HIGH TEMPERATURE RESISTANT PERFORATED POLYMER COMPOSITE MATERIAL AND METHOD FOR MAKING SAME

Abstract: A method for fabricating a perforated high temperature fiber reinforced polymer composite material which includes partially curing a polymer composite material to a viscoelastic state without inducing complete cure. The polymer composite material is formable when further heated, but does not experience excessive resin flow during final curing. According to the methods of the present invention, the polymer composite material may be perforated in the partially cured state, or prior to the partially cured state. A rigid pin mat apparatus of the present invention may advantageously be utilized to perforate the composite material. After the partial curing, the polymer composite material may be contoured utilizing a flexible mat pin apparatus of the present invention, or may be contoured utilizing forming tools generally known to the art. The method may advantageously be utilized with fiber-reinforced polymer composite starting materials, such as graphite/bismaleimide preimpregnated fabric, to produce perforated and contoured high temperature fiber reinforced polymer composite materials for sound suppression applications.

Description: High temperature resistant perforated polymer composite material and method for making same FIELD OF THE INVENTION The present invention relates to methods for fabricating perforated composites and composite laminates, that are capable of being contoured and able to withstand high temperatures to be used for sound suppression 5 applications. Also disclosed are flexible and rigid pin mat apparatuses that may be ' utilized in fabricating perforated composites. BACKGROUND The phrase "fiber reinforced polymer composite 0 material" is generally utilized to describe a composite material that comprises a cured polymeric resin matrix that incorporates reinforcing fibers. The reinforcing fibers are generally incorporated in the form of a woven fiber mat. Perforated fiber reinforced polymer composites are 5 widely utilized as acoustic skins and sound suppression treatments. In particular, perforated fiber reinforced polymer composites are used in the sound suppression systems of aircraft engine nacelles. High temperature polymer composite materials are generally necessary for the inner 0 cowl bondment of a jet engine fan reverser, which is subjected to high temperatures during combustion. Often the acoustic skins and sound suppression treatments require shaping or contouring the high temperature polymer composite material to fit the shape of the engine nacelle. 5 Graphite fiber/bismaleimide preimpregnated fabric is a commercially available fiber reinforced polymer composite material that is able to withstand the high temperature environment of aircraft engine nacelles. Existing methods for fabricating perforated acoustic skins and treatments are 0 not suitable for forming perforated graphite fiber/bismaleimide preimpregnated polymer fabric composites, or composite laminates, due to the high temperature capability and toughness of graphite fiber/bismaleimide preimpregnated fabric. Processing by existing methods leads to extensive fiber damage, gross porosity, delamination and matrix cracking, with a subsequent loss in mechanical and acoustic performance. Additionally, the flexible pin mats are often not capable of withstanding the high temperatures necessary to cure the resins used in the composites. SUMMARY OF THE INVENTION We have discovered ^ methods for producing perforated high temperature fiber reinforced polymer composite materials, and laminates, that overcome the deficiencies of existing methods. As used herein, "high temperature fiber reinforced polymer composite material" is used to describe fiber reinforced composite materials that are able to withstand high temperatures, i.e. above approximately 176.6 β C (350 β F) , without suffering a degradation in performance characteristics. As will be understood from the following description, the methods of the present invention may also be utilized in the formation of perforated fiber reinforced polymer composite materials and laminates that are not required to withstand high temperatures. The term "perforated" is utilized to describe a material that has a plurality of holes. We have also discovered flexible and rigid pin mat apparatuses especially well suited for use in the fabricating of perforated high temperature fiber reinforced polymer composite materials. According to a method of the present invention a three step process is utilized to fabricate a perforated and contoured fiber reinforced polymer composite material or laminate having a high temperature capability. First, resin rheology is utilized to precure or "stage" a flat laminate (or composite) using heat and pressure. The precured, or staged, laminate is cured to a viscoelastic state, without inducing a complete cure, wherein the staged laminate may be handled and formed but still not experience excessive resin flow upon final cure. Second, the staged laminate is subjected to an automated drilling process to create a perforated staged laminate with a high hole density. Third, the perforated staged laminate is contoured, if desired, and subjected to final curing while the perforations are kept open and free of blockage from any excess resin flow. This method overcomes the deficiencies in heretofore existing