Patent Application
20060175730
The present invention relates to the fabrication of composite panels, more particularly to the manufacture of composite panels for use in medical equipment and residential construction.
In U.S. Pat. No. 6,117,376 (the “376 patent”) the inventor of the present invention describes a method of making foam-filled composite products, in which fiber layers, impregnated with a heat activated resin are built up on two mold halves, which are then clamped together to form a pressure vessel. An exothermic foam-forming resin is then poured into the mold cavity. The exothermic foaming reaction of the foam material generates both heat and pressure to consolidate and cure the resin impregnated fiber layers while forming a tight bond between the foam and the fiber layers. Advantages of the method described in the '376 patent includes substantial savings in production costs by obviating the need for expensive autoclaves, vacuum furnaces, heated molds or other capital-intensive equipment.
There are, however, applications where a solid composite panel, rather than a foam-filled composite structure, may be advantageously used. Unfortunately, to manufacture thin composite panels prior to the present invention, one had to resort to the capital equipment-intensive methods such as vacuum furnaces and autoclaves. What is needed, therefore, is a method of making solid composite panels utilizing a self-heating and pressurizing method similar to that taught in the '376 patent.
The present invention comprises a method of manufacturing solid composite panels utilizing an exothermic foam-forming resin to simultaneously consolidate and cure the heat activated resin of the composite panel without forming a foam-filled part. According to an illustrative embodiment of the present invention, composite fabric layers are laid up on the interior surface of a mold half that conforms to the surface configuration of the finished panel. The composite fabric layers may be so-called “pre-preg” sheets, which comprise fabric that has been pre-coated with resin, or they may be dry sheets that are wetted with resin after they have been placed in the mold half. Once the composite fabric layers are laid up, a flexible bladder is placed over the composite layup and the mold closed by means of a rigid cover plate so as to form a cavity inside the mold. A foam-forming resin, is pre-measured so as to expand to more than fill the cavity. The foam-forming resin is injected into the bladder within the mold cavity. The mix begins foaming after a short delay, generating an exothermic reaction that heats the resin impregnated fabric layers above the cure temperature. Simultaneously, the expanding foam generates pressure within the cavity. The heat generated by the exothermic reaction initiates and maintains curing of the wet resin because the thermal energy is largely retained within the mold. The internal pressurization simultaneously consolidates the fabric layers into a substantially solid void-free composite panel. A post-curing interval can be employed to assure full stabilization of the composite product. Thereafter, the mold may be opened and the product released from the mold, and the foam-filled bladder discarded.
Because the mold half and the cover plate have strong pressure resistant outer walls, but only the interior portion of the mold half needs to have a precise, part-forming shape, the outer wall of the mold half and the cover plate can be reused for a variety of similar applications. Virtually no capital equipment is required for the manufacture of the molds or parts in small or large quantities. No external power is required to generate heat or pressure. In consequence, a wide variety of composite parts heretofore requiring massive capital investment for autoclaves, vacuum furnaces and the like can be manufactured on site with minimal equipment and investment.
The present invention will be better understood from a reading of the following detailed description, taken in conjunction with the accompanying drawings figures in which like references designate like elements and, in which:
The drawing figures are intended to illustrate the general manner of construction and are not necessarily to scale. In the detailed description and in the drawing figures, specific illustrative examples are shown and herein described in detail. It should be understood, however, that the drawing figures and the detailed description are not intended to limit the invention to the particular form disclosed, but are merely illustrative and intended to teach one of ordinary skill how to make and/or use the invention claimed herein and for setting forth the best mode for carrying out the invention.
With reference to
In accordance with the present invention, a composite panel such as leg brace 10 can be fabricated on-site at the doctor's office with a minimum investment in capital-equipment by a technician with minimal training. A mold assembly 13 consisting of a lower mold half 12 and an upper mold half such as mold cover 14 comprise the only capital equipment necessary to carry out the molding process. Preferably lower mold half 12 and mold cover 14 are themselves made of carbon fiber composite, ceramic, ceramic-lined metal or other durable material having a low thermal conductivity. Inner surface 16 of lower mold half 12 comprises a female mold portion 18 that conforms to the configuration of outer surface 20 of leg brace 10. Preferably female mold portion 18 comprises a silicone line 22 placed within lower mold half 12. Silicone liner 22 has an outer surface that matches the recess in lower mold half 12 and an inner surface that is cast to conform to the custom dimension of the leg brace being fabricated. Use of replaceable silicone liners within mold half 12 permits lower mold half 12 to be utilized for fabricating a wide variety of shapes and sizes, thus permitting leg brace 10 to be customized to each individual patient.
