Abstract:
A mold assembly ( 100 ) for forming an object, the mold assembly comprising: a first mold portion ( 110 ) comprising an internal surface ( 112 ), an external surface ( 114 ) and an opening ( 116 ) leading from the internal surface to the external surface; and a second mold portion ( 120 ) positionable with respect to the opening ( 116 ) to cover the opening; wherein the first and second mold portions define an interior ( 130 ) for receiving a material to be molded into the object; and wherein the second mold portion comprises a surface ( 122 ) adapted to face the interior of the mold assembly and form a feature on the object.

Description:
This invention relates to a mould assembly and particularly but not exclusively relates to a mould assembly for hot isostatic pressing applications. 
     BACKGROUND 
     Objects or components may be formed by forging. By way of example,  FIG. 1(   a ) shows a gas turbine combustion chamber casing  10 , which may be manufactured from a large single piece forging  20 . ( FIG. 1  shows sections through the combustion chamber casing  10 , which is substantially tubular with a longitudinal axis  30 .) The design of such casings  10  dictates the envelope of the forging  20 . The forging may then be machined to provide the finished product. However, it is typical that approximately 90% of the purchased material has to be removed to produce the finished component. Consequently, the current method of manufacture of gas turbine combustion chamber casings results in a material ‘fly to buy’ ratio of approximately 10%. Whilst this excess material may be recycled, it does not command the same scrap price as the purchase price. This incurs cost in both cycle time to remove the material and consumables. 
     Alternatively, Hot Isostatic Pressing (HIP), for example powder HIP, may be used as a method of manufacture, as it may have a much better ‘fly to buy’ ratio. As shown in  FIG. 1(   b ), powder HIP requires a tool  40 , e.g. a mould, to be manufactured from a large forging. The tool  40  may comprise a plurality of portions  40   a - c , which may be separated to remove the casing  10 . However, any external features on the object or component being moulded need to be machined on the inside of the mould, which causes access problems for both machining and inspection. Furthermore, the machining of internal features deep inside a mould requires the use of long tooling and right angled heads that are not as rigid as standard tooling. This can result in tool ‘push off’ and tool chatter resulting in non-conforming features on the mould. As a result, machining with such tools is generally quite slow and expensive. In addition, if any of the internal features are machined incorrectly then the mould could be scrap. 
     The present invention therefore seeks to address these issues. 
     STATEMENTS OF INVENTION 
     According to a first aspect of the present disclosure, there is provided a method of forming an object, the method comprising: providing a mould assembly comprising first and second mould portions defining an interior for receiving a particulate material to be moulded into the object, wherein the first mould portion comprises an internal surface, an external surface and an opening leading from the internal surface to the external surface, and wherein the second mould portion comprises a surface adapted to face the interior of the mould assembly and form a feature on the object; filling the mould assembly with the particulate material; positioning the second mould portion with respect to the first mould portion to cover the opening; forming the object in the mould assembly by applying heat and pressure to the mould assembly; and simultaneously forming the object and the feature on the object. 
     The method may further comprise securing the second mould portion to the first mould portion. For example, the second mould portion may be securable to the first mould portion by virtue of a freeze fit, a press fit, mechanical attachment (bolts, studs, screws, etc.), adhesives, fusion welding techniques or any other attachment means. The mould assembly may comprise one or more openings and one or more second mould portions. 
     The method may further comprise removing the mould assembly from the object. 
     The step of providing the mould assembly comprises the step of providing the first mould portion having an inner wall and an outer wall defining an annular chamber between them. 
     The opening may be formed by drilling through either the inner or outer wall. 
     The mould assembly may comprise a plurality of openings and a respective second mould portion for each opening. 
     The second mould portion may comprise a protruding portion. The protruding portion may be adapted to engage the opening. 
     The second mould portion may comprise a feature portion. The feature portion may be adapted to form a corresponding feature on the object. The first and second mould portions may be for use in a Hot Isostatic Pressing process, for example a powder Hot Isostatic Pressing process. 
     A turbomachine may comprise the aforementioned object. A gas turbine may comprise the aforementioned object. The object may comprise a casing, for example a combustion chamber casing. 
     The second mould portion may be secured to the first mould portion. The mould assembly may be removed from the object. Hot Isostatic Pressing (e.g. powder Hot Isostatic Pressing) of the material may be used to form the object. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a better understanding of the present invention, and to show more clearly how it may be carried into effect, reference will now be made, by way of example, to the accompanying drawings, in which: 
         FIGS. 1(   a ) and  1 ( b ) show prior art arrangements for forming an object; 
         FIG. 2  shows a partial sectional side view of a mould for a combustion chamber casing according to an example of the present disclosure; and 
         FIGS. 3(   a )-( d ) show a process by which a mould according to an example of the present disclosure may be formed. 
     
