The invention relates to a method and a device for manufacturing a composite part of complex shape. The invention is more particularly intended for the production of a composite part in three dimensions such as a box or a dome, or having a plurality of protruding reliefs. The invention also applies to the production of tubular parts, in particular comprising the junction of several tubes, such as exhaust manifolds. The invention is applicable to many areas, including, but not limited to the achievement of luggage or covers for electronic devices such as tablets or TV screens, carrying helmets or protective equipment,

The prior art in these areas is for example shown in EP 2694 277. This document describes the production of a rectangular shell with flanged edges, the general shape of a box with 5 faces from a plane blank, consisting of a lamination of pre-impregnated fabrics of a thermoplastic polymer. Said fabrics are formed and compacted / consolidated into a composite part reinforced by continuous fibers, by a die punch assembly, while a holding-down device allows to maintain the fibers under tension during shaping of lamination the desired shape. This technique is satisfactory but requires a clipping result of the play performed after forming and consolidation, and does not produce deep drawn.

The invention aims to remedy the drawbacks of the prior art and relates for that purpose a method of a composite component of three-dimensional shape with a thermoplastic matrix and continuous reinforcing characterized in that it comprises the steps

consists in :

at. obtain a prepreg fiber preform of the thermoplastic polymer constituting the matrix corresponding to the shape of the final part, by three-dimensional knitting;

b. placing the preform between the punch and the matrix of matched tooling defining therebetween a sealed closed cavity; vs. close the tooling so as to apply a first pressure to the preform;

d. carry the cavity at the melting temperature of the polymer impregnating the preform while maintaining the first pressure; e. cooling the cavity comprising the preform to a temperature suitable to release by maintaining a second pressure;

f. opening the mold and demoulding the part.

Thus the knitting process allows to obtain a three-dimensional shape of preform, corresponding substantially to the shape of the final part, complete assembly without creases and without shaping by deformation. . The use of a closed cavity associated with the pressure-temperature cycle provides a room directly to the finished dimensions with sharp edges that do not require trimming. The depth of stamping is not limited due to the absence of forming. The preform is pre-impregnated with a thermoplastic polymer, it can be stored for an unlimited period and produced at a remote site processing service. The process is particularly suitable for industrial mass production adapted to the areas covered by the invention.

The term "prepreg" in the context of the invention and applied to the preform obtained by the method of the invention, refers to a dry appearance and flexible preform comprising the polymer constituting the matrix of the future composite part. The means for integrating the polymer to a preform knitted with these characteristics are described in specific embodiments of the invention. The actual impregnation linker percolation between fibers of the polymer melted in the steps d) and e) of the method of the invention.

The invention is advantageously implemented according to the embodiments and variants described below, which are to be considered individually or in any technically operative combination.

According to one embodiment, the method of the invention comprises, before step d) a step to evacuate the molding cavity defined by the punch and the die containing the preform. Thus, the vacuum operation of said cavity and applying the first pressure help ensure degassing of the preform and a good impregnation thereof during melting of the impregnating polymer.

Advantageously, the method of the invention comprises between steps d) and f) a step of:

g. maintaining the temperature for a suitable time for impregnation of the preform by the polymer.

This holding time, depending on the viscosity of the polymer and tau fiber ensures uniform impregnation of the preform.

Advantageously, a second pressure is applied to the preform during the step g). Thus, the application of the second pressure when the polymer constituting the matrix is ​​already thinned, allows for compaction. Maintaining this pressure during cooling is used to calibrate the thickness and shape of the part.

Advantageously, the preform is obtained knitted from a yarn consisting of strands of reinforcing fiber commingled with the strands consisting of the impregnating polymer. This embodiment ensures uniform impregnation of the preform during steps d) and e).

Equivalently, the preform is obtained knitted from a yarn consisting of the coated reinforcing fiber of the impregnating polymer.

According to another alternative embodiment, the preform is knitted from a yarn consisting of the reinforcement and a wire made of the impregnating polymer.

According to one particular case of this latter alternative, the reinforcing fiber is a polymer whose melting temperature is higher than the melting temperature of the impregnation polymer.

According to an alternative embodiment, the preform is knitted using the

knitting technique to signs. This uses the most common knitting technique and the most versatile in terms of feasible forms, however, in many cases, the finish or closing the three-dimensional preform requires the completion of a seam.

