EP2389290A1

EP2389290A1 - Convex woodchip board

Info

Publication number: EP2389290A1
Application number: EP10733692A
Authority: EP
Inventors: Bo Nilsson
Original assignee: SWEDWOOD INTERNATIONAL AB
Current assignee: SWEDWOOD INTERNATIONAL AB
Priority date: 2009-01-20
Filing date: 2010-01-14
Publication date: 2011-11-30
Also published as: SE0950018A1; EP2389290A4; WO2010085198A1; SE534068C2

Abstract

The invention relates to a method for manufacturing a woodchip board (3) comprising a first (21) and second (23) woodchip layer sandwiching an intermediate layer (12) comprising woodchips of other chip size than the first (21) and second (23) woodchip layer. The method comprises the steps involving application of a first quantity (5) of glued woodchips (7) to a lower tool face (9) of a pressing tool (1, 31); application of a filler quantity (11) of glued woodchips (7') to the first quantity (5); application of a second quantity (13) of glued woodchips (7) to the filler quantity (11); compression of said quantities (5, 11, 13) by means of the lower (9) tool face and an upper tool face (15); realization of differentiated times when the glue of the glued woodchips (7) hardens in the first (5) and second quantity (13) respectively, by virtue of the production of these quantities with different thickness and/or by heating of the respective quantities (5, 13) to different temperatures; and removal of the hardened woodchip board (3) from the pressing tool (1, 31).

Description

Convex woodchip board

TECHNICAL FIELD

The present invention relates to a woodchip board according to the preamble to patent claim 1. The invention also relates to a method for producing said woodchip board, which method is defined in patent claim 7.

The invention can be assigned to the manufacturing industry for chipboards, yet is not limited to this but can also relate to other types of wood-based boards, such as MDF and OSB (Oriented Strand Board). Wood-based boards are in turn used in furniture manufacturing, for example.

BACKGROUND ART

Chipboards are currently manufactured for the furniture industry, for example triple-ply boards made of woodchip raw material. The woodchip raw material can be of wood and/or other lignocellulose material and can be constituted, for example, by knife-cut chips from round timber, sawdust or splinter chips. Other chip materials can be flax straw, hemp and bagasse. The chipboards are manufactured under pressure and heat with glue as binder. Following pressing, the board acquires a fixed structure and is then cooled. The board has an upper and a lower side for application of a suitable surface layer, such as paint, plywood, etc. The board can then be used as a furniture component, such as a bookcase, table top, board material for other components which are subjected to deflection forces, etc.

Known chipboards suffer from the drawback, however, that they tend over time to be deflected due to applied load, such as loads from books placed on shelves made of chipboards. This has been a long-known problem. In GB 2 255 095, for example, a method is shown in which it is proposed to create a convex curvature toward the load-bearing side through the use of heat and pressure.

It is also proposed that chipboards be sandwiched between plywood boards, in which the upper plywood board is thicker than the lower one.

The object of the present invention is to refine the notion of the convex curvature, but through the use of a chipboard of different structure.

One object of the invention is to provide a chipboard which can be easily manufactured in convex configuration (and wherein the convexity can be effectively controlled), and which exhibits good characteristics for withstanding loads directed toward the load-bearing side of the chipboard.

It is also desirable to be able to retain the existing manufacturing equipment which is currently present on a production line, without any cost-incurring effort. The prior art suffers from drawbacks such as the complicated process equipment, ineffective use of process equipment in the manufacturing process, the lack of flexibility in a production line, long pressing/gluing time, etc.

It is likewise desirable that the finished chipboard is simple in its structure, has low weight and is dimensionally stable.

DISCLOSURE OF INVENTION The problem is solved by the woodchip board described in the introduction, which is characterized by the distinguishing features defined in patent claim 1.

In this way, a woodchip board has been produced with simple means, wherein the woodchip board can be manufactured in an existing production line in which only one distribution device for distribution of the surface layers (the ratio between the thicknesses of the first and second woodchip layer) needs to be set up in order to create in a woodchip board a convex curvature. By convex woodchip board is meant that the outer face of the woodchip board, corresponding to the outer face of the first woodchip layer, is concave, and that the second, away- facing outer face of the woodchip board, corresponding to the outer face of the second woodchip layer, is convex. The convex woodchip board can also be defined by the fact that the curvature of the first woodchip layer in space has a smaller radius than the curvature of the second woodchip layer. The convex curvature of the woodchip board is realized toward the load- bearing side.