methods and may advantageously be utilized to produce perforated and contoured high temperature fiber reinforced polymer composite laminates able to withstand the stresses of a high temperature environment. According to another method of the present invention, a perforated and contoured high temperature fiber reinforced polymer composite material, or laminate, may be fabricated by the following steps. First, a preimpregnated fiber reinforced polymer composite material, capable upon final cure of withstanding high temperatures, is pressed over a pin mat to form a perforated preimpregnated fiber reinforced polymer composite material. The mat and the pins (projections) are sufficiently rigid to withstand the pressures utilized to force the pins through the preimpregnated composite without deforming. If desired, the pins may fabricated so that they end in a point to facilitate the pressing of the pins through the composite material. Further details relating to the mat are set forth below. The mat/material structure is subsequently staged to a state of partial cure using heat and pressure. The mat and material are then separated thereby leaving a partially cured perforated polymer composite material. The staging of the structure is performed to a state wherein upon separation the material may be formed when heated but does not exhibit substantial flow of the resin during the subsequent completion of the cure. This state is similar, in feel, to a swatch of leather. Finally, the separated perforated polymer composite material, is formed, using a contoured mold, and the cure completed, under heat and pressure, on the contoured mold. The result is a perforated and contoured high temperature fiber reinforced polymer composite material. According to another method of the present invention, a high temperature fiber reinforced polymer composite material may be fabricated by the following series of steps. First, resin rheology is utilized to precure or "stage" a flat laminate (composite) using heat and pressure. The precured, or staged, laminate is cured to a viscoelastic state, without inducing a complete cure, wherein the staged laminate may be handled and formed but still not experience excessive resin flow upon final cure. Second, the staged laminate is subjected to an automated drilling process to create a perforated staged laminate with a high hole density. Alternatively, a die punching process may be utilized to create the perforated stage laminate. Next, two flexible sheets, able to withstand high temperatures are fabricated► The first, "male", sheet includes an array of pin-like projections, also referred to as pins , and the second, "female", sheet includes an array of orifices or holes corresponding to the pin-like projections of the male sheet such that the male and female sheets may be joined. The pin-like projections and holes are located in a manner such that they correspond to the perforations in the perforated staged laminate so that the laminate may be sandwiched between the mats. In addition, the outside dimensions of the mats will generally roughly correspond to, or exceed, the outside dimensions of the laminate. The perforated laminate is sandwiched between the male and female sheets and the resulting structure is contoured into the desired shape. Using heat and pressure, the perforated laminate is cured to a final state in this sandwiched configuration. The sheets are then removed to produce a perforated and contoured high temperature fiber reinforced polymer composite material. Each of the methods of the present invention is especially well suited for use in fabricating perforated and contoured fiber reinforced polymer composite materials from graphite/bismaleimide preimpregnated fabric. This fabric is commercially available from a variety of sources including BASF-Narmco, Hexcel, and US Polymeric. The methods of the present invention may also be utilized to fabricate perforated composite materials from polyimides, high temperature epoxy systems, reinforced thermoplastic systems such as polyether ether ketone, polyphenyl sulfide, polyether-imide, polyamides, polysulfones, polyamide-imide systems and from other starting material systems known to those of ordinary skill in the art. As will be understood from the foregoing description, the methods of the present invention include the formation of a partially cured fiber reinforced polymer composite materials. We have discovered that this step advantageously overcomes the deficiencies in heretofore known methods for forming perforated and contoured fiber reinforced polymer composite materials. We have also discovered an apparatus for use in a process for fabricating perforated fiber reinforced polymer composite materials. The apparatus comprises a plurality of sheets wherein a first sheet has a plurality of pin-like projections (also referred to herein as pins ) , preferably arranged in an array like fashion, extending from a first surface of the sheet and a second sheet has a plurality of orifices or holes arranged in a similar manner to the pin¬ like projections on the first sheet such that the first and second sheets may be juxtaposed to align the orifices and pin-like projections and permit the two sheets to slide together. The first sheet, containing the pins, may be described as he "male" sheet and the second sheet with the corresponding holes, may be described as the "female" sheet. When