Plywood
A structural material made of layers of wood glued together, usually with the grains of adjoining layers at right angles to each other.
Plywood is referred as an assembly of veneer bonded together with the direction of grain in alternate veneer. The direction of these alternate grains is usually placed at right angle.
Plywood is made of three or more thin layers of wood bonded together with an adhesive. Each layer of wood, or ply, is usually oriented with its grain running at right angles to the adjacent layer in order to reduce the shrinkage and improve the strength of the finished piece. Most plywood is pressed into large, flat sheets used in building construction. Other plywood pieces may be formed into simple or compound curves for use in furniture, boats, and aircraft.
Types:
Plywood can be of two types:-
1) Veneer plywood: The plywood in which all the piles are made of veneer up to 7 mm. thicknesses and oriented parallel to the surface of the panel are called veneer plywood.
2) Core plywood: These are the plywood with the cores. It is sub divided in to the following categories:-
a) Wood core plywood: Plywood having a core of solid wood. It is also in to three types-
i. Batten board: Plywood, the core of which is made of strips of solid wood of more than 30 mm. wide.
ii. Block board: Plywood, the core of which is made of strips of solid wood more than 7 mm. wide.
iii. Lamin board: Plywood, the core of which is made of strips of solid wood not wider than 7 mm.
b) Cellular plywood: Plywood, the core of which consists of a cellular construction.
c) Composite plywood: Plywood, the core of which are made of materials other than solid wood or veneers.
1) Veneer plywood: The plywood in which all the piles are made of veneer up to 7 mm. thicknesses and oriented parallel to the surface of the panel are called veneer plywood.
2) Core plywood: These are the plywood with the cores. It is sub divided in to the following categories:-
a) Wood core plywood: Plywood having a core of solid wood. It is also in to three types-
i. Batten board: Plywood, the core of which is made of strips of solid wood of more than 30 mm. wide.
ii. Block board: Plywood, the core of which is made of strips of solid wood more than 7 mm. wide.
iii. Lamin board: Plywood, the core of which is made of strips of solid wood not wider than 7 mm.
b) Cellular plywood: Plywood, the core of which consists of a cellular construction.
c) Composite plywood: Plywood, the core of which are made of materials other than solid wood or veneers.
Advantages of using plywood:
The major advantages of using plywood are as follows-
1. Dimensional stability: Shrinking and swelling or movement along the grain in the longitudinal direction is negligible and much less than those than across the grain. This dimensional instability of wood disappears in plywood.
2. Strength equalization and enhancement: In solid wood strength is high alone the grain and weaker in across the grain. Cross banded nature of plywood imparts higher longitudinal strength to two cross direction. Tensile strength bending and shearing strength and cleavage properties are more or less equal along and across the grain.
3. Balanced constructions: Cross bending wood properties produces stresses in the glue line. If two or even numbers of piles are bonded together there will be a great tendency of the panel to warp or twist due to change in the moisture content. Different species are used in plywood but with careful considerations of specific gravity and thickness to keep the pair of piles closely matched.
1. Dimensional stability: Shrinking and swelling or movement along the grain in the longitudinal direction is negligible and much less than those than across the grain. This dimensional instability of wood disappears in plywood.
2. Strength equalization and enhancement: In solid wood strength is high alone the grain and weaker in across the grain. Cross banded nature of plywood imparts higher longitudinal strength to two cross direction. Tensile strength bending and shearing strength and cleavage properties are more or less equal along and across the grain.
3. Balanced constructions: Cross bending wood properties produces stresses in the glue line. If two or even numbers of piles are bonded together there will be a great tendency of the panel to warp or twist due to change in the moisture content. Different species are used in plywood but with careful considerations of specific gravity and thickness to keep the pair of piles closely matched.
Raw Materials:
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 plywood’s with five or more plies, the inter-mediate layers are known as the cross bands.
Plywood may be made from hardwoods, softwoods, or a combination of the two. Some common hardwoods include Civit, Uriam, kadam, am, koroi, ash, maple, mahogany, oak, and teak. The most common softwood used to make plywood are varieties of pine, cedar, spruce, and redwood are also used.
The type of adhesive used to bond the layers of wood together depends on the specific application for the finished plywood. Softwood plywood sheets designed for installation on the exterior of a structure usually use a phenol-formaldehyde resin as an adhesive because of its excellent strength and resistance to moisture. Softwood plywood sheets designed for installation on the interior of a structure may use a blood protein or a soybean protein adhesive, although most softwood interior sheets are now made with the same phenol-formaldehyde resin used for exterior sheets. Hardwood plywood used for interior applications and in the construction of furniture usually is made with a urea-formaldehyde resin.
Some applications require plywood sheets that have a thin layer of plastic, metal, or resin-impregnated paper or fabric bonded to either the face or back (or both) to give the outer surface additional resistance to moisture and abrasion or to improve its paint-holding properties. Such plywood is called overlaid plywood and is commonly used in the construction, transportation, and agricultural industries.
Manufacturing Process:
The trees used to make plywood are generally smaller in diameter than those used to make lumber. In most cases, they have been planted and grown in areas owned by the plywood company. These areas are carefully managed to maximize tree growth and minimize damage from insects or fire.
