Plastic and nonmetallic article shaping or treating: processes – With step of cleaning – polishing – or preconditioning...
Reexamination Certificate
1998-10-02
2001-07-03
Silbaugh, Jan H. (Department: 1732)
Plastic and nonmetallic article shaping or treating: processes
With step of cleaning, polishing, or preconditioning...
C264S219000, C264S250000, C264S257000, C264S313000, C249S178000, C425SDIG004
Reexamination Certificate
active
06254812
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to methods for forming and curing composite materials, and more particularly, to a method using tooling and having a compliant forming surface for forming and curing of composite materials to form high quality composite parts with tight tolerance control on more than one surface of the part.
BACKGROUND OF THE INVENTION
The use of high strength fiber reinforced composite materials in the manufacture of aircraft and other lightweight structures has increased steadily since the introduction of such materials. Composite materials have a high strength-to-weight ratio and stiffness. These properties make composite materials attractive for use in the design of lightweight structures. Some of the drawbacks to using composite materials have been relatively high fabrication costs and difficulties in manufacturing defect-free parts. Generally, it has been difficult to produce parts formed of high strength composite materials at the same cost as comparable metal parts. It has also been difficult to fabricate composite parts in which tight dimensional tolerances are required on opposing surfaces or multiple surfaces of the formed composite part.
When it is necessary to fabricate composite parts having tight dimensional tolerances on multiple surfaces, it is generally necessary to use matched rigid tooling. In matched rigid tooling, two or more tools having forming surfaces that establish the final part dimensions are used. Once assembled, the forming surfaces of such rigid tooling define the final formed composite part's dimensions. Therefore, matched rigid tooling must be carefully fabricated to ensure that the tools fit together properly to ensure proper part dimensions. Inaccuracies in either the way the tools fit together or in the forming surfaces on any of the tools can result in defects in the formed composite part. Because of the tight machining tolerances required on matched rigid tooling, such tooling tends to be expensive to fabricate, thus increasing overall part fabrication costs.
The difficulties associated with fabricating high quality composite parts with rigid matched tooling is increased by variations in the quality and thickness of uncured composite materials. High-strength composite parts in the aerospace industry are generally fabricated using composite prepreg material. Such prepreg material consists of unidirectional fibers or cloths of graphite, fiberglass, Kevlar®, etc., contained within a matrix material such as an epoxy, bismaleimide, or thermoplastic material. The thickness of individual layers of composite prepreg differs slightly between batches of prepreg and even within the same batch of prepreg. Therefore, the thickness of a workpiece formed of multiple layers of prepreg differs from workpiece to workpiece. Such differences in thickness from workpiece to workpiece can result in differences in the thickness of the final formed composite parts.
When matched rigid tooling is used, the forming surfaces of the tools generally cannot account for the differing thicknesses in the prepreg used. Therefore, if the prepreg used to form the composite part is slightly thicker than normal, portions of the formed composite part will have a lower percentage of resin than desirable. In extreme cases, the increased thickness of the composite prepreg can prevent the matched rigid tooling from fitting together properly, thus leading to tolerance problems on the surface of the formed composite part. Similarly, if the prepreg used is slightly thinner than normal, the use of the matched rigid tooling does not account for the thickness variations, thus resulting in a resin-rich formed composite part.
It would be desirable if a system of composite tooling was developed that could produce composite parts with tight tolerance control on multiple surfaces. At the same time, it would be advantageous if such a tooling method could eliminate the need for expensive matched rigid tooling.
As can be seen from the above discussion, there exists a need for a method of forming composite parts that can maintain dimensional tolerances, while reducing the disadvantages of the prior art. The present invention is directed toward fulfilling this need.
SUMMARY OF THE INVENTION
The present invention is an improved tooling concept for forming composite parts. In the preferred embodiment of the invention, the formation of a composite sine wave span is disclosed. The tooling concept includes one or more tools that have a compliant forming surface that is able to account for slight thickness variations in the composite material used to form the workpiece. The compliant forming surface also allows the tooling concept to account for slight variations in the placement of individual plys of prepreg during the lay up process. The tooling concept allows for such variations, while maintaining dimensional tolerances on the surface of the part and producing a high-quality formed composite part.
In one embodiment of the invention, the tooling system includes first and second tools that are placed in an opposing relationship to at least partially define the geometry of the composite part being formed. At least one of the tools includes a compliant forming surface that is contoured to contact and form one surface of the composite part. The compliant forming surface is supported by a rigid substructure that provides structural integrity to the compliant forming surface. The compliant forming surface forms a covering of a generally constant thickness over the rigid substructure. During forming, the compliant forming surface contacts the composite material and applies a consolidation and forming force to the composite material to produce the composite part. The dimensions of the forming surface of the compliant forming surface defines the final part dimensions of the formed composite part in the areas of the composite part that contact the compliant forming surface.
In accordance with further aspects of the invention, the tooling system is used to form a composite sine wave spar. The sine wave spar tooling includes a rigid lower tool insert and an upper tool insert having a compliant forming surface. The sine wave spar tooling also includes side forming tools that have rigid forming surfaces that define the dimensions of the exterior surfaces of the formed sine wave spars caps.
In accordance with other aspects in the invention, the rigid substructure includes a plurality of recesses. The recesses are sized to accommodate tabs that extend outward from the surface of the compliant forming surface. When assembled, the tabs on the compliant forming surface extend into the recesses to hold the compliant forming surface on the rigid substructure.
A method for forming composite parts using tooling with a compliant forming surface is also disclosed.
The invention's use of tooling having a compliant forming surface placed over a rigid support structure has a number of advantages over prior tooling concepts. The compliant forming surface can deform slightly during curing. This allows the compliant forming surface to accommodate slight thickness variations from composite workpiece to composite workpiece. The compliant forming surface accounts for such thickness variations while maintaining dimensional tolerances on the surface of the formed composite part. In addition, the invention's use of a compliant forming surface reduces tooling costs when compared to similar prior art matched rigid tooling. Because the compliant forming surface can be formed in a casting operation, it is not necessary to closely machine match rigid tools to produce close tolerances on the formed composite part.
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Goodridge Harold M.
Skaggs Kirk D.
Silbaugh Jan H.
Staicovici Stefan
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