The technical field relates generally to the fabrication of thermoplastic components, and more specifically to the heating of thermoplastic components during fabrication.
Typically, tooling in autoclave or hot press processing is a significant heat sink that consumes substantial energy. Furthermore, the tooling may require significant time to heat the composite material to its consolidation temperature and, after processing the composite, to cool to a temperature at which it is safe to remove the finished composite part. Furthermore, even distribution of heat applied to the tooling and the composite material may be difficult, especially when manufacturing a large component.
Fabrication of thermoplastic components may include induction heating. Typically, dies or tooling for induction processing are ceramic because ceramic is not susceptible to induction heating and, preferably, is a thermal insulator (i.e., a relatively poor conductor of heat). Cast 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 work piece. Enclosed in the metal retort, the work piece does not experience the oscillating magnetic field resulting from the induction process. The field is instead absorbed in the retort sheets. Heating is by conduction from the retort to the work piece.
Induction focuses heating on the retort (and work piece) and eliminates wasteful, inefficient heat sinks (e.g., tooling of conventional processes). Induction heating facilitates a reduction in the difference between the coefficients of thermal expansion of the tools and the work piece. Furthermore, this process is energy efficient because significantly higher percentages of the input energy go to heating the work piece than occurs with conventional presses. The reduced thermal mass and ability to focus the heating energy permits the operating temperature to be changed rapidly. Finally, the shop environment is not heated as significantly from the radiation of the large thermal mass of a conventional press, and is a safer and more pleasant environment for the press operators.
Fabrication of thermoplastic components may also include thermoplastic welding. Thermoplastic welding, which can eliminate mechanical fasteners, features the ability to join thermoplastic composite components at high speeds with minimum touch labor and little, if any, pretreatments.
Large scale parts such as wing spars and ribs, and the wing skins that are bonded to the spars and ribs, and/or fuselage sections and support structure may be typically on the order of twenty to thirty feet long, and potentially can be hundreds of feet in length when the process is perfected for commercial transport aircraft. Parts of this size are difficult to produce with perfect flatness. Instead, the typical part may have various combinations of inconsistencies beyond design tolerance. Applying heat to the interface by electrically heating the susceptor in connection with pressure on the parts tends to flatten the inconsistencies, but the time needed to achieve full intimate contact with the use of heat and pressure may be excessive, and can lead to undesirable results.
An existing solution for increasing the rate of production of thermoplastic components is to build more autoclaves and rate tooling when a critical rate of the current tools is reached, or to cap the production capabilities of a manufacturing facility at a certain rate that does not require building new tooling. The existing autoclave based systems have an inherent limit at which production rates above that critical rate will trigger a large increment of capital expenditures to be incurred along with a lag time required to obtain the capital, install the equipment, and ensure the equipment is functional.
Accordingly, there is a need for an apparatus and a system that facilitates rapid fabrication of large thermoplastic components, as well as a related method.