second component in order to define a limit of travel of the pedestals. The surface can be coated with an inorganic layer to assist in the securing of the pedestals of the first core component to the second core component. A suitable inorganic layer may be provided with the ceramic forming liquid dip disclosed above. 
     In some embodiments, the moulding material is formed from a mixture of colloidal and particulate silica and either or both of further optional particulates and organic agents. The moulding material typically sets by drying to form a solid moulding that acts to interlock the two core components. The solidified moulding material may remain stable during the investment casting process, but there may be an acceptable level of sintering that takes place between particles of the moulding material during pre-heat and casting in the casting process. 
     Where the second core component has a cavity, the cavity may be formed during moulding of the second core component. For example, the cavity may be formed using a chill pin. For example the cavity may be formed using a sacrificial insert which is removed before, during or after firing the second core component. Alternatively, the cavity may be formed by subtractive processing before or after firing the second core component. One example of a subtractive process is CNC machining. 
     In some embodiments, the first and second core components may be assembled with one or more spacers to define a gap between them. The one or more spacers may be formed of a sacrificial material. For example, the one or more spacers may be chaplets. For example, the one or more spacers may be formed of wax, e.g. as wax sheets. 
     In some embodiments, a seal element is provided between the first and second core components. The seal element may at least partially cover a gap between at least one of the pedestals and a respective one of the holes at the hole entry side. This has utility for the suppression of leakage of the moulding material through the gap. The seal element may be a sacrificial spacer at least in part defining a gap between the first and second core components. 
     In some embodiments, either or both of the first and second core components may be formed by an additive manufacturing process. Suitable additive manufacturing processes include ceramic 3D printing and stereo-lithography. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       Embodiments of the present disclosure will now be described by way of example with reference to the accompanying drawings in which: 
         FIG. 1  shows a longitudinal cross-section through a ducted fan gas turbine engine. 
         FIG. 2  shows a first and second core components for use in an embodiment of the present disclosure, before assembly. 
         FIG. 3  shows the first and second core components of  FIG. 2  during assembly. 
         FIG. 4  shows the first and second core components of  FIG. 2  after assembly. 
         FIG. 5  shows an alternative embodiment of the present disclosure in partial cross sectional view. 
         FIG. 6  shows a flow chart of a method according to an embodiment of the present disclosure. 
         FIG. 7  shows a flow chart of another method according to an embodiment of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     With reference to  FIG. 1 , a ducted fan gas turbine engine incorporating the features of the present disclosure is generally indicated at  10  and has a principal and rotational axis X-X. The engine comprises, in axial flow series, an air intake  11 , a propulsive fan  12 , an intermediate pressure compressor  13 , a high-pressure compressor  14 , combustion equipment  15 , a high-pressure turbine  16 , an intermediate pressure turbine  17 , a low-pressure turbine  18  and a core engine exhaust nozzle  19 . A nacelle  21  generally surrounds the engine  10  and defines the intake  11 , a bypass duct  22  and a bypass exhaust nozzle  23 . 
     During operation, air entering the intake  11  is accelerated by the fan  12  to produce two air flows: a first air flow A into the intermediate-pressure compressor  13  and a second air flow B which passes through the bypass duct  22  to provide propulsive thrust. The intermediate-pressure compressor  13  compresses the air flow A directed into it before delivering that air to the high-pressure compressor  14  where further compression takes place. 
     The compressed air exhausted from the high-pressure compressor  14  is directed into the combustion equipment  15  where it is mixed with fuel and the mixture combusted. The resultant hot combustion products then expand through, and thereby drive the high, intermediate and low-pressure turbines  16 ,  17 ,  18  before being exhausted through the nozzle  19  to provide additional propulsive thrust. The high, intermediate and low-pressure turbines respectively drive the high and intermediate-pressure compressors  14 ,  13  and the fan  12  by suitable interconnecting shafts. 
     The embodiments of the present disclosure relate to the manufacture of cast metal components with complex internal geometries, for example to turbine blades at the high, and either or both of the intermediate and low-pressure turbines  16 ,  17 ,  18  in  FIG. 1 , the turbine blades having interconnected internal cavities to assist with cooling of the blades in use of the engine. 
     A suitable cast component can be formed according to an embodiment of the present disclosure via investment casting. An assembly of ceramic core components is prepared, this assembly being held in a ceramic shell mould. Molten metal is introduced into the shell mould to fill space between the shell mould and the assembly of core components. The molten metal is allowed to solidify in a known manner to form a desired grain structure for the component (e.g. single crystal or columnar grain structure). The shell mould and the core components are then removed. This can be carried out in a known manner, for example by leaching away the ceramic of the shell mould and core components using a suitable alkaline solution. 
     There now follows a more detailed explanation of the assembly of the core components. A broad outline of the method according to an embodiment of the present disclosure is shown in the flowchart of  FIG. 6 . 
