Abstract:
A mold including: a central core having a side wall with a plane external surface; an internal mold element having an internal surface facing the external surface of the central core, with surfaces inclined to the vertical in order to form an angle of clearance, and a substantially vertical external surface; an external mold element fitted against the internal mold element and having a substantially vertical internal surface facing the external surface of the internal mold element; at least one of the facing surfaces being designed to receive a substantially vertical lateral preform so that the internal mold element and the external mold element are suitable for clamping said lateral preform.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation of and claims priority to PCT application No. PCT/FR2011/051164 filed May 23, 2011, which claims priority to French Patent Application No. 1054039 filed on May 25, 2010, the entire contents of which are both incorporated by reference herein. 
    
    
     TECHNICAL FIELD 
     The invention relates to a mold adapted to receive a preform of a composite part and in which said preform is adapted to be impregnated by injection of resin. 
     BACKGROUND 
     It is known that molding by injection of resin at low pressure (a process commonly called RTM) is a method for putting composites materials into form, in which a thermosetting resin is injected at low pressure, through a textile preform constituted by reinforcements and which is placed in a closed mold. According to this method, a preform is first of all draped which is placed in the mold, the mold is filled with resin, polymerization of the resin is carried out and the composite part obtained is removed from the mold. 
     This type of molding may present difficulties in particular in the case of molding by injection of a complex part, of large size and which has vertical lateral walls, such as an aircraft undercarriage bay. 
     In this type of part, the inside surfaces of the vertical lateral walls have regions of increased thickness for the stiffening of the part. Producing a corresponding preform is difficult to do directly in the mold, in particular for reliable production of the regions of increased thickness, the draping then being carried out blind. 
     When preforms are produced outside the mold then mounted in the mold, the putting in position of the preforms relative to each other (lateral preforms corresponding to the lateral walls of the part, upper preform, etc.) is difficult to ensure on account of the lower stiffness of those preforms before the injection of resin. 
     In both these cases, the removal from the mold of the part obtained is rendered complex due to the negative draft angles generated by the regions of increased thickness of the inside surfaces of the lateral walls. 
     SUMMARY 
     The invention is directed to improving a type of mold for producing parts by the RTM process, by providing a mold adapted to the production of complex parts and which is simple to use. 
     To that end the invention provides to a mold adapted to receive a preform for a part of composite material and in which said preform is adapted to be impregnated by injection of resin to form said part of composite material, said preform comprising at least one substantially vertical lateral preform, characterized in that said mold comprises:
         a central core comprising at least one lateral wall which has an outside wall, which is for example planar;   an inside mold member having an inside surface turned towards said outside surface of the central core, with said surfaces being inclined relative to the vertical to form a draft angle, said inside mold member also having a substantially vertical outside surface;   an outside mold member mounted against said inside mold member and which has a substantially vertical inside surface, such that the inside surface of the outside mold member is turned towards the outside surface of the inside mold member; and   the inside surface of the outside mold member or the facing outside surface of the inside mold member being adapted to receive said lateral preform, such that the inside mold member and the outside mold member are adapted to clamp said lateral preform.       

     It will be noted that in the mold according to the invention, the lateral preform is clamped between two mold members mounted against the central core, such that reliable placing in position of the preform is ensured and that the holding in position of that preform during the injection is also ensured. 
     It will furthermore be noted that the two surfaces of the inside mold member are inclined to each other, with the substantially vertical outside surface and the inside surface forming a draft angle relative to the vertical, which makes it possible to mold a vertical wall with regions of increased thickness and to facilitate removal from the mold by virtue of the draft angle. 
     Furthermore, the design of the mold enables draping of the preform when flat, in optimized conditions. 
     According to other features, taken in isolation or in combination:
         said outside surface of the inside mold member is adapted to receive a layer of material to form a sub-preform of the lateral preform.   a cavity is formed on the inside mold member, on the same side as the outside surface, such that said sub-preform is adapted to cover said cavity;
           said inside surface of the outside mold member is adapted to receive a layer of material to form a second sub-preform of the lateral preform.   the inside mold member and the outside mold member are adapted to clamp the two sub-preforms the uniting of which forms the lateral preform when said mold members are mounted against each other;   the inside surface of the inside mold member bears grooves adapted to form, with the outside surface, flow channels for the resin, when the inside mold member is disposed against said outside surface;   cavities are formed on the outside mold member, on the same side as the inside surface, the sub-preform being adapted to cover the cavities;   the cavities extend in two different directions over the same inside surface of the outside mold member, such that the preform corresponding to the stiffening means associated with the lateral wall extends in two different directions over the same face of said lateral wall;   the central core comprises an upper wall adapted to receive a layer of material to form an upper preform connected to the lateral preform;   cavities are formed in the upper wall, such that a preform of the stiffeners is formed on a face when material is deposited on the upper wall; transverse portions adapted to receive a preform corresponding to upper frame portions are disposed on the upper wall, such that this upper frame portion preform is portion on the opposite side to the preform of the stiffeners relative to said upper disposed;   two lateral preforms are disposed on opposite sides of the central core, with spacers transversely linking the inside mold members associated with each lateral preform.   
