Abstract:
The present invention relates to a mould system and a method for net moulding of a co-cured integrated structure ( 1 ) comprising a skin ( 26 ) provided with stringers ( 4 ) and rib feet ( 2 ) and with a gap ( 6 ) between the stringers ( 4 ) and the rib feet ( 4 ). The mould system comprises a substantially rectangular frame ( 12 ) made of a low heat expanding material, a number of substantially rectangular boxes ( 8 ) made of a heat expanding material, connection plates ( 16 ) made of a heat expanding material and fastening means ( 14 ). By means of the connection plates ( 16 ) the gap ( 6 ) is obtained. A moulding assembly ( 64 ) is created by applying prepregs to the surfaces of the boxes ( 8 ), by connecting the boxes ( 8 ) to each other and by surrounding the connected  boxes ( 8 ) with the frame ( 12 ). The moulding assembly ( 64 ) is treated in a conventional way in an autoclave in order to cure the structure ( 1 ). Thereafter the frame ( 12 ), the boxes ( 8 ) and the connection plates ( 16 ) are removed.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a mould system and a method for net moulding of a co-cured integrated structure comprising a skin provided with stringers and rib feet. The structure can e.g. be used in the fuselage of aircrafts or in boats. 
       BACKGROUND ART 
       [0002]    When manufacturing integrated composite structures, such as skins reinforced by rib feet also called “shear ties” and stringers see  FIG. 1 , the rib feet can either be connected or not connected with a composite layer against the stringers. 
         [0003]    If the rib foot is not connected with the stringer, it is desirable to have a gap between the rib foot and the stringer in order to minimize the stress concentration in that area. It is known today to manufacture aircraft skin structures with a gap between the rib feet and the stringer. 
         [0004]    The structures can e.g. be made of carbon-fibre-reinforced polymer or carbon-fibre-reinforced plastic (CFRP or CRP or often simply carbon fibre). CFRP or CRP is a very strong and light fibre-reinforced polymer which contains carbon fibres. The polymer is most often epoxy, but other polymers, such as phenolic and Bismaleimide are sometimes used. 
         [0005]    In the known fabrication process prepregs are used to create the skin, the stringers and the rib feet. Prepreg is the shortened name used as a reference to composite fabrics that have been pre-impregnated with thermosetting resin but not yet permanently cured. These usually take the form of thin sheets consisting of a combination of a matrix (or resin) and fibre reinforcement. The fibres can have one direction (unidirectional reinforcement) or several directions (fabric reinforcement). If the fibres are unidirectional the solid structure is obtained by placing a number of prepregs on top of each other, with varying fibre direction. The role of the matrix is to support the fibres and bond them together in the composite material. It also keeps the fibres in their position and chosen orientation. The prepregs are mostly stored in cooled areas since activation is most commonly done by heat. 
         [0006]    A structure part is created in two steps, lay-up processing and autoclave processing. In the lay-up process the structure is formed. This is done by applying a number of prepregs, resulting in a stack of prepreg, on a mould. A vacuum bag is placed and sealed over the prepregs and the mould and all air under the bag is evacuated. In the autoclave process the mould covered with the stack of prepreg and the vacuum bag is put into and treated in an autoclave. An autoclave is a pressure vessel which provides the curing conditions for the composite where the application of vacuum, pressure, heat up rate and cure temperature is controlled. In the autoclave a vacuum is still maintained under the vacuum bag, and a heat and a high pressure is applied to the composite structure. 
         [0007]    In the existing fabrication process of aircraft structures, the skin, the stringers and the rib feet are formed and cured separately. The cured stringers and rib feet are bonded to the skin and trimmed to get the desired form comprising the gap between the stringers and the rib feet. 
         [0008]    The problem with the solution according to the prior art is that the trimming and bonding operation is time consuming. There is thus a need for a faster and more efficient way of manufacturing the reinforced structures with a gap between the stringers and the rib feet. 
       DEFINITIONS 
       [0009]    It shall be noted that through this application “longitudinal” refers to the direction being parallel to the stringers and “transversal” refers to the direction being parallel to the rib feet. The stringers are defined as the longitudinal reinforcements and the rib feet are defined as the transversal reinforcements of the structure. 
