Abstract:
This invention relates to a mould assembly for a hot isostatic pressing, HIP, process for fabricating a component, comprising: a first part which includes a shaped surface for forming a first surface of the component, the shaped surface having at least one recess; a second part arranged to move relative to the first part during the HIP process so as to compress a powder in-fill held therebetween, wherein the second part includes a formation configured to focus pressure toward the recess so as to aid consolidation of the powder in-fill at a distal end thereof.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to a mould assembly for manufacturing components using Hot Isostatic Pressing, HIP. In particular, this invention relates to a reusable mould for manufacturing components using HIP. 
     HIP fabrication involves the consolidation of a metal or ceramic powder under high temperature and high pressure conditions. Typically, net-shape HIP processes use a machined consumable mild steel canister as a mould in which a powder in-fill is consolidated into a required component shape. After the HIP process is complete, the consumable canister is removed from the formed component by machining and pickling. 
     The use of consumable canisters is inherently time consuming and materially expensive as each manufactured component requires a new canister. Further, the pickling process requires highly caustic chemicals which have cost and potential safety implications for the technology. 
     The applicants have investigated the use of re-usable moulds in which a substantially incompressible mould is housed within a plain canister. The canister in this instance is still consumable, however, because the features are formed within a re-usable mould, the canister is simpler to design and manufacture. 
     The use of a reusable mould addresses many of the drawbacks of consumable canisters of the prior art. However, using reusable moulds provides new difficulties. 
     SUMMARY OF THE INVENTION 
     The present invention seeks to overcome some of the problems the applicant has discovered with re-usable moulds for HIP processes. 
     In a first aspect the present invention provides a mould assembly for a hot isostatic pressing, HIP, process for fabricating a component, comprising: a first part which includes a shaped surface for forming a first surface of the component, the shaped surface having at least one recess; a second part arranged to move relative to the first part during the HIP process so as to compress a powder in-fill held therebetween, wherein the second part includes a formation configured to focus pressure toward a first end of the recess so as to aid consolidation of the powder in-fill at a second end of the recess. 
     The use of re-usable incompressible moulds for powder HIP processes has the undesirable effect of applying a substantially uniform pressure to the first surface of a component. This is problematic for protrusions on the first surface of the component which are formed in recesses in the shaped surface of a re-usable mould as the pressure applied to the increased volume of powder within the recess can be insufficient. This can lead to incomplete consolidation of the powder in-fill and an inferior component. 
     Having a mould assembly with a second part which includes a formation for focussing pressure toward the recess allows a greater degree of pressure within the recess. This allows for better compaction and consolidation of the powder in-fill. 
     By focussing pressure it is meant that the pressure applied to the recess via the formation is greater than the isostatic pressure applied to the exterior of the mould assembly. 
     The component may be for a gas turbine engine. The gas turbine engine may be an aero engine. The component may be one from a group consisting of fan and compressor casings. 
     The first part may be made from a substantially incompressible material which does not deform during the HIP process so as to be re-usable. 
     The recess in the shaped surface corresponds to a protrusion to be formed on the first surface of the component. The component protrusion may be one of the group which comprises ribs, flanges and bosses. The at least one recess may be round in cross section. For example, the at least one recess may be circular or oval. The at least one recess may be polygonal in cross section. The at least one recess may be regular or irregular in cross section. The recess may be elongate. The longitudinal axis of the recess may be perpendicular to the general plane of the first part. 
     The component may include multiple recesses. The formation on the second part may be configured to focus pressure towards a plurality of recesses. Alternatively, a plurality of formations may be configured to focus pressure towards one or more recesses. 
     The second part may be made from an incompressible material. The incompressible material may be the same as the first part. Although the present invention is principally aimed towards HIP processes which use re-usable moulds, it is to be understood that the invention is not limited to re-usable moulds. 
     The component material may be one of a group of materials consisting of titanium alloys. The titanium alloys may include aluminium and vanadium. 