fit together the male and female sheets fit together form a single unit. In a first embodiment of the apparatus, advantageous for use in a process for fabricating perforated and contoured fiber reinforced polymer composite materials, the male and female sheets are fabricated from a flexible polymer material. This embodiment is hereinafter referred to as the flexible pin mat apparatus. In another embodiment of the apparatus of the present invention, also advantageous for use in a process for fabricating perforated and contoured fiber reinforced polymer composite materials, the male and female sheets are fabricated from a substantially rigid polymer, metal, or alloy material. This embodiment is hereinafter referred to as the rigid pin mat apparatus. Other embodiments of the apparatus of the present invention, not described in detail herein, will be apparent to those of ordinary skill in the art from the following description. As will be understood from the foregoing description of the methods of the present invention, the flexible mat apparatus of the present invention may advantageously be utilized in the fabrication of perforated and contoured high temperature fiber reinforced polymer composite materials. The design of the sheets allows a perforated composite material to be sandwiched between the male and female sheets and contoured and/or cured. The sheets of the flexible mat apparatus of the present invention may be fabricated from any material capable of withstanding the curing temperatures of the resins utilized in the composite materials without degrading. Generally a high durometer, high temperature elastomer is utilized. Preferably the sheets are fabricated from an elastomer or other polymeric material. More preferably the sheets are fabricated from a polymeric material that will not contaminate the fiber reinforced polymer composite material and provide flexibility and dimensional stability at high temperatures. The sheets may be fabricated from a polymer material that allows the sheets to be contoured while joined together. Suitable flexible polymer materials include, but are not limited to, Silastic ® J RTV silicone rubber, manufactured and sold by Dow Corning, Midland, MI; Dapco ® 10 high temperature RTV silicone, manufactured and sold by D Aircraft Corporation, Los Angeles, CA; Dapco ® 60 high temperature RTV silicone, manufactured and sold by D Aircraft Corporation, Los Angeles, CA; 5887 RTV silicone rubber, manufactured and sold by Emerson and Cumming, St. Louis, MO; KE 1604 high strength RTV silicone rubber, manufactured and sold by Shin-Etsu, Chino, CA; TPE-4211-60 advanced thermoplastic elastomer, manufactured and sold by Monsanto Corporation, St. Louis, MO; TPE-4211-80 advanced thermoplastic elastomer, manufactured and sold by Monsanto Corporation, St. Louis, MO; KE 1223 liquid silicone rubber, manufactured and sold by Shin-Etsu, Chino, CA; Ricon ® high vinyl polybutadiene liquid elastomer system, manufactured and sold by Advanced Resins Inc. The male sheet of the flexible mat apparatus of the present invention may be formed in manners known to the art, such as by casting and curing the polymer on a drilled aluminum plate with a mirror image of the desired pin/hole pattern. Similarly, the female sheet of the flexible mat apparatus of the present invention may be formed in manners known to the art such as by casting and curing the polymer on an aluminum sheet with pins corresponding to the pins on the male sheet, or by punching or drilling a preformed solid polymer sheet. The number of pins and the corresponding hole density of the female sheet may be varied depending on the desired hole density of the perforated composite material to be formed utilizing the sheets. Preferably, the diameter of the pins is between 0.0889 and 0.254 cm (0.035 and 0.100 inch), with a tolerance of plus or minus 0.0058 to 0.0127 cm (0.002 to 0.005 inch), and the pins are spaced apart at a center to center distance of 0.1905 to 0.381 cm (0.075 to 0.150 inch), with a tolerance of plus or minus 0.00508 to 0.0127 cm (0.002 to 0.005 inch). Preferably the height of the pins is generally between 0.1905 to 0.381 cm (0.075 and 0.150 inch). The rigid pin mat apparatus of the present invention is similar to the flexible mat apparatus of the present invention except the pins on the male sheet of the rigid pin mat apparatus are sufficiently rigid to withstand the pressures generated in forcing the pins through the composite material being fabricated. Thus the male sheet of the rigid pin mat apparatus should be fabricated from a polymeric material that is substantially rigid when cured. Alternatively, the male sheet of the rigid pin mat apparatus, may be fabricated from a metal or alloy. Additionally, the male and female sheets of the rigid pin mat apparatus of the present invention are fabricated from a material capable of withstanding the curing temperatures of the resins utilized in the composite materials without degrading. Preferably the sheets are fabricated from a material that will not contaminate the fiber reinforced polymer composite material and provide toughness abrasion resistance and dimensional stability. Suitable materials for use in fabricating the male sheet of the rigid pin mat apparatus of the present invention include, but are not limited to: polyetherimide, commercially available from GE Plastics Lowell, MA; polypropylene; Nylon 6/6, manufactured