Abstract:
Surface and thereafter applying a priming coating comprising a polytetrafluoroethylene solution containing a dispersed ceramic filler, wherein the priming coating is dried in two steps by first air-drying the coating, followed by force drying at elevated temperatures, wherein a second polytetrafluoroethylene coating is applied on the dried primer coating, and a third polytetrafluoroethylene coating is applied on the second coating while the second coating has not dried. The coated bond tool is baked to cure the second and third coatings. Prior to application of the priming coating, the metallic tool can be subjected to pretreatment such as pre-baking and grit blasting.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention is directed to an improvement in the fabrication of metallic and advanced composite components, and more particularly, but not by way of limitation to a metallic bond tool having an improved protective release coating for adhesively bonding metallic and advanced composite components on a bonding surface and its method of manufacture. 
     2. Description of the Related Art 
     It is known in the art of manufacturing adhesively bonded metallic and advanced composite components, such as those commonly used in aerostructures, to provide a metallic bond tool having at least one metallic bonding surface for receiving elements of the metallic and advanced composite component to be bonded together by being subjected to elevated temperatures and pressures. Such a metallic bond tool receives a protective release coating on such bonding surface as well as on any complementary item such as a throw in block or other detail that is positioned on such bonding surface to cooperate therewith in the positioning and adhesively bonding together of the metallic and advanced composite component. 
     At times such a metallic or advanced composite component will comprise a layer or layers of a carbon fiber material that has been pre-impregnated with an uncured resin material, such material being commonly known as “prepreg”. Such initial layer or layers of prepreg are then followed by the layup thereon of other elements of the component such as a layer of metallic honeycomb core, detail parts, doublers and an overlay of plies of prepreg. The component is thus assembled on the bonding surface of the metallic bond tool and is subsequently covered by a flexible sheet or bag material which is sealed at its edge to the bonding surface of the bond tool with a suitable sealing material such as tape. The component is then considered to be “bagged”. The bag is provided with various tubes which permit the pressure within such bag to be controlled and to permit the expulsion of volatiles when the bagged component secured to the bonding surface of the bond tool is positioned within an autoclave and subjected to an elevated temperature and elevated pressure in a predetermined timed cycle to cure a bonded component. A typical temperature would be 350° F. at 35 to 70 psi for a predetermined timed cycle. After the part has been cured the bond tool is removed from the oven and the bag material and tape are removed to permit the component to be removed from the bond tool for further processing. 
     Another typical metallic and advanced composite component that is manufactured in this manner is the example of positioning a metallic honeycomb core portion directly upon the bonding surface of the bond tool and then laying up layers of prepreg over such honeycomb core in a preferred orientation and then after performing the desired bagging this part is cured in the manner previously described. After debagging such part it is used in connection with the fabrication of a subsequent component. 
     Quite obviously it is desired to preclude to the greatest extent possible any tendency of the cured component to bond directly in any extent to the bonding surface and to also enhance the removal of the bagging tape and any residue remaining from the bagging and adhesive bonding operation. To accomplish these desired objectives, it is known to apply a protective release agent directly to the bonding surface. One example of such a protective release agent that is commonly used on such bonding tools is hand applied and air dried. However, such a material exudes environmentally undesirable emissions such as dibutylether during application and d-limone during cleaning. Further, such a release material has shown a tendency to transfer to bonding edges of the metallic honeycomb core in applications such as noted above and are difficult or impossible to easily remove thereby requiring the bonded part to be scrapped. 
     To overcome this problem it has been found necessary to provide as a release agent a non stretchable film containing a fiberglass scrim material. While this material precludes the transfer of the release coating material to the edges of the metallic honeycomb core portion, it is been found that such material is relatively easily damaged by the sharp edges of the honeycomb core and must be periodically replaced. To replace such material requires first the removal of such material in a operation that typically requires in excess of 8 worker hours and the subsequent replacement in a an operation that also requires 8-12 worker hours for a typical tool. 
     Other disadvantages of the use of such materials involves the cost and difficulty of the disposal of such hazardous wastes. The removal of such release materials from the metallic bonding surface involves laborious scraping and sanding that result in employee exposure to undesired particulates. 