               

     The invention also relates to a method of molding a part of composite material by injection of resin onto a preform of said part, said preform comprising at least one substantially vertical lateral preform, characterized in that it comprises the following steps:
         disposing a mold member which has a first surface oriented upwardly;   disposing at least one layer of material forming a sub-preform on said first surface;   assembling said mold member with a second mold member, said first surface being turned towards said second mold member, such that said lateral preform formed by at least the sub-preform is clamped between the two mold members;   moving said mold members towards an mold for injection in which is disposed a central core comprising at least one lateral wall which has a planar outside surface;   positioning said mold members such that the lateral preform extends vertically and that the opposite surface to the first surface of the mold member is turned towards the planar outside surface of the central core, with said surfaces being inclined relative to the vertical to form a draft angle;   and   injecting resin into the mold for molding the part.
 
Optionally, proceeding with the injection step, the preceding steps are reiterated to obtain a second lateral preform for the second lateral wall, then the two lateral preforms and the associated mold members are disposed on opposite sides of the central core in said mold for injection, respectively facing a lateral wall of said central core;
       

     The invention also relates to a method for molding a part of composite material, in particular an aircraft undercarriage bay comprising at least one central wall, two vertical lateral walls and stiffening means, by injection of resin onto a preform of said part, characterized in that it comprises the following steps:
         disposing an inside mold member which has an upwardly oriented outside surface, and which comprises a cavity on the same side as the outside surface;   disposing on said outside surface at least one layer of material forming a sub-preform corresponding to a vertical lateral wall;   disposing an outside mold member which has an upwardly oriented inside surface, and which comprises cavities on the same side as the inside surface;   disposing in said cavities of the inside surface at least one layer of material forming a sub-preform corresponding to said stiffening means;   assembling the two mold members, with said two surfaces turned towards each other, such that a lateral preform formed by uniting the two sub-preforms is clamped between the two mold members to form, after injection of the resin, a first lateral wall;   moving said lateral preform and said mold members towards a mold for injection in which is disposed a central core;   reiterating the preceding steps to obtain a second lateral preform for the second lateral wall;   disposing the two lateral preforms and the associated mold members on opposite sides of the central core in said mold for injection, respectively facing a lateral wall of said central core;   transversely connecting the inside mold members by spacers   depositing on an upper wall of said central core, said upper wall linking the two lateral walls, at least one layer of material forming a sub-preform corresponding to the central wall, and   injecting resin into the mold for molding the part.       

     According to other features, taken in isolation or in combination:
         on the upper wall and between the lateral preforms and the associated mold members, transverse portions are mounted which are adapted to receive a preform corresponding to upper frame portions extending in a first direction;   the step of depositing the layer of material in the cavities of the inside surface of the outside mold member comprises a first deposit in said first direction and a second deposit in a perpendicular direction.       

     Furthermore, the invention also relates to a part of composite material obtained by one of the methods of molding by injecting resin as briefly set out above, the part being an aircraft undercarriage bay. 
     The invention also concerns an aircraft comprising at least one aircraft undercarriage bay obtained by one of the methods set out above. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The disclosure of the invention will now be continued with the detailed description of an embodiment, given below by way of illustrative and non-limiting example, with reference to the accompanying drawings, in which: 
         FIGS. 1 and 2  are perspective views of an aircraft undercarriage bay, from different angles; 
         FIG. 3  is a perspective view of the undercarriage bay of  FIG. 1  and of a part of the mold, the undercarriage bay being in course of removal from the mold; 
         FIG. 4  is a rear view of the undercarriage bay and of the mold part represented in  FIG. 3 ; 
         FIG. 5  is a representation of parts of the mold, including in particular a central core, an inside mold member and an outside mold member; 
         FIG. 6  is a representation similar to  FIG. 5 , with the inside and outside mold members supporting parts of the lateral preform; 
         FIG. 7  is a perspective view of the handling means carrying the inside mold member and the outside mold member as represented in  FIG. 6 , when they are assembled, with the lateral preform between those two mold members. 