and sold by the DuPont Corporation, Wilmington, DE; Polysulfone, manufactured and sold by Phillips, Burtlesville, OR. Alternatively, the male sheet of the rigid pin mat apparatus of the present invention may be fabricated from steel, aluminum, or an alloy. The female sheet of the rigid pin mat apparatus may be fabricated from any of these materials or may be fabricated from the materials described above for fabricating the male and female sheet of the flexible pin mat apparatus. If the male sheet of the rigid pin mat apparatus is fabricated from a polymeric material, the sheet may be formed in the, manners described above for fabricating the male sheet of the flexible pin mat apparatus of the present invention. If the male sheet is fabricated from a metal, it may be fabricated in any manner known to the art for fabricating a metal pin mat. Preferably, in either case, the tips of the pins are pointed to facilitate the puncturing of the composite material being fabricated. As in the case of the flexible mat apparatus of the present invention, in the rigid pin mat apparatus of the present invention the number of pins on the male sheet and the corresponding hole density of the female sheet may be varied depending on the desired hole density of the perforated composite material to be formed utilizing the sheets. Preferably, the cross-sectional diameter of the pins is between 0.0889 and 0.1524 cm (0.035 and 0.060 inch), with a tolerance of plus or minus 0.00508 to 0.0127 cm (0.002 to 0.005 inch), and the pins are spaced apart at a center to center distance of 0.1905 to 0.381 cm (0.075 to 0.150 inch), with a tolerance of plus or minus 0.00508 to 0.0127 cm (0.002 to 0.005 inch). Preferably the height of the pins is between 0.1905 and 0.381 cm (0.075 and 0.150 inch) . An advantage of the methods of the present invention is that the methods may be utilized to produce mechanically strong perforated and contoured fiber reinforced polymer composite materials having high temperature resistance and good acoustic properties. Another advantage of the methods of the present invention is that the methods may be utilized ^ to fabricate high temperature fiber reinforced polymer composite material without extensive fiber damage, gross porosity, and matrix cracking which cause a loss in mechanical and acoustic performance. A further advantage of the methods of the present invention is that the methods offer a cost efficient way to produce high temperature perforated fiber reinforced polymer composite materials and laminates. An advantage of the apparatus of the present invention is that the apparatus may be utilized in the methods of the present invention. Another advantage of the apparatus of the present invention is that the apparatus provides a controllable and uniform molded perforation hole size in the high temperature perforated fiber reinforced polymer composite materials and laminates produced using a method of the present invention. The uniform perforation hole size imparts predictable, reliable acoustic performance. A further advantage of the apparatus of the present invention is that the female sheet constrains the pins on the male sheet from moving and changing dimension during the thermal cycle of curing the composite material in the methods of the present invention, thereby easing the removal of the composite material from the sheets. A still further advantage of the apparatus of the present invention is that the sheets help to control resin flow from the composite material sandwiched between the sheets according to a method of the present invention. Additionally, the pins on the male sheet act as physical barriers to prevent the flow of resin into the perforations in the composite material. This allows consolidation pressures during final cure up to 85 pounds per square inch. Pressures in this range form perforated fiber reinforced polymer composites that are free of voids. A still further advantage of the apparatus of the present invention is that the sheets may be fabricated from a material that allows them to be contoured thereby allowing the fabrication of contoured composite materials according to a method of the present invention. An advantage of the apparatus and the methods of the present invention is that the apparatus may be easily and economically fabricated to produce perforated composites of different sizes, and different perforation hole densities and perforation hole arrangements according to a method of the present invention. Further advantages of the methods and apparatus of the present invention will become apparent from the following description. