Wood Bending Utilizing Microwave Heating

Abstract

Wood bending utilizing microwave heating was investigated. In the first place, a creep test for Quercus crispula having various moisture contents during irradiation with microwave was carried out and the effect of the initial moisture content φ₀ on the final creep deflection Y_cf was examined. The relation between Y_cf and φ₀ was expressed by Y_cf=4√φ₀. In the next place, the bending quality of nine kinds of softwoods and hardwoods was compared. The rating of the bending quality was based on the radius of curvature of a bent member. The bending quality varied among the different wood species and as a rule that of hardwoods was better than that of softwoods. Robinia pseudo-acacia and Pinus taeda could be bent to a 3 cm radius in 1 cm thickness without any compressive wrinkles and tensile failures. An attempt was also made to correlate the bending quality with the specific gravity of wood and such mechanical properties of water saturated wood specimens as modulus of elasticity, stress and strain at proportional limit and maximum load and deflection. The mechanical properties were measured by a static bending test at 20°C. A good correlation was found between bending quality and maximum deflection. Furthermore, the anatomical structure in cross section of the bent wood was examined through a microscopic examination. Regular array of cells in radial direction which is common in the untreated wood became disordered by the bending treatment, which suggests that the cell wall materials were softened remarkably during irradiation with microwave.