Plywood – An Engineered Sheet Material

 

Plywood

Plywood is a versatile building material made up of layers of timber veneer glued together in the form of sheets. The idea of plywood originated in 1797 by Mr. Samuel Bentham, a British naval engineer but, its Industrial potential was understood and machines were made for its manufacture only after 50yrs since its invention. In 1865, it was introduced in the US but later in 1928 it was used as a general building material.

Typically plywood has veneers glued with adjacent layers having their wood grains rotated up to 90 degrees onto other. It is commonly referred to as engineered wood. There are different varieties of plywood for different applications such as softwood, hardwood, tropical, aircraft, decorative, flexible, marine, etc;

Most commonly used plywood sheet size is 4 by 8 feet and the thickness range varies from 0.14 to 3 inches ( 4 mm to 25 mm ). Plywood is rated as per the grade system and quality based on resistance to cracking, breaking, shrinkage, twisting and warping. It is used in applications that need high-quality, high-strength sheet material.

Plywood is not an all- purpose material but a major component in almost all building applications. Mostly preferred because it is cheap, strong, does not warp or shrink, does not split easily, smooth uniform finish, etc;

 

Plywood or kryssfaner is manufactured by the following step by step process :

  • Cutting the logs – The large wooden logs, normally stored under water are cut into pieces according to the production requirement.
  • Processing the logs – The pieces of the logs are then heated at around 70-90 degree C temperature for 12-14 hours either under hot water or in a steam chamber.
  • Debarking – The bark/thick skin around the log is removed either by hand tools or by using a debarking machine.
  • Veneer making – Veneers are the few millimeters thick layers of wood. The veneers can be made either by peeling or by slicing. The log is to be loaded to a peeling lathe or a slicer machine for making veneers out of it.
  • Wet clipping – The veneer then cut into pieces of required sizes by using the clipping machine. The clipping machine works like a big scissor.
  • Drying – The veneers are then dried partially by sunlight and partially by a dryer machine. Maintaining required amount of moisture in veneers is very important for producing the good quality plywood.

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  • Dry clipping – After drying again the veneers are clipped using the clipping machine. This step is required to cut the veneers in close tolerance and to remove the drying defects.
  • Resin preparation – Either liquid or the powder resins are used for making glue. Urea formaldehyde (UF) and Phenyl formaldehyde (PF) are the two most common types of resins used in plywood manufacturing. In case of liquid resin, the resin is manufactured normally in house by using a resin kettle or plant.
  • Gluing and assembly – The correct sized veneers are then glued normally by using a gluing machine. The veneers are passed in between two rollers of the gluing machine. The rollers of the gluing machine are kept wetted by the continuous supply of gum. The odd numbers of veneer layers are assembled such a way that the grain directions of the consecutive layers are perpendicular to each other.
  • Hot pressing – The assembled veneer layers are transferred inside the hot hydraulic press which apply specific pressure and temperature for specified duration of time to the veneers assembly and convert it to plywood by curing the resins.
  • Trimming – The circular power saw is used for trimming the plywood boards to give it the required size.
  • Inspection, labeling and dispatching – Inspection is performed to find out the rejected and reparable pieces among the good pieces. The reparable pieces are repaired using putty and color pastes manually. Finally labeling and dispatching is done.

Classification of plywood based on Indian Standards specification number:

For those who want to be assured of quality, the best plywood brands to choose from, will be those that have an legitimate ISI mark, which is a symbol of trust for quality. IS specification numbers are also printed on the boards just above the ISI mark. This is further divided into,

(1) MR Grade- Stands for Moisture Resistant – MR Grade Ply is basically regular plywood. This is also sometimes called Commercial Ply. In layman’s terms this can be used everywhere except in the Bathroom & Kitchen.

(2) BWR Grade- is Boiling Water Resistant Plywood. Some companies call it BWP (Boiling Water Proof) Grade however the Bureau of Indian Standards has officially done away with the BWP terminology. In layman’s terms this wood is also mentioned as Marine Ply & is for use in the Kitchen & Bathroom.

IS: 303 – MR Plywood, BWP/BWR Plywood, Flexi Ply

IS: 710 – Marine Plywood

IS: 10701 – Structural plywood

IS: 5509-1980 – Fire Retardant Plywood

IS: 4990 – Shuttering Plywood

In India, Particle board & MDF are at a stage of nascence where they are bought based on the Brand Name rather than on the ISI Standard. Some manufacturers do cite exterior Grade & Interior Grade MDF/ Particle Board.

Reading the plywood stamp :

When shopping for construction plywood, look for a grading stamp that says “APA The Engineered Wood Association.” This stamp’s meaning varies according to the intended use of the panel. For example, if the plywood is graded for sheathing outside walls, it will include the stud spacing on which it should be attached. For interior remodeling, the appearance of the face veneers is most important. Here are the characteristics of each face grade:

A — A smooth, paint-able veneer free of knots and possessing only neatly made repairs that are parallel to the grain. It can be finished with a clear coat (rather than paint).

