Abstract:
This process for producing a thermoplastic composite part by laying of at least one thermoplastic composite layer on a tool includes laying on the tool a hardened first ply before the laying of the thermoplastic composite layer. The first ply is produced beforehand to the shape of the tool and includes a first material able to experience an attractive force when it is subjected to a magnetic field, and/or to an electrical potential difference, and includes a second material of a thermoplastic resin that is chemically compatible with the resin of the thermoplastic composite layer. The first ply is clamped to the tool by the effect of a magnetic field, and/or of an electrical potential difference, established at the level of the tool. In one form of implementation, the first ply is produced on a first-ply mold that is preferably of convex shape.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is the National Stage of International Application No. PCT/EP2014/077626, having an International Filing Date of 12 Dec. 2014, which designated the United States of America, and which International Application was published under PCT Article 21(2) as WO Publication No. 2015/086834 A1, and which claims priority from, and the benefit of, French Application No. 1362566, filed on 13 Dec. 2013, the disclosures of which are incorporated herein by reference in their entireties. 
     
    
     BACKGROUND 
       [0002]    1. Field 
         [0003]    The presently disclosed embodiment belongs to the field of composites including a succession of superposed plies forming the thickness of a composite part. 
         [0004]    More particularly, the presently disclosed embodiment relates to a process for producing a composite part formed by a stack of plies laid on a matrix or a mold giving a shape to the part. 
         [0005]    More particularly, the presently disclosed embodiment provides a solution to the laying of the first ply deposited on the mold. 
         [0006]    2. Brief Description of Related Developments 
         [0007]    When a composite part including resin-impregnated fibers is produced on a mold, it must be taken into consideration that the part will have to be demolded, i.e. separated from the mold, when all the desired plies have been laid and consolidated by known processes. 
         [0008]    In the case of thermoplastic composites, i.e. composites the resin impregnating the fibers of which is hard at the temperature of use and softens when temperature is increased to a sufficient thermoplastic forming value, each laid ply adheres to the preceding ply on which it is deposited. To maintain the first ply on the mold, in the absence of a tack of the resin that will allow the ply to adhere to the surface of the mold, it is known to clamp the first ply deposited on a mold. Such a solution is effective when the shape of the mold is convex but does not make it possible to guarantee, in the case of a mold of concave shape, that the first ply will perfectly follow the shape of the mold. 
         [0009]    As a result, thermoplastic composite parts of complex shapes produced on a mold including, even if only locally, concave shapes are tricky to produce and there are no solutions that are industrially satisfactory under such conditions. 
       SUMMARY 
       [0010]    The presently disclosed embodiment precisely proposes a process that makes it possible to overcome the constraints of known solutions for producing a thermoplastic composite part by laying layers of material on a tool giving the part its shape. 
         [0011]    In this process for producing a thermoplastic composite part by laying of at least one thermoplastic composite layer on a tool, a step is implemented of laying on the tool a first ply, laid on the tool before the laying of the thermoplastic composite layer. This first ply is a hardened ply produced beforehand to match the shape of the tool and including a first material able to experience an attractive force when it is subjected to a magnetic field, and/or to an electrical potential difference, and including a second material of a thermoplastic resin that is chemically compatible with the resin of the thermoplastic composite layer in order to adhere with said resin, this first ply being clamped to the tool in the step of laying this first ply on the tool by the effect of a magnetic field established at the level of the tool, and/or of an electrical potential difference between the tool and the first ply. 
         [0012]    Thus the clamping of the first ply to the tool is achieved without chemical bonding and the ply is therefore perfectly demoldable and adheres perfectly to the tool at every point of its area because of the homogeneity of the magnetic attraction and/or of the force resulting from the potential difference produced on the first ply, and it is thereby possible to lay composite layers on the free surface of the first ply with a perfect adherence without the first ply moving or deforming on the tool. 
         [0013]    According to one method of implementation, the second material is deposited on said first material in the form of a thermoplastic-resin powder, then the first material and the second material are subjected, clamped on a first-ply mold, in a step of manufacturing the first ply, to a step in which temperature is increased above a melting point and glass transition temperature of the thermoplastic resin, the shape of the first-ply mold being a counterpart of the tool. 
         [0014]    Thus, a relatively thin hardened first ply that preserves the shape of the first-ply mold and therefore the shape of the tool, and that may then be placed on the tool in order to be clamped thereto and receive the thermoplastic composite layers, is produced. 
         [0015]    In one mode of implementation of the process, the first material is deposited on the first-ply mold in a first step and, in a second step, the second material is deposited by spraying thermoplastic-resin powder onto the first material deposited on the first-ply mold. 
