A METHOD AND A DEVICE FOR FORMING A MAT OF MATERIAL

Technical Field

The present invention relates to a method and a device for forming a mat of cellulose-containing material, which mat is intended to form at least one board, disintegrated cellulose-containing material being fed from a feeding out unit in the form of a material flow to a forming unit which comprises at least one forming belt. The feeding out unit is positioned at a higher level than the forming unit. The device comprises at least one feeding down surface extending from the feeding out unit to the forming unit, and the transport of material between the feeding out unit and the forming unit is effected by feeding the material downwards along this feeding down surface. The material flow between the feeding down surface and the forming unit comprises a main material flow and a flow of dust particles which is separated from the main material flow.

Background of the Invention When producing boards of lignocellulose-containing material, such as particle boards and fibreboards according to the dry method (MDF, HDF, LDF etc.), first the lignocellulose-containing material is disintegrated into particles or fibre bundles. Subsequently, these are dried and glue-coated and formed to a continuous mat in one or several forming stations. The mat is pre-pressed and subse- quently finish-pressed to boards under pressure and heat in a continuous or discontinuous press.

The lignocellulose-containing material can be mixed with other materials, for example plastics, waste paper, glass fibres, disintegrated minerals etc., prior to forming or during the forming process. When producing boards, it is essential that the boards have homogeneous properties over the entire board surface. These properties include, inter alia, thickness, transverse tensile strength, bending and breaking strength, painting properties etc. Since these properties are dependent on the density of the board, it is important that the forming is effected with a good precision so that the density of the board is the same over the entire board sur- face. In order to achieve this, the particles or fibre bundles must be distributed as homogenously as possible, both longitudinally and transversely over the entire formed surface, during the forming process.

Conventional forming equipments for forming boards of lignocellulose-con- taining material are often mechanical. They usually comprise at least one feeding out unit comprising a dosing bin, or dosing container, where the material is intermediately stored, and the material is subsequently dosed down, free falling, to a forming unit where the material is laid down on a forming belt, i.e. a conveying belt. Further, when producing fibre boards, a scalping roll is often used subsequent to forming, which mills off any unevenness in the mat surface to improve the forming precision.

A drawback of the conventional forming equipments is that the material during its free fall between the feeding out unit and forming unit easily can be affected by surrounding air flows, both longitudinally and transversely, whereby the forming precision is impaired. Free fall, where this problem is present, arises, as mentioned above, during the fall between the feeding out unit and forming unit and also, depending on machine type, in the fall from the rolls of the forming unit down onto the forming belt.

During a free fall of a material flow in air, two inconvenient phenomena arise. Firstly, it is previously known that a material flow falling freely in air has a tendency to get together and form several material beams. This tendency increases with an increased height of fall. Secondly, co-ejecting air streaming and decelerating, or air-breaking, effects arise. These air streams drag long material and also make the fall shaft, which surrounds the material flow, non-transparent because of the material whirling about.

WO 2004/071731 A1 has solved the above-mentioned problem with the free fall from the feeding out unit to the forming unit by providing a feeding down surface which extends from the feeding out unit to the forming unit, and the transport of material between the feeding out unit and the forming unit is effected by feeding the material downwards along this feeding down surface, whereby a controlled material flow is provided between these units.

However, when using the solution of WO 2004/071731 A1 , it has been shown that dust spots are formed on the bottom side of the boards, e.g. MDF boards, which are produced from the mats formed by way of the feeding down surface disclosed in WO 2004/071731 A1. The dust which forms these dust spots consists mainly of dust particles comprising fibres having a fibre length less than 0.45 mm, whereas the other fibre material mainly comprises fibre and/or fibre bun-

dies having a fibre length greater than 0.45 mm. The inventor has found that these dust spots are formed on the bottom side of the finished boards as a result of the fact that the dust particles are separated from the other fibre material because the dust particles have a velocity, when they leave the feeding down surface, which is smaller then the velocity of the other fibre material, whereby the dust particles fall down on the forming belt before and in front of the other fibre material. It is believed that the reason for this is that the dust particles are slowed down by the friction of the feeding down surface more than the other material. Because of these dust spots on the underside of the finished boards, the boards are less attractive on the market in relation to boards with sides which look the same.

