|Infobox on Plywood|
|Example of Plywood|
|Stowage factor (in m3/t)||2,41 m3/t (crates)|
|Humidity / moisture||Approx. 9%|
|Risk factors||See text|
Plywood is a manufactured wood panel made from thin sheets of wood veneer. It is one of the most widely used wood products. It is flexible, inexpensive, workable, and re-usable, and usually can be manufactured locally. Plywood is used instead of plain wood because of plywood's resistance to cracking, shrinkage, splitting, and twisting/warping, and because of its generally high strength.
Plywood layers (called veneers) are glued together, with adjacent plies having their wood grain at right angles to each other, to form a composite material. This alternation of the grain is called cross-graining and has several important benefits: it reduces the tendency of wood to split when nailed at the edges; it reduces expansion and shrinkage, providing improved dimensional stability; and it makes the strength of the panel consistent across both directions. There is usually an odd number of plies, so that the sheet is balanced—this reduces warping. Because plywood is bonded with grains running against one another and with an odd number of composite parts, it is very hard to bend it perpendicular to the grain direction.
Plywood production requires a good log, called a peeler, which is generally straighter and larger in diameter than one required for processing into dimensioned lumber by a sawmill. The log is laid horizontally and rotated about its long axis while a long blade is pressed into it, causing a thin layer of wood to peel off (much as a continuous sheet of paper from a roll). An adjustable nosebar, which may be solid or a roller, is pressed against the log during rotation, to create a "gap" for veneer to pass through between the knife and the nosebar. The nosebar partly compresses the wood as it is peeled; it controls vibration of the peeling knife; and assists in keeping the veneer being peeled to an accurate thickness. In this way the log is peeled into sheets of veneer, which are then cut to the desired oversize dimensions, to allow it to shrink (depending on wood species) when dried. The sheets are then patched, graded, glued together and then baked in a press at a temperature of at least 140 °C, and at a pressure of up to 1.9 MPa (280 psi) (but more commonly 200 psi) to form the plywood panel. The panel can then be patched, have minor surface defects such as splits or small knot holes filled, re-sized, sanded or otherwise refinished, depending on the market for which it is intended.
Plywood may be made from hardwoods, softwoods, or a combination of the two. Some common hardwoods include ash, maple, mahogany, oak, and teak. The most common softwood used to make plywood in the United States is Douglas fir, although several varieties of pine, cedar, spruce, and redwood are also used.
Plywood for indoor use generally uses the less expensive urea-formaldehyde glue, which has limited water resistance, while outdoor and marine-grade plywood are designed to withstand rot, and use a water resistant phenol-formaldehyde glue to prevent delamination and to retain strength in high humidity.
The adhesives used in plywood have become a point of concern. Both urea formaldehyde and phenol formaldehyde are carcinogenic in very high concentrations. As a result, many manufacturers are turning to low formaldehyde-emitting glue systems, denoted by an "E" rating ("E0" possessing the lowest formaldehyde emissions). Plywood produced to "E0" has effectively zero formaldehyde emissions.
Just as with lumber, there is no such thing as a perfect piece of plywood. All pieces of plywood have a certain amount of defects. The number and location of these defects determines the plywood grade. Standards for construction and industrial plywoods are defined by Product Standard PS1 prepared by the National Bureau of Standards and the American Plywood Association. Standards for hardwood and decorative plywoods are defined by ANSIIHPMA HP prepared by the American National Standards Institute and the Hardwood Plywood Manufacturers' Association. These standards not only establish the grading systems for plywood, but also specify construction, performance, and application criteria.
The outer layers of plywood are known respectively as the face and the back. The face is the surface that is to be used or seen, while the back remains unused or hidden. The center layer is known as the core. In plywoods with five or more plies, the inter-mediate layers are known as the crossbands.
There are two broad classes of plywood, each with its own grading system. One class is known as construction and industrial. Plywoods in this class are used primarily for their strength and are rated by their exposure capability and the grade of veneer used on the face and back. Exposure capability may be interior or exterior, depending on the type of glue. Veneer grades may be N, A, B, C, or D. N grade has very few surface defects, while D grade may have numerous knots and splits. For example, plywood used for subflooring in a house is rated "Interior C-D". This means it has a C face with a D back, and the glue is suitable for use in protected locations. The inner plies of all construction and industrial plywood are made from grade C or D veneer, no matter what the rating.