Here is a typical sequence of operations for processing trees into standard 4 ft by 8 ft (1.2 m by 2.4 m) plywood sheets:
a. Preparation of green veneer: Green veneers are prepared by following steps;
1. Log selection and storage: The log should be straight, cylindrical and defect free. The log with large diameter should be preferred. The logs should store in pond to avoid the bio-deterioration.
2. Pre-treatment: The hardwoods should be heated in boiling water or in streaming chamber. Because, heat soften the texture and makes the veneer pliable.
3. Veneer cutting: After pre-treatment, the bolts are debarked and carried to veneer lathe usually by means of an overhead gantry system to cut it.
4. Slicing of veneer: Then the veneers are converted to slices. Here, two types of slicer are used. The horizontal slicers are used to slice the bigger logs and vertical slicers are used to convert the smaller logs.
5. Veneer handling and green clipping: Then the slices of veneers are transported to the table manually or mechanically. Then the veneers are clipped to the size and the defects are cut off. This procedure is known as green clipping.
b. Drying of veneer: The sliced veneer remains very wet. So, it needs to be dried. The veneers may be dried out by heating in dry kilns, hot plates, etc. Sometimes small veneers are dried under sun.
c. Jointing and splicing of dry veneer: Here, the dried veneers are jointed by glues together to make a wider veneer. The edges of the veneer are also cut perfectly. After jointing the veneer are spliced by the splicer. There may be two types of splicer; taper or tapeless splicer.
d. Processing of plywood: When the veneer is properly spliced and glue layer reached to the desired consistency; the veneer is put under pressure to ensure proper alignment of components and intimate contact between the wood and glue.
e. Mixing and spreading of adhesive: Mixing procedure depends upon the types of adhesive used. The ingredients are measured by weight or volume. A glue spreader is used to spread adhesive to the veneer. Liquid glues can be brushes, paint roller or by spraying.
f. Conditioning and finishing:
I. Conditioning: When the glue is set; then it is ready to trim and sanding. But, before that, some times it needs to be conditioned. The cold pressed plywood sometimes remains wet and if that, then it must be dried out.
II. Trimming and sanding: The plywood panels are trimmed by ripping and cut-off saws which must be maintained in good conditioned and set precisely. Some sheets are run through a set of circular saw blades, which cut shallow grooves in the face to give the plywood a textured appearance. After a final inspection, any remaining defects are repaired. The finished sheets are stamped with a grade-trademark that gives the buyer information about the exposure rating, grade, mill number, and other factors.
Storage:
Plywood is stored under conditions that will not appreciably change the moisture content of panels. Stacking in solid piles with the panels directly over each other with a solid cover over the top of each pile protects the panels against rapid changes in moisture content, warping, dust accumulation and discoloration by light. The edges and ends of panels may be covered by wrappings to protect these from moisture changes. It also helps to protect edges from dirt and mechanical damage.
Quality Control:
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. These standards not only establish the grading systems for plywood, but also specify construction, performance, and application criteria.
Uses of plywood:
Plywood is used wherever a material is required to cover large areas with a light but storage and rigid sheeting.
v House-building, floors, doors, cupboards, concrete, shattering, etc.
v Coach building(railway, tracks, vans, trailers),roofs and walls
v Shipbuilding (boat, decks, cabins)
v Box making
v Aircraft and construction
v Bathroom vanities
v Furniture: Bedroom furniture, hotel furniture, Juvenile furniture, laboratory furniture, Office furniture, School furniture, unfinished furniture, kitchen cabinets and doors, etc.
v Bookcases
v Commercial millwork
v Games instrument
v Manufactured housing
v Recreational vehicles
v Speakers
v Stair parts
v Wall paneling, etc.
Plywood is stored under conditions that will not appreciably change the moisture content of panels. Stacking in solid piles with the panels directly over each other with a solid cover over the top of each pile protects the panels against rapid changes in moisture content, warping, dust accumulation and discoloration by light. The edges and ends of panels may be covered by wrappings to protect these from moisture changes. It also helps to protect edges from dirt and mechanical damage.
Quality Control:
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. These standards not only establish the grading systems for plywood, but also specify construction, performance, and application criteria.
Uses of plywood:
Plywood is used wherever a material is required to cover large areas with a light but storage and rigid sheeting.
v House-building, floors, doors, cupboards, concrete, shattering, etc.
v Coach building(railway, tracks, vans, trailers),roofs and walls
v Shipbuilding (boat, decks, cabins)
v Box making
v Aircraft and construction
v Bathroom vanities
v Furniture: Bedroom furniture, hotel furniture, Juvenile furniture, laboratory furniture, Office furniture, School furniture, unfinished furniture, kitchen cabinets and doors, etc.
v Bookcases
v Commercial millwork
v Games instrument
v Manufactured housing
v Recreational vehicles
v Speakers
v Stair parts
v Wall paneling, etc.
Plywood industry of Bangladesh:
Plywood industries are not distributed around the whole country; most of all the factories are distributed in the Chittagong Division because of the raw materials are available in this division. Only one factory is in Srimangal district in Sylhet Division.
The factories are distributed in Chittagong Sadar, Cox’s Bazar, Patiya, Rangamati, Kaptai, etc.
Plywood industries are not distributed around the whole country; most of all the factories are distributed in the Chittagong Division because of the raw materials are available in this division. Only one factory is in Srimangal district in Sylhet Division.
The factories are distributed in Chittagong Sadar, Cox’s Bazar, Patiya, Rangamati, Kaptai, etc.
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