IMPROVED METHOD FOR MANUFACTURING A SKIN FOR AN AERONAUTIC ENGINE

Description

Technical Field

[0001] This presentation concerns the field of skins used in aircraft engines, particularly, but not exclusively, in engine casings or acoustic panels. More specifically, this presentation concerns a manufacturing process for such a skin, and a skin obtained by this process. Previous technique

[0002] As is well known, the skins used in aircraft engines, for example in engine casings or acoustic panels, are deposited by automated draping of thermosetting matrix composite. More precisely, a skin is draped onto a draping tool, specifically onto the surface (or substrate) of said tool. This draping is performed by well-established dispensing tools, such as robots called "AFP" (Automated Fiber Placement), which successively deposit parallel, pre-impregnated strands or strips in multiple layers, called "plies," or by "ATL" (Automated Tape Layer), which deposits pre-impregnated sheets wider than the strips deposited in the "AFP" technique. These sheets are deposited one after the other. The skin is then cured in an autoclave for several hours.

[0003] Currently, thermoplastic matrix composite materials are also used and offer numerous advantages over thermosets. In particular, the chemical bonds in a thermoplastic resin are reversible. The resin simply needs to be heated to melt and can thus be reused (in contrast, a thermosetting resin, once polymerized, can no longer be used, or is very difficult to reuse). Thermoplastic materials can therefore be reused "indefinitely." They can also be stored at room temperature. DE 101 29 514 A1 discloses an example of a process involving the draping of a thermoplastic material.

[0004] Among the thermoplastic resins used in the aerospace industry and compatible with engine environments, so-called "semi-crystalline" resins are employed. Semi-crystalline materials offer the advantages of resistance to chemicals, fire, and abrasion. They also possess superior mechanical properties.

[0005] However, to achieve the target properties of this material, particularly the desired level of crystallinity, it is necessary to control the thermal cycle during processing. During skin draping, the aforementioned dispensing tools provide the necessary pressure and heat for adhesion of the last layer deposited to the one being deposited, but they do not allow control of the thermal cycle and therefore the level of crystallinity. To address this, thermal cycle management is now achieved through a second process: applying a thermal cycle in an autoclave or oven.

[0006] These autoclave and oven consolidation methods generate high energy consumption and are expensive. Furthermore, the manufacturing process for these hides is fragmented, which slows down the production cycle.

[0007] Therefore, there is a need to at least partially address the aforementioned drawbacks. Description of the invention

[0008] The present exposition relates to a method for manufacturing at least one skin, in particular an acoustic panel for an aircraft engine, comprising draping a thermoplastic material onto a surface of a draping tool, by means of a deposition tool configured to exert pressure on the thermoplastic material and to heat the latter during its draping, in which the draping tool includes a thermal regulation device configured to locally heat the surface of the draping tool.

[0009] It is understood that the skin is deposited by draping directly onto the draping tooling, more precisely onto the surface (or substrate) of said tooling.

[0010] The skin draping is performed by a dispensing tool, which can be an "AFP" or "ATL" robot, through successive deposits of pre-impregnated thermoplastic reinforcement strips or sheets. The dispensing tool may, for example, include a dispensing head that provides the pressure and heat necessary for adhesion between the strips, particularly between the last layer deposited and the one being deposited.

[0011] Furthermore, the thermal regulation system of the draping tool allows for the localized application of heat to the tool's surface, thus heating or cooling the thermoplastic material already deposited on that surface. This localized and targeted modulation and additional heat input enables control over the cooling of the newly deposited thermoplastic material, which has just been heated by the deposition tool. More specifically, thanks to this localized heat input, the cooling temperature gradient is controlled. The cooling rate of the thermoplastic material, heated by the deposition tool, can therefore be controlled, notably being slower than if the thermoplastic material were immediately exposed to ambient temperature.It is therefore understood that the local heat input by the thermal regulation device is preferably carried out at a temperature lower than the heat supplied by the dispensing tool, and higher than the ambient temperature. A local heat input to the surface of the draping tooling before the thermoplastic material is dispensed can also allow for a gradual temperature increase of said surface prior to dispensing.

[0012] The local application of heat makes it possible to target the areas that need to be heated or cooled more or less quickly, in order to obtain the desired degree of crystallinity depending on the areas of the skin, and its thicknesses, which are not necessarily homogeneous over its entire surface, as they depend on the number of folds.

