Abstract:
Methods of forming a collapsible mandrel are disclosed herein. An example method disclosed herein includes forming a layup on a substrate by providing a first layer of rubber, positioning a reinforcement strip on the first layer of rubber, providing first and second release films in spaced apart from each other adjacent the reinforcement strip to expose a portion of the reinforcement strip, providing a second layer of rubber overlying the exposed portion of the reinforcement strip, and laminating the first and second layers of rubber to the reinforcement strip. The release films to prevent the first and second layers of rubber from adhering to each other and the reinforcement strip such that non-adhered ends of the first and second layers of rubber and the reinforcement strip define flaps. The method includes placing the layup in a cavity of a mold forming part of a tool assembly, consolidating the layup when the layup is in the cavity of the mold, and curing the layup to form the collapsible mandrel.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
       [0001]    This application is a continuation of U.S. patent application Ser. No. 12/777,610 filed on May 11, 2010, entitled “COLLAPSIBLE MANDREL EMPLOYING REINFORCED FLUOROELASTOMERIC BLADDER,” which is hereby incorporated herein by reference in its entirety. 
     
    
     TECHNICAL FIELD 
       [0002]    This disclosure generally relates to mandrels on which parts may be formed, and deals more particularly with a collapsible mandrel employing a reinforced fluoroelastomeric bladder for laying up and/or curing composite parts. 
       BACKGROUND 
       [0003]    Inflatable bladders are sometimes used as mandrels to produce fiber reinforced resin parts. Multiple plies of fiber reinforced resin are laid up over the mandrel in order to form the plies into the desired part shape. The mandrel may be removed from the layup after the layup is compacted or cured by collapsing the mandrel to reduce its cross sectional shape so that it can be withdrawn from the part. 
         [0004]    In some applications, the ability of the bladder to apply pressure uniformly over the layup may be dependent on the bladder&#39;s dimensional stability. Maintaining dimensional stability of the bladder may be particularly problematic in the case of long tubular composite part layups. 
         [0005]    One type of known mandrel used for layup and curing of composite parts employs a reinforced silicon rubber bladder, however this type of bladder demonstrates relatively rapid, continuous shrinking over time with repeated use and therefore may not provide uniform pressure during cure cycles. In the case of parts having relatively strict dimensional requirements, reinforced silicon rubber bladders may be used only once because of their inherent problems with thermal growth and post-cure shrinkage. 
         [0006]    Another type of mandrel uses nylon tubular bagging film to provide autoclave pressure during curing to the internal cavity of a part. However, bagging film does not have the required structural strength and rigidity to support a part during the layup process. 
         [0007]    Accordingly, there is a need for a collapsible mandrel exhibiting improved dimensional stability over repeated uses, and which possesses the necessary strength and rigidity to allow the mandrel to be used for part layup. 
       SUMMARY 
       [0008]    The disclosed embodiments provide a collapsible mandrel and method for making the same comprising a reinforced fluoroelastomeric rubber bladder that may exhibit minimal shrinkage over repeated uses and which can be employed for use in multiple cure cycles, thereby lowering recurring tooling cost. The collapsible mandrel provides structure and support during green part layup and provides the proper shape and autoclave pressure during curing with minimal thermal expansion and virtually no post-cure shrinkage. The mandrel is capable of collapsing under applied vacuum for ease of extraction from an enclosed part. Improved dimensional control during part curing may be achieved due to a lower coefficient of thermal expansion (CTE) of the fluoroelastomeric rubber. The lower gas permeability rate of fluoroelastomeric rubber at elevated temperature may also contribute to improved part quality by reducing the possibility of porosities in the part. 
         [0009]    The disclosed method may also reduce volatiles in the fluoroelastomeric rubber which may contribute to maintaining dimensional stability of the mandrel over repeated uses. The collapsibility of the mandrel allows parts to be laid up having complex profiles while preventing lock-in of the bladder within the part due to a varying part profile. 
         [0010]    According to one disclosed embodiment, a collapsible mandrel is provided comprising an inflatable bladder. The bladder includes inner and outer layers of fluoroelastomeric rubber having a reinforcement sandwiched therebetween. The reinforcement may include a coating of fluoroelastomeric rubber thereon. The bladder includes collapsible sidewalls, and the reinforcement is discontinuous in each of the sidewalls. In one embodiment, the reinforcement comprises fiberglass. The reinforcement may include at least two generally rigid elongate members in each of the sidewalls arranged substantially edge-to-edge with each other. 