       SUMMARY 
       [0010]    The object of the present invention is to provide an inventive method for net moulding a structure for e.g. aircrafts, boats or other crafts comprising a skin provided with at least two stringers and at least one rib foot and a gap between the stringers and the at least one rib foot, where the previously mentioned problems are avoided. This object is achieved by mould system according to claim  1 . The claim reveals a mould system for net moulding of a co-cured structure comprising a skin provided with at least two stringers and at least one rib foot and a gap between the stringers and the at least one rib foot. 
         [0011]    The mould system comprises a substantially rectangular frame made of a low heat expanding material. The frame comprises a first longitudinal frame part, a second longitudinal frame part, a first transversal frame part and a second transversal frame part. 
         [0012]    The mould also comprises at least one row unit wherein each row unit comprises a row of at least two substantially rectangular boxes made of a heat expanding material. Each box has a bottom, a first longitudinal side wall, a second longitudinal side wall, a first transversal side wall and a second transversal side wall. The at least two boxes in the row are positioned such that the first transversal side wall of a first box is facing the second transversal side wall of a second box. 
         [0013]    The mould further comprises at least two connection plates connecting the at least two boxes in the row of boxes along the first longitudinal side walls and along the second longitudinal side walls by means of fastening means. Each connection plate is adapted to be positioned on the first longitudinal side walls or on the second longitudinal side walls of at least two adjacent boxes in the row, and each connection plate is adapted to be connected to each of the longitudinal side walls on which the connection plate is positioned. Each connection plate is also adapted to extend along at least a part of each of the first longitudinal side walls on which the connection plate is positioned. 
         [0014]    A first longitudinal side of the at least one row unit is constituted by the first longitudinal side walls of the boxes in the row unit and by the connection plates being connected thereto and a second longitudinal side of the row unit is constituted by the second longitudinal side walls of the boxes in the row unit and of the connection plates being connected thereto. An external surface of the first longitudinal side walls of the boxes and of the second longitudinal side walls of the boxes and an external surface of the connection plates in the at least one row unit have a form such that the first longitudinal side and the second longitudinal side of the at least one row unit form flat surfaces. 
         [0015]    The advantage with the mould system is that a whole structure can be formed and cured as one net moulded unit, which minimises the time for trimming of the structure, as will be described in the method. 
         [0016]    Said object is further achieved by a mould system according to first and a second embodiment. This claim reveals a mould system wherein a flat surface of a longitudinal side of the at least one row unit may be obtained by at least one box in the row unit having a form such that an external surface of at least one of the corresponding longitudinal side walls of the box is flat. 
         [0017]    The advantage with the mould system according to this embodiment is that the length of the connection plates is flexible and that one connection plate can connect two or more boxes to each other. 
         [0018]    Said object is further achieved by a mould system according to a third embodiment. This claim reveals a mould system wherein a flat surface of a longitudinal side of the at least one row unit may be obtained by at least one box in the row unit having a form such that at least one of the corresponding longitudinal side walls of the box has a recess in at least one of its respective ends, wherein a depth of the recess is equal to a thickness of a connection plate. 
         [0019]    The advantage with the mould system according to this embodiment is that the connection plates can have a specified length for connecting two boxes to each other regardless of the longitudinal length of the boxes. 
         [0020]    Said object is further achieved by the mould system wherein the boxes are made of aluminium. 
         [0021]    Said object is further achieved by the mould system wherein the frame is made of invar. 
         [0022]    Said object is further achieved by the mould system wherein the boxes are provided with threaded holes for insertion of a tool for removal of the boxes. 
         [0023]    Said object is further achieved by the mould system wherein the height of each connection plate is substantially equal to or higher than the height of the at least two boxes. 
