Abstract:
A method for the production of a hollow profile, which is a cockpit cross-member for a motor vehicle, involves braiding endless fibers around a core that forms the inner contour of the hollow profile to produce a fibrous hollow structure. After the braiding, the fibrous hollow structure is, in the elastic state, removed from the core in a non-destructive manner, molded into an end contour by application of internal pressure and overmolded with a plastic.

Description:
BACKGROUND AND SUMMARY OF THE INVENTION 
       [0001]    Exemplary embodiments of the invention relate to a method for the production of a hollow profile and a hollow profile component. 
         [0002]    For lightweight construction considerations, components made from fiber-reinforced plastics are increasingly used in motor vehicle construction. Typically, such components can only be produced in the form of plates or hollow profiles that run in a straight line and do not have an undercut. Complexly-shaped components, for example cockpit cross-members, which have to bear a plurality of attachment parts such as the steering console, the central console, the airbag holder and the tunnel brace, are therefore designed to have multiple parts, even in lightweight construction, and are subsequently joined together. During the construction of hollow profile components, no firmly bonded connection between the individual regions of the component is possible here, such that the stability is not always optimal. 
         [0003]    German patent document DE 10 2007 057 198 A1 discloses producing complexly-shaped fiber-reinforced hollow profile components by applying resinated endless fibers to a lost mold in a positive and stress-optimized manner, which can take place, for example, by weaving, braiding, stitching or sewing. Then the fibrous material is cured under formation of the desired reinforced hollow girder and the lost mold is removed destructively. 
         [0004]    During this procedure, the production of complexly-shaped hollow bodies is also enabled, but is afflicted with several disadvantages. In particular, wet, i.e. resin-impregnated, fibers are difficult to treat, wherein, for example, the processing machines also have to be cleaned regularly. Also, the provision of a lost core for the production of each individual hollow profile is both time-consuming and cost-intensive. The same applies for the destruction and disposal of the lost core. 
         [0005]    Exemplary embodiments of the present invention are directed to a method that enables the production of complexly-shaped hollow profiles from fiber composite materials in a particularly simple and economical manner. Exemplary embodiments of the present invention are also directed to a hollow profile component that is particularly resistant to exertions of force during the driving operation and which, at the same time, can be produced particularly economically and simply. 
         [0006]    In such a method for the production of a hollow profile, in particular a cockpit cross-member for a motor vehicle, endless fibers are braided around a core that represents the inner contour of the hollow profile to be produced, to form a fibrous hollow structure. Here, provision is made according to the invention for the fibrous hollow structure to be removed, in the elastic state, from the core without destruction after the braiding, to be molded into an end contour by application of internal pressure and to be overmolded with a plastic. Also, after the overmolding, the workpiece can, in the elastic state, be removed from the core. 
         [0007]    In other words, the inner contour of the hollow profile can be represented by a durable core, in contrast to the prior art. Due to the elastic nature of the braided fibrous hollow structure, a complexly-shaped durable core, which has, for example, undercuts, branches or suchlike, can be removed from the fibrous hollow structure without destruction. The additional effort of providing newer and newer lost cores, as well as the laborious destruction and disposal of the cores, is therefore dispensed with. 
         [0008]    The core preferably has at least one branch. This enables the production of particularly complexly-shaped hollow profiles, which, as well as the actual force-absorbing and force-conducting carrier structure, comprises additional functional elements that are formed as a single part and are firmly-bonded, such as additional struts, consoles or suchlike. 
         [0009]    In a further embodiment of the invention, the fibrous hollow structure is brought into a shape that is close to the end contour before the application of internal pressure by means of at least one handling device, in particular a robot. This enables the inner space of the fibrous hollow structure to be reliably and completely applied with pressure, without folds, kinks or suchlike in the fibrous hollow structure having a negative effect on the design. With this, a particularly procedurally-reliable molding of the fibrous hollow structure into the desired end contour is thus possible. 
         [0010]    Advantageously, hybrid rovings made from reinforcing fibers and thermoplastic matrix fibers are used as endless fibers. Alternatively, reinforcing fibers that are coated with thermoplastic matrix material, so-called towpregs, can also be used. In both cases, the matrix material is thus inserted into the braid in the fixed or paste-like state. The problems in processing resin-impregnated reinforcing fibers are hereby dispensed with. Carbon fibers, glass fibers or suchlike can, for example, be used as reinforcing fibers. Several types of reinforcing fibers, for example steel or aramid fibers, can also be interwoven in a single roving as well as the carbon fibers. PA or PPA, for example, can be used as the thermoplastic matrix material. In both cases, a very fine, homogeneous distribution of reinforcing and matrix fibers can be achieved, which later enables a faster and improved consolidation due to the short flow path of the matrix material. The corresponding hybrid rovings or towpregs additionally enable a particularly accurate, axially parallel configuration of the fibers without twists or knots, which configures the force flow particularly well in the hollow profile. 
