By: Troy
Dubois
Updated
November 18, 2000
Introduction to the Manufacturing
of Plywood
Selecting the Log
Stripping the Bark
Peeling the Log
Making a Continuous
Ribbon of Wood
Cutting and Stacking
Gluing the Wood
Pressing the Wood
Trimming, Sanding, and
Finishing
Conclusion to the Manufacturing
of Plywood
Plywood is a versatile
wood product, which makes it in great demand by contractors. America
alone uses more than sixteen thousand million square feet every year which
is meaningless until it is compared to a twelve-foot-wide pathway stretching
from earth to behond the moon (The International Book of Wood 44).
The following illustration shows what a common piece of plywood looks like.
(Public Textures. http://art.net/Studios/Visual/Jeremy/photo/public_texture/index.html)
Plywood is a building material made from layers of thinly sliced wood, which is glued together. Each layer called a ply is arranged so that the direction of the wood fibers or grain is placed at right angles to the layer next to it (http://online.anu.edu.au/Forestry/wood/plywood/intro.html Plywood & Veneer Production).
(Panel Products. http://www.cyberyard.com/cybyrdnational/pages/howtolibrary/panelproducts.html)
This placement of the layers equalizes a panel’s strength and prevents splitting (Wood 7). Plywood is usually made of three plies, but five, seven, nine, or even more plies may be used in some situations. The most common sizes of plywood in the United States are 4 feet wide and eight feet long and ¼ to ¾ inches thick. The dimensions and thickness can be different according to how the plywood is to be used (Ettlinger 241).
Two types of plywood are interior and exterior. Interior plywood is made with glues that are moisture-resistant. This means that the glue in the interior of the plywood has a limited resistance to water. On the other hand exterior plywood is designed to better withstand conditions resulting from moisture and humidity. This means that the adhesive of exterior plywood is capable of withstanding a certain amount of moisture before it starts to decay. Exterior plywood is also made with waterproof glues (Wood 169). For more information from The Forest Products Society click on the following hyperlink. http://www.forestprod.org/
The manufacturing
process of making plywood consists of several steps. These steps
include: Selecting the log, Stripping the bark, Peeling the log,
Making a continuous ribbon of wood, Cutting and stacking, Gluing, Pressing,
and Trimming, Sanding, and Finishing the wood.
Plywood is made from
several different species of trees. Most softwood plywood is made
of Douglas-fir or southern pine. Hardwood plywood is actually made
from over eighty different species of woods. Some of the most popular
woods are oak, red gum, poplar, birch, cherry, and walnut (Wood 10).
Plywood and oriented strand board have many of the similar processes.
Attached is a hyperlink of some birch trees in the raw material form.
These trees could be used for plywood or oriented strand board. Hyperlink:
http://www.xylo.for.ulaval.ca/osb/sld027.htm
The first step in the manufacturing
process of plywood is the selecting of the logs. The logs that are
selected are chosen for their straightness and roundness. They also
must be relatively free from knots and decay (World Book 586).
Selecting the log. Figure 1
The International Book of Wood,
Pg. 42.
The selected logs are then stripped of their bark. According to the following picture, the logs are fed in from the right by chain conveyors. The logs are going to be stripped so therefore, they are rotated by ridged wheels as the cutting head on the track reverses the log from end to end (The International Book of Wood 44). This is also known as debarking. The following illustration demonstrates this portion of the manufacturing process. Two additional illustrations of this debarking process are located at the following hyperlinks.
http://www.xylo.for.ulaval.ca/osb/sld031.htm
http://www.xylo.for.ulaval.ca/osb/sld032.htm
Stripping the bark. Figure 2
The International Book of Wood,
Pg. 42.
The next step of the manufacturing process is peeling the log. The log sections, which show the marks of the debarking knives, are fed into the lathe loader where the log revolves around on a huge lathe against a long cutter blade which is reduced to a 6-inch core or sawed into lumber or chipped (44). The following illustration shows the operation of the peeler.
Peeling the log. Figure 3
The International Book of Wood,
Pg. 44.
Following the peeler process, the next step in the manufacturing process of plywood is taking the logs and making them into a continuous ribbon of wood. The cutting edge of the lathe is forced against the spinning log. The wood is unwound in a continuous ribbon varying in thickness depending on how its used (World Book 586). The following illustration is a good example of this process.
Making a continuous ribbon of wood. Figure 4
The International Book of Wood,
Pg. 44.