The Object of the Invention

The object of the present invention is thus to prevent that said dust spots are formed on the underside of produced boards.

Summary of the Invention The object of the present invention is attained by providing a device of the kind mentioned in the introductory part of the description, having the features which are mentioned in the characterizing portion of claim 1 , and by providing a method the kind mentioned in the introductory part of the description, having the steps which are mentioned in the characterizing portion of claim 11. Hereby, the dust particles are effectively mixed with the other material of the material flow between the feeding down surface and the forming belt before the material ends up on the forming belt, whereby the dust is not separated and concentrated deepest down in the mat closest to the forming belt. Hereby, the problem with dust spots on the underside of the produced boards is overcome. Further, the effect of the air flow on the material flow from the feeding down surface to the forming belt has the additional positive effect that a general mixing of the material in the material flow is provided, so that the underside of the finished board does not become patchy or speckled, which has shown to be a disadvantage when using said feeding down surface. Instead, the sides of the fin- ished board look the same. Boards having similar looking sides are sold at a higher price in relation to boards having different-looking sides, especially sides which have spots, are patchy and/or speckled.

Said air flow can be guided towards the material flow so that the air flow hits the material flow at a specific position, i.e. that the material flow is subjected to, or affected by, an air flow at a specific position between the feeding down surface and the forming belt, advantageously between the feeding down surface and the forming unit, more precisely immediately after the material flow has left the feeding down surface, but also other positions are possible. The material flow can also be subjected to an air flow at several different positions. The material flow may also be subjected to an air flow along the entire distance between the feeding down surface and the forming belt, or only along the entire distance between the feeding down surface and the forming unit. Further advantageously, the air flow, such as its velocity and direction, is adapted to the thickness and velocity of the material flow and to the quality of the material, such as the size and weight of the material, etc. When the material has landed and lies on the forming belt, it should not be subjected to the air flow. The feeding down surface of the device can be a conveying belt, a layer of rolls or a sliding plate, but the separation of the material flow, which leaves the feeding down surface in the direction towards the forming belt, into a main material flow and a flow of dust particles which is separated from the main material flow, is most prominent when the feeding down surface is in the form of a sliding plate. At the fall of material in connection with the sliding plate, the motion of the material flow downwards is affected by the gravity, whereas in connection with a conveying belt and layer of rolls, the motion of the material flow downwards is also affected by the working speed of these. The sliding plate can comprise several partial plates positioned next each to another. According to an advantageous embodiment of the device according to the present invention, the guiding means is provided between the feeding down surface and the forming belt so that the material flow passes above the guiding means.

According to a further advantageous embodiment of the device according to the present invention, the guiding means is movable in a vertical direction and in relation to the forming belt. This is advantageous since flexibility when producing mats having different thicknesses is obtained, because the distance between the guiding means and the forming belt is set based on the requested thickness/height of the mat on the belt.

According to another advantageous embodiment of the device according to the present invention, the device comprises a flexible partition wall for closing the opening defined by the forming belt and the guiding means, and the partition wall is adjustable to different gaps between the forming belt and the guiding means, whereby material is prevented from passing under the guiding means. Advantageously, the partition wall has a substantially vertical extension.

According to other advantageous embodiments of the device according to the present invention, the guiding means is provided at the lower side of the feeding down surface, whereby the material flow is subjected to the air flow immedi- ately when the material leaves the feeding down surface, and a good mixing of the main material flow and the dust particle flow is obtained, or the guiding means is even pivotally attached to the lower side of the feeding down surface. By the pivotal attachment of the guiding means to the feeding down surface, the position of the guiding means in relation to the horizontal plane can be the same at different heights and distances from the forming belt.

