Plywood is an extremely versatile product, used for a wide range of structural, interior and exterior applications. It is an economical, manufactured sheet of wood with precise dimensions and durability. This article will address all the aspects of plywood, exploring its structural characteristics, its types, the history, applications, production process, sizes, and grades. Most of your woodworking projects need plywood in large quantity. To study the types of plywood according to their application is the must for all the woodworkers.
What is Plywood?
Plywood is a composite material, composed of individual plies and veneers of wood. Wood veneers are assembled and bonded together to produce a flat sheet of wood. The sheet materials are glued together with adjacent layers of wood grain rotated to ninety degrees of each other.
In other terms, it’s a designed wood from a family of manufactured boards consisting of medium density fiberboards and chipboard (particle board). The fiber sheets are composed of cellulose cells, long, strong and thin which are wood fiber sheets and are bound by resin. The modification of the grain is known as cross-graining, possessing several benefits.
For instance, it reduces the plywood’s tendency to split when nails are hammered at the end, decreases expansion or shrinkage and makes strength to be consistent across the panel. There are often several plies which make the sheet to be balanced, and this decreases warping.
Structural Characteristics of Plywood
Engineered wood products such as plywood have a defined and standardized structural characteristics suited for your woodworking projects. Plywood’s versatility has ensured its continuous important role in construction, woodcraft and furniture making.
Plywood is manufactured from hardwood, softwood trees or both. Some types of hardwoods include ash, maple, mahogany or oak. The most popular softwood used in making plywood is Douglas fir, in addition to pine, cedar, and redwood.
Composite plywood can also be designed and engineered with a core of solid timber pieces or particleboards containing wood veneer for the face and back. Composite plywood is most preferred when thick sheets are needed.
Among some of the properties of plywood is high strength, high panel shear, flexibility, moisture resistance, impact resistance and chemical resistance. Plywood contains a structural strength of the wood material it is manufactured from.
This strength is also obtained from the plywood’s laminated design. As stated earlier, the grains of each veneer are laid and rotated against each other at ninety degrees angles. This aspect makes the whole sheet resistant from splitting or breaking when nailed at the edges. The design also gives the sheet uniform strength and more stability.
Also, plywood has more strength to weight ratio than cut lumber, hence making it ideal for webbed beams, flowing and shear walls. Plywood is nearly as twice resistant to shearing than regular wood.
How is Plywood made?
Plywood is manufactured with an odd number of layers, which make it tough to bend. The angles in the veneer grains set against each other may vary from ninety degrees. A veneer can be laid at thirty to forty degrees against the other, a feature that increases the plywood’s strength in all directions.
The cross lamination provided by this design increases the panel shear which is significant in fabricated beams and bracing panels. Flexibility is another distinct property of plywood. Unlike real wood or cut timber, plywood can be manufactured to fit every requirement.
A veneer’s thickness can vary from a few millimeters to several inches, where the veneer’s numbers can increase from three onwards, increasing a sheet’s thickness. The extra layers of veneer add more strength on the plywood. Few layers create a thin veneer which is more flexible and used in paneling and ceilings.
Plywood is moisture resistant. The type of adhesive used in binding the adhesives contributes to the resistance of humidity and moisture. Most plywood also contains a layer of paint or varnish, which increases water damage resistance.
This makes plywood suitable for exterior use such as marine construction, cladding, and sheds. The veneers are also fit for holding concrete while it sets. Moisture and humidity resistance is significant in interior applications and floors. Cross-lamination of the veneers ensures that the plywood does not warp, shrink or expand due to water and extreme temperatures exposure.
Plywood is also chemical, impact, and fire resistant. The veneers are treated with a preservative that does not corrode with exposure to chemicals, hence making plywood suitable for cooling towers and chemical works. Plywood’s impact resistance is an attribute of its high tensile strength which is derived from the panels’ cross lamination.
A cross-lamination distributes force over a large area thus reducing tensile stress. Plywood can, therefore, overload by more than twice its designated load, a critical aspect that protects the wood in short-term seismic activity or strong winds. The impact resistance is also useful since plywood can be used in flooring and concrete formwork.
Plywood is treated with fire resisting chemical coating and combined with non-combustible materials like fibrous cement and plasterboard to make it fire resistant. This makes it highly suitable and ideal for construction of fire resistant structures. With an additional high thermal and sound insulation, plywood offers insulating properties required on floors, ceilings, roofs and wall cladding.
This greatly reduces heating and cooling costs. On the cost aspect, plywood can be manufactured from wood chips, shavings, and tree parts unsuitable for the milling of blocks, beams, and boards, hence making plywood relatively cheaper than its equivalent real wood.
History of Plywood
Plywood has its origins in the lamination of veneers 3,500 years ago in Egypt during Pharaohs time when wooden articles were made from sawn veneers and glued together in a crosswise design.
