Plywood consists of layers of thin veneers or plies glued together under huge pressure with the grain directions at right-angles in each successive layer. This allows the production of large sheets of dimensionally stable material in which most properties are approximately equalized across the surface with the strength of wood parallel to the grain predominating. Shrinkage and swelling are very small, being about 0.18% along the grain and 0.27% across it over the range of 7-20% moisture content (Joyce, 1970). Normally an uneven number of plies is used to give a balanced construction but even-numbered plywoods, in which the grain of the central pair of veneers is parallel, are made. Plywood is highly resistant to splitting parallel to the face grain but splits relatively easily in its thickness. Although cross-ply laminations of wood are known from antiquity, plywood made from thin veneers and used as a substitute for solid wood is essentially a modern material that owes much to the development of economical, moisture-resistant adhesives.
Where one outermost ply is of better quality than the other, the better-quality veneer is referred to as the face and the other as the back. Where they are of equal quality, both are referred to as faces. The central ply is referred to as the core and in plywoods with more than three plies those lying immediately beneath the face and back are referred to as cross bands or cross banding. Manufactured boards in which the central plies are replaced by particle board or wood core strips glued together side by side typically retain only the face veneer and cross banding, or less commonly the face veneer only. Examples illustrated in Figure 2.22 include blockboard, in which the core strips may not exceed 25 mm, and laminboard, in which the strips must not exceed 7 mm wide.
Plywood is graded according to the strength and stiffness of the face veneers, the quality of the face veneers including the size and num-
Figure 2.22 Examples of manufactured board construction: (a) three ply (thicker veneer core); (b) multiply (seven-ply); (c) laminboard (veneered outer layers, laminated core); (d) blockboard (veneered outer layers, blocked core); (e) five ply particle board core
Figure 2.22 Examples of manufactured board construction: (a) three ply (thicker veneer core); (b) multiply (seven-ply); (c) laminboard (veneered outer layers, laminated core); (d) blockboard (veneered outer layers, blocked core); (e) five ply particle board core ber of defects and repairs, and the suitability of the material for interior or exterior use, or immersion in water etc. The precise form in which this is expressed differs between countries.
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