Abstract:
A textile armature that can be used for making composite materials or parts includes a central layer containing fiber segments of a first type of synthetic material previously submitted, before shaping it into a layer, to a process imparting a permanent crimp; outer layers including a mixture of segments of chemical fibers previously submitted to a process imparting a permanent crimping, and of segments of reinforcing fibers, at least some of the segments of chemical fibers of the outer layers penetrate along a portion of their length into the central layer. First segments of chemical fibers of the outer layers are bonded at least partially between them and to the other fiber segments of the textile armature.

Description:
TECHNICAL FIELD OF THE INVENTION 
       [0001]    The present invention concerns textile armatures used as products for reinforcing composite articles, i.e. articles based on resin (polyester or other resin) reinforced by a reinforcement textile armature. 
         [0002]    Reinforcement textile armatures are known from the document EP 0 395 548 which describes the use of two textile reinforcement layers disposed on opposite sides of a central layer consisting of a mat based on fibers with permanent undulations. The reinforcement textile layers and the central layer are stitched/knitted together. 
         [0003]    The document EP 0 694 643 for its part describes a textile armature used to produce composite articles and consisting of at least two reinforcement textile layers as such disposed on opposite sides of a central layer providing the thickness of said material, said layers being stitched/knitted together, which armature includes against at least one of its external faces a synthetic fiber veil, said veil being attached either by gluing it to the outside of the complex or by stitching the various layers together. 
         [0004]    Stitching/knitting techniques are relatively slow and lead to slow fabrication rates, of the order of 3 to 5 meters per minute, that cannot be speeded up. 
         [0005]    The textile armatures of the above documents also have non-uniform deformation capacities over their surface because of the use of stitching/knitting means to fasten the various textile layers to each other. 
         [0006]    Also, the presence of stitching/knitting rows induces surface appearance defects in the finished part obtained after impregnating the textile armature with resin. 
         [0007]    The document EP 0 659 922 discloses the use of two reinforcement textile layers disposed on opposite sides of a central layer consisting of a mat based on fibers with permanent undulations with, on at least one of the reinforcement textile layers, a fibrous veil based on chemical fibers with permanent crimping and with a linear density lower than that of the fibers of the central layer. The fibrous veil or veils, the reinforcement textile layers and the central layer are needle-punched together. 
         [0008]    The needle-punching technique is still slow, leading to fabrication rates of the same order as the stitching/knitting technique. Also, the connection between the layers of the armature is often insufficient. 
         [0009]    Furthermore, to ensure sufficient cohesion for the product to be handled, forceful needle-punching must be used, which breaks the glass fibers. 
       SUMMARY OF THE INVENTION 
       [0010]    A first problem addressed by the invention is providing a textile armature that is of relatively low cost because it is quick to fabricate and that has good multidirectional deformation capacities when used to produce composite materials or parts. 
         [0011]    At the same time, the present invention seeks to provide a textile armature that can be impregnated with resin easily and homogeneously when using press molding, injection molding or vacuum molding techniques. 
         [0012]    Another aspect of the invention seeks to provide a textile armature of the above kind that can be easily cut without fraying. 
         [0013]    To achieve the above and other objects, the invention proposes a textile armature usable for the production of composite materials or parts, comprising:
       a central layer based on chopped fiber sections of a first type of synthetic material that have received before their formation into a layer a treatment conferring on them a permanent crimp,   external layers disposed on opposite sides of the central layer,       
 
         [0016]    wherein:
       the external layers include chopped chemical fiber sections that have previously received a treatment communicating to them a permanent crimp and chopped reinforcement fiber sections,   at least some of the chopped chemical fiber sections penetrate over a part of their length into the central layer,   the chopped chemical fiber sections include at least first chopped chemical fiber sections including at least one surface layer of a thermoplastic material having a melting point lower than or equal to that of the chopped fiber sections of the central layer,   the first chopped chemical fiber sections of the external layers adhere at least partly to each other and to the other chopped fiber sections of the textile armature.       
 
         [0021]    Such a textile armature has good deformation capacities in a number of directions. More particularly, such an armature has no preferential deformation direction and no direction in which deformation is prevented. The textile armature of the invention has homogeneous cohesion between its various textile layers in all directions. 