According to further advantageous embodiments of the device according to the present invention, the longitudinal extension of the feeding down surface is adjustable, and/or the feeding down surface is pivotally attached to the feeding out unit. If the feeding down surface has both an adjustable longitudinal extension, which can be shortened and lengthened, and is pivotally attached to the feeding out unit, the guiding means can, for example, be moved in a frame in a strictly vertical direction. This is described in more detail in the detailed description of embodiments.

According to an advantageous embodiment of the method according to the present invention, the air flow is guided transversely to the direction of motion of the material flow between the feeding down surface and the forming belt. Hereby, the material flow is efficiently subjected to the air flow and an effective mixing of the main material flow and the dust particle flow is obtained.

According to a further advantageous embodiment of the method according to the present invention, the guidance of an air flow is effected so that the material flow between the feeding down surface and the forming belt is subjected to said air flow from below, the results of which is that an efficient mixing of the main material flow and the dust particle is obtained, and at the same time, the material flow can be guided more effectively towards the forming unit.

According to advantageous embodiments of the method according to the present invention, the air flow is guided by a guiding means for said guidance of an air flow, which advantageously is supplied with pressurized air, and advantageously, the guiding means is provided between the feeding down surface and the forming belt so that the material flow is passed above the guiding means.

According to other advantageous embodiments of the method according to the present invention, the method comprises the step of adjusting the vertical position of the guiding means and its distance to the forming belt based on the requested thickness of the mat on the forming belt, and/or the step of guiding an air flow towards the entire width of the material flow, the result of which is that the entire width of the material flow is subjected to the air flow.

Further, the present invention provides a plant for producing boards of cellulose-containing material, which plant comprises a device for forming a mat of cellulose-containing material, said device comprising the special features which are mentioned in any of the claims 1 to 10.

The present invention also provides a method for producing boards of cellulose-containing material, which method comprises the step of forming a mat of cellulose-containing material from disintegrated material, the method comprising the special steps which are mentioned in any of the claims 11 to 16. Such a designed method and such a designed plant, respectively, for producing boards provides the advantages which have been disclosed above.

As described above in the disclosure of the background of the invention, "material" does not only refer to material containing cellulose but also possible additions of any material, such as plastics, waste paper, glass fibres, disintegrated minerals etc. The material can be of different sizes and forms, and comprise fibres and/or fibre bundles, e.g.

Further advantageous embodiments of the device according to the present invention emerge from the dependent claims, and further advantages with the present invention emerge from the detailed description of embodiments.

Brief Description of the Drawings

The present invention will now be described, for exemplary purposes, in more detail by way of embodiments and with reference to the enclosed drawings, in which:

Fig. 1 is a schematic side view of a prior-art mechanical forming equipment for boards, comprising a feeding down surface between the feeding out unit and the forming unit;

Fig. 2 is an enlarged view of the feeding down surface of Fig. 1 ; Fig. 3 is a cross-section side view of an embodiment of the device according to the present invention, having the guiding means in a first position;

Fig. 4 shows the device of Fig. 3 having the guiding means in a second position; Fig. 5 is a schematic view of an embodiment of a the guiding means according to the present invention, seen from below;

Fig. 6 shows the section C-C of Fig. 5;

Fig. 7 shows a part of the section B-B of Fig. 5;

Fig. 8 shows a part of the section D-D of Fig. 5; Fig. 9 shows a part of the section A-A of Fig. 5; and

Fig. 10 is a flow chart illustrating an embodiment of the method according to the present invention.