The woodchip board can be cut into suitable widths to form highly tensioned shelves, in which the first woodchip board is facing downward. A shelf of 700-800 mm length is produced, preferably having an arc height above the chord of about 1.5-2 mm, through adapted thickness of the first woodchip layer in relation to the thickness of the second woodchip layer. Experiments conducted by the Applicant have shown that a ratio of 75-80% thickness of the first woodchip layer compared to 20-25% thickness of the second woodchip layer (where 100% is the total thickness of the first and second woodchip layer combined) gives a satisfactory arc height and adequate resistance to forces which strive to deflect the shelf. Nor does an arc height of 1.5-2 mm in such a shelf affect the esthetic impression. When the shelf is loaded, a deflection will occur, but the greater compressive and tensile strength of the thicker, first layer will enable the shelf to withstand this load to a high degree and not be significantly deflected. Nor does a deflection of 1.5-2 mm from the chord here affect the esthetic impression. The shelf can thus be loaded to the point where the total maximum deflection distance (arc height plus deflection) can be as much as 3-4 mm, without this affecting the esthetic impression.

Preferably, the first woodchip layer is about four times thicker than the second woodchip layer.

By means of a thereby obtained large arc height and high compressive and tensile strength of the first woodchip layer, a greater load can be applied to the woodchip board. Likewise, the woodchip board could satisfactorily withstand deflection in damp climates, since a damp external environment can in itself make the woodchip board softer and thus less resistant to deflection.

Expediently, the first woodchip layer is about three times thicker than the second woodchip layer.

This ratio of distribution of the quantity of woodchips in the first and second woodchip layer gives rise to an optimal arc height for withstanding down-load, at the same time as the second woodchip layer can satisfactorily withstand impact and mechanical damage.

Preferably, the chips are not oriented in the longitudinal direction in the respective layers of the finished woodchip board, but rather the chips are arranged in a number of different directions. A strength is thus built up in all directions. Preferably, the woodchip board is coated with a surface layer.

In this way, by plastics film or board plywood applied to the outer face(s) of the woodchip board, the woodchip board can be made more resistant to deflection.

Expediently, the woodchips in the intermediate layer have a coarser chip size than in the first and second woodchip layer.

A woodchip board can hence be produced which is light, at the same time as it exhibits good resistance to deflection / permits a relatively large deflection distance.

Preferably, a shelf is made of a woodchip board according to any one of claims 1 to 5, wherein the upper face of the shelf, designed to receive articles, such as books, is convex.

In this way, a shelf has been produced which ensures an esthetic impression over time even when high load is applied to the shelf from above over a long period. A shelf resting on supports fixed in the two end faces of a bookcase must be as straight as possible when books are placed on the shelf. Otherwise, the esthetic impression would be affected. The shelf must therefore be able to withstand forces which strive to deflect the shelf. This is ensured by such a shelf.

The problem is solved also by the method described in the introduction, which method is characterized by the steps defined in patent claim 7. One embodiment entails that the second quantity of glued woodchips is applied to the filler quantity in such a way that the thickness of this second quantity, by means of distribution members, is greater or less than the thickness of the first quantity. By the method of producing one of the surface layers (the first woodchip layer comprising the first quantity of woodchips or the second woodchip layer comprising the second quantity of woodchips) with greater thickness than the second surface layer, a highly tensioned woodchip board is produced following the removal of the woodchip board from the pressing tool. The greater quantity of woodchips is given a greater density following the compression of the quantities in the manufacture. This is realized by the fact that, in the compression step, the first quantity, which is thicker than the second quantity, is compressed with the aid of a lower tool face and the filler quantity itself (that is to say, the intermediate layer acts automatically as a tool face, since the second quantity is thinner than the first quantity and the thicker, first quantity has to be compressed over a longer distance. The filler quantity thus acts as a counterstay. The density is thus greater in the thicker layer, which, in turn, gives a greater tensile strength and compressive strength (strength) in the thicker layer than in the thinner one. The thicker surface layer naturally comprises, apart from a greater quantity of wood chips, a greater quantity of glue than the thinner surface layer. In the compression step, the thicker surface layer is compressed with the aid of the tool faces, at the same time as, therefore, the filler quantity (intermediate glued wood chips) acts as a counterstay. At the same time, this phenomenon is obtained by the fact that the thicker surface layer can be more easily compressed due to the greater quantity of glue, and thus the creation of a greater density in the thicker layer (and a greater built-in tensile and compressive strength) than the thinner layer is promoted. Following the compression step and completed hardening of the two surface layers, different tensile and compressive strengths (strengths) have been built into the respective surface layers. That is to say, the thicker surface layer has a built-in greater tensile and compressive strength than the thinner surface layer. This means that, following the removal of the woodchip board from the pressing tool, the woodchip board assumes a convex form. This occurs due to the fact that the built-in different tensile and compressive strengths in the respective surface layers act in such a way that the thicker surface layer having the greater tensile and compressive strength contracts to a greater degree than the thinner surface layer, whereby the woodchip board is curved after having left the pressing tool. The curvature is such that the curvature of the thicker surface layer in space has a smaller radius than the curvature of the thinner surface layer.

