METHOD FOR PRODUCING A THIN-WALLED ACOUSTIC COMPONENT

Description

Technical Field

[0001] The present invention relates to the general field of acoustic attenuation structures or panels. It relates more particularly to acoustic attenuation structures used to reduce noise produced in aircraft engines such as in gas turbines or their exhausts. Prior art

[0002] Acoustic attenuation structures typically consist of an acoustic surface plate or skin permeable to the acoustic waves that are to be attenuated and a reflective solid plate or skin called a "closing plate", a cellular body, such as a honeycomb or a porous structure, being arranged between these two walls. In a well-known manner, such panels form Helmholtz-type resonators that make it possible to attenuate acoustic waves in a certain frequency range. Acoustic attenuation structures of this type are described in particular in documents US 5,912,442 and GB 2,314,526.

[0003] These acoustic attenuation structures are limited to simple cell shapes such as those of the cells of a classic NIDA ® type structure. With this technology, the processed frequency is of the order of a quarter of the inverse of the cavity height. To process low frequencies, very high cavities are therefore required which, in the case of propulsion systems, considerably increase their drag. In addition, the acoustic performances obtained are limited to the absorption of a very narrow frequency range.

[0004] One solution to increase the acoustic attenuation frequency range is to superimpose two cell bodies with different cell shapes and dimensions. This solution has the disadvantage of significantly increasing the size and drag of the acoustic attenuation structure.

[0005] Another known solution consists of placing open cone truncated sections in cells as described in documents EP 0 738 865 and FR 3 082 987. Although this solution makes it possible to reduce the size of the acoustic attenuation structure, the mass of the structure remains significant and, consequently, penalizing for uses in aircraft where control of the overall mass is always sought.

[0006] Document DE 11 2015 000089 T5 describes a method for manufacturing a shock absorber for a vehicle comprising a plurality of hollow elements each having a shape gradually tapering between a base and a top, the hollow complex elements being connected to each other by one or more adjacent edges, the method comprising:- heating a film of thermoplastic material,- shaping the heated film of thermoplastic material in a tool comprising a mold having a plurality of molding cavities having a shape corresponding to the shape of the hollow complex acoustic elements of the acoustic component to be manufactured, the heated film of thermoplastic material being shaped locally in each of the molding cavities,- cooling the mold to a third temperature below the glass transition or melting temperature of the thermoplastic material,- the demolding of a complex multi-element panel comprising a plurality of hollow complex acoustic elements., Statement of the invention

[0007] The main aim of the present invention is therefore to propose a solution for acoustic attenuation structures which do not have the aforementioned drawbacks. According to the invention, this aim is achieved by means of a method for manufacturing an acoustic component comprising a plurality of hollow complex acoustic elements each having a shape which gradually narrows between a base and a top, the hollow complex acoustic elements being connected to each other by one or more adjacent edges, the method comprising: heating a film of thermoplastic material to a first temperature above the glass transition or melting temperature of the thermoplastic material of said film, shaping the heated film of thermoplastic material in a tool comprising a mold having a plurality of molding cavities having a shape corresponding to the shape of the hollow complex acoustic elements of the acoustic component to be manufactured, the heated film of thermoplastic material being shaped locally in each of the molding cavities, at least the mold being maintained at a second temperature above the glass transition or melting temperature of the thermoplastic material and below the first temperature, cooling the mold to a third temperature below the glass transition or melting temperature of the thermoplastic material,demolding a complex acoustic multi-element panel comprising a plurality of hollow complex acoustic elements.

[0008] The method of the invention makes it possible to economically, reliably and repeatably obtain acoustic components equipped with complex shaped acoustic elements with a very low wall thickness, typically less than 1 mm. The acoustic component thus obtained allows the manufacture of thinner and therefore lighter acoustic attenuation structures while respecting the required specifications in terms of size, mechanical performance and mass.

