EP1202848B1

EP1202848B1 - Method and device for compressing wood

Info

Publication number: EP1202848B1
Application number: EP00931857A
Authority: EP
Inventors: Martin Wiklund; Girma Kifetew
Original assignee: Primwood AB
Current assignee: Primwood AB
Priority date: 1999-05-17
Filing date: 2000-05-17
Publication date: 2003-10-15
Anticipated expiration: 2020-05-17
Also published as: EP1202848A1; WO2000069606A9; DE60005968D1; AU4967400A; DE60005968T2; SE9901796L; SE515328C2; ATE251976T1; WO2000069606A1; SE9901796D0

Abstract

The invention relates to a method and a device for providing a hard piece of wood using compression treatment in order to compress a disjoint timber of wood (1). The timber of wood has been prepared such that it comprises a piece of wood having standing annular rings perpendicular to the flat sides of the piece of wood. The piece of wood is placed between two plates (3, 4), which have a fixed placement in relation to each other and have a distance to each other adapted to the thickness of the piece of wood between its flat sides. The piece of wood is compressed between its side surfaces in radial direction to the annular rings of the wood using increasing compression force until a wanted compression has been reached.

Description

The present invention relates to a method of compressing wood of the kind disclosed in the preamble of claim 1, and a device to realise the method.

BACKGROUND

The hardness of wood is a function of the density of the wood. This means that heavier kinds of wood are harder than lighter kinds of wood. This is true even within one and the same piece of wood. For example, the density of the early wood is about 1/3 of the density of the late wood (∼300 kg/m³ and ∼900 kg/m³, respectively) for softwood, such as pine and spruce, grown in Sweden. Transition wood between early and late wood is provided, and the density transition will therefore be continuous. However, the transition between late and early wood is abrupt.

It is a prior knowledge to compress the wood in order to increase the density of the wood with this measure. The normal procedure has then been to compress the timber from the flat sides of plane-parallel pieces of timber in order to get the surfaces of the flat sides as hard as required. An example of such a method and device is known from SE-A-505,408.

It is then self evident that the early wood, which comprises large air filled channels (lumen) in the cells, is the easiest to compress. These channels have thin cell walls, which can deform easily without damaging them.

The late wood has cells with thick walls and rather small lumen and can therefore not be compressed to a great extent before the cell walls are crushed or broken.

This implies among other things that when the compressing load during compressing should be parallel with the annual rings of the wood (i.e. tangential) or in any event with a relatively small angle, for example <45°, the late wood will be crushed. A great part of the fibre walls will then be crushed. FIG 1 illustrates a piece of wood treated with this kind of compression. This piece of wood has got a crush of a late wood layer at A. A shear fracture between early and late wood, or in the early wood, as shown at B, represents a common problem. The piece of wood shown in FIG 1 is cut such that its flat sides are parallel to a diametrical section through a tree trunk wood. The annual rings lie essentially parallel to the flat sides, at least in the middle of the piece of wood since they are arcuate.

A known way to settle the problems mentioned above is to apply the load on both the flat sides and the edge sides. In order to provide a compression giving an end product having as even hardness as possible it is known to apply a so called iso-static compression of the piece of wood. However, this has also proven to provide internal tension, which can cause different kinds of fractures. Different kinds of heat treatment and dehumidification of timber wood during compression have been tested, which for example is described in the article THE SIMULTANEOUS DRYING AND DENSIFICATION OF SAPWOOD, by J.G. Haygreen and D. H. Daniels, published as Sci. Jour. Paper No 6745 of the Univ. of Minn. Agr. Exp. Sta.

Knots in the wood have an essentially higher density (and hence hardness) than the rest of the piece of wood. The knot direction, and hence knot fibre direction, lies practically radial in the tree trunk, and has a certain angle against the longitudinal direction of the tree trunk, most often 70° to 90°. If the knots are to be compressed in their longitudinal direction, a strong force is required. This causes great cross-damages in the material, partly because of the knot as such and partly because of the cross grain around the knot. Solutions to this problem have been tried by establishing the compression power by means of relatively soft plates of rubber, which admit the hard knots to be pressed into the plate of rubber and providing a proportionately even pressure on the wood surface. Thereby, isostatic compression provides the best compression result.