To this end according to the implementation variants of this embodiment, the method object of the invention comprises prior to step b) a step of sewing for closing the contour of the preform. This embodiment allows the preparation of outline preforms closed ready to be used, even if the knitting technique does not provide this feature directly during knitting.

Alternatively, the method comprises, between steps b) and c) a step of:

h. close the contour of the preform by a weld.

This embodiment takes advantage of the constitution of the preform incorporating the impregnating polymer. This welding is carried out before step b) or when the preform is placed on the punch or the die, in order to ensure precise positioning of said solder.

According to another alternative embodiment, the preform is knitted using the technique of the transferred stitch. This technique produces, depending on the variant, a single or double jersey fabric and allows a three-dimensional preform in one piece without seam or joint, at the cost of higher complexity knitting.

According to an advantageous embodiment of the method of the invention, the first and the second pressure are applied to the preform by varying between two values, the gap between the punch and the die, at the closed cavity. This embodiment provides more precise control of the thickness and therefore the calibration of the preform.

The invention also relates to a tooling for carrying out the process according to the invention, which equipment comprises:

x. a punch made of an electrically conductive material; there. a paired matrix to the punch so as to form a cavity between the molding surfaces of the punch and the die, and made of an electrically conductive material;

z. an induction circuit for the heating of the molding surfaces of the punch or die;

u. a high-frequency current generator for supplying the induction circuit.

The use of induction as tooling autonomous heating mode reduces cycle times and produce parts in large series.

According to one embodiment, the cavity defined between the punch and the die has a taper widening towards the base of the punch. Thus, the pressure on the preform is controlled by the relative movement of the die and punch, which, depending on the conicity along the flanged edges, allows to control the value of the gap in the closed cavity on all sides of the preform. .

Advantageously, the punch or die comprises a cooling circuit for circulation of a fluid. This arrangement reduces the cycle time for producing a part by accelerating step f) of the process.

Advantageously, the molding surfaces of the punch and of the die defining the cavity are made of a ferromagnetic material whose Curie point is equal to the melting temperature of the polymer impregnating the preform. This embodiment simplifies the control of the temperature in the cavity, in particular to prevent burning when the preform of reinforcing fibers are prone to this phenomenon.

According to an embodiment of the tooling for carrying out the method of the invention, the latter comprises a matrix consisting of a thermally conductive material and an inductor extending in a cavity formed in said matrix wherein the volume of the molding cavity is variable independently of the approximation of the punch and the matrix. This embodiment is suitable for the implementation of a preform comprising electrically conductive fibers or not. Varying the volume of the molding cavity ensures compaction and calibration thickness of the finished part.

According to a variant of this embodiment of the tooling object of the invention, the punch comprises an inflatable bladder. This variant makes it possible to ensure uniform pressure over the entire surface of the preform in particular during the steps e) and f) of the process.

According to another variant of this embodiment of the punch tooling includes a movable part driven by the approximation of the matrix and the punch. This variant makes it possible to control the air gap so the thickness of the finished part in the molding cavity.

According to another variant, the punch is constituted by a flexible sheet upon which is applied a gas pressure for carrying out the steps c) to f) of the method of the invention.

Advantageously the cover comprises on its face in contact with the preform, filler of a material sensitive to induction heating. Thus the tarp participates in uniform heating of the preform.

The invention is explained below according to its preferred embodiments, in no way limiting, and with reference to Figures 1 to 5, wherein:

- Figure 1 shows an example in a sectional view an embodiment of a tool, shown in the open position, for the implementation of the invention

- Figure 2 shows two examples of preform knitted in a half shell comprising zones trihedron connection:

- Figure 3 illustrates in a sectional view in the open position and another embodiment of a tool for implementing the method object of the invention;

- Figure 4 shows a block diagram of the object of the invention;

- and figure 5 shows in a perspective view an embodiment of a preform comprising knitted closed contour faces 5 and the connection zones between the trihedral faces.

Knitting techniques are known in the prior art and allow for a three-dimensional assembly of a plurality of son interwoven in loops or meshes. Depending on the mode used knitting, complex preforms are made in a single knitting operation. In other knitting techniques the contour of the workpiece can not be closed and requires in this case a closing step by sewing, or more preferably by welding as discussed below.