[0009] Furthermore, during the shaping step, the mold is maintained at a second temperature higher than the glass transition or melting temperature of the thermoplastic material and lower than the first temperature. As explained in detail below, this makes it possible to form complex hollow acoustic elements with very thin walls.

[0010] According to a first particular aspect of the method of the invention, the film of thermoplastic material has a constant thickness.

[0011] According to a second particular aspect of the method of the invention, the film of thermoplastic material has a plurality of portions of excess thickness present at determined locations on the film, said locations corresponding to the locations of the molding cavities of the mold. The volume of material present in each portion of excess thickness is defined as a function of the wall thickness of each complex acoustic element to be manufactured.

[0012] According to a third particular aspect of the method of the invention, the molding cavities have a pyramidal, conical, spiral, funnel or hopper shape.

[0013] According to a fourth particular aspect of the method of the invention, the latter further comprises a step of manufacturing a film of thermoplastic material from granules of thermoplastic material.

[0014] According to a fifth particular aspect of the method of the invention, the film made of thermoplastic material is obtained by stacking and / or welding several unitary films joined together.

[0015] According to a sixth particular aspect of the method of the invention, the complex acoustic elements (121) of the acoustic component (120) have a wall thickness of less than 1 mm.

[0016] The invention also relates to a method for manufacturing an acoustic attenuation structure comprising the following steps: production of an acoustic component according to the method of manufacturing an acoustic component of the invention, production of a complex acoustic multi-element panel comprising the acoustic component and a plurality of partitions forming acoustic cavities, each complex acoustic element of the acoustic component being housed in an acoustic cavity so as to form an acoustic cell, assembly of a face of the complex acoustic multi-element panel with an assembly face of an acoustic skin.

[0017] The hollow complex acoustic elements of the acoustic component can be advantageously combined with partitions, which makes it possible to lower the acoustic frequencies to be treated. It is thus possible to produce acoustic attenuation structures capable of attenuating lower frequency sound waves while having a reduced footprint.

[0018] According to a particular aspect of the method for manufacturing an acoustic attenuation structure, the acoustic attenuation structure further comprises a closing skin covering the horizontal face of the complex acoustic multi-element panel opposite the horizontal face covered by the acoustic skin, the method comprising assembling the part joining the complex acoustic multi-element panel and the plurality of partitions with the closing skin. Brief description of the drawings

[0019] [ Fig. 1 ] There figure 1 is a schematic sectional view of a thermoforming tool showing a step in the manufacture of an acoustic component according to one embodiment of the invention, [ Fig. 2 ] There figure 2 is a schematic sectional view of a thermoforming tool showing another step in the manufacture of an acoustic component according to an embodiment of the invention, [ Fig. 3 ] There figure 3 is a schematic sectional view of a thermoforming tool showing another step in the manufacture of an acoustic component according to an embodiment of the invention, [ Fig. 4 ] There figure 4 is a schematic sectional view of a thermoforming tool showing another step in the manufacture of an acoustic component according to an embodiment of the invention, [ Fig. 5 ] There figure 5 is a schematic perspective view of a thermoplastic material film used to manufacture an acoustic component according to one embodiment of the invention, [ Fig. 6 ] There figure 6 is a schematic perspective view of a thermoplastic material film used to manufacture an acoustic component according to another embodiment of the invention, [ Fig. 7 ] There figure 7 is an enlarged detail view of a portion of the thermoplastic material film of the figure 6 , [ Fig. 8 ] There figure 8 is an exploded schematic perspective view of an acoustic attenuation structure according to one embodiment of the invention, [ Fig. 9 ] There figure 9 is a schematic sectional view of the acoustic attenuation structure of the figure 1 once assembled. Description of the embodiments

[0020] THE figures 1 à 4 illustrate a method of manufacturing an acoustic component in accordance with one embodiment of the invention.

[0021] There figure 1 illustrates a thermoforming tool 200 comprising a mold 210, a counter-mold 220 movable in a double direction D, heating panels 230 and a frame 240 supporting the mold 210. The frame 240 is provided with side clamps 241 and 242 to hold a film of thermoplastic material 10 during the steps of forming an acoustic component. The film of thermoplastic material can also be deposited on the tool by an automaton or manually. It can also be supplied by a conveyor system.

