- Plywood history
- Plywood production technology
- Characteristics and classification of plywood
- How to choose plywood
In this Article: The History of Plywood Creation production technology; classification and characteristics of plywood; Criterias of choice.
Plywood surpasses in popularity all other wood-based composite lumber – in addition to construction and furniture industries, it is used in car building, aircraft and automobile construction, in ship construction, and even in the creation of missiles. The reasons for such a wide field of application have not changed for more than a century – plywood has a relatively low weight, its cost, with higher strength characteristics, is significantly lower than the price of industrial wood. Consider this wood-laminated board, which, according to the popular expression, is capable of “flying over Paris”.
The first samples of plywood, i.e. glued together thin plates of wood, found by archaeologists during the study of pyramids in Egypt and dated to 3,500 BC. – because of the economy of wood, the ancient Egyptian carvers were forced to take such a step, because wood in ancient Egypt was in short supply. In turn, the absence of their own wood and the high cost of imported wood forced the ancient Greeks and, subsequently, the Romans to resort to plywood, while its cost two millennia ago was far from low, because natural resin was used to glue the wooden plates and all production work was carried out manually.
It is noteworthy that almost all the inventors of the first veneer machines – the main raw material for plywood production – were somehow connected with Russia. The first model of the peeling machine at the end of the 17th century was created by mechanical engineer Samuel Bentham, who had previously served Catherine II at the invitation of Prince Potemkin. At the end of his 10-year service in Russia, Bentham returned to England and received patents for several of his inventions at once. However, the machine invented by the Englishman was not noticed by the manufacturers of those times..
Emmanuel Nobel Sr.
A really effective prototype of all modern shelling machines was created by the Swedish engineer-inventor Emmanuel Nobel, father of Alfred Nobel, founder of the Nobel Prize and inventor of dynamite. The model of a rotary (rotary) lathe created by him at the end of the 18th century made it possible to remove veneer of a certain and constant thickness from a wooden block, due to which the plywood “sandwich” became homogeneous in structure and thickness..
Emmanuel Nobel lived and worked in Russia from 1838 to 1859, the inventions created by him during this period were intended for the military industry and were highly valued by Emperor Nicholas I.
At the beginning of the 19th century, Russian industrialist Dietrich Martin Luther, who owned a pencil manufactory in Reval, Estonia (present-day Tallinn), invented his peeling machine, a larger version of a pencil-making machine. He received a patent for his invention in 1819.
The first plywood, the production of which was based on the machine of Dietrich Martin Luther, was created by the Estonian furniture maker Alexander Luther, his namesake. At the end of the 19th century, he decided to use veneer sheets glued together as seating for Viennese chairs – the furniture turned out to be light, durable and inexpensive, thanks to which there was a steady demand for it.
Almost simultaneously with the furniture maker Luther, plywood was created by the Russian inventor – Ogneslav Stepanovich Kostovich, who was engaged in the creation of aircraft and was in dire need of a structural material for their construction. In 1881, he invented arborite, a material consisting of veneer sheets glued together across the fibers. The peeling machine and glue for the production of arborite plywood were also invented by Kostovich independently, and his plywood was highly resistant to moisture and not subject to decay.
At the beginning of the 20th century, domestically produced plywood became widespread on the territory of Russia – by 1915 more than 50 factories for its production were opened with an annual volume of about 24,000 m3. The areas of application of plywood have expanded – in addition to furniture production and aircraft construction, it was used to create weapons, missiles, musical instruments and even technical pipelines..
Today Russia is one of the world’s largest producers of plywood and products from it, the annual production volume is about 3 million m3 of this wood-laminated board.
Plywood production technology
The following main stages in the production of plywood are distinguished: preparation of raw materials, obtaining veneer, creating plywood. Each of the three stages, in turn, consists of several technological operations, which we will consider in detail.
The preparation of wood raw materials begins with hydrothermal treatment – the logs intended for the production of veneer are laid in pools, where they are completely flooded with water heated to 40 ° C, covered with a lid and kept for 24 hours in summer or for two days in winter. To compensate for the water absorbed by the wood, a certain amount of warm water and condensed steam is constantly added to the pool. This operation is necessary to increase the plasticity of wood, otherwise, when peeling blocks, the cut veneer will tend to take the cylindrical shape of the log – hydrothermal treatment allows you to obtain a relatively flat veneer sheet, while its surfaces will be smoother and with fewer cracks.
Debarking logs in a debarking machine
The next preparatory operation is debarking logs, during which the bark is completely removed from their surface. Then the logs pass through the camera of the metal detector, then the conveyor system delivers them to the sawing machines. After cutting the logs by 1,300-1,600 mm in length and removing substandard fragments, they are delivered to the peeling site. Waste – bark and chips remaining after sawing logs and processing veneer at all subsequent stages – are sent to shredding machines, then go to the production of chipboard.
