Abstract:
Thickness gradients in large, cobonded composite structures resulting from gravity-induced resin migration during curing is substantially reduced by rotating the structure during the resin infusion and curing stages. The layup for the structure is placed on a rotatable tool fixture and vacuum bagged. The tool fixture is mounted on a central support tube provided with motors for rotating the tool fixture about the axis of the tube. The tube has internal passageways that deliver resin to the bagged layup and carry away excess resin from the layup using vacuum pressure. The resulting composite structures exhibit thickness gradients less than 10%.

Description:
FIELD OF THE INVENTION 
       [0001]    This invention generally relates to vacuum assisted resin transfer molding, and deals more particularly with a method and apparatus for reducing thickness gradients in molded composite parts caused by gravity-induced settling of the resin during curing. 
       BACKGROUND OF THE INVENTION 
       [0002]    Vacuum-assisted resin transfer molding (VARTM) is being used more frequently to mold large composite structures, such as sections of aircraft. VARTM is a variant of traditional resin transfer molding (RTM), possessing advantages over conventional RTM by eliminating matched-metal tooling costs, reducing volatile emissions and allowing for low injection pressures. In VARTM, vacuum pressure is used to force liquid resin into dry composite reinforcements that have been laid in a sealed mold, often in the form of a preform. The mold can be a one sided tool with a vacuum bag, a two sided matched tool with a vacuum seal, or a soft bag enclosing the entire structure to be molded. Vacuum pressure is used to pull or drive resin into the mold, thus VARTM is sometimes referred to as a vacuum infusion process. The selection of materials, arrangement of mold gates/vents and the selection of processing parameters often have a significant impact on product quality and process efficiency in VARTM. 
         [0003]    When molding relatively large structures, such as an aircraft fuselage, gravitational effects on resin flow behavior can create undesirable thickness gradients in the finished structure. These gradients, which may approach 25% or more, result from the fact that the force imposed by gravity tends to draw the flowing resin downwardly toward the bottom of the molded structure during the curing process, until the resin is sufficiently cured to terminate its flow. As a result, wall thickness of the structure measured in bottom portions of the structure can be significantly greater than wall thickness near the top of the structure. Thickness gradients due to resin migration not only reduce the integrity of the molded structure, but also result in a structure that is unnecessarily heavy, since in order to achieve a minimum wall thickness at the top of the structure, wall thickness near the bottom of the structure is greater than necessary. In the case of aircraft structures, thickness gradients of the type described above make it difficult to produce complete fuselage sections having integral stringers and co-bonded fuselage frames. 
         [0004]    Accordingly, there is a need in the art for an improved method and apparatus for manufacturing composite structures using VARTM which overcomes the deficiencies of the prior art discussed above. The present invention is directed to satisfying this need. 
       SUMMARY OF THE INVENTION 
       [0005]    In accordance with one aspect of the invention, a composite fiber structure is formed by injecting resin into composite fiber layup, and rotating the resin injected layup to reduce gravity-induced migration of the resin through the layup. The thickness of the structure preferably has a vertical gradient less than about 10%. Rotation of the layup is commenced when resin injection begins, and is continued until the structure is cured sufficiently to terminate resin flow. 
         [0006]    In accordance with another aspect of the invention, a method is provided for manufacturing a composite structure exhibiting reduced resin thickness gradient caused by gravity. The method comprises steps of: placing a composite fiber layup on a forming tool; introducing resin into the layup; applying a vacuum to the layup while the resin is being introduced; and, rotating the tool as the resin is introduced into the layup in order to reduce settling of the resin. The layup is formed by placing a dry preform on a female mandrel, and resin is introduced into the layup through a main support tube which mounts the tool for rotation on a pair of supports. The main support tube is also used to draw a vacuum on the structure in order to infuse the resin into the layup. 
         [0007]    According to still another aspect of the invention, apparatus is provided for producing a composite fiber structure comprising a forming tool having the shape of the structure and upon which a laminate layup may be disposed. A mounting assembly is provided for mounting the tool for rotation about a central axis of rotation. An air tight flexible membrane covers the layup and a resin injection system is provided for injecting resin through the membrane into the layup. A vacuum system produces a vacuum within the membrane and urges the resin to pass through composite fibers forming the layup. A motor rotates the tool about the central axis of rotation at a rate which reduces gravity induced settling of the resin in the layup during curing. 
         [0008]    Various additional objects, features and advantages of the present invention can be more fully appreciated with reference to the detailed description and accompanying drawings that follow. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]      FIG. 1  is a perspective view of a section of an aircraft fuselage manufactured in accordance with the method and apparatus of the present invention. 
           [0010]      FIG. 2  is longitudinal cross sectional view of the apparatus of the present invention used to manufacture the fuselage shown in  FIG. 1 . 
           [0011]      FIG. 3  is an enlarged view of a section of the apparatus shown in  FIG. 2 , designated by the letter “A”. 