The early Romans and Greeks also used plywood and veneers on furniture. In the mid-1800 the modern plywood was used in constructing pianos, furniture, and tea chests. The early plywood was originally utilized due to the shortage of fine wood, and therefore thin sheets of high-quality wood had to be glued over a substrate of low-quality wood for a cosmetic effect and additional incidental structural benefits.
The method of plywood invention has repeatedly occurred throughout the course of history. Veneering as a technique was used by high-quality English furniture makers in the eighteenth and nineteenth centuries. Furniture prices were reduced due to this technique, in addition to utilizing the available high-quality materials and improved construction stability.
Modern plywood was invented by Immanuel Nobel in the mid-19th century in the United States. The modern plywood was characterized by veneer cut on the rotary lathe from softwood logs.
The earliest applications of mass-produced modern plywood were recorded in Portland, Oregon in 1907, when the Portland Manufacturing Company owned by Thomas Autzen developed a bonding technology that reduced the drying and manufacturing process of plywood. John Stilgoe theorized the 4’*8’ dimension of a standard sheet of plywood, stating that it was due to space required a mule to move into a barn.
Types of Plywood
The different varieties of plywood are manufactured for different applications. Among the different types of plywood include softwood plywood, hardwood plywood, tropical plywood, aircraft plywood, decorative plywood, flexible plywood and marine plywood.
Softwood plywood is manufactured from Douglas fir, spruce wood, pine, fir lumber, cedar or redwood. Softwood Plywood is easy to work with when conventional working tools are used. Softwood plywood has a light color, giving a plywood effect that can either be painted or varnished as it seems fit.
Softwood is highly resistant to humidity and moisture, making it ideal for projects where the environment is an issue of concern. The key features of softwood plywood include; lightweight and dimensionally stable, environmentally friendly, ability to withstand impact and other bruising forms, strong and rigid, available with square edges and tongue and groove profiles and can act as a load-bearing construction and at the same time as a stiffening element. These softwood plywoods are best suited for beginner woodworking projects.
Softwood is used as a sub-floor in the majority of residential homes. It is also used for concrete and building forms and hence it is manufactured with a special one-inch thickness. Softwood is mass-produced with a standard dimension of 48-by-96-inch sheets and available in 0.75 0.5 and 0.25-inch thickness.
The typical applications of softwood plywood are; construction of home floors, roofs and walls, wind bracing panels, fencing, packages and boxes and vehicle internal bodywork. The prominent grain structure of softwood such as spruce plywood is eliminated using coating solutions that mask it. The coated softwood plywood are used in concrete shuttering panels and ready to paint surfaces in construction.
Hardwood plywood is made from angiosperm trees wood and usually used for demanding end uses. Hardwood plywood has excellent strength, resistance to creep and stiffness.
It also has a high planar shear strength and resistance to heavy impact, which makes it ideally suitable for heavy duty wall and floor structures. Hardwood plywood has a distinct face and back. For instance, it is used for cabinet doors which have to possess distinct appearances on the back and the face.
In some cases, the core material is separated from the face, where for instance one has a maple plywood with a veneer core. Hardwood plywood is made up of three more plies of hardwood such as red oak or mahogany, laid on top of each other. The grain of each ply or panel runs perpendicularly on the side of it. Hardwood is more widely used than softwood, with a wider range of furniture such as desks, chairs, doors, handrails, etc.
Most hardwood plywood has a phenolic resin film coated component that renders it ready to install. For instance, they are used for making panels of concrete formwork systems, container floors, heavy wear subjected floors such as in factories, floors, and also walls and roofs of transport vehicles.
Birch plywood is used for special applications as a structural material such as wind turbine blades and liquefied natural gases insulation boxes. A combination of smooth surfaces, accurate thickness, and durability make birch plywood suitable for end uses like high-end loudspeakers, woodworking plan, furniture, musical instruments, sport and playground equipment, and die-cutting boards.
First developed in the Eastern hemisphere, tropical plywood is made up of tropical wood from the Asian region. Tropical plywood is characterized by superiority over softwood plywood regarding its density, strength, layers and high quality.
Tropical plywood is sold at premium prices in the market due to its high standards of quality, increasing demand, and diminishing supply. Tropical plywood is mainly used in Britain, Japan, the United States, Korea, Dubai, and Taiwan among other countries. It is a preferred choice among constructors in many regions due to its superior quality and low costs.
Regarding usefulness, it lies in between softwood plywood and hardwood plywood. Tropical plywood’s main applications are making common plywood, woodworking projects, concrete panels, floor bases, structure panels, container floors, and laminated veneer lumber.
Aircraft plywood is manufactured from ultra-thin birch veneers, following the standards of Germanischer Lloyd (GL). Aircraft plywood is characterized by lightness, extreme strength and flexibility, making it easy for constructors to work with. It can be painted, stained and finished like real wood.
