Abstract:
A method for fibre placement with the aid of a roller is provided. The roller is able to pivot about a rotation axis and to lay a plurality of first pre-impregnated fibres on second pre-impregnated fibres of a laying surface by rolling over said laying surface. The first fibres being distributed along a lower generatrix of the roller and being in contact with an outer surface of said roller over a contact arc. The method includes cooling the first pre-impregnated fibres in contact with the roller so as to enable them to slide on the outer surface of the roller and heating the second pre-impregnated fibres of the laying surface that are located at the front of the roller so as to promote the adhesion of the first fibres to the second fibres of the laying surface.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims priority to French Patent Application No. 12 59739, filed Oct. 12, 2012, which is incorporated herein by reference in its entirety. 
       TECHNICAL FIELD 
       [0002]    This application pertains to a method for fibre placement with the aid of a temperature-regulated roller. According to one embodiment, a piece made of composite material is produced from a preform of fibres subjected to a polymerization cycle. The technical field relates more specifically to preforms produced from fibres pre-impregnated with resin which are subsequently polymerized during a step of consolidation without addition of resin. 
       BACKGROUND 
       [0003]    When the surface of the mould is developable, it is possible to use draping machines which are intended to lay sheets of fibres on the mould, it being possible for the dimensions of said sheets of fibres to meet those of the piece. In this case, each layer may consist of a single sheet. 
         [0004]    When the surface of the mould is not developable, it is possible to use a fibre placement machine for simultaneously laying a plurality of fibres positioned side-by-side. Thus, a fibre placement machine makes it possible to lay a strip of fibres upon each pass. 
         [0005]    As a variant, a placement machine may make it possible to simultaneously lay a plurality of fibre ribbons. The expression “fibre ribbon” is understood to mean a group of fibres. Generally, a ribbon is planar and has a small width, of around a centimetre or less. To give an order of magnitude, a fibre ribbon may have a width of around 12 mm, 6 mm or 3 mm. 
         [0006]    In the rest of the description, a fibre is understood to mean an elongate element which may consist of a single fibre or of a plurality of fibres in the manner of a ribbon. The term “preform” is understood to mean a volume of fibres disposed in a predetermined arrangement, obtained in particular by superposing layers of fibres on top of one another on a surface of a mould. 
         [0007]    A laying surface corresponds to the surface of the mould in the case of the first layer of fibres or to the last layer laid in the case of the other layers. 
         [0008]      FIG. 1  shows a mould  10  on which a preform  12  has been formed with the aid of a fibre placement machine  14 . 
         [0009]    This machine  14  comprises a fibre store  16 , a mobile placement head  18 , supported by a robot (not shown) such as an articulated arm, for example, and also means  20  for feeding fibres  22  from the store  16  to the placement head  18 . 
         [0010]    The placement head  18  comprises a roller  24  that is able to pivot about a rotation axis  26  moving above the laying surface  28 , and means  30  for heating the laying surface at the front of the roller  24  in order to activate the resin of the fibres  22  positioned by the roller so as to promote the adhesion of the fibres to the laying surface in order that they stay in their positions. 
         [0011]    In the rest of the description, the expression “longitudinal direction” is understood to mean a direction parallel to the rotation axis of the roller. The longitudinal plane corresponds to a plane containing the rotation axis. A transverse plane corresponds to a plane perpendicular to the rotation axis. A radial direction is a direction perpendicular to the rotation axis. 
         [0012]    According to an embodiment illustrated in  FIG. 2 , the roller  24  comprises a cylindrical body  32  made of a single block with two spindle ends  34  at each end, said spindle ends being mounted in a pivoting manner in bearings  36  of a support  38  secured to the placement head. The spindle end  34 /bearing  36  guidance makes it possible to define the rotation axis  26 . 
         [0013]    The support  38  exerts a force on the roller  24  in the direction of the laying surface  28  such that, at a lower generatrix  40 , the roller  24  exerts a force on the fibres  22  to be placed in the direction of the laying surface  28 . 
