When making furniture, you cannot do without curved parts. You can get them in two ways - sawing and bending. Technologically, it would seem easier to cut out a curved part than to steam, bend and then hold it for a certain time until it is completely ready. But sawing has a number of negative consequences.
First, there is a high probability of cutting fibers when working with a circular saw (this is what is used with this technology). The consequence of cutting the fibers will be a loss of strength of the part, and, as a consequence, of the entire product as a whole. Secondly, sawing technology requires more material consumption than bending technology. This is obvious and no comment is required. Third, all curved surfaces of sawn parts have end and half-end cut surfaces. This significantly affects the conditions for their further processing and finishing.
Bending allows you to avoid all these disadvantages. Of course, bending requires the presence of special equipment and devices, and this is not always possible. However, bending is also possible in a home workshop. So, what is the technology of the bending process?
Technological process The production of bent parts includes hydrothermal treatment, bending of blanks and their drying after bending.
Hydrothermal treatment improves the plastic properties of wood. Plasticity is understood as the ability of a material to change its shape without destruction under the influence of external forces and to retain it after the action of the forces is eliminated. Wood acquires its best plastic properties at a humidity of 25 - 30% and a temperature in the center of the workpiece at the time of bending of approximately 100°C.
Hydrothermal treatment of wood is carried out by steaming in boilers with saturated steam. low pressure 0.02 - 0.05 MPa at a temperature of 102 - 105°C.
Since the duration of steaming is determined by the time it takes to reach a given temperature in the center of the steamed workpiece, the steaming time increases with increasing thickness of the workpiece. For example, to steam a workpiece (with an initial humidity of 30% and an initial temperature of 25 ° C) with a thickness of 25 mm to achieve a temperature in the center of the workpiece of 100 ° C, 1 hour is required, with a thickness of 35 mm - 1 hour 50 minutes.
When bending, the workpiece is placed on a tire with stops (Fig. 1), then in a mechanical or hydraulic press the workpiece together with the tire is bent to a given contour; in presses, as a rule, several workpieces are bent simultaneously. At the end of bending, the ends of the tires are tightened with a tie. The bent workpieces are sent for drying along with the tires.
The workpieces are dried for 6 - 8 hours. During drying, the shape of the workpieces is stabilized. After drying, the workpieces are freed from templates and tires and kept for at least 24 hours. After holding, the deviation of the dimensions of the bent workpieces from the original ones is usually ±3 mm. Next, the workpieces are processed.
For bent blanks, peeled veneer, urea-formaldehyde resins KF-BZh, KF-Zh, KF-MG, M-70, and particle boards P-1 and P-2 are used. The thickness of the workpiece can be from 4 to 30 mm. Blanks can have a wide variety of profiles: corner, arc-shaped, spherical, U-shaped, trapezoidal and trough-shaped (see Fig. 2). Such blanks are obtained by simultaneously bending and gluing together veneer sheets coated with glue, which are formed into packages (Fig. 3). This technology makes it possible to obtain products of a wide variety of architectural forms. In addition, the production of bent-laminated veneer parts is economically feasible due to the low consumption of timber and relatively low labor costs.
Layers of plots are smeared with glue, placed in a template and pressed into place (Fig. 4). After exposure under the press until the glue has completely set, the assembly retains its given shape. Bent-glued units are made from veneer, hardwood and coniferous species, made of plywood. In bent-laminated veneer elements, the direction of the fibers in the veneer layers can be either mutually perpendicular or identical. Bending of veneer, in which the wood fibers remain straight, is called bending across the grain, and in which the fibers bend, bending along the grain.