         [0022]    Because the chemical fibers adhere to the textile armature central layer fibers that they penetrate, it is possible to ensure efficacious interconnection of the armature layers by means of a relatively small number of such chemical fibers penetrating the central layer. The penetration of a relatively small number of chemical fibers can then be achieved using a preliminary needle-punching or light needle-punching technique, consisting of less dense needle-punching. Preliminary needle-punching is much faster than knitting/stitching and avoids breaking the reinforcement fibers, especially when they are glass fibers. 
         [0023]    The permanent crimp of the chopped fiber sections of the central layer allows easy deformation of the textile armature when used in molding techniques. Moreover, the permanent crimp of the chopped fiber sections of the central layer preserves free spaces between the chopped fiber sections, thus conferring an aerated character on the armature, which encourages flow of resin when using press molding, injection molding or vacuum molding techniques. 
         [0024]    The adhesion to each other of the chopped chemical fiber sections in the external layers efficiently limits the risk of fraying of the textile armature when cut. Avoided in particular is fraying of chopped glass fiber sections that reinforce the textile armature. 
         [0025]    The chemical fibers have a permanent crimp enabling homogeneous external layers to be produced from a homogeneous mixture of chopped reinforcement fiber sections and chopped chemical fiber sections, for example. The crimp of the chemical fibers avoids “settling” of the mixture of fibers because of their different relative densities or sections. In a first embodiment, a homogeneous mixture can thus be obtained leading to the production of homogeneous external layers: in the external layers, the chemical fibers and the reinforcement fibers are then mixed in a generally homogeneous manner. 
         [0026]    The reinforcement fibers can be glass fibers or plant (hemp, sisal, flax, etc.) fibers. 
         [0027]    The chopped fiber sections of the central layer can preferably be of polypropylene, polyester or polyamide. This is because these fibers are very widely used in the textile industry, of relatively low cost, easy to spin and easy to shape to produce chopped fiber sections with an elastic permanent crimp. 
         [0028]    For a more regular central layer, the chopped fiber sections of the central layer can have at least two different unitary linear densities. 
         [0029]    The chopped chemical fiber sections present in the external layers advantageously have a cross section of smaller diameter than the chopped reinforcement fiber sections. This diameter difference encourages entrainment and penetration along at least part of the length of at least some of the chopped chemical fiber sections of the external layers into the central layer by a preliminary needle-punching process. During preliminary needle-punching, the needles chosen to entrain the chopped chemical fiber sections have a small cross section, so that they do not entrain the chopped reinforcement fiber sections much, if at all. This prevents breaking of the fragile chopped reinforcement fiber sections that reinforce the textile armature, specially in the case of reinforcing glass fibers. 
         [0030]    The first chopped chemical fiber sections of the external layers can advantageously be of a thermoplastic material with a melting point lower than that of the chopped fiber sections of the central layer. 
         [0031]    In one advantageous embodiment of the invention, the first chopped chemical fiber sections of the external layers can be of polyethylene. 
         [0032]    Polyethylene is a material very widely used in the textile industry, of relatively low cost and having a low melting point. It provides thermal bonding at lower energy cost. 
         [0033]    In another advantageous embodiment of the invention, it can be provided that:
       the first chopped chemical fiber sections of the external layers are two-component chopped fiber sections having a central core of a first component and an external sheath of a second component,   the melting point of the first component of the central core is higher than that of the second component of the external sheath.       
 
         [0036]    The use of such two-component fibers produces a thermal bond to connect them to the central layer without risk of manifest or accidental damage to the external layers: only the external sheath of the two-component fibers is softened and participates in thermal bonding, their core remaining undamaged and retaining its mechanical properties. 
         [0037]    In some cases, in particular in the case of reinforcement glass fibers, it can be relatively difficult to ensure sufficiently homogeneous mixing of the reinforcement fibers and the chemical fibers in the external layers. In this case, the external layers can advantageously be produced in a stratified form, comprising an external stratum essentially of chopped chemical fiber sections, an internal stratum essentially of chopped chemical fiber sections, and an intermediate stratum essentially of reinforcement fibers. The connection between the strata is provided by preliminary needle-punching followed by surface softening of the first chopped chemical fiber sections, some chemical fibers of the external stratum passing through the intermediate and internal strata of the external layer to penetrate into the central layer of the textile armature. 
