How to bend hardwood. Wood bending. Tips for bending lumber using hydrothermal preparation

Bending wood with steam. Or how to bend a strong, unbending oak into the shape you need without any problems.

I have been working with flexible wood for 13 years now and during this time I have built many steaming chambers and tested them in action. different systems steam generation. What you are reading now is based on reading literature and personal practical experience. Even mostly experience. I usually worked with oak and mahogany (mahogany). Had a little deal with thin birch veneer. I have not tried other breeds, because I am engaged in the construction and repair of boats. Therefore, I can not automately judge the work with other breeds cedar type, pine, poplar, etc. And since I didn’t do it myself, I can’t judge it. I write here only about what I personally experienced, and not just read in a book.

After this introduction, let's get down to business...

To begin with, there are a few basic rules that are always respected.

By steaming the wood to bend it, you soften the hemicellulose. Cellulose, on the other hand, is a polymer that behaves like resins - thermoplastics. (Thanks to John McKenzie for the last two suggestions).

To do this, you need heat and steam at the same time. I know that in Asia people bend wood and just over the fire, but that wood is definitely quite wet - usually freshly cut. Shipbuilders in ancient Scandinavia harvested materials for plating their ships and put them in a salt water swamp to keep them flexible until they needed to be put to work. However, we are not always able to obtain freshly harvested wood for this purpose, and excellent results can be achieved using ordinary 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 is about steaming time: one hour for every inch of wood thickness.

As I found out, along with the probability of understeaming the workpiece, there is also the probability of oversteaming it. If you have steamed an inch board for an hour and it cracked when you tried to bend it, you should not conclude that there was not enough time. There are other influencing factors that explain this, but we will return to them later. Longer steaming of the same workpiece will not give positive result. It is not bad in such a situation to have a workpiece of the same thickness as that intended for bending and which is not a pity. Preferably from the same board. It is necessary to steam them together and after the supposedly necessary time to get a test sample and try to bend it into shape. If it crackles, 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 carpenters-carpenters will shudder at these words. But the fact remains that fresh wood bends better than dry wood. I can take a two meter inch board from white oak, clamp one end of it in a workbench and bend it to any curvature I need - fresh wood is so malleable. However, of course, she will not remain in this state and she will still have to soar.

In shipbuilding, the main evil is rot. If you are concerned about this issue, then take note that the mere fact of steaming fresh wood eliminates its tendency to rot. Therefore, you don’t have to worry - the frames of the boats are usually made from fresh oak bent under steam and do not rot if cared for. It also means that at least blanks for the Windsor chair can be made in this way. However, I have also worked a lot with air-cured oak and the result was also excellent.

When selecting wood for bending, one thing should be avoided - the oblique layer. If you try to bend such a workpiece may burst.

Therefore, regarding the moisture content of wood, the rules are as follows:

Fresh wood is best.
Air-dried wood is a second good option.
Wood after the dryer is the third and very far from the first two options.

If all you have is after the dryer and nothing else to get - well, then you have no choice. I dealt with this as well. But still, if you can get air-dried wood, it will be much better. Just this week I was bending 20mm thick walnut planks for the transom of my yacht. The blanks were dried for several years and their bending went absolutely smoothly.

Steam chambers.

It is absolutely useless, and even harmful for the bending result, to strive to make an absolutely sealed chamber. The steam must leave her. If you do not provide steam flow through the chamber, you will not be able to bend the workpiece and the result will be as if you had been steaming it for only five minutes. This is familiar to me after all my experiences.

Chambers come in a variety of shapes and sizes. It should be large enough so that the workpiece is, as it were, in a suspended state and steam flows around it on all sides. A good result will be obtained from pine boards with a section of the order of 50 x 200. One way to provide "hanging" the workpiece is to drill through holes in the side walls of the chamber and drive hardwood round rods into it. With their help, the workpiece will not touch the bottom and the area of \u200b\u200bclosed wood will be minimal. However, you should not make a chamber of such a size that the amount of steam generated is not enough to fill its volume. The chamber should be such that it is humid inside and the steam rolls in waves. This means that the dimensions of the chamber must correspond to the capabilities of the steam generator (well, or vice versa).

When I needed to bend a five meter mahogany board with a section of about 200 x 20 for the new cabin of my yacht, I made a chamber from pine boards with a section of 50 x 300. A 20 liter metal tank acted as a steam generator. The power source was a propane burner. The thing is absolutely wonderful, because it is convenient and mobile. Performance 45000 BTU (1 BTU ~ 1 kJ). This is an aluminum cylinder on three legs and with one burner with a diameter of 200 mm.

