Abstract:
The invention relates to a method and a moulding core for producing a fibre composite component ( 34 ), in particular in aerospace, comprising the following method steps: introducing a core sleeve ( 9 ) into a moulding tool ( 2 ) for establishing an outer geometry of a moulding core ( 27 ) to be formed; filling the core sleeve ( 9 ) that is introduced with a vacuum-fixable filling material ( 21 ); applying a vacuum to the core sleeve ( 9 ) and consequently vacuum-fixing the filling material ( 21 ) for forming the moulding core ( 27 ); and at least partly laying at least one semifinished fibre product ( 33   a,    33   b ) on the moulding core ( 27 ) that is formed, for the shaping of the fibre composite component ( 34 ) to be produced.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to a method for producing a fiber composite component, in particular for aerospace, to a molding core for producing such a fiber composite component and to a fiber composite component with at least one stringer which is produced by means of such a molding core and/or such a method. 
       BACKGROUND OF THE INVENTION 
       [0002]    Although it can be applied to any desired fiber composite components, the present invention and the problems on which it is based are explained in more detail below with reference to two-dimensional stringer-stiffened carbon fiber reinforced plastic (CRP) components, for example skin shells of an aircraft. 
         [0003]    It is generally known to stiffen CRP skin shells with CRP stringers in order to withstand the loads occurring in the aircraft sector with the lowest possible additional weight. In this respect, a distinction is made essentially between two types of stringers: T and Ω stringers. 
         [0004]    The cross section of T stringers is made up of a base and a stem. The base forms the connecting surface with respect to the skin shell. The use of skin shells stiffened with T stringers is widespread in aircraft construction. 
         [0005]    Ω stringers have something like a hat profile, its ends being connected to the skin shell. Ω stringers may either be adhesively attached in the cured state to the likewise cured skin shell, or be cured wet-in-wet at the same time as the shell. The latter is desired, because it is more favourable from technical aspects of the process. However, supporting or molding cores are necessary for the wet-in-wet production of skin shells stiffened with Ω stringers, in order to fix and support the dimensionally unstable semifinished fiber products in the desired Ω shape during the production process. Skin shells with Ω stringers have the advantage over T stringers that they allow better infiltration during an infusion process for introducing a matrix, for example an epoxy resin, into the semifinished fiber products. Infusion processes are inexpensive in comparison with other known methods for producing fiber composite components, such as the prepreg process for example, because it allows the use of lower-cost semifinished fiber products. 
         [0006]    However, there is the problem with the production of Ω stringers that the material used at present for the supporting or molding core is cost-intensive and can only be removed with difficulty after the forming of the Ω stringers, with the result that the material remaining in the stringers contributes adversely to the weight of the fiber composite component, and consequently to the weight of the aircraft. Furthermore, it is problematic that the material remaining in the stringers contributes adversely to the overall weight of the aircraft. 
       SUMMARY OF THE INVENTION 
       [0007]    It is one of the objects of the present invention to provide a lower-cost and lighter fiber composite component, in particular for aerospace. 
         [0008]    Accordingly, a method for producing a fiber composite component, in particular a stringer on a base part in aerospace, is provided, comprising the following method steps: introducing a core sleeve into a molding tool for establishing an outer geometry of a molding core to be formed; filling the core sleeve that is introduced with a vacuum-fixable filling material; applying a vacuum to the core sleeve and consequently vacuum-fixing the filling material for forming the molding core; arranging the molding core on a base part comprising semifinished fiber composite products; at least partly laying at least one semifinished fiber product on the molding core that is formed, in order to shape the fiber composite component to be produced; and introducing a matrix into the semifinished fiber products with the molding core and subsequently curing the same at least partially. 
         [0009]    Also provided is a molding core for producing a fiber composite component, in particular a stringer on a base part, in particular in aerospace, comprising a core sleeve, which forms an outer surface of the molding core, and a vacuum-fixable filling material, which is vacuum-fixed by means of the core sleeve. 
         [0010]    Also provided is a fiber composite component with at least one stringer, in particular for aerospace, which is produced by means of the molding core according to the invention and/or the method according to the invention. 