         [0014]    The roller can pivot freely about the rotation axis  26 . Its rotational movement about the rotation axis  26  results from the rolling of the roller  24  on the laying surface  28 . 
         [0015]    According to one embodiment, the cylindrical body  32  is made of elastomer coated with Teflon film. 
         [0016]    The fibre store  26 , also known as a creel, comprises a plurality of spools of fibres. The store  16  comprises means for exerting a constant tension on each of the fibres, regardless of the speed or acceleration of the placement head  18 . 
         [0017]    If the tension is not sufficient for a given fibre  22 , the latter causes axial compression of the fibre  22  when it is compacted by the roller  24 , causing folds in the fibre, as illustrated in  FIG. 3A , these remaining included in the preform in the form of small undulations which impair the mechanical characteristics of the piece produced. 
         [0018]    If the tension is too high for a given fibre  22 , this causes bridging at a hollow in the piece, as illustrated in  FIG. 3B , this bridging remaining included in the preform and resulting in a defect which may impact on the mechanical characteristics of the piece produced. 
         [0019]    Fibre placement is carried out at ambient temperature greater than or equal to about 20° C. In this context, each fibre is unwound by the simultaneous combination of two effects: 
         [0020]    The first effect results from the rolling of the roller on the fibre and should in theory make it possible to unwind a quantity of fibre equal to the distance covered by the roller if the fibre were only in contact with the roller at the lower generatrix  40  of the roller. 
         [0021]    The second effect results from the adhesion of the fibre to the roller over a certain contact arc length A (visible in  FIGS. 3A and 3B ). 
         [0022]    This latter effect generally results in defects for the following reasons. 
         [0023]    Generally, the trajectory of the placement head is not necessarily rectilinear and may describe a curve. In this case, for a given displacement speed of the centre of the roller, the displacement speed Vext of a first end of the roller is greater than the displacement speed Vint of a second end of the roller in a curve. However, taking the second effect caused by the adhesion of the fibres to the roller into account, the tangential speed of each fibre is constant at the lower generatrix of the roller and equal to the rotational speed of the roller θ multiplied by the radius of the roller R. 
         [0024]    Consequently, if Vext is greater than θ·R, the fibre Fext laid at this location is stretched too much, this tending to cause defects of the bridging type as illustrated in  FIG. 3B . In parallel, if Vint is less than θ·R, the fibre Fint laid at this location is not stretched enough, and this tends to cause defects of the folding type as illustrated in  FIG. 3A . 
         [0025]    In order to limit the appearance of these defects, a first solution consists in laying dry fibres which are not pre-impregnated. In this case, the fibres can slide over the roller and be stretched adequately. However, this solution makes it necessary to use other polymerization techniques which have to allow the impregnation of the fibres with a resin. 
         [0026]    These polymerization techniques are generally more complex to implement. In addition, since the fibres are not pre-impregnated they can slide with respect to one another in the preform and ultimately cannot be positioned correctly. 
         [0027]    According to a second solution, it may be conceivable to reduce the length of the contact arc A in order to reduce the frictional forces between the fibres and the roller. 
         [0028]    However, this solution may be difficult to implement since it makes it necessary to change the architecture of the placement head. In addition, a certain length of the contact arc is necessary to ensure guidance of the fibres. Thus, the reduction in the length of the contact arc results in deterioration in the precision of positioning the fibres in the longitudinal direction. 
         [0029]    Document U.S. Pat. No. 6,390,169 proposes a third solution which consists in using a segmented roller  42  as illustrated in  FIGS. 4A and 4B . Each roller segment  44  is dedicated to one fibre and may have a rotational speed different from those of the other segments. Thus, the rotational speed of each roller segment is adapted to the speed of movement of the point of said segment in contact with the laying surface. As a result, each fibre is stretched correctly. 