         [0016]    In this method for producing the first ply, the thermoplastic resin may be uniformly distributed over the first material and with a controllable area density to obtain the desirable amount of thermoplastic resin, in order to give, to the first ply, after the temperature increase, the desired solidity and thermoplastic-layer binding qualities. 
         [0017]    The first material mainly consists of woven or nonwoven wires and/or of mats, and/or of powders, that are deposited to form one or more superposed layers. 
         [0018]    It is thus possible to form the first material on the first-ply mold by laying webs of wires or wovens or interlaced fibers the flexibility of which allows the curvatures of the first-ply mold to be followed, while obtaining a desired resultant solidity and thickness. The deposition of a material made of powder allows a uniform layer to be deposited on the first-ply mold. In this case the powder of the first material may be sprayed in mixture with the thermoplastic-resin powder of the second material, or even coated with the resin of the second material. These various methods may be combined, in particular to take into account local first-ply characteristics dictated by the shape of the part to be produced. 
         [0019]    In one aspect, the first material is deposited on the first-ply mold in widths juxtaposed with a minimum overlap to avoid a gap between the neighboring edges of two juxtaposed widths. 
         [0020]    It is thus possible to lay the first material in strips of dimensions smaller than a width of the first-ply mold, this on the one hand proving to be easier when the mold is of large width, and on the other hand allowing the first material to be better matched to non-developable mold shapes. 
         [0021]    When the second material is deposited on the first material, advantageously a thermoplastic-resin powder electrostatic-spraying process is implemented. 
         [0022]    The first material is advantageously formed from a ferromagnetic material, for example a metal such as a steel, a ferritic stainless steel or a nickel-containing alloy, chosen for its electrical conduction and/or magnetic properties. 
         [0023]    In one method of implementation, the first ply is produced in at least two portions that are juxtaposed on the tool when the first ply is laid on the tool. It is thus possible to produce, for parts of large size, a first ply the portions of which remain of sizes compatible with the production and manipulation of a thin element. 
         [0024]    Advantageously, when the first ply is produced in a plurality of portions, the constituent portions of the first ply are defined so as each to be produced on partial first-ply molds of mainly convex shape. 
         [0025]    It is thus possible to produce each portion under deposition conditions such that the first material is deposited on a convex mold, this in practice proving to be less complex and less constraining in terms of clamping, if a mechanical clamp is used, but also in terms of the tensioning required to prevent defects in adherence to the mold. 
         [0026]    The first material is for example clamped to the first-ply mold, in the step of manufacturing said first ply, by the effect of a magnetic field established at the level of said first-ply mold, and/or of an electrical potential difference relative to the first-ply mold. 
         [0027]    An effective clamp is thus obtained by exploiting the electrical conduction and/or magnetic properties of the first material, these properties moreover being required to maintain the first ply on the tool. 
         [0028]    To prevent the first ply from moving on the tool during the deposition of the thermoplastic composite layers of the part, the first ply is maintained clamped on the tool by the effect of a magnetic field established at the level of said tool, and/or of an electrical potential difference relative to the tool, during the laying of the thermoplastic composite layers superposed on the first ply. 
         [0029]    When the thermoplastic composite layers required to form the part have been deposited, the magnetic field, and/or the potential difference, established at the level of the tool is cancelled, and if needs be reversed, in the step of demolding the part produced on the tool. 
         [0030]    The forces ensuring the clamping are thus cancelled, a reversal of the magnetic field and/or of the clamping potential difference cancelling if needs be a cancellation of the residual forces resulting from a hysteresis effect, so that the part may be demolded without adherence to the tool making this demolding difficult and without the risk of damage that adherence of the resin to the tool could induce. 
       DESCRIPTION OF THE DRAWINGS 
       [0031]    The description and the drawings of an exemplary implementation of the presently disclosed embodiment will allow the aims and advantages of the presently disclosed embodiment to be better understood. It is clear that this description is given by way of example, and is non-limiting in character. 
     
    
     
         [0032]    In the drawings: 
           [0033]      FIG. 1  illustrates the main steps of the process for one mode of implementation; 
           [0034]      FIG. 2 a    schematically illustrates the retention, of the first material of a first ply on a first-ply mold and the spraying of the second thermoplastic-resin powder material onto the first material; 
           [0035]      FIG. 2 b    schematically illustrates the step of melting the second thermoplastic-resin powder material in a heated chamber; 
           [0036]      FIG. 2 c    schematically illustrates the laying of a thermoplastic layer to produce the part on the first ply retained on a tool by magnetic forces established at the level of the tool and/or generated by a potential difference. 
