1. Field of the Invention
The present invention relates to a method and to an apparatus for making structural reinforcement preforms for resin transfer molding (RTM) and reaction injection molding (SRIM) processes for structural composites, and is more specifically concerned with techniques for making such structural reinforcement preforms in accordance with the directed fiber process.
2. Description of the Prior Art
As set forth in my aforementioned patent application Ser. No. 446,859, filed Dec. 6, 1989, and in my later application Ser. No. 552,253, filed Jul. 12, 1990, U.S. Pat. No. 5,217,656, issued Jun. 8, 1993, in making preforms according to the directed fiber process, it has been the practice to spray chopped fibers with a binder resin onto a form that has air pulled therethrough to locate and hold the fibers in place. The form with the fibers and the binder resin is then moved into a hot air plenum chamber, dried and/or cured to set the binder resin. Utilizing this process, a great deal of processing space is required for heating, drying, curing and cooling the preforms. The process has also therefore required large ovens and other equipment for handling the preforms.
In making thermoformed preforms, it has heretofore been the practice to coat a continuous strand fiber mat, during its manufacture, with a thermoplastic binder. The mat is supplied in roll form. The mat is unrolled and provided as a plurality of overlying flat sheets to vary the preform thicknesses and clamped into a holding frame at the edges thereof. The frame is then placed in an oven with radiant heaters which slowly heat the reinforcement mat and the thermoplastic binder from both sides. The thermoplastic binder softens in response to heating and, while soft, the frame is transferred into a cold mold which is then operated to press the reinforcement mat into the desired shape. Cooling causes the thermoplastic binder to stiffen and hold the thermorformable mat in its new shape.
As pointed out in my previous applications, these processes are slow, require a great deal of space and a large amount of energy for heating and cooling.
As also pointed out in my previously-mentioned applications, design flexibility is limited in that in order to meet the strength requirements of one area, an unnecessary use of material in other areas is required (layering) which also increases thickness and weight. Also, neither of the aforementioned processes permits the designer to add subassemblies such as ribs or closed sections to maximize design properties.
In my aforementioned applications, I proposed a new system which eliminates the necessity for large rooms and constantly operating ovens, cooled presses and the like and permits design flexibility with respect to the provision of subassemblies (reinforcement ribs, closed sections, and attachment and/or reinforcement members), while at the same time saving on energy and materials.
These new processes, as disclosed in the aforementioned applications, utilize specifically-developed and tailored binders along with directed energy systems for rigidizing the composite forms and attaching structural components to the preforms and is entirely compatible with RTM and SRIM resin systems, i.e. polyesters, vinyl esters, urethanes, epoxies, phenolics and acrylics. These new processes are designed to be fully automated and to enable specific distribution and placement of numerous types of reinforcements and the like, where necessary, for the required structural properties of a preform. There is also a complete freedom of design inherent in the process which permits the most desirable reinforcement type and/or structures including closed structural shapes and varied wall sections to meet design criteria.
In the process disclosed in the aforementioned application Ser. No. 446,859, filed Dec. 6, 1989, mats of reinforcement material are cut into a desired shape as a two-dimensional planar development of a desired preform. The cut mats are then coated with a binder which is responsive to electromagnetic energy, either microwave radiation or ultraviolet radiation, and the cut mats are placed in a three-dimensional mold and pressed to replicate the desired shape of the preform. While in the mold, the shaped mats are subjected to the appropriate electromagnetic radiation, either microwave or ultraviolet radiation, to cure the binder resin and provide rigidity in a matter of seconds, rather than minutes or hours as with the heat-curable processes. At this point, the preform may be considered a finished product for use in a further molding operation (RTM, SRIM) or may be viewed as a carrier preform for the attachment of subassemblies such as structural reinforcement members and the like before being used in a further molding operation (RTM, SRIM).
When the preform is considered to be a carrier preform, the same is removed from the mold to a station where a designated area or areas of its inner and/or outer surfaces or that of a subassembly or subassemblies are provided with a further coating of an electromagnetic energy-curable binder, a reinforcement member or the like is moved into intimate contact with the preform at the coated area or areas and the appropriate electromagnetic radiation (microwave or ultraviolet) is applied to energetically stitch (cure the binder) the member to the carrier preform. When the final attachment has been made by such energetic stitching, the preform is a finished product in itself ready for use as a structural reinforcement preform as a part of a further molding process for making a structural composite.
As pointed out in the latter application Ser. No. 552,253, particularly with respect to the handling of reinforcement material supplied on rolls that must be unrolled and individually cut into the desired shape and individually stacked in registry in the mold, handling is simplified and registration is inherent when such layers are tacked together prior to or contemporaneously with cutting (termed energetic basting) by applying the binder to superposed webs prior to cutting, pressing the superposed webs together to increase the surface contact of the binder with the fibers of adjacent webs and curing the binder in local spaced zones so as to tack the webs together either before or during the cutting operation.
This technique has also proved efficient in that the tacking is localized and there is sufficient binder remaining for curing to rigidize a multi-layer mat in a desired three-dimensional shape after first cutting a two-dimensional development of that shape from the tacked webs.
After the preform has been rigidized, energetic stitching techniques may still be employed to attach subassemblies, such as reinforcement and/or attachment members, thereto.