I recently found a $50 160,000 BTU propane torch in West Marine's catalog and purchased that as well. With its help, I can bend the frames even for the "Constitution".

When I say "one hour of steaming per inch of thickness" I mean one hour of SERIOUS CONTINUOUS steaming. Therefore, the boiler must be designed to provide steam for the required time. I used a new 20-liter fuel tank for this purpose. It is possible to put blanks into the chamber only when the installation has reached full capacity and the chamber is completely filled with steam. It must be absolutely guaranteed that the water does not run out prematurely. If this happens and you have to add water, it is better to quit this business. Adding cold water will slow down the generation of steam.

One way to make the most of the water is to have the chamber at a slight slope so that the condensation water inside flows back into the boiler. But at the same time, it is necessary that the fitting through which steam enters is closer to the far wall. Another way is to make a siphon system that ensures the replenishment of its level as the water boils away.

Here is a photo of such a system:

In the picture you see wooden camera located slightly at an angle. Directly below it is the boiler of the steam generator. They are connected to each other by means of a hose from the radiator. If you look closely, you can see an L-shaped pipe coming out at the base of the boiler on the left. This is hard to see in the photo, but its vertical part is actually translucent and thus we will know about the water level inside the boiler. To the left of the cauldron is a white bucket containing make-up water. Look closely and notice the brown tube connecting the bucket to the vertical part of the pipe - the level gauge. Since the bucket is on a rise, the siphon effect is observed: with a drop in the water level in the main boiler, water enters it from the bucket. It can be topped up from time to time, but do it very carefully so that it does not rush quickly into the boiler and cool it too much.

To minimize the need to add water during the steaming process, it is better to start work with a bucket filled to the top. I myself prefer to leave a small air gap in the boiler.

Many chambers have a door at the end, through which you can move the workpieces if necessary and remove them if necessary. For example, if you're in the business of making bent frames and you'd like to do it in as little as a day, you fire up the boiler and (when you're at full capacity) put the first blank inside. After 15 minutes, put the second. After another 15 - the third and so on. When it's time for the first one, you take it out and bend it. I assume this procedure will take less than 15 minutes. When she sits still, the second one is already on the way ... and so on. This allows you to do a great job and avoid oversteaming.

The door has another important function. It doesn't even have to be made of hard material - on my small camera, just a hanging rag serves for this purpose. I say "hanging" because the steam must come out from the end (since steam flow is needed). It must not be allowed that excess pressure will be created in the chamber, making it difficult for steam to enter inside. And besides, the picture itself wooden box, from which steam pours out in clubs, looks quite cool - passers-by just get stupefied. The second purpose of the door is to prevent cold air from entering the chamber from below the workpieces.

So, we will assume that our wood is being cooked (with pleasant smell) and the templates are ready. Try to organize everything in such a way that the operation of removing the workpiece from the chamber and bending it is quick and smooth. The most important thing here is time. You have a matter of seconds for this. As soon as the wood is ready, quickly take it out and immediately bend it. How fast does human dexterity allow. If pressing against the template takes time, simply bend with your hands (if possible). For the frames of my yacht (which have a double curvature), I took the blanks out of the chamber, put one end into the clamp and bent this end and then the second one simply with my hands. Try to provide more fold than is necessary for the template, but not by much. And then attach it to the template.

But once again I repeat - the curvature of the wood must be given immediately - within the first five seconds. With every second that wood cools, it becomes less pliable.

The length of the blanks and the curvature at the ends.

It is practically impossible to make blanks of the exact length and expect that it will be possible to provide a bend at the ends. You just don't have the power to do it. For this reason, if you need a workpiece a meter long, and its thickness is more than 6 mm, you'd better cut off a piece of two meters and bend it. I'm just going by the assumption that you don't have a hydraulic press in your workshop - I certainly don't have one myself. When cutting a blank with a margin, remember that the shorter it is, the more difficult it will be to bend.
And if it is with a margin, then the end of the real part will have a large curvature - for an inch oak board, the last 150 mm are absolutely straight. Depending on the radius required at the end, it may be necessary to resort to wood carving in such places and take into account the required thickness when choosing a material.

Templates.

After steaming the workpiece and clamping it on the template, it is necessary to wait a day for complete cooling. When the clamps are removed from the workpiece, it straightens somewhat. The degree of this depends on the structure and type of wood - it is difficult to say in advance. If the workpiece already has some natural curve in the desired direction that can be exploited (I try to do this whenever possible), the degree of straightening will be less. Therefore, if you need a certain curvature of the final product, the template should have more curvature.