       
    
    
     DETAILED DESCRIPTION 
       [0037]      FIG. 1  synoptically shows the main steps of the process of the presently disclosed embodiment. 
         [0038]      FIGS. 2 a , 2 b  and 2 c    schematically illustrate an exemplary implementation of the process. 
         [0039]    In the process  100 , a first step  110  for producing a thermoplastic composite part including a stack of layers  13  includes producing a hard first ply  10  having, on the one hand, physical properties allowing it to temporarily and reversibly adhere to a tool  30  on which the part must be formed and, on the other hand, physico-chemical properties that allow it to bind with the thermoplastic resin of those layers of the part which must be deposited on said first ply. 
         [0040]    The first ply  10  is produced on a first-ply mold  20 . 
         [0041]    The first ply  10  is produced with a shape that is as close as possible if not identical to the shape of the tool  30  on which the thermoplastic composite layers from which the part must be formed must be laid. 
         [0042]    Furthermore, the first ply is as thin as possible so that its thickness is relatively small relative to the thickness of the part to be produced. 
         [0043]    The first-ply mold  20  is of a shape suitable for facilitating the production and demolding of the first ply  10  from said first-ply mold. In particular, if the part to be produced is mainly concave, the first-ply mold  20  will have a complementary, mainly convex shape. 
         [0044]    To produce the first ply  10 , a complex is formed including at least one first material  11  having electromagnetic properties and at least one second material  12  of a thermoplastic resin. 
         [0045]    In a first step  111  of the production of the first ply, the first material  11  is deposited on the first-ply mold  10 , the shape of which first-ply mold is a geometric counterpart of the tool  30  on which the part must be formed of the part. 
         [0046]    The first material  11  is advantageously a metallic material formed from thin, woven or nonwoven, wires of a few tens of microns the flexibility of which allows said metallic material to follow the shapes of the first-ply mold  20 . 
         [0047]    Depending on the electromagnetic properties of the first material  11 , said first material is retained on the first-ply mold  20  by means of a magnetic field and/or an electrical potential difference, said first-ply mold being produced so that the wanted magnetic field, and/or the wanted electrical potential difference, i.e. that required to ensure the first material is retained on the first-ply mold while closely following the shape of said first-ply mold, is produced on its surface. 
         [0048]    The first material  11  may also be retained, conjointly or not with the magnetic field and/or the potential difference, by mechanical means such as clamps for example (not shown). Specifically, mechanical clamping alone may prove to be sufficient to place the first material against the first-ply mold  20  in particular if the shape of said first-material mold is exclusively convex. 
         [0049]    In a second step  112  of the production of the first ply  10 , fine resin-powder particles of the second material  12  are deposited on the first material. The second material is for example deposited by electrostatic spraying. 
         [0050]    The spraying is for example carried out using a spray gun and a solution in dispersion of the resin powder if necessary. 
         [0051]    The second material  12  is deposited in a sufficient amount to ensure, in the subsequent steps of the process, binding of the fibers of a first layer of the thermoplastic composite of the part despite the shear forces generated during the laying of this first layer, the first layer being laid while being deformed to closely follow the shape of the tool and to orient the fibers in the desired directions. 
         [0052]    The second material  12  is advantageously deposited with an area density comprised between 20 g/m 2  and 150 g/m 2 . 
         [0053]    In a third step  113  of the production of the first-ply  10 , the first-ply mold  20 , on which are deposited the first material  11 , which is retained closely against the mold by electrostatic or magnetic forces and/or forces generated by the potential difference, and/or by mechanical clamping, and the second material  12 , which is deposited in powder form on the first material, is subjected to a temperature increase, for example in a heating device  22 , for example an infrared oven, or an air oven, or an autoclave, to raise the particles of the second thermoplastic-resin material  12  above their melting point and glass transition temperature, this causing the second material to adhere to the first material. 
         [0054]    When the second material  12  has bonded to the first material  11 , the temperature is returned to room temperature. 
         [0055]    In a fourth step  114  of the production of the first ply, the first ply  10  is demolded and separated from the first-ply mold  20  after the electromagnetic field of the mold and/or the potential difference and/or the clamps retained up to the third step  113  have been released and if needs be reversed. 
         [0056]    At the end of the first step  110  of the process, a hard first ply  10  of the first material impregnated with the second material is thus formed. 
         [0057]    It will be noted that the obtained first ply  10  has a small thickness, substantially the thickness of the wires or of the metal sevens of the first material, but that nonetheless, despite the fact that the first ply is not very stiff because of its small thickness, the static shape thereof remains the shape acquired in the first-ply mold because of the resin of the second material heated to a melting point. 
