General Discription Plywood is a flat panel built up of sheets of veneer called plies, united under pressure by a bonding agent to create a panel with an a
dhesive bond between plies. Plywood can be made from either softwooeds or hardwoods. It is always constructed with an odd number of layers with the grain direction of adjacent layers perpendicular to one another. Since layers can consist of a single ply or of two or more plies laminated such that their grain is parallel, a panel can contain an odd or even number of plies but always an odd number of layers. The outside plies are called faces or face and back plies, and the inner plies are called cores or centers. The core may be veneer, lumber, or particleboard, with the total panel thickness typically not less than 1.6 mm or more than 76 mm. The plies may vary in number, thickness, species, and grade of wood. To distinguish the number of plies (individual sheets of veneer in a panel) from the number of layer(number of times the grain orientation changes), panels are sometimes described as three-ply, three-layer or four-ply, three-layer. The outer layers (face and back) and all odd numbered layers (centers) generally have their grain direction oriented parallel to the length or long dimension of the panel. The grain of even-numbered layers (cores) is perpendicular to the length of the panel. Characteristics The alternation of grain direction in adjacent plies provides plywood panels with dimensional stability across their width. It also results in fairly similar axial strength and stiffness properties in perpendicular directions within the panel plane. The laminated construction distributes defects, markedly reduces splitting when the plywood is penetrated by fasteners (compared with splitting of solid wood) and improves resistance to checking.
7 Plywood Compared with solid wood, the chief advantages of plywood are that the properties along the length of the panel are more nearly equal to properties along the width, there is greater resistance to splitting, and the form permits many applications where large sheets are desirable. The use of plywood may result in improved utilization of wood. Plywood can cover large areas with a minimum amount of wood fiber because plywood that is thinner than sawn lumber can be used in some applications. The properties of plywood depend on the quality of the different layers of veneer, order of layer placement, adhesive used, and control of bonding conditions. The grade of the panel depends upon the quality of veneers used, particularly of the face and back. The type of panel refers to the durability of the adhesive-to-wood bond and depend upon the adhesive-bonded joint, particularly its water resistance, and upon veneer grades used. Generally, face veneers with figured grain that are used in panels where appearance is important have numerous short, or otherwise deformed, wood fibers. These may significantly reduce strength and stiffness of the panels. On the other hand, face veneers and other plies may contain certain sizes and distributions of knots, splits, or growth characteristics that have no undesirable effects on strength properties for specific use, such as sheathing for walls, roofs, or floors. The plywood industry continues to develop new products. Hence, the reader should always refer directly to current specifications on plywood and its use for specific details. Processing After trees are felled and bucked to length, the logs are graded and sorted to make the most appropriate and efficient use of the wood fiber. For softwood plywood, “saw logs” were shipped to lumber mills. Because of the dwindling availability of the clear, large-diameter peeler logs on which the plywood industry was founded, this practice has changed. Today, the higher grades of softwood peeler logs are sent to sawmills, and with few exception, plywood is made from low-grade sawlogs or peeler logs.
7 Plywood This change came about because of the increasing demand for clear sawn lumber, and it has been made possible by innovations in veneer and plywood manufacturing and testing practices that ensure that panels are suitable for their intended use. Logs delivered to a veneer mill are sorted by grade and species, then debarked and crosscut into peeler blocks. Peeler blocks are often heated or conditioned by steaming or immersion in hot water prior to peeling, which makes them easier to peel, reduces veneer breakage, and results in smoother, higher quality veneer. The heated blocks are then conveyed to a veneer lathe. To maximize veneer yield, each block is gripped on the ends at the block’s geometric center. While rotating at high speed, the block is fed against a stationary knife parallel to its length. Veneer is peeled from the block in a continuous, uniformly thin sheet, much like unwinding a roll of paper towels, but at a speed of up to 4.1 m/s. Depending on its intended use, veneer may rage in thickness from 1.6 to 4.8 mm for softwood plywood and much thinner for hardwood and decorative plywood. After being peeled to a diameter from 127 to 51 mm, the peeler core is ejected from the lathe. Peeler cores may be sawn into standard 38 mm by 89 mm lumber, used for fence posts, and landscape timgers, or chipped into usable widths and defects are removed. The wet veneer is then dried to an avaerage moisture content that is compatible with the adhesive system being used to bond the panels. Since it is critical that veneer moisture content be low at the time adhesive is applied, each sheet is metered as it exits the dryer. Pieces that are too wet or dry are rerouted to be re-dried or reconditioned, repectively. Properly dried veneer is then sorted into one of as many as 15 to 20 different grades according to the size and number of knots and other natural and processing defects. Each grade has a specific use; some veneer requires special processing before it is assembled into plywood. After grading and/or processing, the veneer is taken to the lay-up area.
7 Plywood Adhesive is applied to veneers in the lay-up area by spray, curtain coating, roller coating, extrusion, and recently, foaming. Veneer is laid up into plywood by hand, machine, or a combination of both. Hand lay-up is the oldest method, and it is still the only practical way of making plywood for some applications. With this method, the face, back, and center veneers are hand-placed by workers called sheet turners. After being coated on both sides with adhesive, the alternating core plies are placed by hand or machine. The lay-up process is almost completely automated in newer plywood plants, although the narrow strips used for cores may still be placed manually. Because veneers are laid up, narrow strips are sometimes joined into full-width sheets with hot-melt adhesive coated fiberglass thread so that they can be handled by machine. Also, veneers may be upgraded by puncing out knots and other defects and replaceing them with wood plugs or synthetic patches. Once assembled, panels are conveyed from the lay-up area to the pressing area. Panels are first subjected to cold prepressing to flatten the veneers and transfer the adhesive to uncoated sheets; panels are then hot pressed. After hot pressing, panels are solid-piled or hot-stacked to ensure complete curing of the adhesive, then sawn to size. Panels are then graded with regard to the product standard under which they were manufactured. Knotholes and splits on the faces and backs of some panels may be repaired with wood plugs or with synthetic patches (by filling the holes and splits with what is essentially liquid plastic that quickly hardens). Those panels that do not meet the specification are downgraded or rejected. Panels needing further processing are sent to the finishing area where, depending on their intended use, they may be sanded to thickness, profiled with tongue and groove edges, surface textured, scarf- or finger-jointed, oiled and edge-sealed, or given other treatments. The panels are then ready for shipping.
Although plywood is an engineered wood product, it is also used as a component in other engineered wood products and systems in applications such as prefabricated I-joists, box beams, stressed-skin panels, and panelized roofs. Plywood has high strength-to-weight and strength-to-thickness ratios, and its stiffness and strength are more equal in width and length than are stiffness and strength of solid wood. Plywood also has excellent dimesional stability along its length and across its width. Minimal edge-swelling makes plywood perhaps the best choice for adhesive-bonded tongue-and groove joints, even where some wetting is expected. Because the alternating grain direction of its layers significantly reduces splitting, plywood is an excellent choice for uses that call for fasteners to be placed very near the edge of a panel. In uses where internal knotholes and voids may pose a problem, such as in small pieces, plywood can be ordered with a solid core and face veneers. Some plywood panels are designed for special uses, including marine decorative underlayment and concrete form and special exterior applications. The treating of plywood with preservatives and fire retardants is done by manufacturers outside of the plywood industry. Plywood is easily presure treated with water borne preservative alnd fire retardant, and treated plywood is readily available for use for use where such protection is needed.