Abstract:
A method of forming a unitary, composite structural member, and a member formed in accordance with the process. The process involves stitching a plurality of warp knit panel sections together to generally form a plurality of independent panel sections. The sections are placed within the dies of a molding tool such that a rib portion of each section aligns. Inflatable bladders are then slipped into voids formed in between the various panel sections of the assembly. The bladders are inflated to hold the panel sections in the shape of the final product and to hold the rib portions in contact with one another. Resin is then infused into the panels that make up the assembly. The assembly is then cured. When the panels are removed from the molding tool a unitary, complexly shaped, composite structural member is formed. The invention provides the advantage of not requiring any subsequent manufacturing step such as bonding or mechanical fastening of two or more sub-panels in order to form the finished structural part.

Description:
FIELD OF THE INVENTION  
         [0001]    This invention relates to molding processes and apparatus&#39;, and more particularly to a molding process and apparatus for producing large and complex structural components especially well suited for aircraft structural components through a single step molding process.  
         BACKGROUND OF THE INVENTION  
         [0002]    Due to the size and complexity of structural components such as aircraft wings, sections of aircraft fuselage, etc., the formation of such structures using composite materials in a single step molding procedure has historically not been possible. Until recently, a process capable of holding critical dimensional features within narrow tolerance ranges for large complex composite structures did not exist.  
           [0003]    In previous manufacturing operations, typically the complex part is broken down into multiple pieces of simple design that are subsequently assembled into a single large complex structure. The cost to manufacture the structure thus increases significantly through the additional manufacturing steps needed to fabricate separately and then assemble (i.e., often bond adhesively or mechanically attach) two or more independent component parts to form a single overall structure.  
           [0004]    Accordingly, it would be highly desirable to provide a molding process in which separate structural elements (i.e., such as skins and stiffening elements) could be secured to one another in a continuous fashion in a preliminary manufacturing step and then subsequently molded in a single molding step to form a unified structure. Such a process would significantly reduce the cost and time associated with producing complex built-up structural components of military and commercial aircraft such as wing boxes and other internally-stiffened structural components which heretofore have been manufactured through a plurality of separate molding processes to produce independent parts, which are then subsequently secured together by bonding, riveting or other mechanical means. Such a single step molding process to produce unified composite assembly structure also offers the potential to increase damage tolerance for the structure in view of the additional strength that would be expected from molding several independent component parts together in a single molding step to produce joints having more uniform load paths that are absent of stress concentrations over what would be possible with subsequently bonded or mechanically-attached part interfaces.  
         SUMMARY OF THE INVENTION  
         [0005]    The present invention is directed to a molding process in which two or more dry fiber material preforms are stitched together to form perform assemblies that represent an approximate shape of the final component. The stitched-together preform assemblies are then placed within a suitable tool that maintains the part loft. A plurality of inflatable bladders are then placed inside the preform assemblies to serve as internal vacuum bags. Inflating the bladders serves to urge the dry fiber material forms against interior surfaces of the tool. In one preferred embodiment each dry material form comprises a portion of rib, and the two portions are urged into contact with one another as the bladders are inflated.  
           [0006]    After inflating the bladders, the entire stitched-together assembly is then infused with resin. Various areas of the stitched-together preforms, such as possibly ribbed elements of the assembly, are formed by balancing the force applied to the preform through bladder pressure.  
           [0007]    Once the stitched-together dry fiber preform assembly is fully infused with resin, it is then cured inside a suitable oven for a predetermined period of time to allow the resin infused into the stitch-together assembly to thoroughly cure. When removed from the tool, the stitched-together assembly forms a finished, unitary part. The bladders may then be extracted through small holes formed at various portions of the assembly such as holes within rib webs of the assembly if the assembly includes such webs. The resulting unitized structure thus forms a single piece structural assembly that is co-cured and reinforced with z-direction stitching. Accordingly, no subsequent manufacturing steps involving bonding of individual molded details or elements or mechanical fastening of individual molded panels together is required. The co-curing and the stitching of the independent dry fiber material forms together to create a single piece unit further eliminates local stress concentrations at the interfaces of independent component sections of the assembly and results in a more durable, light weight structure that is especially well suited for use in high-performance aircraft manufacturing applications.  