           [0012]      FIG. 4  is an end view of the apparatus shown in  FIG. 2 . 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0013]    Referring first to  FIG. 1 , a section of an aircraft fuselage generally indicated by the numeral  10  is essentially square in cross section and has walls formed from a laminated composite material well known in the art of aircraft construction. The fuselage  10  has an outer skin  16  which is co-bonded with a series of longitudinally spaced, transversely extending frame members  12 , and a series of laterally spaced, longitudinally extending stringers  14 . The frame members  12 , stringers  14  and skin  16  are preferably formed using VARTM, in which these components are co-cured and co-bonded to form a highly rigid, rugged, unitized structure. In accordance with the present invention, the wall thickness of the skin  16  is substantially uniformed throughout the height of the fuselage  10 , thus providing a structure which possesses high structural integrity with minimum weight. 
         [0014]    The fuselage  10 , or similar large composite structure, is manufactured using the apparatus shown in  FIGS. 2-4 . A two piece tool  18  forms an elongate, hollow female mold having an internal surface defining a female mandrel corresponding to the shape of the fuselage skin  16 . Tool  18  is of a type sometimes referred to as an OML (outside mold line) tool, in which the outside surface of the tool acts as the mold surface that forms the inside surface of the composite structure being molded. The two sections of the tool  18  are releasably held together by a suitable fasteners and are joined at a split line  52 . After the finished structure is fully cured, the two halves of the tool  18  are removed so that the structure can be withdrawn from the tool  18 . The tool  18  is held on a pair of tool supports  20  which in turn are journalled for rotation on a main support and distribution tube  26  which passes through the length of the tool  18 , and is mounted on a pair of tool stands  22  supported on a base  24 . A pair of motors  38 , which may be electric or hydraulic, are secured to the tube  26  and are drivingly connected to the tool supports  20  so as to rotate the entire tool  18  around the tube  26 . 
         [0015]    The main support tube  26  is hollow, providing a passageway throughout its length to convey fluids. The input end  34  of the tube  26  is coupled with a suitable source (not shown) of resin. The exit end  40  of the main tube  26  is connected with a suitable source (not shown) for creating negative pressure, typically less than one atmosphere. 
         [0016]    A composite layup, preferably in the form of a skin preform  32  is installed on the inner-mandrel surface of the tool  18 . The preform  32  may comprise multiple layers of matting formed of composite fibers; the composition, thickness and the number of layers will depend on the particular application. Generally L-shaped frame members  12  are next installed within the tool  18 . Frame members  12  may comprise pre-cured, composite components which are held in place and located by series of frame locator tools  28  that are secured to the interior face of the tool  18 . A pair of L-shaped support clips  42 , also formed of pre-cured composite material are installed on opposite sides of each of the frame members  12 , in contact with the inside face of the skin preform  32 . An air tight, flexible membrane in the form of a vacuum bag  44  is disposed over the assembly comprising the skin preform  32 , clips  42  and frame members  12 . Bag seals  50  are provided where necessary, to provide an air tight seal between the bag  44  and frame members  12 . 
         [0017]    As shown in  FIG. 2 , a plurality of radially extending resin-injection tubes or conduits  30  are connected between the main support tube  26  and the bag  44 . The resin injection tubes  30  are positioned near the entrance end  34  of tube  26  and function to deliver resin from tube  26  into the sealed layup. The resin flows through tubes  30  into the bag  44 , and then along the inner surface of the preform  32 , covering clips  42  and frame members  12 . Air is evacuated from the bag  44  by means of a series of vacuum tubes or conduits  38  positioned near the exit  40  of the main tube  26 . Vacuum tubes  38  are connected through the main tube  26  and the bag  44 , thus placing the vacuum source in communication with the interior of the bag  44 . 
         [0018]    The vacuum created within bag  44  evacuates air from the bag, and the residual negative pressure forces the flowing resin to be infused into the layers of the skin preform  32 . Excess resin is carried through the vacuum tubes  38  to the main support tube  34  and thence through the exit end  40  of the tube  26 . The vacuum source then draws air through the exit  40 , evacuating air from the bag  44  and creating internal negative pressure which draws resin into the main tube at the entrance  34 . The resin flows through the main tube  26  into the resin injection tubes  30 , entering the bag  40  and flowing over the surface of the entire layup. The negative pressure within the bag  44  causes the resin to be infused into the layup. Excess resin is carried away by the vacuum tubes  38  through the exit  40  of the main tube  26 . 
         [0019]    As resin begins to enter the main tube  26 , motors  38  are turned on, causing the entire tool  18 , and thus the layup, to rotate. The rate of rotation will depend upon the size of the tool  18 , the composition of the resin as well as the layup. However the rotational rate should be chosen such that the tendency of the resin to settle due to gravity is offset or neutralized as a result of the layup being periodically inverted. In other wards, the forces imposed by gravity on the layup and the resin are periodically inverted such that the resultant vertical force acting on the resin over a period of time is zero. As a result of this rotational technique, gravity induced sagging or settling of the resin is materially reduced, resulting in thickness gradients less than 10% throughout the entire structure. 
         [0020]    Rotation of the tool  18  is continued through the entire cure cycle, or at least until the resin has hardened sufficiently to preclude settling. After curing, the tool  18  is removed and the formed composite structure is removed from the tool  18 . The resulting structure, in this case a fuselage section, has integral stringers and co-bonded fuselage frames forming a substantially unitized structure wherein the skin and other components have an essentially uniform thickness throughout the structure. 
         [0021]    Although this invention has been described with respect to certain exemplary embodiments, it is to be understood that the specific embodiments are for purposes of illustration and not limitation, as other variations will occur to those of skill in the art.