PROCESS AND DEVICE FOR MANUFACTURING A COMPOSITE PART ENABLING THE EVACUATION OF GAS FROM THE COMPOSITE PART

Abstract

Disclosed is a process for manufacturing a part, named composite part, formed from at least one composite material including at least one layer of a reinforcing structure impregnated with a polymer matrix within which the reinforcing structure extends, wherein: a composite blank of the composite part is prepared; and a step of curing the polymer matrix of the composite blank is carried out, exerting a pressure on at least one pressed face of the composite blank. The pressure is exerted in the form of a pressure gradient applied to the pressed face of the composite blank so as to cause the gas present within the composite part to flow from a zone of maximum pressurization to a gas evacuation zone. Also disclosed is a device for manufacturing such a part.

Claims

1-15. (canceled)

16. A process for manufacturing a part, named composite part, formed from at least one composite material comprising at least one layer of a reinforcing structure impregnated with a polymer matrix within which the reinforcing structure extends, wherein: a blank, named composite blank (1), of said composite part is prepared, said composite blank having at least one state, named non-cured state, in which the polymer matrix is at least partially non-solidified, a step of curing the polymer matrix of said composite blank is carried out, during at least part of said curing step, pressure is exerted on at least one face, named pressed face (2), of said composite blank, wherein: said pressure is exerted in the form of a pressure gradient, said pressure being applied to said pressed face of said composite blank between at least one first zone, named zone of maximum pressurisation, of said pressed face and at least one second zone, named gas evacuation zone (4), of said pressed face, said zone of maximum pressurisation being different from said gas evacuation zone (4) and said pressure gradient being oriented towards said zone of maximum pressurisation so as to cause any gas present within the composite blank to flow from said zone of maximum pressurisation to said gas evacuation zone (4), said pressure gradient is applied with the aid of at least one member, named pressurisation member (8), brought into contact with said zone of maximum pressurisation and into contact with said gas evacuation zone (4) of said pressed face of said composite blank, a pressurisation member is used having an elastic deformation rigidity varying between a zone, named distal zone, of this pressurisation member coming into contactafter application of said deformation pressurewith said zone of maximum pressurisation of said pressed face, and a zone, named edging zone, of this pressurisation member coming into contactafter application of the deformation pressurewith said gas evacuation zone (4) of said pressed face of the composite blank (1), the rigidity of the edging zone of said pressurisation member being greater than the rigidity of the distal zone.

17. The process according to claim 16, wherein in order to apply said pressure gradient: a face, named pressurisation face, of at least one member, named pressurisation member (8), which is elastically deformable, is disposed oriented towards the pressed face (2) of said composite blank and at a non-zero distance from the pressed face of the composite blank, then a face, named pressurised face, opposite to said pressurisation face, of said pressurisation member is subjected to a pressure, named deformation pressure, adapted to elastically deform said pressurisation member, said pressurisation member, its arrangement with respect to the pressed face and said deformation pressure being chosen so that the pressurisation face of the pressurisation member (8) is brought into contact with said pressed face of said composite blank at least between the zone of maximum pressurisation and the gas evacuation zone (4) and applies said pressure gradient to the pressed face.

18. The process according to claim 17, wherein at least one pressurisation member (8) is used having, prior to application of said deformation pressure, a thickness varying between said distal zone and said edging zone, the thickness of the edging zone of said pressurisation member being greater than the thickness of the distal zone.

19. The process according to claim 16, wherein at least one pressurisation member (8) is used having a modulus of elasticity at least in traction orthogonally to the direction between said distal zone and said edging zone, varying between said distal zone and said edging zone, said modulus of elasticity of the edging zone being greater than said modulus of elasticity of the distal zone.

20. The process according to claim 16, wherein the pressurisation member (8) is disposed at a predetermined distance from the pressed face (2) of the composite blank (1) such that, under the effect of the deformation pressure, the relative elongation (L/L) of the pressurisation member (8) varies between a zone, named distal zone, of this pressurisation member coming into contactafter application of said deformation pressurewith said zone of maximum pressurisation of said pressed face, and a zone, named edging zone, of this pressurisation member coming into contactafter application of the deformation pressurewith said gas evacuation zone (4) of said pressed face of the composite blank (1), the relative elongation (L/L) facing the edging zone being greater than the relative elongation (L/L) facing the distal zone.

