Abstract:
A method of prescoring foam board for making composite panels includes impressing a pattern of ridges projecting from a surface of a die into at least one face surface of the foam board. The pattern of ridges on the die forms a pattern of indentations having three series of parallel linear indentations, each series being angularly offset from each series other by 60°. The ridges on the embossing die are pressed into the foam sufficiently deep that the the pattern of indentations impressed in the board surface after whatever springback occurs remain deep enough to enable volatiles generated in an interior zone of a panel to escape when drawn out by a vacuum pump when the foam board is heated under pressure between two face sheets of resin-preimpregnated fabric to form a composite panel.

Description:
This is a divisional of application Ser. No. 08/468,693 filed on Jun. 6, 1995, now U.S. Pat. No. 5,698,153 which is a division of Ser. No. 08/235,594 filed on Apr. 29, 1994, now U.S. Pat. No. 5,589,016. 
    
    
     This invention relates to a prescored foam board used in fabrication of composite panels, and more particularly to a method of manufacturing composite honeycomb core panels with a foam edge borders without manufacturing defects caused by outgasing from resin pre impregnated materials used for the face sheets of the panel. 
     BACKGROUND OF THE INVENTION 
     A modification of airplane interior panels to improve the flammability resistance of the panels involves a change of the resin chemistry used in the resin preimpregnated materials used to form the face sheets of the panels. A phenolic resin system provides the desired reduction in flammability, but also creates manufacturing problems because of the increased volume of volatiles produced during the curing of the resin. This increased volume of volatiles must be accommodated in the manufacturing process to prevent generation of excessive pressure within the interior of the panel which prevents bonding of the face sheets of the panel to the panel core materials. 
     The panels are made up from a honeycomb core surrounded by a foam border, all bonded rigidly together between a face sheet, made of one or more plies of resin preimpregnated fibrous material (“prepreg”), on each side of the panel. The elements that make up the panel are bonded together by the resin in the prepreg that forms the face sheets and the resin in the honeycomb core. The foam border is provided to give a clean uniform solid edge surface around the panel which the honeycomb material would not provide, allowing for sculpted or contoured edges. 
     Although the foam border performs its intended function as desired, it also tends to establish a barrier against egress of volatiles generated or otherwise present during the curing of the resin in the preimpregnated materials that form the face sheet and/or the honeycomb core during manufacturing. This is especially true when the panels are cured in an oven in which the heating of the panel to cure the resin in the preimpregnated face sheets tends to heat the panel first around its peripheral edges and on top, thereby flowing the resin at the peripheral edges first and establishing a seal between the face sheet and the foam border at a relatively early time in the cycle before all the volatiles in the other parts of the panel have been generated. Then, as the heating progresses and the interior and underside portions of the panel are heated in the oven, the resin cures, generating volatiles in the interior of the panel that can be trapped therein by the seal established earlier around the peripheral edge between the foam border and the face sheet. 
     One solution for the need to vent the interior of the panel during the generation of volatiles during curing of the resin is to score the foam border radially to provide gas channels leading from the interior of the panel to the outside edge of the foam border. This technique usually proved successful, but because of the variation in scoring depth and spacing between scoring marks, failures believed to be caused by excessive interior pressure within the panel during curing still occurred at an unacceptable rate. Also, a panel, appearing to have acceptable quality in the factory, would occasionally develop a delamination once at altitude because of the lower air pressure at altitude compared with the gas pressure within the sealed panel. Finally, the hand scoring technique was a very time consuming, costly, nonreproducable, and tedious task which the factory workers disliked. Moreover, when the hand scoring was done with a saw, it produced undesirable foam dust and created small cracks and tears in the foam which tended to act as stress risers, thereby lowering the strength that the foam edge border is expected to provide. 