           [0008]    Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples are intended for purposes of illustration only and are not intended to limit the scope of the invention. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]    The present invention will become more fully understood from the detailed description and the accompanying drawings, wherein:  
         [0010]    [0010]FIG. 1 is a simplified perspective view of a section of a part manufactured in accordance with the method of the present invention, wherein the part represents an exemplary stiffened box structure;  
         [0011]    [0011]FIG. 2 a  illustrates the orientation of the bladders within one half of a molding tool, with the upper half of the molding tool and the upper panel section removed to better illustrate the bladders;  
         [0012]    [0012]FIGS. 2 b - 2   e  illustrate the independent steps of molding the independent preform details or elements that form the overall assembly in a single molding step; and  
         [0013]    [0013]FIG. 2 f  is a simplified illustration of the molding tool residing within an oven while being cured. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0014]    The following description of the preferred embodiment(s) is merely exemplary in nature and is in no way intended to limit the invention, its application, or uses.  
         [0015]    Referring to FIG. 1, there is shown a structure  10  formed in accordance with a preferred method of the present invention. In this example, structure  10  forms a stiffened box structure, but it will be appreciated immediately that the method of the present invention is not limited to the manufacturer of just stiffened box structures or even just aircraft subassemblies. The method of the present invention can be used to form unified, composite structural assemblies that are suitable for use in a wide variety of applications and for forming a wide variety of structural components. The method of the present invention, however, is especially useful in aircraft manufacturing applications as it significantly reduces the cost and time associated with manufacturing large complex structural parts which heretofore could only be manufactured with a plurality of manufacturing steps often involving bonding and/or riveting steps subsequent to the molding of independent sections of a structural assembly.  
         [0016]    With further reference to FIG. 1, the exemplary unified, composite assembly  10  can be seen to include skin portions  12  and  14  which are secured by stitching  16  and  18 , respectively, to portions  20  and  22 , respectively of a ribbed portion  24  of a structural rib assembly  26 . Each portion  20  is comprised of at least a pair of panel portions  20   a  and  20   b  which are stitched together by stitching  28 . Similarly, panel portion  22  includes portions  22   a  and  22   b  that are secured together by stitching  30 . Stitching  16  secures panel portions  20   a  and  20   b  to skin  12 , while stitching  18  secures panel portions  22   a  and  22   b  to skin  14 .  
         [0017]    A particularly unique feature of the rib assembly  26  is the “tongue-in-groove” arrangement by which portions  20  and  22  are coupled together. In this example, portion  22  includes sections  22   c  and  22   d  which receive an end portion  32  of section  28  of panel  20 . Further stitching  34  is then used to secure the tongue portion  32  within panel sections  22   c  such that the single cocured rib  26  is formed. Fillet inserts  36  are also typically inserted into the voids formed in between skin  12  and section  28 , and in between skin  14  and section  30  prior to stitching the skins  12  and  14  to their respective panel sections  20  and  22 .  
         [0018]    Stitching  16 ,  18 ,  28 ,  30  and  34  preferably comprises a common thread material such as Kevlar™. It will be appreciated, however, that suitable thread material may be employed such as Vectran™.  
         [0019]    It will also be appreciated that the exemplary part  10  shown in FIG. 1 will typically incorporate two or more rib structures  26 , thus producing a plurality of internal void sections, three of which,  38 ,  40  and  42  are shown in FIG. 1. Of course, the more rib sections  26  that are included, the more voids that will be produced. However, the composite structural assembly  10  is not limited to the use of only two rib sections  26 , but may employ only a single rib  26  or possibly three or more ribs  26 . Furthermore, the length of the skins  12  and  14  are limited only by the dimensions of the tool that is used to form the structural assembly  10 , as will be described in greater detail momentarily.  