When designing bent-laminated veneer units that bear significant loads during operation (chair legs, cabinet products), the most rational designs are those with bending along the fibers in all layers. The rigidity of such knots is much higher than knots with mutually perpendicular directions of wood fibers. With the mutually perpendicular direction of the veneer fibers in the layers, bent-laminated units up to 10 mm thick are constructed, which do not bear large loads during operation (box walls, etc.). In this case, they are less susceptible to change in shape. The outer layer of such units must have a lobar direction of the fibers (bending along the fibers), since when bending across the fibers, small lobar cracks appear at the bending points, which exclude good finish products.
Acceptable (radii of curvature of bent-laminated veneer elements depend on the following design parameters: veneer thickness, number of veneer layers in the package, package design, bending angle of the workpiece, mold design.
When manufacturing bent-profile units with longitudinal cuts, it is necessary to take into account the dependence of the thickness of the bent elements on the type of wood and the thickness of the bent part.
In the tables, the elements remaining after the cuts are called extreme, the rest - intermediate. The minimum distance between cuts that can be obtained is about 1.5 mm.
As the bending radius of the slab increases, the distance between the cuts decreases (Fig. 5). The width of the cut depends on the bending radius of the slab and the number of cuts. To obtain rounded nodes, a groove is selected in the slab after veneering and sanding in the place where the bend will be. The groove can be rectangular or " dovetail" The thickness of the remaining plywood jumper (bottom of the groove) should be equal to the thickness of the facing plywood with an allowance of 1-1.5 mm. A rounded block is glued into the rectangular groove, and a strip of veneer is inserted into the dovetail groove. Then the plate is bent and held in the template until the glue sets. To give the corner greater strength with inside You can put a wooden square.
How to bend a board, plywood, fiberboard, MDF with your own hands
Often during the process repair work There is a need to obtain curved surfaces of products made from wood. How to bend a board so that the bend is strong and does not crack during the bending process? Well, if you have already decided to do major renovation with your own hands, then you should not retreat in the face of such difficulties. In this article we will talk in detail about how to give wood material a curved shape.
How to bend a tree?
No, our task is not at all to bend an innocent plant. We are talking about wood building materials. How to bend a tree so that it bends and does not break? Bending method wooden products known since ancient times: to give wood a shape, only heat and moisture are needed, under the influence of which the plasticity of the material increases with all the ensuing consequences. How to bend a tree? Hold it in hot water (the higher the temperature, the faster the processes occur) or steam ( a steam generator can be made from a kettle or use an iron). The higher the temperature, the faster the wood gives way and you can start bending it. Moistened and heated wood can be bent under the influence of a load (the ends of the board are placed on supports), and a load is placed in the place of the future bend. Dried wood perfectly retains the minimum radius of curvature that was achieved during the bending process. Now we know how to bend wood, we can dwell on this issue in more detail.
Wood reaction to external influences
The fact is that wood reacts differently to bending. The convex part is subject to tension, the concave part is subject to compression. Moreover, the material also reacts differently to steaming. For example, the ability to compress increases by as much as a third, but the ability to stretch - just a couple of percent. That’s why you shouldn’t think about how to bend a board more than two centimeters thick at home. It is also necessary to take into account that different types wood reacts differently to bending. For example, such species as oak, larch, and maple bend poorly, but beech, ash, and walnut bend well. So before you think about how to bend the board, decide on the type of wood from which it is made.
How to bend plywood, fiberboard, MDF
At home, plywood is bent by increasing its humidity, then ironing it (an iron is required), and fixing it in a template. Any template can be used frame element and it is not at all necessary that its shape should be curvilinear. The product is attached to the template using tape. Can be clamped bent plywood between two spacers, give it a bent shape using ropes, tying them around the product in several places along the radius of curvature. Plywood can only be used after it has dried. It seems that we have figured out how to bend plywood - let's move on.
How to bend fiberboard? The technique is the same as in the previous case! How to bend MDF? In this case, you can go two ways: either bend thin sheets (no more than 5 mm) and glue them together, or use flexible MDF. in which there are transverse slots on one side. The thickness of such sheets is usually 8 mm. When bending, they are placed on top of each other with their milled sides, and then glued together. That's all!