[0022] The mold 210 comprises molding cavities 211 intended to cooperate with teeth 221 for the formation of the acoustic component from the thermoplastic material film 10. The molding cavities 211 of the mold 210 have a shape corresponding to the shape of the hollow complex acoustic elements of the acoustic component to be manufactured.

[0023] The method begins with the heating panels 230 heating the thermoplastic material film 10 to a first temperature above the glass transition or melting temperature of the thermoplastic material of the film as illustrated in figure 1 . More specifically, in the case of an amorphous thermoplastic material, the film is heated to a temperature above the glass transition temperature of the material while, in the case of a semi-crystalline thermoplastic material, the film is heated to a temperature above the melting temperature of the material.

[0024] According to an alternative embodiment, the film of thermoplastic material may be heated to the first temperature above the glass transition or melting temperature of the thermoplastic material of the film before its placement above the mold, for example by heating the side of the film in an infrared oven.

[0025] The film is heated to the first temperature preferably after its placement above the mold because this allows better positioning of the film in the thermoforming tool.

[0026] Once this first temperature has been reached by the film, the film is shaped into thermoplastic material 10. To this end, the counter-mold 220 is moved towards the mold 210 as illustrated in the figure 2 and until the teeth 221 of the counter-mold 220 cooperate completely with the molding cavities 211 as shown in the figure 3 The heated thermoplastic material film is shaped locally in each of the molding cavities 211. During this shaping step, the mold 210 and, possibly the counter-mold 220, are preferably maintained at a second temperature higher than the glass transition or melting temperature of the thermoplastic material and lower than the first temperature.

[0027] This makes it possible to form complex hollow acoustic elements with very thin walls. By keeping the thermoforming tool at a temperature above the glass transition or melting temperature of the thermoplastic material, material flows are facilitated during the forming by stamping of the film. This avoids tearing the thermoplastic film, which initially has a relatively low thickness, for example of the order of 1 mm to 2 mm.

[0028] Once the plastic material film 10 is completely deformed in the cavities 210 as illustrated in the figure 3 , the mold 210 and, possibly the counter-mold 220, are cooled to a third temperature lower than the glass transition or melting temperature of the thermoplastic material making it possible to set said material in the shape of an acoustic component 120 to be manufactured. As soon as the temperature of the thermoforming tool is lower than at least 10°C of the glass transition or melting temperature of the thermoplastic material, the formed acoustic component can be demolded.

[0029] The acoustic component 120 is then demolded by moving the counter-mold 220 in a direction opposite to that of the mold 210 and by opening the side clamps in order to release the acoustic component.

[0030] We then obtain an acoustic component 120 as illustrated in figures 8 And 9comprising a plurality of hollow complex acoustic elements 121 each having a shape gradually narrowing between a base 122 and a top 123 and a wall thickness E 121 less than 1 mm ( figure 9 ). The shape of the molding cavities of the mold defines the shape of the hollow complex acoustic elements of the acoustic component. In the example described here, the molding cavities 211 have a pyramidal shape allowing hollow complex acoustic elements 121 of the same shape to be formed. The molding cavities and, therefore, the resulting hollow complex acoustic elements may have other shapes such as a conical, spiral, funnel or hopper shape.

[0031] In the example described above, the acoustic component is shaped by thermoforming between a mold and a counter-mold. This thermoforming method corresponds to the stamping or drawing technique using a male and female mold, also called "positive and negative."

[0032] However, the shaping of the acoustic component from a heated thermoplastic material film can be achieved by other methods such as: vacuum thermoforming, a method in which a vacuum, created between the material and the mold, forces the material to take on the shape of the mold under the effect of atmospheric pressure (approximately 1 bar); pressure thermoforming or blow molding, a method in which shaping is ensured by pressure forcing the air between the material and the mold.