         [0011]    To overmold the fibrous hollow structure, a short-fiber-reinforced plastic, particularly preferably a thermoplastic, is preferably used. Thus, particularly high strength can be achieved. As well as the overmolding itself, a fusing of the matrix material of the hybrid rovings or towpregs thus takes place at the same time, such that a homogeneous hollow profile body, which is reinforced by both long and short fibers, arises, which has excellent mechanical properties. 
         [0012]    In a further embodiment of the invention, wall strengths with local differences are produced by braiding the fibrous hollow structure. This enables a flux-optimized adaptation of the hollow profile to the actual operating stresses, such that, in the case of particularly low component weight, a particularly high level of resistance to stresses occurring during the driving operation is achieved. 
         [0013]    Advantageously, before the application of high internal pressure, the fibrous hollow structure is heated above the glass transition temperature and to just before the melting point of the matrix material, such that this is capable of optimum flow and can be molded and the end contour is adapted optimally. 
         [0014]    Advantageously, during the overmolding of the fibrous hollow structure, at least one depositor is, in addition, overmolded as well. Such depositors, which can also be produced from fiber composite materials, may also form functional components such as consoles, girders, supports, struts or suchlike on the hollow profile. The overmolding can thus, according to known methods, be carried out in common injection molding tools. 
         [0015]    The invention furthermore relates to a hollow profile component, in particular a cockpit cross-member for a motor vehicle, which has a hollow profile having at least one branch, which is reinforced by a continuous, branched fiber network. Here, provision is made according to the invention for the at least one branch to form a functional part, in particular a console, a tunnel brace or suchlike. By using a continuous, branched fiber network, a particularly stable hollow profile component is obtained. At the same time, by using the at least one branch to form the functional part, a particularly high level of functional integration can be achieved. It is hereby possible to dispense with a non-firmly bonded connection of the functional parts, for example by overmolding or other mechanical joining methods that would potentially weaken the hollow profile component. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWING FIGURES 
         [0016]    The invention and its embodiments are to be illustrated in greater detail below with the aid of the figure. Here are shown: 
           [0017]      FIG. 1  a perspective view of an exemplary embodiment of a cockpit cross-member according to the invention; 
           [0018]      FIG. 2  a cross-sectional depiction of a hybrid roving; 
           [0019]      FIG. 3  a cross-sectional depiction of a towpreg; 
           [0020]      FIG. 4  a braiding machine that can be used within the framework of an exemplary embodiment of a method according to the invention; 
           [0021]      FIG. 5  a braid produced by means of the braiding machine according to  FIG. 4 ; 
           [0022]      FIGS. 6   a  and  6   b  are perspective views of a durable core that can be used within the framework of an exemplary embodiment of the method according to the invention; 
           [0023]      FIG. 7  a sectional depiction through a branching region of an exemplary embodiment of a cockpit cross-member according to the invention; 
           [0024]      FIG. 8  a schematic view of a robotic system for aligning a fibrous hollow body produced within the framework of a method according to the invention into a location that is close to the end contour; 
           [0025]      FIG. 9  a schematic depiction of potential positions for depositors during the overmolding of the fibrous hollow body according to  FIG. 8 ; 
           [0026]      FIG. 10  a molding tool for overmolding the fibrous hollow body; 
           [0027]      FIG. 11  a view of a detailed structure of the exemplary embodiment of a cockpit cross-member according to the invention; 
           [0028]      FIG. 12 ,  13  two alternative views of a depositor for forming a steering console in one exemplary embodiment of a cockpit cross-member according to the invention; 
           [0029]      FIG. 14 ,  15  two perspective views of a further depositor for a steering console for one exemplary embodiment of a cockpit cross-member according to the invention, and 
           [0030]      FIG. 16  a perspective view of a depositor for a support structure for a passenger airbag of one exemplary embodiment of a cockpit cross-member according to the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0031]    A cockpit cross-member denoted as a whole by  10  for a motor vehicle comprises a transverse strut  12 , which is formed as a hollow profile, as well as a tunnel brace  14 , which is also hollow profiled, which supports the cockpit cross-member  10  on the tunnel of the motor vehicle. To create a particularly stable cockpit cross-member  10 , the transverse strut  12  and the tunnel brace  14  are produced as single-part, branched hollow bodies from a fiber-reinforced plastic. Further overmolded attachment parts, such as a support frame  16  for a passenger airbag or a steering console  18 , are applied to the transverse strut  12 . Also, fastening consoles  20  for the lateral fastening of the cockpit cross-member  10  are connected to the cockpit cross-member  10  as overmolded plastic parts. 
         [0032]    Hybrid rovings, as depicted in  FIG. 2 , are applied for the production of such a branched fiber composite hollow profile. Such a hybrid roving  22  comprises a plurality of reinforcing fibers  24 , for example carbon fibers, which are bundled together with matrix fibers  26  made from a thermoplastic material such as PPA. Here, both a regularly alternating fiber arrangement  28  and a disordered fiber arrangement  30  are possible. The advantage of hybrid rovings  22  is that the matrix material is already contained in the preform. Due to the very fine, homogeneous distribution of the reinforcing and matrix fibers, the matrix material is already located in the braid before the braiding process. This enables a fast and particularly reliable consolidation due to short flow paths of the subsequently fused matrix fibers  26 . In addition, the fibers  24 ,  26  are arranged axially parallel and without twists or knots, which significantly increases the resilience of the material. Alternatively, the so-called towpregs  32  depicted in  FIG. 3  can also be used. Here, these are reinforcing fibers  24  that are coated with a sheathing  34  made from matrix material. Particularly short flow paths also arise here during the subsequent consolidation. 