There are three known ways to obtain veneer – by peeling a block on a circular machine, planing tree trunks peeled from bark and by cutting a block into strips. The latter two methods are significantly less productive and therefore less widespread than peeling; they are used only for obtaining plywood from valuable species of deciduous trees. Next, consider obtaining veneer by peeling.
The prepared blocks are fed to the peeling machines, where they are cross-cut, as a result of which a veneer strip is produced, the width of which depends on the length of the original block, and the length depends on its diameter and the thickness of the cut veneer. The veneer strip, in turn, is cut into sheets of 1,300 by 1,420 mm and 1,300 by 2,740 mm (the length of the original block is 1,300 mm), as well as 1,600 by 1,740 mm and 1,600 by 3,420 mm (the length of the block 1 600 mm). Sheets of cut veneer are stacked and sent for drying, waste (fragments of “torn” veneer) – for shredding into chips.
In the process of drying, stacks of veneer laid on a roller conveyor undergo air heat treatment, then piecewise sorting of veneer sheets is performed using a scanner and a moisture meter. Wet and surface-defective veneer is separated from dry and flawless veneer, after which it is sent for re-drying, repair or temporary storage. The repair of veneer sheets with defects is carried out as follows – on special machines, substandard fragments are cut out, in their place are inserted strips of high-quality veneer, carefully selected in color and texture. The thickness of the strips inserted for replacement and the species of wood from which they are obtained must fully correspond to the repaired veneer sheet – the minimum width of the inserted strips is 450 mm, their moisture content is lower than that of the veneer sheet, by about 2-4%. Conditioned veneer is aged for 24 hours in a special warehouse, during this period a uniform moisture level is established in the wood. Veneer sheets subjected to repair operations are stored for no more than 8 hours, after which they are sent to the plywood production site.
Veneer trims and sheets with defects that impede high-quality gluing when building a plywood “sandwich” (long cracks, rot areas, significant holes from fallen knots and curved sheet trimming along the edges) are edge glued on the cutting and picking machine line. Built on a conveyor after cutting out defective areas, a web of veneer strips of a given size stacked to each other is subjected to butt gluing. During this operation, heated glue threads are introduced into the slots between the joints, then pressed against the rollers, and the veneer strips to be joined are tightly shifted towards each other. A strip of ribbed veneer strips is cut into sheets of a given size using saws.
The fibers in the veneer sheets obtained by peeling are directed exclusively transversely, and high-quality plywood can be obtained only by alternating veneer sheets with transverse and longitudinal fiber orientation. To comply with this condition and obtain veneer with longitudinal fibers, dry peeled veneer sheets are cut in half, then a groove is formed in each of them for a “mustache” connection, then a resin-based adhesive layer is applied to the chamfer of one of the halves. Before laying under the press, the joint line of the obtained veneer with longitudinal fibers is warmed up by an infrared ray.
At this stage, the preparation of the veneer is complete and the actual production of the plywood begins..
First step– assembly of packages from veneer sheets, selected according to a given grade of the future plywood sheet. The first and last in the package are veneer sheets with transversely directed fibers, and since the number of sheets in the package is usually odd, then every second sheet has a longitudinal direction of fibers. If the package contains an even number of veneer sheets, then two sheets with longitudinally directed fibers are placed in its middle, which avoids violations of the strength and geometry of the plywood surface. The formed pouches are cold pressed for better fit of the sheets forming them and imparting stiffness to the pouches..
Second phase– gluing packages in a hot press and receiving plywood. A layer of resin-based glue is applied to both planes of even veneer sheets in a special machine, then the packages are placed in a hot press, where the glue is glued and cured – for this operation, which takes place at a temperature of 110-130 ° C and a pressure of 1 to 1.2 MPa, takes about 10 minutes. Finished plywood sheets are stacked in piles up to 1.5 m in height in the workshop, where they are kept for a day for uniform cooling, relieving stress inside the sheets and achieving a uniform level of humidity.
In the third stageplywood sheets are cut to a given size, surfaces are sanded to achieve the same thickness and smoothness over the entire plane, and sorted by quality.
Characteristics and classification of plywood
According to the origin of the veneer that makes up plywood, it is subdivided into coniferous and birch. Birch plywood, the price of which is higher than coniferous veneer plywood, is most often used in furniture production, coniferous – in construction. At the same time, the inner layers of plywood, as a rule, are formed by hardwood veneer and only two outer layers – birch veneer or one of the conifers (spruce, pine, larch, fir and, in rare cases, cedar).