5 shows an example of a preform (500) of three-dimensional shape comprising faces 5 and four areas (501) of connection between the trihedral faces, these connection regions are commonly referred to as "trunk corner". In order to see the three dimensional shape of the knitted preform (500) thereof is here shown resting on a support, which support is for example the punch tooling. The knitting technique in this embodiment allows a continuity of the fibers over the entire surface of the preform, the contour is closed. The strands are continuous, including in the zones of connection trihedron (501) which are not developable areas, that is to say, they can not be flattened while maintaining the length of the strands. As the figure shows,

The knitting step of the object method of the invention is advantageously, but not exclusively, implemented by means of a knitting machine picked dish that offers the most versatility in terms of attainable shapes and closed contour. Forms achievable include box shapes including corners of trunk, shapes substantially in the shape of a dome or cap, such a helmet, tubular forms including pipe connections, or even a combination of these different forms, optionally comprising recesses.

According to the prior art, the three-dimensional knitting is used for producing dry fibrous preform subsequently impregnated with a thermosetting resin by a method employing a transfer liquid resin into a mold, such as RTM (Resin Transfer Molding ).

However, this process is not suitable for mass production. The method object of the invention implements a knitted fiber preform, comprising the polymer itself which will form the matrix of the composite part.

To this end, the knitting step of the method of the invention uses reinforcing fibers such as glass fibers, carbon, aramid, metal, polymers, or natural fibers such as fibers of flax, coconut, sisal, jute, bamboo or, where appropriate previously sized and spun, or a combination of such fibers, combined with the thermoplastic polymer constituting the

matrix of the future composite part.

Examples, said polymer is introduced comêlé the reinforcing fiber, for example in the form of said fiber strands, yarns with strands of said polymer and optionally twisted, or in the form of reinforcing fibers coated with said polymer, or still knitting son said polymer with the son consist of reinforcing fibers.

Whatever the embodiment, the lack of tackiness of the thermoplastic polymer enables its implementation in conjunction with the reinforcing fibers during the knitting step.

As non-limiting examples, said thermoplastic polymer is a polyetherketone (PEK), a polyetheretherketone (PEEK), polyetherimide (PEI), thermoplastic polyester, polyphenylene sulfide (PPS), polyamide (e.g. PA6 or PA6-6), an acrylonitrile butadiene styrene (ABS).

According to other examples, said polymer is selected from biopolymers such as:

- polyamides (PA), including 1 PA1

- polyethylene (PET) biosourced;

- the polylactic acid (PLA);

- or of the bio-based polyesters

The combination of the method of production of the preform, which interleaves the fibers and provides continuity of this interleaving on any form, and a thermoplastic polymer, allows the production of lightweight parts particularly resistant to impacts, and depending on the choice of polymer, temperature resistant and flame. Thus the method of the invention is particularly suitable for the production of components subjected to this type of load, such as luggage items, personal protective equipment such as helmets, harness, shields, elbow pads or knee pads, protective shells or light shielding members.

Figure 1, according to an exemplary embodiment of the method of the invention, suitable for the implementation of a preform of which the reinforcing fibers are not electrically conductive, the preform (100), previously knitted and comprising fibers reinforcement and the thermoplastic polymer impregnation of the final part is placed on the portion forming the punch of the tool.

Figure 2, according to an exemplary non-limiting implementation, the preform (100a) is shaped shell, comprising areas of trihedral connection, called "trunk corners". As an illustrative example, there is a half-shell suitcase. According to an embodiment, said preform (100a) comprises a recess (201) made directly during knitting of said preform. In comparison with techniques of the prior art for the implementation of the thermoplastic composite materials, such as stamping or compression fit, the technique for obtaining the preform by knitting preserves in the connecting regions, in particular the corners of trunk, a tau tau comparable reinforcement to reinforcement in the rest of the room while avoiding the formation of folds in those areas.

Returning to Figure 1, if the knitting technique allows the production of such a preform of a closed contour, while, the tooling in the open configuration, said preform (100) is simply pushed onto the punch (1 10) capping said punch with said preform (100), after having, if necessary, spraying a release agent on said punch. Such an operation is easily performed manually or by means of a manipulator or a robot. The implementation of the tooling self-heating as presented below, that allows the punch is at room temperature during this step.

Figure 2, in another embodiment, the knitted fabric used technique does not allow to close the contour of the preform (100b) and said preform comprises a contour interruption delim ited by two open edges (21, 1 212) .