         [0011]    According to another disclosed embodiment, a collapsible mandrel for laying up and curing composite parts is provided. The mandrel comprises a substantially flexible, pressurizable bladder adapted to inflate when pressurized. The bladder includes an inner layer of rubber, and outer layer of rubber, and a middle layer including a substantially rigid reinforcement that provides the bladder with rigidity when the bladder is pressurized and which collapses when the bladder is depressurized. The inner and outer layers of rubber are a fluoroelastomeric rubber, and the reinforcement is coated with a fluoroelastomeric rubber. The reinforcement includes fiberglass members arranged side-by-side to form flexible butt joints allowing the reinforcement to flex. 
         [0012]    In accordance with still a further embodiment, a method is provided of making composite parts, comprising reinforcing a flexible bladder with substantially rigid strips. The method further includes inflating the bladder, and laying up a composite part over the reinforced inflated bladder. The bladder is deflated and removed either after the layup has been completed or after the layup has been cured. The bladder is deflated by using negative air pressure to collapse the sidewalls of the bladder along the edges of the reinforcement strips. Reinforcing the bladder may be performed by coating the strips with rubber and placing them in side-by-side relationship between two layers of rubber. 
       BRIEF DESCRIPTION OF THE ILLUSTRATIONS 
       [0013]      FIG. 1  is an illustration of a perspective view of a collapsible mandrel according to the disclosed embodiments, shown in its fully inflated state. 
         [0014]      FIG. 2  is an illustration of a sectional view taken along the line  2 - 2  in  FIG. 1 . 
         [0015]      FIG. 3  is an illustration of an isometric view of a typical part that has been laid up and cured using the collapsible mandrel shown in  FIG. 1 . 
         [0016]      FIG. 4  is an illustration similar to  FIG. 3  but showing the mandrel installed and inflated within the part shown in  FIG. 3 . 
         [0017]      FIG. 5  is an illustration of an isometric view similar to  FIG. 4  but showing the mandrel having been collapsed and in the process of being removed from the part. 
         [0018]      FIG. 6  is an illustration of a sectional view similar to  FIG. 2  but depicting the sidewall having been partially collapsed. 
         [0019]      FIG. 7  is an illustration of an isometric view showing a partially completed layup used to form the collapsible mandrel shown in  FIG. 1 . 
         [0020]      FIG. 8  is an illustration of a sectional view of the layup shown in  FIG. 7 , but exploded to better show the relationship of the layers to each other. 
         [0021]      FIG. 9  is an illustration of an isometric view of a layup similar to  FIG. 7 , but depicting the application of the inner layer of fluoroelastomeric rubber. 
         [0022]      FIG. 10  is an illustration similar to  FIG. 8  but showing the inner layer of rubber having been applied to the layup. 
         [0023]      FIG. 11  is an illustration of an isometric view of a mold used to form the collapsible mandrel. 
         [0024]      FIG. 12  is an illustration of a sectional view of the layup after it has been placed in the mold shown in  FIG. 11 , wherein the layers of the layup are exploded to better show their relationship to each other. 
         [0025]      FIG. 13  is an illustration of a perspective view of the layup after it has been placed in the cavity of the mold shown in  FIG. 11 . 
         [0026]      FIG. 14  is an illustration similar to  FIG. 12 , but showing a release film having been placed between the reinforcement and the inner layer. 
         [0027]      FIG. 15  is an illustration similar to  FIG. 14 , but showing a forming mandrel having been placed in the partially formed layup. 
         [0028]      FIG. 16  is an illustration of a sectional view of the area designated as “A” in  FIG. 15 . 
         [0029]      FIGS. 17-20  are illustrations of sectional views of the layup depicting successive steps in the assembly method. 
         [0030]      FIG. 21  is an illustration of a sectional view of a fully assembled layup having the forming mandrel installed therein. 
         [0031]      FIG. 22  is an illustration of a perspective view of the mold shown in  FIG. 11 , wherein the layup has been fully assembled and the lid has been placed on the mold in preparation for vacuum bagging. 
         [0032]      FIG. 23  is an illustration similar to  FIG. 21  but showing the pressure applied by the forming mandrel to the layup during the compaction and curing process. 
         [0033]      FIG. 24  is an illustration of a perspective view showing a person removing the forming mandrel from the cured layup. 
         [0034]      FIG. 25  is an illustration of a flow diagram of a method for making the collapsible mandrel. 
         [0035]      FIG. 26  is an illustration of a flow diagram of aircraft production and service methodology. 
         [0036]      FIG. 27  is an illustration of a block diagram of an aircraft. 
