- Features of the behavior of plywood when bending
- Required tools and equipment
- Bending and soaking
- Machining methods
- Bending by templates
- Manufacturing of composite parts
Few people know that plywood bending is a very common operation in carpentry; even fewer craftsmen are familiar with the correct technique. We propose to consider the features of this material and its behavior when bending, as well as the basic techniques and devices for bending plywood at home.
Features of the behavior of plywood when bending
The veneer from which plywood is made is nothing more than a set of stretched fibers. They are tough and durable, and the space between them is filled with a soft binder. To give the sheets additional strength and compensate for warpage in the plywood layers, the veneer has different grain directions in adjacent layers. Due to this, plywood is a tough material that is difficult to bend..
Usually, plywood with an even number of veneer layers is used for bending, that is, it does not have a predominant fiber direction. In the opposite case, the direction of bending should be calculated across the minimum number of longitudinal fibers, or vice versa – along, if a requirement for increased strength is imposed on the part.
The thinner the plywood, the easier it is to bend it. Sheets with three layers easily bend across the fibers of the outer layer, even in a cold state
When the fibers are parallel to the fold line, they do not interfere with the curvature of the plane. However, the layers located across are under significant stress. What is characteristic, the fibers can stretch quite strongly, the necessary conditions are provided by their softening due to heating or moistening.
At the same time, the layers located at the inner bending radius experience compressive stress, which is not compensated by the elasticity of the material. If the bend is sufficiently steep, folds may form on the inside of the bend. Sometimes the stress at this point is high enough to cause delamination – tearing of the adhesive layer or even the fibers themselves. This characteristic behavior can be compensated for by a number of techniques to which this article is devoted..
Required tools and equipment
As always, you will not be able to do without quality equipment and tools. Since the main technology involves softening the sheet, molding it, and then drying it in an immobilized position with taking shape, then at least a set of clamps is needed for bending. Ideally, there should be at least a dozen of them, but you can bend plywood in stages, rearranging the fasteners.
In this case, along each line across the bend, the workpiece is fixed in three places: at the highest point of the radius and at the edges. In other words, with due diligence, you can do with six clamps, but keep in mind that the accuracy of observing the dimensions of the finished part may be significantly lower than required.
Plywood is almost always bending according to templates. The exception is sheets of arbitrary radius, which are pre-bent before sheathing frame structures. For the risers of stairs, chairs, armchairs and other pieces of furniture, the shape of the bent parts is known with high precision. Therefore, first you need to make a spatial figure, to which the softened sheet will be attached while drying and taking the required shape..
There are many types of equipment that provide technological steaming or soaking of glue. Humidity and temperature must be kept within acceptable limits, otherwise the glue may irreversibly lose its strength properties. At home and for one-time work, you can use air heaters or underfloor heating films. It is better to wet it not directly, but through a hygroscopic material that can accumulate and gradually release moisture, for example, through foam rubber, hemp sacking or felt. Also, all the available adjustable pulling and squeezing aids such as lashing straps or hand jacks are included..
Bending and soaking
When bending plywood, an important rule applies: the higher the sheet thickness and the steeper the bending radius, the more trouble it will take to soften. If you do not take into account the difference in the quality of plywood, then after soaking, you can reach a bend radius of the order of 50–70 values of the sheet thickness. For tighter bends, other methods are recommended..
Soaking or steaming is aimed at softening the fibers in the veneer and making the glue more plastic with the expectation that after drying it will grab and securely fix the new shape of the part. At the same time, oversaturation of wood with water, as well as sharp moistening and drying, is not permissible.
Small sheets of thin plywood are easiest to soften in a steam bath in a regular saucepan or in a juicer
For proper soaking, plywood must be kept in a relative humidity of 90-100% for 2 to 12 hours, depending on the thickness. Periodically you need to check if there are any veneer delamination at the ends or bulges in the center. Bending attempts can be made from time to time. If the part relatively easily takes shape and at the same time no crackling is heard, then the plywood only needs to be firmly fixed until it dries completely.
For long parts, it is easier to prepare a box of any suitable material and then soften it with steam from a steam generator
Wetting is recommended when the plywood is laid on a flat base and wrapped on each side with a layer of absorbent material. If, at the same time, the workpiece is not wrapped tightly in film, you should control the humidity level and periodically re-sprinkle the burlap with water. Heating up to 50-60? С during wetting helps to speed up the process and make the glue swelling reversible. It is categorically impossible to overheat the plywood, so you should constantly monitor the temperature.