How much bigger?

Here we are dealing with pure black magic and I personally cannot give you any figures. One thing I know for sure: it is incomparably easier to unbend an excessively bent workpiece than to bend a cold, unbent one (provided that you do not have a giant lever).

Warning. If you are bending blanks for lamination, the template should be exactly the shape of the blank in the laminate - I have rarely had a lot of flex in well bent laminated wood.

There are endless options for bending templates. And it doesn't matter which one you choose if you happen to own a clamp factory - you can never have too many. If wood is bent with a thickness of more than 12 mm, the template must have a significant mechanical strength- he will experience quite high loads. You can see how it looks in the photo at the beginning of the article.
Quite often, when bending, people use a metal strip on the outside of the bend. This helps to evenly distribute stresses along the length of the workpiece and avoid cracks. This is especially true if the fibers are located outside at an angle to the surface.

Well, that's probably all my thoughts at the moment.

In the manufacture of furniture, you can not do without curvilinear parts. You can get them in two ways - sawing and bending. Technologically, it would seem that it is easier to cut a curved part than to steam, bend and then withstand it for a certain time until it is fully ready. But sawing has a number of negative consequences.

Firstly, there is a high probability of cutting the fibers when working with a circular saw (it is used in this technology). The result of cutting the fibers will be the loss of strength of the part, and, as a result, of the entire product as a whole. Secondly, sawing technology involves a greater consumption of material than bending technology. This is obvious and no comment is required. Thirdly, 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 avoids all these disadvantages. Of course, bending involves the presence of special equipment and fixtures, and this is not always possible. However, bending is possible in the home workshop. So, what is the technology of the bending process?

Technological process manufacturing 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 property of a material to change its shape without destruction under the action of external forces and retain it after the action of forces is eliminated. Wood acquires the best plastic properties at a moisture content of 25 - 30% and a temperature in the center of the workpiece by the time of bending of about 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 the set temperature in the center of the steamed workpiece, the steaming time increases with increasing thickness of the workpiece. For example, for steaming a workpiece (with an initial humidity of 30% and an initial temperature of 25°C) with a thickness of 25 mm to reach a temperature in the center of the workpiece of 100°C, 1 hour is required, and a thickness of 35 mm - 1 hour and 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 pulled together with a coupler. The bent blanks are sent for drying along with the tires.

The workpieces are dried for 6-8 hours. During drying, the shape of the workpieces stabilizes. After drying, the blanks are freed from templates and tires and kept for at least 24 hours. After holding, the deviation of the dimensions of the bent blanks from the original ones is usually ± 3 mm. Next, the blanks are processed.

For bent blanks, peeled veneer, urea-formaldehyde resins KF-BZh, KF-Zh, KF-MG, M-70, 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: angular, arc-shaped, spherical, U-shaped, trapezoidal and trough-shaped (see Fig. 2). Such blanks are obtained by simultaneous bending and gluing together veneer sheets lubricated with glue, which are formed into packages (Fig. 3). This technology makes it possible to obtain products of a wide variety architectural forms. In addition, the production of bent glued parts from veneer is economically feasible due to the low consumption of timber and relatively low labor costs.

Layers of plots are smeared with glue, laid in a template and pressed in (Fig. 4). After exposure under the press until the glue has completely set, the knot retains the shape given to it. Bent glued knots are made from veneer, from hardwood and conifers, from plywood. In curved glued veneer elements, the direction of the fibers in the veneer layers can be either mutually perpendicular or the same. The bending of the veneer, in which the wood grains remain straight, is called a bending across the grains, and in which the fibers are bent - bending along the grains.

When constructing bent glued veneer assemblies that carry significant loads during operation (legs of chairs, cabinet products), the most rational designs are those with a bend along the fibers in all layers. The rigidity of such knots is much higher than knots with mutually perpendicular direction of wood fibers. With a mutually perpendicular direction of the veneer fibers in the layers, bent glued knots up to 10 mm thick are designed that do not carry heavy loads during operation (walls of boxes, etc.). In this case, they are less susceptible to form change. The outer layer of such knots should have a fractional direction of the fibers (bending along the fibers), since when bent across the fibers, small fractional cracks appear at the bending points, which exclude a good finish of the product.

Permissible (curvature radii of curved veneer elements depend on the following design parameters: veneer thickness, number of veneer layers in a package, package design, billet bending angle, mold design.

In the manufacture of 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.

