The Effects of Moisture Content and Machining of Wood on RF Gluing Processes

Theoretical and practical aspects of radio frequency (RF) gluing problems stemming from improper moisture content and machining are discussed. It was shown theoretically that RF power absorbed by wood tends to increase with increases in its moisture content. This relationship has important practical significance in electronic gluing based on selective heating of glue lines. If wood moisture is too high, the wood rather than the glue lines becomes heated, and insufficient glue curing may result. It is, recommended that in RF gluing, wood moisture content be in the range of 5 to 10 percent and an operating frequency of about 5 megacycles per second be used. Frequencies above this value tend to accentuate wood heating at higher moisture levels. Machining requirements for electronic edge-gluing are similar to those required of any edge-gluing operation, namely: warp-free wood; square, straight edges; uniform thickness; smooth machine surfaces, free of torn or loose fibers; and uniform wood moisture content. Out-of-square board edges in electronic edge-gluing can result in a non-uniform voltage drop across the glue line which may disproportionately increase heating time for satisfactory curing of the adhesive. Machined surfaces having ridges or grooves area primary factor in causing RF burning. In edge-gluing panels with extreme variations in board thickness, edge straightness, or moisture content may reduce voltage across glue lines thereby resulting in improper curing.

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