Another embodiment entails that the pressing temperatures for respectively the first and the second quantity are different. A convex woodchip board having the same thickness in the first and second quantity can thus be obtained by applying higher temperature to the quantity which is required to acquire a greater pretensioning. The greater pretensioning creates a convex woodchip board when the woodchip board is removed from the pressing tool. The quantity having the greater pretensioning corresponds to the layer which forms the concave face of the woodchip board.

Preferably, the step involving application of a second quantity of glued woodchips to the filler quantity is realized in such a way that the thickness of this second quantity is less than the thickness of the first quantity.

In this way, the lower surface layer has been made thicker than the upper surface layer, whereby the upper outer face of the woodchip board, following the removal of the woodchip board from the pressing tool, acquires a convex form. That is to say, the woodchip board is curved with an arc upward after it has left the pressing tool. This is advantageous, since the following handling steps, such as the cutting of the woodchip board into suitable shelves and the fitting of these between end faces of bookcases (or packing of the shelves into packs with the aid of robots) can take place without the shelves having to be turned up and down. The aim is, of course, that the shelf with its convex top side is fitted in the bookcase. The shelves can thus always have their convex face upward throughout the production line, without any extra turning step. Even when a fitter is faced with the parts of a construction kit in a pack, he will be spared the need to turn over the shelf, which, in itself, can facilitate the fitting. This is time-saving and hence cost-effective.

Expediently, the method comprises the further step of applying fluid, such as pure water, to at least one of said quantities prior to the compression step.

In this way, the temperature can be raised in the compression step by the water being evaporated by a heat supplied to the glued woodchips. With the aid of the created water vapor, heat can be effectively conducted further into the quantities of glued woodchips. This promotes hardening of the glue and accelerates this. Advantageously, fluid, such as pure water, is applied only to the outer face of the thicker quantity of glued woodchips (and no fluid to the second, thinner quantity), wherein the hardening occurs faster in the thicker quantity. The result of this is that the hardening further acquires a favorable condition in this thicker surface layer. Following the compression step and completed hardening of the two surface layers, different compressive and tensile strengths (strengths) have been built into the respective surface layers. That is to say, the thicker surface layer, with the aid of the supplied water, has a maintained, built-in greater compressive and tensile strength than the thinner surface layer. In this way, the ratio between the thick and thin surface layer (for example, 40% thin surface layer and 60% thick surface layer, that is to say the thicker surface layer is 1.5 times thicker than the thinner one) can be balanced, at the same time as the thicker surface layer can be maintained with sufficiently high compressive and tensile strength. This means that, despite the balanced ratio, the thicker layer contracts to a greater degree than the thinner surface layer, whereby the woodchip board is curved after having left the pressing tool, even though the thinner surface layer can virtually be of the same thickness as the thicker surface layer.

Preferably, the step involving compression of said quantities comprises the supply of heat to the tool face bearing against the thicker quantity of glued woodchips.

In this way, the temperature, in the compression step, can be raised in the thicker surface layer. This promotes the hardening of the glue in this thicker surface layer and accelerates the hardening thereof. Thus, the thicker surface layer can be built in with even greater tensile strength. In this case, the thicker surface layer contracts to an even greater degree than the thinner surface layer, whereby the woodchip board is satisfactorily curved after having left the pressing tool.

A manufacturer of the woodchip board can thus control the convexity for the ready-pressed woodchip board through the use of various parameters, such as the distribution ratio between the thickness of the first and second quantity of glued woodchips (upper and lower surface layer), the application of water to the upper and lower surface layer in different quantity, the temperature difference of the upper and lower tool face of the pressing tool. An operator can cost-effectively set up distribution members in a production line such that the desired distribution between the first and second quantity of glued woodchips is realized and he can adjust the supply of water via nozzles onto the respective surface layer, and can also, with an adjustment control, adjust the supply of heat to the respective tool face.