         FIG. 8  is a representation of the mold in course of assembly, with the central core and the lateral mold members assembled; 
         FIG. 9  is a representation of the mold in course of assembly, with the preform of the central wall and the upper mold member assembled on the central core; 
         FIG. 10  is a representation of the assembled mold, with metal members covering the preforms of the central wall and of the frame portions. 
     
    
    
     DETAILED DESCRIPTION 
     As illustrated in  FIGS. 1 and 2 , a undercarriage bay  1  which constitutes an example of a part of composite material comprises a central wall  2  extending longitudinally and to which two lateral walls  3  connect transversely. Each of the walls comprises stiffeners, here of Omega-section, and frame portions. I will be understood here that the portions of frame are stiffeners of larger size than the Omega-section stiffeners. The bay  1  has the shape of a shoe with a hollow inside forming a housing for a member of the undercarriage bay of an aircraft. 
     Viewed from the side, the central wall  2  is of parabolic shape, with a substantially horizontal upper portion  5 , and a front face  6  of rounded shape which downwardly continues the front end edge of the upper portion  5  and which extends over substantially the whole height of the undercarriage bay  1 . 
     The upper portion  5  has an outside surface  5 ′ adapted to receive frame upper portions  43 , and an inside surface on which stiffeners  44  are integrally formed, which are visible in  FIG. 4  and of which there are for example three here. It may be observed here that the stiffener forming means are disposed on opposite faces of the same wall. The upper portion  5  has a profile which is slightly dome-shaped in a transverse section plane, the center of portion  5  being higher than the sides. 
     The lateral walls  3  are vertical. They each have an inside surface  7  turned inwardly of the bay  1  and an outside surface  8  turned outwardly of the bay. 
     The lateral walls  3  comprise a skin of varying thickness, such that each inside surface  7  comprises regions of increased thickness  70 . Here, the region of increased thickness extends from a corner of an inside surface  7  to an opposite corner, leaving regions of lower thickness  71  on opposite sides of the region of increased thickness  70 , and making removal from the mold difficult on account of negative drift angles. 
     The outside surface  8  bears frame lateral portions  41 , for example in the form of a beam of U-shaped cross-section, which are transversely arranged, and horizontal stiffeners  42  here for example of Omega-shaped cross-section, which extend between two successive frame lateral portions  41 . It may be observed here that the stiffener forming means are disposed on the same face of a wall. The upper ends of the frame lateral portions  41  project beyond the upper portion  5 . 
     The frame upper portions  43 , transversely arranged, which are carried by the outside surface  5 ′, are joined to the upper ends of the frame lateral portions  41 . 
     The two lateral walls  3  and the central wall  2  are joined by connecting brackets not shown. 
     Such an undercarriage bay  1  is formed from composite material, and is obtained by injection of resin, for example at low pressure, onto an overall preform, or rough, of the bay. By way of example, the preform, also termed textile preform, is a structure constituted by reinforcements, with carbon fibers linked by an epoxy powder and/or thermoplastic to give cohesion to the preform. The overall preform comprises two lateral preforms of which only one  103  is represented ( FIG. 7 ), an upper preform  104 , a preform, not shown, for each connecting bracket, preforms for upper stiffeners, not shown, and preforms for upper portions of the frame  105 , which are assembled before the injection into a mold. 
     Each lateral preform  103  is provided to be impregnated with resin to form a composite lateral wall  3  and also to form the frame lateral portions  41  and the stiffeners  42  carried by the wall  3 , after polymerization of the resin. 
     The upper preform is adapted to be impregnated with resin and to form the composite central wall  2  when the resin has polymerized. 
     The mold according to the invention in particular enables the correct assembly of the preforms in relation to each other and enables the support of that assembly before and during the injection of resin in an autoclave in which the mold is placed. 
     As illustrated in particular in  FIGS. 3 to 6  and  8  to  10 , a mold  10  for manufacturing a part of this type comprises a central core  11 , on which are mounted a first lateral mold member  12 , a second lateral mold member  13  and an upper mold member  14  (visible in  FIG. 10 ). 
