Abstract:
An apparatus is disclosed that includes a bolster plate with insertable nodal heating elements that provide heat at their rectangular ends and when driven under pressure against opposing pins with rectangular faces, cure portions of a pre-impregnated fabric. Using half-period translations, ply-upon-pin repositions, and shape-maintaining pins, a method of making honeycomb core composite articles using the above apparatus is disclosed.

Description:
FIELD OF THE INVENTION  
         [0001]    This invention relates to processes of manufacturing lightweight structural cores and their resulting products and in particular to methods of manufacturing honeycomb cores and their resulting products.  
         BACKGROUND  
         [0002]    Composite articles incorporating honeycomb cores are commonly used for fabricating aerospace structures due to their advantageous strength to weight ratio. Honeycomb core composite articles are typically comprised of upper and lower composite skins or layers, i.e., fiber reinforced resin matrix laminates that are separated and stabilized by the honeycomb core. Due to the high bending stiffness and compressive strength properties of honeycomb cores composite articles, i.e., the honeycomb core functions as a shear web and spaces the composite skins from the bending neutral axis, honeycomb core composite articles have particular utility in aerospace applications such as aircraft fuselage panels and door structures. The high strength and low weight of such construction results in lower overall aircraft system weight.  
           [0003]    For example, in commercial aircraft, nearly all of the movable control surfaces, wing and tail leading and trailing edge fixed surfaces, doors, and interior cabin structures employ panels formed of honeycomb cores. Although more expensive than simple structures, the honeycomb core panel possess equal strength at higher stiffness, lower weight, and is resistant to higher natural vibration frequencies. Such resistance is very important when structural elements are employed in close proximity to jet and rocket engines. Moreover, the honeycomb core must have small enough cell sizes to provide stabilization of the facings against premature buckling. In addition, the core must be sufficiently tough and abuse resistant to enable the same to be easily handled in a fabrication shop.  
           [0004]    The honeycomb core panel possesses equal strength at higher stiffness, lower weight, and is resistant to higher natural vibration frequencies. Such resistance is very important when structural elements are employed in close proximity to jet and rocket engines. Such structural panels generally comprise inner and outer composite skins, which are formed from materials such as Aluminum or composite materials such as fiberglass, graphite, embedded in a resinous matrix, e.g., epoxy, having a honeycomb core material interposed therebetween. Fiber can also be constructed of any other materials having a very small diameter and high strength and stiffness. Resins may typically consist of an epoxy, polycyanate, bismaleimide, and the like. The strength and stiffness of the resin matrix also affects the strength of the finished composite structure. For example, stronger resins such as epoxies usually yield a higher strength composite structure than lower strength resins such as polyester.  
           [0005]    Where high damage tolerance and abuse resistance are requires, Aramid honeycombs are employed. For example, KEVLAR® fiber can be used advantageously for this purpose. KEVLAR® is a registered trademark of E. I. du Pont de Nemours &amp; Co., Wilmington, Del. for an aromatic polyamide fiber of high tensile strength. Besides weight and strength, KEVALAR® has a slightly negative axial coefficient of thermal expansion similar to graphite, which means KEVLAR® laminates can be made thermally stable. Unlike graphite, KEVLAR® is very resistant to impact and abrasion damage and can be used as a protective layer on graphite laminates.  
         SUMMARY  
         [0006]    The preferred process embodiment of the present invention results in a honeycomb composite core article. The preferred fabric material can be any thermo set polymer impegnated fabric. In one aspect, the fabric can be KEVLAR® fabric. In other embodiments, the prepreg fabric can be a unidirectional fabric or it may comprise multiple plies of uncured unidirectional material. The fabric or layer of material may be a sheet of resin impregnated felt, a sheet of stitched unidirectional fabric, a sheet of chopped-fiber reinforced molding material or a sheet of unreinforced sheet molding compound.  
           [0007]    The uncured material is aligned into a bolster plate. The fabric is articulated under, and held in place by rectangular parallelepiped pins secured by hooks or other fastening means. The bolster plate applies heating elements, preferably consisting of resistive electric heating strips mounted on the top of an insulating material inserted into slots in the bolster plate, against portions of the fabric, giving the fabric shape and preparing it for node bonding. The heating element supports, in conjunction with the rectangular pins, are also used to concentrate pressure at the nodes. The bolster plate with fabric and rectangular pins is inserted into a press. The press drives the bolster plate and fabric nodal surfaces into the node surfaces of the honeycomb article in-progress, where the node surfaces are supported by previously enclosed pins. The heating elements are used, for the first ply, to cure the pre-impregnated material at each of its nodes or, for subsequent plies, to co-cure the pre-impregnated material at each of its nodes with the previously cured honeycomb layer. Once curing conditions are met, pressure is released, the bolster plate is withdrawn, the previously enclosed pins are removed, and the above steps are repeated cyclically, with appropriate translation of cured surface material at each cycle, until a desired height of the honeycomb core is produced.  
