Types of plywood
Plywood, which is also called wood-laminated board (Chipboard), is a multilayer building material that is made by gluing pre-prepared (peeled) veneer, sometimes in composition with other materials. The number of veneer layers is usually odd (over three). To increase the strength of the chipboard sheet, the veneer layers are superimposed so that the wood fibers of each new layer are perpendicular to the bottom sheet. If the number of veneer layers in the plywood is even, then the two middle layers must necessarily have the same fiber direction. The plywood sheet has outer (front and back) and inner layers of veneer, which differ in quality and sometimes wood species. Types of plywood are classified by design and technological features that determine its operational qualities. Depending on the scope of application, plywood of general purpose is distinguished, which is made of three or more layers of veneer and is used in the manufacture of furniture, containers, and construction. Plywood of this type, depending on the adhesive used, can be of the following grades: plywood of increased water resistance using phenol-formaldehyde adhesives; medium water resistant plywood with urea-formaldehyde adhesives; medium water resistant plywood using albuminocasein glues.
The veneer of the outer and inner layers of plywood is made from wood of various species. Moreover, the type of plywood according to the material is determined by the type of wood from which its outer layers are made. In this case, the veneer layers, which are located symmetrically relative to the longitudinal axis of the plywood section, must be of the same thickness and from the same wood species.
Depending on the quality of the veneer of the outer layers, plywood is divided into five different grades - A / AB, AB / B, B / BB, BB / S, S / S. Also, according to standards, the following combination of face layers is allowed: A / B, A / BB, AB / BB, B / C. Plywood can be sanded or sanded on one or both sides. The roughness of unpolished plywood from hardwood should not exceed 200 microns, polished - 80 microns, and from coniferous wood - not more than 300 and 200 microns, respectively.
Plywood is produced in sheets with a length of 1220 to 2440 mm, a width of 725 to 1525 mm and a thickness of 1.5-18 mm. Plywood longer on one side than 1800 mm is called large format. Plywood, in which the larger size coincides with the longitudinal direction of the veneer fibers of the outer layers, is called longitudinal. Other types of plywood are called transverse. The moisture content of the plywood of the FSF and FC grades should be 5-10%, and the FBA grade chipboard should be 6-15%. Humidity of plywood used in furniture production should not exceed 10% according to the standard.
Facing plywood consists, as a rule, of three or more layers of peeled veneer. In this case, one or both of the outer layers of such plywood is made of facing material.
Facing can be an independent procedure or carried out when gluing parquet. Plywood, which is veneered with planed veneer, is divided into different grades, depending on the adhesive used: using phenol-formaldehyde adhesive (FOF) and using urea-formaldehyde adhesive (FOK). For veneering, planed veneers up to 1 mm thick are used. Veneers can be of various grades and textures: first and second grades, radial, semi-radial, tangential and tangential-face.
Plywood of the FOF brand is used in conditions of increased atmospheric effects (for example, in construction, shipbuilding, etc.). In order to give the surface of plywood greater resistance, the veneer is in some cases impregnated with melamine resin. Plywood, which is faced with paper, is made from peeled veneer with a facing coating that performs protective and decorative functions.
There is also decorative plywood, which, depending on the type of facing coating and resins used, is produced in several grades: DF-y, DF-3 with transparent colorless and colored coating based on sulfate paper, DF-2 and DF-4 with opaque paper coating with a drawing on it (usually with an imitation of wood texture).
In reinforced plywood, one or more of the inner layers of veneer is replaced with sheets of rubber, metal, metal nets and other materials that are more durable than veneer. This helps to increase the strength of the material without a significant increase in mass. To increase the elasticity of plywood and reduce its water permeability, it is reinforced with sheet rubber.
Profiled plywood is a layered material, the cross section of the sheet of which during gluing is given a complex shape (for example, trapezoidal or wavy). This allows you to achieve high rigidity of the plywood sheet along the profile.
Construction plywood is made from softwood veneer (larch and pine) 2-4.5 mm thick or from combined materials. A combined type of plywood is made with alternating layers of veneer from coniferous wood with a thickness of 2 mm and birch veneer with a thickness of 1.5 mm or only from alternating layers of birch veneer of different thicknesses. The outer layers of plywood are made of 1 mm thick birch veneer.
The quality of the outer layers of coniferous wood plywood is divided into six different types, depending on whether it is intended for transparent or opaque finish or for use in structures hidden from cutting. Combined plywood is produced in seven different grades. Construction plywood is produced using high water-resistant adhesives of the FSF and FC brands. It can be either polished or unpolished. The plywood used for construction is produced mainly in large formats with a size of 2440 by 1220 mm, thickness from 8 to 19 mm. Depending on the materials used, different areas of application of chipboard are distinguished. So, plywood from coniferous wood is intended for the construction of prefabricated panelboard, frame and mobile type structures, it is used in car building and for the construction of wooden houses. Combined plywood is used as sheathing material. And chipboard from birch, alder veneer and coniferous wood is produced for export.