However, a piece of wood having protruding knots and with a heavily deformed section is obtained after the compression. Wood gain is lost since there most often is a need to use a flat parallelepiped piece of wood. Another disadvantage is that those side surfaces, which one often wishes to get hard (e.g. for floors) to a great extent are tangential surfaces, i.e. such surfaces where a great deal of the damages mentioned above preferably are obtained. Investigations have shown that most people, from esthetical and tactile reasons, best appreciate the radial surfaces of radially sawn lumber and knot free surfaces with thin annual rings.

Radial compression of wet timber and various complicated heating and drying treatments for a quite particular Japanese kind of wood has been suggested in the article RADIAL COMPRESSION OF SUGI WOOD (CRYPTOMERIA JAPONICA D. DON), BY w. Dwanto et al, published by Division of Wood Biomass Science, Wood Research Institute, Kyoto University, Gokasho, Uji Kyoto 611, Japan.

OBJECTS OF THE INVENTION

An object with the invention is to provide a method and device, which remove the problems stated at the introductory part of this specification.

Another object with the invention is to provide a method and device providing compressed, hard pieces of wood without splits and cross damages.

Still another object with the invention is to provide a method and device providing compressed hard pieces of wood having an esthetical attractive surface structure.

The objects mentioned above are full-filled by using a method having the characteristics mentioned in the characterising part of claim 1. Further features and developments of the method and a device to perform the method are disclosed in the rest of the claims.

The invention thus relates to a method for providing a hard piece of wood using compression treatment in order to compress a disjoint timber of wood, which having the features according to claim 1.

A device to provide for carrying out the method having the features of claim 10 is also part of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described in more detail below with reference to the enclosed drawings, where

FIG 1: illustrates a compression in accordance with a prior known method, described above,
FIG 2: illustrates schematically a first embodiment of a device according to the invention,
FIG 3: illustrates a second embodiment of a device according to the invention,
FIG 4: shows a diagram, by means of which the invention is explained,
FIG 5: illustrates a third embodiment of a device according to the invention,
FIG 6: shows a longitudinal section through a first embodiment of a press having feeding of a longitudinal timber of wood to be treated and batch wise compression of the wood,
FIG 7: illustrates a longitudinal section through a second embodiment of a press having feeding of a longitudinal timber of wood to be treated and continuous compression of the wood, and
FIG 8: illustrates a longitudinal section through a third embodiment of a press having feeding of a longitudinal timber of wood to be treated and continuous compression of the wood.

DESCRIPTION OF PREFERRED EMBODIMENTS

FIG 2A shows a wood board 1 in the form of a piece of wood having vertical annual rings. Such kind of wood is made for example by means of quarter or star sawing. Also the centre part of a tree trunk obtained when using through or square sawing can be used, for example having the pith and a part of the juvenile wood cut away. The wood cut in this way is dried at least to the fibre saturation point, but preferably longer, for example to ca. 10% moisture content. Thereafter, the knots with the surrounding cross grain are cut away. Wood pieces are joined as needed, for example with finger joint.

The wood piece 1 prepared in this way is placed between two

plates

3 and 4. Each plate has a glossy surface turned towards the wood and preferably has a

coating

5 and 6, respectively, providing a low friction to the wood. The purpose of having the glossy surfaces of the

plates

3 and 4 is to give such as low force of friction from these plates as possible when the wood board is compressed during the compression to come. The distance between the

plates

3 and 4 should be approximately as great as the thickness of the board. Preferably, the distance could be controllable, e.g. by means of a steering wheel or the like (not shown). It is also an advantage to be able to move away one of the

plates

3, 4 during working moments with insertion of an uncompressed wood board and unloading of a compressed wood board. However, the positions of the

plates

3 and 4 are essentially fixed, when a wood board is placed between them in order to be compressed.

