Plywood structure and production

Plywood is a wood-based sheet material for construction. It consists of several thin layers of wood (veneer) tightly glued together so that the directions of fiber of adjacent veneers are mutually perpendicular.

Birch veneer is mainly used for the production of plywood. Other wood species widely used for this purpose are alder, beech, and pine. However, special types of plywood providing increased resistance to bending and torsion are also produced.

Industrial plywood is also called "special plywood". It means that the plywood attained peculiar characteristics as a result of some technological processes and is intended for use in one industry or several industries. Thus, large-size plywood (1220x2440; 1525x3050), which is widely used in construction in Europe and North America, can be considered as special plywood. Laminated plywood fits this definition as well. It also includes bakelized plywood, as well as other types of plywood with specific areas of application, e.g. plywood overlaid with fiberglass, sheet metal, granulated cork, reinforced plywood, bioresistant plywood, composite plywood as well as plywood with textured surface, etc.

Plywood with a symmetrical structure about the middle layer (center) has excellent strength properties and resisance to waviness. Such “sandwich” structure is a characteristic of industrially produced plywood, which is usually glued from an odd number of layers and is accordingly called three-layer, five-layer, seven-layer plywood etc. As a rule, a sheet of plywood is collected from veneer of one wood species. However, plywood can also be produced from mixed veneer to apply some specific properties. Manufacturers also provide a wide range of plywood panels with decorative finishes for the producton of furniture, doors, walls, etc.

The main quality characteristics of plywood are:
• wood species
• grade
• dimensions
• thickness
• type of glue
• processing

Usage
Different in quality and thickness plywood sheets (of sizes 1220 x 2440 mm) of various kinds are stored and sold for structural applications.
Such panels are widely used for:
• wall cladding,
• flooring,
• construction of walls,
• roof building,
• construction of formwork, etc.

Suppliers provide the industries with structural panels in cut-to-size formats. Flat surfaces made of plywood can be faced with expensive decorative veneers to be used in high-quality furniture production. As a rule, such plywood is made in a special workshop of a furniture company, since the quality and appearance of furniture depend significantly on the compatibility of face veneer and wood species of the base or frame.

Plywood from both hard and soft woods is available in several types and grades, which differ in application, durability, appearance and cost. For example, a manufacturer can offer different types of water-proof plywood for exterior works, shipbuilding, formwork. Ready-made plywood panels with the thickness of 6 to 25 mm are used for contruction; 3 to 30 mm plywood has found many applications in furniture industry from the backs of mirrors and bottoms of drawers to the countertops of writing-tables. Generally speaking, plywood of various thicknesses and types of surface quality can be used in many areas. For example, cheap thin plywood is utilized for production of packing for containers.

Plywood is mostly produced in the form of flat sheets or panels, but there are some techniques for production of bent parts from it. Commonly, slightly curved plywood is pressed by clamping it between a pair of hot molds. There is also special equipment for production of shaped blanks for cabins and berths, aircraft parts, etc. from plywood.

Production
Adhesives used for plywood production are prepared in automatic mixers and fed into the grooves of glue rollers, made from metal or hard rubber, which are rolled on both sides of every second layer of veneer. Then the packages of veneer coated with glue are laid-up and glued together in a hydraulic or pneumatic press.
If the glue is applied at room temperature (from 21 °C to 27 °C), pressed plywood is usually stacked and kept for 6 hours or even more for the glue to harden. The most effective way of pressing plywood glued with synthetic thermosetting adhesives is hot-pressing. It takes only a few minutes to produce several sheets of plywood in a hydraulic press, which is a multi-tiered set of plates with steam heating to the temperature 120-150 °C. Laid-up and pre-pressed packages (future sheet of plywood) are laid between each pair of such plates. There are also presses with electrically heated tool resistors or with high-frequency electric field.

Metallized plywood or plywood with metal coating is used when solid and dense surface is necessary, for example, in truck bodies, or if there are special requirements for wear resistance and fire safety of the material.

Another option is plywood of high density. For its production the veneer is compacted significantly during hot pressing.

Grades of plywood
Plywood is divided into five grades, depending on the appearance of the outer layers:
• E (elite)
• I
• II
• III
• IV.

The grade of plywood is indicated by the combination of front and reverse veneer layers: grade of the front layer / grade of the reverse layer, for example: E / I, I / I, I / II, II / II, II / III, II / IV, III / III, III / IV, IV / IV, etc.

Grade I - up to 3 pin knots, sound knots up to 15 mm, light knots up to 6 mm, 2 closed cracks no more than 200 mm long per 1 meter, normal color change no more than 5%. Other wood defects are not allowed.

Grade II - pin knots, sound accrete knots up to 25 mm, light knots up to 6 mm, 2 closed and open cracks 2 mm wide, up to 200 mm long, normal color changes, wood inserts are allowed.

Grade III - sound knots, light knots up to 6 mm, 2 closed cracks, open cracks 2 mm wide, up to 300 mm long, normal color changes, wooden plugs, indentations, ripples are allowed.

Grade IV - light knots up to 40 mm are allowed without limit of quantity, open cracks with a width of not more than 10 mm without restriction, wooden plugs, indentations, ripples are allowed.

Thickness

Plywood is a very popular material and it is widely used in furniture industry.

More than that, plywood is so popular due to its light weight, easy handling, resistance to changes in temperature. In addition, the aesthetic properties of plywood are high; it is easily combined with any other material. Plywood surpasses chipboard, fiberboard and MDF for many characteristics, such as thermal conductivity, ecological safety and strength.

It is much more economical to produce plywood than metal or plastic products; it is calculated by comparing the cost of plywood production with its operational characteristics. There are several different ways of plywood production. Types of plywood are divided by methods of veneer production: sawn plywood, sliced plywood and peeled plywood.

Sawn plywood is made only from valuable wood species. Raw materials for plywood production are cut into strips up to 5 mm thick. The consumption of raw materials in the production of sawn plywood is very high, and productivity is low. For this reason, at the moment, sawn plywood has almost ceased to be produced anywhere in the world.

It is clear from the name of the type that sliced plywood is produced by slicing the stock. At the same time, the veneer has a thickness of up to 3.5 mm. Nowadays, such veneer is produced mainly from valuable species of deciduous woods. The production of sliced plywood is effective to a fairly high degree, but still not as effective as production of peeled plywood.

Peeling is the most productive plywood manufacturing technology. Veneer thickness when peeling is 1.2-1.9 mm.

Plywood is made from one or several types of wood, but it is considered to be made of the same wood species as its outer layer. If plywood is made of only one species of wood, it is called homogeneous plywood. If several species are used, it is called combined plywood. In addition, plywood is divided into several types: glued, bakelized (characterized by high water resistance) and plywood for decorative purposes. Glued plywood can be of exterior grade which stands for plywood with a high water resistance index, and of interior grade, i.e. plywood with an average water resistance.

Exterior waterproof plywood is widely used for construction. Besides, plywood is a very popular material for such industries as shipbuilding, rail vehicles construction, commercial vehicles manufacturing. It is also used as reusable formwork in construction industry.