Plywood (German: Furnier, from French fournir – supply, lay) – sheet building material, which usually consists of several thin layers of wood (veneer) tightly glued in thickness, and the directions of the fibers in adjacent veneer layers are, as a rule, mutually perpendicular.
For the production of plywood mainly use birch veneer, to a lesser extent alder, beech, pine and other species. However, special types of plywood are also produced, designed for increased resistance to bending and torsion.
Industrial plywood – “special plywood”. Such plywood is considered special, which has acquired special properties as a result of any technological processes and is used in a certain area or several areas. Partly special can be considered large-format plywood (1220×2440; 1525×3050), which is widely used in construction in Europe and North America. Laminated plywood also fits this definition. This includes bazar plywood (“Finnish”, “waterproof”), as well as other brands of plywood with a narrow scope: veneered with fiberglass, sheet metal, cork chips, reinforced, biostable (antiseptic), composite, electrothermal, with a relief surface, etc. d.
Plywood with a structure symmetrical about the middle layer (center) best resists warping. Such a “sandwich” type structure is 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, etc. A sheet of veneer plywood is drawn from veneer layers of one tree species. Joinery veneered products with cladding and decorative finishes are widely used in the manufacture of furniture, doors, walls, etc.
The main quality parameters of plywood are as follows:
- sheet format
- type of gluing
Plywood sheets (dimensions 1220 x 2440 mm) of various kinds, different in quality, purpose and thickness, are stored and sold as ready-made building panels.
- Such panels are widely used in:
- wall cladding
- construction of partitions
the construction of various gutters, formwork, etc. In a cut form, they are used in many industries. In high-quality furniture industry products, flat surfaces are made of plywood, usually lined with expensive decorative veneers. Such plywood is made, as a rule, in a special workshop of a furniture company, since the quality and appearance of furniture significantly depend on the compatibility of facial 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 purpose, durability, appearance and cost. Among these types, for example, plywood for external works, for shipbuilding, for formwork and moisture-proof plywood are distinguished. In aircraft construction, plywood with a thickness of 1 mm or more is used; in house-building, ready-made plywood panels with a thickness of 6 to 25 mm are used; for the manufacture of furniture, plywood is used with a thickness of 3 mm (three-layer low-grade for the backs of mirrors and bottoms of drawers) to 30 mm (five-layer high-grade for countertops of desks). For products of various industries, plywood is required from 3 mm thick (for shaped trays and gutters) to 38 mm (for printing plates on linoleum). From cheap thin plywood, kits for containers that are stitched with nails are universally produced.
Most plywood is produced in the form of flat sheets or panels, but there are mechanized methods of production and bent products from it. Plywood can be glued by bending it along simple contours, for example, blanks for seats in public places or cases of radio equipment, etc. Typically, slightly curved plywood is pressed by clamping it between a pair of hot molds. Shaped blanks for cabins and cabins, aircraft parts, etc. are also produced from plywood.
Adhesives are prepared in mechanized mixers and in the manufacture of plywood, pairs of glue rollers (metal or hard rubber) are fed into the grooves, which are rolled on both sides of every second layer of veneer or lumber. Then, veneered with veneer glue packets are clamped and glued together in hydraulic or pneumatic, and sometimes in screw (least desirable) presses.
If the glue is applied at room temperature (from 21 ° to 27 ° C), then after that the sheets of glued plywood are usually folded into stacks and kept in them for 6 hours or more to harden the glue. Pressing plywood with synthetic thermosetting adhesives is most effective at elevated temperatures – by hot gluing, when it takes only a few minutes to produce finished sheets. For this, hydraulic presses are made in the form of a multi-tiered set of tool plates with steam heating from 120 ° to 150 ° C; between each pair of such plates a pressed package is laid – the future sheet of plywood. There are presses with electrically heated tool resistors or high-frequency electric field.
For the production of bent plywood, flat tool press plates are supplied with an appropriate pair of metal molds. In the manufacture of shaped plywood with a complex profile of one of the molds, an injection volume is used in an airtight shell, which in all directions presses on one side the molded plywood placed on a rigid stationary mold. For the polymerization of resin adhesives, either heated molds are used, or hot steam is pumped into the injection volume. Since the ductility of wood increases when it is heated, glues hardening at room temperature cannot be used to produce high-quality hard-profiled plywood.