         [0024]    Said object is further achieved by a method for net moulding of a co-cured structure comprising a skin provided with at least two stringers and at least one rib foot and a gap between the stringers and the at least one rib foot. The method comprises the following steps:
       providing at least two substantially rectangular boxes made of a heat expanding material, wherein each box has a bottom, a first longitudinal side wall, a second longitudinal side wall, a first transversal side wall and a second transversal side wall,   applying a first layer of prepregs onto an underside of the bottom and on an external side of at least one of the first transversal side wall and the second transversal side wall of each of the at least two rectangular boxes thereby obtaining at least two boxes having a bottom and at least one of the first transversal side wall and the second transversal side wall covered with a first layer of prepregs,   forming at least one longitudinal row unit by, for each row unit
           positioning at least two of said boxes covered with a first layer of prepregs next to each other, with a covered transversal side wall of a first box facing a covered transversal side wall of a second box, thereby obtaining a row of boxes,   connecting the at least two boxes in the row of boxes along the first longitudinal side walls and along the second longitudinal side walls by means of at least two connection plates and fastening means,   forming a first longitudinal side of the at least one row unit constituted by the first longitudinal side walls of the boxes in the row unit and of the connection plates being connected thereto, and forming a second longitudinal side of the row unit constituted by the second longitudinal side walls of the boxes in the row unit and of the connection plates being connected thereto,
               wherein an external surface of the first longitudinal side walls and of the second longitudinal side walls of the boxes and an external surface of the connection plates in the at least one row unit have a form such that an external surface of the first longitudinal sides and an external surface of the second longitudinal sides of the at least one row unit form flat surfaces,   
               forming a bottom of the at least one row unit constituted by the bottom of the boxes in the row unit,   
           applying a second layer of prepregs onto an external surface of the bottom and an external surface of the first longitudinal side and of the second longitudinal side of the at least one row unit, thereby obtaining at least one row unit having a bottom, the first longitudinal side and the second longitudinal side covered with a second layer of prepregs,   forming of the skin by applying a third layer of prepregs onto a plate made of a low heat expanding material,   positioning the at least one covered row unit on the skin,
           when more than one row unit, positioning a first row unit next to a second covered row unit with the first longitudinal side of the first row unit being positioned next to the second longitudinal side wall of the second row unit,   
           providing a substantially rectangular frame made of a low heat expanding material, wherein the frame comprises a first longitudinal frame part, a second longitudinal frame part, a first transversal frame part and a second transversal frame part,   positioning the frame on the skin, such that it surrounds the at least one covered row unit and such that the first longitudinal frame part is positioned next to a first longitudinal side of at least one covered row unit and that the second longitudinal frame part is positioned next to a second longitudinal side of at least one covered row unit, thereby obtaining a moulding assembly comprising the plate, the frame, the at least one row unit, and the different layers of prepregs,   loosening of the fastening means,   covering the moulding assembly with a vacuum bag for conventional curing,   positioning the moulding assembly and the vacuum bag in an autoclave and exerting a pressure and applying a heat,   removing the frame, the connection plates and the boxes.
 
Said object is further achieved by the method according wherein after the step of positioning the at least one covered row unit on the skin the method comprises the further step of:
   applying a fourth layer of prepregs onto an external surface of a bottom and on an inner side wall of the first longitudinal frame part and of the second longitudinal frame part.       
 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0044]    The present invention will now be described in detail with reference to the figures, wherein: 
           [0045]      FIG. 1 .  FIG. 1  shows an integrated structure with a gap between the stringers and the rib feet, created in accordance with the present invention; 
           [0046]      FIG. 2 .  FIG. 2  shows an enlarged view of the gap between the stringers and the rib feet. 
           [0047]      FIG. 3 .  FIGS. 3A and 3B  shows a side view of a number of boxes covered with a first layer of prepreg according to the invention. 
           [0048]      FIG. 4 .  FIGS. 4A and 4B  shows a view seen from above of a number of boxes covered with a first layer of prepreg according to the invention. 
           [0049]      FIG. 5 .  FIG. 5  shows a view seen from above of a row unit according to a first embodiment of the invention. 
           [0050]      FIG. 6 .  FIG. 6  shows a cross section view of the row unit of  FIG. 5  seen from the cut A, according to the invention. 
           [0051]      FIG. 7 .  FIG. 7  shows a view seen from above of a row unit covered with a second layer according to a first embodiment of the invention. 
