GB2136837A

GB2136837A - Method of and device for spreading particle boards

Info

Publication number: GB2136837A
Application number: GB08321687A
Authority: GB
Inventors: Karl Meier; Hofmattstrasse Hans Knoepfel
Original assignee: VERSATEC AG
Current assignee: VERSATEC AG
Priority date: 1983-03-18
Filing date: 1983-08-11
Publication date: 1984-09-26
Also published as: NL190926C; NL190926B; DE3309743A1; GB8321687D0; NL8302633A; IT8323935A1; DE3309743C2; IT8323935A0; IT1169473B; GB2136837B

Abstract

A method of spreading ligno-cellulose and binding agent containing material consisting of fine and coarse particles in the manufacture of cement-bound particle boards uses apparatus comprising a metering belt (1) supplying the material to a spreading roller (2) from which the particles are entrained and directed to a forming belt (6) disposed beneath the metering belt (1) by an airstream (14) generated by an air guide roller (3) situated between the belts (1, 6). The belts (1, 6) and air guide roller (3) are located in a spreading chamber (K) which is substantially closed by a baffle wall (4) and an air guide wall (5). The fine particles are entrained directly by the airstream (14) onto the forming belt (6) whilst the coarse particles fly farther and are deflected by baffle wall (4) onto the forming belt (6) to give better distribution of the coarse and fine particles in the preform (10). <IMAGE>

Description

SPECIFICATION Method of and device for spreading particle boards This invention relates to a method of spreading bulk material consisting of particles containing ligno-cellulose and wetted with binding agents and consisting of coarse and fine particles such as wood chips, wood fibres or similar, particularly cement-wetted wood chips, for the manufacture of cement-bound particle boards, in which the bulk material is spread from a metering belt by means of mechanical spreading means, for example a spreading roller or similar, in a particle stream in the direction of the forward movement of a lower-lying forming belt, being spread onto the latter to form a preform.The invention also relates to a device for implementing the above method having a metering belt which accepts the bulk material consisting of fine and coarse particles and with a mechanical spreading means such such as a spreading roller or similar, disposed in the end region of the metering belt, and with a lower-lying moving forming belt for accepting the preform which is to be spread.

As is known, the spreading of the bulk material consisting of coarse and fine particles, e.g. coarse and fine chips, for the manufacture of wood particle boards, particularly cement-bound wood paticle boards, is one of the most important process steps in the industrial mass production of wood particle boards.

The spreading of the bulk material onto a forming belt, which is situated in the spreading direction below the spreading device, serves to make the so-called preforms, i.e. a particle cake or particle mat, whose quality is decisive with regard to the final properties of the boards being manufactured. It is of particular importance in this connection that the preform is spread with maximum accuracy and uniformity, since only in this way is it possible for the required bulk material to be used in an economic manner. Furthermore, the demand for accuracy and uniformity of the spreading operation is very significant from the technological point of view so that it possible to manufacture boards of a predetermined, required quality in terms of structure and strength.Moreover, the uniform distribution of the bulk material over the cross section of the board is of decisive importance with regard to the degree and uniformity of the strength properties of the finished cementbound wood particle board.

The known spreading methods can be divided into two categories in the fabrication of particle boards.

The first group comprises installations in which the bulk material is spread according to the so-called air separation method. In this system, the particle mat coming from the metering hopper reaches a delivery roller, for example a spiked roller, which scatters the particles, thereby causing the disintegration of the mat. The falling particles then fall through an air stream which distributes them onto the forming belt in such a way that the finest particles are deposited in the outer layers and the coarse particles in the middle layer. This process of air separation spreading works satisfactorily for particle boards, but exhibits a considerable disadvantage in the case of cement-bound wood particle boards. The known method only results in satisfactory spreading if the cement-wetted particles do not exceed a certain overall moisture content.Although such cement-wetted particles can still be satisfactorily spread, there result boards with relatively low strength values.

Attempts have already been undertaken to eliminate this disadvantage in the case of three-layer boards in that the dry covering layers are spread by air separation while the middle layer of the board is spread in a different manner so that the moisture in the middle layer can be kept higher. Nevertheless, the results are not on a par with the boards in which all layers can be produced with sufficient moisture. In addition, the separate spreading of particles of differing moisture content for the covering layers and the middle layer is only economical in larger plants, especially as it also requires increased investment.

