Radio-Frequency Heating Applied to Wood Gluing

Dielectric heating by radio-frequency power has been established as a practical means for rapidly gluing wood with thermosetting adhesives. To the radio engineer, such an application involves the correlation of power concentration, frequency, voltage gradient, dielectric constant, and power factor for the determination of reasonable operating parameters. An equation is developed showing the interdependence of these factors. Pertinent data on the dielectric properties of several kinds of wood are included covering a suitable range of frequency and for the temperatures and moisture contents commonly encountered. The nonuniform heating likely to occur in large presses because of standing waves on the electrodes can be eliminated by "multiple tuning" in which each of several inductors tunes a section of the press electrodes to parallel resonance. A coupling network is evolved for feeding a tuned press so that variations in press capacitance during the gluing cycle become unimportant to the loading of the oscillator. Means for measuring the temperature at the glue line during the application of radio-frequency power are discussed.