The operator can easily set said parameters for the desired convexity by means of the control unit.

BRIEF DESCRIPTION OF DRAWINGS

The present invention will be explained in greater detail below with reference to the accompanying drawings, in which, in schematic representation:

fig. Ia shows a manufacturing method according to a first embodiment, in which a cycle press is used to produce a convex chipboard; fig. Ib shows the compression of glued woodchips in layers of different thickness; fig. Ic shows an enlargement of a portion in fig. Ib; fig. 1 d shows a finished convex chipboard according to the first embodiment; fig. 2 shows a production line having a continuous pressing apparatus for manufacturing a woodchip board according to the first embodiment; fig. 3a shows a bookcase having a woodchip board as a shelf according to the first embodiment; fig. 3b shows a woodchip board according to the prior art; fig. 4 shows a manufacturing method according to a second embodiment; and fig. 5 shows a manufacturing method according to a third embodiment.

MODE(S) FOR CARRYING OUT THE INVENTION

The present invention will now be described in the form of illustrative embodiments. For the sake of clarity, components without relevance to the invention have been omitted from the drawing. Same details which are shown in more than one figure can in certain cases have no reference notation, but can correspond to those which have a reference notation.

Fig. 1 shows a manufacturing method according to a first embodiment, in which a cycle press 1 is used to produce a convex woodchip board 3. The method entails that a first quantity 5 of glued woodchips 7 is applied to a lower tool face 9. Then a filler quantity 11 of glued woodchips 7' is applied to the first quantity 5 to form an intermediate layer 12 of woodchips. This intermediate layer 12 is constituted by coarser woodchips T than the woodchips 7 of the first quantity 5. After this, a second quantity 13 of glued woodchips 7 is applied to the filler quantity 11 in such a way that the thickness of this second quantity 13, by means of a distribution member (not shown), is less than the thickness of the first quantity 5. The distribution member comprises a woodchip discharge member (not shown), such as a dispenser, which distributes the quantity of woodchips 7 from a storage/gluing area for woodchips.

In fig. Ib it is shown how the lower tool face 9 with applied quantities 5, 11, 13 is raised toward an upper tool face 15 of the cycle press 1. The tool faces 9, 15 press together the quantities 5, 11, 13. A compression (shown with arrows P) takes place under heat to produce a predetermined thickness of the finished woodchip board. The glued woodchips 7, 7' are pressed together one against the other and the glue in these hardens, whereupon the woodchips are bonded together. In the compression step, the first quantity 5 (which is thicker than the second quantity 13) is compressed with the aid of the lower tool face 9 and the filler quantity 11 (that is to say, the intermediate layer 12 acts automatically as a tool face, since the second quantity 13 is thinner than the first quantity 5 and the thicker, first quantity 5 has to be compressed over a longer distance). The filler quantity 11 thus acts as a counterstay. The Applicant has noted that the thicker, first quantity 5 is more easily compressed than the thinner, second quantity 13 due to the greater quantity of glue in the first quantity 5, which is time-saving and which gives a greater compression of the first quantity 5, which in turn generates higher density in the first quantity 5 in comparison to the thinner, second quantity 13. Once the three quantities 5, 11, 13 have hardened, the first quantity 5 will thus acquire a built-in greater tensile strength (strength or pretensioning) than the thinner, second quantity 13, since the thicker, first quantity 5 has a greater density than the thinner, second quantity 13. That is to say, the first quantity 5 strives to contract with greater force than the second quantity 13. This ratio is illustrated in schematic representation in fig. Ic. The tensile and compressive forces are illustrated with the arrows 17, 17'. The first quantity 5 thus contracts to a greater degree than the second quantity 13, whereby the woodchip board 3 is curved after having left the cycle press 1, in which the thicker, first layer has a concave face and the second, thinner layer has a convex face.

In fig. Id it is shown how the hardened woodchip board 3 assumes its defined convexity following its removal from the cycle press 1. The dash-dot line 19 shows a straight line to illustrate the bending of the woodchip board 3. The built-in greater tensile forces 17 (the tensions) in the thicker, first quantity 5 relative to the lesser tensile forces 17' (the tensions) in the thinner, second quantity 13 produce a convexity in the woodchip board 3 removed from the cycle press 1. The convex woodchip board 3 has a curvature which can be defined by the fact that the curvature of the first quantity 5 in space has a smaller radius than the curvature of the second quantity 13. The convex curvature of the woodchip board 3 is realized toward the load-bearing side, that is to say the first quantity 5 is facing away from the load-facing second quantity 13.