     In  FIG. 2 , first core component  102  and second core component  104  are shown separately, before assembly. 
     First core component  102  has an array of pedestals  106  extending from one principal surface  108 . Additional pedestals  110  are provided also extending from the principle surface  108  but towards the leading edge  112  of the first core component  102 . Pedestals  106  have a generally cylindrical trunk portion  114  and a protruding portion  116  with a re-entrant shape  118 . 
     Second core component  104  has a shape generally complementary to the first core component, a space between them (described in more detail below) intended to have a thin aerofoil shape. Second core component  104  has an array of holes  120  intended to receive pedestals  106  and an array of additional holes  122  intended to receive additional pedestals  110 . 
     Second core component  104  has a central cavity  124  defined by internal surface  126 . 
     The first  102  and second  104  core component are assembled to mate in the positional relationship illustrated in  FIG. 3 . The pedestals  106  of the first core component  102  extend through the respective holes  120  of the second core component  104  to protrude into internal cavity  124 . Some, but not all, of the pedestals  106  abut against the opposing internal surface  126  of the second core component, thereby limiting the travel of the first core component  102  towards the second core component  104  and thereby defining the extent of the gap  130  between the first core component  102  and the second core component  104 . As can be seen in  FIG. 3 , the protruding part  116  of the pedestals  106  extents into the internal cavity of the second core component  104 . 
     In the second component, the holes  120  have a hole entry side  132  and a hole exit side  134 . 
     As shown in  FIG. 4 , a moulding material  140  is applied in order to encapsulate the protruding portions  116  of the pedestals  106  extending from the hole exit side  134  with the moulding material  140  to secure the pedestals  106  with respect to the holes  120 . In turn, the first core component  102  is secured with respect to the second core component  104 . The moulding material  140  is filled into cavity  124  of the second core component and therefore fills the space around the protruding portion  116  of the pedestals  106 , including the re-entrant shape. This provides a particularly secure fixing of the pedestals  106  within the moulding material  140 . 
     The resultant assembly of core components, shown in  FIG. 4 , can then be used as described above in conjunction with a shell mould (not shown) for investment casting of the metal component having internal interconnected cavities defined by the arrangement of the core components. This approach allows core components of complex shape to be assembled efficiently without necessarily requiring precise dosage of adhesive, because the internal cavity  124  of the second core component  104  can simply be filled with the moulding material, and yet the approach allows the assembly to have substantial strength to withstand the investment casting process. 
     The pedestals  106  can be formed integrally with the first core component  102 , in the sense that they are formed during moulding of the first core component using a suitable mould. Additionally, however, the shape of the pedestals  106  may be finished by machining, in order to ensure precision and accuracy in their shape and dimensions. 
     The holes  120  may be formed via moulding of the second core component  104 . The holes may additionally be finished by machining, in order to ensure accuracy of shape and dimensions. 
     Accuracy of shape and dimensions of the pedestals  116  and holes  120  assists in the reduction of leakage of the moulding material  140  into the gap  130  between the two core components. Furthermore, such accuracy assists in preventing the metal during casting entering a clearance gap between the pedestals  116  and holes  120 . 
     As can be seen in the illustrated embodiment the first core component  102  is provided with pedestals  116  and the second core component  104  is provided with holes  120 . However, in alternative embodiments (not shown), each core component may have an array of pedestals and holes, for engagement with a corresponding array of holes and pedestals in the other core component. 
     Surface  126  of the second core component  104 , being the surface against which the pedestals  116  of the first core component  102  abut may be partially or fully machined. This can further improve the accuracy of control of the gap between the first and second core components. 
     The core components  102 ,  104  may be fired prior to assembly together. Alternatively, the core components  102 ,  104  may be assembled in an as-moulded condition or in a partially fired condition. 
     It is advantageous in some embodiments for at least one of the core components to be partially fired or de-binderized, then dipped in an inorganic, ceramic-forming liquid and then fully fired. After firing, the ceramic forming liquid may provide sufficient adhesion between the pedestals and holes to allow the assembled core components to be secured together during the firing process. This approach allows the application of the moulding material to encapsulate the protruding portions of the pedestals to be carried out after assembly and firing of the first and second core components. 
     The ceramic forming liquid dip may be, for example, ethyl silicate, colloidal silica, colloidal alumina, colloidal yttria, or any other suitable substance which penetrates the pores of a core component leaving behind a residue which forms a ceramic material during the core firing or casting process. 
     Similarly, where the pedestals  106  formed on the first core component  102  abut with surface  126  of the second core component  104  in order to define a limit of travel of the pedestals, surface  126  may be coated with an inorganic layer as described above to assist in the securing of the pedestals of the first core component  102  to the second core component  104 . 
