Abstract:
A method of constructing large, unitary, fiber-reinforced Polymer composite containers using a vacuum assisted resin transfer molding process. The method allows for the construction of container systems with only two separately molded parts—an open box consisting of a base (i.e., floor), 2 sidewalls and 2 endwalls, and a cover (i.e., roof). The method results in a structure which maintains the continuity of the reinforcement fibers across the junction between the floor, side, and end walls corners. This method can be applied to very large composite structures such as railcar bodies, intermodal containers, and shelters.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation of application Ser. No. 08/872,282 filed Jun. 10, 1997, now abandoned which is a continuation-in-part of application Ser. No. 08/672,175 filed Jun. 27, 1996, now abandoned, which is a continuation-in-part of application Ser. No. 08/609,671 filed Mar. 1, 1996 now abandoned, which in turn claims the benefit of U.S. Provisional Application No. 60/000584 filed Jun. 28, 1995. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates to a method of constructing unitary fiber reinforced resin composite containers using a vacuum assisted resin transfer molding process. 
     Fiber reinforced composite containers such as railcar bodies, intermodal containers, and truck bodies which incorporate materials such as foam and balsa cores are desirable because they are lightweight, corrosion resistance, and provide excellent thermal insulation. To make these containers competitive with metal structures, the manufacturing costs must be minimized without sacrificing structural performance. Typically, high stress levels are generated in the corners of these containers due to large bending moments. For applications such as railcar bodies, high compressive stresses in the floor are experienced due to fork lift truck wheel loading. 
     Composite containers can be produced in several ways. One method consists of assembling a series of flat panels (e.g., pultrusions) using a secondary adhesive bonding approach. This manufacturing approach results in joints at the corners of the structure and a discontinuity of fiber reinforcement. This invention eliminates fiber discontinuities at the corners and adhesive bonding of multiple panel sections. 
     Typically, composite parts and structures which are exposed to the environment need to be protected from UV degradation and weathering, as well as being made aesthetically pleasing. The effects of UV and weathering are currently reduced or eliminated by: incorporating UV absorbers into the resin; incorporating fillers into the resin; pigmenting the outer resin layers of the composite part; gel coating the surface prior to molding; and/or painting the finished surface. Of these methods, gel coats (which are applied prior to the composite fabrication process), and paint (which is applied as a secondary operation after the composite fabrication process) are the most effective. However, the application of gel coats or paints results in the emission of VOCs. Both gel coating and painting operations also require a large capital expenditure both in spraying and ventilation systems and equipment. Gel coated parts must be layed up and molded within a reasonable time after the gel coat is applied, in order to obtain a good bond between the gel coat and the part being molded. However, large, structural parts may take several days to lay up, many more during the prototyping stages. Automation and process improvements will speed up the lay-up time, but this time will probably still be too long for a gel coating operation. Aesthetically pleasing surfaces are usually accomplished by either gel coating or painting. 
     SUMMARY OF THE INVENTION 
     This invention defines an approach for constructing large composite containers using a vacuum assisted resin transfer molding process. The method is applied to large composite containers such as rail car bodies which may be as large as 68 feet long by 10 feet wide by 12 feet high. The approach consists of molding only two (2) individual parts which are subsequently joined. An open box with 2 or 4 sides and a floor is fabricated in one molding step with the top or roof molded in a separate operation. 
     The materials (e.g., fabric and cores) are initially positioned on three (3) or five (5) separate molds which are supported by casters and oriented horizontally. After the individual sections are laid-up, the two (2) sidewalls and optionally two (2) endwalls are attached to the base floor mold. The sidewalls and endwall molds are then rotated into a vertical position and bolted together. The hinging mechanism allows for the mold segments to be attached to the base mold and rotated freely into a vertical position. The hinge also provides for a secondary lateral movement to seat the sidewall molds to the base mold. 