     Thus, it is evident that a need exists for an improved release coating for the bonding tools used in the manufacture of advanced composite components, as for the aerospace industry, which overcomes the numerous problems associated with the known release coatings and which in addition to providing a release agent that has better release properties also is significantly more durable and thereby eliminating many of the environmental and safety issues presented by the release coatings presently employed. 
     It is therefore an objective of the present invention to provide a new and improved metallic bond tool having an improved protective release coating for adhesively bonding metallic and advanced composite components on a bonding surface and its method of manufacture. 
     SUMMARY OF THE INVENTION 
     The foregoing problems are overcome and other advantages are provided by a new improved metallic bond tool having an improved ceramic release coating. 
     Briefly stated, the present invention contemplates a process for making a metallic bond tool having a protective release coating for adhesively bonding metallic and advanced composite components in an autoclave at an elevated temperature and elevated pressure. The process includes the initial step of cleaning a metal bond tool having at least one metallic bonding surface for receiving elements of a metallic or advanced composite to be adhesively bonded together. This cleaning step is performed by heating the metallic tool to an elevated temperature to burn off any foreign residue or machining oil that might be present on the bond tool. The bond surface is then grit blasted with a suitable grit and blown dry with clean dry air. The bond surface is then lightly sanded as necessary to remove any remaining imperfections and blown clean. 
     The bond surface then receives a sprayed coat of a ceramic reinforced tetrapolyfluoroethylene primer solution to a desired uniform thickness. The primer coat is then air dried for a predetermined time and thereafter force dried in an oven at an elevated temperature that is below any temperature that would materially effect the metal characteristics such as by annealing such bond tool. The primer coat is then followed by application of a polytetrafluoroethylene containing mid coat solution that is sprayed on the bond tool surface in two box coats. The mid coat application is then followed by the application of a top coat of a polytetrafluoroethylene containing solution while the mid coat is still wet. The coated bond tool is air dried for a predetermined period of time and then cured in an oven at an elevated temperature that is sufficient to cure the mid and top coats and yet is below any temperature that might materially effect adversely the metal characteristics of the metallic bond tool. 
     The coated metallic bond tool is then buffed with a heavy organic cloth material and is then rinsed with water to remove any polytetrafluoroethylene dust generated by such buffing. Any bonding aids that would normally be secured to the bonding surface to assist in locating and positioning elements of the advanced composite component to be adhesively bonded together, such as throw in blocks, are coated in the same manner. Thus, the present invention provides a metallic bond tool that is provides an improved bonding surface that has excellent durability, does not transfer to the component being bonded together and easily releases elements such as tapes used to bag the advanced composite component for curing without leaving undue residue on the tool. Any residue left on the tool can be cleaned off within a few minutes thereby enabling the improved bond tool to be immediately recycled into production. 
     The foregoing has outlined rather broadly the more pertinent and important features of the present invention in order that the detailed description of the invention that follows may be better understood so that the present contribution to the art may be more fully appreciated. Additional features of the invention will be described hereinafter which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and the disclosed specific embodiments may be readily utilized as a basis for modifying or designing other structures and methods for carrying out the same purposes of the present invention. It should be also be realized by those skilled in the art that such equivalent constructions and methods do not depart from the spirit and scope of the invention as set forth in the appended claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing aspects and other features of the invention are explained in the following description, taken in connection with the accompanying drawings, wherein: 
     FIG. 1 is a perspective of a metallic bond tool of the type employed in the practice of the present invention. 
     FIG. 2 is a perspective of the metallic bond tool of FIG. 1, the bonding surface of which has been coated with an improved ceramic release coating in accordance with the present invention. 