     The moulding material  120  is formed from a mixture of colloidal and particulate silica and either or both of further optional particulates and organic agents. The moulding material sets by drying to form a solid moulding that acts to interlock the two core components  102 ,  104 . The solidified moulding material typically remains stable during the investment casting process. 
     Cavity  124  in the second core component  104  is formed during moulding of the second core component, although other approaches may be used for forming cavity  124 , such as by using a chill pin or sacrificial insert, or by subtractive processing (e.g. CNC machining) before or after firing the second core component. 
     In some embodiments, either or both of the first  102  and second  104  core components may be formed by an additive manufacturing process. Suitable additive manufacturing processes include ceramic 3D printing and stereo-lithography. As will be understood, it is in principle possible to manufacture a shape corresponding to the assembled core components using advanced additive manufacturing processes. However, it is advantageous to form the first and second core components separately and then assemble them, because this allows for the individual components to be inspected, and defective components removed prior to assembly. Another advantage is that when firing the core components, firing powder may adhere to the surface of the components or may be difficult to remove due to the complex nature of the desired core geometry. Assembly of simpler core components in the fired condition enables the firing powder removal step to be accomplished with less difficulty, and ultimately enables the formation of more complex cooling schemes for the cast metal component. 
       FIG. 5  shows an alternative embodiment of the present disclosure, in schematic partial cross sectional view. First  202  and second  204  core component are assembled to mate in the positional relationship illustrated in  FIG. 5 . Pedestal  206  (only one is shown, but in further examples, a plurality of pedestals may be provided) of the first core component  202  extend through respective hole  220  of the second core component  204 . Hole  220  and pedestal  206  are shown in cross sectional form. As can be seen in  FIG. 5 , protruding part  216  of the pedestal  206  extents into an internal cavity  224  of the second core component  204 . 
     The travel of the first core component  202  towards the second core component  204  is limited by spacer  250 , described in more detail below. This therefore defines the extent of the gap  230  between the first core component  202  and the second core component  204 . 
     In the second component, the hole  220  has a hole entry side  232  and a hole exit side  234 . 
     A moulding material (not shown in  FIG. 5 ) is applied in order to encapsulate the protruding portion  216  of the pedestal  206  extending from the hole exit side  234  with the moulding material to secure the pedestal  206  with respect to the hole  220 . In turn, the first core component  202  is secured with respect to the second core component  204 . The moulding material is filled into cavity  224  of the second core component and therefore fills the space around the protruding portion  216  of the pedestal  206 , including the re-entrant shape  218 . This provides a particularly secure fixing of the pedestal  206  within the moulding material. 
     The spacer  250  is a sacrificial spacer, formed for example from wax. In  FIG. 5 , spacer  250  is not shown in cross sectional form. The spacer  250  defines the width of the gap  230  between the first and second core components. The spacer can be formed around the pedestal  206  before the first core component is brought to the second core component. Alternatively, the spacer can be formed around the hole  220  at the hole entry side  232  of the second core component. The spacer  250  is therefore provided between the first and second core components and at least partially covering a gap  219  between the pedestal  206  and its respective hole  220  at the hole entry side  232 . 
     The spacer  250  assists in the reduction of leakage of the moulding material into the gap  230  between the two core components. 
     As mentioned above, the spacer can be fitted on the pedestal or fitted on the second core component. The spacer may be formed by over-moulding directly onto the pedestal or onto the second core component. Alternatively, the spacer can be formed, moulded, machined, or 3d printed separately, and then positioned over the pedestal prior to core assembly, or positioned over the hole prior to core assembly. If it is desired to position the spacer over the hole, the spacer may be provided with additional location features, for examples hooks that extend to the edge of the core component, or links that extend to an adjacent spacer. 
     In the embodiment described above, the sacrificial spacer may be formed of wax. In other embodiments, the sacrificial spacer may be formed of plastic, resin, rubber or any other organic material which will disappear during the investment casting process either by melting during the wax removal phase, dissolving in the condensed water of a de-wax autoclave, or evaporate or combust during the shell pre-fire before casting. 
     In the embodiment described above, component  250  is described as a spacer. However, component  250  may be considered to be a seal element. In this case, the seal element need not function to define the limit of the travel of the first core component  202  towards the second core component  204 . For example, the seal element may be deformable. The seal element may be formed of rubber, for example. In this case, separate spacers (not shown) may be included to define the limit of the travel of the first core component  202  towards the second core component  204 . Therefore, the function of the seal element can be to reduce or prevent the leakage of the moulding material into the gap between the two core components. 
     While the invention has been described in conjunction with the exemplary embodiments described above, many equivalent modifications and variations will be apparent to those skilled in the art when given this disclosure. Accordingly, the exemplary embodiments of the present disclosure set forth above are considered to be illustrative and not limiting. Various changes to the described embodiments may be made without departing from the spirit and scope of the invention. 
     All references referred to above are hereby incorporated by reference.