With an increase in the bending radius of the plate, the distance between the cuts decreases (Fig. 5). The width of the cut depends on the bending radius of the board and the number of cuts. To obtain rounded nodes, in the slab, after veneering and grinding, a groove is selected in the place where the bend will be. The groove can be rectangular or dovetail. The thickness of the remaining plywood bridge (the bottom of the groove) should be equal to the thickness of the facing plywood with an allowance of 1-1.5 mm. A rounded bar is inserted into the rectangular groove with glue, and a strip of veneer is inserted into the dovetail groove. Then the plate is bent and kept in the template until the glue sets. To give the corner greater strength in it with inside you can put a wooden square.

When bending wood, there are many things to consider: fresh wood is best suited for this business, it takes a certain amount of time to steam the wood, which depends on its thickness.

Processes for changing the properties of wood to meet requests: technology and properties Wood is a natural polymeric composite material that changes its properties under mechanical and chemical action. Knowing the patterns of material change, it is possible to create them purposefully, giving the qualities required by the consumer. This is called the wood modification process. It is necessary when chipboard production, MDF, OSB, WPC and other wood-based materials, where chopped wood mixed with a polymer binder is pressed to obtain a homogeneous material of standard sizes.
The proposed wood modification technology changes the properties of wood in the array, that is, to the entire depth of the processed material, without resorting to its grinding. This is achieved by the fact that the molecules of the modifier, i.e., the substance that contributes to changing 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 means of 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 allows not to grind 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 is preserved with all its positive properties, the texture stands out brighter, you can change the color (lamination is not required).
So, the modifier in the dissolved state must penetrate into the cellular, be chemically active for the components that make up the wood 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 carbamide. Carbamide is soluble in water, including that which is contained in a bound state in wood, which means that by impregnating wood aqueous solution carbamide, we, paradoxically, “dry” it, “taking away” part of the wood moisture for hydrophilic urea. Carbamide or urea actively reacts with such components of the wood substance as lignin, hemicelluloses, and extractives.
And since the polycondensation reaction occurs in the macromolecules of the wood substance, the wood mass acquires new parameters set by the manufacturer. useful qualities while retaining the positive old ones. The solution of carbamide is not harmful, chemically neutral, moreover - urea grade A 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 trademark "Destam" or "Lignoferum" in the production of bearing shells. In the production of building and joinery products, thermally modified wood is also currently used, the technology of which is similar to the proposed one, except that the chemical modification of the wood substance is carried out in the absence of carbamide due to the polycondensation of the 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 the production of mechanochemically modified wood consists in the impregnation of the original wood of any species and any humidity with a modifier solution. Impregnation can be carried out by the method of "hot-cold baths" - 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 wood. It should be noted that it is more economical to use low-value breeds, since their operational properties after modification are superior to those of expensive breeds.

How to bend wood correctly and in what way?

Currently, manufacturers of wooden products prefer to do without this operation, and if they use bent elements, then from plywood. It's easier to bend plywood. It should be noted, however, that furniture makers natural wood have long ceased to pamper the buyer. All furniture is made of wood or fiberboard. Products made of bent wood, whether it be a chair or something else, are without a doubt stronger, lighter and more elegant.

Choice of wood

The success of bending largely depends on the type of wood chosen. Almost any breed can be bent, but elm, oak, beech, 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. Do not take old (by age) wood. How younger tree, the more flexible it is. From the desired breed, you need to choose pieces without cracks, without knots. At the very least, knots should not be in the place 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 workpiece

Blanks for bending wooden elements are best obtained not by sawing, but by splitting round timber. The direction of the split should be along the chords of the circle in order to exclude a fragile and unsuitable for bending core. prepared in this way wooden bars and planks will not flake off 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 chipped, and outer side the fold coincided with the outer part of the former round timber. Chipped workpieces are processed with planers to the right 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 need a metal container of a certain size. In it, the blanks will be “steamed” in their entirety or only in the place of the bend. The second is preferable, since it is more convenient to take the workpiece simply with your hands (without devices), which you cannot do if the workpiece is all steamed.

If a this species work is supposed to be put on stream, then you can make a special metal container with a sealed lid and two holes for placing the bent part inside the "steam room". This whole simple structure must be tightly closed in order to reduce the release of steam to the outside. Place a rubber gasket under the cover. It is impossible to screw tightly, swelling or even an explosion under steam pressure can occur. A sufficiently heavy lid will ensure tightness and at the same time work safety valve when the pressure is too high.

It is difficult to specify the time for complete steaming. It depends on the type of wood, the section of the blanks, the degree of dryness of the blanks. Just from time to time you need to take out the workpiece and try it for bending. The readiness of the workpiece will be felt immediately by bending compliance.

