Plywood and related products

Timber which is scientifically prepared in a factory is termed as Industrial timber. Such timbers or timber forms possess desired shape, appearance, strength, etc.

Veneers

These are thin sheets or slices of wood of superior quality. They are obtained by rotating a log of wood against a sharp knife of rotary cutter or saw. The edges of veneers are joined and sheets of decorative designs are prepared. This process is called as veneering. Usually veneers are used to produce plywood, batten boards and laminate boards. The main use of veneers is to improve the appearance of inferior woods. They are used to create an impression that the whole piece is made of expensive wood/timber. There are a few types of veneers available, each serving a particular purpose.

• Rawveneer has no backing on it and can be used with either side facing up. It is important to note that the two sides will appear different when a finish has been applied, due to the cell structure of the wood.
• Paper backedveneer is as the name suggests, veneers that are backed with paper. The advantage to this is it is available in large sizes, or sheets, as smaller pieces are joined together prior to adding the backing. This is helpful for users that do not wish to join smaller pieces of raw veneers together. This is also helpful when veneering curves and columns as the veneer is less likely to crack.
• Phenolic backedveneer is less common and is used for composite, or manmade wood veneers. Due to concern for the natural resource, this is becoming more popular. It too has the advantage of being available in sheets, and is also less likely to crack when being used on curves.
• Laid upveneer is raw veneer that has been joined together to make larger pieces. The process is time-consuming and requires great care, but is not difficult and requires no expensive tools or machinery. Veneers can be ordered through some companies already laid up to any size, shape or design.
• Reconstituted veneeris made from fast-growing tropical species. Raw veneer is cut from a log, and dyed if necessary. Once dyed, the sheets are laminated together to form a block. The block is then sliced so that the edges of the laminated veneer become the “grain” of the reconstituted veneer.
• Wood on WoodAlso called 2-ply is a decorative wood veneer face with a utility grade wood backer applied at an opposing direction to the face veneer.

Plywood

Plywoods are thin boards that are prepared from thin layers of wood or veneers. Plywood is a manufactured wood panel from the family of manufactured boards (such as medium-density fiber-board (MDF),particle board (chipboard), etc.) made from thin sheets of wood veneer. Plywood layers (called veneers or plies) are glued together, with adjacent plies having their wood grain rotated relative to adjacent layers up to 90 degrees.

All plywoods bind resin and wood fiber sheets (cellulose cells are long, strong and thin) 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 all 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 of the surface ply.

A typical plywood panel has face veneers of a higher grade than the core veneers. The principal function of the core layers is to increase the separation between the outer layers where the bending stresses are highest, thus increasing the panel’s resistance to bending. As a result, thicker panels can span greater distances under the same loads. In bending, the maximum stress occurs in the outermost layers, one in tension, the other in compression. Bending stress decreases from the maximum at the face layers to nearly zero at the central layer. Shear stress, by contrast, is higher in the center of the panel, and zero at the outer fibers.

Types of plywoods

• Softwood plywood – Softwoodpanel is usually made either of cedar, Douglas fir or spruce, pine, and fir (collectively known as spruce-pine-fir or SPF) or red wood and is typically used for construction and industrial purposes. The most common dimension is 1.2m × 2.4m or the slightly larger imperial dimension of 4 feet × 8 feet. Plies vary in thickness from 1.4 mm to 4.3 mm. The number of plies depends on the thickness and grade of the sheet but at least three are required as the minimum odd number of plies. Roofing can use the thinner 5/8″ (15 mm) plywood. Subfloors are at least 3/4″ (18 mm) thick, the thickness depending on the distance between floor  Plywood for flooring applications is often tongue and groove; This prevents one board from moving up or down relative to its neighbor, so providing a solid feeling floor when the joints do not lie over joists. T&G plywood is usually found in the 1/2″ to 1″ (12–25 mm) range
• Hardwood plywood – Hardwoodplywood is made out of wood from angiosperm trees and used for demanding end uses.
• Hardwood plywood is characterized by its excellent strength, stiffness and resistance to creep. It has a high planar shear strength and impact resistance, which make it especially suitable for heavy-duty floor and wall structures. Oriented plywood construction has a high wheel-carrying capacity.Hardwood plywood has excellent surface hardness, and damage- and wear-resistance.
• Tropical plywood – Tropicalplywood is made of mixed species of tropical wood. Originally from the Asian region, it is now also manufactured in African and South American countries. Tropical plywood is superior to softwood plywood due to its density, strength, evenness of layers, and high quality. It is usually sold at a premium in many markets if manufactured with high standards. Tropical plywood is widely used in the UK, Japan, United States, Taiwan, Korea, Dubai, and other countries worldwide. It is the preferred choice for construction purposes in many regions due to its low cost. However, many countries’ forests have been over-harvested, including the Philippines, Malaysia and Indonesia, largely due to the demand for plywood production and export.
• Aircraft plywood – High-strength plywood also 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 severalWorld War II fighter aircraft. Although the British-built Mosquito bomber, nicknamed “The Wooden Wonder”, was constructed of a plywood monocoque, this was formed in moulds from individual veneers of birch, balsa and birch, rather than machined from pre-laminated plywood sheets.
• Structural aircraft-grade plywood is more commonly manufactured from African mahogany or American birch veneers that are bonded together in a hot press over hardwood cores of basswood or poplar. Basswood is another type of aviation-grade plywood that is lighter and more flexible than mahogany and birch plywood but has slightly less structural strength.
• Decorative plywood (overlaid plywood) – Usually faced withhardwood, including ash, oak, red oak, birch, maple, mahogany, Philippine mahogany (often called lauan, luan or meranti and having no relation to true mahogany), rose wood, teak and a large number of other hardwoods. However, Formica, metal and resin-impregnated paper or fabric bonded are also added on top of plywood at both side as a kind of ready for use in the decoration field. This plywood is a lot easier to dye and draw on than any other plywoods.
• Flexible plywood – Flexible plywood is very flexible and is designed for making curved parts. In the UK this is sometimes known as “Hatters Ply” as it was used to makestovepipe hats in Victorian times . It is also often referred to as “Bendy Ply” due to its flexibility.
• However these may not be termed plywood in some countries because the basic description of plywood is layers of veneered wood laid on top of each other with the grain of each layer perpendicular to the grain of the next. In the U.S.A, the terms “Bender Board” and “Wiggle Board” are commonly used.
• Marine plywood – Marine plywood is manufactured from durable face and core veneers, with few defects so it performs longer in humid and wet conditions and resists delaminating and fungal attack. Its construction is such that it can be used in environments where it is exposed to moisture for long periods. More recently, tropical producers have become dominant in the marine plywood market.Okoumé from Gabon is now the accepted standard for marine plywood, even though the wood is not very resistant to rot and decay. Each wood veneer will be from tropical hardwoods, have negligible core gap, limiting the chance of trapping water in the plywood and hence providing a solid and stable glue bond. It uses an exterior Water and Boil Proof (WBP) glue similar to most exterior plywoods.
• Other plywoods – Other types of plywoods include fire-retardant, moisture-resistant, wire mesh, sign-grade and pressure-treated. However, the plywood may be treated with various chemicals to improve the plywood’s fireproofing. Each of these products is designed to fill a need in industry.