     FIG. 3 is a flow chart of the process employed to provide the bonding surface of a metallic bond tool with an improved ceramic release coating. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to the drawings in detail and in particular to FIGS. 1 and 2, the reference character  10  generally refers to a metallic bond tool used in the practice of the present invention. The metallic bond tool  10  includes a suitable support base  12  adapted to be lifted by a fork lift means (not shown) for transport and positioning in a suitable autoclave. The support base  12  is provided with a substructure  14  comprising a plurality of welded ribs to which is affixed the bonding surface  16 . The metallic bond tool  10  is illustrated as being exemplary of a bond tool used in the aerostructures industry to make advanced composite and metallic component that are adhesively bonded together on the bonding surface  16  in an autoclave at a predetermined cycle of elevated temperatures and pressures. FIG. 2 illustrates the metallic bond tool of FIG. 1 in which the bonding surface  16  has received a ceramic reinforced release coating  18  in accordance with the present invention. 
     In the practice of the present invention it must be stressed that certain facilities must be used and constant attention be given to maintaining cleanliness of the bond tool itself and of its surroundings. To employ the present invention, a suitable spray booth with a dry filter or water wash system must be provided as well as a grit blasting facility capable of using 80-120 grit aluminum oxide. An oven for receiving an object the size of a metallic bond tool and rated to 800° F. must also be provided. 
     Referring now to the flow chart of FIG. 1, a metallic bond tool  10  to which the improved release coating is to be applied to the bonding surface  16  is subjected to a pre-inspection to determine what steps must be taken to insure its cleanliness. If the metallic bond tool  10  has been previously utilized in bonding operations the previously utilized release coating must be removed as well as any obviously flammable materials adhering to the bonding surface  16 . If the metallic bond tool  10  has not been provided with a release coating and used in production it still must be cleaned because of the residue remaining on the bonding surface  16  from machining oil used in the fabrication of such surface  16 . 
     This initial cleaning is provided by pre-baking the metallic bond tool  10  in a suitable oven at a temperature of 750±10° F. for a period of 3-5 minutes. After the bond tool  10  has been allowed to cool it is then moved to the grit blast area. After the pre-bake operation has been completed it should be stressed that the tool  10  as well as other parts of the tool  10  to be coated should by handled with clean white gloves. 
     Following the pre-bake operation to burn off any foreign residue such as a previous release coating or machine oil, the bonding surface  16  of the bond tool  10  is subjected to grit blasting with 80-120 grit aluminum oxide at a pressure of 40-60 psi. The air used for grit blasting should be clean dry factory air. Following the grit blasting the bond surface  16  of an aluminum bond tool  10  should be lightly sanded with 400-600 grit sand paper to remove any remaining aluminum oxide particles and any surface imperfections. The bonding surface  16  of any steel bond tool  10  should be lightly sanded with 360 grit sand paper to remove any remaining aluminum oxide particles and any surface imperfections. 
     The bond tool  10  should be blown with clean dry air to make sure that all residue is blown from the bond surface  16  of the tool and care should be taken not to handle the bonding surface  16  to avoid introducing any contaminants thereto. 
     The next step is to apply by spraying, by means of a dedicated sprayer, a primer coat of a ceramic reinforced polytetrafluoroethylene primer on to the bonding surface  16  which has been suitably masked to avoid over spray. The primer is preferably TEFLON® PRIMER—BLACK made by the DuPont Co. of Wilmington, Del. 19098 and identified by it as Product Code 857-101. The Material Safety Data Sheet for this product indicates that it contains the following ingredients: 
     POLYTETRAFLUOROETHYLENE 
     AMORPHOUS SILICA 
     POLYAMIDE-IMIDE POLYMER 
     TETRAFLUOROETHYLENE/ PERFLUORINATED VINYLETHER 
     WATER 
     TRIETHYLAMINE 
     FURFURYL ALCOHOL 
     METHYL PYRROLIDONE 
     CARBON BLACK 
     SODIUM ALUMINUM SULPHO-SILICATE (ULTRAMARINE BLUE) 
     ALUMINUM OXIDE 
     This primer should be applied by a suitable spray gun to a dry film thickness of approximately 0.6-0.7 mils (15-18 microns). Following application of this primer to the bonding surface  16  the bond tool  10  should be air dried for a minimum of 40-60 minutes in a contamination free area and then placed in an oven. The temperature of the oven should then be increased from ambient to a temperature of 250-300° F. and held there for a time period of 15-25 minutes of force dry the primer. The tool  10  is then removed from the oven and permitted to cool to ambient temperature. This temperature is well below any temperature that would materially adversely effect the metal characteristics of the metallic bond tool  10 . 