Workpiece bending

It is best to bend the workpiece in a template. The blank, bent and dried in the template, will provide the part configuration we need. Especially if you need not one, but several completely identical parts.

With a certain skill, you can do the same as a shower for a horse harness is made - the steamed blank is bent and the ends are tied with a rope. In this form, leave until completely dry. It is necessary to dry bent parts in a ventilated place protected from the sun. Attempting to artificially accelerate drying by heating can lead to cracking of the wood.

It should be noted that after removing the part from the template, it “loses” a little, i.e. straightens up. Given this property, the blanks must be bent a little “cooler” so that the desired shape is obtained upon release. How much "cooler" is a matter of experience. Much depends on the section of the workpiece, the type of wood, the degree of its steaming before flexible.

Press for bending wood materials

Manufacturer ORMA, Italy

Purpose
This equipment is designed 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, sledges, school furniture.

Specifications:

The complete bending kit includes
- Steaming chamber - a reservoir for moistening blanks with a condensate collector, complete with a steam generator (a separate generator for each autoclave)
- Pre-bending press (required depending on tasks and performance)
- Bending and stabilization press (selected depending on the tasks and productivity), depending on the complexity of the product, it can be equipped with additional side cylinders. The possible total force is vibrated 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

Norms and structural strength of bent wood

In addition to the traditional use of bent, building structural elements made in this way are increasingly being used today. Usage load-bearing elements from bent wood allows you to create new interesting views architectural solutions, which, in combination with optimal economic indicators such structures explains the increased interest in them from the side 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 with their simultaneous gluing (curved glued products). The process of bending wood is based on its ability, under certain conditions, under the influence of external loads, to change its shape and keep it in the future.

It is clear that to cut a product from the board large sizes and curvature is almost impossible, therefore, in order to make a bent board or beam at home for building a beautiful or a dome crowning a decorative turret at home, you should prepare everything you need for bending wood. Just as website optimization allows you to increase the rating of an Internet resource, so the choice quality material for bending improves its result. Selected as preparation unedged board or timber without knots, with an oblique layer of not more than 10% of the surface area. The best varieties woods with increased plasticity are hornbeam, maple, beech, oak, ash and elm.

After the material is selected, it is possible to start the bending process, the main stages of which are: hydrothermal treatment, blank bending and product drying. Optimal parameters, at which the bending takes place with the highest quality, are the moisture content of the wood 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 surely arouse the interest of a wide audience, since the simplicity of this process is incomparable with the result obtained. Hydrothermal consists in steaming or boiling the workpiece in hot water.

Steaming is technically more difficult, so at home it is easier to organize the boiling of wood in the cooking tank right size. The workpiece removed from the brewing tank should immediately be secured to the rail with clamps while the wood is warm. Otherwise, stresses will arise in its outer layers, leading to cracks.

Flexible plywood and its application

Flexible plywood (plywood for bending) is now in great demand, due to the fact that it is comfortable material for the manufacture of structures in which rounding is necessary. The use of such bending plywood is effective and expedient, since it takes any necessary shape. Its flexibility allows you to embody the most daring fantasies of designers and produce the most fashionable and modern furniture, whether it's an original wardrobe for your living room, pretty shelves for the kitchen or modern and comfortable office furniture.
Such plywood is made from tropical trees, mainly from CEIBA wood, but sometimes flexible plywood is also made from other woods: Parika, Keruing. Flexible (bending) plywood is, as a rule, a 3-layer board, which is glued together in the transverse or longitudinal versions of the jackets.

Flexible plywood, can be used for all types of bends, even very small radii. It does not need to be heated and treated with water. The self-supporting construction of the bending plywood makes the use of structural and special supports unnecessary. Unique designer models, rounded structures and complex shapes with multiple radii that cannot be created from traditional materials are made 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.). Thicker sheets can be obtained by gluing several sheets of thinner bending plywood together.

tropical plywood High Quality is a combination modern technologies and traditional materials. A product designed to satisfy the most sophisticated needs modern manufacturers furniture and joinery. Flexible plywood (bending plywood) is cheaper than pre-made wood molds. Significant time savings, less labor intensity and greater profitability - these are its advantages over any other way to change the shape of plywood.

In addition to flexible plywood, our company offers another unique product - Ultralight plywood. The range of use of this plywood is also quite wide: it is the production of door panels, the production of cabinet furniture, sofas, armchairs, shelves. Ultralight plywood is a new product on our market, it is 1.8 times lighter than birch. This plywood is well veneered with veneer, finished with films and varnishes, and most importantly, it allows you to significantly reduce the weight of the finished product!