21. The process according to claim 16, wherein at least one bladder (10) adapted to be brought into contactin an inflated statewith said pressed face of said composite blank at least between said zone of maximum pressurisation and said gas evacuation zone (4) is used as a pressurisation member (8).

22. The process according to claim 21, wherein said pressure is applied to the pressed face using at least one bladder (10) comprising: at least one internal cavity (14), said internal cavity comprising at least one inlet orifice (12) for a fluid for inflation of said bladder, at least one flexible membrane, said bladder being adapted to be able to conform to said composite blank when the bladder (10) is in contact with said composite blank (1).

23. The process according to claim 21, wherein the bladder (10), being in a state inflated to a pressure lower than the deformation pressure, is disposed at a predetermined distance from the pressed face (2) of the composite blank (1), said predetermined distance varying between said distal zone and said edging zone, said predetermined distance facing the edging zone being greater than the predetermined distance facing the distal zone.

24. The process according to claim 21, wherein said bladder is inflated with the aid of gaseous nitrogen.

25. The process according to claim 16, wherein a thermal treatment of said composite blank is carried out simultaneously with said curing step.

26. A device for manufacturing a part, named composite part, formed from at least one composite material comprising at least one layer of a reinforcing structure impregnated with a matrix within which the reinforcing structure extends, said device comprising: a blank, named composite blank (1), of said composite part, said composite blank having at least one state, named non-cured state, in which the polymer matrix is at least partially non-solidified, said composite blank (1) having at least one face, named pressed face, of said composite blank, on which a pressure is exerted, further comprising at least one member (8) for pressurisation of said composite blank, said pressurisation member (8) being adapted to be able to exert said pressure in the form of a pressure gradient, said pressure being applied to said pressed face of said composite blank between at least one first zone, named zone of maximum pressurisation, of said pressed face and at least one second zone, named gas evacuation zone (4), of said pressed face, said zone of maximum pressurisation being different from said gas evacuation zone (4), said pressurisation member (8) being brought into contact with said zone of maximum pressurisation and into contact with said gas evacuation zone (4) of said pressed face of said composite blank and said pressure gradient being oriented towards said zone of maximum pressurisation so as to cause the gas present within the composite blank to flow from said zone of maximum pressurisation to said gas evacuation zone (4), said pressurisation member having an elastic deformation rigidity varying between a zone, named distal zone, of this pressurisation member coming into contactafter application of said deformation pressurewith said zone of maximum pressurisation of said pressed face, and a zone, named edging zone, of this pressurisation member coming into contactafter application of the deformation pressurewith said gas evacuation zone (4) of said pressed face of the composite blank (1), the rigidity of the edging zone of said pressurisation member being greater than the rigidity of the distal zone.

27. The device according to claim 26, wherein said pressurisation member (8) comprises at least one inflatable bladder (10) having at least one face, named pressurisation face (17), adapted to be disposed, after inflation of the bladder by a pressure, named deformation pressure, in contact with the pressed face of said composite blank (1), and at least one face, named pressurised face (19), opposite to said pressurisation face.

28. The device according to claim 27, wherein said bladder (10) is formed of at least one material selected from the group of elastomeric materials.

29. The device according to claim 26, further comprising a pressurisation support (20) adapted to keep said composite blank (1) in position when said pressurisation member (8) is disposed in contact with said composite blank (1).

30. The process according to claim 17, wherein at least one pressurisation member (8) is used having a modulus of elasticity at least in traction orthogonally to the direction between said distal zone and said edging zone, varying between said distal zone and said edging zone, said modulus of elasticity of the edging zone being greater than said modulus of elasticity of the distal zone.

31. The process according to claim 18, wherein at least one pressurisation member (8) is used having a modulus of elasticity at least in traction orthogonally to the direction between said distal zone and said edging zone, varying between said distal zone and said edging zone, said modulus of elasticity of the edging zone being greater than said modulus of elasticity of the distal zone.

32. The process according to claim 17, wherein the pressurisation member (8) is disposed at a predetermined distance from the pressed face (2) of the composite blank (1) such that, under the effect of the deformation pressure, the relative elongation (L/L) of the pressurisation member (8) varies between a zone, named distal zone, of this pressurisation member coming into contactafter application of said deformation pressurewith said zone of maximum pressurisation of said pressed face, and a zone, named edging zone, of this pressurisation member coming into contactafter application of the deformation pressurewith said gas evacuation zone (4) of said pressed face of the composite blank (1), the relative elongation (L/L) facing the edging zone being greater than the relative elongation (L/L) facing the distal zone.