     The central core  11  comprises a base  15  and a shell  16  having a shape substantially equivalent to that of the undercarriage bay  1 . 
     The shell  16  has two lateral walls  17  which issue from the base  15  and an upper wall  18  transversely connecting the two lateral walls  17 , remotely from the base  15   
     Each lateral wall  17  has a planar outside surface  19 . As can be seen more easily in  FIG. 4 , the planar outside surfaces  19  are inclined to the vertical (the dashed line diagrammatically representing the inclination relative to the full line representing the vertical), while converging towards each other on moving away from the base  15 . 
     The upper part of the lateral walls  17  comprises a cavity  20  ( FIGS. 5 and 6 ) adapted, as explained later, to receive an edge  27  of the corresponding lateral mold member. 
     The upper wall  18  has an upper outside surface  21  and comprises three cavities  22  apparent on that outside surface  21  to enable the molding of the stiffeners  44  on the inside surface of the upper portion  5 , as will be described later. 
     It will be noted that the cavities  20  and  22  stop axially at a specific distance from the back edge of the shell  16 . 
     Reinforcing means  23  ( FIGS. 4 ,  5  and  6 ) are provided inside the shell to reinforce the central core  11 . 
     The first lateral mold member  12  comprises an inside mold member  24  and an outside mold member  25 , which are adapted to be hinged relative to each other (as can be seen in  FIGS. 5 to 7 ). 
     The inside mold member  24  comprises a wall  26  which has a substantially planar inside surface  28  ( FIG. 7 ) and an outside surface  29 . The inside mold member  24  comprises a cavity  30  which is formed on the same side as the outside surface  29 , and which is of similar shape and size to the shape and size of the region of increased thickness  70  of the inside surfaces of the lateral walls of the bay  1 . 
     The outside surface  29  comprises on it upper part a cavity  45  adapted to receive a preform corresponding to the connecting brackets referred to earlier. 
     The inside surface  28  comprises on its upper part an edge  27  projecting transversely, which is adapted to be accommodated, as explained later, in the cavity  20  of the corresponding lateral wall  17 . 
     The inside surface  28  comprises a resin supply network, formed by grooves  31  formed in the wall  26  and opening onto the inside surface  28  ( FIG. 7 ). The flow of the resin is made possible when the inside surface  28  is mounted against the central core  11  and flow channels are thus formed between the inside surface  28  and the central core  11 . 
     The two surfaces  28  and  29  are not parallel to each other, such that, as will be explained later, when the inside mold member  24  is positioned in the mold, the inside surface  28  forms an angle other than zero with the vertical while the outside surface  29  extends vertically. 
     Male hinge members  32  are disposed projecting from an edge of the inside mold member  24  ( FIGS. 5 and 6 ). 
     The outside mold member  25  comprises a support plate  33  and a lateral wall  34 . The lateral wall  34  has an inside surface  35  provided with cavities  36 . The cavities  36  are of a number and have shapes corresponding to the number and to the shapes of the frame portions  41  and of the stiffeners  42  that it is desired to obtain after molding on the lateral wall  3 . Here, as illustrated in  FIG. 5 , cavities  36  cross each other in the manner of a mesh on the inside surface  35  to reproduce the desired layout of the stiffeners  42  and of the frame lateral portions  41  on the undercarriage bay illustrated in  FIG. 1 . The fact that the cavities  36  are disposed on a surface of a single mold member makes it possible subsequently to obtain stiffener forming means (of the frame portions and of the stiffeners) on the same face of a wall of the undercarriage bay. 
     The support plate  33  and the lateral wall  34  are adapted to be disconnected from each other, in particular when mold extraction takes place as explained later. 
     The outside mold member  25  comprises female hinge members  37  adapted to receive the male members  32 . 
     The two mold members  24  and  25  are hinged at the location of the support plate  33  and are adapted to pass from a flat position, in which they are at 180°, to an assembled position in which they are pressed against each other, the inside surface  35  being located facing the outside surface  29 . 
     The outside surface  29  and the inside surface  35  are adapted to receive respectively a sub-preform  50  and  51  ( FIG. 6 ), formed from at least one layer of material constituted by reinforcements, such that when the members  24  and  25  are assembled, the lateral preform  103 , which is formed by uniting the sub-preforms  50  and  51 , is clamped between the two members  24  and  25 . 
     As will be explained later, such a mold design enables the sub-preform or sub-preforms  50  and  51  to be draped onto the mold member or members independently, laid flat, while having a vertical position ensured by the lateral preform  103  in the mold. 