           [0052]      FIG. 8 .  FIG. 8  shows a cross section view of the row unit of  FIG. 7  seen from the cut B, according to the invention. 
           [0053]      FIG. 9 .  FIG. 9  shows the mould system according to a first embodiment of the invention seen from above. 
           [0054]      FIG. 10 .  FIG. 10  shows a cross section view of the mould system of  FIG. 9  seen from the cut C, according to the invention. 
           [0055]      FIG. 11 .  FIG. 11  shows a cross section view of the mould system of  FIG. 9  seen from the cut D, according to the invention. 
           [0056]      FIG. 12 .  FIG. 12  shows a view seen from above of a row unit according to a second embodiment of the invention. 
           [0057]      FIG. 13 .  FIGS. 13A and 13B  show a view seen from above of a row unit according to a third embodiment of the invention. 
           [0058]      FIG. 14 .  FIG. 14  shows a perspective view of the mould system with two row units. 
           [0059]      FIG. 15 .  FIG. 15  shows a flowchart of the moulding method according to the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0060]      FIG. 1  shows an integrated structure  1  which is manufactured by using the mould system and the method of the present invention. The structure can be used in e.g. aircrafts but it can be used also in other applications such as e.g. boats or crafts. The structure comprises a skin  26 . The skin  26  is reinforced with longitudinal stringers  4  and transversal rib feet  2 , which projects from the skin  26 . The stringers  4  are substantially perpendicular to the rib feet  2 . The rib feet  2  and the stringers  4  are spaced from each other by a gap  6 . This gap  6  makes the structure more flexible and prevents the structure  1  from breaking when exposed to high forces.  FIG. 2  shows an enlarged view of the gap  6  between the rib feet  2  and the stringers  4 . 
         [0061]      FIGS. 3-11  aim at describing the manufacturing method of the integrated structure  1  and the mould system that is used for the manufacturing method according to a first embodiment of the present invention.  FIGS. 3 and 4  show the first step of creating the rib feet. This step is denoted by step  1  in  FIG. 15 .  FIG. 3A  shows a side view of four boxes  8  and  FIG. 4A  shows the four boxes  8  from above.  FIG. 3B  show a side view of two boxes  8  and  FIG. 4A  shows the two boxes  8  from above. The boxes  8  are hollow with the opening upwards. Each box  8  comprises a bottom  30  and four side walls, a first longitudinal side wall  46 A, a second longitudinal side wall  46 B, a first transversal side wall  44 A and a second transversal side wall  44 B. The boxes  8  are made of a heat expanding material, like e.g. aluminium or steel. A number of prepregs forming a first layer  10  is applied on the boxes. The first layer  10  is applied on the underside of the bottom  30  of the boxes  8  and on the external side of at least one of the transversal side walls  44 A and  44 B of the boxes  8 , thereby forming boxes that have a prepreg covered bottom surface  30  and at least one prepreg covered transversal side wall  44 A or  44 B. In all figures a number of prepregs positioned one upon the other and forming a thick layer (stack of prepreg) is illustrated by a thick, black line. In all figures there is a small clearance between the boxes  8  and the layers. This clearance only serves for illustration purpose in order to make the figures clearer. There is no clearance in the actual embodiments of the invention. 
         [0062]    The second step of the manufacturing method is to create at least one row  18  of boxes. This step is denoted by step  2  in  FIG. 15 . A row  18  of boxes  8  is formed by positioning a number of covered boxes  8  next to each other, just as in  FIGS. 3A ,  3 B,  4 A and  4 B, so that the covered transversal side wall  44 A of a first box  8  and the covered transversal side wall  44 B of a second box  8  are facing each other. The first layers  10  covering the first transversal side wall  44 A of the first box and the second transversal side wall  44 B of the second box  8  will later form one rib foot. The number of boxes  8  in one row  18  depends on the desired number of rib feet  2 . Two boxes  8  are necessary in order to obtain one rib foot, three boxes  8  makes two rib feet and so on. (Compare  FIGS. 3A and 4A  with  FIGS. 3B and 4B .) The row  18  of boxes  8  has two transversal sides  47 A and  47 B (constituted by the covered/non covered transversal side wall  44 A of a first end box and the second covered/non covered transversal side wall  44 B of a second end box of the row), a bottom  49  (constituted by the covered bottom  30  of the boxes in the row), and two longitudinal sides, a first longitudinal side  45 A and a second longitudinal side  45 B (constituted by the longitudinal side walls  46 A and  46 B of the boxes  8 ). 