     Accordingly, a need has existed for a prescored foam board material which can be used to make foam borders for composite panels, with an optimum scoring pattern which will provide gas flow channels from the interior of the panel to the exterior for venting of volatiles within the panel during fabrication. This pattern should be equally effective regardless of the angle at which the foam pieces are cut when the foam border is cut from the foam board stock. The scoring pattern should not weaken the foam board excessively. If possible the scoring of the gas channels in the foam surface should relax and at least partially fill in during the latter stages of manufacturing so that the panel interior can be sealed against intrusion of moisture or other contaminates during use in the airplane. Finally, the prescored foam should be no more expensive and preferably less expensive than the manual scoring method. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an object of this invention to provide an improved prescored foam board having a pattern of indentations that provides venting of volatiles generated within a composite panel during fabrication to prevent excessive gas pressure within the panel that could interfere with proper bonding of the panel elements during fabrication. Another object of the invention is to provide an improved method of making a composite panel using prescored foam borders to vent volatiles in the panel during curing of the resin used to make the panel. Still another object is to provide a method of making a prescored foam board to emboss an indentation pattern into the surface of the foam board used as a foam border on composite panels, which process preserves the strength of the board, avoids the generation of dust, and avoids creation of stress risers in the foam board which could affect its strength or service life. Yet another object of the invention is to provide a composite panel which is sealed around its periphery against intrusion of contaminates during use but which, during fabrication, had venting channels for escape of volatiles generated or otherwise present in the resin used in the materials used to make the composite panel. Still another object of the invention is to provide a method of prescoring foam board to produce gas channels for venting of volatiles generated within the core of the composite panel, which method is reliably effective in venting such volatiles and which also enables the channels to relax and facilitate sealing of the peripheral border of the panel against intrusion of contaminates. 
     These and other objects of the invention are attained in a prescored foam used in panel fabrication wherein the surface of the foam board has three series of parallel linear indentations, each of which lies at a angle 60° offset from the other two series and which are rolled, stamped sawn or otherwise impressed into the surface of the foam by a laser, ultrasonic knife, patterned roller, flat die, sawing system, or the like. A roller embossing system is preferred because it which ramps the indentations into the board without creating stress risers. During fabrication, the gases generated within the core of the panel are vented through the indentations in the surface of the foam board out to the exterior of the panel, preventing the generation of excessive gas pressure within the panel. A vacuum is drawn inside the panel to draw the face sheets tight against the honeycomb core for reliable bonding. During heating, the foam material relaxes and springs back to partially fill in the indentations, and resin from the pre impregnated material which will form the face sheets flows to completely fill the partial indentations to reseal the peripheral edge of the panel. Any volatiles remaining or generated after the edge of the panel is sealed by the flowing of resin from the face sheet material are accommodated by the vacuum previously drawn in the interior of the panel to pull the face sheets against the honeycomb core material. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     The invention and its many attendant objects and advantages will become better understood upon reading the description of the preferred embodiment in conjunction with the following drawings, wherein: 
     FIG. 1 is a perspective view of a panel with a top sheet pealed backshowing the two plies of the top sheet and revealing the interior honeycomb core material and the foam border edge showing the pattern of indentations formed in the surface of the foam board; 
     FIG. 2 is a plan view of the structure shown in FIG. 1; 
     FIG. 3 is an enlarged plan view of a corner section of the honeycomb core element and a portion of the foam border edge shown in FIG. 2; 
     FIG. 4 is an exploded elevation of the elements that make up a panel shown in FIGS. 1 and 2, shown assembled in a vacuum bag for consolidation and bonding in an oven; 
     FIG. 5 is a plan view of a foam board from which individual pieces are cut to form the foam edge frame for a composite panel; 
     FIG. 6 is a plan view of a typical cell dimension for the pattern of prescored indentations in the foam sheet shown in FIG.  5 . 
     FIG. 7 is a graph showing the approximate timed sequence of various parameters related to the process for making the composite panel; 
     FIG. 8 is a schematic elevation of a roller mechanism for rolling the pattern of indentations shown in FIG. 5 into a rigid foam board; 
     FIG. 9 is an exploded elevation, similar to FIG. 4, showing an alternative embodiment using electric heaters instead of an oven; and 
     FIG. 10 is a schematic plan view of an electric heater for use in the assembly shown in FIG.  9 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Turning now to the drawings, wherein like reference characters designate identical or corresponding parts, and more particularly to FIG. 1 thereof, a panel  30  is shown having a portion of a top sheet  31  pealed back to illustrate the interior construction of the panel. The panel  30  is made of interior components covered on both sides by face sheets  31 . The interior components include a planar core element such as a honeycomb core element  32 , and an edge border  34  of a closed cell foam such as polyurethane foam disposed around the honeycomb core element  32 . The edge border element  34  can be made of a plurality of separate pieces  35  jointed and/or glued together at their junctions  36 , as illustrated in FIG. 2, or can be a single piece cut from a large piece of foam board. Preferably, the foam edge border  34  is made up of a plurality of separate pieces  35  since the advantages of a single piece construction are outweighed by cost of the waste that would be caused by making the edge border  34  out of one piece without junctions  36 .. The separate pieces preferably include the corner instead of terminating at the corner, and are jointed together with a dovetail joint for strength and security against shifting apart during assembly and before bonding between the face sheets  31 . The honeycomb core element  32  is made of phenolic resin, epoxy or other resin system preimpregnated Nomex paper formed into hexagonal cells  37  oriented with the cell axis perpendicular to the plane of the panel, as illustrated in FIG.  3 . Alternatively, the honeycomb could be made with metallic foil or thermoplastic film. 