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When bending wood, you need to take into account many points: fresh wood is best suited for this task; you need to steam the wood for a certain amount of time, which depends on its thickness.
Processes for changing the properties of wood to meet demands: technology and properties Wood is a natural polymer composite material that changes its properties under mechanical and chemical influence. Knowing the patterns of material changes, you can create them purposefully, imparting the qualities required by the consumer. This is called the wood modification process. It is necessary in the production of chipboard, MDF, OSB, WPC and others wood materials, where shredded wood mixed with a polymer binder is pressed to produce a homogeneous material of standard sizes.
The proposed wood modification technology changes the properties of wood in the massif, that is, to the entire depth of the processed material, without resorting to grinding it. This is achieved by the fact that the molecules of the modifier, i.e., a substance that helps change the properties of wood, are comparable in size to the molecules of the wood substance and less than the size of the intercellular spaces in it. Therefore, by diffusion or forced impregnation under pressure, the modifier penetrates the entire thickness of the impregnated product, and then, under the influence of temperature and pressure, reacts with natural chemicals found in the wood substance.
Thus, the technology makes it possible not to chop wood, not to use expensive polymer binders, and to achieve the same effect that was achieved with MDF production, for example, but in a cheaper way. At the same time, it retains all its positive properties, the texture stands out brighter, you can change the color (lamination is not required). 
So, the modifier must penetrate into the cells in a dissolved state, be chemically active for the components that make up the woody substance, and, reacting with these components, purposefully change the physical and operational properties material. The most suitable substance for this is urea, because in the previously mentioned MDF or OSB, the most applicable binders are urea. Urea is soluble in water, including that contained in a bound state in wood, which means that by saturating the wood with an aqueous solution of urea, we, paradoxically, “dry” it, “taking” part of the wood moisture onto hydrophilic urea. Urea or urea actively reacts with components of wood substances such as lignin, hemicelluloses, and extractives. 
And since the polycondensation reaction occurs in the macromolecules of the wood substance, the solid wood acquires new characteristics specified by the manufacturer useful qualities, preserving the positive old ones. The urea solution is not harmful, chemically neutral, moreover, grade A urea according to GOST 6691-77 is used as a feed additive for livestock. Urea-modified wood is certified (GOST 24329-80) and is mainly used under the Destam or Lignoferum trademarks in the production of bearing shells. In the production of construction and joinery products, thermally modified wood is currently also used, the technology of which is similar to that proposed, except that the chemical modification of the wood substance is carried out in the absence of urea due to the polycondensation of decomposition products of lignin, hemicelluloses, extractives and xylans. 
Due to thermal degradation, the physical and mechanical properties of thermally modified wood are partially reduced. The technological process for producing mechanochemically modified wood consists of impregnating the original wood of any species and any moisture content with a modifier solution. Impregnation can be carried out using the “hot-cold bath” method - diffusion or in an autoclave - forced. Then drying is carried out, if necessary - with compaction (pressing), and heat treatment, which fixes the new properties of the wood. It should be noted that it is more economical to use low-value rocks, since their performance properties after modification exceed the properties of expensive rocks
How to bend wood correctly and in what ways?
Currently, manufacturers of wooden products prefer to do without this operation, and if they use bent elements, they are made from plywood. It's easier to bend plywood. It should be noted, however, that furniture makers natural wood They stopped pampering the buyer a long time ago. All furniture is made from wood board or fiberboard. Products from bent wood, be it a chair or something else, is without a doubt stronger, lighter and more elegant.
Wood selection
The success of bending largely depends on the type of wood chosen. Almost any species can be bent, but elm, oak, beech, etc. have the best flexibility. If carefully dried wood is needed for carpentry work, then in our case it is better to use freshly harvested wood. You should not use old (aged) wood. How younger tree, the more flexible it is. From the desired rock you need to select pieces without cracks or knots. At the very least, there should be no knots in the area of the intended bend. It is important that the wood is straight-grained, without strands, cross-layers and “screws”. It is best to prepare not sawn boards and beams, but solid round timber.
