Coating processes – Particles – flakes – or granules coated or encapsulated – Fluidized bed utilized
Reexamination Certificate
1998-11-24
2001-01-09
Meeks, Timothy (Department: 1762)
Coating processes
Particles, flakes, or granules coated or encapsulated
Fluidized bed utilized
C427S424000, C427S214000
Reexamination Certificate
active
06171649
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to preparing pre-coated, aluminum alloy-based components. In particular, the present invention relates to the use of fluidized-bed coating processes to coat aluminum-alloy aircraft structural components.
It has recently been discovered that the corrosion protection and ease of installation and processing of certain aluminum-alloy aircraft components can be improved by pretreating the alloys with an organic coating prior to installation. Such pretreatment reduces processing time and commensurate cost, while obviating conventional use of toxic solvents that pose difficult and expensive disposal problems.
Such advances are the subject of commonly owned U.S. Pat. No. 5,614,037 the entire contents of which are incorporated by reference herein. As disclosed therein, it has been the practice to coat some types of fasteners with organic coatings to protect the base metal of the fasteners against corrosion damage. In the usual approach, the fastener is first fabricated and then heat-treated to its required strength. After heat-treatment, the fastener is etched with a caustic soda bath or otherwise cleaned to remove the scale produced in the heat-treatment. Optionally, the fastener is alodined or preferably anodized. The coating material, dissolved in a volatile carrier liquid, is applied to the fastener by spraying, dipping, or the like. The carrier liquid is then evaporated. The pre-coated fastener is heated to an elevated temperature for a period of time to heat treat the alloy, and simultaneously cure the coating. The finished fastener is then used in the fabrication of the aircraft structure.
As explained in U.S. Pat. No. 5,614,037, it has not been the practice to coat high-strength, aluminum-alloy fasteners and other structural components with curable coatings, because it is observed that the normally accepted curing treatment for the coating can adversely affect the strength of the components. The uncoated aluminum-alloy components are therefore more susceptible to corrosion than would otherwise be the case.
The absence of the coating means that aluminum components such as rivets, fasteners, etc., must be installed using a wet-sealant compound for purposes of corrosion protection and ease of installation. The wet-sealant compound typically contains toxic components and therefore requires precautions for the protection of the personnel using it and for environmental protection. It is also messy and difficult to work with, and requires extensive cleanup of the area around the fastener using caustic chemical solutions.
The procedural advances described in U.S. Pat. No. 5,614,037 address many of these needs. However, for certain aircraft components, including fasteners, coating uniformity is essential. While coatings can be applied in many ways, many known application techniques including rack and barrel systems, conveyer systems or plasma vapor deposition do not provide the required tolerances and uniformity for irregularly-shaped aluminum alloy aircraft components such as rivets and other fasteners. For example, if coatings are deposited too thickly to aluminum alloys, the coating's resilience (i.e., the strength or integrity of the coating) is weakened. If too thin a coating is deposited, corrosion protection and overall component performance may suffer. Further, coating application methods where one layer of coating is delivered over many separate applications is unacceptable since the multiple bond layers introduced between coating applications cannot be reliably cured to produce the proper cross-linking necessary to achieve the optimum strength levels required.
Such coating tolerances were previously difficult to obtain using known methods for small, irregularly shaped aluminum-alloy components, such as rivets and other fasteners.
Therefore, there exists a need for an improved approach to the application of corrosion inhibiting coatings to aluminum-alloy aircraft structural components, such as rivets, fasteners, small and irregularly-shaped components, etc., and the mechanical, aircraft structures attached to these aforementioned components.
SUMMARY OF THE INVENTION
The present invention provides an improved method for pre-coating and pre-treating aluminum-alloy, aircraft components such as fasteners, rivets, small and irregularly-shaped components, etc., and the mechanical, aircraft structures attached to these aforementioned components. Heat-treatable components are heat-treated to obtain acceptable mechanical properties and also are protected by a cured organic coating. Cold-worked and/or non-heat-treatable components have a coating applied to the component and cured while still achieving the desired deformation state in the final component. The application of the coating does not adversely affect the desired formal properties of the finished component.
In one preferred embodiment of the present invention an aluminum-alloy fastener is prepared by providing an aluminum-alloy component precursor to an apparatus having a fluidized bed or mist. The apparatus is activated to deliver gas at a predetermined velocity to create an airstream. The airstream impacts the components to suspend and raise them vertically. A coating in a vapor, atomized state is introduced to the apparatus such that the coating impacts the suspended components and is deposited on the component uniformly to a controlled thickness during a predetermined period of time. The component remains exposed within the gas flow airstream to dry the coating after the vaporized coating is discontinued in the apparatus. The gas flow is then turned off, and the uniformly-coated components are then removed from the apparatus.
The preferred coating apparatus according to one embodiment is a fluidized-bed apparatus comprising three vertically-contiguous sections. The upper section constitutes an expansion zone. The intermediate section has an upwardly extending first wall. The lower section includes a second wall, inwardly-tapered frustoconically as it extends downward and terminates in a bottom gas inlet. The intermediate and lower sections constitute a fluidized-bed vessel. The second wall has an interior wall surface being divided into an upper portion and lower portion. The upper portion is defined by the area of vertical movement of a rotor disc, widening frustoconically in an upward direction. The rotor disc is substantially flat and can be moved vertically. The apparatus has a vacuum assembly positioned above and in fluid communication with the upper section to pull a gas flow into the vessel through an annular gap around the periphery of the rotor disc. The apparatus further has a valve for releasing material to be treated into the fluidized bed above the rotor disc. A plurality of nozzles positioned below the expansion zone introduce the coating to the material to be treated.
The components preferably are small, irregularly-shaped aircraft structural components including fasteners, rivets, hinges, fittings, etc. Any small, irregularly-shaped component that can be provided to a fluidized-bed or mist apparatus and raised to a suspended state via directed gas pressure and flow can be treated according to the present invention. It is preferred that the components remain completely suspended in the airstream during the coating process. However, it is understood that the components may randomly contact the chamber walls or floor. Therefore, the present invention also contemplates the coating of completely suspended and substantially suspended aircraft structural components.
Small components are able to be coated in the fluidized-bed or mist apparatus in batches or lots of from about 25,000 to about 30,000 pieces per batch or more for running times of from about 10 to about 15 minutes or longer depending upon the volume and density of the vaporized coating desired to be released into the apparatus and the mass and density of the components.
In a further preferred embodiment, a coating is delivered to the surface of aluminum-alloy aircraft components without the compone
Byrd Norman R.
Keener Steven G.
Alston & Bird LLP
Meeks Timothy
Strain Paul D.
The Boeing Company
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