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 depicts a male sheet of an embodiment of the apparatus of the present invention. Figure la is a blow-up of a section of Figure 1. Figure 2 depicts a female sheet of an embodiment of the apparatus of the present invention. Figure 2a is a blow-up of a section of Figure 2. Figure 3 depicts a perforated fiber reinforced polymer composite material sandwiched between a male and a female sheet of the apparatus of the present invention prior to final curing of the composite material. Figure 4 depicts a high temperature perforated fiber reinforced polymer composite material manufactured according to a method of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Figures 1 and 2 depict an embodiment of the apparatus of the present invention. Referring to Figure 1, both the flexible pin mat apparatus and the rigid pin mat apparatus of the present invention comprise a first male sheet 10, having a plurality of pins 20. The pins 20 are depicted in Figure 1 arranged in a variable 16 x 16 or greater array wherein each pin-like projection is equally spaced from adjacent pins. As shown in Figure 1 the outside dimensions of the array are smaller than the dimensions of first sheet 10, however the present invention should not be considered limited to this configuration. Similarly, although Figure 1 depicts a symmetric array of substantially circular pins, the pins the sheet may include any number and arrangement of pins. The pins may be substantially circular in cross section or may be of any other shape, such as a polygonal shape or an elliptical shape. The cross sectional shape chosen for the pins depends on the shape chosen for the perforations ip. the composite material fabricated utilizing the apparatus according to a method of the present invention. Additionally, the pins of the rigid pin mat apparatus of the present invention may be fabricated with pointed ends to facilitate the puncturing of the composite material. Figure la depicts a portion of the sheet depicted in Figure 1. Figure 2 depicts a second sheet 30, having orifices 40 corresponding to the pins 20 shown in Figure 1. Figure 2a depicts a portion of the sheet depicted in Figure 2. As shown in Figure 2, orifices 40 are arranged in a similar configuration to the pins of the first sheet in order that the first and second sheets may be juxtaposed and pins 20 slid into orifices 40. Figure 3 depicts an elevation view of such an arrangement with a perforated fiber reinforced polymer " composite material, 50, sandwiched between the sheets 10 and 30, prior to final curing of the composite material. Other details concerning the sheets of the present invention will be apparent to one of ordinary skill in the art from the following description of the use of the apparatus of the present invention in the methods of the present invention. The male sheet of the flexible and rigid pin mat apparatuses may be fabricated by casting a high durometer, high temperature elastomer, such as the ones listed above, on a drilled aluminum plate having the desired hole pattern. As will be understood from the description of the methods of the present invention contained in the preceding and following paragraphs, the hole pattern of the aluminum sheet should correspond to the desired hole pattern of the perforations in the composite material being fabricated. The female sheet may be fabricated by casting off the male sheet, curing and separating the two sheets. Once cured the male and female sheets should be capable of being heated to a temperature higher than the temperature necessary to final cure the composite material being fabricated without losing their dimensional tolerance or stability. The male ,sheet of the rigid pin mat apparatus may also be fabricated from steel or another metal or metal alloy in manners generally known for fabricating metal pin mats. The female sheet of the rigid pin mat apparatus may be fabricated from a high temperature, high durometer elastomer casted off the male sheet. The methods of the present invention are described below in sequential fashion following the order set forth above. In broad terms the methods of the present invention comprise partially curing a polymer composite material to a viscoelastic state without inducing complete cure. The polymer composite material is formable when further heated, but does not experience excessive resin flow during final curing. According to the methods of the present invention, the polymer composite material may be perforated in the partially cured state, or prior to the partially cured state. The rigid pin mat apparatus of the present invention may advantageously be utilized to perforate the composite material. After partial curing, the polymer composite material may be contoured utilizing the flexible mat pin apparatus of the present invention, or may be contoured utilizing forming tools generally known to the art. After contouring, curing of the polymer composite material is completed to yield a perforated polymer composite material. This method may advantageously be utilized with fiber- reinforced polymer composite starting materials, such as graphite/bismaleimide preimpregnated fabric to produce perforated and contoured fiber reinforced polymer composite materials. The methods of the present invention are set forth in greater detail in the following paragraphs. According to a method of the present invention a three step process is utilized to fabricate a perforated and contoured high temperature fiber reinforced polymer composite material or laminate. First, resin rheology is utilized to precure or "stage" a flat laminate (or composite) using heat and pressure. The starting material is generally a preimpregnated fabric such as graphite fiber/bismaleάmide preimpregnated fabric. One or more plies may be laminated and processed in manners known in the art. Additionally, surfacing plies such as adhesives, acoustic screens and core isolators may be readily integrated into the laminate and processed. The material combination to be perforated is consolidated in an identical manner to either traditional autoclave or compression mold processing. According to the method of the present