         [0038]    Another advantage of this embodiment is that the exterior surface of the textile armature consists essentially of chemical fibers. The thermoplastic material surface layers of these chemical fibers adhere to each other and thus oppose fraying of or damage to the reinforcement fibers upon subsequent handling of the textile armature when not yet embedded in resin. 
         [0039]    The possibility of eliminating the chemical fiber internal stratum will be noted. 
         [0040]    According to a first variant of both embodiments, the chopped chemical fiber sections of the external layers can include only first chopped chemical fiber sections. 
         [0041]    Alternatively, according to a second variant of both embodiments, the chopped chemical fiber sections of the external layers can include a mixture of first chopped chemical fiber sections and second chopped chemical fiber sections. The second chopped chemical fiber sections are of a material having a melting point higher than the melting point of the thermoplastic material of the first chopped chemical fiber sections and a price very much lower than the price of the chemical fibers from which the first chopped chemical fiber sections are fabricated. For example, the chopped second chemical fiber sections are of polyamide or polyester. 
         [0042]    Another aspect of the invention proposes a method of fabricating a textile armature usable for the production of composite materials or parts, including the following successive steps: 
         [0043]    a) providing a central layer based on chopped fiber sections of a first type of synthetic material that have received before their formation into a layer a treatment communicating to them a permanent crimp, 
         [0044]    b) disposing on opposite sides of the central layer an external layer including chopped reinforcement fiber sections and chopped chemical fiber sections with a permanent crimp including at least first chopped chemical fiber sections having at least one surface layer of a second type of thermoplastic synthetic material with a melting point lower than or equal to that of the first type of synthetic material, 
         [0045]    c) effecting preliminary needle-punching to cause the chopped chemical fiber sections to penetrate into the central layer over part of their length, 
         [0046]    d) heating the textile armature to soften at least superficially and to render adherent the chopped chemical fiber sections. 
         [0047]    Such a fabrication process is easy to implement with technical means (machines, tooling, etc.) known and very widely used in the textile industry. The process is therefore of relatively low cost. The use of thermal bonding also allows a higher production rate than using bonding by stitching/knitting or by needle-punching. 
         [0048]    During the step d), the heating can preferably be obtained by circulating hot air through the textile armature. This heating method provides thermal bonding of the fibers to each other as far as the heart of the textile armature and avoids intensive heating of the lower and upper faces of the textile armature on which are disposed the external layers including chopped chemical fiber sections. This therefore avoids excessive melting of the chopped chemical fiber sections of the external layers and the formation of a continuous layer of thermoplastic material after cooling, which continuous layer would compromise good impregnation of the textile armature by the resin on its use in press molding, injection molding or vacuum molding techniques. 
         [0049]    Alternatively, during the step d), the heating can be effected by high-frequency microwave radiation or by infrared radiation of appropriate wavelength to soften the first chopped chemical fiber sections. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0050]    Other objects, features and advantages of the present invention will emerge from the following description of particular embodiments, which is given with reference to the appended figures, in which: 
           [0051]      FIG. 1  is a diagrammatic view in longitudinal section of a textile armature of a first embodiment of the invention during its fabrication; 
           [0052]      FIG. 2  is a diagrammatic view in longitudinal section of the  FIG. 1  textile armature during a preliminary needle-punching operation; 
           [0053]      FIG. 3  is a diagrammatic view in longitudinal section of the  FIG. 2  textile armature during a heating operation; 
           [0054]      FIG. 4  shows in perspective a chopped chemical fiber section of a two-component structure; and 
           [0055]      FIG. 5  is a diagrammatic view in longitudinal section of a textile armature of a second embodiment of the invention. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0056]    In a first embodiment shown in  FIG. 1 , a textile armature  1  of the invention includes three successive textile layers  21 ,  3  and  22 . The central layer  3  is based on chopped fiber sections  3   a  of a first type of synthetic material. The chopped fiber sections  3   a  have been treated to confer a permanent crimp on them before they are formed into a layer. 