33. The process according to claim 18, wherein the pressurisation member (8) is disposed at a predetermined distance from the pressed face (2) of the composite blank (1) such that, under the effect of the deformation pressure, the relative elongation (L/L) of the pressurisation member (8) varies between a zone, named distal zone, of this pressurisation member coming into contactafter application of said deformation pressurewith said zone of maximum pressurisation of said pressed face, and a zone, named edging zone, of this pressurisation member coming into contactafter application of the deformation pressurewith said gas evacuation zone (4) of said pressed face of the composite blank (1), the relative elongation (L/L) facing the edging zone being greater than the relative elongation (L/L) facing the distal zone.

34. The process according to claim 19, wherein the pressurisation member (8) is disposed at a predetermined distance from the pressed face (2) of the composite blank (1) such that, under the effect of the deformation pressure, the relative elongation (L/L) of the pressurisation member (8) varies between a zone, named distal zone, of this pressurisation member coming into contactafter application of said deformation pressurewith said zone of maximum pressurisation of said pressed face, and a zone, named edging zone, of this pressurisation member coming into contactafter application of the deformation pressurewith said gas evacuation zone (4) of said pressed face of the composite blank (1), the relative elongation (L/L) facing the edging zone being greater than the relative elongation (L/L) facing the distal zone.

35. The process according to claim 17, wherein at least one bladder (10) adapted to be brought into contactin an inflated statewith said pressed face of said composite blank at least between said zone of maximum pressurisation and said gas evacuation zone (4) is used as a pressurisation member (8).

Description

[0065] Other aims, features and advantages of the invention will become apparent upon reading the following description of one of its preferential embodiments given by way of non-limiting example and with reference to the attached figures in which:

[0066] FIG. 1 is schematic, longitudinal cross-sectional view of a device in accordance with the invention, comprising a pressurisation member,

[0067] FIG. 2a is a schematic, longitudinal cross-sectional view of a device in accordance with the invention, in which an inflatable bladder of variable thickness is used as a pressurisation member, said bladder being in a partially inflated state,

[0068] FIG. 2b is a schematic, longitudinal cross-sectional view of a device in accordance with the invention, in which an inflatable bladder of variable thickness is used as a pressurisation member, said bladder being in an inflated state,

[0069] FIG. 3a is a schematic, longitudinal cross-sectional view of a device in accordance with the invention, in which an inflatable bladder of variable modulus of elasticity is used as a pressurisation member, said bladder being in a partially inflated state,

[0070] FIG. 3b is a schematic, longitudinal cross-sectional view of a device in accordance with the invention, in which an inflatable bladder of variable modulus of elasticity is used as a pressurisation member, said bladder being in a inflated state,

[0071] FIG. 4 is schematic, longitudinal cross-sectional view of a device in accordance with the invention, comprising a pressurisation member.

[0072] FIGS. 1 to 4 illustrate different steps of shaping and curing of the polymer resin forming the matrix impregnating a reinforcing structure of the composite blank 1.

[0073] FIGS. 1 to 4 illustrate a composite blank 1 comprising a plurality of layers superimposed one on another, each layer of the composite blank 1 being formed of a reinforcing structure impregnated with a matrix comprising at least one polymer resin. The composite blank 1 has a parallelepiped shape and comprises two free main faces, including one face, named upper face 2, and one face, named lower face 3, opposite to said upper face. The composite blank 1 also has a gas evacuation zone 4 comprising at least one border of said composite blank 1. The composite blank 1 is, for example, formed of an epoxy resin and of mats of carbon fibres superimposed one on another in parallel with said pressed face. The pressure is exerted on the composite blank in a direction normal to the longitudinal direction of said mats of fibres.

[0074] The device in accordance with the invention illustrated in FIG. 1 comprises a composite blank 1, a pressurisation member 8 and a pressurisation support 20 adapted to keep said composite blank in position when a pressure is applied to the composite blank 1. The pressurisation member 8 has a pressurisation face 15 in contact with the pressed face 2 of said composite blank, and a pressurised face 16 opposite said pressurisation face 15, the pressurisation member being elastically deformable. The pressurisation member 8 illustrated in FIG. 1 is formed of a single material and has a block shape, of which the thickness (shortest distance between the pressurisation face 15 and the pressurised face 16) is constant.

