Plywood is a type of engineered board made from thin sheets of wood, called plies or wood veneers. The layers are glued together, each with its grain at right angles to adjacent layers for greater strength. There are usually an odd number of plies, as the symmetry makes the board less prone to warping , and the grain on the outside surfaces runs in the same direction. The plies are bonded under heat and pressure with strong adhesives, usually phenol formaldehyde resin, making plywood a type of composite material. Plywood is sometimes called the original engineered wood.
The adhesives used in plywood has become a point of concern, due to the off gassing of the formaldehyde. Both urea formaldehyde and phenol formaldehyde are carcinogenic, so their use is undesirable. Many manufacturers are turning to “Greener Products” as government regulations become stronger against the use of these adhesives.
A common reason for using plywood instead of plain wood is its resistance to cracking, shrinkage, twisting/warping, and its general high degree of strength. In addition, plywood can be manufactured in sheets far wider than the trees from which it was made. It has replaced many dimensional lumbers on construction applications for these reasons.
A vast number of varieties of plywood exist for different applications. Softwood plywood is usually made either of Douglas fir or spruce, pine, and fir, and is typically used for construction and industrial purposes. Decorative plywood is usually faced with hardwood, including red oak, birch, maple, lauan (Philippine mahogany) and a large number of other hardwoods.
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 most common varieties of softwood plywood come in three, five or seven plies with a metric dimension of 1.2 m × 2.4 m or the slightly larger imperial dimension of 4 feet × 8 feet. Plies vary in thickness from 1/10″ through 1/6″ depending on the panel thickness. Roofing can use the thinner 5/8-inch plywood. Subfloors are at least 3/4-inch depending on the distance between floor joists. Plywood for flooring applications is often tongue and grooved. The mating edge will have a “groove” notched into it to fit with the adjacent “tongue” that protrudes from the next board. This keeps the boards from slipping past each other providing a solid feeling floor when the joints do not lie over joists. Tongue & groove flooring plywood is typically 1″ in thickness.
High-strength plywood, known as aircraft plywood, is made from mahogany and/or birch, and uses adhesives with increased resistance to heat and humidity. It was used for several World War II fighter aircraft, including the British-built Mosquito bomber which was nicknamed the wooden wonder.
Certain plywoods do not have alternating plies. These are designed for a specific purpose. One such plywood is known as “Bendy Board”. This is very flexible and is designed for making curved parts. However these may not be termed as plywood in some countries because the basic description of plywood is layers of veneered wood laid on top of each other with the grain perpendicular on each layer.
Other types of plywoods are fire retardant, moisture resistant, marine grade, sign grade, pressure treated, and of course the hardwood and softwood plywoods. Each of these products are designed to fill a need in industry.
In addition to the glues being brought to the forefront, the wood resources themselves are becoming the focus of manufacturers, due in part to energy conservation, as well as concern for our natural resources. There are several certifications available to manufacturers who participate in these programs. FSC certified, Leeds Certified, FSI certified, and Greenguard certified. Many of these programs offer tax benefits to both the manufacturer, as well as the end user.
Plywood production
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 peeled into sheets of veneer which are then cut to the desired dimensions, dried, patched, glued together and then baked in a press at 140 °C (280 °F) and 19 MPa (2800 psi) to form the plywood panel. The panel can then be patched, resized, sanded or otherwise refinished, depending on the market for which it is intended.
History
Plywood has been made for thousands of years; the earliest known occurrence of plywood was in Ancient Egypt around 3500 BC when wooden articles were made from sawn veneers glued together crosswise. This was originally done due to a shortage of fine wood. Thin sheets of high quality wood were glued over a substrate of lower quality wood for cosmetic effect, with incidental structural benefits. This manner of inventing plywood has occurred repeatedly throughout history. Most high quality English furniture makers working in the eighteenth and nineteenth centuries (and since) have used veneering as a technique. In addition to making the most out of the highest quality materials available, it reduces prices and improves stability of construction. The irregularities of grain which confer decorative interest often result in uncontrollable warping and cracking if any attempt is made to use the wood in thicknesses much greater than those characterizing cabinet-making veneers (typically 1-2mm).
Modern plywood, in which the veneer is cut on a rotary lathe from softwood logs, is of relatively recent origin, invented by Immanuel Nobel. The first such lathes were set up in the United States in the mid 19th century. Plywood has been one of the most ubiquitous building products for decades.