A first fabric layer 24 is placed inside lower mold half 12 within the recess defined by female mold portion 18 of silicone liner 22. First fabric layer 24 may be made from any of a wide variety of structural fabric materials such as carbon fiber, aramid fiber, glass fiber and ceramic fiber is either dry or “pre-preg” (pre-impregnated with resin) sheets. In the illustrative embodiment, first fabric layer 24 comprises a pre-preg sheet of woven bi-directional 0.006 inch bi-directional carbon fiber, coated with a B-Staged (i.e., heat activated) epoxy resin sold by Cass Polymers, of Charlotte Mich. under the trade mark PROBUILD, however other fibrous materials and resin systems such as 0.006 inch uni-directional carbon fiber, glass fiber, aramid fiber, ceramic fiber or other flexible fabric materials coated with heat-activated resins may be used without departing from the scope of the present invention.
One or more immediate fabric layers such as fabric layer 26 are positioned atop first fabric layer 24 within lower mold half 12. Fabric layer 26 also may comprise any of a wide variety of structural fabric materials such as graphite fiber, aramid fiber, glass fiber, or ceramic fiber, but in the illustrative embodiment comprises a pre-preg sheet of uni-directional graphite material such as C.F. 7440-44-0 made by Hexcel Composites of Stamford Conn., coated with a B-Staged epoxy such as the aforementioned PROBUILD epoxy resin. Depending on the application, additional layers of uni- and bi-directional structural fabric may be added to achieve the strength and rigidity needed for the particular application. Once the proper number of layers have been built up within lower mold half 12, a mold release layer 28 is positioned over the structural fabric layers 24 and 26. Depending on the desired finish, mold release layer 28 may be any number of film or woven sheets designed for such purposes such as mylar, which yields a glossy surface or polytetrafluoroethylene mold release sheets manufactured by Precision Coatings, Inc. of Walled Lake, Mich. Mold release layer 28 freely releases from the resins that are cured in direct contact with its surface while having the additional advantage of avoiding contamination of the resin surface with release agents such as wax based materials.
A bladder 30 is then place atop mold release layer 28. In the illustrative embodiment, bladder 30 comprises a polyethylene bag having a volume substantially in excess of the cavity defined by inner surface 16 of lower mold half 12. Bladder 30 is then filled with a pre-measured portion of an exothermic foam-forming resin 34 such 20-10 foam resin manufactured by Polytech Foam Products, Inc. a division of FAI International, of Richmond Calif. Typical foam resins expand to form a foam having a density from 2 pounds per cubic foot to 24 pounds per cubic foot and can be selected based on the desired pressure and heat to be exerted on the fabric layers. Before the foam material expands significantly, mold cover 14 is attached to mold lower half 12 by means of screws, clamps or otherwise such that inner surface 16 of lower mold half 12 forms a substantially sealed interior volume 32.
The foam-forming resin 34 begins to expand rapidly and builds up interior pressure within interior volume 32 thus consolidating fabric layers 24-26 into a solid void-free body. The reaction of the foam-forming resin 34 is also strongly exothermic and heats the foam to an excess of 300° F. as it expands. The heat from the foam-forming resin 34 is conducted easily through bladder 30 into the resin surrounding the fabric layers 24-26. The insulating properties of lower mold half 12 and mold cover 14 retain the heat within the interior volume 32 such that the temperature in the resin surrounding fabric layers 24-26 remains above the heat activation/curing temperature for a pre-determined period of time long enough to ensure full curing of the resin.
Once the resin surrounding fabric layers 24-26 is fully cured, mold cover 14 is removed and the cured part consisting of fabric layers 24-26 is removed from lower mold half 12. Bladder 30 containing the now-cured foam-forming resin 34 is then separated from cured leg brace 10 and is discarded, as is mold release layer 28. The final part is then used as-cast or is finished with paint, gel coat or other surface finish.
With reference to
Since exothermic foam-forming resin 34 assumes whatever irregular shape is present within interior volume 32 of lower mold half 12, it is possible to use the exothermic foam reaction to consolidate and heat multiple parts at once. With reference to
With reference to
Although certain illustrative embodiments and methods have been disclosed herein, it will be apparent from the foregoing disclosure to those skilled in the art that variations and modifications of such embodiments and methods may be made without departing from the spirit and scope of the invention. Accordingly, it is intended that the invention should be limited only to extent required by the appended claims and the rules and principals of applicable law.