     HIPing of titanium alloys requires a thermal soak in the order of 900 degrees centigrade with pressures in the order of 100 MPa to 140 MPa. Hence, it is necessary to use a mould material which can withstand this temperature and pressure without deforming or compressing. Hence, the first part may be made from a substantially incompressible material which does not deform during the HIP process so as to be re-usable. The first part material may be one of a group of materials consisting of high temperature capable nickel alloys. 
     The Nickel alloys may include chronite and turbine blade casting alloys. 
     A further advantage of the Nickel alloy should be that it does not bond to itself or a titanium component or a canister alloy during or after HIPing. Hence, the first and second parts of the mould assembly can be separated from the formed component without damage. 
     The formation may be convex or concave. The formation may be a protrusion. The protrusion may extend from a body of the second part toward the first part when the mould is assembled. The protrusion may be elongate. The longitudinal axis of the protrusion may be perpendicular to the general plane of the second part. The protrusion may be round in cross section. For example the cross section may be circular or oval. The protrusion may be polygonal in cross section. The cross section of the protrusion may regular or irregular. The protrusion may be cylindrical or cubiodal. The protrusion may have a constant cross sectional area along its length. The cross sectional area of the protrusion may increase along its length relative to the recess. 
     When assembled, the protrusion may have a cross sectional area at the proximal face which is greater than an open end of the recess. The cross sectional area of the proximal face may be the same as the open end of the recess. The cross sectional area at the proximal face may be smaller than the open end of the recess. The recess and protrusion may be concentrically aligned. 
     The proximal face of the protrusion may include features or formations to help focus the pressure towards the at least one recess. For example, the proximal face of the protrusion may be concave. Alternatively, the proximal face may include a formation or feature which distributes the pressure across a recess. 
     The mould assembly may include a sealed canister in which the first part and second part are housed for the HIP process. The second part may contact a canister so as to be subjected to the isostatic pressure during the HIP process. The contact may be direct. Alternatively, the contact may be via an intermediate member. The second part may include a pressure plate which contacts the canister so as to be subjected to the isostatic pressure during the HIP process. The pressure plate and formation may be integrally formed. The pressure plate may be round or polygonal. The pressure plate may be integral to the canister which houses the mould assembly and powder in-fill when in use. The formation may be an integral part of the canister. 
     The canister may have a lid so as to seal it. The lid may be attached to the canister via any suitable means such as welding. The canister may be a mild steel canister. The canister may have a wall thickness of less than 6 mm. Alternatively, the canister may have a wall thickness less than 5 mm. The canister may have a wall thickness of less than 4 mm. The canister may have a wall thickness greater than 1 mm. The canister may have a wall thickness greater than 2 mm. The canister may have a wall thickness greater than 3 mm. The canister may include an annex for housing parts of the mould assembly. The annex may be attached to the canister via a welding. The annex may be integral to the canister. The annex may form part of the lid of the canister. 
     The mould assembly may further comprise a seal which defines a void around at least a portion of the second part, the seal preventing ingress of powder in-fill into the void prior to the application of isostatic pressure in the HIP process. 
     The void may be provided so that the second part can move into the void toward the first part during the HIP process. The seal may be rigid. The seal may be a plate. The seal may be a flexible membrane. The flexible membrane may be a foil. The foil may be made from mild steel. The thickness of the foil may be in the range of between 50 microns to 1000 microns. 
     The formation may pass through the seal. The seal may envelope second part. The seal may envelope the pressure plate and the formation so as to separate the pressure plate and formation from the powder in-fill. 
     The formation may have a proximal face which is larger than the open end of the recess. The proximal face may be substantially the same size as the proximal face of the open end of the recess. The protrusion may be dimensioned so as to fit within the recess during the HIP process such that a cavity can be formed in the rear of the component protrusion. 
     The second part may be movable from a first pre-pressure position to a second post-pressure position, wherein when the second part is in the post-pressure position at least a portion of the protrusion sits within the recess such that a component protrusion formed within the recess includes a cavity in a rear side thereof. 