The finished woodchip board 3 thus comprises a first 21 and second 23 woodchip layer sandwiching the intermediate layer 12 comprising woodchips 7' of larger chip size than the first 21 and second 23 woodchip layer. A woodchip board 1 can thereby be produced with lighter weight, at the same time as it exhibits good resistance to deflection and/or permits a relatively large deflection distance.

The method of producing one of the surface layers (the first woodchip layer 21 comprising the first quantity of woodchips 5 or the second woodchip layer 23 comprising the second quantity 13 of woodchips) with greater thickness than the opposite surface layer gives rise to a highly tensioned woodchip board 3 following the removal of the ready-pressed woodchip board from the cycle press. This phenomenon arises from the fact that the thicker, first woodchip layer 21 , during the hardening, automatically hardens faster in the compression operation than the glue in the thinner woodchip layer 23, due to the greater density in this thicker surface layer 21. The thicker, first woodchip layer 21 comprises, apart from a greater quantity of woodchips, naturally a greater quantity of glue than the thinner, second woodchip layer 23, which accelerates the hardening relative to this second woodchip layer 23. Following the compression step and completed hardening of the two woodchip layers 21, 23, different compressive and tensile strengths (strengths) have been built into the respective woodchip layers 21, 23. That is to say, the thicker, first woodchip layer 21 has a built-in greater tensile strength than the thinner, second woodchip layer 23. This means that, following the removal of the woodchip board 3 from the cycle press 1, the woodchip board 3 assumes a convex form. This occurs due to the fact that the built-in different compressive and tensile strengths in the respective woodchip layers 21, 23 act in such a way that the thicker, first woodchip layer 21 having the greater tensile strength contracts to a greater degree than the thinner, second woodchip layer 23, whereby the woodchip board 3 is curved after having left the cycle press 1.

By controlling the ratio between the thicknesses of the first 5 and second 13 quantity, the convexity of the ready-pressed woodchip board 3 can be determined. If a woodchip board 3 with great convexity is desired, the distribution member is controlled to distribute glued woodchips 7 in such a way that one of the first 5 or second 13 quantity of woodchips acquires a substantially greater thickness than the second one. If, on the other hand, a concave woodchip board 3 is desired, the distribution member is controlled from a control room (not shown) to dispense glued woodchips 7 in such a way that the first quantity 5 is thinner than the second one.

In this example, the first woodchip layer 21 is about four times thicker than the second woodchip layer 23. By means of a thereby obtained large arc height of the woodchip board 3 and high compressive and tensile strength of the first woodchip layer 21, a greater load can be applied to the woodchip board 3. Likewise, the woodchip board 3 can satisfactorily withstand deflection in damp climates, since a damp external environment can in itself make the woodchip board 3 softer and thus less resistant to deflection. The upper face 24 of the woodchip board is convex and its lower face 26 is concave.

We now turn to figs. 2, 3a and 3b. The convex woodchip board 3 is advantageously used for a shelf 27, which is shown in fig. 3. The upper face 24 of the shelf 27, designed to receive articles, such as books 29, is convex. In fig. 2, a continuous press 31 is shown. This is used to manufacture the woodchip board 3, which is then cut by means of a crosscutting machine 33 in order to determine the dimensions of the shelves 27 in terms of length and width. The continuous press 31 works roughly according to the same principle as the cycle press 1 in fig. 1. That is to say, instead of the tool faces 9, 15 merely being guided vertically toward each other, the tool faces 9, 15 press the quantities 5, 11, 13 together, while the quantities, at the same time, are transported horizontally.

Under the influence of heat, the first 5 and the second 13 quantity of glued woodchips 7 are pressed together, with an intermediate (filler quantity 11) quantity of glued woodchips therebetween. The first quantity 5 is applied more thickly than the second quantity 13. The first quantity 5 is controlled by means of a first distribution member (and first dispenser 35) to be thicker than the second quantity 13. The second quantity 13 is applied by means of a controllable second dispenser 37 so as to produce the desired thickness of this second quantity 13. The filler quantity 11 is controlled by means of a third dispenser 39 to the desired thickness. All controlling of the thicknesses of the various quantities 5, 11, 13 takes place from a control room (not shown). Controlling of the gluing of the woodchips 7, 7', and adjustment of heat of the tool faces 9, 15, also takes place there. When the finished woodchip board 3 leaves the continuous press 31 , it will assume a convex form, that is to say will bulge upward. The woodchip board 3 is cut into finished shelves 27.

