Plywood: The Production Process
Plywood is a wood-based panel product comprised of a collection of veneers that are glued together with a resin. To sum up the production process; logs are peeled into veneers, the veneers are lathered in glue, laid up so that the grain direction of each veneer is perpendicular to its adjacent veneer, pressed together, trimmed and sanded.
But there is more to the production process than just that…
Step 1: The Forest
It’s important to us that all raw logs used in our plywood are sourced from legal and sustainable forest concessions. We only use mills who use the Timber Trade Federation’s Responsible Purchasing Policy to ensure this.
Once trees reach an acceptable level of maturity, they are felled by trained harvesters. Depending on the operation of the mill, vehicles like those in the image above can be used to select and fell trees using satellite imagery.
Step 2: Transport to the Mill
Logs are transported from the forest concession to the mill to be processed.
Transportation comes with its own environmental issues in some instances, as forest areas are cleared to make room for roads and therefore new trees cannot grow.
Step 3: The Log Pond
On arrival at the mill, logs are stored in the Log Yard. Mills will use various methods to retain the moisture in the logs, many opting for sprinkler systems.
Logs will then eventually make their way to the Log Pond. Logs are submerged in water for an extended period of time so that they are easier to cut down to size and easier to peel. Some mills in colder climates have heated log ponds as the heat also improves the quality of the peeling; they refer to this as hydro-thermal processing.
Step 4: De-barking the Logs
Before the logs are cut and peeled, the bark must be removed. Mills use industrial machines such as this to de-bark logs as they continue along the production line.
Step 5: Cutting the Logs
De-barked logs then move on to be cut to size. The size that the logs are cut to is usually dependent on the production at the time of cutting; finished panel size & grain direction play a part in the cutting of logs.
Step 6: Peeling the Logs
Logs are peeled using a rotary lathe. This peels the log in a manner similar to that of a pencil sharpener except the blade is completely parallel to the log at the time of cutting. You can see this process in action in the video above, and see them coming out of the back of the peeling machine straight onto a conveyor in the video below.
Step 7: Sizing and Grading
Once they’ve been peeled the resultant sheets are fed to the next step on a conveyor, as in the image below.
Following peeling, the veneers are moved along the production line in long streams. They need to be cut to size and go through an initial grading process. Grading is especially important in regards to Birch Plywood so the majority of mills use scanning technology to check for defects in the veneers, cutting to the required sizes and then separating potential face veneers from core veneers.
Unlike mills from Eastern Europe and the Baltics, countries like China will peel the veneers in smaller squares and then stitch them to the appropriate size as a more cost effective (but quality reducing) method.
Step 8: Drying the Veneers
At this point, the veneers are still wet from being soaked in the log pond. The veneers must be dried for a variety of reasons; from protecting the wood from fungal decay to increasing the mechanical properties of the finished board.
Most mills use large, industrial dryers, often connecting to the log peeler via conveyor belt; however, more frugal methods can also be used. For example, some log peelers in China leave the veneers out in the open to dry throughout the day.
Step 9: Repairing Defects
Once dry, veneers need to be repaired where defects exist. In the case of Birch Plywood for example, open knots (where branches used to be) can be ‘plugged’ in. Splits in veneers on plywood can also be filled in and mis-sized veneers can be finger-jointed together (see above).
Some mills have a machine that scans the veneers and automatically plugs open holes or knots. (See below).
Step 10: Application of Glue and Lay-Up
Veneers are run through a gluing machine which essentially rolls the glue onto the face and back of the veneer. They are then placed on top of an unglued veneer so that the stack alternates; Glued, Unglued, Glued, Unglued and so on.
Step 11: Cold Pressing
Cold Pressing occurs after glue has been applied in order to prepare the veneers for Hot Pressing. This works to flatten out the veneers and ensure the glue is distributed across the veneers evenly.
Step 12: Hot Pressing (Daylight Press)
This part of the production process is where the actual panels begin to take shape. Multiple panels are loaded into the daylight press. The Daylight Press then compresses and maintains heated pressure on the boards for a long period of time. This creates and maintains required contact between the glue and veneers. It also decreases tension in the glue line and the thickness of glue layer.
Step 13: Trimming, Sanding and Finishing
Following Hot Pressing, the board is left to stabilise and cool down before further processing. Then it is a case of trimming down any excess veneer so the board has square edges, then the boards are most commonly sanded using a large, industrial sander.
Step 14: Quality Control
The final product has to be assessed for quality but it wouldn’t be very efficient just to wait until the end of the process to find a large problem with production. For this reason, mills control their production by carrying out a number of tests on different sections of the production process e.g. testing moisture levels, formaldehyde release, durability, etc.
Many mills have what is called a Factory Production Control certificate which means they have been audited by a third party and had their production process approved to relevant European standards.
Step 15: Packaging
Finished products are then stacked up and banded together. Any relevant CE marks are printed on the packaging.