Efficient Composite Curing by Intelligent Microwave Processing

Our ultimate aim is to develop an intelligent and highly efficiency microwave curing process for composite materials that, enhanced by nanoscale fillers, will optimise the cure of polymer matrix composites, and allow the very rapid processing of potentially very large structures with dramatic reductions in consumed power. The rapid heating of composite materials has the potential to not only reduce cycle time, but also improve material properties, such as interlaminar shear strength and reduce void content. By lowering the minimum resin viscosity, rapid heating provides better fibre wet-out and improved fibre-matrix adhesion. However, inducing the necessary rapid temperature changes using conventional conduction heating is a very energy intensive process and therefore not particularly practical, especially for large structures. Microwave heating offers a far more efficient approach and evidence exists that shows that microwave heating can actually enhance the properties of cured composites. This is particularly true for thick section laminates, where more uniform heating can be achieved. It is far from clear, however, if these improvements are due to rapid heating, or are related to other non-thermal field related effects associated with microwave heating, or a combination of both.

Conventional microwave heating of composites is believed to be effective mostly due to microwave absorption by the carbon fibres. However, resins have been seen to absorb to a lesser extent, but their absorbance reduces as cross-linking increases, so if we can increase absorption by resins we can further enhance heating efficiency. Moreover, attempts to microwave cure components have thus far been limited to large multi-mode cavity ovens where the electric field intensity can vary greatly across a component, making an even and controlled heat application difficult if not impossible to achieve. A commercial microwave oven, developed by Vötsch Industrietechnik GmbH, uses a hexagon configuration, with 24 magnetrons, to promote a homogeneous field, however, once a component or material are placed within the cavity the field homogeneity will be disturbed.

Our eventual aim is to develop an intelligent, variable frequency, solid-state microwave curing process that can administer microwave heating via a remote scanning applicator, curing material as it moves. The device will traverse the surface of a component, simultaneously heating and monitoring the cure state. The nature of the approach allows simultaneous monitoring of reflected microwave power such that the applied field can be adaptively controlled to maintain high-efficiency microwave delivery and as a result, optimise the cure regardless of thickness and lay-up. In doing so, we can optimise curing, whilst minimising process time and energy usage.