Thereafter, a pressure force is imposed on both tangential sides of the wood board 1 in the direction of the arrows P1 and P2. The pressure force is provided by a press arrangement having

pressure providers

7, 8 and a

pressure foot

9, 11 on the tangential surfaces. The two pressure providers are controlled by a control unit 11, which also can measure the total pressure, to which the wood board 1 is exposed. It is to be noted that the way shown tangential surfaces pressure from both directions is to be preferred. However, it is also possible, as shown in FIG 3, to have a fixed holder-on 12 at one side of the wood board and apply the pressure P3 against its other radial surfaces using the same kind of

pressure arrangement

8, 10 as in FIG 1 controlled by a control unit 13.

The compression can go on until a strong increase of the pressure resistance is provided. This occurs when the wood board has got a density in the order of 1200 kg/m³. A early back occurs when the pressure force is released, but remaining density for pine and spruce wood is ca 800 kg/m³, i.e. somewhat lower than the original density of the late wood.

A diagram of the pressure force as a function of compression is shown in FIG 4. During a first phase having increasing pressure force, up to BR1, no essential compression is provided. Thereafter, a strong compression using a small extra pressure force is provided up to BR2, whereupon the pressure force has to be increased essentially in order to get a small increase of the compression. At an automatic pressure equipment controlled by the

control unit

11 or 13 the compression is finished when the pressure force has reached a predetermined level that according to observed results is known to represent the maximal compression before an extra pressure increase will lead to a hardly noticeable effect. It is also possible to measure the distance between the radial surfaces of the wood board during the pressure procedure. The measurement result is fed to the

control unit

11, 13, which stops the pressure increase when this distance begins to decrease with a predetermined rate.

Instead of releasing the pressure force after obtained compression so that a back swelling is provided the pressure force could be maintained constant during a certain time. The material is floating during this time, and a certain tension relaxation is obtained. It is also possible to complete with for example chemical or thermo-mechanical locking, which means that the piece of wood 1 for instance has been impregnated with some chemical substance before compression, which substance hardens during compression. Or else that heat is provided, particular during the end phase having constant pressure.

After that the wood board 1 in FIG 1 has been removed from the pressure device the piece of wood board can be attached, for example glued, on a fixating bed, e.g. a plate of wood 14. This fixation may be made only directly on one side of the wood board. However, as shown in FIG 2B, two compressed wood boards 1, 1' could be attached on each side of the fixating bed in order to get symmetry such that the composite wood board does not bend. The fixating bed strives against the effect provided when the compressed wood board has a possible tendency to swell in the compression direction. The bed 14 should not be made thicker than the glue holding it in place, it should be so thin that it does not press itself away from the glue. These kind of wood boards will advantageously be used for flooring, it is then important that the material does not warp. The bed 14 could in use, be protruding somewhat at one side and be notched somewhat at the other side, such that pieces of wood board having groove and tenon are created in this way.

Compression of a wood board in one piece is shown in the embodiment in FIG 2. FIG 3 illustrates that a piece of wood 1 before the compression could be sawn into radial sliced veneer (shown sparsely for clearness), which thereafter are laid one of top of the other between the

plates

15 and 16. Thereafter the compression is provided from the radial surfaces, which results in hard veneer disks, which can be used for example for wear and tear layers on floors, kitchen shelves, kitchen shutters etc.

In spite of the possibility shown in FIG 3 it is to be preferred to saw, or disjoint in some other way, an already compressed wood board to radial sliced veneer disks, which can be used for wear and tear layers. Since the piece of wood board is not compressed in tangential direction, i.e. between the

plates

3 and 4 in FIG 2, no increase material losses are obtained compared with cutting the veneer before the compression.

FIG 5 shows that two

triangular blocks

17 and 18 have been glued together such that the glued common block 20 has got vertical annual rings. Two

blocks

19 and 20 provided in this way have thereafter, having an insert 21 between them, been placed between fixedly placed

plates

23 and 24. After that the blocks are compressed from the tangential surfaces in order to get a harder and stronger material.

