Technical field

The present invention relates to a method of the type stated in the preamble to claim 1 as well as an autoclave and an apparatus for heating wooden objects to be butted in connection with solid bending of such wooden objects, e.g. when manufacturing furniture, by which method the objects are treated by means of heat and steam for a predetermined period of time before they are bent and/or moulded in the desired shape.

Background Art

Solid bending of wood is a well known process used when shaping i.a. objects for the manufacture of furniture. Solidbending is e.g. usedwhenmanufacturingthe so-called bent-wood chair.

Not all types of wood are useable for solid bending as the structure of the wood has to meet certain requirements in order for it to be applicable for this process. One of the best and most frequently used types of wood for this purpose is beech, but a large number of other types of wood such as birch, ash, rubber tree, teak a.o. may also be used for solid bending.

Briefly, solid bending of wood traditionally takes place by heating the objects in an autoclave by means of steam and moistening them if required. This heating process may take up to several hours.

Following the heating and steaming of the object for a sufficient period of time, said objects are taken out of

the autoclave and bent and moulded in the desired shape in a special press whichusually bends the wood over a form by means of a bending strap, whereupon the object is held in the bent shape by nailing a board or the like to the ends of the object in order to hold it in the bent shape after having removed it from the press.

When a sufficientlylargenumber of objects have beenbent, said objects are placed in a traditional drying plant, where they are left for the number of days necessary to dry them to the desired degree of dryness, whereupon the objects proceed in the finishing process, which will not be described in greater details in this specification, as it does not form part of the invention.

By the above known method it is usually only possible to bend objects in one plane. However, it is possible, under certaincircumstances andbymeans ofvery costly specially made fixtures and bending tools to bend in more planes.

By the method described it may also be difficult to bend complicated configurations in one plane.

Furthermore, it has been known for long that by butting the solid wood desired to be bent, said wood is made more plastic and thereby easier to bend to complicated con¬ figurations in more planes. By the butting, the section of the wooden object to be made bendable is compressed. Said compression which takes place in the longitudinal direction of the fibres causes bindings between the individual fibres tobedetached andthe fibres arethereby able to slide relative to each other. When the fibres are no longerbound togethertransversely, it is substantially easier to bend the wooden object. Such plastification by means of capacitiveHF electrodes orbymeans ofmicro-wave antennas is described in DE 31 06 304.

However, the method is encumbered with the drawback that

the wood is weakened to some extent, thereby having less strength inthebent construction also outsidethe sections actually to be bent, which limits the applicability especially for load-bearing constructions, such as chairs.

To reduce said weakening to a minimum, only the section(s) of the wood desired to be bent are heated while the section of the wood not to be bent is not heated. By means of the methods conventionally used for heating objects, e.g. by means of steam in an autoclave, it is complicated and expensive to shield the sections of the objects not desired to be affected by the heating.. Selective heating by applying two brush electrodes opposite each other and in direct contact with the section of the object to be bent and applying a highvoltage (dc or ac) is however described in US PS 1,994,607.

According to the invention, selective heating may be obtained in a very simple manner by means of HF heating. It is, however, already known to use HF heating in some processes for the treatment of wooden products.

Swedish printed accepted application No. 423,931 thus discloses a method of drying wooden products, where the drying is effected by arranging the products in a closed drying chamber where they are exposed to a microwave field from one or more microwave generators. The air in the chamber is adjusted as to temperature and moisture to allow the drying to take place by the moisture of the products being made to migrate from the middle of the products towards the boundary surfaces. This implies that the temperature of the air must be kept at a lower value than the temperature of the products and that from the start the humidity of the air should be kept at such a high value that the drying of the surface layers of the products which might impede the migration of the moisture is prevented.

Furthermore, as described in i.a. US PS 2,231,457 and

Swedish printed accepted application No. 319,129 it is known from the wood-working industry to use a HF generator and a set of capacitive HF electrodes to heat wood, e.g. in connection with glueing.

Description of the Invention

According to the invention a method of the type described inthe introductionis suggested, saidmethodbeing charac¬ terised in that the electrode members are placed in such a manner that the high frequency capacitive field is arranged across the object(s) substantially onlv in the section(s) oftheobjec (s) which areto bebutted, whereby only the section later to be bent is heated while the rest of the wooden object(s) is/are not heated.

Thereby a plurality of advantages are obtained: the wood to be heated is heated and even faster than hitherto known at that. Energy is saved, as only the wood to be heated is subjected to the heatingprocess and the undesired side effects of the heating process are avoided, that is the weakening of the sections which do not need to be heated and which therefore keep their strength by the method according to the invention.

The invention further relates to an autoclave and an apparatus for carrying out the method according to the invention and the autoclave is characterised in that it is made of an electrically non-conductive material with small or substantially no dielectric losses and provided with holding means for the mounting of a number of ex¬ changeable electrodeplates above andbelow or on each side of the autoclave.

An autoclave to be used when carrying out the method according to the inventionmay furthermore be characterised in that it is provided with a number of firmly mounted or displaceable electrodeplates situated opposite each other

inside or on the outside of the autoclave, said plates each optionally being connectable to an HF generator.

Brief Description of the Drawings

The invention is described in greater details below on the basis of some examples and with reference to the accom¬ panying drawings, in which

Fig. 1 shows an example of an autoclave adapted for carrying out the method according to the invention,

Fig. 2 is a sectional view of an example of an embodiment of an autoclave for carrying out the method according to the invention,

Fig. 3 is a sectional view of an example as in Fig. 2 but with the electrodes being placed differently,

Fig. 4 is a sectional view through a second example of an embodiment of an autoclave for carrying out the method according to the invention,

Fig. 5 is an oblique plan view of an example of a row of electrode plates, and

Fig. 6 is an oblique plan view of the example of Fig. 5 when said electrode plates are pushed together.