         [0011]    One advantageous effect that may be achieved by one embodiment of the present invention is that the fiber composite component can be produced by means of a lower-cost molding core. In this embodiment, instead of a cost-intensive material, an extremely low-cost fixable filling material, such as quartz sand for example, may be advantageously used for the molding core. 
         [0012]    The “vacuum-fixable filling material” is to be understood in the present case as meaning a filling material which has in particular granular elements, which hook into one another when a compressive pressure is applied to the filling material. The form of the filling material is defined in a fixed state for as long as the compressive pressure on the filling material is maintained. If the compressive pressure is removed, the elements are no longer in engagement and changing of the geometrical form of the filling material is enabled. 
         [0013]    According to one particular embodiment of the invention, before filling, the core sleeve is held open by gravitational force and/or by a sucking means acting on an outer surface of the core sleeve. Consequently, the core sleeve can be easily filled. The sucking means are formed particularly advantageously by the same means as are used for the vacuum fixing of the core sleeve. 
         [0014]    An elastic core sleeve with a somewhat smaller diameter than the molding core to be formed or a core sleeve with a somewhat greater diameter may be used for example, tolerable fold formations possibly occurring in the latter case. 
         [0015]    In the case of a further embodiment of the invention, the vacuum-fixable filling material in the molding tool is compacted and/or shaken before the vacuum fixing. “Compacting” is to be understood as meaning compacting of the grains of the filling material. 
         [0016]    The molding tool may be formed such that it can be divided in the longitudinal direction, in order to ensure easy removal of the molding core. 
         [0017]    Compacting and/or shaking before the vacuum fixing is conducive to engagement of the elements of the filling material and therefore leads to a fixed defined geometrical form of the filling material when the compressive pressure is applied. 
         [0018]    “Vacuum fixing” is to be understood as meaning the provision of a negative pressure at an opening of the core sleeve. As a result of the negative pressure, the core sleeve draws itself firmly around the fixable filling material and consequently fixes it. The opening is subsequently closed or sealed by, for example, welding and/or adhesive bonding. Alternatively, a vacuum generating device may remain permanently connected until after the curing of the CRP component. As a result, process monitoring is better possible, such as for example the detection of leaks in the core sleeve. Furthermore, the vacuum can be maintained in the core sleeve when there are leaks in the latter. 
         [0019]    According to a further embodiment of the invention, reinforcing means are arranged in the region of transitions, to be formed with a sharp edge, of the outer geometry of the molding core to be formed, inside and/or outside the core sleeve. One of the effects of these reinforcing means, in particular corner profile parts of an approximately triangular cross section, is that they increase the edge strength of the molding core. Further functions of the corner profile parts may be that of increasing the radii, for example in the region in which the stringer adjoins the base part, providing a design appropriate for the fibers, influencing the resin distribution and protecting the core sleeve from damage. 
         [0020]    Although other embodiments of the corner profile parts are conceivable within the scope of the present invention, a configuration with a braided CRP gusset or extruded plastic gusset is particularly suitable. The CRP gusset has a coarsely woven CRP braided tube with a centrally drawn-in fiber strand, which can easily be pressed into a triangular shape. The CRP gusset cures with the fiber composite component. The extruded plastic gusset may be optionally left in the component or removed. The configuration described above with the CRP gusset or plastic gusset offers the following advantages: the undefined formation of resin pockets in the corners is avoided. As a result, on the one hand weight can be saved and on the other hand sharp kinks in the laid CRP fabric can be avoided. Furthermore, there are possibilities for optimized fiber orientation in the stringer and/or base part as a result of increased radii. Furthermore, unwanted rapid advancement of the resin during the infiltration is prevented, since any cavities are filled by the molding core with the corner profile parts. Furthermore, easier and more reliable removal from the mold is ensured. Jamming of the core in the corners is less likely to occur. Furthermore, the sensitivity of a brittle or soft molding core during handling is reduced, since loads are shifted to the corner profile parts. Furthermore, mold core production is simplified, since the molding core no longer has to be formed with a sharp edge. In addition, the core sleeve undergoes less stress with corner profile parts arranged on its outside, since the core sleeve no longer forms sharp edges at which the core sleeve material could suffer fatigue. 