An alternative to wetting softening is to steam the plywood. In this case, the sheet is fixed over a container of boiling water or not far from it. The disadvantage of this method is that the humidity rises throughout the room with concomitant condensation. The advantage of this method is the uniformity of softening and a low risk of waterlogging or overheating..
The vacuum table speeds up the process of softening plywood, as well as the subsequent forming according to the template
A pre-prepared frame structure or template is covered with a softened sheet of plywood, fixed at key points of each bend. The plywood is clamped with clamps or self-tapping screws with a washer sequentially from one edge to the other, while the more often temporary fasteners are installed, and the harder it is, the lower the likelihood that after removal from the template the part will change shape.
In some cases, it makes no sense to resort to softening: for example, if a part is fixed on a sufficiently rigid frame, it will take on the desired shape over time due to natural changes in humidity. Another particular case is the need to bend plywood to a radius less than the allowable for a certain thickness. Here it is worth using both the softening of the sheet and its mechanical processing..
One method of pre-folding is to make a series of cuts parallel to the roll axis on the inside of the bend. The cuts are best done with a taper cutter using a linear guide. The depth of the grooves should be no more than 3/4 of the thickness of the workpiece, while as a result, at least two layers of veneer should remain intact.
If the cuts are facing inside the part and are hidden from view, it makes no sense to calculate their number and pitch. The subsequently formed scars can, if necessary, be smoothed out with automotive filler and sanded to a smooth state. With due diligence, you can calculate the reduction in arc length during bending using known bending angle and radius. In this case, the number of cuts will be equal to the quotient of dividing the arc reduction by the thickness of the cutter in the widest part of the entry. The required number of cuts must be evenly distributed over the entire bend radius.
Before fixing on the template, the cuts formed on the plywood are filled with a quality wood glue, for example Titebond 2. The protruding glue can be immediately removed with a spatula or sanded after drying. While the glue cures, the plywood is rigidly fixed on the template.
You can also ease the softening of thick plywood by perforating the workpiece. Holes with a diameter of 2–3 times the thickness of plywood are made with a slot cutter or a Forstner drill in a checkerboard pattern with a pitch of about 80–100 mm between them. The degree of perforation can be higher, it is determined both by the flexibility of the plywood to bend and the required final strength of the part. Naturally, this bending method is only suitable for hidden technological elements that do not form the finishing plane. If desired, the bent part can be sheathed on one or both sides with veneer or thin plywood, having previously softened them.
Bending by templates
In the simplest case, pieces of hard plywood that have the shape of a bend profile can serve as a template. Since most of the parts are bent along one contour on both sides, the templates are made in pairs, and then fastened together with spacers slightly less than the width of the part. The templates can also be different if it is necessary to bend a part of an irregular shape, that is, curved along two axes.
Another type of template is a frame made of beams, which does not form an exact bend contour. In this case, the transverse beams are located at the pressure points, that is, on the most protruding and concave parts of the part’s profile. For fastening at intermediate points, you can add an arbitrary number of jumpers to the frame.
When making the frame, the type of clamping devices used must be taken into account. So, on plywood inserts, holes for the jaws of the clamps can be provided, and the frame itself can be equipped with additional stiffening ribs in the direction of the clamping and tensile loads. If the part has a uniform bend along its entire length, it can be fixed without a frame, for example, tightened with a rope, cable, a chain with a screw hitch or using sling belts.
Manufacturing of composite parts
It is possible to bend parts of complex shape by alternately gluing layers of steamed veneer on a workpiece of the required shape. In practice, this is a method to avoid fiddling with thick parts, because on sharp bends it is much easier to deal with thin sheets due to the small values of the allowable radius..
In the simplest case, the base is formed by a relatively thick perforated plywood sheet, the permissible bending radius of which is obviously lower than the required one. Such a detail, most likely, will not have the required strength, therefore, it is subsequently “sheathed” with several more thin layers. In each case, an adhesive joint is made over the entire plane, the sheets are pre-softened so that they do not straighten the shape of the core.
Sheathing can be performed not only for the purpose of giving strength. In some cases, one or more outer layers pursue a purely decorative function. For example, thin veneer sheets can hide traces of fixing a workpiece to a template, while plywood can mask perforations or cuts. It is also possible to paste over the part with plastic, laminated veneer and other finishing materials that do not retain their shape well.