     
    
     DETAILED DESCRIPTION 
       [0037]    Referring first to  FIGS. 1 and 2 , the disclosed embodiments relate to a collapsible mandrel  30  which includes an inflatable, flexible bladder  35 . The bladder  35  includes four sidewalls  32  and two endwalls  34 , however the bladder  35  may have more or less than four sidewalls  32 . The mandrel  30  also includes a pressure fitting  36  which is adapted to be coupled with a source (not shown) of pressurized fluid such as air for inflating the bladder  35 , and with a vacuum source for deflating the bladder  35 . In the illustrated embodiment, the mandrel  30  is elongate and possesses a generally trapezoidal cross section, however other cross sectional shapes are possible. 
         [0038]    Referring particularly to  FIG. 2 , the walls  32  of the bladder  35  each comprise a middle layer  49  sandwiched between inner and outer layers  42 ,  44  respectively. Each of the inner and outer layers  42 ,  44  comprises a flouroelastic rubber, such as Vitron® which is readily commercially available. A fluoroelastomer is a special purpose fluorocarbon-based synthetic rubber that has wide chemical resistance and superior performance, particularly in high temperature applications. The fluoroelastomeric rubber has a relatively low coefficient of thermal expansion, thus providing the mandrel  30  with good dimensional stability. The thickness of the inner and outer layers of  42 ,  44  fluoroelastomeric rubber will depend upon the particular application. The middle layer  49  comprises a reinforcement which may include elongate strips  46  of substantially rigid material, such as, for example and without limitation, a woven fiberglass having a coating  51  of fluoroelastomeric rubber on each side thereof. The fiberglass reinforcing strips  46  not only provide the mandrel  30  with structural rigidity, but also reduce the tendency of the bladder  35  to shrink over repeated uses. 
         [0039]    As will be discussed later in more detail, the fiberglass reinforcement strips  46  are arranged side-by-side and edge-to-edge to form a butt joints  48  between the strips  46 . The butt joints  48  function as hinges that allow the reinforcement strips  45  to be swing and fold angularly with respect to each other when the mandrel  30  collapses. The design and location of the butt joints  48  allow the bladder  35  to collapse under negative air pressure or vacuum in a predictable manner, enabling easier extraction of the bladder  35  from the part. The thickness of the coatings  51  may depend upon the application, as well as the exact material makeup of the reinforcement strips  46  and their surface finishes. The coatings  51  aid in bonding the reinforcement strips  46  to the inner and outer layers  42 ,  44  of fluoroelastomeric rubber. 
         [0040]    Referring now also to  FIGS. 3 and 4 , when inflated, the reinforced collapsible mandrel  30  may be used as a tool for laying up a composite part such as the elongate, generally tubular composite part  38  which includes a relatively long tubular opening  40 . In the illustrated embodiment, the opening  40  has a trapezoidally shaped cross section, substantially matching that of the mandrel  30 . By virtue of the reinforcement strips  46 , the mandrel  30  possesses sufficient rigidity to remain dimensionally stable while multiple plies (not shown) of composite material are laid up over the sidewalls  32  of the mandrel  30  during the layup process. The mandrel  30  may optionally be used to maintain the shape of the part  38  while it is being cured using autoclave processing or other curing techniques. 
         [0041]    Referring to  FIGS. 5 and 6 , following the layup and/or curing process, the mandrel  30  may be removed from the part  38  by deflating the bladder  35  using negative pressure to draw fluid (e.g. air) from the bladder  35  through the pressure fitting  36 . As the bladder  35  deflates, negative pressure within the bladder  35  causes the endwalls  34  and sidewalls  35  to flex inwardly as shown in  FIG. 5 , collapsing and drawing away from the part  38 . As the bladder  35  begins to deflate and collapse, adjacent pairs of the reinforcement strips  46  fold relative to each other along their mutual edges at joints  48 , as shown in  FIG. 6 . With the bladder  35  partially collapsed, as shown in  FIG. 5 , the mandrel  30  may be pulled from the tubular interior  40  of the part  38 . 
         [0042]    The collapsible mandrel  30  may be made according to a method that will now be described with reference to  FIGS. 7-24 . Referring first to  FIGS. 7 and 8 , a layup  56  is formed on a suitable substrate  50  by first laying down a layer  44  of fluoroelastomeric rubber. Next, strips  46  of fiberglass cloth or other reinforcement are laid down on top of the rubber layer  44 , in aligned, edge-to-edge contact forming butt joints  48 . The reinforcement strips  46  are coated with fluoroelastomeric rubber prior to being laid down on the rubber layer  44 . A strip of release film  52  is interposed between an outer edge of the rubber layer  44  and one of the reinforcement strips  46 . Two strips of release film  54 , which may comprise FEP, are placed on top of the fiberglass strips  46 , in spaced apart relationship to each other, leaving a portion  45  of the reinforcement strips  46  exposed. 