         [0021]    “Semifinished fiber products” are to be understood as meaning woven or laid fabrics and fiber mats. These are provided with a matrix, for example an epoxy resin, and subsequently cured, for example with the aid of an autoclave, to form a fiber composite component. 
         [0022]    A release layer, which reduces adhesive attachment of the cured fiber composite component, may be applied to the core sleeve. This facilitates removal of the core sleeve after the at least partial curing of the portion of the fiber composite component that is created by means of the molding core. 
         [0023]    According to a further embodiment of the invention, the molding core is arranged on a base part comprising semifinished fiber composite products and/or is at least partially surrounded by semifinished fiber products to form at least a portion of the fiber composite component. Consequently, base parts, for example skin shells, pressure domes, etc. with Ω stringers can be advantageously formed. As an alternative or in addition, separate fiber composite components, which are defined entirely in their form by the molding core, can also be produced. 
         [0024]    According to a further embodiment of the invention, after the ending of the vacuum fixing, the fixable filling material is removed from the core sleeve, in particular is shaken out, flushed out and/or extracted by suction. The removal of the vacuum means that the fixable filling material is no longer fixed and can be removed from the core sleeve out of an opening of the core sleeve. This may take place for example by means of a suction tube, which is pushed into the filling material and sucks it out of the core sleeve. As soon as filling material has been adequately removed from the core sleeve, it is possible, for example in the production of an Ω stringer, for the core sleeve to be drawn out from it in the longitudinal direction of the Ω stringer. Consequently, the core then no longer contributes to the overall weight of an aircraft and consequently no longer adversely affects its payload. 
         [0025]    According to one particular embodiment of the invention, the molding core is formed with at least one undercut. This undercut may lie in the longitudinal direction of the molding core, for example. Consequently, stringers of variable cross section in their longitudinal direction can be produced by means of such a molding core. It may also be advantageous that, after removal of the vacuum fixing and the filling material, the core sleeve can be advantageously removed from the molding core with an undercut. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0026]    The present invention is explained in more detail below using preferred exemplary embodiments and with reference to the attached figures of the drawings, in which: 
           [0027]      FIG. 1  shows a state of the method in the production of a molding core according to an exemplary embodiment of the present invention that is given by way of example; 
           [0028]      FIG. 2  shows a further state of the method in the production of the molding core according to the exemplary embodiment that is given by way of example; 
           [0029]      FIG. 3  shows yet another state of the method in the production of the molding core according to the exemplary embodiment that is given by way of example; 
           [0030]      FIG. 4A  shows a section along a sectional line A-A from  FIG. 3  according to the exemplary embodiment; 
           [0031]      FIG. 4B  shows a section along the sectional line A-A from  FIG. 3  according to a further exemplary embodiment of the present invention; 
           [0032]      FIG. 5  shows a state of the method in the production of a fiber composite component according to the exemplary embodiment that is given by way of example; and 
           [0033]      FIG. 6  shows a further state of the method in the production of a fiber composite component according to the exemplary embodiment that is given by way of example. 
       
    
    
       [0034]    In the figures, the same reference numbers refer to identical or functionally identical components unless otherwise stated. 
       DETAILED DESCRIPTION OF THE INVENTION  
       [0035]      FIGS. 1 to 3  show a number of states of the method according to an exemplary embodiment of the present invention. 
         [0036]    Arranged in a device  1  is a molding tool  2 . The molding tool  2  may have a cross section  3  such as that represented in  FIG. 4A . The cross section  3  of the molding tool  2  is formed such that it is essentially trapezoidal. A wall  4  of the molding tool  2  is provided with a multiplicity of small holes  5 , in order to prevent sucking in of the core sleeve  9 . The introduction of a ventilating film between the core sleeve  9  and the wall  4  to ensure an evenly distributed vacuum is similarly conceivable. 
         [0037]    The holes  5  are connected by means of a channel  6  to a hose  7 . The hose  7  is in turn connected to a first vacuum pump (not represented). The first vacuum pump makes it possible to generate a negative pressure in an interior space  8  of the molding tool  2 . 