Specifications

Direction of bend Across the grain: along the width

Composition Hot pressed tropical wood with thermal setting adhesive

Density 300-400 kg/cu.m.

Thickness 5mm, 8mm, etc.

Dimensions 2500/2440 mm x 1220 mm etc. order

Bending radius For 5 mm thick, min. 7 cm for 8 mm thick min. 10 cm

Elasticity
Perpendicular to fibers: 210 N/mm2
Parallel to fibers: 6300 N/mm2
(For 5mm panel at 10% humidity)

Store panels horizontally in a clean, shady, dry area.

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 either during the initial molding or in a separate, final step.

You can use any wood glue.

Panels must be transported on a hard, flat surface. Individual panels can be rolled up, but they should not be stored in this position for long periods of time.

Bending is widely used in industries such as shipbuilding. To begin with, there are a few basic rules that are always respected.

To do this, you need heat and steam at the same time. In Asia, people bend wood and just over a fire, but that wood is definitely quite wet - usually freshly cut. Shipbuilders in ancient Scandinavia harvested materials for plating their ships and put them in a salt water swamp to keep them flexible until they needed to be put to work. However, we are not always able to obtain freshly harvested wood for this purpose, and excellent results can be achieved using ordinary 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 is about steaming time: one hour for every inch of wood thickness.

Be aware that along with the probability of understeaming the workpiece, there is also the possibility of oversteaming it. If you have steamed an inch board for an hour and it cracked when you tried to bend it, you should not conclude that there was not enough time. There are other influencing factors that explain this, but we will return to them later. Longer steaming of the same workpiece will not give a positive result. It is not bad in such a situation to have a workpiece of the same thickness as that intended for bending and which is not a pity. Preferably from the same board. It is necessary to steam them together and after the supposedly necessary time to get a test sample and try to bend it into shape. If it crackles, then let the main workpiece steam for another ten minutes. But no more.

Wood:

As a rule, the best option will be if you can find freshly cut wood. I understand that the carpenters-carpenters 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 of white oak, clamp one end of it in a workbench and bend it to any desired curvature - fresh wood is so malleable. However, of course, she will not remain in this state and she will still have to soar.

When selecting wood for bending, one thing should be avoided - the oblique layer. If you try to bend such a workpiece may burst.

Therefore, regarding the moisture content of wood, the rules are as follows:
1.Fresh wood is best.
2. Air-drying wood is the second good option.
3. - the third and very far from the first two options.

If all you have is after the dryer and nothing else to get - well, then you have no choice. But still, if you can get air-dried wood, it will be much better.

If you decide to decorate the room with wood or start creating beautiful furniture in a classic style - then you will need to make curvilinear parts. Luckily, wood is a unique substance, as it allows the experienced craftsman to play with the shape a bit. It's not as difficult as it seems, but it's not as easy as you'd like.

Previously, the site already had a publication on plywood bending. In this article, we will understand the principles of bending massive board and timber, we will find out how they do it in production. We will also bring helpful tips from professionals who will be useful to the home craftsman.

Why bending is better than sawing

A curvilinear wooden part can be obtained in two ways: by bending an even workpiece, or by cutting out the required spatial shape. The so-called "cutting" method attracts users with its simplicity. For such manufacture of parts and structures, you do not need to use complex devices, you do not have to spend a lot of time and effort. However, in order to cut out a curvilinear wooden product, you have to use obviously too large a workpiece, and a lot of valuable material will be irretrievably lost as waste.

But the main problem is the performance of the resulting parts. When cutting a curved part from ordinary edged lumber, the wood fibers do not change their direction.
As a result, transverse cuts fall into the zone of radii, which not only worsen the appearance, but also significantly complicate the subsequent finishing of the product, for example, its milling or fine grinding. In addition, on the most vulnerable mechanical impact rounded areas, the fibers run across the section, which makes the part prone to breaking in this place.

Whereas when bending, the opposite picture is usually observed, when the wood only becomes stronger. On the edges of the curved beam or board do not go "end" sections of the fibers, so subsequently you can process such workpieces without restrictions, using all standard operations.

What happens in wood when it bends

Bending technology is based on the ability of wood, while maintaining its integrity, to change its shape within certain limits as force is applied, and then retain it after removal of mechanical stress. However, we all know that without preparatory activities lumber is elastic - that is, it returns to its original state. And if the applied forces are too great, then the beam or board simply breaks.