     The first part may comprise an insert having a plurality of pieces which combine to provide the recess and a holding piece having at least one cavity in which the insert is mateably received. 
     During a HIP process the mould and constituent powder in-fill expand and contract during the thermal cycle. If the thermal expansion of the mould is greater than that of the component material, any protrusions will be compressed and frictionally retained within the tooling when cooled. Subsequent separation often leads to damage of the tooling or component. This problem is greater for large component manufacture and for components which include multiple protrusions. 
     Having a holding piece with an insert which can be split into multiple pieces allows the tooling to be disassembled after the HIP process is complete. Hence, each piece of the mould can be pulled obliquely away from the surface of a manufactured component rather than being tangentially slid off a protrusion against any frictional retention. 
     The insert may comprise a facing surface against which a portion of the component is formed and wherein a portion of the parting line between the insert and holding piece has a draft angle in the range of between 10 and 60 degrees with respect to the a facing surface of the insert. 
     The holding piece may include a facing surface against which a portion of the component is formed in use. 
     The cavity which mateably receives the insert may be an aperture which passes through the holding piece from a first side which faces the component to an exterior second side such that a force can be applied to the insert from the exterior second side so as to remove it from the holding piece after use. 
     A portion of the parting line between the insert and holding piece may have a draft angle with regard to the facing surface of the insert in the range of between 10 and 60 degrees. Preferably, the draft angle is substantially 45 degrees. Having a parting line with a draft angle of 45 degrees allows the insert to be separated from the holding piece more readily. 
     The insert may include a parting line which dissects the insert into pieces. There may be two or more insert pieces. The insert pieces may be symmetrical. The insert pieces may be similar in size and shape. The insert pieces parting line may be flat so as to not be interlocking. The insert pieces parting line may extend perpendicularly from the facing surface of the insert. 
     The cavity which mateably receives the insert may be an aperture. The aperture may pass through the holding piece such that a force can be applied to the insert from an exterior surface of the mould assembly so as to remove the insert from the holding piece after use. 
     The insert may include a through-hole so as to expose the powder in-fill to the exterior of the mould assembly. The through-hole may comprise portions of the or each insert piece. Having a though-hole in the insert allows a pressure to be applied directly to the second end of the recess via the canister which can aid consolidation and provide shorter HIP process times. 
     The insert may include a recess having walls which extend at substantially 90 degrees from the facing surface of the insert. Alternatively, the walls may extend from the facing surface at an angle less than 90 degrees. The cross sectional area of the recess may increase along the length of a recess such that a formed protrusion can have an overhang with respect to the facing surface. 
     In a second aspect the present invention provides a method of fabricating a component using a mould assembly in a HIP process, the mould assembly including: a first part which includes a shaped surface for forming a first surface of the component, the shaped surface having at least one recess; a second part arranged to move relative to the first part during the HIP process so as to compress a powder in-fill held therebetween, wherein the second part includes a formation configured to focus pressure toward a first end of the recess so as to aid consolidation of the powder in-fill at a second end of the recess, wherein the method includes the steps of: enclosing the first part and second part within a canister so as to provide the assembled mould assembly in which the first and second part can be moved relative to each other during the HIP process; filling the canister with a powder in-fill which will form the component; 
     evacuating the canister; applying a thermal and pressure cycle to the canister so as to move the second part relative to the first part such that the powder in-fill is consolidated within the recess; removing the canister from the component and mould assembly; 
     removing the first part and second part from the component. 
     The method of the second aspect may include the step of providing a seal which defines a void around at least a portion of the second part. The seal may prevent ingress of powder in-fill into the void prior to the application of isostatic pressure in the HIP process. 
     The mould assembly used in the method of the second aspect may further comprise an insert comprising a plurality of pieces which combine to provide the recess in which a protrusion of the component can be formed and a holding piece having at least one formation in which the insert is mateably received. In this case, the method may further include the steps of: mateably inserting the pieces of the insert into the holding piece formation to provide the first part; and, removing the holding piece from the component and insert pieces and, individually removing the insert pieces so as to remove the first part after the canister has been removed from the component and mould assembly. 