Detailed description of preferred embodiments of an apparatus to be used when carrying out the invention

Figs. 1, 2 and 3 show an example of an apparatus to be used when carrying out the invention.

By the process described below only the sections 50 of a number of wooden objects 12 desired to be bent are heated and during said heating the wood must retain its moisture

andpossiblybemoistened furtherby addingaqueous vapour.

The heating may therefore advantageously take place in an autoclave 10, where temperature and moisture may be controlled and adjusted.

The wooden objects 12 later to be bent are placed on supports 14 in the autoclave 10 and arranged in such a manner relative to each other that all the sections to be heated are substantially in a "column" above each other and/or in a "row" next to each other. Thereby the entire "column" or "row" of bending sections may be heated simultaneously by placing an HF electrode at each end of the "column" or "row" and applying an HF field over the bending sections while the rest of thewood is not affected by the HF field.

The autoclave 10 is preferably made of a material with small dielectric losses, e.g. a thermostable plastic material. The same applies to the supports 14.

By manufacturing the autoclave in a material with small dielectric losses, such as thermostable plastics, it is possible to heat the wood inside the autoclave by applying an HF field through HF electrodes placed on the outside of the container wall in simple fixtures.

The advantage of placing the electrodes on the outside of the autoclave is that it is thereby made easier to change the position of the electrodes.

As shown in Figs. 2 and 3 the electrodes 20-23 may be positioned differently. The positioning of the electrodes is of course selected in such a manner that the HF field 36 passes through exactly the sections 50 of the wood to be bent.

As indicated in Figs. 5 and 6, the electrode plates may

advantageously be manufactured in the form of more small plates 20-32, said plates being somewhat narrower but in the full width of the autoclave and adapted to be mounted preferably displaceably in continuation of each other in more optionally overlapping electrode sections. The electrodes may thereby quickly be adjusted to heat exactly the sections desired to be bent.

The electrodes 20-32 may be fastened to holders mounted independently of the autoclave. Preferably, electrode holders 40 have been provided on the autoclave per se. The holders 40 are preferably manufactured in a thermostable, substantially non-loss insulating material in order not to affect the electric field 36. The holders may be shaped in many different ways.

In an advantageous embodiment two longitudinal carrying rails 40 have been provided above and below the autoclave, a number of rectangular electrode plates 20-33 being dis- placeable on said carrying rails in the longitudinal direction ofthe autoclave and optionallybeingpushedmore or less together or being stacked, as shown in Fig. 6.

A HF generator 16 is connected to at least two of the electrode plates 21, 22, 23, 24 - 33, which only are of the extension necessary to heat the sections of the objects desired to be bent. As the electrodes are made of metal (aluminium, brass, copper) in the form of thin plates having a length and a width covering the section to be treated, it is simple and inexpensive to carry out selec¬ tive heating and as a consequence of the HF energy simul¬ taneously heating the objects all over their entire cross section, said heating may take place in a few minutes in contrast to conventional heating by means of steam which often takes hours.

More types of autoclaves may be used for HF heating. The drawing shows an autoclave 10 with a pivoting cover 18 at

δ one end which is merely shown as an example. Autoclaves which are opened in another manner, e.g. where the upper portion may be tipped up, may also be used. The autoclave may also have a rectangular or quadratic cross section, which configuations, relative to their sectional areas, may contain more objects than autoclaves with, a circular cross section.

As, at the same time, heating by means of HF takes place very quickly, the emission of moisture from the objects is mimimal.

For security reasons, the autoclave is provided with a safetyblow-off valve 34, preferably a valve with a little aperture, in order to prevent desiccation.

In the preferred embodiment said valve 34 opens when the pressure rises slightly above the atmospheric pressure whereby no special requirements are made to the pressure resistance of the autoclave.

Alternatively, the HF electrodes may be placed inside the autoclave as indicated in Fig. 4 and carry the HF supply lines 42, 44 through the wall of the autoclave in gastight lead-ins 35. If the electrodes are mounted on the inside, the autoclave may be constructed of both plastics and metal.

In stead ofusing exchangeable or displaceable electrodes, it may, especially in the embodiment where the electrodes are mounted inside the autoclave, be advantageous for the HF electrodes to be divided into a suitably large number, e.g. as shown 2 x 7 sections firmly mounted and which may optionally be connected to the generator, whereby the extension of the HF field applied depends on which elec¬ trode members are connected to the HF generator. In Fig. 4 the electrode members 24, 26, 28 are interconnected and connected to one generator terminal (not shown) via the HF supply line 42 and the electrode members 25, 27, 29 are

also interconnected and connected to the second generator (not shown) via the HF supply line 44. By means of this arrangement, the middle section 50 of the wooden object is heated.

The autoclave is preferably provided with sensors to measure temperature, moisture and possible pressure. Such sensors may be positioned in many places and they are not indicated on the drawing.

On all Figures the electrodes 20-33 are positioned above and below the wooden objects. This position is only chosen by way of example.

It is obvious that it is also within the scope of the invention to position the electrodes on each side of the autoclave or obliquely in an angel. Only, the electrodes should be placed in such a manner that the sections of the wooden objects desired to be heated are in the field between the electrodes.