Making a blank 
Blanks for bending wooden elements It is best obtained not by sawing, but by splitting round timber. The direction of splitting should be along the chords of the circle to eliminate the core, which is fragile and unsuitable for bending. Wooden blocks and planks prepared in this way will not flake during bending. The future part is marked so that the direction of the bend coincides with the radius of the round timber from which the workpiece was split, and the outer side of the bend coincides with the outer part of the former round timber. The chipped workpieces are processed with planes until required sizes with a small allowance for finishing.
Steaming the workpiece
To give the workpiece the best plasticity, it must be steamed. To do this, you will need a metal container of a certain size. The whole workpiece will be “steamed” in it or only at the bend. The second is preferable, since it is more convenient to take the workpiece simply with your hands (without tools), which cannot be done if the workpiece is all steamed.
If this type of work is intended to be put on stream, then a special metal container can be made with a sealed lid and two holes for placing the bendable part inside the “steam room”. This entire simple structure must be tightly closed to reduce the escape of steam to the outside. Place a rubber gasket under the cover. Do not screw it tightly; it may swell or even explode under steam pressure. A fairly heavy lid will ensure a tight seal and still work safety valve with excessive pressure increase.
It is difficult to indicate the time for complete steaming. It depends on the type of wood, the cross-section of the workpieces, and the degree of dryness of the workpieces. You just need to take the workpiece out from time to time and test it for bending. The readiness of the workpiece can be felt immediately by its flexibility in bending.
Workpiece bending
It is best to bend the workpiece using a template. A blank bent and dried in a template will provide the part configuration we need. Moreover, if you need not one, but several completely identical parts.
With a certain skill, you can do as they do a shower for horse harness - the steamed workpiece is bent and the ends are tied with a rope. Leave it in this form until completely dry. Bent parts should be dried in a ventilated place protected from the sun. Trying to artificially speed up drying by heating can lead to cracking of the wood.
It should be noted that after removing the part from the template, it “gives up” a little, i.e. straightens up. Taking this property into account, the workpieces need to be bent a little “steeper” so that upon release you get required form. How “cooler” is a matter of experience. Much depends on the cross-section of the workpiece, the type of wood, and the degree of steaming before bending.
Press for bending wood materials
Manufacturer ORMA, Italy
Purpose
This equipment is intended for bending (bending) wood materials. Before bending, the blanks are steamed in specialized chambers. Stabilization of the workpiece is carried out by high-frequency current.
This equipment has found wide application in the manufacture of chairs, sleds, and school furniture.
Specifications: 
Complete bending kit includes
- Steaming chamber - a reservoir for moistening workpieces with a condensate collector, complete with a steam generator (a separate generator for each autoclave)
- Pre-bending press (necessary depending on tasks and productivity)
- The bending and stabilizing press (selected depending on the tasks and productivity), depending on the complexity of the product, can be equipped with additional side cylinders. The possible total force varies from 30 to 120 tons. Specific pressure up to 7.5 kg/cm2
- Electronic frequency generator – with the ability to work on two presses for bending and stabilization
Standards and structural strength of bent wood
In addition to the traditional use of bent, today construction materials are increasingly used structural elements made in this way. Usage load-bearing elements from bent wood allows you to create new interesting views architectural solutions, which, combined with the optimal economic indicators of such structures, explains the increased interest in them from outside practical application not only in industrial, but also in private housing construction.
There are two ways to make a curved structure from bent wood: sawing it out of boards, bending timber (solid bent products) or layers of wood and simultaneously gluing them together (bent-glued products). The process of bending wood is based on its ability under certain conditions under the influence of external loads change its shape and retain it in the future. 
