Plywood is a sheet material manufactured from thin layers or "plies" of wood veneer that are glued together with adjacent
layers having their wood grain rotated up to 90 degrees to one another. All plywood bind resin and wood fibre 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 in 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.
Types of Plywood:
Softwood plywood is usually made either of cedar, Douglas fir or spruce, pine, and fire
Hardwood plywood 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.
Usually faced with hardwood like ash, oak, birch, maple, mahogany, rosewood, teak and a large number of other hardwood.
Flexible plywood is designed for making curved parts
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. 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 .
Other types of plywood include fire-retardant, film faced 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.
History of Medium Density Fibreboards (MDF)
Medium-density fibreboard (MDF) is an engineered wood product made by breaking down hardwood or softwood residuals into
woodfibres, often in a defibrator, combining it with wax and a resin binder, and forming panels by applying high temperature
and pressure. MDF is generally denser than plywood. It is made up of separated fibers, but can be used as a building material
similar in application to plywood. It is stronger and much denser than particle board.
Over time, the term MDF has become a generic name for any dry process fibre board. MDF is typically made up of 82% wood fibre,
9% urea-formaldehyde resin glue, 8% water and 1% paraffin wax and the density is typically between 500 kg/m3 and 1,000 kg/m3 .
The density of the board, when evaluated in relation to the density of the fibre that goes into making the panel, is important.
A thick MDF panel at a density of 700–720 kg/m3 may be considered as high density in the case of softwood fibre panels, whereas a
panel of the same density made of hard wood fibres is not regarded as so. The evolution of the various types of MDF has been driven
by differing need for specific applications.
History of Particle Boards
Particle board – also known as particleboard, low-density fibreboard (LDF), and chipboard – is an engineered wood product
manufactured from wood chips, sawmill shavings, or even sawdust, and a synthetic resin or other suitable binder, which is pressed
and extruded. Oriented strand board, also known as flakeboard, waferboard, or chipboard, is similar but uses machined wood flakes
offering more strength. All of these are composite materials that belong to the spectrum of fiberboard products.
Particle board is cheaper, denser and more uniform than conventional wood and plywood and is substituted for them when cost is
more important than strength and appearance. Though it is denser than conventional wood, it is the lightest and weakest type of
fiberboard, except for insulation board. Different grades of particleboard have different densities, with higher density connoting
greater strength and greater resistance to failure of screw fasteners.A major disadvantage of particleboard is that it is very prone
to expansion and discoloration due to moisture, particularly when it is not covered with paint or another sealer. The advantages of
using particleboard over veneer core plywood is it is more stable, (unless it gets wet), much cheaper to buy, and somewhat more
convenient to use. Also nowadays laminated particle board and melamine faced particle board is also available to make it more durable and attractive.