[0013] This process allows for precise control of the material's crystallinity level, ensuring material integrity and optimal properties. It also provides excellent adhesion between the fibers and the material matrix, without the need for additional energy-intensive autoclave heating cycles. This enables cost reduction and faster manufacturing cycles.

[0014] In some embodiments, the draping tooling comprises a plurality of cells arranged under the surface of said draping tooling on which the skin is manufactured, the thermal regulation device being configured to heat each cell individually.

[0015] The term "cells" refers to cavities, or compartments, located beneath the surface of the draping tool. By individually heating each of these cells, it is possible to locally heat the surface of the draping tool. This allows for preheating specific areas of the tool's surface just before the thermoplastic material is deposited, thus controlling the temperature gradient experienced by the material and improving the cooling of the newly deposited thermoplastic material during the skin manufacturing process.

[0016] In some embodiments, local heating of the draping tooling surface is carried out by induction, via a heat transfer fluid or by forced air.

[0017] Furthermore, when the draping tooling comprises multiple cells arranged beneath its surface, each cell can be heated individually by induction, via a heat transfer fluid, or by forced air. In particular, localized forced-air heating allows for significant variation in the amount and duration of heat applied with each deposited ply, which is especially advantageous given the high thermal insulation capacity of thermoplastic resins. This technique enables dynamic temperature variation compatible with automated draping speeds ranging from 0.1 m / min to 60 m / min. Moreover, forced-air heating results in low energy consumption due to its ability to provide localized dynamic temperature variation.

[0018] In some embodiments, local heating of the draping tooling surface is carried out by forced air, each cell being supplied with forced air via a duct, each duct being equipped with a heating element individually controlled by the thermal regulation device.

[0019] In some embodiments, the thermal regulation device includes a control unit configured to synchronize the local heating of the draping tooling surface with the movement of the dispensing tool.

[0020] In particular, when the draping tooling comprises multiple cells arranged beneath its surface, the control unit can individually control each cell, and specifically each heating element, to deliver heat locally based on the position and movement of the dispensing tool. This synchronization further improves the control of local cooling of the thermoplastic material that has just been deposited and thus heated by the dispensing tool, thereby allowing for greater control over the material's degree of crystallinity and consequently improving its material health.

[0021] In some embodiments, a dispensing surface is defined as the contact surface between the dispensing tool and the surface of the draping tooling, the dispensing tool is configured to heat the surface of the draping tooling to a first temperature downstream of the dispensing surface, and the thermal control device is configured to locally heat the surface of the draping tooling upstream of the dispensing surface to a second temperature lower than the first temperature, an upstream downstream direction being defined with respect to the direction of movement of the dispensing tool with respect to the surface of the draping tooling.

[0022] It is understood that during the skin manufacturing process by draping, the deposition tool moves relative to the surface of the draping tooling, and the deposition surface therefore moves accordingly. This deposition surface can, in particular, be the contact surface between a compaction roller and the surface of the draping tooling. Thus, upstream of the deposition surface is the pre-impregnated strip(s) of thermoplastic material that has just been deposited by the deposition tool during its movement, and downstream of the deposition surface is the region of the draping tooling surface just before the said strip(s) are / are deposited.

[0023] Heating this downstream region to the first temperature facilitates deposition and improves the adhesion of the thermoplastic strips to each other. Furthermore, locally heating the surface of the draping tool, upstream of the deposition surface where the pre-impregnated thermoplastic reinforcement strip is located (the material having just been deposited and heated by the deposition tool), to a second temperature lower than the first, cools this material while reducing the temperature difference it experiences, thus controlling its cooling.

[0024] In some embodiments, the thermal regulation device is configured to locally regulate the surface temperature of the draping tooling by local heating gradients between 30°CMmin and 100°C / min.

[0025] It is therefore possible to heat or cool the thermoplastic material locally by means of heating ramps of varying speeds, depending on the desired level of crystallinity.

[0026] In some embodiments, the thermoplastic material is a polyetheretherketone, a polyetherketoneketone, or a polyaryletherketone. These so-called "semi-crystalline" thermoplastic resins have the advantage of being particularly high-performing and compatible with the aeronautical engine environment.