Detailed Description of Embodiments

Fig. 1 shows a prior-art forming equipment for boards, as disclosed in WO 2004/071731 A1. The material is fed in via an inlet 101 to a feeding out unit 102, here in the form of a dosing bin 102. The feeding out unit 102 has an upper feeding back equipment 103, which feeds material, which has been fed in, to the left in the feeding out unit 102, feeding out rolls 104 and a bottom conveying belt 105 which feeds the material to the right towards the outlet of the feeding out unit 102. The material is fed from the feeding out unit 102 via the feeding out rolls 104 to the upper portion of a feeding down surface 106 provided in a fall shaft region 107 and extending between the feeding out unit 102 and a forming unit 108, where Fig. 2 shows an enlargement of said feeding down surface 106. The material is transported between the feeding out unit 102 and the forming unit 108 by feeding the material downwards along and on this feeding down surface 106, which here is in the form of a sliding plate 106. The term "forming unit" is used as a generic term for all the different parts and devices which can be used for forming a mat 201 , for example a forming belt 102, rolls 110, a forming station 111 , a scale 112 etc. The

device comprises a diffusion roll 202 which the material leaving the feeding down surface 106 passes. The diffusion roll 202 further adapts the speed of the material flow to the speed of the forming belt 109. In the forming station 111 , a mat 201 is formed on the forming belt 109 which transports the mat 201 of material to the right in the figure. The weight of the mat 201 can be continuously checked by a scale unit 112 and data from the scale unit 112 can be used for controlling the speed of the bottom conveying belt 105. The sides of the mat 201 are supported by sidewalls 113 before the entry into a pre-press 114.

Fig. 3 shows a cross-section of an embodiment of the device according to the present invention. The device comprises a feeding out unit 302 and a forming unit 304 with a forming belt 306, on which a mat is formed and which is adapted to transport material placed thereon to the right in the figure, and a rotatable diffusion roll 308, the purpose of which is the same as for the diffusion roll of Fig. 2, provided in connection with the forming station 311 with its rotation axis substantially parallel to the plane of the forming belt. The diffusion roll 308 is movable in a vertical direction by way of its attachment to a guiding element 309 which is provided in a first guiding device 313 in which the guiding element 309 is adapted to be slid- ingly moved in a vertical direction in the direction towards or away from the forming belt 306. The first guiding device 313 is provided in connection with the forming station 311. In Fig. 3, the guiding element 309 is in a first position adjacent to the forming belt 306.

The feeding out unit 302 is supplied with disintegrated cellulose-containing material and the material is subsequently fed to the forming unit 304, which is positioned at lower level, in the form of a material flow. The device comprises a feeding down surface 310 in the form of a sliding plate 312, 214 which extends from the feeding out unit 302 to the forming unit 304, which sliding plate 312, 314 is adapted to transport material between the feeding out unit 302 and the forming unit 304 by feeding the material downwards along and on this sliding plate 312, 314. The feeding down surface is limited by a lower side which is adjacent to the forming unit 304, an upper side which is adjacent to the feeding out unit 302 and two longitudinal sides which extend between said upper and lower sides. The sliding plate 312, 314 comprises two partial plates 312, 314 which are positioned next to each other, an upper partial plate 312 which is pivotally attached, via a first pivot axis 315 which extents substantially parallel to the plane of the forming belt,

to the feeding out unit 302, and a lower partial plate 314 which is slidingly mounted to the upper partial plate 312. The partial plates 312, 314 lie substantially in the same plane. The longitudinal extension of the feeding down surface 310 is adjustable, i.e. its length can be reduced or extended, as the two partial plates 312, 314 are slidable in relation to each other along their longitudinal axes. The lower partial plate 314 is guided under the upper partial plate 312. When the material flow has left the feeding down surface 310, the material flow is divided into a main material flow, in most cases comprising fibres and/or fibre bundles having a fibre length greater than 0.45 mm, and a flow of dust particles, which is separated from the main material flow, in most cases comprising fibres having a fibre length less than 0.45 mm. The device includes guiding means 316 for guiding an air flow towards the material flow between the feeding down surface 310 and the forming belt 306, more precisely towards the material flow between the feeding down surface 310 and the diffusion roll 308, to capture said dust particles and blow these into the main material flow before the material reaches the forming belt 306. The guiding means 316 is provided between the feeding down surface 310 and the forming belt 306 so that the material flow passes above the guiding means 316, and the guiding means 316 is pivotally attached to the lower side of the feeding down surface 310, which is also the lower side of the lower partial plate 314, via a second pivot axis 317 which extents substantially parallel to the plane of the forming belt. The guiding means 316 includes a first tubular element 318 provided with two inlets for pressurized air, and a second tubular element 318 provided with outlets for the air flow towards the material flow, where the first tubular element 318 has a larger cross-sectional area than the second tubular element 320. Be- tween the tubular elements 318, 320, there are openings for through-flow of air flow, and the tubular elements 318, 320 extend along the lower side of the feeding down surface and along the entire width of the feeding down surface 310. The guiding means 316 is adapted to guide an air flow towards the entire width of the material flow, the result of which is that the entire width of the material flow is sub- jected to the air flow. The tubular elements 318, 320 extend substantially parallel to the plane of the forming belt. The tubular elements 318, 320 are described in more detail in connection with Figs. 5 to 9.