It is clear that cutting a product from a board large sizes and curvature is almost impossible, therefore, in order to make a bent board or beam at home for the construction of a beautiful or dome crowning a decorative turret of a house, you should prepare everything necessary for bending wood. Just as website optimization allows you to increase the ranking of an Internet resource, so does the choice quality material for bending improves its result. An unedged board or timber without knots, with a cross-layer of no more than 10% of the surface area, is selected as a blank. The best varieties woods with increased plasticity are hornbeam, maple, beech, oak, ash and elm.
After the material has been selected, you can begin the bending process, the main stages of which are: hydrothermal treatment, bending the workpiece and drying the product. Optimal parameters conditions at which bending takes place with the highest quality are the wood moisture content in the range of 25-30% and the temperature in the center of the workpiece from 80 to 90°C.
Professional promotion of sites dedicated to the intricacies of wood bending technology will certainly arouse the interest of a wide audience, since the simplicity of this process is incomparable with the result obtained. Hydrothermal involves steaming or boiling the workpiece in hot water.
Steaming is technically more complex, so at home it is easier to organize the boiling of wood in a cooking tank of a suitable size. The workpiece removed from the cooking tank should be immediately secured to the tire using clamps while the wood is still warm. Otherwise, stress will arise in its outer layers, leading to cracks.
Flexible plywood and its application
Flexible plywood (bending plywood) is now in great demand due to the fact that it is comfortable material for the manufacture of structures that require rounding. The use of such bending plywood is effective and expedient, since it can take any necessary shape. Its flexibility allows you to embody the wildest fantasies of designers and produce the most fashionable and modern furniture, be it a closet with original design for your living room, cute shelves for the kitchen or modern and comfortable office furniture. 
This plywood is made from tropical trees, mainly from CEIBA wood, but sometimes flexible plywood is made from other woods: Parika, Keruing. Flexible (bending) plywood is, as a rule, a 3-layer board that is glued together in the transverse or longitudinal design of the shirts.
Flexible plywood, can be used for all types of bends, even with very small radii. No need to heat or treat with water. The self-supporting design of bending plywood makes the use of structural and special supports unnecessary. Unique design models, rounded designs and complex shapes with multiple radii that cannot be created from traditional materials are produced quickly and easily. Flexible plywood fulfills almost all thickness requirements by increasing the number of layers of material (for example, increasing the thickness to 10mm, 15mm, 16mm, 18mm, 20mm, etc.). Greater sheet thickness can be obtained by gluing several sheets of thinner bending plywood together.
High quality tropical plywood is a combination modern technologies And traditional materials. A product created to satisfy the most sophisticated needs of modern furniture and joinery manufacturers. Flexible plywood (bending plywood) is cheaper than pre-made wood forms. Significant time savings, less labor intensity and greater profitability are its advantages over any other method of changing the shape of plywood.
Except flexible plywood Our company offers another unique product - this is Ultra-light plywood. The range of use of this plywood is also quite wide: this is the production of door panels, the manufacture of cabinet furniture, sofas, armchairs, shelves. Ultra-light plywood is a new product on our market; it is 1.8 times lighter than birch. This plywood can be veneered well, finished with films and varnishes, and most importantly, it can significantly reduce the weight of the finished product!
Specifications
Bend direction Across the grain: along the width
Composition Hot-pressed tropical wood with heat-set adhesive
Density 300-400 kg/cub.m.
Thickness 5 mm, 8 mm, etc.
Dimensions 2500/2440 mm x 1220 mm, etc. by department. order
Bending radius For 5 mm thick, minimum 7 cm for 8 mm thick, minimum 10 cm
Elasticity
Perpendicular to grain: 210 N/mm2
Parallel to grain: 6300 N/mm2
(For 5mm panel at 10% humidity)
Store panels horizontally in a clean, shaded, dry place.
Apply glue to the panels, fixing the desired shape. After the glue dries, the panel will retain its shape. H.P.L. or the plywood can be glued both during the initial molding and at a separate, final stage.