Birch aviation plywood is made from high-quality thin birch veneer and is used for the manufacture of light aircraft, musical instruments. Bakelized plywood is made by gluing sheets of peeled birch veneer with the mutually perpendicular direction of the fibers in adjacent layers of phenol-formaldehyde resins. Such plywood can be of various grades, depending on the adhesive used and the method of its application: FSB, FBS1 (the outer layers are impregnated with an alcohol-soluble resin), PBV and FBV1 (the outer layers are impregnated with a water-soluble resin, and the alcohol-soluble resin is applied to the inner layers). For the manufacture of such plywood, veneer grade B for the exterior and grade BB for the inner layers are used. Such plywood is produced at high pressure with the use of a large amount of glue, so it is characterized by high strength.
For the manufacture of plywood, veneer is used from birch wood and less often than other hardwoods, and for the inner layers veneer from coniferous wood is often used. The choice of raw materials is determined by its prices and technological properties of materials. In this regard, the price-quality ratio of birch and coniferous wood is the most optimal. Sometimes other types of wood are used (for example, hornbeam, beech, poplar, maple, alder), but chipboard from them is more expensive. The technology for the production of plywood includes several main stages. Initially, wood is prepared for peeling. To do this, the raw material is first soaked in order to equalize the total humidity. All this is done almost manually. After soaking, the logs are transported to the peeling line. At the second stage, veneer sheets are made from pre-soaked logs. The size of the finished sheets depends on the size of the future plywood sheet. Veneer sheets are cut using rotary shears. To optimize the production process, all waste that is not used for the further production of plywood goes to the chipper, and then it is used for a heat-generating plant. At the third stage, the veneer is dried in a dryer - a multi-storey roller system, which provides for the floor-by-floor passage of the veneer from entrance to exit for a certain time.
However, the most useful are grain breads, which are prepared by extrusion. The production technology of such breads is slightly different. First, a wet mixture is made from grains, flour and eggs. Then it goes to tanks filled with hot air. Under its influence, loaves with a porous consistency resembling dense briquettes from tightly adhered and swollen grains are obtained.
The time is set using a special program, and the drying process itself takes place with the participation of a heat-generating unit that operates on peeling waste. Layers are first sorted, defective sections are cut out, after which all layers are glued together. When passing through the surface control system, the sheets are sorted by humidity, then impregnated with glue and stacked in layers into the so-called “sandwich”. Resin, bakelite film, adhesives based on phenol-formaldehyde resins, etc. are used for gluing veneers. Sheets with a large amount of scrap or low quality are sorted separately. All defective areas are also cut out of the panel, and patches are glued to their place. Then such sheets are used in the middle of the plywood sheet. At the penultimate stage, pre-prepared and glued veneer layers are sent under a thermal press. Under the influence of weight and temperature, the final form of plywood is formed. At the final stage, plywood, depending on its application, is given the required shape. Sheets are laminated and packaged.
The production of plywood for certain types and purposes may differ slightly from this scheme. So, for example, for the manufacture of peeled veneer with a tangel structure, special machines are used. When processing from log-churak, which is previously heat-treated, the upper thin layer of wood is carefully removed in a spiral. Such a veneer is laid in layers in a special way. The direction of the fibers of each layer in the plywood sheet is directed at the same angle from the layer that is in the center. The number of veneer layers in such plywood, as a rule, is also odd - from three to twenty-three.
Decorative plywood has a beautiful texture, since peeled sheet usually includes parts of several annual rings of the original wood material. Sometimes, for the manufacture of such plywood, a half-tree peeling method is used, in which slices are made parallel to the axis of the log diametrically to it (in other cases, around the circumference). Conical peeling allows you to achieve unusual slices, like chips in a machine for mechanical sharpening of pencils. Also, layers for decorative plywood can be planed (the so-called wave cutting). In this case, the peeling knives are ground in a wave-like fashion.
Decorative plywood, which is used mainly in the manufacture of furniture, is packaged in packs of sheets obtained from one tree trunk in the same way of cutting or peeling. Glued plywood is made in large sheets of various thicknesses. The most expensive plywood varieties are veneered on one or both sides with veneers of more valuable wood species.
To organize own production of plywood, you will need several special machines, including a calibration machine (to remove the top layer of wood from the log), a peeling machine (to remove veneer) and a cutting machine, which cuts the finished plywood sheet into the given dimensions; thermal press, areas for workshops and warehouses for storing finished products. All production facilities and sites must comply with the general requirements for labor safety. Separate production sites and equipment, the operation of which may be associated with the presence of harmful and dangerous health factors, are taken out to separate rooms. They should be equipped with ventilation and heating systems, purification of air containing dust and gases, have an appropriate level of natural and artificial lighting.
An enterprise that carries out a full cycle of plywood production should have the following employees: a shop manager, craftsmen, tillers, veneer cutters, a dryer, a sorter, baggers, a hot press operator, a woodworking machine tool operator, an adhesive production operator, and sorters-packers, mechanics, energy, technologists, cleaners, drivers. In addition, to work in the office, you will need purchasing managers, sales managers, an estimate engineer, an accountant, a cashier, a lawyer, an economist, a financial director, a marketing and advertising manager, a household manager, a system administrator, an office manager, etc. Of course, small manufacturing companies cost less and fewer employees. Some of them do not even spend money on equipment for the production of plywood, buying a finished veneer and gluing it almost manually to the finished sheets. Some are engaged in the production of plywood of a certain type (for example, laminated or bent). The presence of such a narrow specialization allows you to get by with a small starting capital and more likely to recoup your investments in the business.
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