What Is Plywood?
Plywood is an “engineered” wood that comes from thin layers of wood veneer combined to form sheets of various thicknesses. We arrange the veneer sheets so that the grain is at 90° to successive layers. Plywood comes from the class of wood belonging to medium-density fiberboard (MDF) and chipboard.
You can learn more about manufactured woods in another of our posts. It deals with the advantages and disadvantages of plywood, MDF, oriented strand board (OSB) and particle board.
The resultant plywood boards created by bonding the various layers together becomes extremely strong. It is due to the “cross-graining” of the multiple layers. The sheets are also added in an odd number, which somehow reduces expansion and shrinkage. The cross-graining also minimizes the possibility of plywood splitting at the edges when driving nails into it.
Some varieties of plywood have the successive layers arranged at multiple angles. It adds a lot to the strength of the boards. Understandably, this plywood grade is more costly than the plywood with layers whose grains are 90° from each other. But the type of plywood with multiple angles also has a higher level of durability.
History of Plywood
Veneers were hand-cut until the late 1700s. Then, an Englishman named Sir Samuel Bentham introduced several machines for the production of veneers. In some of his discussions, Sir Bentham mentioned combining multiple layers of veneer using glue to form a solid board. It was the first time that someone mentioned the word “plywood”.
Despite Sir Bentham’s efforts and idea, it wasn’t until a hundred more years that veneers started being used in the furniture industry and other areas. In the late 1800s, plywood began to find use indoors, and for vehicles, railroad cars, and airplanes.
The name “plywood” did not appear initially; on the contrary, names like “pasted wood” used in sarcasm created negative hype for this material. To boost the image of this material, manufacturers started calling it “plywood.”
From 1928 onwards, plywood of standard sizes began to be manufactured in the United States. Plywood became an accepted material for building. With the introduction of better adhesives and improved manufacturing techniques, it became a more accepted material in several different fields.
As you will be aware, plywood plays a crucial role in woodworking, and even substitutes lumber in many scenarios. The plywood manufacturing business is a multi-billion-dollar business today.
How Plywood Is Made
There is a lot of information available on what plywood is and its various uses. Plywood plays a vital role in multiple fields like shipbuilding, domestic and commercial furniture, and construction, to name a few. It has become a favorite material among woodworkers due to its aesthetics, durability, and ease of use. But you may have some questions about its manufacturing process. Let us take a closer look at the manufacturing process of plywood:
Log Selection And Segregation
The process of manufacturing plywood begins with selecting the logs. We produce plywood from both hardwood and softwood logs. Hence, we identify and segregate hardwood and softwood logs according to the various wood species.
It helps us ascertain the potential for making veneer for plywood and the potential aesthetics and physical qualities of the wood. Some of the commonly-used varieties of wood that we use for making plywood are cedar, pine, birch, mahogany, maple, ash Douglas fir, and spruce.
Once suitable logs are segregated and stored in their designated areas, they need to be conditioned. It results in the production of a good-quality veneer during the peeling process. This stage involves immersing the logs in water or spraying them.
Bark Removal And Length Cutting
There are two processes involved at this stage. Firstly, we measure the selected logs for cutting. Then, after cutting the logs to the required lengths, they undergo a “debarking” process. A specialized machine called a debarking machine shaves off the bark of the logs. The method of debarking also helps to improve the cross-sectional uniformity of the logs.
Peeling and Sizing
A rotary lathe machine rather like a giant potato peeler now peels thin layers of wood from the logs. While we clip some veneers to size as they come out of the lathe, others come out as continuous sheets to be sized later.
This is the stage where we remove sections with visual defects. Cutting off the defects reduces the size of the veneers. It does not make a lot of difference, because we join smaller sheets of veneer to form full-size sheets while adding the plywood layers.
The Drying Process
The veneer sheets, once formed, still have a considerable amount of moisture content. Therefore, to get rid of the moisture, there needs to be a drying process. Bonding of the veneer sheets will not be adequate if they contain moisture.
We pass the veneer sheets through dryer rollers that are heated by steam. The steam comes from burning the wooden waste from the previous processing. The rollers can also get their heat from an electric heater. The veneer sheets are considered ready for the next stage when the moisture content is between 6% and 14%.
Once the veneers have adequately dried, we trim them into sheets typically of dimensions of 4′ X 8′. We then arrange the surfaces side-by-side, ready to be joined together. The orientation of the veneers is important, and we segregate the veneers into “face” veneers, “back” veneers, and “core” veneers depending on the position in which the plywood will be finally used. Now, the plywood sheets are ready to be bonded together with an adhesive
Once the sheets lie properly side-by-side, we bond them together with the use of adhesives. The adhesives will vary according to the type of plywood in production. We usually use urea-formaldehyde resins to create ordinary grade plywood. Marine plywood, on the other hand, needs melamine or phenol-based adhesives for its production
The veneer layers pass through a mechanical glue spreader to ensure uniform spreading of the glue on the alternate veneer layers. The veneers now lie one on top of each other to form panels. The thickness of the plywood depends on how many layers it contains.
Once we apply the adhesive, the plywood sheets pass through a cold hydraulic press. This procedure takes about 20 minutes, during which the adhesive solidifies, and the layers of veneers become a solid mass. It also serves to flatten the plywood.
After passing through the cold press, the veneers pass through a hot press, which lasts for a few minutes. The duration and amount of heat applied depends on the species of wood, design of the plywood, and the thickness of the veneer.
By this stage, the plywood panel almost reaches the stage of final completion. A quality checker picks samples randomly and conducts tests for bonding, mechanical strength, and formaldehyde emission. There will be a visual check as well, and if there are any defects, we cover them by using a filler.
Sanding and trimming
Once quality control passes the plywood, we sand its surface to ensure smoothness and uniform thickness. The sanding needs to be done based on individual requirements. Depending on those requirements, we sand one or both faces and, if required, maybe the sides. Final trimming takes place after the sanding stage.
Packing, stacking and stamping.
At last, the plywood is now ready for dispatch. The stamping process involves providing technical specifications about the material. And other relevant identification information will also appear. Once the stamping work ends, we stack the plywood sheets horizontally. These stacks of plywood will either go into storage or load onto trucks for transportation to the market.
Now you know all the details of the different stages of plywood manufacturing. The journey of the material from the forests to the finished product is a long and winding one. By understanding more about the plywood manufacturing process, you can appreciate working with this material better. With this additional knowledge, you can work better with plywood to get the best out of it in all your woodworking projects.