[0033] Generally speaking, any process that allows a film of thermoplastic material to be heated and pressed onto a mold in the shape of the acoustic component to be manufactured can be used.

[0034] Furthermore, in accordance with the invention and regardless of the tooling used, it must include means for regulating at least the temperature of the mold in order to maintain it at the second temperature described above during the shaping of the thermoplastic film. For this purpose, the tooling may include thermal regulation means using, for example, induction or pulsed air, the thermal regulation being able to be controlled by zone in the tooling. The tooling and in particular the mold is preferably made with a metal having a high heat transfer capacity such as, for example, aluminum.

[0035] The thermoplastic film material used to manufacture the acoustic component may be selected, but not limited to, the following materials: polyaryletherketones (PAEK) such as polyetheretherketone (PEEK) and polyetherketoneketone (PEKK), polyetherimides (PEI), polycarbonate (PC), polyphenylene sulfide (PPS), polysulfones (PSU). The thermoplastic material may be filled or unfilled.

[0036] The acoustic component manufactured according to the method of the invention may have a planar shape like the acoustic component 120 illustrated in figures 8 And 9or a curved shape having a single or multiple curvature. In the latter case, the curvature(s) may be produced directly during the shaping of the film with a mold having a suitable geometry or after the shaping step by conforming or hot forming. Concerning the conforming or hot forming of the acoustic component after its shaping, this operation may be carried out by placing the flat component on a tool having the curvature(s), the tool comprising heating means for heating the component to a temperature above the glass transition temperature of the thermoplastic material and means for applying a uniform force to the component.

[0037] The thermoplastic material film used to manufacture the acoustic component may have a constant thickness such as the film 10 illustrated in figure 5 The film can be made in a single piece or obtained by welding several unitary films joined together like unitary films 11 and 12 of the figure 5 which, once welded, forms a film of thermoplastic material 10 of constant thickness E 10 . The film of thermoplastic material can be manufactured from granules of thermoplastic material by injection or any other method suitable for forming a film.

[0038] The thermoplastic material film used to manufacture the acoustic component may also have a variable thickness such as the film 20 illustrated in figures 6 And 7 . More precisely, the thermoplastic material film 20 has a plurality of extra thickness portions 22 having a thickness E 22 greater than the thickness E 21 of the connecting portions 21 of the film 20 present between the extra thickness portions 22 ( figure 7 ). The excess thickness portions 22 are present at determined locations on the film, these locations corresponding to the locations of the molding cavities of the mold. The excess thickness portions 22 constitute a surplus of material which, once stretched or flowed into the molding cavities during the shaping of the acoustic component, will make it possible to form hollow complex acoustic elements with a wall thickness close to that of the connecting portions 21. The volume of material present in each excess thickness portion is defined as a function of the wall thickness of each complex acoustic element to be manufactured. The excess thickness portions may have a circular, annular or hexagonal shape and be distributed uniformly on the film. In the example described here, each excess thickness portion 22 extends between connecting portions 21 and an opening or cavity 222 present in the center of the excess thickness portion.Each opening or cavity 222 is intended to be aligned with the center of the molding cavities of the mold. According to a particular characteristic, the central opening or cavity 222 forms a hole. This makes it possible to obtain the outlet orifice of each complex acoustic element immediately after shaping. This avoids an additional machining operation. The diameter of the opening or cavity 222 is preferably greater than the diameter of the end of the teeth 221 of the counter-mold 220. By way of non-limiting example, in the case where the diameter of the end of the teeth of the counter-mold is 5 mm, the hole formed by the central opening has a diameter of between 6 mm and 7 mm.