     The sidewall and endwall molds are designed so that a small portion of the floor is included. This allows the entire corner geometry to be incorporated and creates a vertical joint with the floor mold. This section of the mold also provides a lip which effectively contains the lay-up during mold rotation. At the interfaces between molds, a seal is provided to maintain the vacuum integrity of the assembled mold because if air leaks along the mold, surface finish defects and possibly structurally weakened areas (depending on the intensity and location of the leak) will occur. Indexing keyways are also provided to ensure alignment between sections. 
     This invention relates to the material lay-up method used to maintain the fiber continuity across the corners of the composite container. Although the five sections are laid up separately, fiber continuity is maintained by the inclusion of additional material in each lay-up beyond the size of the section. This added material is initially folded back on itself and then unfolded after the molds are assembled. The layers of unfolded fabric extend the required distance and are interleaved (overlapped) with fabric in the adjacent section to effectively transfer the load around the corner. 
     Metal caul plates may be positioned over the inside of the lay-up to improve surface definition and smoothness. These caul plates are held in place by retainers at the top of each section to prevent movement during the rotation of the molds. 
     After the molds are assembled and the fabric is unfolded in each corner, the entire lay-up is vacuum bagged. A pre-seamed vacuum bag is used which replicates the inside of the box. A continuous seal is created at the top edge of the sidewalls and endwalls. A resin matrix is then infused into the box using a vacuum assisted resin transfer molding process. Once the resin has cured the box is now a unitary structure. 
     The sidewalls can have openings within the sidewalls of the size and shape suitable for accommodating structural elements, such as doors and windows, or suitable for the installation of miscellaneous systems and/or equipment, such as mechanical refrigeration units. The composite box can comprise means for supporting ancillary structures appendable to the composite box, such as metal attached plates, mounting studs/threaded attachments, bearing plates, brackets, beams, fittings, hinges, lateral beams, transverse beams, floor stringers, corner rails, and/or posts. The composite box can further integrate internal co-molded hollow elements integrated within the walls of the composite box suitable for running utilities, such as co-molded ducting and/or conduits for air flow, water, and miscellaneous systems, such as electrical wiring. 
     The method of forming a composite box having end, side, and bottom walls comprises forming a plurality of cores, each having a peripheral surface, length and width dimensions extending end-to-end and side-to-side, respectively, positioning said cores end-to-end and side-by-side in respective mold segments that define said end, side, and bottom walls while encapsulating the cores in fabric material; assembling the mold segments to interface with one another to form a box, said fabric material overlapping with fabric material in adjacent mold segments; sealing the interface between the mold segments enclosing said molds in a hermetically sealed bag having inlets and outlets; connecting a source of uncured resin to said inlets evacuating said molds through said outlets; forcing said uncured resin through said inlets to said outlets to fill the mold between said core and said mold to impregnate said fiber material; curing said resin to form a Composite box; and removing the mold segments. Preferably, the resin is vinyl ester or polyester. 
     In forming the composite box of the present invention, openings can be included within the walls of the composite box of a size and shape suitable for accommodating structural elements. The cores can be positioned in order to create an opening within the sidewalls suitable for accommodating structural elements. Means for supporting ancillary structures to the walls within the composite box can be appended to the walls of the composite box. Hollow elements having endings at the surface of the walls of the composite box can be co-molded within the composite box, and the endings of the hollow elements can be filled with an easily-removable substance, such as clay, preventing the hollow openings from filling with resin during the process of filling the molds. 
     The composite box structure comprises end, side, and bottom walls, each of said walls, including a plurality of cores having a peripheral surface, length and width dimensions; the cores are positioned end-to-end and side-by-side to define said end, side, and bottom walls; a layer of fiber material encapsulates the cores and bridges the adjacent edges of said end, side, and bottom walls; and a cured resin material saturates said layer of fiber material to form a unitary composite box structure. 
     The composite box also includes fiber material caps along the length of said core, side-by-side positioning of said cores, and resin saturated fiber material form I beam reinforcements in said end, side, and bottom walls and said top. 