         [0057]    To guarantee increased durability of the elasticity of the crimped chopped fiber sections  3   a , the latter chopped fiber sections can advantageously be fabricated from single-strand fibers. 
         [0058]    The chopped fiber sections  3   a  can have the same unitary linear density. It is nevertheless possible for the chopped fiber sections  3   a  to have at least two different unitary linear densities for greater regularity of the central layer  3 . Thus the central layer  3  can include chopped fiber sections  3   a  of 110 dTex and chopped fiber sections  3   a  of 70 dTex, for example. 
         [0059]    On opposite sides of the central layer  3  are disposed external textile layers  21  and  22 . In this embodiment, the external textile layers  21  and  22  are based on a mixture of chopped reinforcement fiber sections  4  and chopped chemical fiber sections  7 . 
         [0060]    The chopped chemical fiber sections  7  have been treated beforehand to confer a permanent crimp on them. 
         [0061]    The external textile layers  21  and  22  are obtained from a homogeneous mixture of chopped chemical fiber sections  7  and chopped reinforcement fiber sections  4  such as glass fibers or plant fibers. Such a homogeneous mixture can be obtained by means of an appropriate cutter and then will be deposited by gravity onto the two faces of the central layer  3 . 
         [0062]    The homogeneity of the mixture is also a result of the permanent crimp of the chopped chemical fiber sections  7 , which imparts a “grab” to the mixture and prevents a phenomenon known as “settling” caused by the different relative densities or sections of the chopped reinforcement fiber sections  4  and the chopped chemical fiber sections  7 . 
         [0063]    Good results have been obtained with external layers  21 ,  22  produced from a homogeneous mixture containing from 90% by weight of chopped glass fiber sections  4  and 10% by weight of chopped chemical fiber sections  7 . 
         [0064]    The chopped chemical fiber sections  7  of the external textile layers  21  and  22  include at least first chopped chemical fiber sections  70  in a thermoplastic material having a melting point less than or equal to that of the chopped fiber sections  3   a  of the central layer  3 . In the state represented in  FIG. 1 , the textile armature  1  has no secure cohesion or connection allowing it to be transported. 
         [0065]    Once provided with its three textile layers  21 ,  3  and  22 , the textile armature  1  undergoes a preliminary needle-punching treatment shown in  FIG. 2 . During this preliminary needle-punching operation, needles  8  cause at least some of the chopped chemical fiber sections  7  (including the first chopped chemical fiber sections  70 ) of each external textile layer  21  and  22  to penetrate over part of their length into the central layer  3 . The direction of movement of the textile armature  1  is indicated by the arrow  12  and the direction of movement of the needles is perpendicular to that direction  12  and to the surface of the textile armature  1 . 
         [0066]    The needles  8  used have barbs  8   a  of appropriate size for preferential entrainment of the chopped chemical fiber sections  7 , and in particular the first chopped chemical fiber sections  70 , and do not entrain the chopped reinforcement fiber sections  4  of the external textile layers  21  and  22 . In practice, the chopped chemical fiber sections  7 , in particular the first chopped chemical fiber sections  70 , have a smaller diameter than the chopped reinforcement fiber sections  4 . For example, chopped chemical fiber sections  7  of approximately 2 to 6 denier and chopped reinforcement fiber sections  4  of approximately 40 Tex minimum are used. 
         [0067]    It is to be understood that the thicknesses and dimensions of the lines representing the chopped fiber sections  3   a ,  4 ,  7  and  70  in  FIGS. 1 to 3  are not representative of the actual thicknesses and dimensions of the chopped fiber sections  3   a ,  4 ,  7  and  70 . The thicknesses and dimensions used in  FIGS. 1 to 3  are merely intended to make it easier for the reader to appreciate the difference between the various fibers  3   a ,  4 ,  7  and  70  and textile layers  21 ,  3  and  22 . 
