Plywoods are boards which are prepared from thin layers of wood or veneers. Veneers are thin sheets of slices of wood of superior quality. The thickness of veneers varies from 0.4 to 6 mm.
To make plywood, round logs of 1.5–2.5 m lengths are first cut. They are then steamed and veneers are cut from these logs with a wide sharp knife. About three or more veneers in multiples of odd numbers are glued one over the other. Care is taken to place the direction of the grains in one sheet at right angles to the other. This arrangement is needed to provide adequate longitudinal and transverse strengths. The arranged veneers with adhesives in between are pressed under a pressure of 0.7–1.40 N/mm2 under hot or cold conditions (IS: 303–1989).
Three-ply plywood is made using only three plies. The thickness varies from 6 to 25 mm (Fig. 10.10). Multiply plywood is made using more than three plies but keeping the number of veneers used as odd in number.
Figure 10.10 Typical three-ply plywood
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