invention, the assembly of plies, in the case of a laminate, or the starting material in the case of a preimpregnated fabric without additional layers, is heated under sufficient pressure and temperature, and for a sufficient time to advance the resin to a semi-cured point in the resin's rheological profile wherein the laminate is formable when further heated, but does not experience excessive resin flow upon further curing. Such resin flow would block the acoustic flow path by partially or completely blocking the perforations, and thereby severely limit the acoustic performance of the laminate. Generally, in order to reach the semi-cured state the pressure utilized ranges from 344.75 KPa to 586.075 KPa (50 psi to 85psi) and the material is heated to a temperature of from 93.3°C to 148.8°C (200 β F to 300°F). Depending on the pressure and temperature utilized, the time to reach the semi-cured state ranges from 15 mins to 1 hour. Once the semi-cured state is reached the material is perforated by boring or drilling. For example, a multiaxis horizontal boring machine with automatic tool changer may be utilized to machine multiple holes in the semi-cured material. Drilling with both diamond and carbide tools, as well as end mills, is also possible. Depending on the starting materials utilized, the semi-cured material may be drilled without lubricants to prevent contamination of the composite material laminate. Additionally, multiple semi- cured laminates or composite materials may be drilled or bored simultaneously by stacking the individual laminates or composite materials and utilizing a fixture to apply even compacting pressure to the stack. The number, size and pattern of the holes may be varied depending on the acoustical performance desired in the final product. Good acoustical performance may be achieved with the following hole pattern: Hole Density = 7,000 holes/0.0929 square meters (square foot) Laminate Porosity = 33% Hole diameter initial (semi-cured) = 0.26416 cm (0.104 inch) Hole diameter final (final cure) = 0.23876 cm (0.094 inch) Center to Center hole distance = 0.3683 cm (0.145 inch) Array Type = plain square (equal number of rows and columns) . After drilling the semi-cured composite material or laminate is placed in a tool and if a contour is desired in the final product, the semi-cured composite material or laminate is formed using pressure and heat if required. Methods of forming and contouring are known to those of ordinary skill in the art and include vacuum assisted or pressure forming. The composite material or laminate is final cured to the desired configuration using pressure and heat to produce the composite part. As set forth above, if the proper semi-cured state is achieved there will be no significant resin flow during final cure and therefore the perforations drilled into the composite material or laminate will remain clear. In another method of the present invention, the flexible pin mat apparatus of the present invention is utilized during contouring and final curing of the composite material or laminate. According to this method of the present invention a fiber reinforced polymer composite material is produced as follows. First the starting material, generally a preimpregnated fabric such as graphite fiber/bismaleimide preimpregnated fabric, is perforated. One or more plies of the starting material may be laminated and processed in manners known in the art. Additionally, surfacing plies such as adhesives, acoustic screens and core isolators may be readily integrated into the laminate and processed. Alternatively, individual plies may be perforated and then laminated in a subsequent operation. The starting material or laminate may be perforated in a variety of ways that are within the skill of one having ordinary skill in the art. For example the material or laminate is perforated by feeding the fiber-reinforced composite material, with a separator on both sides, through two sets of feeder rolls and a punch. One feeder roll is located upstream of the punch and the other is located downstream on the exit side of the punch. The rolls serve to feed the material automatically through the punch that can be made of either single or multiple rows of pin punches which have a matching die set. Alternatively, the starting material or laminate may be semi-cured and then perforated by drilling with a multiaxis horizontal boring machine with automatic tool changer in the manners described in the preceding paragraphs. For good acoustical performance the perforations may be configured as set forth above. After perforation the composite material or laminate is placed on the male sheet of the flexible pin mat apparatus of the present invention. The flexible pin mat utilized has an array of pins having a center to center distance equal to the center to center hole distance in the perforated composite material or laminate to allow the composite material of laminate to slide onto the male sheet. Similarly, the array type on the male sheet should be similar to the array type of the perforations. However, the array on the male sheet could contain additional columns and/or rows and therefore be larger. After the perforated composite material or laminate is placed on the male sheet of the flexible pin mat apparatus the female sheet of the flexible pin mat apparatus is placed over the top of the composite material or laminate to sandwich