     The cavity which mateably receives the insert may be an aperture which passes through the holding piece, and the method of the second aspect may comprise the further step of: applying a force to the insert from the exterior of the mould assembly so as to separate the holding piece and insert. 
    
    
     
       BRIEF DECRIPTION OF THE DRAWINGS 
       Embodiments of the present invention will now be described with the aid of the following drawings in which: 
         FIG. 1  shows a cross section of a mould assembly for a HIP process. 
         FIGS. 2 a  and 2 b    show a cross section of a mould assembly according to the present invention before and during a HIP process, respectively. 
         FIG. 3  shows an alternative embodiment of the mould assembly according to the present invention. 
         FIGS. 4 a  and 4 b    show cross sections of another embodiment of the present invention and the resulting component, respectively. 
         FIGS. 5 a  and 5 b    show a reusable HIP mould assembly in cross-section before and after HIPing respectively. 
         FIGS. 6 a  and 6 b    show embodiments of the first part of a reusable HIP mould in cross section. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Throughout the following description reference is made to proximal and distal ends, surfaces and sides of various parts. Generally, the terms proximal and distal are in relation to the powder in-fill such that proximal relates to the end, side or face closest to or within the powder in-fill and distal relates to the end, side or face furthest from the powder in-fill. 
       FIG. 1  shows a HIP mould assembly  10  comprising a canister  12  in the form of a mild steel box, a reusable mould  14  having a recess  16  and a powder in-fill  18 . The powder in-fill  18  is introduced to the canister  12  via a filling tube  20  and fills the void defined between the upper  22  and cavity  24  surfaces of the reusable mould  14  and the walls  26  of the canister  12 . 
     The powder in-fill is consolidated during a HIP process so as to form a component and includes the constituent materials which make up the component. In the present embodiment the formed component is a titanium alloy which is a particularly useful material for gas turbine engine components due to the low density and low high temperature creep. The titanium alloy is Ti6/4. Suitable particle sizes for HIPing with titanium alloys typically range from 50 to 250 microns. Of course the skilled person will appreciate that other materials can readily be used in HIP manufacturing as is known in the art. 
     The mould  14  is a substantially incompressible block of nickel alloy, for example, a high temperature turbine blade casting alloy, having a shaped surface which has been machined to provide the shape of the component which is desired. The shaped surface includes a first surface  22  and a recess  16  which corresponds to a protruding feature  28  on the first surface of the component in the form of a boss. The boss extends perpendicularly from the first surface  30  of the component and has a planar distal face. The skilled man will appreciate that the geometry of the mould needs to be calculated to allow for the thermal expansion of the mould at the HIP temperature and the contraction of the cooled component. 
     The canister  12  is a mild steel vessel in which the mould can be placed prior to being sealed shut, typically by having a lid welded in place. The canister  12  needs to be of a suitable thickness so as to maintain the sealed environment for the mould  14  and powder in-fill  18  during the HIP process. This thickness will vary according to the material and dimension of the component being produced but is typically in the order of a few millimetres. 
     To form a component from titanium alloys, it is necessary to use a high temperature soak, typically in the range of 900 degrees. Hence, the reusable mould  14  needs to be of a suitable material to withstand the necessary high temperature. Nickel alloys are generally suitable for making reusable moulds for the HIP process. Further, nickel alloy components tend not to bond to the titanium alloy component which helps with the separation of the mould  14  and component after the HIP process is complete. 
     To form a component, the mould  14  is loaded into the canister  12  which is then sealed. The powder in-fill  18  is injected into the canister  12  via tube  20  so as to fill the void which is defined by the walls  26  of the canister  12  and the facing surface of the mould  14 . Any air which remains in the canister  12  is evacuated from the void using a vacuum pump. A typical evacuation pressure is 1.3 Pa. The canister is placed within a pressure vessel which is evacuated and filled with an inert gas such as Argon. The canister is then subjected to a temperature soak of approximately 920 degrees under an external isostatic pressure of approximately 120 MPa to 140 MPa for between 2 and 4 hours, before being cooled and removed from the pressure vessel. 