           [0008]    The process maintains any fabric orientation bias from −45/+45 degrees for high shear stiffness to a simple 0/90 degree orientation designed for good compression properties. The forming pins, having right rectangular faces, provide for high and even bond pressure during the cure cycle. The fabric and resulting product are co-cured one ply at a time, thereby eliminating bond preparation, minimizing processing time, and allowing for the build up of a core to a desired height. The freestanding post-cure process allows for heat-forming, if required. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]    For a further understanding of the nature and objects of the present invention, reference should be made to the following detailed description, taken in conjunction with the accompanying drawings, in which like elements are given the same or analogous reference numbers and wherein:  
         [0010]    [0010]FIG. 1 illustrates a first ply of fabric drawn over the bolster plate and its groove-mounted heating elements;  
         [0011]    [0011]FIG. 2 illustrates the bolster plate with the first ply of fabric and pins being inserted into the press;  
         [0012]    [0012]FIG. 3 illustrates in cross-section the alignment with the fixture pins;  
         [0013]    [0013]FIG. 4 illustrates in cross-section the pressing and heating of the first ply;  
         [0014]    [0014]FIG. 5 illustrates in cross-section the translation and repositioning of the first ply and the alignment of the second ply;  
         [0015]    [0015]FIG. 6 illustrates in cross-section the second ply being co-cured with the first ply;  
         [0016]    [0016]FIG. 7 illustrates in cross-section the translated first and second plies and the alignment of the third ply;  
         [0017]    [0017]FIG. 8 illustrates in cross-section the iterative properties of the process and apparatus;  
         [0018]    [0018]FIG. 9A is a partial flowchart of the preferred method;  
         [0019]    [0019]FIG. 9B is a partial flowchart of the preferred method; and  
         [0020]    [0020]FIG. 9C is a partial flowchart of the preferred method. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0021]    [0021]FIG. 1 illustrates an embodiment of the present invention, the first par being a bolster table  100  including a bolster plate  102  with a ply of material  106  that has been applied across heating elements  103  and held in position by bolster plate pins  101  which are in turn held in place by grooves  104  and hooks and  105 . The heating elements  103  are mounted in grooves  117  of the bolster plate  102 . The top portion of an individual heating element  103  preferably includes an electric strip heater  118  mounted atop a compliant insulating layer, preferably of 90 durometer silicone rubber. The bolster plate is preferably made ot steel. The pins  101  of the several embodiments of the present invention are preferably stainless steel rectangular parallelepipeds. Other pins providing a rectangular contact surface and able to withstand the heat and pressure required are also acceptable. FIG. 2 illustrates an apparatus  200  having the bolster plate  102  with fabric and bolster plate pins  101  being placed into the press  207 . Heating elements  103  in the bolster plate are aligned with a set of fixture pins  208  in a grooved alignment fixture  209  in the press  207 .  
         [0022]    [0022]FIG. 3 illustrates the bolster plate  102  with fabric  106  and bolster plate pins  101  aligned with the fixture pins  208  in the grooved alignment fixture  209  in the press  207  where the heating elements  103  provide heat and concentrate pressure during each nodal cure cycle.  
         [0023]    [0023]FIG. 4 illustrates the node curing of the first ply  106 . The press  107  is adjusted to apply the appropriate amount of pressure at the contact surfaces, or nodes, of the fabric between each fixture pin  208  and each heating element  103 . Once each portion of the ply  401  between the fixture pin  208  and heating element  103  (i.e., the node  402 ) has been pressed and heated a sufficient time for the node  402  to be cured, the electric strip heater  118  are switched off, and the press  207  is retracted. The bolster plate pins  101  are then removed. Many fabrics are applicable to the ply or plies of the several embodiments of the present invention. The prepreg fabric can be one or more plies of uncured unidirectional material and preferably the fabric is KEVLAR® or a similar aromatic polyamide fiber of high tensile strength. In addition, the fabric may be a sheet of resin impregnated felt, a sheet of stitched unidirectional fabric, a sheet of chopped-fiber reinforced molding material or a sheet of unreinforced sheet molding compound.  
         [0024]    [0024]FIG. 5 illustrates the newly formed ply  401  being shifted laterally  510  the distance of one-half cell, before being reinserted into the apparatus  200  on top of the fixture pins  208 . The bolster plate  102  is removed and prepared for the next ply. The second ply  501  is aligned into the bolster plate  102  in a manner as done with the initial alignment of the first ply and then the second ply  501  is held in position by the reinserted bolster plate pins  101 . The bolster plate  102  is reinserted into the press  207 .  
         [0025]    [0025]FIG. 6 illustrates the co-curing of the first ply  401  and the second ply  501 . Pressure (e.g., 40 pounds per square inch) generated by the press  207  and heat from electric strip heater elements  118  are applied to the top surface and bottom surface of each of the nodes  601 , thereby curing the nodes  601 .  