The guiding means 316 is provided in a second guiding device 322 in which the guiding means 316 is adapted to be slidingly moved in a vertical direc-

tion towards or away from the forming belt 306, whereby the guiding means 316 is positionable at different distances to the forming belt 306. The distance between the guiding means 316 and the forming belt 306 is advantageously between 50 and 150 mm, but is set based on the requested thickness of the mat to be formed on the forming belt 306. Since the feeding down surface 310 both has an adjustable length and is pivotably attached to the feeding out unit 302, and the guiding means 316 is pivotably attached to the feeding down surface 310, the position of the guiding means 316 in relation to the horizontal plane can be the same at different heights and distances to the forming belt 306, and at the same time, the guiding means 310 can be moved within the second guiding device 322 in a strictly vertical direction of movement and/or in a direction of movement perpendicular to the plane of the forming belt 306. The guiding means 316 is connected to the guiding element 309, which provides for that the movement of the guiding means 316 a certain distance gives a movement of the guiding element 309 a cor- responding distance. The result of this is that the position of the guiding means 316 in relation to the diffusion roll 308 is always the same. In Fig. 3, the guiding means 316 is in a first position adjacent to the forming belt 306.

The device includes a flexible partition wall 324 for closing the opening defined by the forming belt 306 and the guiding means 316, which partition wall 324 extends in a substantially vertical direction. The partition wall 324 is adjustable to different gaps between the forming belt 306 and the guiding means 316, whereby material is prevented from passing under the guiding means 316. The partition wall 324 is formed by a portion of a flexible belt 326, the width of which corresponds to the width of the feeding down surface 310. The first end portion of the belt 326 is attached to the guiding means 316 and its second end portion is attached to a weight 328 which is provided in vertically extending and tubular guiding unit 330 and is adapted to be moved, i.e. elevated and lowered, in the guiding unit 330. The guiding unit 330 is attached adjacent to the forming belt 306. The belt 326 extends around a guiding roll 332 which is mounted in the forming station 311 and is adjacent to the forming belt 306. The weight 328 stretches the belt 326 so that the belt is kept stretched. In Fig. 3 the weight 328 is positioned at a lower position.

Fig. 4 shows the device of Fig. 3 having the guiding means 316 in a second position which is at a longer distance to the forming belt 306 compared to the

first position of the guiding means 316 shown in Fig. 3. The guiding element 309 provided with the diffusion roll 308 is also moved a distance corresponding to the distance of movement of the guiding means 316 to a second position which is at a longer distance to the forming belt 306 compared to the first position of the guiding element 309, whereby the position of the guiding means 316 in relation to the position of the diffusion roll 308 is the same. The longitudinal extension of the feeding down surface 310 has been reduced by the fact that more of the lower partial plate 314 has been moved under the upper partial plate 312, and the angle formed between the feeding down surface 310 and the plane of the forming belt 306 has changed. The vertical extension of the partition wall 324 has been extended with a certain length in order to close the larger opening which is formed between the forming belt 306 and the guiding means 316, whereby the extension of the belt 326 within and along the guiding unit 330 is reduced with a corresponding length, and the weight 328 is moved to an upper position, but is still stretching the belt 326 so it is kept stretched.