     Once cooled, the canister is removed via a combination of machining and pickling before the component taken from the mould and machined to provide the finished article. 
     Because the re-usable mould is incompressible the pressure applied to the first surface of the component is uniformly distributed across the width of the mould as viewed in  FIG. 1 . Hence, there can be insufficient pressure to consolidate the volume of powder in-fill within the recess. Hence, in particularly there can be a lack of consolidation at the distal end of deep recesses which can lead to an inferior component being produced. 
       FIG. 2 a    shows a mould assembly  210  according to the present invention. In addition to the assembly shown in  FIG. 1 , the mould assembly  210  includes a first part  214 , a second part  232 , a guide member  234  and a seal  236  in the form of a mild steel foil wrap. The second part  232 , guide member  234  and seal  236  are all housed within an annex  250  of the canister  212 . 
     The first part  214  has a shaped surface  222  which has been machined to include a recess  216  of the appropriate dimensions for creating a component with a boss  228 . The skilled person will appreciate that the exact dimensions required of the recess  216  will in part be determined by the thermal expansion and shrinkage of the first part  214  and the component during and after the HIP process. 
     The second part  232  includes a pressure plate  238  in the form of a Nickel alloy disc. The pressure plate  238  has a distal surface  240  which is in contact with the canister  212  via the foil seal  236  such that the pressure on the outside of the canister  212  results in a force to the pressure plate  238 . On the proximal side  242  of the pressure plate  238  there is an integral formation in the form of a protrusion  244 . The protrusion  244  is an elongate cylindrical member which extends perpendicularly in relation to its longitudinal axis from the centre of the proximal side  242  of the pressure plate  238  toward the recess  216  in the first part  214  of the mould assembly  210 . 
     The force placed on the pressure plate  238  via the isostatic pressure is transferred to the proximal face  248  of the protrusion. The surface of the pressure plate  238  in contact with the canister  212  is substantially larger than the proximal face  248  of the protrusion  244  such that the pressure exerted by the protrusion  244  on the powder in-fill  218  in the vicinity of the proximal face  248  is greater than the isostatic pressure on the pressure plate  238 . In this way, the isostatic pressure is focussed towards the recess  216 . 
     The pressure plate  238  and protrusion  244  rest in a first position prior to the application of the isostatic pressure. When isostatic pressure is applied to the exterior of the canister  212 , the pressure plate  238  and protrusion  244  are forced toward the recess  216  in the first part  214  of the mould assembly  210  until coming to rest at a second position once the compaction and consolidation process of the powder in-fill  218  is complete. 
     The mould assembly  210  includes a guide member  234 . The guide member  234  is a plate of a similar size and shape to the pressure plate  238  with a central aperture which snugly receives and supports the protrusion  244  as it passes from the first position to the second position. 
     Prior to the HIP process the pressure plate  238 , protrusion  244  and guide member  234  are set within an annex  250  of the canister  212 . The annex  250  is sealed from the main canister chamber by a seal  236  in the form of a mild steel foil having a thickness of approximately  200  micrometres. The foil seal  236  envelopes the pressure plate  238 , protrusion  244  and guide member  234  so as to prevent ingress of the powder in-fill  218  prior to the HIP process and thereby provides a void  237  between the pressure plate  238  and guide  234  into which the pressure pate can move under isostatic pressure. The dimensions of the canister annex  250  walls are such that they are forced inward so as to collapse during the movement of the pressure plate  238  from the first position towards the second position, as shown in  FIG. 2   b.    