     The composite box can be constructed with a co-molded layer molded to the top and side and bottom walls. The co-molded outer surface would protect against UV degradation and weathering, and therefore will have enhanced UV stability and enhanced abrasion, impact, and wear resistance. The co-molded layer can be either a film or sheet of material which will bond chemically and/or mechanically with the resin system used in fusing the composite portion of the box. Co-molded layer could be, inter alia, a sheet made of acrylic or polyvinylidene fluoride. The co-molded surface should, in addition, guard against fiber print-through and offer an aesthetically pleasing surface with multiple color options, which should not fade over the expected product life. 
     A railroad car may be built comprising a box, as defined above, mounted to a wheeled frame structure reinforced with steel I beams that interact with the I beam reinforcements in said bottom wall. The composite box can also be used for cargo and shipping containers, truck trailer bodies, modular housing, and insulated refrigeration rail cars. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows five separate molds containing materials positioned for forming a box. 
     FIG. 2 shows the five separate molds of FIG. 1 in the shape of a box. 
     FIG. 3 is a cross-sectioned view of FIG. 2 taken along line  3 — 3  with the addition of vacuum bagging and its relationship to a roof section. 
     FIGS. 4 a  and  4   b  are enlarged partial views of FIG. 3 showing alternate embodiments of the improved seal of this invention. 
     FIG. 4 is a cut away view of a mold. 
     FIG. 5 shows a cross section of a mold. 
     FIG. 6 shows the incorporation of co-molded openings within the walls of the composite box. 
     FIG. 7 shows a cross section of the walls of the composite box. 
     FIG. 8 shows a cross section of a mold having a co-molded hollow element. 
     FIG. 9 shows a cross-section of a typical composite lay-up in a mold, including the co-molded film or sheet. 
     FIG. 10 shows a cross-section of the fabric lay-up co-molded to a plastic sheet. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIGS. 1,  2 ,  3  and  8 , five separate molds are shown to include two end walls  10 ,  12 , two side walls  14 ,  16  and a floor  18 . These molds contain cores covered with fabric as best shown on FIG.  3 . For example, cores  20  are placed in a mold, with fabric  22  covering the cores. Since a key to this invention is to maintain the continuity across the junction between the floor side and end wall corners of the box, additional material  22   a  is included in each lay up which is beyond the size of the mold. This material is initially folded back on itself and then unfolded after the molds are assembled and interleaved with material in adjacent molds to transfer load around corners (FIGS. 1,  2 , and  3 ). 
     FIG. 4 is a cut away view of the floor mold  18  showing the cores  20  assembled within the mold. The cores can be of various materials suitable for the particular use being made of the composite box. A preferred material for the cores is foam which has the added feature of providing insulation where the box is used as a refrigerated rail car. 
     After the molds are assembled and sealed at their interfaces, the entire box is vacuum bagged. A pre-seamed vacuum bag  30  is used, and a continuous seal  32  is created at the top edges of the end wall and side walls. At the interface between mold sections a chemical barrier seal  34  is provided (FIGS. 3 and 4A) to provide vacuum integrity of the assembled mold sections. 
     Referring now to FIGS. 3 and 4, a vacuum outlet  40  is connected to a vacuum source (not shown) and to the interior of the bag  30 . A source  44  of uncured resin  46 , such as tank  44  containing vinyl ester or polyester, is connected through bag  30  via tubing  45  and T shaped fittings  48  placed in channels  50  of core  20 . Channels  50  are connected to a distribution network either in or on the surface of core  20 . 
     In operation, the bag  30  is evacuated forcing uncured resin  46  through channels  50  and grooves  52  to fill the mold and the bag  30  to impregnate fiber material  22 ,  22   a  and  24 . The resin is allowed to cure. 