Fig. 5 shows the guiding means 316 of Fig. 3 seen from above, comprising the first tubular element 318 provided with a first inlet 502 and a second inlet 504 for pressurized air, and the second tubular element 320 which is joined together with the first tubular element 318. The second tubular element 320 is located be- tween the first tubular element 318 and the forming station 311 (see Fig. 3). The tubular elements 318, 320 are advantageously made of a suitable metal.

Fig. 6 shows the section C-C of Fig. 5. Here it is shown that the cross-sections of both the first and second tubular elements 318, 320 are rectangular, and the first tubular element 318 has a larger cross-section area than the second tu- bular element 320, which provides for an advantageous pressure balancing. Between the fist and second tubular elements 318, 320 there are openings 601 for through-flow of air flow from the first tubular element 318 to the second tubular element 320, and the second tubular element 320 is provided with outlets 602 for the air flow towards the material flow. Fig. 7 shows only a part of the section B-B of Fig. 5, corresponding to the distance E of Fig. 5. The first tubular element 318 is provided with ten openings 702, 704, 706, 708, 710 for the outflow of air flow, which are formed as elongated slits, and between each two openings 702, 704, 706, 708, 710 there is a distance, which provides for the rigidity of the structure.

Fig. 8 shows a part of the section D-D of Fig. 5, corresponding to the distance E of Fig. 5, and the side of the second tubular element 320 which abuts the first tubular element 318 and its openings 702, 704, 706, 708, 710 for the outflow of airflow. The second tubular element 320 is also provided with ten openings 802, 804, 806, 808, 810 for the inflow of airflow, the dimensions and positions of which correspond to the openings 702, 704, 706, 708, 710 for the outflow of airflow of the first tubular element 318, so that when the first tubular element 310 is joined together with the second tubular element 320, ten openings between the first and second tubular elements 318, 320 are formed for the through-flow of airflow. Fig. 9 shows a part of the section A-A of Fig. 5, corresponding to the distance E of Fig. 5, and the side of the second tubular element 320 which faces the material flow leaving the feeding down surface. On this side, the second tubular element 320 is provided with fifteen outlets 812, 814, 816, 818, 822, 824 for the airflow towards the material flow, which are also formed as elongated slits but have a smaller width than the openings 802, 804, 806, 808, 810 for inflow of airflow of the second tubular element 320, and between each two outlets 812, 814, 816, 818, 822, 824 there is a distance, which provides for the rigidity of the structure.

Fig. 10 is a flow chart illustrating an embodiment of the method for forming a mat of cellulose-containing material, which mat is intended to form at least one board, according to the present invention, by way of a feeding out unit, a feeding down surface, a diffusion roll and a forming unit comprising a forming belt, as described in connection with Figs. 1 and 2. The method comprises the following steps: A guiding means for guiding an airflow is provided, at 901 , between the feeding down surface and the forming belt, and the height of the guiding means and distance of the guiding means to the forming belt are set, at 902, based on the requested thickness of the mat on the forming belt. Disintegrated cellulose-containing material is fed to a feeding out unit, at 903, and subsequently transported from the feeding out unit by feeding the material downwards along this feeding down surface, at 904. When the material leaves the feeding down surface in the direction towards the forming unit, and, inter alia, passes above the guiding means, the material flow is separated into a main material flow and a flow of dust particles which is separated from the main material flow, as described above. An air flow is guided towards the material flow between the feeding down surface and

the forming belt, at 905, to capture said dust particles and blow these into the main material flow before the material reaches the forming belt. The airflow is guided transversely to the direction of motion of the material flow between the feeding down surface and the forming belt, and towards the entire width of the material flow. The guidance of the airflow is effected so that the material flow is subjected to said air flow from below and from the side. Subsequently, the material flow mixed by way of the airflow is treated by a diffusion roll, after which the material flow reaches the forming unit and lands on the forming belt, at 906. After this, subsequent treatment of the mat follows.