the composite material or laminate between the male and female sheets. Figure 3 depicts this configuration with composite material or laminate 50, sandwiched between male sheet 10 and female sheet 30. Generally to form the sandwich structure the male sheet is placed on a flat tool with the pins facing upward. The punched or drilled composite material or laminate is then placed by hand over the pin array and pressed to the bottom of the male sheet. The female sheet is placed over the composite material and pressed downward to sandwich the composite material or laminate between the sheets of the flexible pin apparatus. The entire sandwich assembly may then be placed in a tool and formed to contour using pressure and heat if required. After forming, the composite material or laminate is final cured under pressure. After final cure the male and female sheets are removed leaving the perforated and contoured fiber reinforced polymer composite material or laminate. The pins of the flexible pin mat apparatus prevent residual resin, or fiber, from blocking the perforations during final cure. Additionally, use of the flexible pin mat apparatus allows consolidation pressures as high as 85 pounds per square inch to be utilized during final cure. Pressures in this range form acoustic laminates that are well consolidated and free of voids, even when the composite material or laminate is contoured. As will be apparent to those of ordinary skill in the art, both the perforating (punching) and drilling processes are very controllable and can readily be altered without the long tooling changes and lead times required by existing methods of fabricating perforated fiber reinforced polymer composite materials or laminates. Although the drilling or punching process may remove a portion of the fiber reinforcement, the method of the present invention yields composite materials or laminates with superior mechanical performance to other perforation methods. While not wishing to be bound by any particular theory, it is believed this improvement is attributable, in part, to drilling the composite material or laminate in a semi-cured viscoelastic state, rather than the final cured state, thus causing reduced microstructural damage to the laminates. In another method of the present invention a starting material or laminate, such as the ones described in the preceding paragraphs, is perforated utilizing the rigid pin mat apparatus of the present invention. A restraining tool, such as those known to the art, is utilized to hold the starting material. For perforation the starting material or laminate is stacked on top of the rubber pressure pad that lines the bottom of the restraining tool. As will be recognized by those of ordinary skill in the art, if a laminate is desired, the individual plies may be stacked in the restraining tool. The male sheet of the rigid pin mat apparatus is then placed on top of the plies of starting material or laminate with the rigid pins projecting towards the material. A rigid lid, generally fabricated from metal, that fits inside the restraining tool is then placed on the top of the rigid pin mat, on the side opposite the pin side, and the entire assembly is loaded into a press. Pressure is then applied to the lid and assembly forcing the rigid pins through the starting material or laminate. The male sheet of the rigid pin mat apparatus with the material that now surrounds the pins is removed from the assembly and the female sheet of apparatus is placed over the material to sandwich the material between the male and female sheets. This sandwich assembly is then heated under specific heat and pressure conditions to semi-cure the starting material. If the starting material has not already been completely laminated the heat and pressure will also serve to laminate the plies of the starting materials. As set forth above, the resulting semi-cured material is still formable when heated but does not exhibit substantial flow of the resin during subsequent completion of the cure. At this point the viscoelastic properties of the semi-cured material or laminate are similar to leather. The resulting semi-cured perforated composite material is separated from the male and female sheets of the rigid pin apparatus and formed in manners known to the art, using heat if required. Subsequently, the composite material is cured, on a contoured tool if desired, to yield a perforated fiber reinforced polymer composite material. This method is particularly well suited for use with starting materials, or laminates, requiring cure temperatures that would degrade the flexible pin mat apparatus of the present invention. Additionally, since the composite material is removed from the rigid pin mat in the semi-cured state, the rigid pin mat may be fabricated from polymers that would be degraded by the temperature/pressure conditions necessary to final cure the starting material. As will be apparent from the foregoing description, the methods and apparatuses of the present invention overcome the deficiencies in known methods for fabricating perforated high temperature fiber reinforced composite polymer materials and enable the production of materials able to withstand high temperatures while maintaining excellent acoustic performance. Figure 4 depicts one such high temperature perforated fiber reinforced polymer composite material fabricated according to the present invention.