         [0068]    Preliminary needle-punching differs from needle-punching in that the textile armature  1  is passed between the needles  8  at a higher speed and the density of the needles  8  is lower. By way of illustration, standard needle-punching achieves a fabrication rate of at most approximately 4 meters per minute, whereas preliminary needle-punching achieves a rate of between approximately 8 meters per minute and approximately 20 meters per minute. Still by way of illustrative and indicative example, a needle-punching machine conventionally has a needle density per running meter between approximately 1600 and 32000, whereas a preliminary needle-punching machine has a needle density per running meter between approximately 900 and 1400. 
         [0069]    The low needle density per running meter limits the risk of the chopped reinforcement fiber sections  4  of the textile layers  21  and  22  breaking when the needles  8  cause some of the chopped chemical fiber sections  7  to penetrate the external layers  21  and  22  in the central layer  3 , especially in the case of reinforcement glass fibers  4 . 
         [0070]    The preliminary needle-punching is sufficient to ensure cohesion during transfer of the textile armature blank to the next workstation but insufficient to provide the final cohesion of the textile armature  1 , which is still not transportable to use as a reinforcing product following preliminary needle-punching. 
         [0071]    After the preliminary needle-punching operation shown in  FIG. 2 , the textile armature  1  is heated ( FIG. 3 ). During this heating step, the thermoplastic surface layer of the first chopped chemical fiber sections  70  of the external textile layers  21  and  22  is softened and renders the first chopped chemical fiber sections  70  adherent. The first chopped chemical fiber sections  70  that have been entrained by the preliminary needle-punching needles  8  adhere to the adjacent chopped reinforcement fiber sections  4  of the external textile layers  21  and  22  and adhere to the adjacent chopped fiber sections  3   a  of the central layer  3 . After cooling, the various textile layers  21 ,  3  and  22  of the textile armature  1  are thus interconnected by needle-punched and adhesively bonded fibers of the at least partly thermoplastic first chopped chemical fiber sections  70 . The textile armature  1  can then be transported. The adhesion of the at least partly thermoplastic first chopped chemical fiber sections  70  to the chopped reinforcement fiber sections  4  of the external textile layers  21  and  22  and to the chopped fiber sections  3   a  of the central layer  3  compensates the inability of preliminary needle-punching to ensure sufficient cohesion of the textile armature  1  to render it transportable. 
         [0072]    In one advantageous embodiment of the invention, the chopped fiber sections  3   a  of the central layer  3  can be of polypropylene. Polypropylene is easy to spin and easy to shape to produce chopped fiber sections with permanent elastic crimp. Chopped fiber sections  3   a  in polyester or in polyamide can equally be used. 
         [0073]    In one embodiment of the invention, the chopped chemical fiber sections  7  include first chopped thermoplastic polyethylene fiber sections  70 . First thermoplastic chopped chemical fiber sections  70  of any other material having a melting point lower than that of the chopped fiber sections  3   a  of the central layer  3  can equally be used. 
         [0074]    Using polyethylene is advantageous because of its low melting point. The chopped polypropylene, polyester or polyamide fiber sections  3   a  of the central layer  3  are not damaged much, if at all, by the heating operation and in this case retain all their physical and technical characteristics. On the other hand, the first chopped chemical fiber sections  70  of polyethylene drawn by preliminary needle-punching into the external textile layers  21  and  22  are softened by heating and adhere to the adjacent fibers of the textile layers  21  and  22  and to each other. 
         [0075]    Heating is adjusted to soften and render the thermoplastic first chopped chemical fiber sections  70  adherent, but without melting them. This avoids the formation of uniform exterior layers on the upper and lower faces of the textile armature  1  that are impermeable to the resin. Such uniform and impermeable layers would compromise good impregnation of the textile armature  1  by the resin during a subsequent step of shaping by press molding, injection molding or vacuum molding. 
         [0076]    In another advantageous embodiment of the invention, to avoid all risk of formation of exterior layers of low permeability to the resin, two-component thermoplastic fibers can be used as the first chopped chemical fiber sections  70 , as shown in  FIG. 4 , having a central core  7   a  and an external sheath  7   b , the melting point of the central core  7   a  being higher than that of the external sheath  7   b.    