    
    
     DETAILED DESCRIPTION 
     With reference to  FIG. 2 , a mould assembly  100  for forming an object, e.g. a component or article, according to an example of the present disclosure, may comprise a first mould portion  110  and a second mould portion  120 . The first and second mould portions may define an interior  130  for receiving a material to be moulded into the object. The moulded object may be a component. In particular, the object may be a component for a gas turbine engine, for example a casing section. The object may be a casing section for a combustion chamber. 
     The first mould portion  110  may comprise an internal surface  112  and an external surface  114 . The internal surface  112  may correspond in shape to the desired shape for the object to be moulded. The first mould portion  110  may further comprise an opening  116 , e.g. bore or hole, leading from the internal surface  112  to the external surface  114 . 
     The second mould portion  120  may be positionable with respect to the opening  116  to cover (e.g. seal, close or conceal) the opening. For example, as shown in  FIG. 2 , the second mould portion may at least partially be inserted into the opening  116  of the first mould portion  110 . The second mould portion  120  may be secured to the first mould portion  110 . In particular, the second mould portion  120  may be secured to the exterior surface  114  of the first mould portion  110 . For example, the second mould portion may be secured to the first mould portion by virtue of one or more of a freeze fit, a press fit, mechanical attachment (bolts, studs, screws, etc.), adhesives, fusion welding techniques or any other attachment means. A fluid tight seal may be provided between the first and second mould portions. 
     In the example shown in  FIG. 2 , the second mould portion  120  may comprise a protruding portion  124 . The protruding portion  124  may be adapted to engage the opening  116 , for example so that the second mould portion  120  is insertable into the opening  116  of the first mould portion  110 . The protruding portion  124  may be disposed about the perimeter of the opening  116 , e.g. to provide a tight fit against the opening. The second mould portion  120  may therefore act as a plug or bung blocking the opening  116 . The second mould portion  120  may be further secured to the first mould portion  110  by any of the above-mentioned attachment means. 
     The second mould portion  120  may comprise a surface  122  adapted to face the interior of the mould assembly  100  and form a feature e.g. a boss, on the object. The feature formed on the object may correspond in shape to the surface  122  of the second mould portion  120 . In particular, the second mould portion  120  may comprise a feature portion  126 . The feature portion  126  may be adapted to form the corresponding feature on the object. In the example shown in  FIG. 2 , the feature portion  126  may comprise a recess  128  between sides of the protruding portion  124  and a protrusion  127  set back in the surface  122 . Accordingly, the protrusion  127  and the recess  128  may form a corresponding recess and protrusion in the object to be formed. 
     The protruding portion  124  of the second mould portion  120  may be blended, e.g. rounded, at an end  125  of the protruding portion which may be adjacent to the internal surface  112  of the first mould portion  110 . A smooth transition between the internal surface  112  of the first mould portion  110  and the surface  122  of the second mould portion  120  may thus be provided. 
     The second mould portion  120  may comprise an abutment surface  121 . The abutment surface  121  may abut the external surface  114  of the first mould portion  110  adjacent to the opening  116 . The abutment surface  121  may thus limit movement of the second mould portion  120  with respect to the opening  116 . The protruding portion  124  and abutment surface  121  may be arranged such that the end  125  of the protruding portion lines up with the internal surface  112  of the first mould portion  110 . 
     The opening  116  may comprise a slot, e.g. an elongate slot, and the second mould portion  120  may be elongate to engage the slot. The first mould portion may be substantially tubular and the opening  116  and/or second mould portion  120  may be orientated in a longitudinal, circumferential or any other direction. 
     In an alternative embodiment (not shown), the opening may comprise a blind bore. For example, the blind bore may be provided on the internal surface of the first mould portion. At least a part of the second mould portion may fit inside the internal blind bore. 
     With reference to  FIG. 3 , the mould assembly  100  may be formed in one or more stages. As shown in  FIG. 3(   a ), the first mould portion  110  may be formed by machining the internal surface  112  from a workpiece. The first mould portion  110 , and hence the object to be formed, may be substantially tubular. Referring to  FIG. 3(   b ), the opening  116  may then be formed by machining, e.g. drilling or boring, the opening  116  into the first mould portion  110 . The opening  116  may be machined from either the internal or external surface  112 ,  114  of the first mould portion  110 . The opening  116  may be positioned where a feature, e.g. a boss, is required on the finished object. 
     The second mould portion  120 , shown in  FIG. 3(   c ), may be machined separately from the first mould portion  110 . The second mould portion  120  may have the required localised finished form machined onto it more easily than machining the internal surface  112  of the first mould portion  110 . The protruding portion  124  and/or feature portion  126  may also be machined into the surface  122  of the second mould portion  120 . Referring to  FIG. 3(   d ), the second mould portion  120  may be placed over the opening  116 , and in the example shown, fitted into the opening  116 . The opening  116  may thus be sealed such that any material placed in the mould assembly  100  may not leak through the opening  116  from the interior  130 . The second mould portion  120  may be positioned with respect to the first mould portion  110  to achieve the correct orientation of the feature to be formed on the object. 
     The present disclosure may provide an improvement to the powder HIP method of manufacture, by using inserts in moulds or tools to negate the requirement to complete any complex internal machining. The present disclosure allows complex external casing features to be produced using the powder HIP method. It removes the requirement for complex, difficult to access internal features and negates the requirement for long flimsy arbours when machining at depth in components. The complex forms can be more easily and accurately machined into the second mould portion. 
     Furthermore, any errors during the manufacture of the second mould portion will only scrap the second mould portion and not the rest of the mould assembly. A further advantage is that the openings, which are machined into the first mould portion may all be manufactured from the outside making the manufacturing of the tool much easier. 
     The present disclosure may be applied to any moulding or casting method for example, Hot Isostatic Pressing and in particular powder Hot Isostatic Pressing.