         [0038]    A preferably elastic and/or flexible core sleeve  9  is introduced into the interior space  8  of the molding tool  2 . The core sleeve  9  is formed longitudinally with an upper opening with respect to the effective direction of gravitational force. The core sleeve  9  is preferably produced from a plastic, in particular a polyamide and/or a PTFE plastic. The core sleeve  9  is aligned with its longitudinal axis L along the longitudinal axis of the molding tool  2  and protrudes at its upper end, which has the opening  10 , from a closable opening  11  of the device  1 . 
         [0039]    Preferably, a portion of the core sleeve  9  that has the opening  10  is subsequently pushed through an opening  15  of a plate  12  and fastened there by means of a clamping ring  18 . 
         [0040]    According to the present exemplary embodiment, the closable opening  11  of the device  1  is closed by the plate  12  and the latter is closed in a sealing manner with respect to the interior space  8  of the molding tool  2  by means of suitable fastening means  13   a,    13   b.    
         [0041]    Before or after that, the first vacuum pump is switched on to generate a vacuum in the interior space  8  of the molding tool  2 . This has the effect that an outer surface  16  of the core sleeve  9  is sucked against an inner surface  17  of the molding tool. Consequently, the core sleeve  9  comes to lie with its outer surface  16   a  snugly against the inner surface  17  of the molding tool. The arrows  22  and  23  indicate the direction of flow of the air. 
         [0042]    The molding tool  2  is advantageously provided with such a number of holes  5  that the wall  16   a  of the core sleeve  9  lies completely against the inner surface  17  of the molding tool and against the plate  12  for closing the opening  11 . Instead of holes, in the case of a multipart molding tool  2 , gaps, in particular in the region of corners of the cross section of the molding tool, between the multiple parts may be used for applying the vacuum. A multipart molding tool  2  has the advantage of easy introduction of the core sleeve  9  and easy removal of the molding core  27  produced from the molding tool  2  and may be advantageously used for this invention. 
         [0043]    Subsequently, a vacuum-fixable filling material, for example quartz sand, is fed to the opening  10  of the core sleeve  9  by means of a feeding device  19 . The feeding device  19  may be formed for example by a hopper  20 , to which the quartz sand is fed by means of gravitational force and/or compressed air. 
         [0044]    The filling of the core sleeve  9  with quartz sand  21  is stopped as soon as the core sleeve  9  is adequately filled, that is to say for example just below the opening  10 . 
         [0045]    A vacuum sealing device  24  is coupled in a sealing manner to the opening  10  of the core sleeve. The vacuum sealing device may in this case have at least to some extent the same components as the feeding device  19 . 
         [0046]    In a further step, the first vacuum pump is switched off and then allows flowing of the air in the direction of flow opposite to that shown in  FIG. 2 , indicated by the arrow  25 . 
         [0047]    Then, a second vacuum pump (not represented) is switched on and sucks air out of the core sleeve, indicated by the arrow  26 , and consequently generates a second vacuum inside the core sleeve  9 . The wall  16   a  of the core sleeve  9  then stretches itself firmly around the fixable quartz sand  21  and compresses it. 
         [0048]    On account of the prior pretensioning of the core sleeve  9  by applying the vacuum to the outer surface  16   b  of the core sleeve  9  by means of the first vacuum pump, the core sleeve  9  does not form any folds during the contraction by means of the second vacuum. 
         [0049]    The compression of the quartz sand  21  has the effect that the individual grains of the quartz sand engage in one another, and consequently the molding core  27  is formed. As a result of the low compressibility of the quartz sand, the outer shape of the molding core  27  changes only minimally under the effect of the contraction of the wall  16   a  of the core sleeve  9 . 
         [0050]    In addition, the molding tool  2  may be shaken in the longitudinal direction of the core sleeve  9 , for example before the application of the second vacuum, in order to achieve compaction, and consequently improved strength. This increases the resistance to point loads of the molding core  27  that is formed. 