Returning to Figure 1, if the knitting technique does not allow to obtain a knitted preform closed contour, then, the preform being positioned on the punch (110), positioning said preform in tooling is stabilized performing at least two weld points joining the open edges of the contour discontinuity. The presence of the polymer in the knitted preform allows for such welding. Said welding is carried out for example by means of a soldering iron., Alternatively, it is in the form of dots or line of welding by other means known from the prior art, such as by laser or ultrasonic.

According to this mode of implementation of the tool, the punch comprises

advantageously positioning means of the open edges of the preform relative to each other, for example in the form of barbs or hooks threaded through the meshes. Thus, the preform is perfectly positioned on the tooling.

The tooling is comprised of at least two parts (1 10, 120) separable between an open configuration and a closed configuration defining a punch (110) and a die (120). The punch and die are paired so that they define between them, at their molding surfaces and upon closure of the tooling, a gap (e), exaggerated in the figure, corresponding to the final thickness of the produced part. Thus, the space between the punch and the die defined a molding cavity in which is located the preform (100).

According to an exemplary embodiment, means (160) allow to evacuate the molding cavity comprising the preform (100) once the tool closed.

According to an exemplary embodiment, the punch and the die are made of a ferromagnetic material and are coated on their outer faces, outside of the molding surfaces of a continuous layer (1 11, 121) of an electrically conductive material and not ferromagnetic, such as copper. Upon closure of the tooling, the parts of the tooling forming the punch and the die are separated from each other by an electrically insulating material layer (130). This layer of insulating material also seals off the cavity formed between the punch and the die upon closure of the tooling.

The punch assembly matrix is ​​inserted into an induction circuit formed by two half-turns (141, 142) one of which (141) is integrated in the punch and the other (142) to the matrix. Closing the tool has the effect of electrically connecting the two half-turns. The coil thus formed, surrounding the tool, is connected to a high-frequency current generator (not shown), so that the supply of said coil causes the circulation of currents induced on the surfaces of the tooling. The induced currents circulating in a reduced thickness of material at the surface of the tooling. Thus, said induced currents circulating in the coating layers (121, 1 1 1), and due to the cut performed by the insulating layer (130) between the two parts of the tool,

Said molding surfaces being made of a ferromagnetic material

rapidly rise in temperature due to the movement of such high-frequency induced currents, and release their heat to the preform (100).

Under the effect of temperature, the polymer comprised in the knitted preform is heated to its melting temperature, and the preform being confined in a closed cavity and sealed, said polymer impregnates the preform uniformly.

closing pressure applied to the mold so that the air gap (e) between the punch and the die, calibrate the thickness of the final part. The use of induction heating, in the configuration of the mold according to this embodiment makes it possible to concentrate the heating of the molding surfaces of the cavity without the need to heat the entire mass of the tool.

Said molding surfaces rapidly rise in temperature, said temperature is controlled, e.g., by selecting the ferromagnetic material forming the mold based on its Curie temperature.

Advantageously, the punch (1 10) and / or the matrix (120) of the tool comprise channels (151, 152) for circulation of a coolant, eg water, allowing a rapid cooling of the surfaces slinky.

Thus, the molding cavity and the preform (100) being brought to a temperature at least equal to the melting temperature of the polymer comprised in said preform, said temperature is maintained for a suitable time between a few seconds to 1 minute to ensure impregnation uniform of the preform by the polymer.

Said holding time is a function of the fiber reinforcement rate and viscosity of the molten polymer to the holding temperature. More the polymer is more viscous and the fiber content and the higher the holding time is important. The holding time is easily determined by trial.

After the appropriate time of maintenance, the power supply to the coil (141, 142) is stopped, and the molding cavity is cooled by the circulation of a heat transfer fluid in the channels (151, 152) of the tool cooling .

During the steps of heating, holding and cooling, the tooling closing pressure is maintained.

According to an implementation variant of a second pressure, higher than the first pressure, is applied to the preform at the end of heating and during

the maintaining step. This second pressure is applied, for example, by relatively moving the punch relative to the die. According to an exemplary implementation, the punch and the die have a slight taper, widening towards the base of the punch so that the relative movement applies said pressure, by reducing the air gap (e) over all surfaces of the preform.

When the cooling of the molding cavity by the flow of coolant reduces in temperature thereof to a temperature below the glass transition temperature of the polymer, or more generally a temperature where rigidity of the polymer is sufficient to provide handle piece thus produced without deforming, the mold is opened and the part is demolded.