     The tool  10  is then moved back into the spray booth for application to the bonding surface  16  by spraying of a product identified as TEFLON® TOPCOAT—BLACK, Product Code No. 857-202 made by the DuPont Co. of Wilmington, Del. 19890. This product is noted in the applicable Material Safety Data Sheet as having the following ingredients: 
     POLYTETRAFLUOROETHYLENE 
     TETRAFLUOROETHYLENE/PERFLUORINATED VINYLETHER 
     ACRYLIC POLYMER 
     DIETHYLENE GLYCOL MONOBUTYL ETHER 
     OLEIC ACID 
     WATER 
     TRIETHANOLAMINE 
     AROMATIC HYDROCARBON 
     OCTYLPHENOXYPOLYETHOXYETHANOL SURFACTANT 
     CARBON BLACK 
     ALUMINUM OXIDE 
     This mid coat is applied with a suitable dedicated spray gun in two box coats to a dry film thickness of between 0.3-0.4 mils (10-13 microns) with the appearance of the bonding surface  16  after such application having a semi-wet look. 
     A top coat is then applied by a suitable dedicated spray gun to the semi-wet mid coat. This top coat is identified by the DuPont Co. as being TEFLON® TOPCOAT—CLEAR having a Product No. 857-301. This top coat is noted in the applicable Material Safety Data Sheet as having the following ingredients: 
     POLYTETRAFLUOROETHYLENE 
     ACRYLIC POLYMER 
     DIETHYLENE GLYCOL MONOBUTYL ETHER 
     OLEIC ACID 
     WATER 
     TRIETHANOLAMINE 
     AROMATIC HYDROCARBON 
     OCTYLPHENOXPOLYETHOXYETHANOL SURFACTANT 
     The noted top coat should be applied on the wet mid coat as a wet spray to a dry film thickness of 0.3-0.4 (8-10 microns) thereby resulting in a total dry film coating thickness for the bonding surface  16  of 1.3-1.6 mils (33-40 microns). 
     Following the application of the top coat to the bonding surface  16  the bond tool should be air dried in a contamination free area for 40-60 minutes, during which time all masking used to prevent over spraying of the base  12  or the substructure  14  should be removed. 
     The coated bond tool  10  should then be placed in a suitable oven having an ambient temperature and the temperature of the oven should then be increased to 775+10° F. and held there for a period of 10-15 minutes The upper temperature of the oven during such curing period is below a temperature that would materially adversely effect metal characteristics of the metallic bond tool  10  such as annealing. The oven should then be turned off and the doors opened to permit the coated bond tool  10  now having a bond surface  16  having the appearance seen in FIG. 2 to cool to a temperature where it can be safely removed from the oven. 
     The bonding surface  16  of the bond tool  10 , after it has been allowed to cool to ambient temperature, should then be polished or buffed by a heavy organic cloth material such as cotton or wool. To ensure that any polytetrafluoroethylene dust is throughly removed from the bond tool  10 , it should be thoroughly rinsed with water to ensure that all such dust is removed from the base  12 , substructure  14  and the bonding surface  16 . 
     Thus, the present invention provides a metallic bond tool that has an improved release coating on its bonding surface that has an excellent release property as well as durability and abrasion resistance. The present invention provides increased durability of the coating of the bonding surface by providing a coating having a coefficient of thermal expansion commensurate with that of the metal to the bonding surface  16 . While the invention has been described with respect to the particular bonding surface of a metallic bond tool, the invention would also contemplate being used on other parts of the bond tool, such as throw in blocks, that are customarily used in the adhesive bonding of metallic and advanced composite aerostructure components. 
     The present disclosure includes that contained in the appended claims as well as that of the foregoing description. Although this invention has been described in its preferred forms with a certain degree of particularity, it is understood that the present disclosure of the preferred form has been made only by way of example and numerous changes in the details of construction and combination and arrangement of parts and method steps may be resorted to without departing from the spirit and scope of the invention.