         [0063]    The third step is shown in  FIGS. 5 and 6 . This step is denoted by step  3  in  FIG. 15 . The aim of this step is to connect the boxes to each other in order to obtain a rigid row unit  20 . This is performed by positioning two long connection plates  16  in the form of straight rulers on each of the respective longitudinal sides  45 A and  45 B of the row and connects each connection plate  16  to each of the boxes  8  by means of fastening means  14 . The fastening means  14  can e.g. be screws or bolts. In addition to making a rigid row unit  20  the connection plates  16  creates the gap  6  between the stringers  4  and the rib feet  2 . Without the connection plates  16  the stringers  4  would abut against the rib feet  2 . Therefore the connection plates  16  are very fundamental in the present invention. The row unit  20  has two transversal sides, a first transversal side  47 A and a second transversal side  47 B (the same sides as for the row of boxes), a bottom  49  (the same bottom as for the row of boxes), and two longitudinal sides, a first longitudinal side  48 A and a second longitudinal side  48 B. The first longitudinal side  48 A of the row unit  20  is constituted by the first longitudinal side walls  46 A of the boxes  8  and of the connection plate  16  connecting the boxes  8  along these longitudinal side walls  46 A. The second longitudinal side  48 B of the row unit  20  is constituted by the longitudinal side walls  46 B of the boxes  8  and of the connection plate  16  connecting the boxes along these longitudinal side walls  46 B. The connection plates  16  are made of a heat expanding material such as aluminium or steel. According to this embodiment the length of the connection plates  16  is substantially equal to the length of the row unit  20 . The height of the connection plates  16  is substantially equal to the height of the boxes  8  and the thickness of the connection plates  16  is substantially equal to the width of the desired gap  6 . The connection plates  16  in this embodiment are positioned on and cover each of the longitudinal sides  45 A and  45 B of the row  18  of boxes  8 . They connect the boxes  8  to each other, and thereby create a rigid row unit  20 . This has the advantage that several boxes  8  can be handled as one unit, which makes it easier to apply additional prepregs. Depending on the desired number of stringers  4  one or several rigid row units  20  can be created. 
         [0064]    In the fourth step of the manufacturing method a second layer  11  of prepregs is applied on the rigid row units  20 . This step is denoted by step  4  in  FIG. 15 . For each row unit  20  the second layer  11  is applied on the bottom  49  of the row unit  20  (onto the first layer  10  covering the bottom surface  30  of the boxes  8 ) and on the first and the second longitudinal sides  48 A and  48 B of the rigid row unit  20 . Since the connection plates  16  are positioned on the longitudinal sides  48 A and  48 B of the row unit  20 , this means that the second layer  11  is applied on the bottom  49  of the row unit  20  and on each of the connection plates  16 . The result of this step is that the bottom  49  of the row unit  20  and the first and second longitudinal sides  48 A and  48 B of the row unit  20  are covered by prepregs.  FIGS. 7-8  illustrate how this step is performed. 
         [0065]    The fifths step in the manufacturing procedure is to create the skin  26 . This step is denoted by step  5  in  FIG. 15 . The skin  26  is created by placing a number of prepregs on the surface of a plate  40  made of a low heat expanding material. The plate  40  can have a plain surface but the surface can also be curved. The form of the plate  40  depends on the desired form of the skin  26 . The prepregs forms a third layer  50  constituting the skin  26 . This third layer is shown in  FIGS. 10 and 11 . 