         [0051]    The molding core  27  that is formed has the cross section  28  shown in  FIG. 4A . 
         [0052]    In addition, as shown in  FIG. 4B , reinforcing elements in the form of corner profiles  29   a,    29   b  may be arranged within the core sleeve  9  before the application of the second vacuum. 
         [0053]    In a further step, according to the present exemplary embodiment, the opening  10  of the core sleeve  9  is closed, for example welded, by means of the vacuum sealing device. Alternatively, it may be provided that the vacuum remains connected for as long as vacuum fixing is required. Consequently, quality control can be carried out, for example the detection of leaks in the core sleeve  9 . If the vacuum sealing device  24  is then moved away, for example upwards, the second vacuum is retained within the core sleeve  9 . The plate  12  is then lifted off in a further method step. After the opening of the molding tool  2 , the dimensionally stable molding core  27  is removed from it and arranged on a panel  31  comprising a number of fiber mats. 
         [0054]      FIGS. 5 and 6  show further states of the method in the production of a fiber composite component according to the exemplary embodiment that is given by way of example. 
         [0055]    Two molding cores  27  of an approximately trapezoidal cross section, which have been produced in particular by the method described above, are arranged with their base  32  lying on a base part  31 . The base part  31  has at least one layer of a semifinished fiber product. 
         [0056]    In a further step, further semifinished fiber products are laid flat on the molding cores  27 . The semifinished fiber products  33   a,    33   b  thereby lie with a middle portion on the outer surface  16   b  of the molding cores  27  and with their ends on the base part  31 , that is to say for example on the skin of an aircraft. 
         [0057]    Various production methods may be used for producing the fiber composite component. The infusion process is preferably chosen, in order to introduce a matrix, that is to say for example epoxy resin, into the semifinished fiber products  31 ,  33   a,    33   b.  The prepreg process can similarly be used here. 
         [0058]    In a further step, the base part  31  is advantageously cured with the molding cores  27   a,    27   b  and the semifinished fiber products  33   a,    33   b  under the effect of heat and pressure in an oven or autoclave, depending on the process used. It is important here that the core sleeve  9  reliably withstands the process temperature and the process pressure. 
         [0059]    The semifinished fiber products  33   a,    33   b  cure for example in a suitable oven or autoclave (not represented) to form stringers  35   a,    35   b.  After curing, the at least partially cured fiber composite component  34  consequently has the two Ω stringers  35   a,    35   b.    
         [0060]    In a further method step given by way of example, after the core sleeve  9  is opened at the end face, a suction tube  38  may be introduced into the opening  10  of the core sleeve  9 . The suction tube  38  sucks the quartz sand  21  out of the core sleeve  9 , and consequently out of the stringer, illustrated in an exemplary fashion in  FIG. 6  for the stringer  35   a.    
         [0061]    Once the quartz sand  21  has been largely removed, the core sleeve  9  can be drawn out of the stringer in the longitudinal direction, represented in an exemplary fashion in  FIG. 6  for the stringer  35   b.  This is also possible whenever the stringer  35   a,    35   b  has undercuts in the longitudinal direction, that is to say recesses in the stringers that extend transversely to the longitudinal direction of the stringers. Removal of the core sleeve  9  or the molding cores  27  from the mold is consequently ensured in an easy way. After that, the fiber composite component  34  can be further processed or used directly. 
         [0062]    The invention is not restricted to the specific method represented in the figures for producing a fiber composite component in aerospace. 
         [0063]    For example, the individual sequence of individual method steps of the production method according to the invention can be changed in various ways. The form taken by the individual method steps can also be modified. For example, flushing out of the quartz sand instead of extraction by suction may be carried out before the removal of the core sleeve from the mold. The molding core may also be drawn or pressed out of the stringer as a whole in the longitudinal direction of the said stringer. 
         [0064]    Furthermore, the geometry of the molding core can be modified in various ways. 
         [0065]    Furthermore, it is also possible for a number of molding cores to be used to form a single molding core, around which fiber composite mats are placed. This allows a more complex geometry to be created by means of the multiplicity of molding cores and consequently more complex fiber composite components to be produced.