     A description will now be made of the mold when it is assembled, before the injection of resin, with reference to  FIGS. 3 to 10 . 
     The first and second lateral mold members  12  and  13  form a mirror image of each other relative to the central core  11 , such that from the central core  11  and towards the outside of the mold, there are successively encountered a lateral wall  17 , an inside mold member  24  whose inside surface  28  is turned towards the lateral wall  17 , a lateral preform  103  clamped between the outside surface  29  of an inside mold member  24  and the inside surface  35  of an outside mold member  25 , and lastly the outside mold  25 . 
     Furthermore, spacers connect substantially transversely together the inside mold members  24  of each lateral mold member, providing a stiffening function. A first spacer  38  is fastened behind the inside mold members  24 , in their upper corner, and a second spacer  39  ( FIG. 3 ) is fastened at the front of the inside mold members  24 , in the bottom portion. 
     An upper preform  104  (visible in  FIG. 9 ) of the central wall  2 , is disposed on the upper outside surface  21  of the upper wall  18 , with a preform for the upper stiffeners  44  being disposed in the cavities  22  (visible in  FIG. 8 ). 
     An upper mold member  14  comprises metal plates  140  (visible in  FIG. 10 ) which are adapted to cover the upper preform  104 , and transverse portions  141  disposed above the upper preform  104  and which are adapted to receive a preform  105  (visible in  FIG. 9 ) corresponding to the frame upper portions  43 . The upper preform  104  corresponding to the central wall is clamped between the central core  11  and the upper mold member  14 . It is observed here that the preform for the upper stiffeners  44  is disposed remotely from the preform  105  for the frame upper portions  43  relative to the upper preform  104  for the central wall  2 . 
     The upper mold member  14  transversely joins the lateral preforms  103 , with the preform  105  disposed as to extend from and in alignment with the upper ends of the lateral preforms. 
     A description will now be made of the use of such a mold with reference to  FIGS. 5 to 7  for the manufacturing of a composite part such as an aircraft undercarriage bay. 
     The inside mold member  24  and the outside mold member  25  are for example laid out flat. The inside mold member  24  is disposed such that the outside surface  29  is oriented upwardly, while the outside mold member  25  is disposed such that the inside surface  35  is oriented upwardly ( FIG. 5 ). 
     It should be noted that laying flat is not imperative. To facilitate the draping of the material (depositing of a layer or layers) the mold member must not however be close to the vertical. An inclination that would be liable to induce natural slippage of the layer of material on the mold member must therefore be avoided. This inclination depends in particular on the coefficient of friction between the material of the layer and that of the mold. 
     Thus, for the coefficient of friction of carbon on steel (low coefficient of the order of 0.10 to 0.15), the inclination is for example from 6 to 9°. 
     At least one layer of material is deposited, here several layers, forming a sub-preform  50  on the outside surface  29  (the layers of material are represented grayed in  FIGS. 6 to 9  to facilitate understanding). The layers of material espouse the shape of the outside surface  29  and in particular of the cavity  30  corresponding to the region of increased thickness of the lateral wall  3 , such that a portion of the layers of material is set back relative to the plane in which said layers mainly extend. 
     Deposition is also carried out of layers of material forming a sub-preform  51  on the inside surface  35 . This sub-preform  51  will be accommodated in the cavities  36  on the inside surface  35  and will correspond to the stiffeners  42  and to the frame portions  41  in the undercarriage bay to be obtained. 
     As was described earlier, the cavities  36  here form a mesh. The deposit of the layers of material takes place here in two different directions with a first deposit in each cavity which extends in a given direction, then a second deposit in each cavity which extends in a different direction. 
     It is to be noted that the deposit of the sub-preforms  50  and  51  is made simple here by the laid flat position of the two mold members  24  and  25  when that deposit is made. 
     The male hinge members  32  and the female hinge members  37  are associated in such a manner as to assemble the two mold members  24  and  25 , then they are folded against each other, thereby forming the first lateral mold member  12 . The outside surface  29  and the inside surface  35  are turned towards each other, and the lateral preform  103  formed by uniting the sub-preforms  50  and  51  is clamped between the mold members  24  and  25 . 
     This assembly facilitates the handling of the members thus assembled. 