The veneer face and back of the aircraft plywood does not have any open defects. The standard size for this plywood is 1500*1500 with a varying thickness of (0.4, 0.6, 0.8, 1.0, 1.2, 1.5, and 2.0 mm)
A traveling boat is subject to plate action from water waves and floating debris. Marine plywood is designed in a way these forces are absorbed. For these and other similar reasons, marine plywood is the closest to real solid wood.
It is better than solid wood regarding quality and grading out of defects. Marine plywood is made using veneers of the highest quality grade. The veneers are free of defects, eliminating any core gaps produced from cutting the sheet. The high-grade veneers are used in every layer with durable faces and use an exterior Water and Boil Proof (WBF) glue that increases its ability to perform longer in humid and wet conditions and resist delamination and fungal attacks. Marine plywood is compliant with BS 1088 (British’s Standards for marine plywood).
Flexible plywood is commonly known as bendy plywood or ‘flexiply.’ It is extensively used in shop fittings and furniture where curves are desirable. Plywood with flexible characteristics require specially treated veneers. The plywood is characterized by thinner inner core veneer sandwiched between two thicker ‘tenderized’ veneers. If you are handling any higher cost woodworking projects then you can use such flexible plywoods for better workability.
This special design of these particular veneers offers a large degree of flexibility and a cost effective method of constructing rounded columns and curved structures with small radii without the necessity of water or heat-forming. Flexible plywood is available in both long grain (column wraps) and cross grain (barrel wrap) alignments. A wide variety of finished thicknesses can be made by bonding more than one sheet of flexible plywood together.
Production of Plywood
The manufacturing of plywood has seven process steps which include log preparation, peeling, clipping, drying, cross-bands, and grading. During log preparation, logs are first debarked after they are delivered from the plantation/forest.
A machine scrapes the bark from the logs. Before peeling, the logs are conditioned by water sprays, immersing in cold or heated or a steam treatment used. Conditioning ensures that the logs are at high and consistent moisturized content which facilitates the yielding of smooth veneer which will have less tendency to split or tear, hence higher yields and quality.
During the peeling process, the logs are loaded and centered on the peeler block in the spindles of veneer lathe. The peeler block is centered with the log’s axis so as to obtain an optimum veneer recovery.
This is done by a manual or ‘x-y charging’ system that uses a laser scanner to measure the block in three dimensions and calculate the largest perfect cylinder in the block. After a lathe is located, it effectively rotates the block against a blade, producing a continuous veneer ribbon with standard thickness.
The veneer’s ribbon passes from the lathe through clipping machines that cut the veneer into size or smaller strips to remove the defective material. After clipping, the wet veneer is fed through a drier to reduce the moisture content to 8%.
The purpose of drying is to find the optimum moisture content required for gluing, which also depends on the species of the trees, the density of the veneer, the type of adhesive and gluing procedure to be utilized. In a mechanical drier, the veneer is passed through a long chamber of hot air. Small strips of veneer are then joined into full-size sheets through gluing, stitching or perforated tape. Cross-bands are the core veneers running across the panels at right angles.
They are produced by smaller lathe or cutting full veneer sheets into two. The dried, clipped and joined veneer are then graded in preparation for use in plywood manufacturing.
Plywood fabrication involved layup, glue mixing, glue spreading, repressing, hot pressing, trimming, filling, sanding and quality assurance. The adhesives used for manufacturing plywood are of synthetic resin, all on a thermo-setting.
The adhesives have a defined bond tests series categorized by their durability. The A bond includes Phenol, Resorcinol, and Tannin, which have fully weather resistant properties. B bond is relatively weaker and partially weather resistant.
Sizes and Grades
The most common plywood thickness ranges from 1/8 to 3.0 inches. The standard size regarding width and length is four by 8 feet. Specialized plywood for forming concrete can range from 15/64 to 13/16 inches with other multiple formats existing. Aircraft plywood, on the other hand, is usually available at 1/8 inch thickness and upwards.
Grading of plywood varies with the country of origin. The most popular quality standards are the British Standard and American Standard (ASTM). Grade A describes a face and back veneer practically free from all defects. Grade A/B means that the face veneers are free from all defects.
However, the reverse veneers contain small few knots and discolorations. Grade A/BB means the face veneer is free from all defects with jointed veneers, large knots, and plugs on the reverse side. Grade B describes a plywood with both sides of the veneers containing a few small knots and discolorations. The other subsequent grades include BB, C/D, WG, X, and WBP.
Conclusion on Plywood Study
One thing is very clear from above article is plywood is manufactured with different manufacturing processes and too with different reinforcements. This opens the wide variety of applications of plywood and can be used for any of your woodworking project. If you have any questions regarding this article comment below.