METHOD AND ARRANGEMENT FOR THE PRODUCTION OF LIGNOCELLULOSE-CONTAINING BOARDS

Abstract: In a method and an arrangement for continuously producing lignocellulose-containing boards, the material is disintegrated into particle and/or fibre form, glue-coated, dried, and formed into a mat (1). The mat is pressed into board form (3) in a continuous steam injection press (2) and the board then passed through an after-conditioning zone (4). A specific volume of air having a specific moisture content and temperature is drawn through the board (3) in the after-conditioning zone (4) by suction.

Description: METHOD AND ARRANGEMENT FOR THE PRODUCTION OF LIGNOCELLU- LOSE-CONTAINING BOARDS The present invention relates to a method of producing continuously lig- nocellulose-containing boards in accordance with the preamble of claim 1 , and to an arrangement for carrying out the method in accordance with the preamble of claim 5. Methods of producing lignocellulose-containing board are well known to the art and have found wide use in practice. The manufacture of such boards in- eludes the following main method steps: disintegration of the raw material into particles and/or fibres of appropriate size, drying the particles and/or fibres to a determined moisture quotient and glue-coating the material either prior to or subsequent to said drying process, shaping the glue-coated material to form a mat, which may comprise several layers, and optionally cold pre-pressing the mat, pre- heating said mat, water-spraying mat surfaces, etc., and heat pressing the mat in a discontinuous press or in a continuous press while subjecting the material simultaneously to pressure and heat so as to obtain a finished board. It is difficult to control the quality of the boards produced in accordance with this known method with respect to the moisture content, temperature and dimensional stability of the boards. When the boards leave the heat pressing in the production process, they have a temperature in excess of 100°C and a corresponding vapour pressure. The temperature of the board surfaces falls rapidly to beneath 100°C as the enclosed moisture is vaporised by virtue of a so-called flash effect. The boards are then cooled in so-called cooling wheels. As a result, the boards will obtain a moisture content of about 6-7% after intermediate storage of the board over a day or two. In many applications this creates a problem in environments that have a higher average relative humidity, since the boards will take up moisture when used and therewith undergo dimensional changes, as in the case of all lignocellulose-containing materials. One way of counteracting this is to spray water on the boards as they leave the press. Another known phenomena is that boards produced in this way obtain mutually different moisture contents in the surface layers relative to the core layer. If the boards are used for some type of sur ace treatment for instance, such as lamination, without having earlier equalised the difference in moisture content, the dimensions of the board may change when this equalisation takes place in time so that the surface layer loosens. In order to achieve desired equalisation between the various layers, it is customary to store the boards for a number of weeks. Another known problem is that the boards are not dimensionally stable when they leave the press. This is noticeable primarily because the boards shrink or swell over a process that can take one or more days. Consequently, calibration grinding of the boards is not normally undertaken until the boards have been stored in intermediate storage locations over a number of days. Another known problem is that the boards are too hot to be stacked and stored when leaving the hot press. If the boards are too hot when stacked, the glue joints may begin to break down and the boards consequently weakened. This problem is normally alleviated by keeping the boards in a so-called cooling wheel in which the board temperature is lowered by natural convection. It will be evident from the aforegoing that the conventionally used press technique and board production technique involve a number of cost-inducing handling stages and intermediate storage subsequent to the actual board manufacturing process. Accordingly, the object of the present invention is to stabilise a board with respect to its moisture content, temperature and dimensional stability in a continuous process, and therewith avoid cost-inducing handling and storage of the board. Because dimensional stability is achieved, the boards may also be ground or sanded down to a final thickness directly after manufacture. This object is achieved with the method and the apparatus defined in the characterizing clauses of respective claims. The invention will now be described in more detail with reference to the accompanying drawing, which is a schematic longitudinal section view of plant constructed in accordance with the invention. The plant illustrated in the drawing is based on the plant disclosed in SE 504 638, which describes a continuous steam injection process. A mat 1 formed from lignocellulose-containing material is fed into a continuous steam injection press 2 and there pressed into boards 3. The boards exiting from the continuous steam injection press 2 enter an after-conditioning zone 4. In the illustrated example, the zone 4 includes two after-conditioning units 5 and 6. The boards can be transferred directly to a grinder 7 from the after-conditioning zone 4, for grinding of the board to a final thickness. In accordance with the invention, each after-conditioning unit 5, 6 comprises an air supply unit 8 that includes a suction fan 9 and a heater 10. A steam or water supply device 11 may also be provided for moistening the air. The air is sucked into the two air supply units at 12. As will be seen from the drawing, the air is delivered from above in the case of the after-conditioning unit 5, and from be- neath in the case of the after-conditioning unit 6. Thus, as the boards exit from the continuous steam injection press 2 they pass into the after-conditioning zone 4 in which air is sucked through the boards with the aid of negative pressure in an amount determined in relation to board production and at a specific moisture content and temperature. In the first after- conditioning unit 5, the air is sucked down through the board, whereas in the after- conditioning unit 6 the air is sucked through the board in the opposite direction, i.e. upwards. However, this double air flow in mutually opposite directions is not necessary in order to achieve an effect since in certain cases the throughflow of air in only one direction will suffice, meaning that only one after-conditioning unit will be required. It can be mentioned by way of example that a board having a density of 600 kg/m 3 and a thickness of 16.6 mm is cooled from 100 to 60°C in 60 seconds when applying a subpressure of 15 kPa. By way of another example, it can be mentioned that a board having a density of 600 kg/m 3 and a thickness of 32 mm is correspondingly cooled in 80 seconds. It will also be noted that board having a thickness of 10 mm and a density of 650 kg/m 3 and produced in accordance with the invention in a pilot plant obtained a stable thickness after having passed through the after-conditioning zone. Measurements made one or more days after manufacture showed that boards which had passed through the after-conditioning zone retained their thickness, whereas boards that had not passed through said zone were often liable to shrink up to 1 mm within a day or two, in the same way as conventionally manufactured board shrinks. Conventionally produced boards have an enhanced density at their surfaces. However, because it is possible to produce in the steam injection press boards that do not have an enhanced surface density, the air throughflow and therewith conditioning of the board and lowering of its temperature can be effected more quickly than in the case of conventional board handling techniques.

