Stock material or miscellaneous articles – Structurally defined web or sheet – Honeycomb-like
Reexamination Certificate
1999-04-27
2001-06-12
Lorin, Francis J. (Department: 1775)
Stock material or miscellaneous articles
Structurally defined web or sheet
Honeycomb-like
C427S207100, C427S430100
Reexamination Certificate
active
06245407
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to non-metallic honeycomb structures which are heat formed (thermoformed) into a variety of different shapes. More particularly, the present invention is directed to such structures which include a resin impregnated composite core which is coated with one or more matrix resins to improve the structural strength of the honeycomb.
2. Description of Related Art
Honeycomb structures are well known and widely used in many applications where a high strength and lightweight material is required. The combined features of lightweight and strength found in honeycomb structures makes them particularly well-suited for use in aircraft and other applications where high strength and low weight are required. Honeycomb structures have been made from a wide variety of materials including metals, such as aluminum. Composite materials made from resin impregnated fibers and papers have also been widely used in honeycomb structures. Thermosetting resins have typically been used as the resin matrix of choice in cases where the honeycomb is used as part of a structural member where optimum honeycomb strength is required.
One common process for fabricating honeycomb structures involves bonding multiple sheets of material together along specially oriented node lines. The node lines are offset between different layers in such a way that a honeycomb structure is formed when the layers are expanded. This type of process is commonly referred to as “expansion” process. The expansion process is not suitable for fabricating honeycomb structures in some instances where certain thermoset matrix resins are used. For example, sheets made from certain materials which are too stiff or porous cannot be formed into honeycomb structures using the expansion process.
A fabrication process or method commonly referred to as the “corrugation” process has been used to form high strength thermoset honeycomb structures in those situations where the expansion process cannot be used. The corrugation process involves initially shaping sheets of uncured thermoset or thermoplastic material into a corrugated configuration. The corrugated sheets are cured at high temperature to form stiff corrugated sheets which are then bonded together to form the honeycomb core. The honeycomb core is then optionally coated with a matrix or dip resin. The honeycomb core is generally cut into numerous flat panels which can be used “as is” or further processed in accordance with conventional honeycomb fabrication techniques. For example, the honeycomb core may be sandwiched between sheets of various materials to form extremely strong structural panels.
In many instances, it is desirable to take the high strength thermoset honeycomb core and shape it into non-planar structural elements. This is accomplished by heating the honeycomb core until it becomes sufficiently soft to allow it to be molded or otherwise shaped into the desired configuration. A number of problems have been experienced during the heat forming of thermoset panels into non-planar shapes. For example, the final structural strength of thermoset honeycomb cores can be adversely affected by the high temperatures required to soften the cured resin. In addition, many of the high strength adhesives that are used to bond the panels together become weak at temperatures below those required to soften the thermoset material. As a result, the cores become uncontrollably distorted and weakened during the heat forming process. This problem is especially acute for heavy density and/or small cell honeycomb cores.
In view of the above, there is a need to provide honeycomb cores which are sufficiently strong to be useful as structural panels and which can be heat molded into non-planar shapes without unduly distorting or otherwise adversely affecting the honeycomb structure or strength.
SUMMARY OF THE INVENTION
In accordance with the present invention, a honeycomb structure is provided wherein the heat formability of the structure is improved by coating the honeycomb core with a dip resin which is composed of phenolic and polyamide resins. It was discovered that use of the combined phenolic/polyamide dip resin not only increases the heat formability of the honeycomb core, but also increases the overall strength of the honeycomb.
Honeycomb structures of the present invention include a honeycomb core which is composed of core fibers which are impregnated with a core resin. The surface of the core is coated with a dip resin which is made up of phenolic resin and polyamide resin. Dip resins composed of polyamide and phenolic resins were found to increase the yield stress of the core over dip resins which employed phenolic resin alone. In addition, the node strength of the core was increased. Further, the heat formability was increased, i.e., planar honeycomb cores made using a phenolic/polyamide dip resin could be thermoformed into structures having much smaller radii of curvature than similar cores utilizing a phenolic dip alone.
The present invention is well-suited for use in improving the formability of honeycombs which include interleaf layers (bisector sheets) and are made with carbon, glass or aramid fibers wherein the fibers are impregnated with a phenolic resin. Honeycombs which include interleaf layers are generally identified as heavy density honeycombs.
The dip resins of the present invention are resin solutions which include a resin portion which contains from 10 to 95 weight percent phenolic resin and 5 to 90 weight percent polyamide resin. The dip resin is applied to the honeycomb core in accordance with known dipping procedures and may be advantageously used as a replacement for dip resins in other honeycomb fabrication processes where a dip resin is needed or desirable.
The above-discussed and many other features and attendant advantages of the present invention will become better understood by reference to the following detailed description when taken in conjunction with the accompanying drawings.
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Caldwell Mark S.
Lee Emi
Petrisko Robert
Wang Yen-Seine
Bielawski W. Mark
Hexcel Corporation
Lorin Francis J.
Oldenkamp David J.
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