         [0030]    Even though it limits the risks of the appearance of defects of the bridging or folding type, this solution is not entirely satisfactory since the roller is relatively complex. Furthermore, since each segment has a large radius of around 35 mm, the roller does not make it possible to correctly apply the fibres to the laying surface if the latter has a hollow having a radius of curvature less than 35 mm. 
         [0031]    Other objects, desirable features and characteristics will become apparent from the subsequent summary and detailed description, and the appended claims, taken in conjunction with the accompanying drawings and this background. 
       SUMMARY 
       [0032]    Thus, the present disclosure aims to remedy the drawbacks of the prior art. 
         [0033]    To this end, the various teachings of the present disclosure provide a method for fibre placement with the aid of a roller that is able to pivot about a rotation axis and to lay a plurality of first pre-impregnated fibres on second pre-impregnated fibres of a laying surface by rolling over said laying surface, said first fibres being distributed along a lower generatrix of the roller and being in contact with an outer surface of said roller over a contact arc, characterized in that it comprises cooling the first pre-impregnated fibres in contact with the roller so as to enable them to slide on the outer surface of the roller and in heating the second pre-impregnated fibres of the laying surface that are located at the front of the roller so as to promote the adhesion of the first fibres to the second fibres of the laying surface. 
         [0034]    This solution provides the same advantages as the dry fibres in the region of the roller. In addition, since the second fibres of the laying surface are heated, the first pre-impregnated fibres laid by the roller adhere and stay in their positions in the preform until the consolidation step. 
         [0035]    In one example, the outer surface of the roller is cooled to a temperature Tf less than or equal to about 12° C. 
         [0036]    Advantageously, the outer surface of the roller is cooled by circulating a heat transfer fluid in the roller. 
         [0037]    In one example, the roller comprises a fluid circuit with a supply of heat transfer fluid at a temperature for lowering the outer surface of the roller to the temperature Tf and keeping it at this temperature, and an outlet for evacuating the heat transfer fluid. 
         [0038]    A person skilled in the art can gather other characteristics and advantages of the disclosure from the following description of exemplary embodiments that refers to the attached drawings, wherein the described exemplary embodiments should not be interpreted in a restrictive sense. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0039]    The various embodiments will hereinafter be described in conjunction with the following drawing figures, wherein like numerals denote like elements, and wherein: 
           [0040]      FIG. 1  is a perspective view of a placement machine according to the prior art; 
           [0041]      FIG. 2  is a front view of a roller of a placement machine according to the prior art; 
           [0042]      FIG. 3A  is a side view of the roller from  FIG. 2 , which causes a first type of defect; 
           [0043]      FIG. 3B  is a side view of the roller from  FIG. 2 , which causes a second type of defect; 
           [0044]      FIGS. 4A and 4B  are front views of a roller according to the prior art in contact with contact surfaces having different profiles; 
           [0045]      FIG. 5  is a longitudinal section through an exemplary embodiment of a roller of a fibre placement machine according to the various embodiments of the present disclosure; 
           [0046]      FIG. 6A  is a longitudinal section through another exemplary embodiment of a roller of a fibre placement machine according to various embodiments of the present disclosure; 
           [0047]      FIG. 6B  is a cross section through the roller from  FIG. 6A ; and 
           [0048]      FIG. 7  is a longitudinal section through another exemplary embodiment of a roller of a fibre placement machine according to various embodiments of the present disclosure. 
       
    
    
     DETAILED DESCRIPTION 
       [0049]    The following detailed description is merely exemplary in nature and is not intended to limit the present disclosure or the application and uses of the present disclosure. Furthermore, there is no intention to be bound by any theory presented in the preceding background or the following detailed description. 
         [0050]      FIGS. 5 ,  6 A,  6 B and  7  show a roller  50  of a fibre placement machine that is able to pivot about a rotation axis  52 . 