         [0077]    The two-component first chopped chemical fiber sections  70  can include a polyamide, polyester or polypropylene central core  7   a  and an external sheath  7   b  of copolyester, polyethylene or any other material having a melting point less than that of the chopped fiber sections  3   a  of the central layer  3 . In particular, good results have been obtained using a central core of polyester and an external sheath of copolyester or a central core of polypropylene and an external sheath of polyethylene. Other pairs of materials can be used in the form of coaxial two-component fibers: polypropylene and copolypropylene; polypropylene and ethyl vinyl acetate. 
         [0078]    Because the central core  7   a  has a higher melting point than the external sheath  7   b , the risk of accidental complete melting of the thermoplastic first chopped chemical fiber sections  70  of the external textile layers  21  and  22  during fabrication of the textile armature  1  is avoided. 
         [0079]    This is also an effective way to limit the risk, during the heating step, of the thermoplastic first chopped chemical fiber sections  70  being completely melted by excessive or poorly controlled heating, forming uniform layers impermeable to the resin by spreading of their constituent material over the upper and lower faces of the textile armature  1 . The core of the two-component fibers is not damaged much, if at all: thus the external textile layers  21  and  22  are not damaged. 
         [0080]    Furthermore, using two-component thermoplastic first chopped chemical fiber sections  70  with an external sheath  7   b  and a central core  7   a  reduces the polyolefin content of the textile armature  1 . This proves advantageous, the resin being not particularly compatible with polyolefins. In fact, the resin adheres badly to polyolefin fibers. 
         [0081]    If two-component thermoplastic first chopped chemical fiber sections  70  are used, then the two-component thermoplastic first chopped chemical fiber sections  70  can advantageously have an external sheath  7   b  of copolyester or polyethylene. The chopped polypropylene, polyester or polyamide fiber sections  3   a  of the central layer  3  are thus not affected by heating. In fact, copolyester and polyethylene have melting points lower than those of polypropylene, polyester or polyamide. It is then possible to heat the textile armature  1  to a temperature just sufficient to soften the copolyester or polyethylene of the two-component thermoplastic first chopped chemical fiber sections  70  without softening or otherwise affecting the polypropylene, polyester or polyamide chopped fiber sections  3   a  of the central layer  3 . 
         [0082]    In a second embodiment, shown in  FIG. 5 , a textile armature  1  of the invention again includes three textile layers  21 ,  3  and  22  in succession. The central layer  3  has the same structure as the central layer  3  of the  FIG. 1  embodiment described above. 
         [0083]    In this second embodiment, shown in  FIG. 5 , the difference lies in the structure of the external layers  21  and  22 . In this case, the external layers  21  and  22  are stratified layers each comprising a respective external stratum  21   c  or  22   c , a respective internal stratum  21   a  or  22   a  and a respective intermediate stratum  21   b  or  22   b.    
         [0084]    The external strata  21   c  and  22   c  and the internal strata  21   a  and  22   a  consist essentially of chopped sections of chemical fibers such as the chemical fibers  7  of the  FIG. 1  embodiment (which include at least first chopped chemical fiber sections  70  including at least one thermoplastic material surface layer). 
         [0085]    The intermediate strata  21   b  and  22   b  consist essentially of reinforcement fibers such as the reinforcement fibers  4  of the  FIG. 1  embodiment. 
         [0086]    During preliminary needle-punching, chopped chemical fiber sections  7 , including first chopped chemical fiber sections  70 , are drawn through the strata by the needles until they penetrate into the central layer  3 . The subsequent heating bonds the first chopped chemical fiber sections  70  to the other fibers and connects the layers and strata. 
         [0087]    In a variant of this second embodiment, an internal stratum  21   a ,  22   a  can be omitted. 
         [0088]    The textile armature  1  of the invention can easily be fabricated at low cost by a method including the following steps in succession: 
         [0089]    a) providing a central layer  3  based on chopped fiber sections  3   a  of a first type of synthetic material that have received before their formation into a layer a treatment communicating to them a permanent crimp, 
         [0090]    b) disposing on opposite sides of the central layer  3  a respective external layer  21  and  22  including chopped reinforcement fiber sections  4  and chopped chemical fiber sections  7  with a permanent crimp including at least first chopped chemical fiber sections  70  having at least one surface layer  7   b  of a second type of thermoplastic synthetic material with a melting point lower than or equal to that of the first type of synthetic material, 
         [0091]    c) effecting preliminary needle-punching ( FIG. 2 ) to cause the chopped chemical fiber sections  7 , in particular the first chopped chemical fiber sections  70 , of each external layer  21  and  22  to penetrate into the central layer  3  over part of their length, 
         [0092]    d) heating the textile armature  1  ( FIG. 3 ) to soften at least superficially and to render adherent at least said first chopped chemical fiber sections  70 . 