     Face sheets  31  are bonded on each side of the core element  32  and the surrounding edge border  34 . The face sheets  31  are each made of one or more, preferably two, plies  38  of resin impregnated woven, braided, or unwoven fibrous material such as fiberglass fabric, Kevlar, graphite or the like material, commonly referred to as “prepreg”, which are bonded together and bonded to the interior components with the impregnating resin in the prepreg. The resin selected for the use in the prepreg is phenolic resin, chosen for its physical characteristics, including flame retardance and resistance to flammability, light weight and low emission of noxious fumes in the presence of flame, as well as its mechanical properties of strength and stiffness. Prepregs using other polymer systems also suffer similar panel core pressure manufacturing problems and will benefit from this invention. 
     Although the phenolic resin has desirable physical properties, its use in the manufacture of the panel  30  has created manufacturing problems because of the large volume of volatiles which it generates during the curing process. In manufacturing the panel  30 , the elements  40  which make up the final panel, namely the interior elements consisting of the honeycomb core element  32  and the surrounding foam edge border  34 , and the two prepreg sheets  38  on each side of the interior elements, are assembled as illustrated in FIG. 4, between two sheets  42  and  42 ′ of a release film, which assembly in turn is sandwiched between two sheets  44  and  44 ′ of breather material. The entire assembly is then covered with a vacuum bag  46  of a heat resistant gas impervious material such as nylon for phenolic resin prepregs, PVC for epoxy prepregs or a silicon blanket sealed to a top surface  50  of a table  52  with a conventional sealing material  48 . A series of vacuum ports  54  communicates through a strip  45  of breather material, which connects all the ports  54 , with the surface of the table  52  by way of a gas pipe  56  and is connected by a coupling  58  to a source of vacuum such as a vacuum pump  60  through a vacuum line  62 . 
     In operation, the assembly shown in FIG. 4 is sealed in the vacuum bag  46  to the top surface  50  of the table  52  by the seal material  48 . The vacuum pump  60  is connected to the coupling  58  by way of the vacuum line  62  and air is evacuated from within the sealed enclosure within the vacuum bag  46 . The withdrawal of air from within the vacuum bag  46  has the dual advantage of causing atmospheric air pressure exerted on the top surface of the bag  46  to press downward on the assembled elements within the bag, exerting a downward pressure on the top plies  38  pressing them against the core elements  32  and  34  and pressing those against the bottom plies  38 ′. This pressure will be effective in promoting bonding of the elements  40  together when the assembly is heated in an oven to flow the resin in the prepreg plies  38  and  38 ′. The second advantage of the vacuum in the bag  46  is that the air within the cells of the honeycomb element  32  is evacuated, establishing a low pressure reservoir which can accommodate the residual out gassing from the phenolic resin which may later be trapped by sealing of the panel elements around the peripheral edge, as described below. 
     After the vacuum has been established within the vacuum bag  46 , the vacuum line is disconnected from the coupling  58 , and the table  52  with the vacuum bag  46  and its assembled elements is transported into an oven along with numerous similar tables and assembled vacuum bags. The vacuum within the vacuum bag  46  is maintained by a one way valve  64  in the vacuum line  56 , preventing intrusion of air back into the vacuum bag  46 . In the oven, the coupling  58  is reconnected to another vacuum line  62  connected to the vacuum pump  60  to withdraw volatile gases generated or otherwise present during curing of the phenolic resin in the oven, and to maintain the consolidating pressure exerted by the atmosphere through the bag  46  on the plies  38  and  38 ′. 
     In the oven, power to resistance heating elements  66  is applied and air is circulated through a recirculating air supply line  68  to be heated by the elements  66  and circulate within the oven around the vacuum bag  46  to heat the bag and its contents and also heat the table  52 . As the temperature rises, the resin preimpregnated in the prepreg sheets  38  and  38 ′ begins to become less viscous and flows, wetting the prepreg sheets, the facing edges of the honeycomb core element  32 , and the foam edge border  34 . During this period, the phenolic resin in the prepreg sheets  38  produces significant volumes of volatiles, primarily water vapor, formaldehyde and free phenol which must have an escape path from the interior of what will become the panel when it is completely bonded together. Unless an escape path is provided for these volatiles, they will generate sufficient pressure within the core of the panel between the face sheets  31  and  31 ′ to interfere with complete bonding of the sheets  38  and  38 ′ to the honeycomb core element  32  and possibly the foam edge border pieces  34 . For that purpose, a pattern of indentations shown in FIG. 5 is provided on the surface of the foam edge border pieces. 