The second group comprises those installations which work with gravity spreading. In this method, the particles coming from the metering hopper are delivered to the rotating spreading devices, for example rotating rollers, spiked rollers or similar, e.g. by means of a metering belt, whereby the spreading devices, such as rollers or similar, break up the bulk material on the metering belt and cast it in the direction of the moving forming belt which is underneath the spreading device.

The motion of the individual particles results in a certain separating effect in that the heavier coarse particles assume longer trajectories and the lighter, fine particles assume shorter trajectories. The trajectory, as well as the degree to which the bulk material is broken up, are determined by the peripheral speed of the rollers which are provided with spikes or cams.

In many cases use is made of an entire system of such spreading rollers in order to obtain satisfactory breaking up and separation of the particles. Nevertheless, this method has numerous deficiencies, such as uneven separation, short spreading angles, reduction in weight at the edges of the board, build-up at the walls etc. The most disadvantageous effect is that very often individual coarse chips get into the covering layer consisting of finer chips, with the result that the uniform structure of the fine covering layers is destroyed. A further disadvantage of the known gravity spreading methods is that the rotation of the spreading devices results in uncontrollable air streams which entrain in particular the fine and superfine particles so that there is inaccuracy of spreading.The uncontrolled air streams also result in undefinable flow conditions in the spreading space above the forming belt. All these disadvantages have an even greater impact when spreading cement-bound wood particle boards.

The object of the invention is to create a method of spreading bulk material containing lignocellulose and consisting of coarse and fine particles, particularly cement-wetted particles, said method avoiding the aforementioned disadvantage and in which the fine and coarse particles are separated uniformly and with a fine transition from each other over a large area so that the occurrence of internal stresses during pressing is largely reduced and the stability of the boards and their strength characteristics are enhanced. The object of the invention also includes the creation of a device enabling the method according to the invention to be implemented.

The invention provides a method of spreading bulk material consisting of coarse and fine particles, for the manufacture of cementbound particle boards, in which the bulk material is spread from a metering belt by means of a mechanical spreading means, in a particle stream in the direction of the forward movement of a lower-lying forming belt, being spread onto the latter to form a preform, wherein the fine particles are at least partially separated out of the particle stream by means of a forced air flow and are brought onto the forming belt by a shorter route than said coarse particles.

The invention also provides a device for implementing the above method with a metering belt which accepts the bulk material consisting of fine and coarse particles and with a mechanical spreading means disposed in the end region of the metering belt, and with a lower-lying, moving forming belt for accepting the preform which is to be spread, wherein disposed below the spreading means is a device which serves to generate a forced air flow directed downwardly towards the forming belt.

In such a process, a particle stream is formed by the spreading device following the breaking up of the metered bulk material mass on the metering belt. From this particle stream the fines are separated out by means of a specially generated forced air flow, and these fines are then brought onto the forming belt by the shortest route, ensuring that the finest particles are already directed onto the forming belt before any other particles are deposited on it. Consequently, the finest particles get into the outermost layers of the board so that the surface of the boards is always certain to be fine and smooth.

Moreover, optimum spreading results are achieved if, either alone or in conjunction with the described generation of a forced air flow, with separation of the fine and superfine particles, the actual particle stream generated by the spreading device with the further4lying, coarse particles, particularly those flying too far, is retarded at a rear point of the actual spreading chamber, preferably by mechanical means, and is diverted directly to the forming belt by the shortest route. Retardation can be advantageously achieved by a baffle wall which, starting at the spreading device, extends forward in a curve and downwardly until just above the surface of the preform.In this method for spreading the cement-wetted particles, it is important for the particles to be cast from the metering belt, preferably a fine material belt, by a spreading roller into a special spreading chamber whose front wall forms a specially shaped baffle wall such that a long wedge can be formed on the forming belt by some of the coarser particles impacting on this baffle wall.So that, in contrast to the known designs, the particle stream generated by the spreading device is prevented from entraining much uncontrolled air which conveys the fines too far away, thus bringing about a considerable deterioration in the desired separation, there is a reduction of the air stream through the formations of a specific and directed forced air flow and a changing of its direction, whereby this forced air flow can be generated by an air guide roller in the form, for example of a toothed disc roller.

Another important feature of the method is that the actual particle stream and the separating air stream for the fines, which is generated by forced air and deflected to the forming belt, as well as the mechanical retardation and deflection of the further-flying larger particles, all take place in a space which is at least substantially enclosed from the point of view of fluid mechanics such that the air streams circulating in it are largely controlled ble and desired. This circulation of the air in a space which is substantially enclosed provides the important advantage that, when spreading cement-wetted particles, this constantly circulating air can only absorb a limited amount of moisture from the particles, as a result of which the undesired drying of the cementwetted particles is largely prevented. This fact alone results in a considerable improvement of the properties of the finished cement-bound particle board.

