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The two main products produced from veneer are plywood and Laminated Veneer Lumber (LVL). Veneers are also used to as surface finishes on lower grade wood or wood products such as particleboard to improve appearance. These veneers are either peeled or sliced, depending on the desired grain appearance.
Plywood
Plywood is generally produced in panels. Most is used in the construction industry where its ability to withstand large racking forces that may be imposed on a structure (e.g. by an earthquake) makes it ideal for bracing walls. Plywood is used in the boat building industry for the same reason.
Specialty plywood products are numerous but form a small portion of the total ply market. Some are not manufactured in exactly the same way as panel plywood, but the basic plywood concept is the same: aligning the grain perpendicular to adjacent veneers to form a stronger and more stable product.
Manufacturing Processes
Plywood is made by gluing together one or more veneers to both sides of a veneer or solidwood core. The grain of alternate layers is arranged at right angles. Expansion or contraction within the plane of the board of one veneer is restricted by the wood fibres in the adjacent veneer. The resulting panel has similar strength and shrinkage properties in the two directions and the large dimensional and strength variations that occur across the grain in solid wood are eliminated.
The plywood manufacturing process has three stages: veneer manufacture; clipping, drying and upgrading; panel lay-up, pressing and finishing.
Veneer Manufacture
Plywood panels are made from rotary peeled softwood veneers of 2-6 mm thickness. Rotary peeling involves rotating a log about its central axis against a knife lying parallel to the grain. The log or bolt is mounted on a set of chucks that hold it in place and provide rotating force. As it is turned a thin sheet of veneer is peeled off through the gaps between the nosebar and the knife-edge as a long continuous ribbon. Veneer of uniform thickness and smooth finish is important, as is maintaining a continuous sheet.
Logs are often preconditioned or heated through to make peeling easier and enhance surface finish. In New Zealand, most veneer production uses radiata pine, which is soft and pliable and does not need preconditioning.
Higher quality logs are used where appearance is important, such as the exterior layer. Veneer can be produced using the rotary peeling technique or by slicing the logs. Sliced veneer has a more attractive grain and may use species of much higher value. It is therefore sliced very thinly, 0.25-2 mm, to increase the face area of veneer and reduce costs. |