     To form a component, the mould is loaded into the canister  212  which is then sealed. The powder in-fill  218  is injected into the canister  212  via tube  220  so as to fill the void which is defined by the walls  226  of the canister  212  and the shaped surface  222  of the first part  214  of the mould assembly. Any air which remains in the canister  212  is evacuated using a vacuum pump. A typical evacuation pressure is 1.3 Pa. The canister  212  is placed within a pressure vessel which is evacuated before being filled with an inert gas such as Argon. The canister  212  is then subjected to a temperature soak of approximately 900 degrees under an external isostatic pressure  252  of approximately 120 MPa to 140 MPa for between 2 and 4 hours before being cooled and removed from the pressure vessel. 
     The isostatic pressure  252  creates a force on the pressure plate  238  which causes it to move from the first position toward the second position and recess  216 . The relationship between the pressure plate  238  and protrusion  214  is such that force applied to the larger area of the pressure plate  238  via the canister  212  wall is transferred to the smaller area of the proximal face  248  of the protrusion  244 . This results generally in a redistribution of the isostatic pressure  252  on the exterior of the canister  212  to a focussed area of pressure beneath the proximal face  248  of the protrusion  244 . As the proximal face  248  of the protrusion  244  moves toward the recess  216 , the powder in-fill  218  at its distal end is compacted into and consolidates within the recess  216 . This ensures that the consolidation within the recess is sufficient to provide a homogeneous protrusion on the first surface of the component. 
     The redistribution of pressure from the canister  212  to the proximal face  248  of the protrusion  244  largely deprives the area under the guide member  234  of a compacting pressure. However, once the pressure plate  238  contacts the guide member  234  it forces the guide member  234  toward the first part  214  of the mould assembly  210  so as to compact and consolidate the powder underneath the guide member  234 . As this occurs, the isostatic pressure  252  on the pressure plate  238  is no longer focussed toward the recess  216  via the protrusion  244  but is spread uniformly across the proximal face of the guide member  234  and protrusion  244 . 
       FIG. 3  shows mould assembly  310  similar to the assembly shown in  FIG. 2 a  and  b   . Thus there is a canister  312  having a fill tube  320  and annex  350 , a first part  314 , second part  332  and seal  336 . The second part  332  includes a pressure plate  338  and a protrusion  344  as per the previous embodiment. However, in this embodiment the guide member is omitted such that the travel of the protrusion  344  from the first position to the second position is guided by the isostatic pressure on the canister  312  and canister annex  352  walls and the contact with the powder in-fill  318 . 
     The seal  336  is in the form of a mild steel foil which is similar to the embodiment in  FIGS. 2 a  and 2 b   . The foil is wrapped around the pressure plate  338  and protrusion  344  so as to prevent ingress of the powder in-fill  318  into the space beneath the pressure plate  338  and provide a void  337  into which the pressure plate  338  can move under the isostatic pressure. 
     Once the isostatic pressure is applied during the HIP process the pressure plate  338  moves from the first position into the void  337  provided by the foil seal  336 . As the pressure plate  338  moves the powder in-fill  318  lying adjacent the pressure plate  338  to flow and re-distribute under the compacting force, thereby spreading out to fill the void  337 . Whilst this process is on going the force applied via the canister annex  312  wall on the distal face of the pressure plate  338  is largely focussed beneath the proximal face of the protrusion  348 . Once, the powder has redistributed to substantially fill the void  337 , the pressure plate  338  contacts and compacts the powder beneath it such that consolidation can take place. It will be understood that the foil seal  336  is of sufficient thickness and strength that it deforms and ruptures under the force of the pressure plate  338  such that the powder in-fill  318  can spread. 
       FIG. 4 a    shows an assembly similar to the assembly shown in  FIG. 3 . Thus, there is shown a mould assembly  410  including a first part  414  having a recess  416  in which a protrusion  438  of a component  411  can be formed. The mould assembly  410  also includes a second part  432  having a pressure plate  438  and a protrusion  444  which are separated from the powder in-fill  418  by a void  437  provided by a foil seal  436 . 
     The dimensions of the protrusion  444  in the mould assembly  410  of  FIG. 4 a    are such that it can fit within the recess  416  of the first part  414  when in the second position. In this way a hollow  415  is formed in the rear of the protrusion  428  as shown in  FIG. 4   b.    