[0039] Each portion of excess thickness is intended to form by stretching or creep a hollow complex acoustic element 121 having a shape that gradually narrows between its base 122 and its top 123. We therefore move from an initial flat surface Spl corresponding to the portion of excess thickness that extends between connecting portions 21 and the central opening or cavity 222, to a larger projected surface Spr corresponding to the surface of the hollow complex acoustic element 121. The ratio between the initial flat surface Spl and the projected surface Spr defines a slenderness factor Fe that can be between 2 and 5. The principle is that of mass conservation. Thus, the initial flat surface Spl and the initial thickness E 22 of each portion of excess thickness 22 is defined as a function of the final projected surface Spr and thickness E 121 of each hollow complex acoustic element 121 according to the following formula: Spl × E 22 = Spr × E 121

[0040] In the case where it is desired to produce an acoustic component of constant thickness Ec, the thickness E 22 of the excess thickness 22 can be determined according to the following formula: E 22 = Spr / Spl × Ec = Fe × Ec

[0041] In this case, the thickness E 21 of the connecting portions 21 which are not intended to be stretched or crept corresponds substantially to the final thickness of the acoustic component 120.

[0042] By way of non-limiting example, the thickness E 21 of the connecting portions 21 may for example be 0.5 mm while the thickness E 22 of the extra thickness portions 22 may be 1.6 mm.

[0043] Such a film of thermoplastic material with variable thickness can be manufactured by calendering, machining, stacking of unit wires, injection molding, etc.

[0044] We now describe in relation to the figures 8 And 9a method of manufacturing an acoustic attenuation structure in accordance with one embodiment of the invention. The acoustic attenuation structure 100 here comprises an acoustic skin or plate 110, an acoustic component 120 manufactured as described above, a plurality of partitions 131 and a closing skin or plate 140.

[0045] The closing skin 140 corresponds to a solid surface intended to reflect the sound waves entering the acoustic attenuation structure. The closing skin may be a constituent element of the acoustic attenuation structure as in the example described here or correspond to a structure of an object, for example an aircraft engine. In the latter case, the acoustic attenuation structure of the invention does not comprise a closing skin and is directly mounted on the structure of the object.

[0046] The acoustic skin 110 has the function of allowing the sound waves to be attenuated to pass through inside the acoustic attenuation structure 100. For this purpose and in the example described here, the acoustic skin 110 comprises a plurality of perforations 111.

[0047] The acoustic component 120 is formed in a single piece and extends in length and width in a horizontal direction DH and in height in a vertical direction DV. The acoustic component comprises a plurality of hollow complex acoustic elements 121 each having a shape gradually narrowing between a base 122 and a top 123.

[0048] In the example described here, the complex acoustic elements 121 have a pyramidal shape. The base 122 of each complex acoustic element 121 is in continuous contact with the base of the adjacent complex acoustic elements so as to form a continuous network of edges 124.

[0049] In the embodiment described here, the plurality of partitions 131 is made in a single piece, namely a network of ribs 130 which, once assembled with the complex acoustic multi-element panel 120, forms the partitions around the complex acoustic elements 121.

[0050] Still in the example described here, the acoustic attenuation structure 100 is produced by assembling the complex acoustic multi-element panel 120 with the plurality of partitions 131 in a single piece, the upper edge 131a of the partitions 131 being fixed, for example by gluing or welding, on the lower portion 122b of the bases 122 of the complex acoustic elements 121 ( figure 9 ). The plurality of complex acoustic elements being formed in a single piece within the complex acoustic multi-element panel and the plurality of partitions also being formed in a single piece, the assembly between these two elements is greatly facilitated by the self-positioning of the complex acoustic elements with the partitions.

[0051] The closing skin 140 is fixed, for example by gluing or welding, on the lower edge 131b of the partitions 131 while the acoustic skin 110 is fixed, by gluing or welding, on the upper portion 122a of the bases 122 of the complex acoustic elements 121 corresponding to the exposed surface of the edges 124. Thus, the acoustic skin and the closing skin are each fixed on a perfectly flat support in the horizontal direction DH, which makes it possible to ensure very good sealing between the skins and the assembly of the complex acoustic multi-element panel with the plurality of partitions.