The next step of the process is the cutting and the stacking of the ribbons of wood. The standard size for the pieces of wood to be cut is 4’ X 8’. The thickness of the plywood will be determined later when the sheets are glued and pressed together to a varying thickness of which ¼” to ¾” is the most common. The following hyperlink shows the process of cutting the sheets to the proper length and width.
As the sheet emerges from the peeler it is scanned automatically and then it is stacked green and is prepared for the transferring to the drying ovens (The International Book of Wood 44). This next picture shows the wood after it is cut to rough size and stacked green and ready to be sent to the drying ovens. The moisture content of commercial plywood should not be lower than 6 percent nor greater than 14 percent at the time it leaves the factory (Wood 173).
Stacked for drying ovens. Figure 5
The International Book of Wood,
Pg. 44.
The next step in making plywood is the gluing of the plies together in order to determine the desirable thickness of the plywood sheet. Modern methods of manufacturing use synthetic plastics such as urea resins or phenol-formaldehyde for bonding the plies together. These glues are mixed mechanically and then spread on alternate layers of lumber by passing between pairs of grooved metal or hard rubber rollers (Ball 26). The following picture shows the plywood sheets moving through the rollers as the adhesive is being applied. Further pictures of the gluing procedure are located at the following website about plywood under the topic of Application of Adhesive.
http://online.anu.edu.au/Forestry/wood/plywood/glueing.html#aa
Gluing procedure. Figure 6
The International Book of Wood,
Pg. 44.
The following step in manufacturing process is the procedure of pressing the glued sheets together to a desired thickness. One example of a hot press machine is found at the following hyperlink.
Other examples of pressing machines are located at the following hyperlink.
http://online.anu.edu.au/Forestry/wood/plywood/pressing.html#pp
Hydraulic or pneumatic
presses squeeze the plies together with heat and pressure or with just
pressure only. When heat is used the glue hardens within a few minutes.
The glue solidifies as the plies are pressed together; and once the pressure
is released, the boards are considered dry (Wood 49). The hydraulic
presses are equipped with a series of steam-heated platens with a temperature
from 250 degree F to 300 degree F. Each pair of platens has a sheet
of plywood between them that is pressed to the desired thickness.
Sometimes a screw method is also used but this method is not as desirable
(Ball 26).
Lastly the sheets
have to go through a process that gets them ready to be shipped out for
market. This processes involves trimming, sanding and finishing the
sheets. This process also takes the sheets down to the proper size
that is desirable to the consumer. This is what makes the final smooth
edges that are seen in the modern lumberyard. The finished panels
of plywood are then divided into two groups indicating whether they are
for interior or exterior use. Plywood may carry a quality grade,
which is indicated with the letters A through C, with A being the highest
quality. This grade is based on requirements set by U.S. Commercial
Standard CS - (35-36) (Wood 468).
These are
the steps of the manufacturing process of plywood from the original type
of tree chosen, clear to the completed sheets of plywood stamped and ready
to be shipped to your local hardware stores and lumber yards. This
process shows how the plywood, which the customer buys, comes from a variety
of different tree species. It also shows the mechanics of how the
plywood you buy is made into widths, lengths and a variety of different
thicknesses depending on the use of the material. Hopefully, this
illustrates the process of how a tree is manufactured into the plywood
that is found in a local lumberyard.
Final illustration of a stack of plywood. Figure 7
Ball, John. (1976). Carpenters and Builders Library. Indianapolis: Howd W. Sams and Co., Inc.
Bramwell, Martyn. The International Book of Wood. (1976). New York: Simon and Schuster.
Compton’s Encyclopedia. (1990). “Plywood”. Chicago: Encyclopedia Britannica, Inc.
Engleman, Jeremy A. Public Textures.
http://art.net/Studios/Visual/Jeremy/photo/public_texture/index.html
Ettlinger, Steve. (1989).
The Complete Illustrated Guide to Everything Sold in Hardware Stores.
New
York: Macmillian
Publishing Co.
Oriented Strandboard (OSB): Raw Material,
Manufacturing Process, Properties and Uses. (03/12/98).
http://www.xylo.for.ulaval.ca/osb/index.htm
Panel Woods. http://www.cyberyard.com/cybyrdnational/pages/howtolibrary/panelproducts.html
The Forest Products Society. (2000). http://www.forestprod.org/
Wood, Andrew. Plywoods of the World. (1963). Edinburgh: W. and A.K. Johnston and G. W. Bacon Limited.
The World Book Encyclopedia. (1997). “Plywood”. Chicago: World Book, Inc.
Young, Robert M. (2000).
Plywood and Veneer Production.
http://online.anu.edu.au/Forestry/wood/plywood/intro.html