Plywood
Plywood is a very promising material used in furniture making, aircraft engineering and shipbuilding, as well as in interior finishing and construction.
Plywood is a wood layered material, in particular, several glued sheets of peeled veneer.
Peeled veneer is, in its turn, a thin layer of wood in the form of a tape obtained after billet peeling using veneer machines. To make the future veneer properly flexible, billets are boiled in water heated to 50–60 °С before peeling.
As a result of machining, the tape of the specified width and thickness is obtained, which is then cut with special scissors into sheets of the desired size, dried, trimmed and graded. Then the plywood is made, the number of which layers may range from 3 to 23 (or from 4 to 40 mm), depending on the desired final thickness.
Two basic rules are observed in plywood making, in particular:
- Symmetry of the veneer sheet relative to the core stock.
- Parity of the number of layers (although this rule is not always observed).
In most cases, plywood is further treated with special resin and varnish to make the veneer sheet strong and water-resistant.
In terms of water resistance, there are three types of plywood:
- FK – this plywood is glued with urea resin. As a rule, this typical variant of plywood is used indoors;
- FSF – in this case, phenolic resin is used, which makes it possible to use veneer sheets not only indoors, but also outdoors;
- FB, or bakelized plywood, is the most resistant to aggressive conditions and sea water.
In some cases, plywood is also laminated, which makes it highly resistant to various natural phenomena and chemical conditions.
Plywood is classified by the degree of surface machining:
- NSh – typical, unpolished;
- Sh1 – plywood polished on one side;
- Sh2 – the best quality plywood by the degree of machining – polished from both sides.
Plywood and, more exactly, its veneer are usually made of birch or softwood.
Birch plywood has high strength due to physical and mechanical properties of the wood. Furthermore, it looks very aesthetic due to the beautiful structure and warm shades.
Softwood plywood is primarily made of pine, very attractive and harmonious; in addition to high strength, it is light-weight, which is appreciated in house-building.
Plywood can also be combined. In this case, the layers of pine and birch veneer alternate each other. Such plywood is not only attractive but has no less attractive low price, which makes it optimal for use in furniture making, interior finishing of premises and implementation of different structural solutions.
Another important factor in the selection of plywood is its grade, which is determined by the number of different snubs per square meter of the outer sheet surface.
Grade I – high-quality plywood produced in accordance with the strictest requirements. It has absolutely no defects. The maximum allowable defect is subtle brown flecks and some big fused snubs, not more than 8 mm in diameter.
Grade II – this plywood may have patches in the form of veneer inserts; but such plywood is covered with paint and various materials without any pretreatment.
Grade III – it typically includes defective products not assigned to Grade II. It is used in the manufacture of various containers and packaging, visually hidden structures. It is not very attractive but has good strength characteristics.
Grade IV – may include any manufacturing defects. The main thing is proper gluing, other defects are allowed in any quantity. It is suitable for solid containers and packaging.
In any case, plywood has a number of clear advantages compared with lumber. This is certainly strength in all directions, which is quite sufficient for many applications, good flexibility, and possible manufacture of large sheets. Other advantages may include low warping and cracking of plywood and ease of transportation. The ecological properties of the product are also attractive and important.