In this example, the first woodchip layer 21 is about three times thicker than the second woodchip layer 23. This distribution of the quantity of woodchips 7 in the first 21 and second 23 woodchip layer gives rise to an optimal arc height

B to withstand down-load, at the same time as the second woodchip layer 23, as a result of its higher density, can satisfactorily withstand impact and mechanical damage. The arc height B is defined as the perpendicular distance from the chord K.

The woodchip board 3 is thus cut into suitable widths to form highly tensioned shelves 27, in which the first woodchip layer 21 is facing downward. A shelf 27 of 700-800 mm length is produced, preferably having an arc height B above the chord K of about 1.5-2 mm. The arc height can be adjusted from the control room by adapting the thickness of the first quantity 5 in relation to the thickness of the second quantity 13 by means of the dispensers 35, 37. Experiments conducted by the Applicant have shown that a ratio of 75-80% thickness of the first woodchip layer compared to 20-25% thickness of the second woodchip layer (where 100% is the total thickness of the first and second woodchip layer combined) gives a satisfactory arc height B and adequate resistance to forces which strive to deflect the shelf. The intermediate layer 12 comprising coarser woodchips 7' is roughly twice as thick as the first woodchip layer 21. An arc height of 1.5-2 mm of such a shelf 27 does not affect the esthetic impression. When the shelf 27 is loaded, a deflection will occur, but the greater compressive and tensile strength of the thicker, first woodchip layer 21 will enable the shelf 27 to withstand this load to a high degree and not be significantly deflected. Nor does a deflection N (see fig. 3 a) of 1.5-2 mm from the chord K affect the esthetic impression. The shelf can thus be loaded to the point where the total maximum deflection distance (arc height B plus deflection N) can be as much as 3-4 mm, without this affecting the esthetic impression.

Since the woodchip board 3, already in the continuous press 31, is manufactured with the convex upper face 24 upward, the woodchip board 3 can be cut into shelves 27, which are then inserted in a bookcase 41 without needing to be turned up and down, which is advantageous from a time perspective. That is to say, in the production of shelves 27, it is effective already in the continuous press 31 to arrange for the first quantity 5 applied to the lower tool face 9 to be thicker than the second quantity 13.

The shelves 27 are coated with a surface layer (not shown), such as a plastics film, for, inter alia, the esthetic impression. In this way, as a result of the plastics film (or board plywood) applied to the upper 24 and lower 26 face of the woodchip board 3, the woodchip board 3 is made more resistant to deflection.

The shelf 27 has thus been produced which ensures an esthetic impression over time even when high load is applied to the shelf 27 from above over a long period. The shelf 27 resting on supports 43 fixed in the two end faces of the bookcase 41 must be as straight as possible when books 29 are placed on the shelf 27. Otherwise the esthetic impression would be affected. The shelf 27 must therefore be able to withstand forces which strive to deflect the shelf 27. The chips 7 are not oriented in the longitudinal direction in the respective layers of the finished woodchip board. The chips are arranged in a number of different directions. A strength is thus built up in all directions. Fig. 3a shows the bookcase 41 comprising shelves 27', 27". The upper shelf 27' bears no load and the lower shelf 27" bears the load of some books. An arc height of 1.5-2 mm of the shelf 27' (without load) does not affect the esthetic impression. When the shelf 27" is loaded, a deflection N occurs, but the greater compressive and tensile strength of the thicker, first layer 21, will enable the shelf 27" to withstand this load to a high degree and withstand continued deflection. A deflection N from the chord of 1.5-2 mm does not affect the esthetic impression. The shelf 27 can thus be loaded to the point where the total maximum deflection distance (arc height B plus deflection N) can be as much as 3-4 mm without this affecting the esthetic impression.

Fig. 3b shows a woodchip board 3' according to the prior art. The known shelf 3' comprises an upper and a lower woodchip layer of same thickness, with an intermediate woodchip layer between the upper and lower woodchip layer. The shelf 3 ' is straight when it is without load. When the shelf 3 ' is loaded with the same load as in fig. 3a, a deflection N of 3-4 mm from the chord will affect the esthetic impression.