         [0033]    The reinforcing fibers  24  can be formed as carbon fibers, glass fibers or suchlike. Also, mixed fiber compositions, for example with additional, integrated steel or aramid fibers, are possible. 
         [0034]    To braid the hollow profile around a durable core, a braiding machine  36 , as is depicted in  FIG. 4 , is used. A plurality of braiding wheels  40 , each of which carries a plurality of reels  42 , are arranged around the durable core  38 . The respective hybrid rovings  22  are unwound from the reels  42  and braided around the core  38 . Here, a partial fusing of the material of the matrix fibers  26  can already be achieved by infrared heaters  44 . The use of several braiding wheels  40  enables the production of a multilayer braid. In particular, differences in thickness can also hereby be achieved, wherein several layers of the braid are braided over one another in regions of greater stress. 
         [0035]    The braiding angle depicted with the aid of a section of the braid  46  can, in such braiding processes, be +/−5° to +/−80°. For reinforcement in the 0-degree direction, which is particularly advantageous in the case of bending loads, additional filler yarns can be added to the braiding wheel. These pass into the braid in an extended manner and thus have barely any undulation. Furthermore, so-called UD braiding can be used, wherein hybrid rovings are braided with pure matrix fibers and the matrix is subsequently fused. 
         [0036]    To achieve the branching in the cockpit cross-member  10 , a mold core  48  according to  FIGS. 6   a  and  6   b  is used. The mold core  48  shown in  FIG. 6   a  is constructed in multiple parts and has a branched central piece  50 , which can be combined with end pieces  52  to produce the complete core  48 . As is shown in  FIG. 6   b , the branching can also be achieved by inserting an end piece  52  into a corresponding receiver of the central piece  50 . 
         [0037]    Due to the flexible nature of hybrid rovings  22  or towpregs  32 , it is possible to completely braid such a core  48 , even in the branching region, and to then still release it from the braid  46  in a non-destructive manner. Should, at greater branching angles, as illustrated in  FIG. 7 , no complete braiding of the branching region be possible, this can, if necessary, later have an overmold  54  added to it. 
         [0038]    After the release of the braid  46  from the core  48 , this is, as shown in  FIG. 8 , held by a handling robot  56 . This has a plurality of manipulators  58 , which grip the braid  46  and hold it in a position close to the end contour. In this position, the braid  46  is finally inserted into an injection molding machine  62 , wherein it is, if necessary, provided at several points with depositors  60  made from a thermoplastic material, said depositors being held in the injection molding tool  62  at the corresponding positions. Before the actual overmolding of the braid  46 , the braid  46  has internal pressure applied to it, such that it maintains the desired hollow contour, even during the injection molding. Then the braid  46  and, if necessary, the depositors  60 , are overmolded with a thermoplastic compound which can, if necessary, even contain yet more short fibers for further reinforcement. Here, the thermoplastic compound enters the braid  46  and, at the same time, fuses the matrix fibers  26 , such that a homogeneous plastic body arises with the desired inner fibrous structure. Also, the depositors  60  produced from thermoplastic material, for example fiber-reinforced plastic, are thus connected firmly to the cross-member, such that a single-part cross-member  10  is created with a high level of functional integration. 
         [0039]    When overmolding the braid  46 , as depicted in  FIG. 11 , more reinforcing ribs  64  can additionally be injected as well. 
         [0040]    Finally, yet more examples of various forms of depositors  60  are depicted in  FIG. 12  to  FIG. 16 .  FIGS. 12 to 15  thus show different views of a depositor  60  for the formation of the steering console  18 . The depositor can be constructed from flat organic sheet structures  66 , which can be connected to the braid via a plastic rib structure  68  that forms a hollow space  70 . Corresponding receiving openings  72  serve to bolt the steering console  18  to components that are to be fastened to it. Also, reinforcing ribs  74  can be provided here, which provide the steering console  18  with particularly good strength. 
         [0041]    Finally,  FIG. 16  shows a depositor  60  for the formation of the clamp  16  for a passenger airbag. Here, the depositor  60  consists of a rectangularly peripheral frame  78  made from thermoplastic material, which in turn has a fabric rib structure  68  added to it, which receives the braid  46  that is to be overmolded. Here, a firmly bonded connection can also be generated by melting the rib structure  68  during the overmolding, such that a particularly good grip can be achieved as well here. 
         [0042]    The foregoing disclosure has been set forth merely to illustrate the invention and is not intended to be limiting. Since modifications of the disclosed embodiments incorporating the spirit and substance of the invention may occur to persons skilled in the art, the invention should be construed to include everything within the scope of the appended claims and equivalents thereof.