Plywood is marked according to the applied glue impregnation. Below are some plywood grades and characteristics depending on the impregnation:
- FSF (phenol formaldehyde glue). Achieved good wear resistance, mechanical strength and high resistance to moisture. FSF plywood is in high demand, it is used in various production areas and construction, especially in roofing work. Has a higher formaldehyde emission rate compared to other brands, therefore it is intended for outdoor installation;
- FC (urea-formaldehyde glue). Plywood with this marking is weakly resistant to moisture, is used for interior decoration and in furniture production;
- FB (bakelite varnish). Plywood of this brand perfectly resists any moisture level;
- FBA (albumin or casein glue). With its high environmental characteristics and a complete absence of formaldehyde emissions, FBA plywood is suitable only for dry interior spaces;
- BS (alcohol-soluble bakelite glue). This plywood is also called aviation plywood, because previously used only in aviation and shipbuilding. BS plywood is the most durable, wear-resistant, successfully resists almost any aggressive environment, is not susceptible to moisture;
- BV (water-soluble bakelite glue). This brand of plywood has all the performance characteristics of BS plywood, with the exception of moisture resistance.
More detailed classification and characteristics are given in GOST 3916.1-96 (hardwood plywood, including birch) and GOST 3916.2-96 (softwood plywood).
In terms of their performance characteristics, hardwood and coniferous plywood surpasses any composite panels on wood filler, as well as some types of wood. Any plywood is combustible and cannot withstand prolonged exposure to open flames or high temperatures, is resistant to weak chemical solutions, acids and is not resistant to organic solvents (alcohol, acetone, gasoline, etc.). Otherwise, plywood can only be compared with plywood itself, namely, its deciduous and coniferous species.
Softwood plywood, in particular fully formed by softwood veneer, in addition to phenol-formaldehyde impregnation, also has a natural resin impregnation. Thanks to this, it better resists fungus and moisture decay. Softwood veneer plywood weighs on? less hardwood (birch) plywood.
In turn, birch plywood is superior to coniferous in terms of physical and mechanical characteristics – it is more durable. The texture and pattern of birch plywood create a special “warm” atmosphere in the room, which makes it often used in decorative finishing.
Film faced plywood (marking “FOF”) is finished with thermoplastic films, acquiring high resistance to aggressive chemical environments. At the same time, film faced plywood has a higher price than similar non-laminated varieties; its main buyers are construction and manufacturing companies, as well as furniture manufacturers..
How to choose plywood
The quality of this brand of plywood is assessed by its appearance and marking. Knowing what information is hidden behind the marking, you will be able to assess how this type of plywood corresponds to the one declared by the manufacturer (the values of the abbreviation used to determine the brand of plywood are given above). So, the marking indicates the dimensions and thickness of the plywood sheet in millimeters (for example, 1500x3000x9 or 2440x1220x18), types of grinding (“NSh” – not polished, “Ш1” – one-sided grinding, “Ш2” – double-sided grinding), formaldehyde emission class ( “E1”, “E2” or “E3”). The inscription “pine / pine” or “birch / pine” means: in the first case, that both outer sides of the plywood sheet are made of coniferous veneer; in the second case, one outer side is made of birch veneer, and the other is made of coniferous.
Plywood packaging with markings
The plywood packaging indicates its grade, there are five of them:
- elite (marking “E”). It does not have any defects, it does not go to the public sale, because all batches of plywood of this grade are purchased by machine-building enterprises and furniture makers;
- first grade (marking “I” or “B”). The length of permissible cracks and other minor surface defects is not more than 200 mm, the total number of defects is not more than three;
- second grade (marking “II” or “BB”). Cracks no more than 200 mm, wood inserts (knots), areas of dried glue are acceptable, but no more than 2% of the sheet area. The maximum number of visible defects should not exceed six;
- third grade (marking “III” or “CP”). 6 mm wormholes are possible, but no more than 10 pcs. on m2, cracks up to 300 mm long, areas of dried glue not exceeding 6% of the sheet area;
- fourth grade (marking “IV” or “C”). As the worst quality grade, plywood of this grade can contain an unlimited number of wormholes with a diameter of no more than 40 mm, edge damage with a depth of no more than 5 mm, an unlimited number of fallen out and intergrown knots.
In marking plywood, the grade is usually indicated through an oblique separator (slash), for example, “II / IV” or “I / I”, i.e. in the first case, the grades of the outer surfaces of the plywood are different (one side of the second grade, the second – the fourth), in the second – both sides of the plywood of the same grade (in the example, the first).
Plywood of domestic production and China is presented on the Russian market, the second, as a rule, is cheaper, but at the same time it has a higher emission class and is not allowed to be installed indoors – be careful when choosing!