         [0051]    As known to one skilled in the art, the roller  50  comprises means for connecting it to a placement head. According to one exemplary embodiment, the roller  50  comprises at each end a spindle end  54  that is mounted so as to be able to pivot in a bearing  56  of a support  58  secured to the placement head. The spindle end  54 /bearing  56  guidance makes it possible to define the rotation axis  52 . The placement machine and the connection between the roller and the placement head are not described in more detail since they are known to a person skilled in the art and since the roller  50  according to the various teachings of the present disclosure is more specifically suitable for being mounted on existing fibre placement machines in place of existing rollers. 
         [0052]    According to one embodiment, the roller  50  comprises an approximately cylindrical outer surface  60  which can roll on a laying surface  62  so as to place fibres  64 . As illustrated in  FIG. 6B , each fibre  64  is in contact with the outer surface  60  over a contact arc A which depends on the architecture of the placement head. To give an order of magnitude, this contact arc A has a length which is greater than one quarter of the circumference of the roller and less than half of the circumference. 
         [0053]    The fibres  64  are pre-impregnated with a resin. The fibres and the resin are selected depending on the piece to be produced. 
         [0054]    At ambient temperature, greater than or equal to about 20° C., the pre-impregnated fibres  64  have relatively high tack which gives the fibres a bonding ability and adhesion to the laying surface  62 , allowing the fibres to stay in their positions until the consolidation step. 
         [0055]    According to the various teachings of the present disclosure, the roller  50  comprises means for cooling the pre-impregnated fibres  64  in contact therewith, over at least a portion of the contact arc A in order to reduce the tack of said fibres such that they lose their bonding ability. As a result, if necessary, the fibres  64  can slide over the outer surface  60  so as to compensate for the different lengths of fibre laid from one fibre to another on account in particular of the trajectory of the placement head or of the geometry of the laying surface  62 . 
         [0056]    Advantageously, the roller  50  comprises means for cooling the outer surface  60  which, by conduction, cools the fibres  64  over at least a portion of the contact arc A. The means for cooling the outer surface  60  make it possible to lower its temperature and to keep it at a temperature less than Tf °, starting from which a loss of bonding ability of the resin which impregnates the fibres can be observed. 
         [0057]    By loss of bonding ability, the fibres will be expected to be able to slide on the outer surface  60  if necessary during their placement on the laying surface. 
         [0058]    For the majority of resins used in the aeronautical field, the temperature of the outer surface  60  should be less than or equal to approximately 12° C. 
         [0059]    In one example, the cooling means cool the outer surface around its entire circumference. 
         [0060]    As a complement to the cooling means, the fibre placement machine comprises means for heating the fibres of the laying surface  62  that are located at the front of the roller  50 . According to one exemplary embodiment, these means for heating the fibres are secured to the placement head. By way of example, the means for heating the fibres comprise at least one infrared lamp. 
         [0061]    When the fibres  64  come into contact with the laying surface  62 , the upper part of the fibres is at a temperature less than Tf and does not adhere to the outer surface  60  of the roller  50 , whereas the lower part of the fibres in contact with the heated laying surface  62  adheres to the latter since the pre-impregnated resin on the fibres is “reactivated”, thereby ensuring that the fibres unwind and are positioned correctly. 
         [0062]    In one example, the cooling means use a heat transfer fluid which circulates inside the roller  50  which, to this end, comprises a fluid circuit  66 . 
         [0063]    Advantageously, the means for cooling the heat transfer fluid are located outside the roller. 
         [0064]    In this case, the fluid circuit  66  comprises a supply  68  of heat transfer fluid at a “cold” temperature for lowering the outer surface  60  to a temperature less than Tf and keeping it at this temperature, and an outlet  70  for evacuating the heat transfer fluid. 
         [0065]    This solution has the advantage of allowing the evacuation of calories which the roller  50  accumulates by rolling over a heated laying surface  62 . 