         [0058]    The metallic material of the wires of the first material  11  is chosen from materials having suitable magnetic properties: a good magnetic permeability and/or a high coercive field and/or a magnetization saturation for example, properties generally obtained with ferromagnetic materials. When it is desired to at least partially produce the clamping with a potential difference, the first material is chosen from electrically conductive materials so as to be able to foe brought to and maintained at a desired electrical potential and to make it possible to maintain an electrical potential difference between, on the one hand, the first element  10  and, on the other hand, the tool  30  and if needs be the first-ply mold  20 . 
         [0059]    In a second step  120  for producing the thermoplastic composite part, the first ply  10  produced in the first step  110  is laid on a tool  30  for molding the part to be produced. 
         [0060]    The first ply  10  is retained on the surface of the tool  30  by means of a magnetic field or, for its electrically conductive portion, an electrical potential difference between said tool and said first ply, the tool being produced from a ferromagnetic material so as to produce the magnetic field on its surface and/or from an electrically conductive material to be maintained at the wanted electrical potential difference required to ensure the first ply  10  is retained closely following the shape of said tool. 
         [0061]    The tool is for example produced from a steel or an alloy such as an Invar®. 
         [0062]    If needs be, clamps hold the first ply on the tool  30  in particular on the edges of said tool. 
         [0063]    In a third step  130  for producing the thermoplastic composite part, a first thermoplastic composite layer  13  of the part is laid on the first ply  10  maintained on the tool  30  and the other thermoplastic composite layers are laid in successive layers and consolidated using a known method, for example by means of a laying head  31 , until the number of plies wanted for the part is obtained. 
         [0064]    During the laying of the thermoplastic composite layers, the magnetic field or potential difference ensuring the first ply  10  is retained on the tool  30  is maintained so that, even in concave zones of the tool, the successive thermoplastic composite layers that are applied and that adhere directly or indirectly to the first ply perfectly follow the shape of the tool. 
         [0065]    In a fourth step  140  for producing the thermoplastic composite part, the thermoplastic composite part produced is demolded and separated from the tool  30  after the electromagnetic field of the tool, and/or the electrical potential difference between the tool and the first ply that is incorporated in the part at this stage of the process, and/or the clamps, which were retained during the deposition of the successive thermoplastic composite layers of the part, have been released and if needs be reversed. 
         [0066]    According to various variants of the process such as just described in a first mode of implementation, the process may be adapted depending on constraints related to the shape of the part to be produced or to the materials implemented. 
         [0067]    Thus, the first material of the first ply  10  may be formed by one or more layers of a magnetic metal material. For example from nickel, silver, ferritic stainless steel. 
         [0068]    The first material  10  may be deposited on the first-ply mold  20  in the form of wires in webs, wovens, knits, mats or even of a powder. 
         [0069]    When a plurality of widths are juxtaposed on the first-ply mold  20 , preferably two neighboring widths are placed with a slight overlap in order to avoid interstitial zones that, including only resin, could form a fragile zone in the first ply. 
         [0070]    The deposited weight per unit area of first material  11  is chosen to obtain the sought magnetic and/or potential-difference-generated adherence and to ensure enough mechanical support is given to the second material  12  to form a first ply  10  that is able to be handled and to serve as a bind for the first thermoplastic composite layer of the part. 
         [0071]    The area density of first material  11  is advantageously comprised between 20 g/m 2  and 150 g/m 2 . 
         [0072]    Thus, according to one method of implementation, the first ply  10  that must be deposited on the tool  30  is produced in a plurality of portions that are juxtaposed on the tool. 
         [0073]    This method proves to be advantageous when the first ply  10  is of such a large size that it would be difficult to handle a first ply made up of a single element without running the risk of damaging said first ply, and/or when the shape of the first ply is complex with concave zones and convex zones, clever cutting of the ply allowing, in the latter case, first-ply portions essentially having a simple curvature able to foe produced on convex molds to be produced. 
         [0074]    Thus, according to one method of implementation of the first step  110  of the manufacture of the first ply  10 , the second step  112  of depositing the second material  12  on the first material  11  is carried out before the first step III, the first material deposited on the first-ply mold  20  then already including the thermoplastic-resin powder of the second material. 
         [0075]    Advantageously, the complex formed by the first material and the thermoplastic powder of the second material is produced in an upstream step. 
         [0076]    The process thus allows a thermoplastic composite part to be produced with a first ply that is perfectly clamped to the tool, while ensuring that the resin of the first ply does not adhere to the tool, while avoiding the need for suction techniques that require seal-tight plies and that do not guarantee a homogenous pressure over the surface of the tool, and while avoiding use of the complex and expensive solution that is a counter-mold.