Fig. 4 shows a manufacturing method according to a second embodiment. The method comprises the further step of applying water 45 by means of nozzles 47 to the lower tool face 9 before the first quantity 5 is dispensed by means of the first dispenser 35. Water 45 is thus applied to the first quantity 5 of glued woodchips 7 by spray nozzles onto the lower tool face 9 prior to the compression step. The quantity of water 45 which is applied is 20-100 gr/m2, preferably 40-80 gr/m2. In the compression of the applied quantities 5, 11, 13 between the lower 9 and upper 15 work face, the glued woodchips 7 are supplied with heat by means of lower 49 and upper 51 heating elements. The heat promotes and accelerates the hardening of the glue of the glued woodchips 7. On the side to which water has been applied, that is to say on the first quantity 5, an evaporation of the water 45 by means of the lower heating element 49 will occur, which raises the hardening temperature faster for the first quantity 5 in comparison to the second quantity 13 (since the evaporated water 45 transports the heat more rapidly). Once the glue of the three quantities 5, 11, 13 has hardened, the first quantity 5 will thus acquire a built- in greater tensile and compressive strength 17 (tensile forces/strength) than the thinner, second quantity 13 (see fig. Ic), since the glue which hardens tends to shrink somewhat. That is to say, the first quantity 5 strives in the hardening to contract first, and the second quantity 13 which hardens somewhat later strives in the hardening to contract later and with weaker tensile forces 17'. The built- in compressive and tensile forces 17 in the first hardened quantity 5 are greater than in the second hardened quantity 5, which is shown in fig. Ic. The first hardened quantity 5 therefore contracts to a greater degree than the second hardened quantity 13 when the woodchip board 3 leaves the continuous press 31 , whereby the woodchip board 3 acquires a convex form. With the aid of the created water vapor, heat can be effectively led further into the first quantity 5 of glued woodchips 7. This promotes the hardening of the glue in this quantity and accelerates the hardening. Following the compression step and completed hardening of the quantities 5, 11, 13, different compressive and tensile strengths (compressive and tensile forces 17, 17' or strengths) have been built into the respective quantity 5, 13. That is to say, with the aid of the supplied water 45, the thicker, first woodchip layer 21 has a greater maintained built-in compressive and tensile strength than the thinner, second woodchip layer 12, which is also illustrated in fig. Ic. In this way, the ratio between the thicker, first woodchip layer 21 and the thinner, second woodchip layer 23 (for example 40% thin woodchip layer 23 and 60% thick woodchip layer 21, that is to say the thicker, first woodchip layer 21 is 1.5 times thicker than the thinner, second woodchip layer 23) can be balanced, at the same time as the thicker, first woodchip layer 21 can be maintained with sufficiently high compressive and tensile strength. This means, despite the balanced ratio, that the thicker, first woodchip layer 21 contracts to a greater degree than the thinner, second woodchip layer 23, whereby the woodchip board 3 is curved after having left the continuous press 31, even though the second woodchip layer 23 can have about the same thickness as the first woodchip layer 21. A control unit 53 controls the quantity of water 45 supplied to the first quantity 5 of woodchips 7 and likewise controls the heat of the heating elements 49, 51.

Fig. 5 shows a manufacturing method according to a third embodiment, in which the step involving compression of the quantities 5, 11, 13 comprises heating of the thicker, first quantity 5 at a higher temperature than the thinner, second quantity 13. In this way, the temperature, in the compression step, can be raised in the thicker, first quantity 5. This promotes the hardening of the glue of the first quantity 5 and accelerates the hardening of this first quantity 5. The thicker, first woodchip layer 21 can therefore have still greater tensile strength incorporated. The thicker, first woodchip layer 21 thus contracts to a greater degree than the thinner, second woodchip layer 23, whereby the woodchip board 3 is satisfactorily curved and assumes a convex form after having left the pressing tool. Parameters can be easily set for the desired convexity. For example, the operator can set the dispensers 35, 37, 39 such that the first 5 and the second 13 quantity have the same thickness, and he can set the nozzles 47 such that water 45 is supplied to the one or other quantity 5, 13, and set the heating elements 49, 51 such that heat is distributed equally to the two quantities 5, 13, or merely use the heating elements 49, 51 to distribute heat to the woodchips 7 in such a way that the first quantity 5 is heated more than the second quantity 13. For example, he can shut off the supply of water 47, set the dispensers 35, 37 such that the thicknesses of the first 5 and the second 13 quantity are equal, and set the lower heating element 49 such that higher heat is supplied to the quantity (5 or 13) which is required to acquire greater built-in tensile forces 17.

