Abstract:
An impregnation mandrel for production of a gas turbine casing made from composite material, including: a mandrel having a central wall and two side plates; compaction bars, each including (i) a wedge configured to bear against a fibrous reinforcing part covering angles formed between the central wall and the side plates of the mandrel, and (ii) an attachment flange configured to be attached to the corresponding side plate of the mandrel; a flexible casing forming a vacuum bag and configured to be applied at least against the fibrous reinforcing part covering the central wall of the mandrel; and a mechanism for injecting resin into a space defined between the vacuum bag and the mandrel at one of longitudinal ends of the fibrous reinforcement and for extracting the resin at the opposite end.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to the general field of gas turbine casings, and more particularly holding casings of gas turbine fans for aeronautical engines. 
     In a gas turbine aeronautical engine, a fan casing fulfils a number of functions. It defines the incoming air stream to the engine, supports abradable material with respect to the tip of the fan vanes, supports an optional structure for sound wave absorption for acoustic inlet treatment of the engine and incorporates or supports a holding shield. The latter comprises a trap for catching debris such as ingested items or fragments of damaged vanes thrown out by centrifuge action to prevent them from passing through the casing and reaching other parts of the aircraft. 
     Making a casing for holding a fan made of composite material has already been proposed. Reference could be made to document EP 1 961 923 which describes the production of a casing made of composite material of evolutive thickness, comprising the formation of a fibrous reinforcement by superposed layers of a fibrous texture and densification of the fibrous reinforcement by a matrix. According to this invention, the fibrous texture is made by three-dimensional weaving with evolutive thickness and is wound in several superposed layers onto a mandrel having a central wall of profile corresponding to that of the casing to be manufactured and two lateral flanges of profile corresponding to those of the external flanges of the casing. The resulting fibrous preform is held on the mandrel and impregnation by resin is completed under vacuum prior to polymerisation. The winding on a mandrel of a woven texture of evolutive thickness as described in this document directly gives a tubular preform having the preferred profile with variable thickness. 
     In practice, the resin impregnation step conducted under vacuum requires a supple envelope (or liner) to be applied to all the fibrous reinforcement, and especially at the level of the flanges of the reinforcement which will later form the external flanges of the casing. A difference in pressure is then set between the exterior and the space delimited by the mandrel and the liner containing the fibrous reinforcement. The injection of resin into this space can then start. 
     During this step, it was noted that setting the vacuum tends to generate tension in the layers of fibrous texture positioned at the level of the angles of flanges between the flanges and the central wall of the mandrel, this tension setting causing the fabrics to unstick at the origin of resin compaction and mass defects between the layers. 
     AIM AND SUMMARY OF THE INVENTION 
     The main aim of the present invention is therefore to rectify such drawbacks by proposing a solution for impregnation by vacuum liner ensuring uniform compaction of the fibrous reinforcement, especially at the level of the angles of flanges. 
     This aim is attained by an impregnation mandrel for making a gas turbine casing made of composite material, comprising:
         an impregnation mandrel on which is intended to be held a fibrous reinforcement formed by superposed layers of a fibrous texture, the mandrel comprising a central annular wall the profile of which corresponds to that of the casing to be manufactured and two lateral flanges whereof the profiles correspond to those of external flanges of the casing to be manufactured;   compaction bars each comprising a corner intended to be supported against the part of the fibrous reinforcement covering the angles formed between the central wall and the flanges of the mandrel, and a coupling flange intended to be fixed on the corresponding flange of the mandrel;   a supple envelope forming a vacuum liner intended to be applied at least to that part of the fibrous reinforcement covering the central wall of the mandrel; and   means for injecting resin into a space delimited between the vacuum liner and the mandrel at a longitudinal end of the fibrous reinforcement and for extracting it at an opposite end.       

     The compaction bars of the mandrel according to the invention are positioned once the winding operation is completed and before placing of the vacuum liner. These compaction bars ensure uniform compaction of the part of the fibrous reinforcement covering the angles of flanges prior to setting the vacuum. In this way, any risk of formation of resin mass between the layers of the fibrous reinforcement during this operation for setting the vacuum can be prevented. 
     Also, the compaction bars are intended to be fixed directly on the impregnation mandrel, which properly and repeatedly controls the geometry of the external flanges of the casing to be manufactured. 
     The mandrel preferably comprises at least one resin injection orifice terminating inside the space delimited between the vacuum liner and the mandrel at a longitudinal end of the fibrous reinforcement, and at least one resin extraction orifice placed at the longitudinal end of the fibrous reinforcement opposite to where the resin injection orifice terminates. 
     In this case, the resin injection orifice can be formed in one of the flanges of the mandrel and the resin extraction orifice can be formed in the other flange. Advantageously, the resin injection orifice terminates at the corner of a so-called injection compaction bar, whereas the resin extraction orifice terminates downstream of the so-called extraction opposite compaction bars. 
     The coupling flanges of the extraction compaction bars may comprise grooves ensuring passage of the resin. 