     A foam board  80  of rigid polyurethane closed cell foam is shown in FIG. 5 having a pattern of surface indentations  82  embossed into the surface of the foam board  80 . The pattern is in the form of three series of parallel lines all angularity offset from each other by 60°. A representative set of three lines from each series is illustrated in FIG. 5 as  82 A,  82 B and  82 C. As illustrated, the linear indentations of series  82 A in the board  80  extends parallel to the longitudinal axis of the board, and the two series  82 B and  82 C extend 60° from the series  82 A to the right and to the left respectively. As illustrated, the linear indentations  82 A,  82 B and  82 C all intersect at single points throughout the pattern, but the pattern could have been arranged with only two lines intersecting at any one point. 
     As shown in FIG. 6, the pattern of linear indentations  82  in the board  80  forms a series of contiguous equilateral triangles  84 . For the size panel being constructed, a pattern in which the linear indentations are spaced about 1¾ inches apart produces an equilateral triangle having sides all equal to about 2 inches in length. This was found to produce a satisfactory series of venting channels for the volatile vapors generated inside the panel during curing of the phenolic resin. The width of the indentations is about {fraction (1/16)}th inch and the depth is about {fraction (1/16)}″ to {fraction (3/16)}″ deep. The dimensions could be adjusted to match the vacuum requirements of differing processes, constructions, or resin systems. 
     The indentation pattern shown in FIG. 5 can be cut into the foam board using a saw with a depth gauge and automatic indexing of the board after each saw cut. Likewise, it could be formed by a series of die strokes using a flat or curved die having the pattern shown stamped into the board. These techniques produce adequate patterns of linear indentations, however they also produce foam dust in the case of sawing and small cracks or tears in the foam board in the case of sawing and stamping. In most cases, these problems will not significantly affect the manufacture or use of the panel, so they may be disregarded. However, in situations where they may be considered undesirable, a preferable technique may be used, as illustrated in FIG. 8, using two embossing rollers  90  and  92  having the die pattern shown in FIG. 8 set in their exterior surface. The two embossing rollers  90  and  92  are spaced longitudinally in the direction of motion of the board  80  indicated by the arrow  94 . Preferably, each roller  90  and  92  has a solid idle roller  96  and  98 , respectively, positioned on the opposite side of the board  80  to provide a reaction force to the force exerted by the embossing rollers  90  and  92 . As illustrated, the pattern embossed into the top surface of the board  80  by the roller  90  is offset from the same pattern embossed in the lower surface of the board  80  by the roller  92  so that none of the embossed linear indentations coincide which could form a weak hinge point in the foam board  80 . 
     Besides the advantages of speed and cleanliness provided by the rolling technique for embossing the pattern of indentations  82  into the foam board  80 , the rolling technique also avoids the generation of small cracks and tears in the foam material which occur with the stamping and sawing technique and serve as stress risers which can form the start of fatigue cracks in the material. The avoidance of the stress risers is t believed to occur because of the gradual ramping of the indentations into the material at a small ramp angle Φ. This angle Φ, about 10-35°, preferably about 15°, allows the die surfaces in the rollers  90  and  92  to gradually displace the material in the surface of the foam board  80  and do so in localized areas of the foam board surface. This allows other material immediately upstream of the die contact area to prestrain and distribute the stress laterally, so when that area is strained by the rolling die it does not tend to result in the small tears and cracks that are observed with the use of a flat die. 
     Turning now to FIG. 7, a graph is shown having time as the abscissa and various parameters of the process such as temperature, rate of volatile generation, core pressure, and resin viscosity as separate ordinates. The graph of FIG. 7 shows the change in these parameters as the cycle progresses from beginning to end. At time zero, all of the tables  52  have been loaded into the oven, the vacuum lines  62  connected to all of the couplings  58 , oven door closed and the heater elements  66  energized. The temperature begins to climb and reaches a temperature of about 200° F. in about 12 minutes. At about this temperature, the resin reaches gelling temperature, its viscosity falls, and the resin flows, wetting the surfaces in contact with the prepreg materials. Also at this temperature, the evolution rate of volatiles increases rapidly. Volatiles, primarily water vapor, are generated rapidly and continue to be generated for the next 5 or so minutes. At this phase in the cycle, the cross linking of polymers in the phenolic resin matrix accelerates. As the cross linking proceeds the resin viscosity increases and it becomes more difficult for volatiles generated in the core of the panel to escape through the embossed indentations in the surface of the foam edge border  34 . Eventually, the resin will become viscous enough to prevent further escape of volatiles generated within the core of the panel and the vacuum established by the vacuum pump  60  in the core of the panel will begin absorbing these volatiles. The residual generation of volatiles in the core after the prepreg face sheets  31  bond to the foam edge border to seal the peripheral edge of the panel results in a decrease of the vacuum, or increase in pressure, in the core, as shown in the graph 100C. 