     After removing the vacuum bagging materials and the connections between mold segments, the side and endwall molds are removed. After the molds are separated, they are rotated back to their horizontal position (the molded walls remain) vertical). The box is removed from the base mold. 
     A roof structure  51  is molded separately in one piece using the same method as the box. The roof is molded as a ‘tub’ which incorporates a small portion of both the side and end walls. This allows for the molding of the entire corner of the roof and creates a single horizontal seam between the box and roof. The roof may include a vertical flange  52  which fits inside the interior surface of the box sidewalls. This flange helps align the roof to the box during assembly and allows for the installation of a mechanical fastener into the sidewall. 
     As best shown in FIGS. 4A and 4B, a typical mold section joint consists of a flange  16 A that is normal to the mold surface extending along the entire edge. This flange provides a mating surface to which seals, indexing features, and clamping mechanism may be attached. There exists a pressure differential between the mold surface (low pressure) and the back of the mold (high atmospheric pressure). In order to maintain this pressure differential, a “system” of seals (i.e., a chemical barrier seal  34  and a vacuum seal  60 FIG. 4A) are attached to one or both flanges. The main seal or vacuum seal consists of a closed loop gasket which in the cross section shown in FIGS. 4A and 4B is designated  60 . A vacuum is applied within this closed loop through vacuum part  62 . If the one or both of the active seals should fail, the leak path will follow the path of least resistance and is intercepted by the vacuum source. The main seal  60  needs to be very compliant so that the initial seal can be provided with a relatively light clamping force. Once the vacuum is applied and the “loop” is evacuated through vacuum port  62 , the seals will be compressed by the atmospheric pressure that exists on the back side of the flange. This provides an additional benefit of requiring minimal external clamps to hold the mold segments in position as we now have available to us a clamping mechanism that provides a very uniform clamping pressure along the entire length of the flange area. The embodiment shown as FIG. 4B can provide a satisfactory seal without the need for a chemical barrier seal. This force can be considerable and is a product of the area of the closed loop and the atmospheric pressure (app 14.7 PSI). Some of the materials that may be used for the Active Seal include closed cell foam extrusions of Neoprene, EPDM, Silicone, etc. 
     The cores are shown in FIG. 4 with C-clamps  24  and  26  around the ends of the cores. FIG. 5 shows a cross section of the cores  20  separated by C-clamps  26  and FIG. 8 shows a cross section of the cores  20  separated by a hollow element  27  co-molded and integrated within the walls of the mold suitable for running utilities within the mold. 
     Shown in FIG. 6 are two openings co-molded into the walls. The larger opening  86  is suitable for a door or larger system and/or equipment such as a mechanical refrigeration unit, and the smaller opening  84  is suitable for a window or smaller system or equipment. 
     Supporting ancillary structures can be appended to the walls of the composite box such as the metal attach plate  94  shown in FIG.  7 . In addition, hollow elements  92 , having endings at the surface of the walls of the composite box can be filled with a removable substance (not shown) such as clay to prevent the hollow openings from filling during the process of filling the molds. 
     A typical mold lay-up as shown in FIG. 9 consists of a mold  70 , a film or sheet of surfacing material  72 , the fabric lay-up  74 , a core material  76 , a caul plate  78 , and a vacuum bag  80 . The surfacing material  72  is placed in the mold during the composite lay-up and prior to infusion, and is co-molded with the fabric  74  and core  76  into the final composite structure. 
     A thermosetting resin typically bonds to the plastic sheet by chemical means (it attacks the surface of the plastic, thereby forming a good bond when the resin cures). If the bond strength between the plastic sheet and the cured fiberglass/resin laiminate is not satisfactory by this method (chemical bond) alone, a fabric layer can be imbedded into the plastic sheet prior to the molding operation. This gives the resin something to lock into during the molding process, thus forming a mechanical bond as shown in FIG. 10 between the plastic sheet  72  and the cured fiberglass/resin laminate  82  (in addition to any chemical bond which may also exist).