Then the cycle begins again with a new preform. The workpiece of time between two apertures of the mold depends on the size of the room, the nature of the polymer and fiber content, but generally is between 1 and 5 minutes, essentially a function of time holding temperature.

The use of induction heating ensures that sufficient heating temperature is reached on every cycle to ensure uniform impregnation of the piece. The heating rate of about 2 ° Cs "1 , authorized by the heating mode, also allows the implementation of natural fibers, susceptible to burns without risk of degradation of the fibers.

The implementation of steps before it applies in the same way by initially placing the preform in the matrix rather than the punch and are also suitable for the implementation of a knitted preform with a shape other than that of a half box.

Figure 3 according to another exemplary implementation of the method of the invention, suitable for any type of fiber including electrically conductive fibers, the implementation tool comprises at least two parts, in the form of a punch (310) and a die (320). According to this exemplary embodiment, the matrix (320) is made of a metallic material, good heat conductor such as an aluminum or copper alloy without these examples are limiting.

The matrix (320) includes a conduit assembly (340) into which extend the inductors (341). Said inductors are in the form of brass tubes or Litz cables. According to an exemplary embodiment, the inside said conduits comprising a jacket (342) made of a ferromagnetic material to a thickness of between 0.2 mm and 2 mm. Thus, when the inductor (341) is supplied with high frequency current, induced currents circulating in the ferromagnetic liner (342), causing warming thereof.

Heat is transferred to the matrix and is spread by conduction to the molding surface thereof, which heats up.

Advantageously, said conduits (340) comprising inductors (341) are located at a distance d of the impression of the matrix so as to ensure a uniform temperature on the molding surfaces of said matrix.

Alternatively the matrix is ​​made of a material, for example a steel, ferromagnetic, in this case the lining of the ducts (340) comprising the inductor (341) is not necessary.

Advantageously, the matrix includes conduits (360) for circulating a heat transfer fluid for cooling.

The mold is here shown in an open position. The approximation of the punch (310) and the die (320) and the means (312) sealing it possible to define a sealed cavity in which is included the knitted preform (100).

According to this embodiment, the punch comprises a bladder (350) and means (351) for inflating said bladder. The bladder (350) consists of an elastomer resistant to the fusion temperature of the polymer included in the preform.

For example said bladder is made of silicone loaded atoms. This embodiment allows to compact all the faces of the preform by inflating said bladder, even if the fingerprint does not include taper widening towards the base of the punch, and even includes a reverse taper, for example when the process object of the invention is used for the manufacture of a helmet. In this case, the matrix comprises at least two separable parts to permit demoulding of the final part.

This embodiment, wherein the punch comprises a bladder (350) is also usable in the embodiment of the tool shown in FIG 1. In this case, the surface of the bladder come into contact with the preform (100) during manufacture of the part comprises an electrically conductive coating bonded

electrically to the rest of the punch, to ensure the circulation of induced currents, this coating may be further ferromagnetic.

According to an exemplary use of tools corresponding to this embodiment, the knitted preform (100) is inserted into the cavity of the die (320); The punch has approached the matrix creating a sealed cavity in which is included the preform. The bladder (350) is inflated to a first pressure to make contact with the preform. The inductors (341) are supplied with high frequency current, which has the effect of heating the molding cavity and to bring to its melting point the polymer comprised in the knitted preform.

The bladder inflation pressure (350) is increased so as to ensure compacting of the preform. The temperature is maintained in the molding cavity so as to ensure uniform impregnation of the preform, the hold time being a function of the fiber content and viscosity of the polymer.

Then, feeding of the inductors is stopped and the coolant is fed into the cooling channels (360) so as to cool the mold and the piece thus produced, at a suitable temperature to its demolding.

In the case where the tooling shown in Figure 1 comprises an inflatable bladder having such a conductive coating as described above, that the bladder is bonded to the die or punch, the sequence of such operations as described above is even.

As for the example of Figure 1, other embodiments are possible, wherein the plug does not include a bladder and wherein the heating and cooling means are included in the punch and even in the punch and the die, the preform being disposed over the die rather than in the matrix.

Thus, the combination of the use of a self-heating mold by induction, according to any of the embodiments discussed above or a combination of these embodiments with a knitted preform comprising the impregnation polymer makes it possible to achieve mass in reduced implementation times and consolidation in a single operation, of complex shape comprising a high fiber content.