         [0043]    Due to the relative tackiness of the rubber coating  51  and the rubber layer  54 , the reinforcement strips  46  adhere to the rubber layer  44 , except in the area of the release film  52 . 
         [0044]    Next, as shown in  FIGS. 9 and 10 , a layer  42  of fluoroelastic rubber is placed on the layup  56 , overlying the release film  54  and the exposed portion  46   a  of the reinforcement strips  46 . Due to the presence of the release film  54 , the rubber layer  42  adheres to the reinforcement strips  46  only along the exposed portion  46   a . Thus, the area in which both the inner and outer rubber layers  42 ,  44  are bonded to the reinforcement strips  46  is limited to that shown by the numeral  55  in  FIG. 10 . The layup  56  shown in  FIG. 10  may then be laminated by vacuum bag processing at elevated temperature to laminate the rubber layers  42 ,  44  to the reinforcement strips  46 . 
         [0045]    Referring now to  FIGS. 11 ,  12  and  13 , following the lamination process, the layup  56  may be placed in the cavity  60   a  of a mold  60  forming part of a tool assembly  58  that includes a lid  62 . Forming aids (not shown) may be used to press the layup  56  down into the mold cavity  60   a , and conform the layup  56  to radii  60   b  ( FIG. 13 ) in the mold cavity  60   a . At this point, as shown in  FIG. 12 , the release film  52 ,  54  ( FIG. 10 ) has been removed and the area of full lamination  55  between the reinforcement  46  and the inner and outer layers  42 ,  44  respectively, lies along the bottom of the mold cavity  60   a . Three of the butt joints  48  between the reinforcing strips  46  are respectively positioned roughly in the middle of three corresponding sidewalls  32 , and the outer free ends  64  of the layup  56  extend outside of the mold  60 , acting as flaps. 
         [0046]    Next, as shown in  FIGS. 13 and 14 , a strip of release film  66  is inserted between an edge of the reinforcing strips  46  and the rubber layer  42 , following which, as shown in  FIG. 15  a forming mandrel  68  is inserted into the mold cavity  60   a  and placed on top of the partially formed layup  56 . At this point, the release film  66  is removed. As best seen in  FIG. 16 , the forming mandrel  68  may comprise a core  70  of memory foam which is covered by a nylon cure tube bag  72 , a thin breather  74  and a layer of release film (e.g. FEP)  76 . 
         [0047]    Next, as shown in  FIG. 17 , flap  42   a  forming part of the inner layer  42  is folded over onto the forming mandrel  68 , following which, as illustrated in  FIG. 18 , flaps  42   b  and  46   a  are folded over onto the flap  42   a  resulting in an overlap joint  65 . Then, as shown in  FIG. 19 , flap  46   b  is folded over onto a portion of flap  32   b , resulting in a butt joint  48   a  between the ends of two of the reinforcing strips  46 . Next, as shown in  FIG. 20 , flap  44   a  is folded over onto the reinforcement, overlapping the butt joint  48   a . Finally, as shown in  FIG. 21 , flap  44   b  is folded over onto  44   a  resulting in a second overlap joint  78 . 
         [0048]    Referring to  FIG. 22 , with the various flaps having been closed, the mold  60  may be closed by installing the lid  62 , following which the entire tool assembly  58  may be vacuum bagged (not shown) and subjected to a vacuum and/or autoclave pressure, as shown in  FIG. 23  which results in compacting and curing the layup  56 . Vacuum bag processing of the bladder  35  aids in removing volatiles from the rubber which tends to increase dimensional stability and/or resists shrinkage of the mandrel  30  over multiple uses. Following compaction and curing, as shown in  FIG. 24 , the forming mandrel  68  may be removed from the layup  56  by applying a vacuum to the bagged memory foam core  70  which causes the core  70  to collapse. Once collapsed, a person  79  may grasp an end of the release film  76  and pull the mandrel  68  through one end of the fully cured part layup  56 . 
