Some Fundamentals of High Frequency Gluing

The heating effects of radio-frequency electromagnetic fields with special emphasis on the dielectric heating of wood for curing glues are reviewed. The essential part of the RF generator, the vacuum tube, converts low-frequency input power to high-frequency output power. The resultant high-frequency electromagnetic field of force disturbs the atomic and molecular structure of the substance being treated, and this internal disturbance produces heat within the mass with a resultant internal rise in temperature. The press must be designed with consideration of the physics of the high-frequency field of force and the particular mode desired to fit the load. It is important to provide flexibility in the pressure sources so that the wood is not crushed in the gluing process. Hold-down devices for edge gluing must release their pressure momentarily when the RF power comes on so that breathing can be accomplished. Since considerable vapor pressure may build up during the heating cycle, it is good practice to shut off the RF power before releasing the pressure. An RF electric field pattern is not completely uniform, either parallel to the glue lines or at right angles to them. Thus, the design of electrodes for any given fabrication must be given careful consideration. The generator load and connecting links must be tuned to each other much the same as in a musical instrument. The best generator to date has been the double ender, which has a pair of charged electrodes, while the framework of the press remains neutral. The RF generator can be expected to be 45 percent effective. Compared to conventional heat applications from external sources, this efficiency is very high. The cost per kilowatt approximates $650 on capital investment. Excessively high RF voltages are not suitable for industrial purposes, because the tolerance of wood and glue with respect to voltage is limited. Increasing the voltage gradient beyond a certain tolerance will result in destructive arcs. Major factors governing the behavior of the wood and glue during pressing are: moisture content, species, density, ambient temperatures, distribution of moisture content, and type of glue. Of the four basic glues (urea, phenol, melamine, and resorcinol), ureas require the least RF power while phenols probably require the most. Parallel bonding may be enhanced by adding a small amount of salt to the glues. Any arrangement of electrodes which results in the process of parallel bonding is by far the fastest method for setting glues.

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