Plastic and nonmetallic article shaping or treating: processes – Pore forming in situ – Composite article making
Reexamination Certificate
1999-12-27
2001-05-15
Kuhns, Allan R. (Department: 1732)
Plastic and nonmetallic article shaping or treating: processes
Pore forming in situ
Composite article making
C264S046400, C264S046600, C264S101000, C264S102000, C264S321000
Reexamination Certificate
active
06231794
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to methods of making radar absorber structures, and more particularly to a method for forming absorber structures having reduced weight without compromised structural properties.
2. Description of the Related Art
The use of open-cell foam substrates having a conductive filler to form light-weight broadband microwave absorbers is known. However, such absorbers suffer from a variety of drawbacks.
For one thing, repeated flexure of the absorbers results in reduction of the elasticity, with resultant failure, of the foam, as well as failure or other deleterious effects in the conductive filler. In many situations, the foam absorbers are rendered defective due to improper distribution of the conductive material in the foam. In other situations, the weight of the absorber and conductive filler exceeds the permissible limits of the anticipated application.
U.S. Pat. No. 5,192,810 issued Mar. 9, 1993 to Hill discloses a method and apparatus for manufacturing dimpled structures from elastic cloth. The method permits forming complex polyimide foam shapes through the steps of impregnating a low density, open cell, reticulated foam with polyimide foam precursor, placing the combination in a mold, closing the mold, then heating the assembly to the foaming and curing temperatures of the precursor. As the powdered precursor expands, it spreads throughout the reticulated foam producing a product having substantially uniform density and polyimide foam characteristics. If desired, the reticulated foam can be removed from the product by using a material that outgasses and boils away at polyimide processing temperatures. Radar absorbing materials may be incorporated in the reticulated foam so that the final product will have uniformly distributed radar absorbing components with the desirable high temperature resistance and other properties of polyimide foams.
U.S. Pat. No. 5,845,877 to Justice et al. and issued on Dec. 8, 1998 discloses a sealing assembly for reducing the gap between an aircraft's movable flight control member and adjacent structures. The flight control member and adjacent structures have external surfaces and side surfaces facing each other. First and second flexible bulb seals are mounted on the periphery of the side surface of both the flight control member and the adjacent structure forming an extension of the external surface of both. Each bulb seal includes a compressible foam bulk absorber core. A first sheet of dielectric material is bonded to the bottom surface of the core. A second sheet of flexible resistive material is bonded to the top, and sides of the core. A third sheet of flexible magnetic radar absorbing material is bonded to the second sheet. A fourth sheet of flexible abrasion resistant material is bonded over the third sheet.
Against this background of known technology, the applicant has developed a novel method for forming a low density closed cell foam filled reticulated absorber having significant weight reduction properties and which can be shaped and cured to form simple or complex components at low cost and with little difficulty, while overcoming many of the deficiencies and drawbacks of similar absorbers currently known in the relevant technology
OBJECTS AND SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide an improved method for forming a foam filled reticulated structure containing radar absorbing materials for the attenuation of electromagnetic energy.
Another object is to provide a method for making a radar absorbing structure in which method the foam is loaded into a mold, covered and sealed in the mold, and then, prior to curing, subjected to a vacuum whereby the density of the foam is reduced thereby resulting in a significant reduction in weight of the structure.
Still another object of the invention is to provide a reticulated foam filled absorber which is capable of attenuating electromagnetic energy or radar over a broad range of frequencies.
The above objects are achieved according to the invention by the provision of an improved method for producing a foam filled reticulated absorber structure having electromagnetic energy attenuation characteristics, in which method the foam is deposited in a mold for forming and curing, but before curing is subjected to a vacuum in the mold to assist in the expansion of the foam thereby lowering the density of the foam prior to its being cured. To achieve the radar absorption effect, a conductive material is applied to the outer surface of the foam substrate or caused to be contained within the openings of the reticulated foam structure. The conductive material is preferably comprised of a particulate material, such as carbon powder, or a combination of carbon and a metal such as silver, copper or nickel. Whereas most foam absorbers exhibit densities on the order of 7.8 pounds per cubic foot, the foam product of the invention can be made lighter—on the order of 6.0 to 6.5 pounds per cubic foot —and thinner than conventional foam absorbers. The absorber made according to the present invention is also more durable and reproducible than existing reticulated foam absorbers. The foam absorber product of the invention retains efficient performance characteristics upon mechanical cycling, and is durable and has increased chemical resistance.
Other objects, advantages and features of the invention will become more apparent, as will equivalent structures which are intended to be covered herein, with the teaching of the principles of the invention in connection with the disclosure of the preferred embodiments thereof in the specification, claims and drawings in which:
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Kuhns Allan R.
Lockheed Martin Corporation
Schruhl Robert A.
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