Claims: CLAIMS 1. A method for fabricating a perforated composite material comprising the steps of: curing a preimpregnated starting material, at sufficient temperature and pressure, to a semi-cured viscoelastic state wherein the material may be formed upon further heating but will not yield substantial resin flow upon final cure; perforating the semi-cured material to form an array of orifices; and curing the perforated material at a temperature and pressure sufficient to final cure. 2. The method of claim 1 further comprising contouring the perforated semi-cured material prior to final cure. 3. The method of claim 1 wherein the semi-cured laminate is perforated by drilling and facilitated via a multispindle compaction head. 4. The method of claim 1 wherein the starting material is fiber reinforced. 5. The method of claim 4 wherein the starting material is a graphite/bismaleimide preimpregnated fabric. 6. A method for fabricating a perforated polymer composite material for high temperature sound suppression applications comprising the steps of: pressing a first rigid sheet having an array of rigid pin like projections through a preimpregnated starting material to perforate the starting material; placing a second sheet, having an array of orifices arranged in the same manner as the array of rigid pin like projections on the first sheet over the preimpregnated starting material to sandwich the starting material between the first and second sheets; curing the resulting assembly, at sufficient temperature and pressure, to a semi-cured viscoelastic state wherein the material may be formed upon further heating but will not yield substantial resin flow upon final cure; removing the first and second sheets; and curing the perforated material at a temperature and pressure sufficient to final cure. 7. The method of claim 6 further comprising contouring the perforated semi-cured material prior to final cure. 8. A method for producing a perforated and contoured composite material comprising the steps of: curing a preimpregnated starting material, at sufficient temperature and pressure, to a semi-cured viscoelastic state wherein the material may be formed upon further heating but will not yield substantial resin flow upon final cure; perforating the semi-cured material to form an array of orifices; placing the semi-cured material onto a first flexible sheet having an array of pin-like projections arranged in the same pattern as the array of perforations; placing a second flexible sheet having an array of orifices arranged in the same pattern as the perforations and the pin-like projections over the semi-cured material to sandwich the semi-cured material between the first and second sheets; contouring the resulting assembly; subjecting the assembly to sufficient temperature and pressure to induce final curing of the semi-cured material; and removing the first and second flexible sheets. 9. The method of claim 8 wherein the preimpregnated starting material is a graphite/bismaleimide preimpregnated fabric. 10. A sound suppression apparatus for high temperature applications comprising: a first sheet having a plurality of pin-like projections; a second sheet having a plurality of orifices arranged in the same manner to the pin-like projections on the first sheet such that the first and second sheets may be juxtaposed to align the orifices and pin-like projections and permit the sheets to slide together; wherein the sheet are formed from a material that is able to withstand the temperature and pressure necessary to cure a high temperature polymer composite material without degrading. 11. The apparatus of claim 10 wherein the first and second sheets are fabricated from a flexible polymer material. 12. The apparatus of claim 10 wherein the first sheet is fabricated from a substantially rigid polymer material. 13. The apparatus of claim 12 wherein the pin-like projections are pointed.

Inventors: COUGHLIN MARK GERARD, HALL ROBERT HOWARD, LAUBSCH KENNETH LESLIE, MCAFEE KEVIN ALLYN, COUGHLIN, MARK GERARD, HALL, ROBERT, HOWARD, LAUBSCH, KENNETH, LESLIE, MCAFEE, KEVIN, ALLYN
Applicants: MARTIN MARIETTA CORP, MARTIN MARIETTA CORPORATION
Application year: 1993
Main classification: B29C67
IPC codes: B26F1, B29C33, B29C70, B32B3, B32B38