B — A solid-surface veneer that allows only small round knots, patches, and round repairs. Acceptable for the inside surfaces of a painted shelving unit.

C — Allows small knots, knot holes, and patches. Probably not used for an exposed face inside the house unless you are going for a rustic look. The lowest grade allowed for permanent exterior exposure.

D — This veneer can include large knots and knotholes. Acceptable only for a hidden face, such as the surface of a sheathing panel that faces inside the wall.

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Different varieties of plywood exist for different applications :

Softwood plywood:-

Softwood panel is usually made either of cedar, Douglas fir or spruce, pine, and fir (collectively known as spruce-pine-fir or SPF) or redwood and is typically used for construction and industrial purposes.

The most common dimension is 1.2 by 2.4 mts (3 ft 11 in × 7 ft 10 in) 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. Sub-floors are at least 3/4″ (18 mm) thick, the thickness depending on the distance between floor joists. 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 :-

Hardwood plywood is made out of wood from angiosperm trees and used for demanding end uses. It 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 :-

Tropical plywood 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. It is the preferred choice for construction purposes in many regions due to its low cost.

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. This was formed in molds 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 or from European Birch veneers throughout. 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. Aviation-grade plywood is manufactured to a number of specifications including those outlined since 1931 in the Germanischer Lloyd Rules for Surveying and Testing of Plywood for Aircraft and MIL-P-607, the latter of which calls for shear testing after immersion in boiling water for three hours to verify the adhesive qualities between the plies and meets specifications.

Decorative plywood (overlaid plywood) :-

Usually faced with hardwood, including ash, oak, red oak, birch, maple, mahogany, Philippine mahogany (often called lauan, lauan or meranti and having no relation to true mahogany), rosewood, teak and a large number of other hardwoods.

Flexible plywood :-

Flexible plywood is designed for making curved parts, a practice which dates back to the 1850’s in furniture making.

Aircraft grade plywood, often Baltic birch, is made from 3 plies of birch, as thin as 1/16″ thick in total, and is extremely strong and light. At 3/8″ thick, mahogany 3-ply “wiggle Board” or “Bendy Board” come in 4′ x 8′ sheets with a very thin cross grain central ply and two thicker exterior plies, either long grain on the sheet, or cross grain. Wiggle board is often glued together in two layers once it is formed into the desired curve, so that the final shape will be stiff and resist movement. Often, decorative wood veneers are added as a surface layer.

Marine plywood :-

Marine plywood is manufactured from durable face and core veneers, with few defects so it performs longer in both 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. Okoume 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 plywood.

Marine plywood can be graded as being compliant with BS 1088, which is a British Standard for marine plywood. There are few international standards for grading marine plywood and most of the standards are voluntary. Some marine plywood has a Lloyd’s of London stamp that certifies it to be BS 1088 compliant. Some plywood is also labeled based on the wood used to manufacture it.

Marine plywood is frequently used in the construction of docks and boats. It is much more expensive than standard plywood, which is two to three times as expensive as standard plywood, depending on grade.

Apart from these there are specific demand for the following articles from the consumers :-

Shuttering Plywood, Packaging Plywood, Plywood drums, Flush doors, Commercial & Decorative Plywood, Block Boards, Molded Plywood furniture and chair seats, Laminated picking sticks for cotton and jute textile industry, Sliced decorative veneers of walnut, teak, rosewood etc, Impregnated wood veneers required for heavy chemical industry and electrified railways.

  • Particle board is made of coarser wood particles. It is commonly used as floor underlayment and occasionally for inexpensive cabinets (often with a vinyl wood-print veneer). It also is frequently used as the bottom layer for counter-tops.

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  • OSB, another engineered sheet made of bigger pieces of wood, is stronger than either MDF or particleboard, although not as strong as plywood, and is commonly used for sheathing and roof decking.
  • MDF is made of fine wood fibers that have been pressed and glued together. The resulting sheets are very smooth and flat. MDF paints well and is a good choice for interior shelving and painted cabinetry.

mdf

  • MDO has a paper face designed to take paint beautifully. It is commonly used to make outdoor signs.
  • Block board is cubical stocks of wood sandwiched between two thin sheets of Ply. As is obvious from its construction block board has higher  resistance against warping or bending. It is available in the standard thickness of 16, 19 & 25 mm. Again Block board comes in different specifications that determine its use in interior applications.

block brd

Challenges :

Major challenge being faced by the plywood industry is the

  • Monopolization of certain critical inputs for plywood manufacturing. Face veneer, which provides support and finishing to the plywood, is one such product. Prepared from gurgan and kuring woods imported from Burma and Malaysia, it is a monopoly item.
  • Another threat is invasion of Indian market by Chinese plywood products.
  • Threats from wood substitutes made from agro wastes such as bagasse, rice husk and cotton stalk or from wood that cannot be used as timber, such as twigs and small branches.
  • Low operational efficiency in Indian plywood industry leads to low profitability.
  • Competition has lead to increase in the bargaining power of customers making pricing cut throat.

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