         [0026]    [0026]FIG. 7 illustrates that once the nodes  601  are cured, the fixture pins  208  at the cured surfaces are removed. The bolster plate pins  101 , enclosed by each of the resulting cells  701 , are kept in place. The entire honeycomb core  702  with bolster plate pins  101  is shifted laterally  705  one-half cell, i.e., one-half of a corrugation period, and realigned into the grooved fixture  209  of the press  207 . The first set of bolster plate pins  101  now function as a second set of fixture pins  101  for the next iteration of ply application. The bolster plate  102  is removed and the next ply of material  710  is applied to the bolster plate  102  and the next ply of material  710  is held in place by the first set of fixture pins  208  now functioning as a second set of bolster plate pins  208 . The bolster plate  102  is placed into the press  207  where pressure and heat by way of the heating elements  103  and the first set of bolter plate pins  101 , now functioning as a second set of fixture pins  101 , are applied at the nodal surfaces of the core  702  and the next ply of material  710 . The second set of fixture pins  101  are removed and the entire core  702  translated one-half of a corrugation cycle in a core-centering direction  710  with the second set of bolster plate pins  208  enclosed by the resulting cells of the second layer  501  and the third layer  710  and with the second set of bolster plat pins  208  aligned with a respective heating element  103 . The second set of bolster plate pins  208  now function as the first set of fixture pins  208  and the second set of fixture pins  101  now function as bolster plate pins  101 . Thereafter, the cycle repeats until a core  702  desired core height is achieved (FIG. 8). Once the core has reached the desired height, pins are removed and the core is placed in an oven for final curing.  
         [0027]    The above process is disclosed in flowchart form in FIGS. 9A, 9B and  9 C. The flowchart of FIG. 9A illustrates that the process begins  905  with providing a bolster table  100  including a steel plate  102  having a plurality of transverse rectangular grooves  117  and a plurality of groove-mounted pins  101 , preferably rectangular parallelepipeds, retractably and translationally mounted coplanar with the bolster plate  102  and providing an apparatus for receiving the bolster table  100  including a press  207  and a fixture  209  comprising a plurality of pins  208  that are preferably rectangular parallelepipeds  910 . The process continues  915  with drawing a sheet of prepregnated fabric  106  across the bolster plate  102  having heating elements. Then the flowchart of FIG. 9B illustrates that the process continues with the inserting the bolster plate  102  into the receiving press  207  so that the top surfaces of the heating elements  103  and the bottom surfaces of the fixture pins  208  are substantially aligned  920 . Next, the bolster plate  102  is pressed so that that the heating elements  103  press portions of the fabric of at least one ply  106  against the fixture pins  925 . Next  930 , portions of the fabric of at least one ply  106  are heated by the heating elements  103 . Then  935 , if this is the first ply, the press  207  is retracted, the bolster plate  102  removed, the fabric  106  is removed and translated by one-half of a corrugation period and reinserted into the bolster table  100  so as to rest upon the tops of the bolster plate pins  101 , optionally removing the heating elements  103 , and cleaning the bolster plate as needed  940 . If this is not the first ply  935 , then the flowchart of FIG. 9C illustrates that the press  207  is retracted  942 , the bolster plate pins  101  are removed, the fixture pins  208  are translated one-half of a corrugation period and with them the plies of the article thus formed  702 . Next  945  the bolster plate  102  is pressed such that the hating elements  103  press portions of the article  702  against the first set of bolster plate pins  101 , now functioning as a second set of fixture pins, and heating those portions of the article by way of the heating elements  103 . Next, the press  207  is retracted, the bolster plate  102  and the fixture pins  208 , now functioning as a second set of bolster pins  208 , are removed, and the bolster plate pins  101 , i.e., the second set of fixture pins  101 , are translated by one-half of a corrugation period in a restoring direction, and a third ply is inserted into the bolster table  100  so as to rest upon the tops of the fixture pins  208 , i.e., the second set of bolster plate pins  208 , and thereafter the heating elements are optionally removed and the bolster plate is cleaned as needed  950 . The process iterates  952  until a desired core size  720  is achieved. Thereafter, the article  720  is removed from the apparatus and post-cured  955 .  
         [0028]    The words used in this specification to describe the invention and its various embodiments are to be understood not only in the sense of their commonly defined meanings, but to include by special definition in this specification structure, material or acts beyond the scope of the commonly defined meanings. Thus if an element can be understood in the context of this specification as including more than one meaning, then its use in a claim must be understood as being generic to all possible meanings supported by the specification and by the word itself.  
         [0029]    The definitions of the words or elements of the following claims are, therefore, defined in this specification to include not only the combination of elements which are literally set forth, but all equivalent structure, material or acts for performing substantially the same function in substantially the same way to obtain substantially the same result.  
         [0030]    In addition to the equivalents of the claimed elements, obvious substitutions now or later known to one with ordinary skill in the art are defined to be within the scope of the defined elements.  
         [0031]    The claims are thus to be understood to include what is specifically illustrated and described above, what is conceptually equivalent, what can be obviously substituted and also what essentially incorporates the essential idea of the invention.