Patent Publication Number: US-2005126699-A1

Title: Process for the manufacture of composite structures

Description:
GOVERNMENT RIGHTS  
      This invention was made under National Aeronautics and Space Administration contract No.: NRA8-30, dated 2002. Therefore the United States Government retains the rights granted under this contract. 
    
    
     BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The invention relates to the field of composite structure fabrication procedures and, in particular, to a non-autoclave curing procedure for providing void free parts involving honeycomb sandwich.  
      2. Description of Related Art  
      There are numerous processes for making a honeycomb core composite structure. For example, previously cured cover sheets made up of layers of filamentary material in a resin matrix can be directly bonded by an adhesive to the honeycomb core. However, it is desirable to use pre-impregnated filamentary material as cover sheets and bond to the core as the resin in the cover sheets cure. To insure a good bond between the cover sheets and core, a layer of adhesive is placed therebetween.  
      The actual process involves laying up a preform of the core with face cover sheets on a tool surface or mold. The preform is vacuum bagged. A vacuum is drawn from within the bag, and the assembly is heated to the curing temperatures until the adhesive and resin are cured. Any trapped air or out- gassing from the resin and adhesive are drawn off because of the vacuum.  
      The bagged preform is heated to curing temperatures as the vacuum draws out any gases from the resin and/or adhesive. Alternately, the vacuum-bagged preform can be placed in an autoclave wherein the pressure applied to the preform for consolidation purposes can be increased above atmospheric pressure. This generally reduces the possibility of voids and provides a more uniform cover sheets. However, autoclaves are expensive.  
      Thus, it is a primary object of the invention to provide a process for producing high quality honeycomb sandwich structures.  
      It is another primary object of the invention to provide a process for producing honeycomb core structures without the use of an autoclave.  
      It is a further object of the invention to provide a process for producing honeycomb core structures without the use of an autoclave, but providing equal quality.  
     SUMMARY OF THE INVENTION  
      The invention is a process for making a composite structure having a honeycomb core and face sheets using vacuum bagging techniques without the use of an autoclave. In detail, the process includes the following steps: 
      1. Separately de-bulking first and second covers made of multi-layers of filamentary material sheets pre-impregnated with a resin having a first curing temperature:     2. Forming a preform sandwich assembly by placing the first cover sheets on a mold surface; placing a first layer of adhesive on the first cover, the first layer of adhesive having a second curing temperature less than the first curing temperature of the first cover sheets; positioning a honeycomb core material over said first layer of adhesive; placing a second layer of adhesive on the honeycomb core; and placing the second cover on top of the second layer of adhesive core, said second layer of adhesive having a second curing temperature less than the first curing temperature for the cover sheets;     3. Vacuum bagging the assembly;     4. Drawing a vacuum from within said vacuum bag;     5. Initially heating the assembly at a heating rate of between 0.5±0.1 degree and 1.5±0.1 degrees Fahrenheit per minute until the gel temperature of said adhesive is reached;     6. Holding the temperature at the gel temperature until the layer of adhesive has 70% cured;     7. Raising the temperature to the first curing temperature of the fiber reinforced resin; and     8. Maintaining the temperature at the first curing temperature until the resin has cured. 
 
 The above process is capable of forming composite sandwich structures having a quality equal to those fabricated by use of an autoclave. In addition, it does not require that the cover sheets be previously cured. 
   