         [0049]      FIG. 25  illustrates the overall steps of the method of making the collapsible mandrel described above. Beginning at step  80 , the outer layer  44  of flouroelastic rubber is laid down on a suitable substrate  50  ( FIG. 7 ) following which, at step  82 , the middle reinforcing layer  49  is formed by laying down rubber coated reinforcement strips  46  in side-by-side, edge-to-edge abutment with each other, aligned such that the butt joints  48  are later located respectively approximately midway between each of the sidewalls  34 . At step  84 , release film is installed over the reinforcement strips  46 , leaving a portion of the reinforcement strips  46  exposed. Then, at step  86 , the inner layer  42  of fluoroelastomeric rubber is laid down and the layup  56  is laminated using any of various techniques, including vacuum bag processing at elevated temperatures. The laminated layup  56  is then placed in the mold  60  at step  88  and is swept into the radii of the mold cavity  60   a . Next, a release film  66  is inserted between edge flaps of the inner and middle layers  42 ,  49 . 
         [0050]    A forming mandrel  68  is assembled at step  92  and inserted into the mold  60  over the layup  56  at step  94 , following which the release film  66  may be removed. Next, at step  96 , a flap  42   a  of the inner layer  42  is folded over at step  96 , following which, at step  98 , a second flap  42   b  of the inner layer  42 , and a flap  46   a  of the middle layer  49  are folded over, forming a lap joint  65  between these flaps. Next, at  100 , a second flap  46   b  of the middle layer  49  is folded over, forming a butt joint  48   a  between the middle layer flaps. At  102 , the first flap  44   a  of the outer layer  44  is folded over, following which, at  104 , the second flap  44   b  of the outer layer  44  is folded over, forming a lap joint  78  between the two outer layer flaps  44   a ,  44   b . Then, at  106 , the mold  60  is closed, and at  108 , a vacuum bag is installed over the mold  60  and a vacuum is applied to the layup  56 . The consolidated layup  56  may then be cured at  110 , optionally using autoclave processing. At  112 , the forming mandrel  68  is removed from the cured layup  56 , and the layup  56  is removed from the mold  60 . Finally, at  114 , endwalls  34  and one or more pressure fittings  36  may be installed on the cured bladder. 
         [0051]    Embodiments of the disclosure may find use in a variety of potential applications, particularly in the transportation industry, including for example, aerospace, marine and automotive applications. Thus, referring now to  FIGS. 26 and 27 , embodiments of the disclosure may be used in the context of an aircraft manufacturing and service method  120  as shown in  FIG. 26  and an aircraft  122  as shown in  FIG. 27 . Aircraft applications of the disclosed embodiments may include, for example, a wide variety of structural and non-structural composite parts and components that are generally tubular. During pre-production, exemplary method  120  may include specification and design  124  of the aircraft  122  and material procurement  126 . During production, component and subassembly manufacturing  128  and system integration  130  of the aircraft  122  takes place. Thereafter, the aircraft  122  may go through certification and delivery  132  in order to be placed in service  134 . While in service by a customer, the aircraft  122  is scheduled for routine maintenance and service  136  (which may also include modification, reconfiguration, refurbishment, and so on). 
         [0052]    Each of the processes of method  120  may be performed or carried out by a system integrator, a third party, and/or an operator (e.g., a customer). For the purposes of this description, a system integrator may include without limitation any number of aircraft manufacturers and major-system subcontractors; a third party may include without limitation any number of vendors, subcontractors, and suppliers; and an operator may be an airline, leasing company, military entity, service organization, and so on. 
         [0053]    As shown in  FIG. 27 , the aircraft  122  produced by exemplary method  120  may include an airframe  138  with a plurality of systems  140  and an interior  142 . Examples of high-level systems  142  include one or more of a propulsion system  144 , an electrical system  146 , a hydraulic system  148 , and an environmental system  150 . Any number of other systems may be included. The disclosed method may be employed to fabricate parts, structures and components used in the interior  142  and in the airframe  138 . Although an aerospace example is shown, the principles of the disclosure may be applied to other industries, such as the marine and automotive industries. 
         [0054]    Systems and methods embodied herein may be employed during any one or more of the stages of the production and service method  120 . For example, parts, structures and components corresponding to production process  128  may be fabricated or manufactured in a manner similar to parts, structures and components produced while the aircraft  122  is in service. Also, one or more apparatus embodiments, method embodiments, or a combination thereof may be utilized during the production stages  128  and  130 , for example, by substantially expediting assembly of or reducing the cost of an aircraft  122 . Similarly, one or more of apparatus embodiments, method embodiments, or a combination thereof may be utilized while the aircraft  122  is in service, for example and without limitation, to maintenance and service  136 . 
         [0055]    Although the embodiments of this disclosure have 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.