Birch Throughout plywood Technology and production process
Plywood is a board made from thin sheets of construction veneers which are bonded in layers to create a strong and stable board. Veneers used for plywood construction are cut from a log for constructional or decorative purposes. The two most common methods used for veneer production are called saw and rotary cutting. Constructional veneers on the other hand, are cut using the second method named rotary cutting. The knife blade peels thin sheets of wood and it is an efficient way to produce veneers, as they can be cut to any width. Plywood is most commonly made using an odd number of veneers, three being the minimum. However, the number of veneers used depends on their thickness and the finished plywood board. The veneers are bonded with an adhesive at right angles to one another to reduce the shrinkage and improve the strength.
Various species can be used for plywood production, where the face and core veneers may be made from different species or the same species can be used for the entire board construction. The surface of the plywood board consists of veneers calledface plies. Depending on their quality, the ply of better quality is known as the face and the other is referred as the back. To indicate the quality of the plies, a letter code is used to mark the grade. The grading system uses the letters E, B,BB, and C where grade E ( elite) classifies the best quality and grade C indicated the poorest quality. The grades only refer to theappearance of the face plies and do not indicate the structural performance of the plywood board. Plywood is constructed in awide range of different sizes. The most common dimensions (mm) are: length – 1525, 2440 and 3010 and width – 1220, 1525. The thickness of plywood can range from 3-40 mm depending on the area of use.
A common adhesive is called formaldehyde and is composed of carbon, hydrogen and oxygen. Formaldehyde is both a naturally existing chemical produced by plants and animals and an industrial chemical. An important characteristic of plywood is the amount of formaldehyde emitted. Contact with formaldehyde can cause short-term skin irritation. However, long-term exposure of formaldehyde can cause serious health problems (cancerogenic). Formaldehyde emissions can be rated as E1 (9mg/100g or less), E-0
A common hardwood species used for plywood production is birch. The birch plywood board is classified as one of the most popular types of plywood due to its superior mechanical properties and beautiful appearance. Birch plywood can be used for interior and exterior applications. In the plywood production process, the birch logs need to be straight with a specific dimension. Only 50% of the birch log is utilized during the production process. The core of the birch log cannot be used and therefore waste products are usually sold to other wood working industries. The advantage with birch is its fast rate of growth (30-35years) compared to other wood species, e.g., spruce. At the same time the growth rate cannegatively affect the quality of the birch logs.
Historical review of plywood manufacturing
Plywood production took place already in ancient Egypt where wooden products were made
using sawn veneers . The thin sheets of veneer, which were of high quality, were glued over a substrate of lower quality. This process was common due to the lack of fine woods and the high quality veneer sheets gave the wood a nice appearance.
During the 18th and 19th centuries, furniture manufacturers in England used veneers for production of the highest quality furniture.
Immanuel Nobel invented the modern technique of rotary cut veneer and the first production equipment was constructed in the USA during the 19th century. “Plywood has been one of the most ubiquitous building products for decades.” In the beginning of the 19th century, Thomas J. Autzen was the owner of Portland Manufacturing Company, USA, which was the first plywood production facility. He contributed to the development of the plywood production by reducing the drying and manufacturing process. Thanks to his findings, the new technology had an important role of “making plywood one of the most abundant and affordable building products ever produced”.
Applications of plywood
As illustrated in Figure 11, plywood can be used for many different purposes (Jackson et. al.,1990). Interior plywood comprises furniture and wall panelling, while exterior plywood is appropriate for kitchen fitments and applications around showers and bathrooms. Another type of plywood is structural plywood, which is manufactured for applications where strength and durability are the primary factors. Structural plywood is used in the construction and building industry. Most commonly plywood is used for formwork applications due to its strength,stability and tolerance to changes in temperature and moisture. Finally, marine plywood is used where moisture resistance is needed. Plywood can also beused for transport- and boat construction and packaging manufacture.