     The vacuum liner can be intended to be also applied to the compaction bars and be fixed tightly by its free ends to the flanges of the mandrel. 
     The coupling flanges of the compaction bars are preferably intended to be fixed tightly on the flanges of the mandrel. 
     For each flange of the mandrel, there can be four compaction bars and they can be put end to end angularly to cover the total circumference of the mandrel. 
     Another aim of the invention is a winding machine of a fibrous texture on an impregnation mandrel, comprising a take-up mandrel on which a fibrous texture is intended to be stored, produced by three-dimensional weaving, the take-up mandrel having a substantially horizontal axis of rotation, an impregnation mandrel such as defined previously, the impregnation mandrel having a substantially horizontal axis of rotation parallel to the axis of rotation of the take-up mandrel, electric motors for driving the mandrels in rotation about their respective axis of rotation, and a control unit of the electric motors for driving the mandrels in rotation. 
    
    
     
       BRIEF DESCRIPTION OF THE DIAGRAMS 
       Other characteristics and advantages of the present invention will emerge from the following description, in reference to the attached diagrams which illustrate an embodiment devoid of any limiting character, in which: 
         FIG. 1  is a schematic view and side elevation of a winding machine of a fibrous texture on an impregnation mandrel according to the invention; 
         FIG. 2  is a view of the impregnation mandrel of the winding machine of  FIG. 1  during placing of the compaction bars; 
         FIG. 3  is a sectional view along of  FIG. 2 ; and 
         FIG. 4  is a sectional view of the impregnation mandrel of  FIG. 3  after placing of the vacuum liner. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The invention will now be described within the scope of its application to the production of a fan casing in an aeronautical engine with a gas turbine. 
     An example of a manufacturing process of such a fan casing is described in document EP 1 961 923 which can be referred to. 
     The casing is made of composite material with fibrous reinforcement densified by a matrix. The reinforcement is made of fibres such as carbon, glass, aramide or ceramic and the matrix is made of polymer, for example epoxide, bismaleimide or polyimide. 
     Briefly, the manufacturing process described in this document consists of making a fibrous texture by three-dimensional weaving with chain take-up on a drum (hereinbelow called take-up mandrel) having a profile determined as a function of the profile of the casing to be manufactured. 
     The resulting fibrous texture is then transferred to the mandrel of a resin injection mould (hereinbelow called impregnation mandrel) the external profile of which corresponds to the internal profile of the casing to be manufactured. 
     While the preform is held on the impregnation mandrel, impregnation is then done with resin. For this purpose, a supple envelope (also called vacuum liner) is applied tightly to the preform and the resin is injected into the resulting mould. Impregnation is assisted by a difference in pressure being set between the exterior and the interior of the mould containing the preform (air vacuum). After impregnation, a resin polymerisation step is carried out. 
     The invention applies to any type of winding machine whereof the function is automated transfer of the fibrous texture stored on the take-up mandrel to the impregnation mandrel of the resin injection mould, such as shown in  FIG. 1 . 
     Reference could be made to patent application FR 11 53212 (not yet published) which describes in detail the structure and operation of such a machine. 
     Briefly, the winding machine  10  comprises a frame  12  supporting especially a take-up mandrel  14  and an impregnation mandrel  100  according to the invention. These mandrels are removable, that is, they can be dismantled from the frame. 
     The take-up mandrel  14  receives the fibrous texture  16  produced for example by three-dimensional weaving. It is borne by a horizontal axle  18  one end of which is mounted to rotate on the frame  12  of the winding machine and the other end is coupled to the output shaft of an electric engine  20 , for example an electric motoreducer on alternating current. 
     The assembly constituted by the take-up mandrel  14 , its axle  18  and its electric engine  20  can translate relative to the frame along the axis of rotation of the take-up mandrel. This degree of liberty in translation of the take-up mandrel creates alignment of this mandrel on the impregnation mandrel prior to winding of the fibrous texture on the impregnation mandrel. 
     The impregnation mandrel  100  of the winding machine is intended to receive the fibrous texture stored on the take-up mandrel, in superposed layers. In a way known per se, it has a central annular wall  102  whereof the profile of the external surface corresponds to that of the internal surface of the casing to be made and two lateral flanges  104   a ,  104   b  whereof the profiles correspond to those of the external flanges of the casing at its upstream and downstream ends to enable it to be mounted and linked to other elements. 
     The impregnation mandrel is borne by a horizontal axis  22  which is parallel to the axis of rotation  18  of the take-up mandrel and whereof one end is mounted to rotate on the frame  12  of the winding machine and the other end is coupled to the output shaft of an electric engine  24 , for example an electric motoreducer on alternating current. 
     A control unit  26  is connected to the electric motors  20 ,  24  of the two mandrels and controls the rotation speed of each mandrel. More generally, this control unit controls the assembly of operating parameters of the winding machine, and especially the displacement in translation of the take-up mandrel when motorised. 
