Electric heating – Inductive heating – Specific heating application
Reexamination Certificate
1995-06-05
2001-04-03
Hoang, Tu Ba (Department: 3742)
Electric heating
Inductive heating
Specific heating application
C219S633000, C219S634000, C219S659000
Reexamination Certificate
active
06211497
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to the consolidation and forming of organic matrix composites, more specifically, the present invention relates to methods and apparatus for inductively heating, forming and consolidating resins to make organic matrix composites.
BACKGROUND OF THE INVENTION
Fiber-reinforced resin (i.e., organic matrix) composite materials have become widely used, have a high strength-to-weight or high stiffness-to-weight ratio, and desirable fatigue characteristics that make them increasingly popular in weight, strength or fatigue critical applications.
Prepregs consisting of continuous, woven, or chopped fibers embedded in an uncured matrix material are cut to the desired shape and then stacked in the desired configuration of the composite part. The prepreg may be placed (laid-up) directly upon a tool or die having a forming surface contoured to the desired shape of the completed part or the prepreg may be laid-up in a flat sheet and the sheet may be draped over a tool or die to form to the contour of the tool.,
After being laid-up, the prepreg is consolidated (i.e., cured) in a conventional vacuum bag process in an autoclave (i.e., a pressurized oven). The pressure presses the individual layers of prepreg together at the consolidation/curing temperatures that the matrix material flows to eliminate voids and cures, generally through polymerization.
In autoclave fabrication, the composite materials must be bagged, placed in the autoclave, and the entire heat mass of the composite material and tooling must be elevated to and held at the consolidation or curing temperature until the part is formed and cured. The formed composite part and tooling must then be cooled, removed from the autoclave, and unbagged. Finally, the composite part must be removed from the tooling.
To supply the required consolidation pressures, it is necessary to build a special pressure box within the autoclave or to pressurize the entire autoclave, thus increasing fabrication time and cost, especially for low rate production runs.
Autoclave tools upon which composite materials are laid-up are typically formed of metal or a reinforced composite material to insure proper dimensional tolerances and to withstand the high temperature and consolidation forces used to form and cure composite materials. Thus, autoclave tools are generally heavy and have large heat masses. The entire heat mass of the tool must be heated along with the composite material during curing and must be cooled prior to removing the completed composite part. The time required to heat and cool the heat mass of the tools adds substantially to the overall time necessary to fabricate a single composite part.
In composite parts requiring close tolerances on both the interior and exterior mold line of the part, matched autoclave tooling must be used. When matched tooling is used, autoclave consolidation pressure is used to force the matched tooling together to consolidate the composite material and achieve proper part dimensions. Matched tooling is more expensive than open faced tooling and must be carefully designed to produce good results, adding to part fabrication costs.
An alternative to fabricating composite parts in an autoclave is to use a hot press. In this method, the prepreg is laid-up, bagged (if necessary), and placed between matched metal tools that include forming surfaces that define the internal and external mold lines of the completed part. The tools and composite material are placed within the press and then heated. The press brings the tools together to consolidate and form the composite material into the final shape. Fabricating composite parts in a hot press is also expensive due to the large capital expense and large amounts of energy required operate the press and maintain the tools.
Generally, in hot press operations, to obtain close tolerances, the massive, matched tooling is formed from expensive metal alloys having low thermal expansion coefficients. The tooling is a substantial heat sink that takes a large amount of energy and time to heat to composite material consolidation temperatures. After consolidation, the tooling must be cooled to a temperature at which it is safe to remove the formed composite part thus adding to the fabrication time.
Another contributor to the cost of fabricating composite parts is the time and manpower necessary to lay up individual layers of prepreg to form a part. Often, the prepreg must be laid up over a tool having fairly complex contours that require each layer of prepreg to be manually placed and oriented. Composite fabrication costs could be reduced if a flat panel could be laid-up flat and then formed into the shape of the part.
One method used to reduce the costs of fabricating composite materials is to lay up a flat panel and then place the flat panel between two metal sheets capable of superplastic deformation as described in U.S. Pat. No. 4,657,717. The flat composite panel and metal sheets are then superplastically deformed against a metal die having a surface contoured to the final shape of the part. Typically, the dies used in such superplastic forming operations are formed of stainless steel or other metal alloys capable of withstanding the harsh temperatures and pressures. Such dies have a large thermal mass that takes a significant amount of time and energy to heat up to superplastic forming temperatures and to cool down thereafter.
Attempts have been made to reduce composite fabrication times by actively cooling the tools after forming the composite part. These attempts have shortened the time necessary to produce a composite part, however, the time and energy expended in tool heat up and cool down remains a large contributor to overall fabrication costs.
The present invention is a method and apparatus for consolidating and forming organic matrix composites that avoid some of the above-identified disadvantages of the prior art.
SUMMARY OF THE INVENTION
The present invention is a method and apparatus for consolidation of organic matrix composites using inductive heating. In the present invention, the dies or tooling for the organic matrix composite parts are made from a material that is not susceptible to inductive heating. Examples of usable tool materials are ceramics or resin composites. The tooling is strengthened and reinforced with fiberglass rods or other appropriate reinforcements to withstand the temperatures and pressures used to form the composite materials. Such materials decrease the cost of tool fabrication and also generally reduce the thermal mass and weight of the tooling. Since the tooling used in the present invention is not susceptible to inductive heating, it is possible to use the tooling in combination with inductive heating elements to heat the composite material. The present invention allows the composite material to be inductively heated without heating the tools significantly. Thus, the present invention can reduce the time and energy required to fabricate a composite part.
Graphite or boron reinforced organic matrix composites may be sufficiently susceptible because of their reinforcing fibers that they can be heated directly by induction. Most organic matrix composites require a susceptor in or adjacent to the composite material to achieve the necessary heating. The susceptor is heated inductively and transfers its heat to the composite material.
The present invention reduces the time and energy required to consolidate resin composite prepreg lay-ups for a composite. Because induction focuses the heat on the workpiece rather than the tool, there is less mass to heat or cool. Inexpensive composite or ceramic tooling can also be used. The lower operating temperature of the tools decreases problems caused by different coefficients of thermal expansion between the tools and the workpiece in prior art forming systems. The present invention also provides an improved method for fabricating composite parts to close tolerances on both the internal and external mold line of the part.
In a method for consolidating/or a
Gregg Paul S.
Martinson Howard
Matsen Marc R.
Snyder Robert
Hammar John C.
Hoang Tu Ba
The Boeing Company
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