     The following embodiments shown in  FIGS. 5 a  and 5 b    describe a first part  514  of a mould assembly  510  which can be used with the mould assemblies  10 ,  210 ,  310 ,  410 , of the previously described embodiments. The second part of the mould assembly is not shown in the following embodiments for the sake of clarity. 
     As with the earlier described embodiments,  FIG. 5 a    shows a HIP mould assembly  510  which includes a canister  512  in the form of a mild steel box, a first part of a reusable mould  514  having a plurality of recesses  516   a ,  516   b  and a powder in-fill  518 , which forms a component  517  once consolidated during the HIPing process. The powder in-fill  516  is introduced to the canister  512  via a filling tube  520  and fills the void defined between the upper  522  and recess  524  surfaces of the reusable mould  514  and the walls  526  of the canister  512 . During the HIP process, the temperature soak and pressure consolidate the powder in-fill  518  so as to form a homogenous component  517 . 
       FIG. 5 b    shows the component  517  and first part  514  after the HIP process with the canister  512  removed for clarity. The upper surface  532  of the component  517  as viewed in  FIG. 5 b    is deformed as a result of the isostatic pressure applied during the HIP process. This deformation is typically removed in a subsequent machining step to provide the finished component. 
     The powder in-fill  518  and first part  514  expand during the thermal soak and contract during the subsequent cooling. The first part  514  and powder in-fill  518  (component  517 ) are made from a nickel alloy and a titanium alloy respectively. Hence, they have different coefficients of thermal expansion. Specifically, the nickel alloy of the mould  514  has a higher coefficient of thermal expansion and therefore contracts to a greater degree than the component  517  during the cooling phase of the HIP process. After cooling the component protrusions  528   a ,  528   b , are larger than the mould by an amount two times delta d, as shown in  FIG. 5   b.    
     Because the protrusions  528   530  are entirely surrounded by the recesses  516   a ,  516   b , a compressive force results which grips and retains the protrusions  528   a ,  528   b  within the respective recesses  516   a ,  516   b , as shown in  FIG. 5 b    by arrows  534 . This prevents the mould being readily separated from the component and applying a large force to separate the two can result in damage to the mould  514  and or component  517 . 
     The present invention provides a mould assembly  610  as shown in  FIGS. 6 a  and 6 b   . The mould assembly  610  generally includes a holding piece  636  and inserts  638 ,  640 ,  642 , which form the recesses in the mould assembly  610 , The inserts  638 ,  640 ,  642 , include a plurality of insert pieces  638   a,b ,  640   a,b ,  642   a,b , which are retained in corresponding cavities in the holding piece  636  and which combine to form the recess required for a component protrusion. 
     All of the inserts  638 ,  640 ,  642  are generally a truncated cone shape with the larger end of the cone providing the facing surface  646  for abutting the powder in-fill  618  and the narrow end seated within the holding piece  636 . When the inserts  638 ,  640 ,  642  are located in the holding piece  636 , the facing surfaces  646  of the inserts and the facing surface  644  of the holding piece  636  are flush so as to provide a continuous smooth profile against which the component can be formed. 
     The first insert  638  on the left of the mould assembly  610  as viewed in  FIGS. 6 a  and  b   , has a recess  639  within the conical body in the form of a cylinder having a circumferential side wall and flat circular base surface. The open end of the recess is defined as the first end and the base of the recess is defined as the second end. The insert is mateably received within a cavity  641  in the holding piece in the form of an aperture which passes from the facing surface  644  of the holding piece  636  to a second surface  648  on the exterior of the holding piece  636 . The holding piece  636  and insert  638  mate so as to define a parting line  639   a  along the angled conical face  639  of the insert  638 . The parting line  639   a  of the embodiment is at 45 degrees relative to the facing surface  646  of the insert  638 . Having a parting line  639   a  of 45 degrees between the holding piece  636  and insert  638  allows the two parts to be easily separated after the HIP process is complete. 