[0027] In some embodiments, during the manufacture of the first skin, the tool for depositing successive pre-impregnated reinforcing strips of thermoplastic material, by means of a compaction roller exerting pressure on the thermoplastic material during its draping.

[0028] The application tool applies pressure to the pre-impregnated thermoplastic strips, much like a roll of tape being unrolled using a tape gun or dispenser, via the compaction roller. This pressure from the compaction roller improves the adhesion of the thermoplastic strips to each other, from one layer to the next, that is, from one ply to the next.

[0029] In some embodiments, the deposition tool heats the thermoplastic material during draping via a laser.

[0030] Using a laser facilitates the orientation of the heat source, specifically by directing the laser towards the compaction roller and thus towards the thermoplastic material being laid. This targeted application of heat by the laying tool improves the adhesion between the layers, particularly between the last layer laid and the one being laid. Alternatively, the laying tool can heat the thermoplastic material during laying using a lamp, torch, or any other suitable means.

[0031] This presentation also concerns a manufacturing process for an acoustic panel for an aircraft engine, comprising: the manufacture of a first skin by a process according to any of the preceding embodiments, the manufacture of an acoustic complex comprising a plurality of cells, on the first skin, and the manufacture of a second skin by draping the thermoplastic material over the acoustic complex, by means of the deposition tool.

[0032] It is understood that the first skin is deposited by draping directly onto the draping tooling, more precisely onto the surface (or substrate) of said tooling, while the second skin is deposited onto the acoustic complex, itself formed previously on the first skin, for example by additive manufacturing.

[0033] In some embodiments, the acoustic complex is manufactured using a material compatible with the first and second skin, preferably using the same thermoplastic material as for the first and second skin.

[0034] In some embodiments, the acoustic assembly includes a reinforced thermoplastic material. The thermoplastic material may, for example, comprise a thermoplastic matrix and carbon fiber fillers, thereby enhancing the mechanical strength of the acoustic panel. Brief description of the drawings

[0035] The invention and its advantages will be better understood upon reading the detailed description below of various embodiments of the invention, given by way of non-limiting examples. This description refers to the accompanying figure pages, on which: [ Fig. 1 ] There figure 1 represents a cross-sectional view of a turbojet engine comprising an acoustic panel including a skin according to an embodiment of the invention, in a longitudinal plane of the turbojet engine, [ Fig. 2 ] There figure 2 represents a partial perspective view of an acoustic panel comprising a skin according to an embodiment of the invention, [ Fig. 3 ] There figure 3 schematically represents a dispensing tool moving on a draping tool in a manufacturing process according to an embodiment of the invention, [ Fig. 4 ] There figure 4 schematically represents a view from below of a set of cells of a draping tool used in a process for manufacturing a skin according to an embodiment of the invention, [ Fig. 5 ] There figure 5 schematically represents the different stages of an acoustic panel manufacturing process. Description of the implementation methods

[0036] On the figure 1 A cross-sectional view of a turbojet engine 1 according to an embodiment of the invention is shown, in a longitudinal plane of the turbojet engine 1. The turbojet engine 1 comprises a nacelle 2, an intermediate casing 3, and an inner casing 4. The nacelle 2 and the two casings 3 and 4 are coaxial. The nacelle 2 defines, at one end, an inlet channel 5 for a fluid flow and, at the other end, opposite the first end, an exhaust channel 6 for a fluid flow. The nacelle 2 and the intermediate casing 3 define a primary fluid flow path 7. The intermediate casing 3 and the inner casing 4 define a secondary fluid flow path 8. The primary and secondary fluid flows 7 are arranged along the axial direction of the turbojet engine between the inlet channel 5 and the exhaust channel 6.

[0037] The turbojet 1 further includes a fan 9 configured to deliver an airflow F as a fluidic flow, the airflow F being divided at the fan outlet into a primary flow Fp circulating in the primary duct 7 and a secondary flow Fs circulating in the secondary duct 8. The turbojet 1 further includes at least one acoustic panel 10 configured to attenuate the acoustic waves emitted by the turbojet before these waves escape radially out of the nacelle 2 of the turbojet 1. The acoustic panel 10 is configured to attenuate acoustic waves whose frequency belongs to a predetermined frequency range. In the embodiment illustrated in the figure 1 , panel 10 is integrated into intermediate casing 3 and internal casing 4. Although not shown, other acoustic panels may be integrated into nacelle 2 and internal casing 4 in particular.