In the specimen embodiment of the device according to the invention shown in diagrammatic form in the accompanying drawing, the metering belt 1 is driven by the drive roller 1 6 rotating in the direction of the arrow. The bulk material, which is delivered forwardly in the direction of the arrow 1 7 on the metering belt 1 in a metered bulk material layer 8, comes at the front of the belt 1 into the area of the spreading roller 2 which rotates about a horizontal axis in the direction of the arrow.

This spreading roller 2 which is provided with spikes or protrusions rotates at such a speed that the incoming bulk material 8 is broken up and is cast forward in a particle stream.

For the selective separation of superfine particles and for obtaining a controlled air circulation within the spreading chamber K, there is, below the spreading roller 2, a device for generating a forced air flow 14 which is directed downwardly to the forming belt 6. In the specimen embodiment shown, this device consists of a so-called air guide roller 3 which, for example, may be in the form of a toothed disc roller. The rotation of this air guide roller 3 in the direction of the arrow 18 generates an air flow in the direction of arrows 14'.

With the stated direction of rotation this air flow in the front region of the air guide roller 3 is directed downwardly in the direction 14 of the forming belt 6 and, furthermore, runs along the partiai circumference of the air guide roller 3 as shown by the arrows 14'.

This forced air flow 14 ensures that, immediately after being ejected by the spreading roller 2, fine and superfine particles are separated out of the actual particle stream and are brought downwardly directly onto the forming belt 6 which is driven by the roller 6', before other more coarse particles get there. This guarantees that the fine and superfine particles get into the outermost layer of the finished board, thereby improving the surface quality of the board.

At the same time, the disadvantage of known designs is prevented whereby too much air is entrained with the actual particle stream, causing fines to be taken too far to the front and bringing about a considerable deterioration in the desired separation.

The air guide roller 3, whose diameter is at least as large as the outside diameter of the spreading roller 2, and which lies directly below the spreading roller 2, cooperates below the drive roller 16/metering belt 1 with an air guide wall 5 which is directed downwardly to the forming belt and is arched to the rear, such that, firstly, the spreading chamber K is substantially closed off to the rear from the point of view of fluid mechanics, and, secondly, an inlet gap 9 for the forced air flow is formed in the region of the intake side of the rotating air guide roller 3 and tapers away toward the top in the direction of the drive roller 1 6 of the metering belt 1, as a result of which the speed of the inducted air stream at this point is increased.

Since the spreading chamber space is virtually closed off to the rear by the air guide wall 5, the positioning of the air guide roller 3 in particular results in the fact that the air currents circulating in the spreading chamber K can be controlled even in terms of quantity and direction.

If the spreading chamber is also substantially closed off in its front region by the wall identified as the baffle wall 4, there results inside the spreading station 1 2 a space which itself is substantially enclosed, at least from the point of view of fluid mechanics, in which space a limited air quantity is constantly circulated with a specific flow direction so that this air can only absorb a limited amount of moisture from the cement-wetted particles. As a result of this an undesired drying of the bulk material is largely prevented. The enclosed design of spreading chamber K is also enhanced by the fact that the spreading roller 2 and the drive roller 1 6 of the metering belt 1 nearly close off the space between the walls 4 and 5.

The baffle wall 4 is such that the coarse particles in the particle stream are, upon impact, deflected downwardly to the forming belt 5. The baffle wall 4 is, therefore, designed and shaped in such a way that it interrupts the trajectories of the coarser particles which are cast against it, deflecting these coarser particles downwardly to the forming belt. For this purpose, the baffle wall 4 runs in an arch whereby its definitive form should, of course, be specified through appropriate tests. In a practical arrangement, the baffle wall 4 runs in a convex arch starting at the spreading roller 2 as far as the vicinity of the surface of the preform 10.The position of the baffle wall in relation to the spreading roller and/or in relation to the air guide roller should be specified depending on the existing conditions whereby, for the easy adjustment of the longitudinal distance between spreading roller 2/air guide roller 3 and baffle wall 4, the latter is adjustable and can be changed in its position. Preferably the baffle wall 4 may be at least approximately parabolic in form and may extend at least as far as the downstream end of the preform spreading wedge 7; it may alternatively have an elliptical form.