     The first lateral mold member  12  and the associated lateral preform  103  are then moved towards the central core  11 . As illustrated in  FIGS. 5 to 7 , handling tooling  40  is used for moving the lateral mold member  12 . This tooling carries the outside mold member  25  even before the operation of depositing the layer of material, thereby facilitating the setting to the vertical of the lateral mold member  12  when the inside and outside mold members have been assembled. 
     Similar operations are carried out to form the second lateral mold member  13  and to move it towards the central core  11 . 
     Each of the lateral mold members  12  and  13  is disposed on opposite sides of the central core  11 , by bringing, for the lateral mold member  12 , the inside surface  28  against the outside surface  19  of the core, and by proceeding in the same way for the lateral mold member  13 . The projecting edge  27  of the internal mold members is accommodated in the cavity  20  formed in the lateral wall  17 . 
     The internal mold members of each lateral mold member are connected substantially transversely by the spacers  38  and  39 . 
     The upper mold member  14  is disposed facing the outside surface  21  of the upper wall  18 , while the latter was covered beforehand with at least one layer of material forming the textile preform  104 . The upper mold member  14  is adapted, as described earlier, to clamp the upper preform  104  with the central core  11  and ensure the placing in position of the preforms corresponding to the upper portions of the frame  43 . 
     Each lateral preform  103  is then reliably indexed in relation to the central core and the upper textile preform  104  is disposed directly on the central core  11 . Furthermore, each preform is held in position by the mold members that clamp it, and no subsiding of the preform is therefore to be feared when the resin is injected. 
     The mold is closed, covered with a vacuum bag in which a vacuum is formed and is placed like this in an autoclave. 
     The injection of resin is then carried out. The resin spreads in particular between the central core  11  and the inside mold member  24  by the flow channels formed by the grooves  31  and the outside surface  19  ( FIG. 7 ). 
     The inside mold member of the symmetrical lateral mold member  13  comprises a similar resin supply network, such that the resin spreads all around the central core  11 . 
     The injection of resin, then the polymerization, enable the preforms to be mechanically connected and to obtain an undercarriage bay 
     It should be noted that during injection the support  40  is withdrawn and the plate  33  is kept in position. 
     After a specific time, sufficient for the resin to be polymerized, the mold may be opened and the composite part removed. 
     Removing the part from the mold will now be described. 
     In a first phase, for each of the lateral mold members  12  and  13 , the lateral wall  34  of the outside mold member  25  is removed, the lateral wall  34  being for that purpose disconnected from the support plate  33 . The lateral wall  34  is removed while the support plate  33  remains in position, hinged to the inside mold member  24 . 
     The upper mold member  14  (visible in  FIG. 10 ) is also removed. 
     As illustrated in  FIGS. 3 and 4 , the assembly formed by the inside mold members  24 , the plate  33  of the outside mold members  25  and the spacers  38  and  39  is then moved away from the central core  11 . Rings ( FIG. 3 ) are provided on the plates  33  to anchor the slings for mold extraction and to raise the assembly away from the central core  11 . The draft angles of the surfaces  19  and  28  thus enable easy removal from the mold. 
     When the part and the remaining mold members have been put down again, the spacers  38  and  39  should be removed, then the inside mold members  24 . The part is thus removed from the mold. 
     If the part to mold does not have a region of increased thickness on the inside, the constitution of the mold with a core  11  and an inside mold member  24  is not necessary since removal from the mold is possible. If the part to mold comprises local regions of increased thickness on the inside then the inclined walls are necessary (in order to have a draft angle). The removal from the mold is rendered possible thanks to the use of a core  11 , an inside mold member  24  and the draft angle. 
     In variants not represented, the number and the shape of the stiffeners and of the frame portions may vary such that the arrangement of the associated preforms in the mold varies; for example, it may be that only stiffeners are disposed on the lateral walls while only frame portions are disposed on the central wall, with the stiffeners and the frame portions extending in two different directions; or else, for example, the central wall may have stiffeners and upper frame portions on the same outside surface  5 ′ of the upper portion  5 , with in this case a different shape of the upper mold member to dispose the preform of the upper stiffeners and the preform for the upper frame portions of the same side of the upper preform. 
     In another variant not represented, only one of the mold members is draped, the function of the other mold member merely being to clamp the lateral preform. 
     In other variants not represented, the shape of the regions of increased thickness may vary; it is possible for one and/or the other of the lateral walls not to have any region of increased thickness on their inside surface; the central wall may also have no region of increased thickness on its inside surface. 
     Numerous other variants are possible according to circumstances, and in this connection it is to be noted that the invention is not limited to the examples described and shown.