     When the insert  638  is mateably received within the aperture  641  in the holding piece  636 , as shown in  FIG. 6 b   , the rear of the insert  638  is exposed from the wxterior surface  648  of the holding piece  636 . This allows pressure to be applied directly to the rear surface of the insert  638  from the exterior of the mould assembly  610  once the canister has been removed which aids separation of the holding piece  636  and insert  638 . 
     The second insert  640  is similar to the first insert  638  except that it has a curved circular base so as to provide the corresponding component protrusion with a domed distal end and that it is mateably received within a closed cavity  643 . The closed cavity  643  forms a parting line with the insert which is parallel to the facing surface  646  of the insert  640 . Hence, when the second insert  640  is placed within the closed cavity  643  the holding piece  636  envelopes the rear of insert  640 . 
     The third insert  642  includes a through-hole rather than a closed recess. The through-hole allows the powder in-fill  618  to be exposed from a rear side of the insert  642  such that when it is inserted into the canister  612 , pressure is more effectively applied to the second end of the recess which is in direct contact with the canister  612 . The third insert  642  is situated within a cavity in the form of an aperture  645  in a similar way to the first insert  638 . 
     Each of the inserts  638 ,  640 ,  642  include two insert pieces which are symmetrical about a central parting line  650 ,  652 ,  654 , which dissects each insert  638 ,  640 ,  642 . The parting lines  650 ,  652 ,  654 , between pieces are flat and extend perpendicularly from the facing surface  646  of each insert so as to provide no interlock therebetween. In this way, the inserts  638 ,  640 ,  642 , are held together at the parting lines  650 ,  652 ,  654 , by the holding piece  636  and powder in-fill  618  only. 
     Having multiple pieces within a given insert  638 ,  640 ,  642 , allows the insert to be disassembled from the component protrusion after a component has been formed during the HIP process. Specifically, the arrangement of the insert pieces  638 ,  640 ,  642 , is such that each piece can be removed from the facing surface of the component at an oblique (or perpendicular) angle rather than parallel to and against any frictional retaining force. Hence, the frictional retaining force which results from the differential thermal contraction between the component and the first part  614  of the mould can be negated. 
     The inserts  638 ,  640 ,  642 , and holding piece  636  are made from the same material, Nickel alloy, so as to provide the first part  614  with a uniform thermal expansion and contraction. 
     To form a component, the first part  614  is loaded into the canister  612  which is then sealed. The powder in-fill  618  is injected into the canister  612  via tube  620  so as to fill the void which is defined by the walls  626  of the canister  612  and the facing surface of the first part  614 . Any air which remains in the canister  612  is evacuated from the void using a vacuum pump. A typical evacuation pressure is 1.3 Pa. The canister  612  is placed within a pressure vessel which is also evacuated before being filled with and inert gas such as Argon. The canister is then subjected to a temperature soak of approximately 900 degrees under an external pressure of approximately 120 MPa to 140 MPa for between 2 and 4 hours before being cooled and removed from the pressure vessel. Once the HIP process is complete the canister  612  can be removed by machining and pickling. 
     After the canister  612  is removed, the holding piece  636  can be removed from the component and inserts  638 ,  640 ,  642 , simply by applying a pulling force to the holding piece  636 , and an opposing pushing force to the exterior of the inserts  638 ,  642 , which pass through the holding piece  636  to the exterior side. Once the holding piece  636  is removed, the inserts  638 ,  640 ,  642  are free to separate, the individual insert pieces are removed from the formed protrusions. 
     The skilled person will appreciate that the above described embodiments are demonstrative, not restrictive, and that the scope of the invention is determined by the claims. For example, the invention is primarily described in the context of a re-usable mould. However, the invention could be implemented on a disposable mould. 
     Further, the component and mould materials are not restricted to Titanium alloys and Nickel alloys respectively. Also, although the present invention is described in the context of large components for gas turbine engines, it will be understood that the invention is a generic one which may find application elsewhere.