[0075] In particular, the pressurised face 16 is subjected to a constant pressure such as a gas pressure, and said pressurisation member also has an elastic deformation rigidity varying between said zone of maximum pressurisation of said pressed face and said gas evacuation zone of said pressed face of the composite blank, i.e. a rigidity gradient between said zone of maximum pressurisation of said pressed face and said gas evacuation zone of said pressed face of the composite blank. In the illustrated example where the thickness of the pressurisation member is constant, this rigidity gradient is obtained by a modulus of elasticity gradient between said zone of maximum pressurisation of said pressed face and said gas evacuation zone of said pressed face of the composite blank.

[0076] The pressurisation support 20 can also act as a mould permitting a desired shape to be imparted to the composite blank 1. A step of curing the polymer matrix of the composite blank 1 thus also consists of a step of compression moulding. The pressurisation member 8 makes it possible to apply a pressure gradient illustrated by the arrows 5, 6 and 7 and to cause flowing and evacuation of the gas which may be present in the blank 1 in the evacuation direction 18. The pressure 5 is greater than the pressure 6 which is itself greater than the pressure 7 applied to the zone closest to the gas evacuation zone 4 of the composite blank 1.

[0077] In the embodiments illustrated in FIGS. 2a, 2b, 3a and 3b, the device in accordance with the invention comprises a device for pressurising the composite blank 1 consisting of an inflatable bladder 10 in the form of a balloon comprising a peripheral membrane of flexible material. The bladder 10 comprises an internal cavity 14 comprising an orifice 12 for inlet of a gas for inflation of said bladder. Prior to inflation (step not illustrated), the bladder is in a flexible form in the manner of a fabric of a greater or lesser degree of flexibility and the bladder does not have a predetermined shape. When the bladder is partially inflated (FIG. 2a), the bladder is shaped (rotationally symmetrical about an axis passing via the orifice 12 and corresponding to the orientation of the pressure gradient to be applied to the composite blank 1) and is not in contact with the composite blank 1 nor with a pressurisation support 20. After additional inflation to a pressure, named deformation pressure, as shown in FIG. 2b, the inflatable bladder 10 has an external pressurisation face 17 brought into contact with the pressed face 2 of the composite blank 1, and an internal pressurised face 19 defining the internal cavity 14, this pressurised face 19 being opposite to said pressurisation face. Once inflated, the gas pressure inside the inflatable bladder 10 being applied to the pressurised face of the bladder is greater than the pressure prevailing about and outside the bladder (outside the internal cavity 14).

[0078] The devices illustrated in FIGS. 2 and 3 also comprise the hollow pressurisation support 20 adapted to keep said composite blank in position when the bladder 10 compresses the composite blank 1, the bladder 10 and the composite blank 1 thus being disposed inside said pressurisation support 20. The pressurisation support 20 used in the embodiments illustrated in FIGS. 2a to 3b is in the form of a hollow generally parallelepiped mould, of which one of the sides has an opening permitting the provision of a passage for the orifice 12 for inlet of gas for inflation of said bladder, as well as vents 21 and 22 via which the gasses evacuated by the gas evacuation zone 4 of the composite blank 1 can escape.

[0079] In the embodiment of a device in accordance with the invention illustrated in FIGS. 2a and 2b, the peripheral membrane of said bladder has a gradient of thickness a, b, c (thicknesses increasing from a to c). As shown in FIG. 2b, the thickness varies in a continuous manner, increasing between a zone, named distal zone 9 (in the proximity of the portion of thickness a of said bladder) and a zone, named edging zone 13 of the bladder (in the proximity of the thickness portion c). The edging zone 13 faces the gas evacuation zone 4 of the pressed face 2 of the composite blank 1. The distal zone 9 faces the zone of maximum pressurisation of the pressed face 2. The thickness of the edging zone 13 of the bladder 10 is greater than the thickness of the distal zone 9 of the bladder 10.

[0080] In the embodiment of a device in accordance with the invention illustrated in FIGS. 2a and 2b, a median zone 11 of the bladder having a thickness b located between the distal zone 9 and the edging zone 13 of the bladder 10 is thus such that a<b<c (the value of the thickness a is less than the value of the thickness b which is itself less than the value of thickness c). In this way, a continuous pressure gradient is applied to the pressed face 2 of the composite blank 1 permitting flowing and evacuation of the gasses (gas bubbles . . . ) in an evacuation direction 18. This pressure gradient may or may not be constant (affine variation of the pressure along the blank).

