Stock material or miscellaneous articles – Sheet including cover or casing – Including elements cooperating to form cells
Reexamination Certificate
2002-04-22
2003-05-20
Jones, Deborah (Department: 1775)
Stock material or miscellaneous articles
Sheet including cover or casing
Including elements cooperating to form cells
C428S116000, C428S117000, C052S309400, C052S309500, C052S309800, C052S309900, C052S309110, C052S309130, C052S309140, C052S309150
Reexamination Certificate
active
06565942
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a process for joining honeycomb sandwich panels with thermoplastic welds, particularly double interleaf staggered joints connecting the face sheets of the respective panels.
BACKGROUND ART
The use of composites in primary structure in aerospace applications is limited today because of the relatively high cost. A significant contribution to the total cost is the assembly cost where the precured composite elements are assembled, drilled, and fastened. The necessary design for mechanical fastening complicates the structure, especially in thin sections, because of the need for access to both sides of the bond line.
While composites might be adhesively bonded, cocured, or welded, these connecting processes generally produce bonds that rely upon the resin matrix for strength. The bond line lacks any reinforcing material to help with load transfer. These bonds generally have modest strength, and are susceptible to disbanding with shock impact or other “out of plane” forces affecting the assembly. Such forces often arise in environments prone to vibration.
1. Composite Manufacturing
Fiber-reinforced organic resin matrix composites have a high strength-to-weight ratio (specific strength) or a high stiffness-to-weight ratio (specific stiffness) and desirable fatigue characteristics that make them increasingly popular as a replacement for metal in aerospace applications where weight, strength, or fatigue is critical. Thermoplastic or thermoset organic resin composites would be more economical with improved manufacturing processes that reduced touch labor and forming time.
Prepregs combine continuous, woven, or chopped reinforcing fibers with an uncured matrix resin, and usually comprise fiber sheets with a thin film of the matrix. Sheets of prepreg generally are placed (laid-up) by hand or with fiber placement machines directly upon a tool or die having a forming surface contoured to the desired shape of the completed part or are laid-up in a flat sheet which is then draped and formed over the tool or die to the contour of the tool. Then the resin in the prepreg lay up is consolidated (i.e. pressed to remove any air, gas, or vapor) and cured (i.e., chemically converted to its final form usually through chain-extension or fused into a single piece) in a vacuum bag process in an autoclave (i.e., a pressure oven) to complete the part.
The tools or dies for composite processing typically are formed to close dimensional tolerances. They are massive, must be heated along with the workpiece, and must be cooled prior to removing the completed part. The delay caused to heat and to cool the mass of the tools adds substantially to the overall time necessary to fabricate each part. These delays are especially significant when the manufacturing run is low rate where the dies need to be changed frequently, often after producing only a few parts of each kind. An autoclave has similar limitations; it is a batch operation.
In hot press forming, the prepreg is laid-up to create a preform, which is bagged (if necessary), and placed between matched metal tools that include forming surfaces to define the internal, external, or both mold lines of the completed part. The tools and composite preform are placed within a press and then the tools, press, and preform are heated.
The tooling in autoclave or hot press fabrication is a significant heat sink that consumes substantial energy. Furthermore, the tooling takes significant time to heat the composite material to its consolidation temperature and, after curing the composite, to cool to a temperature at which it is safe to remove the finished composite part.
As described in U.S. Pat. No. 4,657,717, a flat composite prepreg panel was sandwiched between two metal sheets made from a superplastically formable alloy and was formed against a die having a surface precisely contoured to the final shape of the part.
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, but the cycle time for heating and cooling remains long. Designing and making tools to permit their active cooling also increases their cost.
Boeing described a process for organic matrix forming and consolidation using induction heating in U.S. Pat. No. 5,530,227. There, Boeing laid up prepregs in a flat sheet sandwiched between aluminum susceptor sheets. The susceptor sheets were susceptible to heating by induction and formed a retort to enclose the prepreg preform. To ensure an inert atmosphere around the preform during curing and to permit withdrawing volatiles and outgassing during the consolidation, the face sheets are welded around their periphery. Such welding unduly increased the preparation time and the cost for part fabrication. It also ruined the susceptor sheets (i.e., prohibited their reuse) which added a significant cost penalty to each part fabricated with this approach. Boeing described in U.S. Pat. No. 5,599,472 a technique that readily and reliably sealed the susceptor sheets of the retort without the need for welding and permitted reuse of the susceptor sheets in certain circumstances. This “bag-and-seal” technique applies to both resin composite and metal processing.
2. Processing in an Induction Press
The dies or tooling for induction processing in Boeing's induction heating workcell are ceramic because a ceramic is not susceptible to induction heating and, preferably, is a thermal insulator (i.e., a relatively poor conductor of heat). Ceramic tooling usually is strengthened and reinforced internally with fiberglass rods or other appropriate reinforcements and externally with metal or other durable strongbacks to permit it to withstand the temperatures and pressures necessary to form, to consolidate, or otherwise to process the composite materials or metals. Ceramic tools cost less to fabricate than metal tools of comparable size and have less thermal mass than metal tooling, because they are unaffected by the induction field. Because the ceramic tooling is not susceptible to induction heating, it is possible to embed induction heating elements in the ceramic tooling and to heat the composite or metal retort without significantly heating the tools. Thus, induction heating can reduce the time required and energy consumed to fabricate a part.
While graphite or boron fibers can be heated directly by induction, most organic matrix composites require a susceptor in or adjacent to the composite material preform to achieve the necessary heating for consolidation or forming. The susceptor is heated inductively and transfers its heat principally through conduction to the preform or workpiece that, in Boeing's prior work, is sealed within the susceptor retort. Enclosed in the metal retort, the workpiece does not experience the oscillating magnetic field which instead is absorbed in the retort sheets. Heating is by conduction from the retort to the workpiece.
Induction focuses heating on the retort (and workpiece) and eliminates wasteful, inefficient heat sinks. Because the ceramic tools in the induction heating workcell do not heat to as high a temperature as the metal tooling of conventional, prior art presses, problems caused by different coefficients of thermal expansion between the tools and the workpiece are reduced. Furthermore, Boeing's induction heating press is energy efficient because significantly higher percentages of input energy go to heating the workpiece than occurs with conventional presses. The reduced thermal mass and ability to focus the heating energy permits change of the operating temperature rapidly which improves the products produced. Finally, the shop environment is not heated as significantly from the radiation of the large thermal mass of a conventional press. The shop is a safer and more pleasant environment for the press operators.
In induction heating for consolidating and forming organic matrix composite materials, Boeing generally places a thermopla
Anderson David M.
Hahn Steven E.
McCarville Douglas A.
Boss Wendy
Hammar John C.
Jones Deborah
The Boeing Company
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