By means of the control unit 53, a manufacturer of the woodchip board 3 can thus control the convexity for the ready-pressed woodchip board 3 through the use of various parameters, such as the distribution ratio between the thickness of the first 5 and second quantity 13 of glued woodchips 7 (upper and lower surface layer), the quantity of water 45 on the first 5 and/or second 13 quantity, temperature differences of the lower 9 and upper 15 tool face. An operator (not shown) can cost-effectively set up the dispensers 35, 37, 39 in a production line such that a desired distribution between the first 5 and the second 13 quantity of glued woodchips 7 is obtained. He can easily adjust the supply of water 45 via the nozzles 47 to the respective quantity 5, 13, and can adjust the heat of the respective tool face 9, 15 with an adjustment control.

Such an embodiment entails that the pressing temperature for the first 5 and the second 13 quantity is made different. A convex woodchip board with the same thickness of the first 5 and second 13 quantity can thus be obtained by applying a higher temperature to the quantity 5 which is required to acquire a greater pretensioning. The greater pretensioning creates a convex woodchip board 3 once the woodchip board 3 has been removed from the pressing tool 1, 31. The quantity 5 having the greater pretensioning corresponds to the woodchip layer 21 forming the concave face of the woodchip board 3.

By means of the control unit 53, it is possible to control the pretensioning (and thus the convexity of the woodchip board) in a repeatable manner.

The invention should not be considered limited to the above-described embodiments, but rather refinements and/or combinations thereof are possible within the scope of protection defined by the appended patent claims. The finer the woodchips which are used in the quantity incorporated with pretensioning tensile forces, the stronger is the pretensioning, due to the higher density. The invention can be used for the manufacture of bookcases. However, other boards and board components for furniture products, fittings and building products, floor chipboards, etc, can also be produced by and constitute the invention. The intermediate layer can be thicker than the thicker layer, but may also be thinner. The chip size in the first and the second layer is preferably finer than in the intermediate layer, but can also be coarser.

Claims

1. A woodchip board comprising a first (21) and second (23) woodchip layer sandwiching an intermediate layer (12) comprising woodchips (7') of other chip size than woodchips (7) of the first (21) and second (23) woodchip layer, characterized in that the first woodchip layer (21) is thicker than the second woodchip layer (23), wherein a greater tension in the first woodchip layer (21) than in the second woodchip layer makes the woodchip board (3) convex.

2. The woodchip board as claimed in claim 1, wherein the first woodchip layer (21) is about four times thicker than the second woodchip layer (23).

3. The woodchip board as claimed in claim 1, wherein the first woodchip layer (21) is about three times thicker than the second woodchip layer (23).

4. The woodchip board as claimed in any one of claims 1 to 3, wherein the woodchip board (3) is coated with a surface layer.

5. The woodchip board as claimed in any one of the preceding claims, wherein the woodchips in the intermediate layer (12) have a coarser chip size than in the first (21) and second (23) woodchip layer.

6. A shelf made of a woodchip board (3) as claimed in any one of claims 1 to 5, wherein the upper face (24) of the shelf (27), designed to receive articles, such as books (29), is convex.

7. A method for manufacturing a woodchip board (3) comprising a first (21) and second (23) woodchip layer sandwiching an intermediate layer (12) comprising woodchips of other chip size than the first (21) and second (23) woodchip layer, which method comprises the steps:

- application of a first quantity (5) of glued woodchips (7) to a lower tool face (9) of a pressing tool (1, 31);

- application of a filler quantity (11) of glued woodchips (7') to the first quantity (5);

- application of a second quantity (13) of glued woodchips (7) to the filler quantity (11); - compression of said quantities (5, 11, 13) by means of the lower (9) tool face and an upper tool face (15);

- realization of differentiated times when the glue of the glued woodchips (7) hardens in the first (5) and second quantity (13) respectively, by virtue of the production of these quantities with different thickness and/or by heating of the respective quantities (5, 13) to different temperatures; and

- removal of the hardened woodchip board (3) from the pressing tool (1, 31).

8. The method as claimed in claim 7, wherein the step of applying a second quantity (13) of glued woodchips (7) to the filler quantity (11) is realized in such a way that the thickness of this second quantity (13) is less than the thickness of the first quantity (5).

9. The method as claimed in claim 7 or 8, wherein the method comprises the further step of applying fluid, such as pure water (45), to at least one of said quantities (5, 13) prior to the compression step.

10. The method as claimed in any one of claims 7 to 9, wherein the step involving compression of said quantities (5, 13) comprises the supply of heat to the tool face (9) bearing against the thicker quantity (5) of glued woodchips

(7).