[0081] The application of the pressure to the pressed face during inflation of the bladder comprises a first dynamic phase during the inflation of the bladder 10, the non-inflated bladder not initially being in contact with the composite blank 1 and coming progressively into contact with the composite blank 1 during inflation thereof, until an inflated state is reached, at least at the deformation pressure. Then, once the bladder is inflated and the system is at equilibrium, said pressure gradient is maintained in order to prolong and finalise the evacuation of the gasses present within the composite blank 1. However, in come cases, the dynamic pressure variation which results from the exertion of a pressure gradient by the bladder, can be sufficient for the evacuation of the gasses and not necessitate additional maintenance of the pressure.

[0082] Causing the inflated bladder 10 to be brought into contact with said composite part by applying pressure thereto permits evacuation of the gasses present within the thickness of the composite blank 1 in the direction 18 of evacuation of the gasses as far as the gas evacuation zone 4 of the composite blank 1. Bringing the inflated bladder 10 into contact with said composite part by applying a pressure thereto also makes it possible to control the geometry of the part and to control the level of fibres in the part, of the resin (liquid polymer matrix) also being able to be evacuated at the evacuation zone 4.

[0083] In the embodiment of a device in accordance with the invention illustrated in FIGS. 3a and 3b, the external membrane of said bladder has a modulus of elasticity varying in different zones of the membrane of the bladder 10, i.e. a modulus of elasticity gradient. As shown in FIG. 3b, a zone, named distal zone 9, of said bladder 10 has a first modulus of elasticity E.sub.1. A zone, named median zone 11, of the bladder has a second modulus of elasticity E.sub.2, E.sub.2 being greater than E.sub.1. A third zone, named edging zone 13, of the bladder has a modulus of elasticity E.sub.3, E.sub.3 being greater than E.sub.2. The median zone 11 is thus disposed between the distal zone 9 and the edging zone 13 of the bladder 9. The edging zone 13 faces a zone of the composite blank closer to the gas evacuation zone 4 of the pressed face 2 of the composite blank 1 than the zone of the pressed face of the composite blank 1 facing which the distal zone is disposed. The distal zone 9 faces a zone of the composite blank closer to the zone of maximum pressurisation of the pressed face 2 than the zone of the pressed face 2 of the composite blank 1 facing which the edging zone 13 is disposed. In this way, and taking into account the relative values of the moduli of elasticity of these three distinct zones, the same pressure prevailing within the bladder, a discontinuous pressure gradient is applied to the pressed face 2 of the composite blank 1 permitting flowing and evacuation of the gasses in the evacuation direction 18. Such a pressure gradient makes it possible to cause the gasses present within the thickness of the composite blank 1 to flow from the end of the composite blank which is disposed in contact with the distal zone 9 of the bladder to the gas evacuation zone 4 of the composite blank 1.

[0084] As in the embodiment of FIGS. 2a and 2b, the application of the pressure according to this pressure gradient to said pressed face comprises a first dynamic phase during inflation of the bladder 10. Then, once the bladder is inflated, from the deformation pressure, the pressure gradient may or may not be maintained.

[0085] The internal pressure (deformation pressure) within the bladder is, for example, between 0.2 MPa and 1.2 MPa.

[0086] The composite blank can cure at ambient temperature (between 20 C. and 25 C.) or being subjected to a thermal treatment (heating) simultaneously with the application of the pressure gradient. The composite blank, the bladder and the pressurisation support 20 can be placed in an oven or the heating can be provided by a pressurisation medium which is itself, for example, a heating medium.

[0087] The period during which the gasses are evacuated form the composite blank can be, for example, between 30 min and 45 min.