         [0093]    After the step d) of heating the textile armature  1 , there can advantageously follow a step e) of cold-rolling the textile armature  1  to give it a constant and homogeneous thickness. Rolling also encourages compacting of the fibers and bonding of the fibers to each other. 
         [0094]    The chopped chemical fiber sections  7  of the external layers  21  and  22 , and in particular the first chopped chemical fiber sections  70 , can advantageously have a diameter less than that of the chopped reinforcement fiber sections  4 . The preliminary needle-punching of the step c) is then effected by means of needles  8  ( FIG. 2 ) that have barbs  8   a  adapted to entrain preferentially the chopped chemical fiber sections  7 , including the first chopped chemical fiber sections  70 , but not to entrain significantly the greater diameter chopped reinforcement fiber sections  4 . 
         [0095]    During the heating step d), the textile armature  1  enters a hot-air tunnel oven  9 . The hot-air tunnel oven  9  includes a conveyor belt  11  that moves the textile armature  1  through the oven  9  in the direction defined by the arrow  13 . The conveyor belt  11  is perforated. Hot air jets  14  are directed through the textile armature  1  to heat it throughout its thickness in order to cause the first chopped chemical fiber sections  70  which are at least in part of the thermoplastic type to adhere to the chopped fiber sections  3   a  of the central layer  3  and to the chopped reinforcement fiber sections  4  of the external layers  21  and  22 . 
         [0096]    To produce a textile armature  1  conforming to the embodiment of  FIGS. 1 to 3 , a homogeneous mixture of chopped chemical fiber sections  7  and chopped reinforcement fiber sections  4  is obtained by means of a cutter during the step b) and deposited by gravity onto the two faces of the central layer  3 . 
         [0097]    To produce a textile armature  1  conforming to the embodiment of  FIG. 5 , the chemical fiber strata  21   a ,  22   a ,  21   c  and  22   c  are each produced by carding. The internal strata  21   a  and  22   a , if any, are disposed on either side of the central layer  3 , after which the intermediate strata  21   b  and  22   b  of reinforcement fibers cut by a cutter are deposited by gravity, after which the external strata  21   c  and  22   c  are disposed on either side of the assembly formed in this way. 
         [0098]    In a first variant of all the embodiments described above and shown in  FIGS. 1 to 5 , the chopped chemical fiber sections  7  of the external layers  21  and  22  can consist only of first chopped chemical fiber sections  70 . 
         [0099]    In a second variant of each of these embodiments, the chopped chemical fiber sections  7  of the external layers  21  and  22  can consist of a mixture of first chopped chemical fiber sections  70  and second chopped chemical fiber sections  71 . 
         [0100]    The second chopped chemical fiber sections  71  are chosen to have a melting point higher than that of the first chopped chemical fiber sections  70  and are of a material less costly than the material of the first chopped chemical fiber sections  70 . Second chopped chemical fiber sections  71  of polyester or polyamide could be used, for example. 
         [0101]    Firstly, using a mixture of first chopped chemical fiber sections  70  and second chopped chemical fiber sections  71  significantly reduces the fabrication cost of the textile armature of the invention. For example, a mixture containing approximately 50% to 70% by weight of second chopped chemical fiber sections  71  can be used. 
         [0102]    Furthermore, using a mixture reduces the quantity of thermoplastic material in the external layers  21  and  22  to avoid, after the heating step, forming on the surface of the textile armature  1  an impermeable film of thermoplastic material preventing penetration of the resin, whilst conferring on the mixture, thanks to the second chopped chemical fiber sections  71 , sufficient density to be carded by a conventional carding device. 
