Theory and Application of Dielectric Heating

When a dielectric is placed in a rapidly alternating field, the molecules are first stressed in one direction and then in the other at the frequency of the generator. This rapid motion of the individual molecules causes friction, and the friction produces heat. In a relatively thick dielectric, the center section will start heating as quickly as the outer sections. The rate of heating depends upon the loss factor of the material, the square of the applied voltage, and the frequency of the RF power. To keep the voltage down and avoid frequent flashover, the frequency used is relatively high, between 5 and 50 megacycles. Radio-frequency generators are essentially frequency changers for converting 60 cycle power into RF power. Principal components are an electronic rectifier which converts incoming A.C. power to high voltage direct current, oscillator tubes to convert direct current to alternating current in the RF range, electrodes for applying energy to the dielectric, and various control devices. The woodworking industry is rapidly becoming an important user of RF energy. Radio-frequency heating is applied in three ways: 1) In through heating, the electrodes are placed parallel to the glue line and the entire mass of wood is heated. This method could be used in the manufacture of plywood sheets or curved chair seats, etc. 2) In edge gluing, the electrodes are placed perpendicular to the glue lines. Since the wet glue lines have greater conductivity, most of the power flows into the glue line. This method is used in gluing core stock. 3) In stray-field heating, the electrode consists of strips of aluminum or brass, and the power flows from one electrode down to the glue line and back to the adjacent electrode. This method is used to set glue lines that are accessible from only one surface. In all these applications extreme care must be taken to prevent RF radiation that would cause interference with aircraft radios, etc.

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