     With such a machine, winding of the fibrous texture on the impregnation mandrel is done as follows: the free end of the fibrous texture of the take-up mandrel is first fixed on the impregnation mandrel by means of a device for holding by clamping described hereinbelow, then the engines for driving the mandrels in rotation are activated and controlled by the control unit so as to apply adequate winding tension on the fibrous texture. 
     Winding of the fibrous texture in superposed layers on the impregnation mandrel can then start and be executed in the direction of rotation marked by arrow F in  FIG. 1 . By way of example, it might be necessary to effect 4 turns ⅛ to produce a fibrous reinforcement  28  having a thickness conforming to the specifications of the casing to be manufactured. 
     According to the invention, the impregnation mandrel  100  is provided with means ensuring impregnation by resin under vacuum liner on completion of the winding operation. 
     More precisely, as shown in  FIGS. 2 to 4 , the impregnation mandrel comprises so-called angular compaction bars which are intended to be positioned on the mandrel at the level of the parts of the fibrous reinforcement  28  covering the angles formed between the central wall  102  and the flanges  104   a ,  104   b  of the latter. 
     These bars comprise a first series of compaction bars  106   a  intended to be mounted against the part of the fibrous reinforcement covering the angle formed between the central wall of the mandrel and the flange  104   a , and a second series of compaction bars  106   b  intended to be mounted against the part of the fibrous reinforcement covering the angle formed between the central wall of the mandrel and the other flange  104   b.    
     The compaction bars  106   a ,  106   b  of these series cover the entire circumference of the mandrel and are sectored. So, in the example illustrated in  FIG. 2 , each series comprises four compaction bars each extending over 90° approximately and put end to end angularly to cover the total circumference of the impregnation mandrel. Of course, the number of bars per series could be different. 
     Each compaction bar  106   a ,  106   b  comprises a corner  108   a ,  108   b  which is intended to be supported against the part of the fibrous reinforcement covering the angles formed between the central wall  102  and the flanges  104   a ,  104   b  of the mandrel, and a coupling flange  110   a ,  110   b  intended to be fixed on the corresponding flange of the mandrel. 
     Placing the compaction bars on the impregnation mandrel ensures uniform compaction of the fibrous reinforcement at the level of the flange angles. This placing can be ensured by using a specific tool of tension type, for example. 
     Once in place, the compaction bars are fixed on the impregnation mandrel by means of their coupling flanges  110   a ,  110   b  and by means for example of screws  112 . This fixing is made tight by the presence of O-ring joints  114  positioned against an internal face of the coupling flanges about the boreholes made for passage of screws and plugs  116  sealing the openings made in the coupling flanges for passage of these same screws. 
     A supple envelope  118  forming a vacuum liner is then applied to at least that part of the fibrous reinforcement covering the central wall of the mandrel. As shown in  FIG. 4 , this vacuum liner  118  is preferably applied at the same time to the fibrous reinforcement at the level of the central part of the mandrel, but also covers the compaction bars  106   a ,  106   b , at the level of its free ends, to be fixed tightly on the flanges  104   a ,  104   b  of the mandrel. The material used to make the vacuum liner  118  is for example nylon (the choice of material will depend especially on the class of temperature of the resin). 
     The impregnation mandrel further comprises means for injecting resin into the resulting mould. For this purpose, one of the flanges of the mandrel (here the flange  104   a ) comprises at least one resin injection orifice  120  which terminates inside a space  122  delimited between the corner  108   a  of a corresponding compaction bar  106   a  (also called “injection compaction bar”) and the corresponding flange  104   a . In this way, the injection of resin is done at the level of one of the free ends of the fibrous reinforcement  28  held on the mandrel. 
     The resin is extracted at the level of the opposite flange (specifically here flange  104   b ). For this purpose, this flange comprises one or more extraction orifices  124  which terminate in a space delimited between the free end opposite the vacuum liner  118  and the flange  104   b , this space being situated downstream of the corresponding compaction bars  106   b  (also called “extraction compaction bars”). Downstream here means relative to the flow of the resin between the two longitudinal ends of the fibrous reinforcement held on the mandrel. 
     To allow the resin to pass from the fibrous reinforcement  28  to the extraction orifice or the extraction orifices  124 , it is necessary for it to get over the extraction compaction bars  106   b . At the level of their internal face, the coupling flanges  110   b  of the latter also have a plurality of grooves  126  (see  FIG. 3 ) extending radially outwards and dimensioned to allow such passage of resin. 
     Also, it is evident that the extraction orifice or the extraction orifices can be used to set up the vacuuming of the liner  118  by creating a difference in pressure between the exterior and the space delimited by the mandrel and the liner containing the fibrous reinforcement. For this purpose, it might be necessary to place vacuum drainage fabric between the vacuum liner and the flange  104   b  of the mandrel in its part downstream of the extraction compaction bars  106   b  (such fabric prevents discontinuity of the vacuum as far as the extraction orifices). Setting vacuum assists the resin injection operation. 
     Once the vacuum is set, the resin is injected into the mould formed by the impregnation mandrel covered by the vacuum liner. On completion of this operation, a resin polymerisation step is conducted as known per se.