         [0020]    Each of the skins  12  and  14 , and the independent component parts of the rib  26  are formed from a dry fiber material form, typically warp knit fabric. As will be understood, the dry materials forms are typically not impregnated with resin to facilitate the stitching process by avoiding fiber breakage during needle penetration.  
         [0021]    With further reference to FIG. 1, the ribs  26  may be each formed such that they each include at least one opening  41  for allowing an inflatable bladder, which will be explained momentarily, to be removed after the molding process that forms the composite structure  10  is completed. Alternatively, the openings  41  could be formed in end walls of the part (i.e., surfaces of the part perpendicular to the ribs  26 ). The precise shape of the part being formed, and the number of internal voids it incorporates, will determine in part where the opening(s)  41  are most effectively placed.  
         [0022]    Referring now to FIGS. 2 a - 2   f , a description of the molding process used to form the unified composite structure  10  will be described. With reference to FIGS. 2 a  and  2   b , panel sections  12  and  14  are placed adjacent one another such that sections  28  and  30  of ribs  26  are aligned longitudinally with one another. End portions  32  of each section  28  are placed between sections  22   c  and  22   d  of rib section  30  (FIG. 2 b ). A plurality of deflated bladders  44 ,  46  and  48  are placed in the void sections  38 ,  40  and  42  respectively. The panel sections  12  and  14  are then placed within dies  50  and  52  of a suitable rigid molding tool  54 . It will be appreciated, however, that the just-described steps could be reversed, meaning that the skin panels  12  and  14 , together with their rib sections  28  and  30 , could be placed within the dies  50  and  52  initially, and then aligning the sections  28  and  30  as needed within the molding tool dies  50  and  52  to place the skin panels  12  and  14  in position to be molded together to form a single, unitary part. FIG. 2 a  shows the orientation of the bladders  44 ,  46  and  48  within the lower die  52 . In this example, openings  55  enable open ends  44   a ,  46   a ,  48   a  of the bladders  44 ,  46  and  48 , respectively, to extend through a wall  53  of the die  52  so that the bladders can be inflated. Ends  44   a ,  46   a  and  48   a  are sealed around the openings  55  via suitable adhesive or other structure which permits the ends  44   a ,  46   a ,  48   a  to be removed from the tool  54  when the process is completed. In this example, the composite structure  10  can be seen to include lateral end panels  57 .  
         [0023]    With further reference to FIGS. 2 b - 2   d , after the skin panels  12  and  14  are longitudinally aligned, the inflatable bladders  44 ,  46  and  48  are inflated by creating a vacuum within the interior area of the mold tooling  54 . This allows the outside ambient pressure (1 atm) to backfill the bladders with air and fully inflate them. Alternatively, a pressurized air source could be used to inflate the bladders  44 ,  46 ,  48 , as indicated in phantom in FIG. 2 a . With either method, the pressure that the inflated bladders  44 ,  46 ,  48  apply to the surfaces of the skin panels  12  and  14  can be closely controlled to ensure that the panels  12 ,  14  are held firmly against interior surfaces of the tool  54 .  
         [0024]    The bladders  44 ,  46  and  48  may be formed from a variety of light weight, flexible materials, but in one preferred form comprise latex bladders. The bladders  44 ,  46  and  48  are shaped such that when they are substantially or fully inflated they engage the intersurfaces of the skin panels  12  and  14  and the surfaces of sections  28  and  30  of each of the ribs  26 . The bladders  44 ,  46  and  48  urge the flaps  22   c  and  22   d  of each section  30  of each rib  26  against the end portion  32  of each section  28  of each rib  26  such that the sections  22   c  and  22   d  are effectively clamped against opposing surfaces of the end portion  32  of each rib  26 . The bladders  44 ,  46  and  48  further serve to maintain the skins  12  and  14  pressed against interior surfaces of the die portions  50  and  52  of the tool  54 .  