[0038] The following description describes an example in which a skin, manufactured by a process according to the invention, is used in the acoustic panel 10. It should be noted, however, that this example is not limiting, the skin being able to be used for other parts of the turbojet 1, in particular the intermediate casing 3, the inner casing 4 or the nacelle 2, without necessarily being part of an acoustic panel.

[0039] On the figure 2 A partial perspective view of an acoustic panel 10 is shown, comprising an outer skin (hereinafter referred to as the first skin) obtained by a process according to an embodiment of the invention. Acoustic panels 10 are known Helmholtz resonators. Typically, an acoustic panel 10 such as the one illustrated in the figure 2 It includes a honeycomb structure layer. The acoustic panel 10 comprises, in particular, a first perforated skin 12, a second solid skin 14, and an acoustic complex 16, which is a honeycomb core sandwiched between these skins. The acoustic complex 16 consists of a network of honeycomb-shaped cells 18. It should be noted that this honeycomb structure is not limiting; other types of structures, manufacturable by additive manufacturing as described later, may be applicable without departing from the scope of the invention. As for the first perforated skin 12, it is fixed to the acoustic complex 16 and is arranged, within the framework of the invention, on the side of the primary vein 7 of the turbojet 1. This skin is perforated by a plurality of orifices 20, Each orifice 20 opening onto a cell 18 of the acoustic complex 16, several orifices 20 being able to open onto the same cell 18.

[0040] A manufacturing process for a skin according to an embodiment conforming to this exposition, used in the manufacture of such an acoustic panel 10, will then be described with reference to figures 3 à 5 .

[0041] Initially, the first skin 12 is manufactured by automated draping (step S100), specifically, but not limited to, the "AFP" (for "automated fiber placement") or "ATL" (for "automated tape layer") techniques, which are well-known in themselves, by successively depositing parallel, multi-layered, pre-impregnated strands or strips, called "plies." The deposited strips comprise a thermoplastic material TP (more simply referred to as "TP material" in the following description), specifically, but not limited to, a polyetheretherketone PEEK, a polyetheretherketone PEKK, a polyaryletherketone PAEK, or a polyphenylsulfone PPSU.

[0042] These strips of TP material are deposited by a dispensing tool 100, onto a draping tool 200, shown schematically on the figure 3 The dispensing tool 100 comprises a dispensing head 110, fed with TP material in strip form by a feeding device (not shown). A cutting module 120 cuts the TP material strip when it reaches the desired length.

[0043] A compaction roller 130 applies the strip of material TP to a surface S of the draping tool 200 described below, exerting pressure on this strip during its draping. More precisely, during draping, the laying tool 100 moves in the direction D represented by an arrow on the figure 3 The direction of movement of the dispensing tool 100, in direction D, defines, according to this explanation, an upstream-downstream direction, corresponding to a right-left direction on the figure 3 Thus, the region to the right of the compaction roller 130 on the figure 3 More precisely, to the right of a draping surface P, which is the contact surface between the compaction roller 130 and the surface S (the draping surface P itself moving when the deposition tool 100 moves in direction D), corresponds to an upstream region in which the material TP has already been deposited, while the region to the left of the draping surface P in this figure corresponds to a downstream region in which the material TP will be deposited. Thus, during its movement, the compaction roller 130 rolls downstream on the surface S, sandwiching the strip of material being deposited between the compaction roller 130 and the surface S.

[0044] Furthermore, a heat source, specifically a laser 140 directed preferably towards the compaction roller 130 and onto the TP material, heats the TP material strip being deposited in the downstream region. The compaction roller 130 and the laser 140 provide the pressure and heat necessary for adhesion of the TP material strips to each other, particularly between the last deposited ply and the one being deposited. Specifically, the laser 140 can heat the TP material to temperatures between 100°C and 500°C to enable adhesion of the strips, and the compaction roller 130 can exert compaction forces between 200 N and 500 N, and up to 1200 N for in-situ configurations.

[0045] The draping tooling 200 comprises a lower portion 210 and an upper portion 220. The upper surface of the upper portion 220 is the surface S onto which the TP material is deposited by draping during the fabrication of the first skin 12. This surface S has a curvature, shown for illustrative purposes on the figure 3 , but generally corresponding to the final shape of the acoustic panel 10 intended to be manufactured. For example, if the acoustic panel to be manufactured has an annular shape, the surface S will have an equivalent shape, in the form of a circular arc.