      The novel features, which are believed to be characteristic of the invention, both as to its organization and method of operation, together with further objects and advantages thereof, will be better understood from the following description in connection with the accompanying drawings in which the presently preferred embodiment of the invention is illustrated by way of example. It is to be expressly understood, however, that the drawings are for purposes of illustration and description only and are not intended as a definition of the limits of the invention.  
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is cross-sectional view of a completed composite structure comprising a honeycomb core and cover sheets made of filamentary material in a resin matrix made by the subject process.  
       FIG. 2  is a cross-sectional view of a tool use to de-bulk the fiber-reinforced cover sheets prior to curing.  
       FIG. 3  is a cross-sectional view of a typical vacuum bagging stack-up used for making the composite structures by the subject process.  
       FIG. 4  is a cross-sectional view of the apparatus shown in  FIG. 3   
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
      Referring to  FIG. 1 , the process is primarily used to make composite sandwich structures  10  comprising a honeycomb core  11 , with cover sheets  12  and  14  made of layers of filamentary material in a pre-impregnated resin matrix bonded to the core by layers of adhesive  15  and  16 , respectively. The filamentary layers can be made from unidirectional layers stacked at various angles such as 0, 45 and 90 degrees or a combination of angles. The composition of layers will depend upon the particular application. For handling purposes, the individual layers of filamentary are pre-impregnated with a resin and partially cured into pre-impregnated tapes. In this state, particularly if stored at low temperature, they have a significant shelf life. However, when assembled into a cover sheet, they must be de-bulked prior to curing of the resin to insure that the final structure is free of voids.  
      Thus, the individual plies or stacks of plies for the cover sheets, indicated by numerals  12 A and  14 A, prior to the curing process are individually de-bulked in the apparatus  20  shown in  FIG. 2 . For example, cover sheet  14 A is place on a tool  22 , covered with an impermeable cover  24 , which is sealed to the tool sealing tape  26 , typically chromate tape. A vacuum line  27  is provided so that a vacuum can be drawn. This will cause an entrapped air or gases to be withdrawn from the cover  14 A. De-bulking process details will vary, but may include vacuum de-bulking of individual plies or stacks of plies, ultrasonic lamination, or warm or hot de-bulks.  
      Referring now to  FIGS. 3 and 4 , the apparatus, generally indicated by numeral  30 , includes a tool or mold  32  having a smooth surface  34 . A layer of porous separator cloth  36 , having a thickness of 0.005-inch, is placed on the surface  34 . A suitable separator cloth  36  is Armalon, manufactured by the Richmond Company, Norwalk, California. The fiber-reinforced cover sheet  14 A is placed over the cloth  36 . An uncured sheet  16 A of adhesive is placed over the cover sheet  14 A; A honeycomb core  11  is then placed over the sheet  16 A of adhesive. An uncured sheet  15 A of adhesive is placed over the honeycomb core. The then the cover sheet  12 A is placed over the sheet  15 A of adhesive finally forming a composite sandwich structure preform  10 A. A second sheet of separator cloth  38  is placed over the cover sheet  12 A. An aluminum or cured composite caul plate  40 , having a thickness of 0.25 inch is positioned there over.  
      Placed about the periphery of the preform  10 A, is a silicone rubber dam  42  sealed to the surface  34  by double-sided tape  44 . A suitable silicon rubber dam material is supplied by Airtech International Incorporated. A suitable tape  44  is manufactured by Fralock Company, Canoga Park, Calif. The gap  46  between the dam  42  and preform  10  should be less than 0.06 inch. A 0.001 or 0.003-inch thick non-porous sheet  48  of material is positioned over the caul plate  40  and preform  10 A and sealed to dam  42  by a second layer of double sided tape  50 . A suitable sheet  48  is A-4000 Non-porous Armalon manufactured by Airtech International Incorporated, Huntington Beach, Calif. However, fiberglass inserts  52  are positioned on top of the damn  42  creating flow paths there across. Strips of tape  54  are positioned between the fiber glass inserts  52  extend from the sheet  48  to the surface  34  of the tool  32 , thus further securing the sheet  48  to the dam  42 . A suitable fiberglass insert  52  is No.: 181-fiberglass 0.014 inch thick. A suitable tape  54  is 0.002 inch thick Flashbreaker Tape, manufactured by Airtech International Incorporated.  
      Positioned over the sheet  48  and dam  42  is a layer or layers  56  of fiberglass breather cloth (0.014 inch per ply). For example, 2-181 fiberglass manufactured by Richmond Company. A sheet  58  of 0.126-inch thick polyester material is positioned over the layers  56 . Finally, a vacuum bag  60  is positioned over the sheet  58  and sealed to the surface  34  by means of vacuum bag sealed tape  62 . Preferably the vacuum bag material is 0.003-inch thick nylon material obtainable from Airtech International Incorporated. The tape  62  is GS 100 also manufactured by Airtech International Incorporated. Vacuum port lines  64  extend to the tape  62  and are coupled to vacuum pumps (not shown). These sheets are needed to make an air tight vacuum bag without leaving impressions called “mark-offs” in the composites. The lower sheets are used to contain the resin of the prepreg resin flow is restricted to within the sheets. The dams prevent the edges from being pinched off, crushed or made thinner than the surrounding materials. The completed assembly is typical of that used to make composite structures and there are variations both as to the selection of materials, thickness thereof and the sources.  
      With the assembly illustrated in  FIG. 3  and  4 , is then place in an oven and the following cure cycle is followed. 
      1. A vacuum is drawn through the ports  64 , which causes the vacuum bag  60  to contract about compressing the sheets  48 ,  56 ,  58  about the caul plate  40  and preform  10 A. The vacuum level should be at least  25  inches of mercury.     2. The temperature is slowly raised at a rate of 0.5 to 2 degrees per minute; preferably 1 degree per minute until an intermediate temperature is reached and held to provide approximately 70 percent cure of the sheets  15 A and  16 A of adhesive.     3. The intermediate temperature is maintained for a period of approximately one-hour or until the sheets  15 A or  16 A have reached the gel state. This minimizes the formation of bubbles in the sheets of adhesive.     4. The temperature is then raised to and held for the appropriate time at the curing temperature of the first resin in the fiber-reinforced cover sheets  14 A and  14 B until the sheets have cured. At this point the preform  10 A had been transformed into the completed composite sandwich structure  10 . 
 
 Thereafter, the assembly is removed from the oven, and after the assembly has cooled sufficiently, the composite structure  10  can be removed from the tool. 
   

      Numerous structures have been fabricated using this procedure. For example: 
      1. Cover  12  and  14  sheets made 8 to 128 layers of layers of Part no. IM7/977-2 epoxy pre-impregnated graphite fibers manufactured by Cytec Corporation, Anaheim.     2. Kevlar® paper honeycomb core  11 , Part HK362-04896 manufactured by M. C. Gill Company, El Monte, Calif.     3. Sheets  15  and  16  of adhesive were EA9696AL manufactured by Loctite, Bay Point, Calif. 
 
 The process has produced structures equal in quality to autoclave produced structures wherein the cover sheets were pre-consolidated and cured. 
   

      While the invention has been described with reference to a particular embodiment, it should be understood that the embodiment is merely illustrative, as there are numerous variations and modifications, which may be made by those skilled in the art. Thus, the invention is to be construed as being limited only by the spirit and scope of the appended claims.  
     INDUSTRIAL APPLICABILITY  
      The invention has applicability to any industry using composite structures, in particular aircraft manufacturing industries.