         [0066]    The placement machine comprises means for controlling the temperature and the flow rate of the heat transfer fluid in order to obtain the appropriate temperature at the outer surface  60  of the roller. 
         [0067]    According to one embodiment which is illustrated in  FIG. 5 , the roller  50  comprises a hollow cylinder  72  having a thin and rigid wall, having a side wall at each end, each side wall comprising an orifice which is centred with respect to the rotation axis, one of the orifices communicating with the supply  68  of heat transfer fluid and the other orifice communicating with the outlet  70  for the heat transfer fluid. According to this embodiment, the fluid circuit  66  corresponds to the interior of the hollow cylinder  72 . 
         [0068]    According to another embodiment which is illustrated in  FIGS. 6A and 6B , the roller  50  is solid and comprises a fluid circuit  66  having a plurality of longitudinal ducts  76  located close to the outer surface  60 . According to one embodiment, the fluid circuit  66  comprises a central duct  74  which is coaxial with the rotation axis  52  which extends along the entire length of the roller  50 , one of its ends being connected to the supply  68  of heat transfer fluid and its other end being connected to the outlet  70  for heat transfer fluid. 
         [0069]    The longitudinal ducts  76  are distributed around the entire circumference of the roller so as to obtain a temperature which is as homogeneous as possible at the outer surface  60 . The longitudinal ducts  76  extend along virtually the entire length of the roller and are parallel to the central duct  74 . 
         [0070]    As a complement, the circuit  66  comprises radial ducts  78 , one for each end of the longitudinal ducts  76 , which allow the longitudinal ducts to be brought into communication with the central duct. Compared with the embodiment illustrated in  FIG. 5 , this solution improves heat exchanges between the heat transfer fluid and the outer surface of the roller. 
         [0071]    According to another embodiment which is illustrated in  FIG. 7 , the roller  50  comprises a hollow cylinder  80  having a deformable wall. In this case, the fluid circuit  66  corresponds to the interior of the hollow cylinder  80 . 
         [0072]    Advantageously, the roller  50  comprises means for altering its capacity to deform. According to one embodiment, the placement machine comprises means for regulating the pressure of the heat transfer fluid and in this way to modulate the capacity of the hollow cylinder  80  to deform. The higher the pressure, the more the deformation of the roller is limited. 
         [0073]    This solution makes it possible to distribute the force applied to the fibres in a better manner, along the lower generatrix of the roller, and to obtain a uniform pressure on the fibres, in the case of a discontinuity of the laying surface. 
         [0074]    According to one exemplary embodiment, the hollow cylinder  80  comprises a tread strip which is made of deformable material and is reinforced by a reinforcement  82 . This transverse and/or longitudinal reinforcement  82 , obtained by coiling threads for example, makes it possible to avoid radial expansion on account of the pressure and gives the tread strip good flexural rigidity. 
         [0075]    The roller  50  according to various embodiments may also be used in a step subsequent to the placement of the fibres, said step comprising compacting the preform in the hot state. In this case, the roller comprises means for heating the fibres in contact therewith. According to one embodiment, it is possible to use the above-described rollers by circulating a “hot” heat transfer fluid, the temperature and flow rate of which make it possible to heat the fibres to the temperature required for compacting. 
         [0076]    Whatever the application, a roller according to the various embodiments of the present disclosure comprises means for regulating the temperature of the fibres in contact therewith. 
         [0077]    While at least one exemplary embodiment has been presented in the foregoing detailed description, it should be appreciated that a vast number of variations exist. It should also be appreciated that the exemplary embodiment or exemplary embodiments are only examples, and are not intended to limit the scope, applicability, or configuration of the present disclosure in any way. Rather, the foregoing detailed description will provide those skilled in the art with a convenient road map for implementing an exemplary embodiment, it being understood that various changes may be made in the function and arrangement of elements described in an exemplary embodiment without departing from the scope of the present disclosure as set forth in the appended claims and their legal equivalents.