The result is a knot free, lasting and hard material having an attractive appearance because of the visible narrow annual rings, which appears as longitudinal rays. Since the completely compressed material has a rectangular form a minimum of waste material is provided.

In the section shown in FIG 6, having an essential smaller scale than in FIGs 2, 3, 5 and perpendicular to the sections shown in these FIGs, i.e. through the press plates, it is illustrated that the compression could be provided batch wise along the length of a long wood plank 25, or the length of a group of wood planks. This means that each plank, or group of planks, are compressed with stepwise forwardly fed planks, whereby the press is opened between every movement of the plank in the direction of the arrow P5 and is closed during the compression. This means that the compression is made successively. The distance plates of the

kind

3, 4; 15, 16; 23, 24 are provided on each side of the plank in order to prevent the wood material to flow out over the edges. The distance plates are not shown in FIG 6 since they are provided in front of and behind the section shown in the Figure. As shown the plank is fed in through an opening in the right side of the Figure between a press arrangement having two

longish pressure units

26 and 27 having different pressure forces along the plank 25.

As mentioned above the plank is fed through the press arrangement in the direction of the arrow P5 and is then compressed more and more in the beginning (see the section from the beginning to BR1 in FIG 4). After that, the plank is exerted to a pressure, which increased in a lower rate (see the section between BR1 and BR2 in FIG 5). After that, the control unit (not shown in FIG 6) senses that the pressure should not increase any more the pressure is maintained constant in the end of the pressure chain along the plank 25. Thus the pressure along the plank is here controlled to have different pressure power in dependence of the position along the press units. This is possible to make by means of the control unit in a way obvious for the person skilled in the art. In order to feed the planks through the press as free from friction as possible the

press units

26 and 27 could have a coating giving a low friction. This is advantageous even if the press is opened somewhat at each movement forward.

In the embodiment shown in FIG 7, however, a chain of

press plates

27, 28 has been placed between a guide (work support) 29 and 30 each for the belonging

press units

31 and 32, respectively, and the plank 25. The

press plates

27 and 28 are fed in the direction of the arrows such that they help the feeding movement of the plank 27 through the press arrangement. In doing so the press plates do not need to be opened between the batch wise compression, and the pressure proceeding can be provided continuously. Each chain of press plates can be fastened to each other in a ring and be fed around as an endless band.

In order to increase the capacity of the press a plurality of pieces of work could be pressed simultaneously by placing them at the side of each other, or on top of each other. In the later case, shown in FIG 8, an intermediate rigid metal bar 35 should be provided between the

planks

33 and 34 in order to prevent the individual planks to get lasting distortion from its parallelepiped form.

It is also possible to provide a stepwise fed press or a continuously working press, which works with transversely fed pieces of work, which then are fed into the press by means of one of the fixed

plates

3, 4, or 15, 16 or 23, 24 before that the position of this plate is fixed before the start of the press operation. This is shown in the embodiment according to FIG 5 but can be applied to anyone of the other embodiments. At the feeding in, one of the plates 23 is moved outwardly far enough to let the planks be fed in as shown. Thereafter, the plate 23 is moved back and is fixed to its position. The

press feet

9 and 10 are thus not yet inserted in the way shown in FIG 5.

The method according to the invention can be used for each kind of wood where it is a wish to increase its hardness and resistance. A corresponding increase of the tearproof, compression strength and resistance to bending for the wood is provided in the similar way as described above for the hardness.

The method is particularly advantageous for fast-growing wood, which in itself has deficient resistance values, for example fast growing spruce planted on arable land. The method is adapted for treatment of soft as well as hard wood.

The pieces of wood treated to be hard with the method according to the invention are principally thought to be used as floor surface. The hardness has been derived by compression sideways and thus not by compression from the flat sides. For every floor it is essential that it has a pleasant feel. Thereby, it is a wish that the floor shall have a soft feel as it does if it has a low E-module. The E-module at tangential load, i.e. on surfaces along a radial plane is much lower than that on tangential surfaces. The manufacturing method according to the invention, if it is used to making floor boards, thus gives an aesthetically attractive floor having its vertical annual, narrow rings. The floors are also pleasant to walk on.