[0088] In another embodiment of a device in accordance with the invention illustrated in FIG. 4, the device comprises a composite blank 1, a pressurisation member 26 and a pressurisation support 27 adapted to keep said composite blank in position when a pressure is applied to the composite blank 1. This device is disposed in an autoclave (not illustrated) in which a pressure greater than atmospheric pressure prevails, adapted to permit the creation of a pressure gradient by virtue of the contact of the pressurisation member 26 with the pressed face 2 of said composite blank. The step illustrated in FIG. 4 shows the device prior to application of the pressure in the autoclave, the pressurisation member 26 not yet being in contact with the composite blank, the pressurisation member 26 being disposed at a predetermined distance from the composite blank before the pressure within the autoclave is increased. The pressurisation member 26 has a pressurisation face 15 adapted to come into contact with the pressed face 2 of said composite blank when the device is subjected to the pressure prevailing within the autoclave, and a pressurised face 16 opposite said pressurisation face 15, the pressurisation member being elastically deformable. The pressurisation member 26 illustrated in FIG. 4 is formed of a single material and has a block shape of which the thickness (shortest distance between the pressurisation face 15 and the pressurised face 16) is less at its centre. Since the pressurisation member 26 has a variable thickness, it makes it possible to apply a pressure gradient, whereas the pressure applied to its pressurised face 16 is constant and corresponds to the pressure prevailing within the autoclave. Bolts and nuts 24, 25 make it possible to fix the pressurisation member 26 to the pressurisation support 27. A vent 23 formed in the thickness of the pressurisation support 27 makes it possible to connect the device to a source of suction (vacuum). This device makes it possible to cause flowing and evacuation of the gas which may be present in the blank 1 in the evacuation direction 28 and 29 from the centre of the composite blank to the ends 30 and 31 of the composite blank, thus each forming a gas evacuation zone.

[0089] In another embodiment of a device in accordance with the invention (not illustrated), it is possible to cause the relative elongation (L/L) of the wall of the bladder (i.e. the stretch of the wall of the bladder or surface stretching) to be varied, in particular by causing the distance separating the bladder and the composite blank and/or the dimensions and/or the shape of the bladder to be varied, taking account of the shape and the dimensions of the composite blank. The distance separating the bladder and the composite blank designates the distance which separates the bladder from the pressed face of the composite blank when the bladder is in a partially inflated state, prior to inflation to the deformation pressure.

[0090] The greaterlocallythe distance between the bladder and the composite blank prior to inflation, the lowerlocallywill be the pressure applied to the pressed face of the composite blank. The pressure exerted on the pressed face of the composite blank is equal to the internal pressure of the bladder less the force of resistance to the inflation of the bladder per unit of surface area, the force of resistance to the inflation of the bladder being proportional to the relative elongation (L/L) of the wall of the bladder during inflation as well as to the thickness and the Young's modulus of the bladder. It is also possible to cause variation of the dimensions of the bladder, in particular the shape and size of the bladder, in order also to influence this shrinkage prior to inflation.

[0091] Furthermore, in order to create said pressure gradient it is possible to choose a bladder formed of a material which is rigid in a direction (for example, the axial direction (or longitudinal direction) which corresponds to the direction of the pressure gradient). For example, carbon or glass fibres or fibres known under the name of Kevlar are placed in the material forming the wall of the bladder in the axial direction. During inflation, the rigidity will thus vary in the circumferential direction of the bladder. The material forming the bladder can thus be anisotropic.

[0092] In another embodiment of a device in accordance with the invention (not illustrated), the bladder can comprise a plurality of internal cavities (for example, three internal cavities), the pressure being different in each of said cavities in order to exert a discontinuous pressure gradient on the pressed face of the composite blank.

[0093] The invention can comprise a large number of embodiment variants. In particular, it is possible to prepare any type of composite part, even the most complex, using, if necessary, a bladder specially designed and adapted to the geometry of said composite part. The composite blank can be of any shape, for example, a disc, a crown, a strip or even be in the form of a convoluted part.

[0094] Furthermore, one or a plurality of bladders can be used simultaneously to prepare a single composite part. And each bladder can have, in combination, a discontinuous thickness and/or a discontinuous modulus of elasticity and/or a plurality of internal cavities. The peripheral membrane of the bladder used as a pressurisation member in a device or process in accordance with the invention can have, for example, five zones of different moduli of elasticity and each of these five zones can have a continuously varying thickness. Such an embodiment has the advantage of allowing a significant pressure gradient to be achieved while exerting a pressure which increases in the most regular manner possible, without sudden increase of pressure between each zone having a different modulus of elasticity. On the other hand, each zone of the membrane of the same thickness and/or having the same modulus of elasticity can have variable dimensions (identical or different from each other) depending on the pressure to be applied, the type of part, the materials forming the blank.