         [0103]    It should be noted that the textile armature  1  of the invention, in particular its variant with a mixture of first chopped chemical fiber sections  70  and second chopped chemical fiber sections  71 , has proven to be of remarkable benefit in so-called “pre-forming” techniques. 
         [0104]    In this case, the textile armature  1  is heated during the step d) of the fabrication process that conforms it to a required shape. It is heated just sufficiently for the shape obtained to be able to be transported to a resin injection molding machine with a mold corresponding to the shape obtained and enabling the textile armature to be heated again, and slightly more strongly, before or during injection of the resin. 
         [0105]    In the case of preforming a textile armature  1  with exterior layers  21  and  22  in a mixture of first chopped chemical fiber sections  70  and second chopped chemical fiber sections  71 , it has been found advantageous to use approximately 50% by weight or more of first chopped chemical fiber sections  70 . 
       Example 
       [0106]    I) On a conventional carding device, a central layer  3  is produced of chopped crimped monofilament fiber sections  3   a  of polypropylene with a unitary linear density of 110 dTex. The chopped fiber sections  3   a  have a cut length of approximately 90 mm, a crimp of approximately two undulations per centimeter, and a melting point between approximately 170° C. and 180° C. 
         [0107]    The central layer  3  has a mean thickness between approximately 4 and 5 mm and a weight of approximately 250 grams per square meter. 
         [0108]    For greater regularity of the central layer  3 , chopped crimped monofilament polypropylene fiber sections  3   a  with a unitary linear density of 70 dTex can be mixed in a proportion from 10% to 50% by weight with chopped fiber sections  3   a  having a unitary linear density of 110 dTex. 
         [0109]    II) There is deposited on each face of the central layer  3  an internal stratum  21   a  and  22   a  consisting of chopped chemical fiber sections  7 . 
         [0110]    The chopped chemical fiber sections  7  consist of a mixture comprising 70% (by weight) of second chopped crimped chemical fiber sections  71  of polyester and 30% (by weight) of two-component first chopped chemical fiber sections  70 . The two-component first chopped chemical fiber sections  70  have a central core  7   a  of polyester and a thermoplastic external sheath  7   b  of copolyester. The thermoplastic copolyester external sheath  7   b  has a melting point of approximately 110° C. 
         [0111]    The second chopped crimped chemical fiber sections  71  of polyester have a unitary linear density between approximately 3 denier and approximately 6 denier, preferably chosen to be approximately 3.5 denier. 
         [0112]    The two-component first chopped chemical fiber sections  70  have a unitary linear density between approximately 2 denier and 4 denier. 
         [0113]    III) There is deposited on each internal stratum  21   a  and  22   a  an intermediate stratum  21   b  and  22   b  consisting of chopped reinforcement fiber sections  4 . 
         [0114]    The chopped reinforcement fiber sections  4  are chopped glass fiber sections  4  having a unitary linear density of approximately 50 Tex and a cut length of approximately 50 mm. 
         [0115]    The diameter of the elementary fibers of the chopped glass fiber sections  4  is approximately 14 microns. 
         [0116]    The intermediate strata  21   b  and  22   b  have a density close to 450 g/m 2 . 
         [0117]    IV) There is deposited on each intermediate stratum  21   b  and  22   b  an external stratum  21   c  and  22   c  of identical composition to the internal strata  21   a  and  22   a.    
         [0118]    V) The textile armature  1  is introduced by means of a conveyor belt  11  into a roller-type preliminary needle-puncher. The density of the needles is 4.6 per square centimeter, the separation of the rollers is 20 mm, and the depth of penetration of the needles is 12 mm. The belt moves at a speed of 10 meters per minute. 
         [0119]    VI) After the preliminary needle-punching operation, the textile armature  1  is passed through a hot-air tunnel oven  9  having a heating portion 20 meters long at a speed of 10 meters per minute. The temperature of the hot-air tunnel oven  9  is approximately 120° C. 
         [0120]    VII) On exit from the hot-air tunnel oven  9 , cold-rolling gives the textile armature  1  its final thickness, which is close to approximately 4 to 5 mm. 
         [0121]    The present invention is not limited to the embodiments explicitly described and includes variants and generalizations thereof within the scope of the following claims.