Many modifications are possible within the scope of the invention as defined by the accompanying claims.

Claims

A method for providing a hard piece of wood using compression treatment in order to compress a disjoint timber of wood, which has been prepared such that it comprises a piece of wood (1; 19, 20; 25) having vertical annual rings perpendicular to the lradial surfaces of the piece of wood,
characterized by

a) placing the piece of wood (1; 19, 20; 25) between two plates (3, 4; 23,24) and having its radial surfaces turned towards the plates, the plates having a fixedly provided position in relation to each other and having a distance to each other adapted to the thickness of the piece of wood (1; 19, 20; 25) between its radial surfaces;

b) compression (9,10; 10,12) of the piece of wood between its tangential surfaces in radial direction to the annual rings of the wood using increasing compression force until a wanted compression has been reached.
A method according to claim 1, characterized in that the fixedly provided position for the two plates (3, 4; 23,24) is adjustable before the compression operation.
A method according to claim 1 or 2, characterized in that the compression action, after that a wanted compression has been reached, continues with constant compression force during a time fitted for providing fixation of the compression obtained.
A method according to anyone of the preceding claims, characterized by treating the piece of wood (1; 19, 20; 25) before compression with a chemical substance to obtain locking of the compression obtained when once obtained.
A method according to anyone of the preceding claims, characterized by heat treatment during and/or after compression to obtain thermo-mechanical locking.
A method according to anyone of the preceding claims, characterized by gluing one of the longitudinal sides, or both, to a fixating bed, e.g. a plate of wood, after compression.
A method according to anyone of the preceding claims, characterized by sawing the piece of wood (1; 19, 20; 25) to radial cut veneer before the placing between the plates.
A method according to anyone of the claims 1 to 6, characterized by sawing the piece of wood (1; 19, 20; 25) to veneer sheets after the compression.
A method according to anyone of the preceding claims, characterized by feeding the wood material (25) in the form of a plank through a press arrangement giving different compression pressure along the path along which the plank is fed through.
A device for providing a hard piece of wood (1; 19, 20; 25; 33, 34) having vertical annual rings perpendicular to the flat longitudinal sides of the piece of wood using compression treatment in order to provide compression of the piece of wood,
characterized by

a) two plates (3, 4; 13, 14; 23, 24) having a fixed position in relation to each other and having a distance to each other adapted to the thickness of the piece of wood (1; 19, 20; 25; 33, 34) between its flat longitudinal sides and between the two plates the piece of wood is adapted to be placed having its flat longitudinal sides turned towards the plates (3, 4; 13, 14; 23, 24);

b) press means (7, 9, 8, 10; 12, 8, 10; 26, 27; 31, 32) for compressing the piece of wood between its tangential sides in radial direction of the annual rings of the wood and with increasing compression power until a wanted compression has been obtained.
A device according to claim 10, characterized in that the fixed position for the two plates (3, 4; 13, 14; 23, 24) is adjustable before the compression operation.
A device according to claim 10 or 11, characterized by a fixating bed means, for example a wooden sheet (14), glued to the piece of wood (1, 1') after the compression operation.
A device according to anyone of the claims 10 to 12, characterized by pressure means comprising a press arrangement (26, 27; 31, 32) providing different compression power along a plank (25) of wood, or several planks (33, 34) of wood, fed through the press arrangement.
A device according to claim 13, characterized by
a chain of press plates (27, 28) in the press arrangement being placed near-by the plank or the planks, which are compressed, when feeding the plank or planks forward the press plates are adapted to be moved in the same batch wise or continuous rate as the plank or the planks.
A device according to anyone of the claims 10 to 14, characterized by a control unit (11, 13) providing control to the press means (7, 9, 8, 10; 12, 8, 10; 26, 27; 31, 32) in the press arrangement to provide a fitting pressure power for compressing the piece of wood.