         [0025]    Referring to FIG. 2 d , the bladders are shown substantially fully inflated. The flap portions  22   c  and  22   d  are almost fully clamped over the end portions  32  of each rib  26 , and the skins  12  and  14  are being held against interior surfaces of the tool dies  50  and  52 .  
         [0026]    Referring to FIG. 2 e , resin is introduced at one or more suitable openings in the tool  54  from a resin source  56  such that the skin panels  12  and  14  are held tightly against the interior surfaces of the molding tool dies  50  and  52  while the flap sections  22   c  and  22   d  are held tightly over an associated end portion  32 . After the various portions  12 ,  14 ,  28  and  30  of the composite structural assembly  10  are fully infused with resin, the tool  50  is placed in a suitable oven  58  and then cured for a predetermined period of time, as indicated in FIG. 2 f . The period of time may vary considerably, but in one preferred implementation of the present invention it comprises a time period of about three 3 hours. It will also be appreciated that the oven  58  need not be an autoclave, but instead may simply comprise an oven suitable to heat the resin infused panels  12 ,  14 ,  28 , and  30  to a temperature of preferably between about 250 degrees F. (121 degrees C.)-350 degrees F. (176 degrees C.).  
         [0027]    When the assembly comprising panels  12 ,  14 ,  28  and  30  are removed, the fully formed and cured composite structural assembly  10  is formed. Co-curing the skins  12  and  14  to their respective rib sections  28  and  30 , while simultaneously carrying sections  22   c  and  22   d  to the end portion  32  of rib section  28 , eliminates the need to assemble the two skins  12  and  14  to the rib  26  in a separate manufacturing step. Alternatively, it eliminates the need to assemble the rib sections  28  and  30  to one another in a separate, subsequent manufacturing step by bonding or by mechanical fasteners such as rivets. Accordingly, a single, unitary, composite structural member can be created in a single molding step.  
         [0028]    The process of the present invention described above significantly reduces the cost of manufacturing closed, stiffened box structures and other complexly shaped components that would ordinarily have been manufactured with first a molding step, and then a subsequent securing step to secure two or more of the component sections together. Co-curing all of the component sections of the unitary, composite structural assembly  10  further provides a stronger structural part by providing efficient, continuous load paths along the interfaces where two or more components have been molded to one another. Forming the assembly  10  in one step further serves to minimize stress concentrations at the interfaces where two or more independent sections are joined together by eliminating the need for drilling or otherwise forming one or more openings through which fastening elements such as rivets can be installed. The stitching used to form the assembly  10  further helps to provide damage arrestment and further to increase damage tolerance to the assembly. The high-strength stitched interfaces also form stronger joints than what would be possible with co-curing two or more independent panel sections. The process of the present invention further provides for uniform, smooth internal transitions, such as where the flap sections  22   c  and  22   d  are bonded to end portion  32  of rib section  28 .  
         [0029]    A particularly desirable feature of the present invention is how the rib sections  28  and  30  are automatically urged into contact with one another as the bladders  44 ,  46 ,  48  are inflated.  
         [0030]    After the assembly  10  is cured, the bladders  44 ,  46  and  48  may then be removed through openings  55  after the bladders  44 ,  46  and  48  are deflated and rotated in a circular motion to help break the surface tension between the bladders and the inner part surfaces. In this regard it will be appreciated that the bladders  44 ,  46  and  48  are extremely thin-gauge material and relatively inexpensive components that can be discarded after one molding operation is performed. If a multi-cell bladder is employed, then removal will require urging one or more of the cells through one or more openings  41  in the ribs  26 , before removing the bladder from the tool  54 , such as through a single one of openings  55 .  
         [0031]    The description of the invention is merely exemplary in nature and, thus, variations that do not depart from the gist of the invention are intended to be within the scope of the invention. Such variations are not to be regarded as a departure from the spirit and scope of the invention.