Various types of glued plywood are produced:
- with lining kraft paper;
- with and without impregnation;
- suitable for external cladding and veneering.
Metallized plywood, i.e. glued plywood with layers of metal, is used where, according to operating conditions, a solid and dense surface is necessary, for example, in truck bodies, or where, thanks to the metal gasket, the wear resistance and fire safety of the material are increased.
High density plywood is also produced. Usually their sheets are drawn from layers of bazar plywood and pressed by hot gluing. The specific gravity of plywood of this type can reach a value of 1.3, and the wood of such plywood is significantly compacted during hot pressing.
Grades of plywood
Depending on the appearance of the outer layers, plywood is divided into five grades:
- E (elite)
The type of plywood is indicated by the combination of the types of veneer of the front and reverse layers: the variety of the front layer / variety 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 – pin knots up to 3 pcs., Healthy knots up to 15 mm, falling knots up to 6 mm are allowed, closed cracks are allowed no more than 200 mm in the amount of 2 pcs. 1 m wide, a healthy color change is allowed no more than 5%, other wood defects are not allowed.
Grade II – pin knots, healthy fused to 25 mm, falling to 6 mm, closed cracks and 2 mm open in the amount of 2 pcs are allowed. up to 200 mm long, healthy color changes, wood inserts.
Grade III – healthy knots are allowed, falling knots up to 6 mm, closed cracks, 2 mm open cracks in the amount of 2 pcs. up to 300 mm long, healthy color changes, wood inserts, waviness, ripples.
Plywood is a very common material, which is used, including, in the furniture industry. Plywood gained wide distribution due to its light weight, lightness and ease of processing, resistance to sudden changes in ambient temperature. In addition, the aesthetic properties of plywood are quite high, it is easily combined with any other material. According to many characteristics, such as thermal conductivity, environmental friendliness and durability, plywood surpasses chipboard, fiberboard and MDF.
It is much more economical to produce plywood than metal or plastic products, this is calculated by comparing the cost of plywood production with its operational indicators. Plywood is produced in several different ways. Types of plywood are divided by methods of veneer production into: sawn, planed and peeled.
The first type of plywood is sawn plywood, it is made only from valuable types of wood, raw materials for the manufacture of plywood are cut into strips up to 5 millimeters 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 all over the world has almost ceased to be produced.
From the name of the type of plywood, it is clear that planed plywood is produced by planing the workpiece. In this case, the veneer has a thickness of up to 3.5 millimeters. Now, such veneers are produced mainly from valuable species of deciduous trees. Sliced plywood has a fairly high degree of productivity, but this figure is still lower than that of peeled.
Peeling is the most productive plywood manufacturing technology. Veneer thickness for peeling 1.2-1.9 mm. The process of peeling itself resembles the work of a lathe, the difference is that for the workpiece – ridge, the top layer is removed over the entire width. Then a large canvas is cut to the desired format and glued.
Plywood is made from one or several types of wood, but plywood is considered to be made from the species of wood from which its outer layer is made. If plywood is made of only one species of wood, it is called homogeneous. If several species are used, then combined plywood. In addition, plywood is divided into several types: glued, bazar plywood (characterized by high water resistance) and plywood for decorative purposes. Of all three types of plywood, only glued has the following types: FSF – plywood with a high water resistance index, FC and FBA – an average water resistance indicator and FB – limited water resistance.
FSF waterproof plywood is used in construction to protect the exterior of houses (the plywood itself is additionally painted) and the interior (without painting) from moisture. In addition, plywood is used in shipbuilding, carriage and automotive industry as a building material for structures, and is also used as reusable formwork during concrete pouring in the construction industry.
In order to increase the strength and extend the life of the plywood, it is laminated. This is done by applying impregnated phenolic resin to the plywood. For construction purposes, laminated plywood with a mesh surface is made. This plywood is used when pouring concrete. When concrete sets, a mesh pattern is printed on its surface. This trick is used because it is much easier to apply finishing material to mesh recesses in concrete.