         [0066]    In the sixth step the rigid row units  20  covered with a second layer  11  are put onto the third layer  50  constituting the skin  26 . This step is denoted by step  6  in  FIG. 15 . If there are several rigid row units  20 , the rigid row units  20  are positioned on the third layer  50  such that the first longitudinal side  48 A of a first row unit  20  is positioned next to the second longitudinal side  48 B of a second row unit  20 . This is illustrated in  FIG. 9 . The figure also illustrates how the gaps  6  are obtained by means of the connection plates  16 . 
         [0067]    In addition to the mentioned boxes  8 , the connection plates  16  and the fastening means  14 , the mould system comprises a substantially rectangular frame  12  made of a low heat expanding material.  FIG. 9  shows the frame  12 , the rigid row units  20  and the two first layers  10  and  11  seen from above. The frame  12  comprises two longitudinal frame parts, a first longitudinal frame part  54 A and a second longitudinal frame part  54 B, a first transversal frame part  56 A and a second transversal frame part  56 B, wherein each frame part  54 A,  54 B,  56 A and  56 B have a bottom  60  and at least one side wall  62  (see  FIGS. 10 and 11 ). The at least one side wall  62  has approximately the same height as the longitudinal side walls  46 A and  46 B of the boxes  8  but it can also be higher than the longitudinal side walls  46 A and  46 B of the boxes  8 . The frame  12  can either be made as one unit or it can comprise of several parts that are connected to each other to make one unit. 
         [0068]    In step  7  a fourth layer  52  of prepregs is applied on the underside of the bottom  60  and on the inner side of the side wall  62  of at least one of the first longitudinal frame part  54 A and the second longitudinal frame part  54 B. This step is denoted by step  7  in  FIG. 15 . The result of the step is that the underside of the bottom  60  and the inner side of the side wall  62  of at least one of the longitudinal frame parts  54 A and  54 B are covered by prepregs. The step can be omitted, which is shown in  FIG. 15  by the dotted lines. However, this leads to thinner end stringers  4 . 
         [0069]    In step  8  the frame  12  is positioned on the third layer  50  forming the skin. This step is denoted by step  8  in  FIG. 15 . The frame  12  surrounds the row units  20  so that the prepreg covered longitudinal frame part  54 A is positioned next to the prepreg covered longitudinal side  48 A of a first end row unit  20  and the prepreg covered longitudinal frame part  54 B is positioned next to the prepreg covered longitudinal side  48 B of a second end row unit  20 . 
         [0070]      FIG. 10  shows a cross section view seen from a cut C and  FIG. 11  shows a cross section view seen from a cut D in  FIG. 9 . As seen from  FIG. 10 , the stringers  4  are formed by the second layer  11  covering a longitudinal side  48 A of a first row unit  20  and by the second layer  11  covering a longitudinal side  48 B of second row unit  20  being positioned next to the first row unit  20 . Alternatively the stringers  4  are formed by the second layer  11  of one end row unit  20  and the fourth layer  52  on the inner side of the side wall  62  of a longitudinal frame part  54 A or  54 B. It is important that the external surfaces of the longitudinal sides  48 A and  48 B of the row units  20  is flat. If not, the stringers  4  will not be straight. 
         [0071]    Before covering the now prepared moulding assembly  64 , i.e. the frame  12 , the plate  40 , the row units  20  and the layers  10 ,  11 ,  50 ,  52  of prepregs, with a vacuum bag and placing the moulding assembly in an autoclave for curing of the structure, the fastening means  14  are loosened. The reason for this is that the heat expanding boxes  8  shall be able to move and float freely within the rigid frame  12  during the hardening process. In this way the different layers  11  and  12  and  50  and  52  are forced towards the rigid frame  12  by the expansion of the heat expanding boxes  8  and the heat expanding connection plates  16 , and remaining air between the prepregs can be pressed out, making the structure  1  solid and hard. These steps are denoted by steps  9 - 11  in  FIG. 15 . 
         [0072]    The last step is to remove the frame  12 , the connection plates  16  and the boxes  8 . The frame  12  can be removed easily but the connection plates  16  and the boxes  8  are more difficult to remove. Due to their expansion in the autoclave they are now pressed towards the carbon-fibre structure  1 . In order to facilitate the removal, the boxes  8  can be provided with threaded holes  32  shown in  FIG. 14  for insertion of a tool for removal of the boxes  8 . 