[0052] Once assembled, the attenuation structure 100 comprises a plurality of acoustic cells 150 each formed by a complex acoustic element 121 and the partitions 131 which surround it ( figure 9 ). The height H 121 of the complex acoustic elements 121 is less than the height H 150 of the acoustic cells 150. More precisely, the height H 121 of the acoustic cells is between 10% and 99% of the height H 150 of the acoustic cells in the vertical direction. The height H 121 can be between 5mm and 100mm while the base of each element 121 can be inscribed in a circle with a diameter between 5mm and 50mm. In addition, thanks to the manufacturing method of the invention, the hollow complex acoustic elements 121 have a very low thickness E 121, less than 1mm and typically between 0.2mm and 0.5mm.

[0053] The acoustic skin, the plurality of partitions and the closing skin can be produced by injection of a thermoplastic or thermosetting material, whether filled or not, by injection-compression of a thermoplastic or thermosetting material, whether filled or not, or by injection with control of the temperature of the tooling of a thermoplastic or thermosetting material, whether filled or not.

[0054] The plurality of partitions, the acoustic and closing skins as well as the assemblies joining in a single piece the plurality of partitions and the complex acoustic multi-element panel or the plurality of partitions and one of the skins can also be produced by injection of a thermoplastic or thermosetting material, whether filled or not.

Claims

1. A method for making an acoustic component (120) comprising a plurality of hollow complex acoustic elements (121) each having a shape tapering gradually between a base (122) and an apex (123), the hollow complex acoustic elements being connected to one another by one or more adjacent edges, the method comprising: - heating a film of thermoplastic material to a first temperature above the glass transition temperature or melting temperature of said film, - shaping the heated film of thermoplastic material in a tool comprising a mould which has a plurality of mould cavities having a shape corresponding to the shape of the hollow complex acoustic elements (121) of the acoustic component (120) to be made, the heated film of thermoplastic material being shaped locally in each of the mould cavities, at least the mould being kept at a second temperature above the glass transition temperature or melting temperature of the thermoplastic material and below the first temperature, - cooling the mould to a third temperature below the glass transition temperature or melting temperature of the thermoplastic material, - removing a complex acoustic multi-element panel (120) comprising a plurality of hollow complex acoustic elements (121) from the mould.

2. The method according to claim 1, wherein the film of thermoplastic material has a constant thickness.

3. The method according to claim 1, wherein the film of thermoplastic material has a plurality of portions of excess thickness at determined locations on the film, said locations corresponding to the locations of the mould cavities of the mould.

4. The method according to claim 3, wherein the volume of material present in each excess thickness portion is defined as a function of the wall thickness of each complex acoustic element to be made.

5. The method according to any one of claims 1 to 4, wherein the mould cavities have a pyramid, conical, spiral, funnel or hopper shape.

6. The method according to any one of claims 1 to 5, further comprising a step of making a film of thermoplastic material from granules of thermoplastic material.

7. The method according to any one of claims 1 to 6, wherein the film of thermoplastic material is obtained by welding several unit films joined together end-to-end.

8. The method according to any one of claims 1 to 7, wherein the complex acoustic elements (121) of the acoustic component (120) have a wall thickness less than 1 mm.

9. A method for making an acoustic attenuation structure (100) comprising the following steps: - producing an acoustic component (120) according to the method as defined in any one of claims 1 to 8, - producing a complex acoustic multi-element panel comprising the acoustic component and a plurality of partitions (131) forming acoustic cavities (132), each complex acoustic element of the acoustic component being housed in an acoustic cavity so as to form an acoustic cell (150), - assembling a face (122a) of the complex acoustic multi-element panel (120) with an assembly face (112) of an acoustic skin (110).

10. The method according to claim 9, wherein the acoustic attenuation structure (100) further comprises a closure skin (140) covering the horizontal face of the complex acoustic multi-element panel (120) opposite the horizontal face covered by the acoustic skin (110), the method comprising the assembly of the part joining together the complex acoustic multi-element panel (120) and the plurality of partitions (131) with the closure skin (140).