The method of the invention is thus particularly suitable for the production of composite parts for high consumption of markets.

The combination of a high fiber content, of a thermoplastic polymer constituting the matrix and the mode of organization of the fibers in the composite makes these parts particularly resistant to impacts and allows to maintain a rate of material fibers in areas which, according to the methods of the prior art, have a reduced fiber content, particularly the corner trunks in the example shown.

Now these areas, including areas of connection triad or trunk area, are areas particularly stressed on the products covered by the invention, particularly in terms of impact, like a suitcase shell.

Figure 4, in a first step (410) of the method object of the invention, the preform is knitted so as to correspond to the shape of the final part. Said preform comprises the thermoplastic polymer impregnation of the final part or as comêlée in knitted son or as a coating on knitted son or by knitting son son reinforcement and consisting of the polymer of impregnation.

Said polymer is stable so that the preform can be stored without time limit or generated on a remote site processing service.

According to a step of loading (420) the preform thus obtained is mounted on or in the tool according to any of its embodiments, the mold being opened.

According to a particular embodiment, corresponding to the case where the knitting technique does not allow to obtain a preform with a closed contour, a step (425) of welding is carried out directly in the tool in order to maintain the closed contour of the preform .

In a step (430) impregnation / consolidation, the mold is closed, thereby applying a first pressure on the preform and the molding cavity defined between the punch and the die is heated to a temperature greater than or equal to the melting temperature polymer impregnation.

According to a particular embodiment, the method of the invention comprises a step (427) of pumpdown of the cavity comprising the preform after closing the mold.

According to a holding step (440) the molding cavity and the preform are

maintained at the temperature reached in the previous step (430) while maintaining the contact pressure between the molding surfaces of the punch and the matrix and the preform.

According to an alternative setting work, the holding step (440) comprises a compacting step (445) of increasing the pressure on the preform, or by bringing the punch and the matrix or by applying additional air pressure on the expandable means (bladder) of the die or the punch.

According to a cooling step (450) the molding cavity and the preform are cooled by circulating a coolant into the mold while maintaining pressure on the preform.

Cooling (450) is continued until the temperature in the molding cavity is less than or equal to the glass transition temperature of the impregnating polymer.

According to a demolding step (460) the mold is opened and the part removed from the mold. The cycle then restarts the charging step (420) with a new preform.

WE CLAIMS

Method for producing a composite component of three-dimensional shape with a thermoplastic matrix and continuous reinforcing characterized in that it comprises the steps of:

at. obtaining (410) a fibrous preform (100, 500) of the polymer constituting the prepreg matrix corresponding to the shape of the final part, by three-dimensional knitting;

b. placing (420) the preform between the punch (120, 320) and the die (110, 310) matched tooling defining therebetween a sealed closed cavity;

vs. close the tooling so as to apply a first pressure to the preform;

d. carrying (430) the polymer melting temperature to cavity impregnating the preform while maintaining the first pressure; e. cooling (450) the cavity comprising the preform to a temperature suitable to release by maintaining a second pressure;

f. opening the mold and demolding (460) the room.

The method of claim wherein step c) comprises a pumpdown (427) of the cavity.

A method according to claim 1, comprising between steps d) and f) a step of:

g. maintain (440) the temperature for a time suitable for the impregnation of the preform by the polymer;

The method of claim 3, wherein a second pressure is applied (445) on the preform during step g).

The method of claim 1, wherein the preform is obtained knitted from a yarn consisting of strands of reinforcing fiber commingled with the strands consisting of the impregnating polymer.

6. The method of claim 1, wherein the knitted preform is obtained from a wire coated with the impregnating polymer.

7. The method of claim 1, wherein the preform is obtained by knitting son consisting of fiber reinforcement and son made of the impregnating polymer.

8. The method of claim 7, wherein the reinforcing fiber is a polymer whose melting temperature is higher than the melting temperature of the impregnation polymer.

9. The method of claim 1, wherein the preform is knitted using the knitting technique to signposting.

10. The method of claim 1, comprising before step b) a step of sewing for closing the contour of the preform.

11. The method of claim 1, comprising between steps b) and c) a step of:

h. close the contour of the preform (425) by a weld.

12. The method of claim 1, wherein the preform is knitted using the technique of the transferred stitch.

13. The method of claim 4, wherein the first and second pressures are applied to the preform by varying between two values, the gap between the punch and the die, at the closed cavity.