1. What is plywood?
Plywood is made of several 1 mm thick slides of solid wood. Each layer of plywood 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.
2. History and development
The use of thin layers of wood as a means of construction dated to approximately 1500 B.C. When Egyptian craftsmen bonded thin pieces of dark ebony wood to the exterior of a cedar casket found in the tomb of King Tut-Ankh-Amon. This technique was later used by the Greeks and Romans to produce fine furniture and other decorative objects. In the 1600s, the art of decorating furniture with thin pieces of wood became known as veneering, and the pieces themselves became known as veneers.
Until the late 1700s, the pieces of veneer were cut entirely by hand. In 1797, Englishman Sir Samuel Bentham applied for patents covering several machines to produce veneers. In his patent applications, he described the concept of laminating several layers of veneer with glue to form a thicker piece – the first description of what we now call plywood.
In 1928, the first standard-sized 4ft by 8ft (1.2m by 2.4m) plywood sheets were introduced in the United States for use as a general building material.
3. Components of Plywood
Plywood is divided into three parts:
– The outer layers of plywood are known respectively as the face and the back which is solid wood
– The core are several 1 mm thick wooden layers.
– The type of adhesive used to bond the layers of wood together depends on the specific application for the finished plywood. Lumbering worker usually use Urea Formaldehyde (UF) for interior and Phenol Formaldehyde (PF) resin for exterior because of its excellent strength and resistance to moisture.
Plywood may be made from hardwoods, softwoods, or a combination of them. Ash, maple, mahogany, oak, and teak are some common types of hardwood. Pine, cedar, spruce, and redwood are also used.
4. Physical characteristics and features
Average density of plywood: 600 – 700 kg/m3.
The common size for plywood sheets used in building construction is: 1220x2440mm; 1160x2440mm; 1000x2000mm.
Common range in thickness: 3, 4, 5, 6, 8, 10, 12, 15, 18, 20, 25 mm.
Although the core, the crossbands, and outer layers of a plywood panel can be made of different thickness veneers, the thickness of each must be balanced from the center layer. For example, the face and back must be of equal thickness. Meanwhile the top and bottom crossbands must also be equal.
5. Producing Plywood
Preparing the logs
1, Selected trees in an area are marked as being ready to be cut down, or felled. The limbs are removed from the fallen trees with chainsaws.
2, Logs are picked up and placed on a chain conveyor that brings them to the debarking machine. This machine removes the bark, either with sharp-toothed grinding wheels or with jets of high-pressure water, while the log is slowly rotated about its long axis.
3, The debarked logs are carried into the mill on a chain conveyor where a huge circular saw cuts them into sections about 2.5m to 2.6m long, suitable for making standard 2.4m long sheets. These log sections are known as peeler blocks.
Making the veneer
4, Before the veneer can be cut, the peeler blocks must be heated and soaked to soften the wood. The blocks may be steamed or immersed in hot water. This process takes 12 – 40 hours depending on the type of wood, the diameter of the block, and other factors.
5, The heated peeler blocks are then transported to the peeler lathe to be automatically aligned and fed into the lathe one at a time.
6, The long sheet of veneer emerging from the peeler lathe may be processed immediately, or it may be stored in long, multiple-level trays or wound onto rolls. At the same time, optical scanners look for sections with unacceptable defects, and these are clipped out, leaving less than standard width pieces of veneer.
7, The sections and veneer are then sorted and stacked according to grade. This may be done manually, or it may be done automatically using optical scanners.
8, The sorted sections are fed into a dryer to reduce their moisture content and allow them to shrink before they are glued together. Most plywood mills use a mechanical dryer in which the pieces move continuously through a heated chamber.
9, As the sections of veneer emerge from the dryer, they are stacked according to grade. Under width sections have additional veneer spliced on with tape or glue to make pieces suitable for use in the interior layers where appearance and strength are less important.
Those sections of veneer that will be installed crossways – the core in three ply sheets, or the crossbands in five-ply sheets – are cut into length of about 1.3m.
Forming the plywood sheets
10, When the appropriate sections of veneer are assembled for a particular run of plywood, the process of laying up and gluing the pieces together begins. The short sections of core veneer are laid crossways on top of the glued back, and the whole sheet is run through the glue spreader a second time. Finally, the face veneer is laid in top of the glued core, and the sheet is stacked with other sheets waiting to go into the press.
11, The glued sheets are loaded into a multiple-opening hot press.
12, The rough sheets then pass through a set of saws, which trims them to their final width and length.
6. Advantages of plywood
– Plywood is hard and has high mechanical durability.
– Compared to MDF, plywood has better capacity of water resistance, which reduces the blistering when immerged in water.
– Plywood has good capability of screw holding as well as adhesion
– Good moisture-resistance in ventilate environment.
– Plywood is more expensive than MDF and PB
– When cutting the board, the edges are prone to chipping
– In moisture environment, the board surface is easy to warp if not properly treated.
– Concrete Forming
– Molded seats
– Sheathing material