     Since the natural heat flow into an article lying on the table  52  in the oven is from the top and sides of the heated article inward, the top surface and peripheral edges of the panel elements shown in FIG. 2 will tend to be heated first. The center of the panel on the table side tends to reach gelling temperature last. This situation could potentially cause the peripheral edge of the panel elements to be heated to gelling temperature and begin cross linking before the evolution of volatiles in the lower central portion of the panel has commenced. In such a situation, especially in the fabrication of very large panels, this could potentially result in premature sealing of the peripheral edge of the panels before evolution of the volatiles from the ungelled resin, and the development of excessive pressure in the core of the panel could occur. In such a situation, a refinement of the heat application can be used to tailor the sequence of gelling in zones of the panel to prevent premature sealing of the panel periphery and enhance the efficacy of the scoring pattern in the foam edge boarder. 
     To alter the pattern of heating in the oven to heat from the center outward rather than from the peripheral edge inward, as shown in FIG. 4, a pair of heat ducts  110  and  112  is provided above and below each table  52  having suitable gas respective nozzle  114  and  116  disposed over the center of the area in which the panel will be positioned. The nozzles  114  and  116  will direct the heated air in the convection oven against the top and bottom center of the vacuum bag  46  and the table  52  respectively causing the center of the bag assembly and table to heat up first. This arrangement will cause the gelling of the resin in the prepreg sheets  38  and  38 ′ to occur first in the center, thereby leaving open a path for egress of the ensuing volatiles before the peripheral edge of the part becomes sealed when the resin at the peripheral of the part gells. 
     The same zone controlled heating illustrated by the use of the ducts  110  and  112  in FIG. 4 is also achieved in a second embodiment of the invention as shown in FIG. 9, wherein a pair of electrical heaters a top heater element  120 , and a lower heater element  120  shown in FIG. 9, are respectively disposed above and below the same elements  40  through  44  shown in FIG.  4 . An insulator  122  is placed over the top of the top heater element  120  and the entire arrangement is sealed in a vacuum bag  46  to a table  124  by a sticky sealant  48 . 
     The heater  120  is a flexible blanket heater having a series of heater elements  126 ,  128 ,  130  etc. disposed concentrically around the center heater element  126 , as shown in FIG.  10 . Lead wires connect from the peripheral edge of the blanket heater  120  to each of the heater elements and each element is connected to a common ground  132 . The lower heater element  120 ′ can be set into the table  124  with internal leads  134  or can be a separate heater element lying on the surface of the table  124 . 
     In operation, the elements  40  through  44  are assembled on the heater element  120 ′ and covered with the heater element  120  and the, insulation blanket  122 . The vacuum bag  46  is placed over the assembly and sealed to the table  124  with a sealant  48 . The vacuum pipe  56  is connected by its coupling  58  to a vacuum line leading to a source of vacuum  60 , and a vacuum is drawn inside the vacuum bag  46 . After the vacuum is established and the atmospheric pressure is exerting a uniform downward force on the top of the bag  46  to compress all of the elements  40  that will form the panel, the center element  126  in the top and bottom heaters  120  and  120 ′ is energized to begin heating the center of the assembly. After a suitable interval of about 5 minutes, the next concentric radial heater  128  adjacent to the inner heater  126  is energized to begin heating the next radial zone outside the center zone  126 . The heating of the panel elements progresses radially outwardly in this manner. As the gelling and curing of the resin progresses from the center outward, the volatiles generated during the gelling and curing process can escape radially outward through the linear indentations  82  in the foam edge board pieces  34  since the resin at the peripheral edges of the panel has not yet begun to gell and fill the indentations  82 . The elements  128 ,  130  et cetera are energized in a sequence that provides a suitable time period for the generation of the volatiles to progress through its normal maximum generation rate and begin tapering off before the next element raises the temperature to cause the generation of volatiles in the next radially outward zone to peak, so that all of the volatiles can be drawn out from the interior of the panel thorough the vacuum pipe system  56  before the peripheral edge around the foam edge border  34  is heated and sealed. 
     Obviously, numerous modifications and variations of the described preferred embodiments will occur to those skilled in the art in view of this disclosure.