The layers of a wooden blank work differently when bent. Outside the radius, the material is stretched, inside it is compressed, and in the middle of the array, the fibers practically do not experience significant loads and have little resistance to the forces acting on the workpiece (this inner layer is called “neutral”). Under critical deformation, the fibers on the outer radius are broken, and on the inner radius, “folds” are usually formed, which are a fairly common defect in the bending of softwood. The fibers of plastic hardwood or softwood can shrink by 20 percent or more, while the stretch limit is about one to one and a half percent.

That is, to determine the possibility for bending (without breaking) more important indicator will be the limit of relative elongation of the stretched layer. It directly depends on the thickness of the part and determines the radius to be obtained. The thicker the workpiece and the smaller the radius, the greater will be the relative elongation along the fibers. Having data on the physical properties of popular wood species, it is possible to formulate the maximum possible ratio of the thickness and radius of the parts for each of them. In numbers it will look like this:

Bending using a steel bar

Bending without using a bar

These data indicate that softwood lumber, in comparison with dense hardwood, is less adapted to free bending. To work with lumber at aggressive radii, it is imperative to use combined methods of preliminary preparation of parts and mechanical protection.

Tire as an effective way to avoid the destruction of wood during bending

Since the main problem is the breakage of the fibers from the outer radius, it is this surface of the workpiece that needs to be stabilized somehow. One of the most common methods is the use of an overhead tire. The tire is a steel strip with a thickness of half a millimeter to two millimeters, which covers a beam or board along the outer radius and is bent on a template along with the wood. The elastic strip absorbs part of the energy during stretching and at the same time redistributes the breaking load along the length of the workpiece. Thanks to this approach, coupled with moistening and heating, the allowable bending radius is significantly reduced.

In parallel with the use of a steel tire in bending devices and machines, mechanical compaction of wood is achieved. This is done using a pressing roller, which presses on the workpiece along the outer bending radius. In addition, the template mold in such a fixture is often endowed with 3 mm teeth (in increments of about 0.5 cm), oriented towards the workpiece travel.

The task of the jagged surface of the template is to prevent the workpiece from slipping, to prevent the mutual shift of the fibers in solid wood, as well as to create a small depressed corrugation in the concave radius of the part (the fibers are pressed into the array here, therefore, problems with folds are solved).

Pressing with a tire allows you to bend bars and boards from softwood and soft hardwood with a minimum percentage of rejects. Please note that parts from relatively hard rocks become about ten to twelve percent thinner when bending with pressing, and pine and spruce blanks become 20-30% thinner. But the positive aspects of this method include a significant increase in the strength characteristics of the finished product, as well as a significant reduction in the requirements for the presence of flaws and defects in wood blanks.

How to improve the plasticity of wood

In the normal state, lumber has elasticity, significant spatial rigidity and resistance to compression. Wood receives these valuable properties from lignin, a natural "network" polymer that gives plants a stable shape and strength. Lignin is located in the intercellular space and in cell walls, connecting cellulose fibers. In coniferous wood it contains about 23-38 percent, in hardwood - up to 25 percent.

Essentially, lignin is a kind of glue. We can soften it and turn it into a "colloidal solution" if we heat the lumber by steaming, boiling, treating with high frequency current (a household microwave is also applicable for small parts). After the lignin is melted, the workpiece is bent and fixed - cooling down, the molten lignin hardens and prevents the wood from returning to its original shape.

Practice shows that the optimum temperature for bending solid wood (bar, rail, board) will be 100 degrees Celsius. This temperature must be obtained not on the surface, but inside the workpiece. Therefore, in many respects, the time of thermal exposure will depend on how massive the part is. The thicker the part, the longer it will take to heat up. For example, if steaming is used to prepare a 25 mm thick strip (with a moisture content of about 28-32%) for bending, then on average it takes about 60 minutes. It is noteworthy that the exposure time under steam for parts of similar dimensions for any species is approximately the same.

By the way, it is believed that it is also impossible to overheat the part, since lignin after hardening can lose elasticity and become too brittle.

The boiling method is not often used, since the workpiece is strongly and unevenly moistened, and such water-saturated fibers and cells, when bent, can tear, at least with the formation of a pile. Parts after cooking have to dry for too long. But this method shows itself well if you need to process only part of the workpiece for bending.

Steaming allows you to heat the workpiece evenly, and its humidity at the exit tends to approach the optimum. The most suitable humidity for achieving maximum plasticity of sawn timber is considered to be in the range of 26-35 percent (the saturation point of the wood fibers).

To steam wood for bending at home, use home-made cylindrical chambers made of metal / polymer pipes or rectangular wooden boxes. Heating tanks act as a source of steam, electric kettles and other similar devices that can provide a temperature of about 105 degrees and a little pressure. This is always followed by the stage of drying the part (+ exposure of the fixed form) to about fifteen percent and its finishing.