Fiberboards

Fiberboard is a type of engineered wood product that is made out of wood fibers. Types of fiberboard (in order of increasing density) include particle board, medium-density fiberboard, and hardboard. Fiberboard is sometimes used as a synonym for particle board, but particle board usually refers to low-density fiberboard. Plywood is not a type of fiberboard, as it is made of thin sheets of wood, not wood fibers or particles. Fiberboard, particularly medium-density fiberboard (MDF), is heavily used in the furniture industry.

For pieces that will be visible, a veneer of wood is often glued onto fiberboard to give it the appearance of conventional wood.

Fiberboard is also used in the auto industry to create free-form shapes such as dashboards, rear parcel shelves, and inner door shells. These pieces are usually covered with a skin, foil, or fabric such as cloth, suede, leather, or polyvinyl chloride.

Urea-formaldehyde (UF) resins are dominantly used in the medium density fiberboard (MDF) industry because of their low cost and fast curing characteristics. However, pressures on the use of UF resins are mounting steadily due to potential problems associated with formaldehyde emission. On the other hand, phenol-formaldehyde (PF) resins are more durable and do not emit formaldehyde after cure. But the Industry has traditionally shied away from using PF resins due primarily to their higher cost and much slower curing rate than UF resins. However, the press times for PF-bonded fiberboard can be substantially reduced by manipulating the fiber mat temperatures, molecular weight distribution of PF resins and pressing parameters. As a result, the press times for PF-bonded fiberboard can be made comparable to those for UF-bonded fiberboard. Also, the resin content required for PF-bonded fiberboard is less than 5% to achieve a good board quickly. This is considerably lower than that required for UF-bonded fiberboard. Certain types of fiberboard can be considered “green” building products. Consisting of bio-based, secondary raw materials (wood chip or sugarcane fibers) recovered from within 100 miles (160 km) of manufacturing facilities, the binding agent used in this type of fiberboard is an all-natural product, consisting of vegetable starch containing no added formaldehydes.

Fiberboard, classified by ASTM C208, Standard Specification for Cellulosic Fiber Insulating Board, has many benefits and is used in residential and commercial construction. Different uses and applications include

• sound proofing/deadening,
• structural sheathing,
• low-slope roofing,
• sound deadening flooring underlayment,

Hardboards

Hardboard (not to be confused with hardwood), also called high-density fiberboard (HDF), is a type of fiberboard, which is an engineered wood product.  This product is also known under Isorel or Masonite brand names.

It is similar to particle board and medium-density fiberboard, but is denser and much stronger and harder because it is made out of exploded wood fibers that have been highly compressed. Consequently, the density of hardboard is 31 lbs or more per cubic foot (500 kg/m³) and is usually about 50-65 lbs per cubic foot (800–1040 kg/m³). It differs from particle board in that the bonding of the wood fibers requires no additional materials, although resin is often added. Unlike particle board, it will not split or crack.

It is used in construction and furniture. Hardboard is produced in either a wet or dry process. The wet process, known as the Mason Method, leaves only one smooth side while the dry processed hardboard is smooth on both sides. Masonite is produced using the wet process only.

Impreg Timbers

Timbers that are fully or partly covered in resin are known as impreg timbers. Phenol formaldehyde soluble in water is usually the resin employed in its manufacture. The resin fills the spaces between wood cells and by means of a chemical reaction a consolidated mass is developed and then later cured at a temperature of about 150^oC to 160^oC.

It is strong and durable, provides electrical insulation, resists atmospheric and acidic effects and has less expansion tendencies to temperature variances as compared to timber. It is available under various names such as Formica, sungloss, sunmica, etc. and it is used for moulds, furniture, decorative articles, etc.

Compreg Timbers

Similar to impreg timbers except that their curing is carried out under pressure application.

Block boards and Lamin boards

Block boards are boards having a core made up of strips of wood in which the edges are glued together to form a solid sheet, which is then finished with one or two cross-bonded veneers on each face.