To understand the strength of plywood and the factors that impact this, it's first essential to comprehend how plywood sheets are made.
The first step in plywood production is gathering the timber. Plywood can be made from either hardwood or softwood, or a combination. Hardwood plywood may be made of birch, oak, mahogany, or teak, while softwoods such as beech or pine are often used to make softwood plywood.
After removing branches and bark and soaking the timber in water to make it easier to peel, the logs are cut to size. Then, thin veneers are peeled away from the log using a rotary lathe. These sheets are cut to size and graded based on their quality -- veneers with defects can become the core of the plywood, while high grade veneers are used for the face.
The next step is to dry the veneers and check them for defects, plugging holes and filling in splits. Then, they are glued and stuck together, with the grain alternating (known as cross-graining or cross-lamination). The glue that is used may vary on the type of plywood being made -- urea-formaldehyde resin is the typical adhesive used for plywood, although exterior or marine plywood that needs to be resistant to moisture often uses a phenol-formaldehyde resin instead.
The plies are pressed together twice, first without heat and then using heat. Finally, the sheets of plywood are trimmed and sanded, ready for quality control, packaging, and use. In some cases, the finished sheet material may be treated with a chemical to make it more fire-resistant.