         [0073]    According to the first embodiment the connection plates  16  are in the form of rulers and their length is substantially equal to the length of the row units  20 . 
         [0074]    According to a second embodiment the connection plates  16  are designed as shorter rulers and a single connection plate  16  does not cover the entire longitudinal side  48 A and  48 B of a row unit  20 . This has the advantage that the connection plates  16  are more flexible. Instead of positioning a connection plate  16  on each longitudinal side  45 A and  45 B of a row of boxes  8  as in the first embodiment and connect all boxes  8  in the row  18  of boxes  8  to the two connection plates  16 , several connection plates  16  connect the boxes  8  in a row of boxes  8  along the respective longitudinal sides  45 A and  45 B. Each connection plate  16  is adapted to connect one of the respective longitudinal sides  45 A,  45 B of two or more boxes  8  in a row  18  of boxes  8 , but not necessarily of all boxes  8  in the row  18 . This embodiment is shown in  FIG. 12 . As can be seen in the figure six connection elements  16  are used for fixing four boxes  8  to each other in one row unit  20 . Three connection elements  16  have the total length which is substantially equal to the length of the row unit  20 . According to this embodiment the four boxes  8  in  FIG. 12  could also have been connected by e.g. four connection elements  16 , where two connection elements  16  connect three adjacent boxes  8  to each other on each of the longitudinal side  45 A and  45 B of the row  18  of boxes  8 , and two connection elements  16  connect said three boxes  8  to the fourth box  8  on each longitudinal side  45 A and  45 B of the row  18  of boxes  8 . In  FIG. 12  there is a clearance between the connection plates  16 . This clearance is only for illustration. In the invention the connection plates  16  abut against each other. 
         [0075]    Just as in the first embodiment the row unit  20  has two transversal sides, a first transversal side  47 A and a second transversal side  47 B (the same sides as for the row of boxes), a bottom (the same bottom as for the row of boxes), and two longitudinal sides  48 A and  48 B. The longitudinal sides  48 A and  48 B of the row unit  20  are, just like in the first embodiment, constituted by the connection plates  16  and the longitudinal side walls  46 A and  46 B of the boxes  8 . 
         [0076]    According to a further embodiment each connection plate  16  only connects two adjacent boxes  8  to each other. Each of the boxes  8  in a row unit  20  has a recess  58  in the ends of at least one of the first and second longitudinal side walls  46 A and  46 B. Each recess  58  is adapted to receive a part of a connection plate  16 . The recesses can be seen in  FIG. 13B . A perspective view if the mould system according to this embodiment is shown in  FIG. 14 . As mentioned before it is important that the external surfaces of the longitudinal sides  48 A and  48 B of the row units  20  is flat. If not, the stringers  4  will not be straight. By making the depth of the recesses  58  equal to the thickness of the connection plates  16  the external surface of the connection plates  16  aligns with the external surface of the boxes  8  when the connection plates  16  are connected to the boxes  8 . 
         [0077]    The invention is not limited to the described embodiments. The embodiments can be combined and/or further developed without limiting the scope of the invention. The number of boxes  8  in a row unit can be varied according to the desired number of rib feet  2 . The number of row units  20  can be varied according to the desired number of stringers. The three embodiments can be combined so that each row unit  20  can be provided with connection plates  16  according to one embodiment on one longitudinal side  48 A and according to another embodiment on the other longitudinal side  48 B. The embodiments can even be combined within one longitudinal side  48 A or  48 B as seen in  FIG. 13A . 
         [0078]    The invention is not limited to the specific flowchart presented, but includes all variations within the scope of the present claims. The internal sequence of steps for arriving at the structure can of course be varied according to the situation. 
         [0079]    Reference signs mentioned in the claims should not be seen as limiting the extent of the matter protected by the claims, and their sole function is to make claims easier to understand. 
         [0080]    As will be realised, the invention is capable of modification in various obvious respects, all without departing from the scope of the appended claims. Accordingly, the drawings and the description thereto are to be regarded as illustrative in nature, and not restrictive.