One of the earliest applications of mass-produced modern plywood manufacturing in the United States was recorded in Portland, Oregon by the Portland Manufacturing Company. The owner, Thomas J. Autzen helped develop a bonding technology, which greatly shortened the drying and manufacturing process. His early engineering contribution played an important role in making plywood one of the most abundant and affordable building products ever produced.
In India, waterproof plywood is also known as “kitply”. Though Kitply is a brand, it has become a genericized trademark, since the company that makes it pioneered the use of waterproof plywood in India.
The landscape historian John Stilgoe has theorized that the 4′ x 8′ dimensions of a standard sheet are due to the space required for moving a mule into a barn.
Plywood advantages
1. High uniform strength: Wood is 45 times stronger along the grain than across the grain. Crossing the adjacent sheets tends to equalise the strength in all directions.
2. Freedom from shrinking, swelling and warping: Solid wood exhibits considerable movement across the grain but generally negligible shrinkage or swelling in a longitudinal plane. The balanced construction of a plywood panel with the grain direction of adjacent veneers at right angles tends to equalise stress, thus reducing shrinkage, swelling and warping.
3. Non-splitting qualities: Solid wood splits fairly readily along the grain. Plywood by virtue of the crossed laminations can be nailed or screwed near the edges without damage from splitting.
4. Availability of relatively large sizes: Sawn timber can be obtained in fairly long lengths but only in relatively narrow widths. Plywood can be sold in sizes up to 6 ft * 25 ft and by the scarf jointing of small sheets up to 6 ft *40 ft, however 8 ft*4 ft is the most common size.
5. Economical and effective utilisation of figured wood: Twenty sheets of veneer can be sliced from 1 inch of solid wood. When glued to a core of cheaper material a high grade panel is produced. This procedure thus affects distinct economies in the use of figured or the more valuable woods. In addition to facilitating the utilisation of attractive but fragile face veneers to give results which cannot be duplicated in solid construction. More effective utilisation is obtained by the matching of veneer in such a manner that the decorative effect due to the natural figure in the wood is enhanced by the regularity or symmetry of the design.
6. Ease of fabrication of curved surfaces: The trend of modern architectural design is to feature curved surfaces. The desired shapes can be readily fabricated in plywood construction, utilising male and female forms, or a single forming a vacuum press or autoclave
7. Reduction of waste: One of the important aspects in the manufacture of plywood is that it results in the conservation of timber by the elimination of the waste which occurs in sawing e.g. sawdust. Waste is confined to the small core which remains after peeling, from the veneer which is lost in rounding up the log, and the elimination of such defects as knots and splits.
8. Dense woods can be sliced and bonded into plywood panels for use in furniture construction whereas furniture fabricated from solid timber would be far too heavy.
US plywood grades
Plywood grades are determined by a veneer quality on the face and back of each panel. The first letter designates quality of face veneer (best side), while the second letter denotes the surface quality of the back of the panel. The letter “X” indicates the panel was manufactured with scrap wood as the center plies, not “exterior” as is commonly thought. The A-D rating is only good for construction (softwood) plywood, not for hardwood plywoods such as oak or maple.
“A”: Highest grade quality available. Can be defect free or contain small knots, providing they are replaced with wooden plugs (the fillers having a “boat” or an “American football” shape) or repaired with synthetic patch. This grade may contain occasional surface splits that are repaired with synthetic filler. The surface is always sanded and provides for smooth paintable face quality.
“B”: Second highest quality veneer grade. Normally a by-product of downgraded “A” quality veneer. Solid surface, but may contain small diameter knots and narrow surface splits. Normally repaired with wooden plugs or synthetic filler. The surface is normally sanded smooth.
“C”: Considered to be a lower end face quality, but a reasonable choice for general construction purposes. May contain tight knots up to 1½ inches diameter, some open knot holes, some face splits, and discoloration. Some manufactures may repair the defects with synthetic filler. Panels are typically not sanded.
“D”: Considered to be the lowest quality veneer and often used for the back surface for construction grade panels. Allows for several knots, large and small, as well as open knots up to 2½ inches diameter. Open knots, splits, and discoloration are acceptable. “D” grade veneers are neither repaired nor sanded. This grade is not recommended for permanent exposure to weather elements. Plywood applications
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. 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 out of wood. Plywood is currently successfully used in stressed-skin applications. The American designers Charles and Ray Eames and Phil Bolger are famous for their plywood-based furniture