The other class of plywood is known as hardwood and decorative. Plywoods in this class are used primarily for their appearance and are graded in descending order of resistance to moisture as Technical (Exterior), Type I (Exterior), Type II (Interior), and Type III (Interior). Their face veneers are virtually free of defects.
Plywood sheets range in thickness from. 1.6 mm 76 mm. The most common thicknesses are in the 6.4 mm to19.0 mm range. Although the core, the crossbands, and the face and back of a sheet of plywood may be made of different thickness veneers, the thickness of each must balance around the center. For example, the face and back must be of equal thickness. Likewise the top and bottom crossbands must be equal.
The most common size for plywood sheets used in building construction is 4 ft (1.2 m) wide by 8 ft (2.4 m) long. Other common widths are 3 ft (0.9 m) and 5 ft (1.5 m). Lengths vary from 8 ft (2.4 m) to 12 ft (3.6 m) in 1 ft (0.3 m) increments. Special applications like boat building may require larger sheets.
Plywood is used in many applications that need high-quality, high-strength sheet material. Quality in this context means resistance to cracking, breaking, shrinkage, twisting and warping.
Exterior glued plywood is suitable for outdoor use, but because moisture affects the strength of wood, optimal performance is achieved in end uses where the wood's moisture content remains relatively low. On the other hand, subzero conditions don't affect plywood's dimensional or strength properties, which makes some special applications possible.
Plywood is also used as an engineering material for stressed-skin applications. It has been used for marine and aviation applications since WWII. Most notable is the British de Havilland Mosquito bomber, which was primarily made using a moulded sandwich of two layers of birch plywood around a balsa core. Plywood is currently successfully used in stressed-skin applications.
Plywood is often used to create curved surfaces because it can easily bend with the grain. Skateboard ramps often utilize plywood as the top smooth surface over bent curves to create transition that can simulate the shapes of ocean waves.
Shipment / Storage / Risk factors
Plywood is a commodity which is transported in large quantities in seaborne trades throughout the world. It is also a commodity which is highly susceptible to damage and is often insufficiently prepared for shipment.
The manufacture of plywood has been described as the ‘unrolling of logs of wood’. Very long thin sheets are shaved from the log which, after being cut to size, are glued together to form various thicknesses of plys of wood. Thicknesses vary from around 4 mm to 25 mm and the sheets vary in size, the most popular being 96”x 48”. Moisture content of this manufactured product has been found to be about nine percent.
The method of transporting plywood is to stack the sheets into bundles of about fifty sheets or more, depending on the thickness of the plywood, which are secured together with metal strapping bands. Access for ease of handling by fork-lift trucks is usually achieved by attaching wooden bearers to the individual flat metal securing bands across the width of the base of each bundle. It is not unusual for plywood to be transported in a completely unprotected condition. In some trades the plywood is partly packed, and, on rare occasions, it is completely packed and well protected.
Very often when packing is used, it is deficient, failing adequately to protect those areas which are vulnerable to handling damages, such as on the corners of the bundles. One of the most common forms of packing is an arrangement where the stack of plywood is placed upon a wooden frame after the plywood has been enfolded in a plastic sheet. The sides, ends and top are then covered with plywood sheets and then strapped up with flat metal strapping bands. If done properly and with care, this packing can adequately protect the plywood from normal handling and stowage problems. Often, this packing is applied without sufficient care. Any deficiency or tear in the plastic sheeting can allow moisture penetration into the bundle of plywood. This moisture will seep between the plywood sheets causing staining.
The strapping bands are sometimes of inadequate strength and the method of joining them is often unsatisfactory. This results in a lack of rigidity of the bundle causing the plywood sheets to become misaligned during handling in the worst cases the bundle becomes loose with the damage to the edges of the plywood being considerable. If the plywood, end and top packing is too short, corner damage can occur.
It can be seen from the above that it is of the utmost importance that bundles of plywood should be examined by the ship’s staff before loading (break bulk), paying particular attention to the packing of the plywood, if any. Deficiencies in packing should be noted and suitable remarks inserted on the Mate’s receipts and bills of lading. Careful attention should be paid to stowage, to prevent corner damage both during the stowage and in the securing of the stow. The stow should be properly secured to prevent movement of the bundles of plywood during the voyage. Proper ventilation should be carried out during the voyage to minimise any possible staining from condensation. If possible, stowage should be away from the hatch square to prevent the possibility of moisture dripping down on the plywood externally, if the plywood is totally unprotected.
Part of the above article was produced by the Carefully to Carry Committee – the UK P&I Club’s advisory committee on cargo matters.