[0046] The volume formed by the upper portion 220 is divided into a plurality of cells 222 (or chambers, or compartments) independent of each other. To facilitate description, only four cells 222 are shown on the figure 3 Their dimensions, particularly their width, are deliberately exaggerated. In reality, the draping tool 200 comprises in its upper portion 220 a large number of cells 222, of a width less than that illustrated in this figure.

[0047] In this regard, the figure 4 Figure 222 schematically represents an example of the distribution and shape of the cells 222, in a view perpendicular to the surface S (for example, a view from below). These triangular cells 222 are arranged to form a network beneath the surface S. The dashed square represents the TP material of at least a portion of the first skin 12 deposited on the surface S, i.e., just above the cells 222. It is thus possible to heat small areas of the surface S, and therefore small areas of the TP material deposited on the surface S, locally and in a targeted manner, by heating each cell 222 individually, thereby modulating the heat input.

[0048] To achieve this, the draping tooling 200 includes a thermal control device configured to heat each cell 222 individually. The thermal control device includes a compressed air supply unit 230 configured to supply, via a compressed air supply channel 232, a plurality of conduits 234, each communicating individually with one of the cells 222. In addition, each conduit 234 is equipped with a heating element 236, for example a heating resistor, to heat the air flowing in the conduit 234 before its injection into the cell 222.

[0049] The thermal regulation device also includes a control unit 240, configured to control the compressed air supply unit 230, and to individually control each heating element 236. Thus, the control unit is configured to control the conduits 234, and consequently the cells 222 to be supplied with compressed air, while regulating the temperature of the heating elements 236 individually, so as to control the temperature of the pulsed air thus injected into each cell 222.

[0050] Furthermore, although this connection is not illustrated, the control unit 240 can also be connected to the dispensing tool 100, so as to synchronize the heating of the cells 222 with the movement of the dispensing tool 100 during the draping of the first skin 12 in step S100. More specifically, by detecting the position of the dispensing tool 100 at a given moment during its movement, and in particular the position of the draping surface P, the control unit can supply the cells 222 arranged upstream of the draping surface P, so as to target the areas of the surface S where the material TP has just been deposited, and thus control the cooling of the latter.The control unit can also supply the cells 222 arranged downstream of the draping surface P, so as to target the areas of the surface S where the material TP will be deposited, and thus control the temperature rise of the latter, in order to limit the temperature gradient induced by the laser 140.

[0051] It can thus, via the heating elements 236, regulate the temperature of the air supplying said cells 222, and the speed of the heating or cooling ramps, according to the desired final state of the material (level of crystallinity), and also according to the number of plies deposited, and therefore the thickness of the first skin 12 being manufactured. Indeed, when N strips of TP material have been superimposed one on top of the other, strip N+1, deposited on strip N, will be separated from the surface S by said N strips. Given the insulating nature of the TP material, it may be necessary to adjust the local temperature and heating speed of the cells 222 in order to obtain an equivalent level of crystallinity for each ply. For example, the control unit 240 can locally regulate the temperature of the surface S by local heating gradients ranging from 30°C / min to 100°C / min.

[0052] In general, the control unit 240 can be configured to individually control the heating of each cell 222 as a function of the position of the dispensing tool 100, its speed of movement, the heating temperature of the laser 140, and the number of folds deposited on the surface S. For example, when the laser 140 heats the strip of material TP being draped to a temperature T1 downstream of the draping surface P, the control unit 240 can control the heating elements 236 so that the cells 222 located upstream of the draping surface P are heated to a temperature T2 lower than T1, so as to control the temperature gradient ΔT, where ΔT = T1 - T2, taking into account parameters such as the number of folds present at that position and at that time, the trajectory or the speed of movement of the dispensing tool 100.

[0053] In the example described above, the thermal regulation device uses a forced air system to individually heat the cells 222. It should be noted that this example is not limiting, other means such as induction or a heat transfer fluid can be used.

[0054] The S100 step of manufacturing the first skin 12 is completed when all the folds enabling the desired shape and thickness of the skin have been deposited on the surface S of the draping tooling 200, while locally heating this surface S in the manner described above.

