Dielectric heating speeds honeycomb core manufacture

Many composites, particularly those made with glass or aramid fibers in a polymer matrix, tend to be inherently nonconductive. While that can be a good thing, say, for an application that requires electrical insulation, it also makes processing these composites quite energy intensive: Extra time is needed to “soak” the part to bring it completely up to cure temperature. Since 1939, Thermex Thermatron LP (Louisville, Ky.) has designed, engineered and sold radio frequency (RF) generators and microwave power sources for dielectric heating — alternative systems that can significantly reduce cure duration for nonconductive materials without exposing the part to an external heat source.

RF heating has recently been put to use by several honeycomb core manufacturers who must bond sheets of aramid paper and phenolic resin material together to form the core “books” that are later cut and expanded. Working with these manufacturers, Thermex Thermatron designed a press, calculated the material properties and determined the electrical power requirements for an RF process that subjects the core sheet adhesive to an electromagnetic field, which excites the resin’s dipolar molecules. These asymmetrical molecules are positively charged at one end and negatively charged at the other and tend to reorient themselves when subjected to an electromagnetic field. As the field alternates, the dipoles reverse direction, causing friction between themselves and other fixed molecules and, as a result, generate heat at the point of each dipole molecule.

RF cure is reportedly 40 times faster than conventional hot-press core curing methods. According to the company’s composites business development manager, Paul Kennedy: “The adhesive used to bond the sheets together to form the honeycomb core books is cured in about 20 minutes in our 700-ton, 40-kW unit [see photo] as compared to 24 hours in a hot press. It’s becoming the industry standard, and it’s applicable, generally, to composites curing, including pultrusion.”

Kennedy says RF advantages include instant start and stop of process heat, and direct temperature control (the higher the electromagnetic frequency, the faster the dipoles oscillate; the stronger the field, the greater the torque exerted on each dipole, which produces more rotation and higher friction). Additionally, RF systems provide more uniform heating and reduce energy consumption. Savings can be as high as 50 percent, compared with conventional heating methods, says Kennedy, noting that the resin’s chemistry and dipole moment characteristics determine how much electrical energy will be converted to heat and how evenly the product will heat.