Chemical methods of plasticizing wood

It is also known that it is possible to make lumber more pliable using impregnation with various compositions. There are ready-made impregnations that make wood cells more plastic, for example, Super-Soft 2. Some practitioners soak wood in so-called textile conditioners, with similar results.

But rather primitive “recipes” containing ammonia and ethyl alcohol, glycerin, alkalis, hydrogen peroxide, dissolved alum can also be used ... Many of them are extremely simple - they increase the ability of the workpiece to absorb water and help retain moisture in the fibers.

Thin products such as veneer are spray processed, but pre-treatment of normal lumber with chemicals is usually done by full immersion. It takes time for the working substances to get inside the bar or rail, usually it takes from 3-5 hours to several days (although heating helps to reduce the wait).

It is largely because of the duration of the processes that chemical plasticization is not often used, although there are other problems: the cost of chemistry, color changes, the need to provide protection from harmful fumes, the increased tendency of such curved parts to straighten ...

Tips for bending lumber using hydrothermal preparation

Very carefully select the quality of the workpiece for bending. It is better not to use material with cracks, knots (even live and intergrown), fiber inclination. If there are no options for this, then orient the part in the bending fixture (machine or template) so that the defects fall into the concave radius zone, and not into the tension zone on the outer radius. Give preference to the bending method with a bar.
When selecting a workpiece, it is imperative to provide for a change in the size of the part after molding. For example, the thickness of a coniferous bar can be reduced by 30 percent if bending with pressing is performed.
Even if you plan on extensive finishing, don't leave too much material. The thinner the workpiece, the easier it bends without breaking.
If the amount of work is small, then it is better not to cut out blanks, but to prick them from chocks. So it is possible to avoid cutting the fibers and, as a result, marriage during bending.
For bending, it is desirable to use lumber with natural humidity. If dry blanks are used, then preference should be given to those that have not been processed in drying chamber, and dried under a canopy - in an atmospheric way.
After steaming, work with softened wood very quickly, as lignin begins to harden almost immediately, especially in the most vulnerable outer layers of solid wood. Usually you need to focus on a margin of time from half an hour to 40 minutes, so there is no point in making large cameras if you simply do not have time to install all the material from which into templates.
Position the material in the steam chamber so that it is the surfaces facing the outer radius that are unobstructed by the steam jets.
To save time, many carpenters forgo the use of clamp templates. Instead, they use metal staples and wedges, or limit posts, on the templates.
Keep in mind that a curved bar or rail will still tend to straighten. And this straightening always occurs by a few percent. Therefore, when high precision is required in the manufacture of a part, it is necessary to conduct tests and, based on the results obtained, correct the shape of the template (reduce the radius).
After cooling the part in the form, let it stand for a while. Some experienced furniture makers prefer to make an exposure of 5-7 days. The tire, as a rule, is left fixed to the part for all this time.

Despite all their strength and strength, wooden details can be easily and simply bent, if suddenly during the construction process some special and original oval or round parts are required. Tree at proper processing it is easy to change its shape, and you can carry out this procedure on your own, without resorting to the help of professionals.

Types of work

There are two main ways to bend a tree to the desired shape, and one of them is cold, the other is hot. As the name implies, the methods differ only in the use of hot temperatures, in terms of efficiency, both of these methods are exactly the same, just hot way fixing the tree is much faster. For each method, you will need glue, pva or wallpaper, depending on what is at hand, you do not need to buy anything special. And with the help metal parts you can create a kind of press or frame that will hold the tree in the desired shape. In order to bend the beam, it is necessary to lubricate it with glue, strongly and carefully, without fear that the tree will become wet from this. In fact, under the influence of the adhesive solution, all excess moisture will leave the wood, and it will become even more durable and strong, which is extremely important. After the beam has been smeared with glue, it must be fixed with tools in the desired shape, and in the case of a cold work process, simply leave it fixed to dry. If you choose hot, then the timber should be covered with a regular film so that it dries faster and all moisture evaporates.

Durability

Whichever method is chosen, both are effective and work in the same way. The beam is completely fixed in its new form, and will no longer return to the old one. You can bend the wet beam from the glue as much as possible, without fear that it will break. And as a result, you will get an original and beautiful piece of furniture or a detail to create an even more attractive interior of the house or its facade. A beam bent with glue will not even need to be processed for durability, which is extremely convenient. Moisture will not soak through the adhesive solution, and insects will not encroach on such a tree in which there will be much more glue. That is why this method is the most optimal and practical, if you urgently need to bend a wooden beam.