You can use any wood glue.
The panels must be transported on solid flat surface. Individual panels can be rolled up, but they cannot be stored in this position for a long time.
Bending is widely used in industries such as shipbuilding. To begin with, there are a few basic rules that are always followed.
By steaming wood to bend it, you soften the hemicellulose. Cellulose is a polymer that behaves like thermoplastic resins. (Thanks to John MacKenzie for the last two suggestions).
To do this, you need heat and steam at the same time. In Asia, people bend wood over fire, but that wood is definitely quite wet - usually freshly cut. Shipbuilders in ancient Scandinavia prepared hull materials for their ships and placed them in a saltwater swamp so that they would remain flexible until they were ready for use. However, we are not always able to obtain freshly harvested wood for these purposes, and excellent results can be achieved by using conventional air-dried wood. It would be very good if, a few days before the operation itself, you immerse the workpieces in water so that they gain moisture - those Vikings knew what they were doing. You need warmth and you need moisture.
The main rule concerns steaming time: one hour for every inch of wood thickness.
Know that along with the probability of understeaming the workpiece, there is also the probability of oversteaming it. If you hovered an inch board for an hour and when you tried to bend it, it cracked, do not conclude that the time was not enough. There are other influencing factors that explain this, but we'll get to those later. Steaming the same workpiece for a longer time will not give a positive result. In such a situation, it’s a good idea to have a workpiece of the same thickness as that intended for bending and which you don’t mind. Preferably from the same board. They need to be steamed together and after the supposedly required time, take out a test sample and try to bend it into shape. If it cracks, then let the main workpiece steam for another ten minutes. But no more.
Wood:
Usually, the best option it will if you can find freshly cut wood. I understand that the cabinet makers will shudder at these words. But the fact remains that fresh wood bends better than dry wood. You can take a two-meter inch board from white oak, clamp one end of it in a workbench and bend it along any necessary curvature - fresh wood is so pliable. However, naturally, it will not remain in this state and you will still have to float it.
In shipbuilding, the main evil is rot. If you are concerned about this issue, then please note that the very fact of steaming fresh wood eliminates its tendency to rot. Therefore, you don’t have to worry - the frames of boats are usually made from fresh steam-bent oak and do not rot if it is taken care of. This also means that in this way it is possible to make at least blanks for the Windsor Chair. But air-dried oak also gives excellent results.
When selecting wood for bending, one thing to avoid is cross-layering. When trying to bend such a workpiece may burst.
Therefore, regarding wood moisture content, the rules are as follows:
1.Fresh wood is best.
2. Air-dried wood is a second good option.
3. - the third and very far from the first two option.
If all you have is from the dryer and you can’t get anything else - well, then you have no choice. But still, if you can get air-dried wood, it will be much better.
The first thing you need to do is steam wooden block. There are several ways to do this. Sometimes builders limit themselves to steaming in hot water, but it is best to build a special box into which streams of hot and humid air must be released. This is done very simply. Pour water into any saucepan or kettle and bring to a boil. Then a rubber tube is inserted into any hole in this container, and its other end is inserted into a pre-cut hole in the box. This way, moist and hot air will enter the box. As a rule, 1 hour of steaming is enough for the full effect. Try not to over-steam your bar, because... This won't lead to anything good either.
Step 2
Now you can begin the bending process itself. To do this, special templates are used, according to which the bend must be carried out, i.e. One part of the block is leaned against a wooden template and, with the help of strong pressure, it receives the same shape. It is important that one side of the bent bar is firmly secured. The likelihood of cracks appearing is extremely low, but it is still better to bend the block carefully, without strong and sudden pressure.
Step 3
After bending, the block must be left alone and allowed to dry. When it dries, it will not change its shape, so you can simply put it on a dry surface and hit sun rays place. After drying, it can be coated with varnish or paint.