Claims: CLAIMS 1. A method of continuously producing lignocellulose-containing board in which the material is disintegrated into particle and/or fibre form, glue-coated, dried and formed into a mat (1) which is pressed into board form (3) in a continuous steam-injection press (2), and in which the board is thereafter passed through an after-conditioning unit (4), characterized by drawing a determined volume of air of given moisture content and temperature through the board in the after- conditioning zone (4) by suction. 2. A method according to claim 1 , characterized in that said air is first sucked through the board (3) in one direction and then in the opposite direction. 3. A method according to claim 1 or 2, characterized in that the surface lay- ers of the board (3) are given the same density as that of the centre layer in the steam injection press. 4. A method according to any one of claims 1 - 3, characterized in that the board (3) is ground to a final thickness directly after having left the after-condition- ing zone (4). 5. An arrangement for applying the method according to any one of claims 1 - 4 and comprising a steam injection press (2) and an after-conditioning zone (4), characterized in that the after-conditioning zone (4) includes at least one after- conditioning unit (5) that has an air supply unit (8) for the passage of air through a by-passing board. 6. An arrangement according to claim 5, characterized in that the after-conditioning zone (4) includes two after-conditioning units (5 and 6) which are each provided with an air supply unit for the passage of air through said board (3) from mutually opposite directions. 7. An arrangement according to claim 5 or claim 6, characterized by a grinding machine (7) positioned downstream of the after-conditioning zone (4) and functioning to grind the board (3) to its final thickness. AMENDED CLAIMS [received by the International Bureau on 30 June 2000 (30.06.00); original claims 1-7 replaced by new claims 1-5 (1 page)] 1. A method of continuously producing lignocellulose-containing board in which the material is disintegrated into particle and/or fibre form, glue-coated, dried and formed into a mat (1 ) which is pressed into board form (3) in a continuous steam-injection press (2), and in which the board is thereafter passed through an after-conditioning unit (4), characterized by drawing a determined volume of air of given moisture content and temperature through the board in the after- conditioning zone (4) by suction, and grinding the board (3) to a final thickness di- rectly after having left the after-conditioning zone (4). 2. A method according to claim 1 , characterized in that said air is first sucked through the board (3) in one direction and then in the opposite direction. 3. A method according to claim 1 or 2, characterized in that the surface layers of the board (3) are given the same density as that of the centre layer in the steam injection press. 4. An arrangement for applying the method according to any one of claims 1 - 3 and comprising a steam injection press (2) and an after-conditioning zone (4), characterized in that the after-conditioning zone (4) includes at least one after-conditioning unit (5) that has an air supply unit (8) for the passage of air through a by-passing board, and in that a grinding machine (7) is positioned downstream of the after-conditioning zone (4) and functioning to grind the board (3) to its final thickness. 5. An arrangement according to claim 4, characterized in that the after-conditioning zone (4) includes two after-conditioning units (5 and 6) which are each provided with an air supply unit for the passage of air through said board (3) from C mutually opposite directions.

Inventors: ERIKSSON N LENNART, SCHEDIN KURT, SISLEGAARD LARS-OTTO, ERIKSSON, N., LENNART, SCHEDIN, KURT, SISLEGAARD, LARS-OTTO
Applicants: ERIKSSON N LENNART, SCHEDIN KURT, SISLEGAARD LARS OTTO, VALMET FIBERTECH AB, ERIKSSON, N., LENNART, SCHEDIN, KURT, SISLEGAARD, LARS-OTTO
Application year: 2000
Main classification: D21J1
IPC codes: B27N3, B27N7