[0055] Next, the acoustic assembly 16 is manufactured directly onto the first skin 12 by additive manufacturing (step S200), using a tool designed for this purpose. The acoustic assembly 16 is preferably made of the same thermoplastic material as the first skin 12 and can also be reinforced with carbon fibers.

[0056] The second skin 14 is then deposited on the face of the acoustic assembly 16 opposite the face to which the first skin 12 is attached (step S300), so that the first and second skins are substantially parallel to each other. The second skin 14 is also made of the same TP material as the first skin 12, by automated draping, using the same deposition tool 100. However, unlike the first skin 12, the TP material strips of the second skin 14 are heated only by the laser 140 and pressed by the compaction roller 130, so as to make the strips adhere to each other. It should be noted that the pressure applied to this second skin 14 is compatible with the rigidity of the acoustic complex 16. The thermal regulation device of the draping tool 200 described above is not used, the second skin 14 being deposited on the acoustic complex 16, and not on the surface S of the draping tool 200.It should be noted, however, that the control of the crystallinity of the second skin 14 is less critical than for the first skin 12, the latter being in contact with the primary flux Fp and fulfilling the acoustic function.

[0057] Although the present invention has been described with reference to specific embodiments, it is evident that modifications and changes can be made to these examples without departing from the general scope of the invention as defined by the claims.

Claims

1. A method for the manufacture of at least one skin (12), in particular of an acoustic panel (10) for an aeronautical engine, comprising the laying of a thermoplastic material (TP) on a surface (S) of a lay-up tooling (200), via a depositing tool (100) configured to exert a pressure on the thermoplastic material (TP) and to heat the latter while it is being laid, wherein the lay-up tooling (200) comprises a thermal regulation device configured to locally heat the surface (S) of the lay-up tooling (200), the thermal regulation device comprising a control unit (240) configured to synchronize the local heating of the surface (S) of the lay-up tooling (200) with the movement of the depositing tool (100).

2. The method according to claim 1, wherein the lay-up tooling (200) comprises a plurality of cells (222) disposed under the surface (S) of said lay-up tooling (200) on which the skin (12) is manufactured, the thermal regulation device being configured to heat each cell (222) individually.

3. The method according to claim 1 or 2, wherein the local heating of the surface (S) of the lay-up tooling (200) is carried out by induction, via a heat transfer fluid or via pulsed air.

4. The method according to claim 3, wherein the local heating of the surface (S) of the lay-up tooling (200) is carried out by pulsed air, each cell (222) being supplied with pulsed air via a conduit (234), each conduit (234) being equipped with a heating element (236) individually controlled by the thermal regulation device.

5. The method according to any one of claims 1 to 4, wherein, a deposition surface (P) being defined as the contact surface between the depositing tool (100) and the surface (S) of the lay-up tooling (200), the depositing tool (100) being configured to heat the surface (S) of the lay-up tooling (200) to a first temperature downstream of the deposition surface (P), and the thermal regulation device being configured to locally heat the surface (S) of the lay-up tooling (200) upstream of the deposition surface (P) to a second temperature lower than the first temperature, an upstream-downstream direction being defined relative to the direction of movement (D) of the depositing tool (100) relative to the surface (S) of the lay-up tooling (200).

6. The method according to any one of claims 1 to 5, wherein the thermal regulation device is configured to locally regulate the temperature of the surface (S) of the lay-up tooling (200) by local heating gradients comprised between 30°C / min and 100°C / min.

7. The method according to any one of claims 1 to 6, wherein the thermoplastic material (TP) is a polyetheretherketone, a polyetherketoneketone or a polyaryletherketone.

8. The method according to any one of claims 1 to 7, wherein, during the manufacture of the first skin (12), the depositing tool (100) deposits successive prepreg reinforcement strips of the thermoplastic material (TP), by means of a compacting roller (130) exerting a pressure on the thermoplastic material (TP) while it is being laid.

9. The method according to any one of claims 1 to 8, wherein the depositing tool (100) heats the thermoplastic material (TP) during laying using a laser (140).

10. A method for the manufacture of an acoustic panel (10) for an aeronautical engine, comprising: - manufacturing a first skin (12) by a method according to any one of claims 1 to 9, - manufacturing an acoustic complex (16) comprising a plurality of cells (18), on the first skin (12), and - manufacturing a second skin (14) by laying the thermoplastic material (TP) on the acoustic complex (16), by means of the depositing tool (100).