The premetered bulk material is thus delivered on the metering belt 1 to the spreading roller 2 and is cast by the latter in a particle stream in the direction of the specially shaped baffle wall 4. As already mentioned, this baffle wall 4 must be shaped in such a way that a constantly uniform, elongated, well broken-up wedge 7 is formed on the forming belt 6. The wedge 7 represents the lower half of the board which is being produced; the upper half is produced in smaller systems by the movement of the forming belt 6 opposite to direction 1 5 or by a second spreading system disposed preferably as a mirror image of the first.

The position of the air guide roller 3 directly below the spreading roller 2 and the design of this air guide roller 3 such that its diameter is greater than that of the spreading roller 2, has the advantage that the particles falling down more or less vertically from the spreading roller 2 can no longer, as before, get directly onto the forming belt and thus enter the outer fine covering layers. Any particles falling down in this area are accepted by the air guide roller 3 and, as a result of its rotation, are ejected by it and redelivered into the main particle stream so that they get into the middle layer of the preform.Owing to the fact that the spreading chamber K is substantially enclosed from the point of view of fluid mechanics, it is always the same air which is circulated inside this chamber and which thus always has sufficient moisture, as a result of which an undesired withdrawal of moisture from the particles is prevented. Another advantage is that as a result of this enclosed air circulation, comparatively little air can escape into the space of the spreading station 12 and thus into the fabrication area, with the result that less dust can also get into the surrounding areas.

In the diagrammatic representation the trajectories 11 to 11 d of the particles starting at the spreading roller and the deflection paths are drawn as straight lines for the sake of simplicity and in order to illustrate the operating principle of the baffle wall.However, it can be seen that the design of the baffle wall 4 results in a particularly concentrated and directed spreading of the particles inside the spreading chamber K whereby, in conjunction with the directed generation of a forced air flow, and the thus achieved settling of the flow conditions in the spreading chamber, as well as the preseparation of fine and superfine particles, there results a wedge of spread material with optimum stratification of the finest particles in the outer layers and the coarse particles in the middle layers with previously unattained uniformity and accuracy of the control itself. With the method and device according to the invention it is possible to obtain the excellent spreading of cementwetted particles without difficulty, even if the bulk material has a high moisture content.

The position and size of the air guide roller 3 prevents the undesired falling down of the coarse particles from the spreading roller 2 onto the forming belt below, whereby this air guide roller 3 also supports the breaking up of the bulk material by the spreading roller 2 so that, in total, it is possible to obtain preforms for cement-bound particle boards which are of high quality in terms of appearance, strength and stability.

Claims

1. A method of spreading bulk material consisting of coarse and fine particles for the manufacture of cement-bound particle boards, in which the bulk material is spread from a metering belt by means of a mechanical spreading means, in a particle stream in the direction of the forward movement of a lowerlying forming belt, being spread onto the latter to form a preform, wherein the fine particles are at least partially separated out of the particle stream by means of a forced air flow and are brought onto the forming belt by a shorter route than said coarse particles.

2. A method according to claim 1, wherein in order to obtain as short a falling distance as possible the separated fine particles are retarded.

3. A method according to claim 1 or claim 2, wherein the separated fine particles are deflected downwardly toward the forming belt.

4. A method according to any one of claims 1 to 3, wherein coarse particles which travel farthest in the particle stream are retarded at a rear point and are deflected to the forming belt.

5. A method according to any one of claims 1 to 4, wherein the generation of the particle stream, the generation of the forced air flow and the generation of the separating air flow deflected by the latter toward the forming belt as well as the deflection of the farthest-flying coarse particles take place in a space which is substantially enclosed from the point of view of fluid mechanics.

6. A device for implementing the method according to claim 1 with a metering belt which accepts the bulk material consisting of fine and coarse particles and with a mechanical spreading means disposed in the end region of the metering belt, and with a lowerlying, moving forming belt for accepting the preform which is to be spread, wherein disposed below the spreading means is a device which serves to generate a forced air flow directed downwardly towards the forming belt.

7. A device according to claim 6, wherein the device generating the forced air flow is a rotating air guide roller.

8. A device according to claim 7, wherein said guide roller is in the form of a toothed disc roller.

9. A device according to claim 7 or claim 8, wherein the air guide roller has an outside diameter which is at least as large as the outside diameter of the spreading roller.

10. A device according to claim 9, wherein the air guide roller is disposed vertically below the spreading roller.

11. A device according to any one of claims 7 to 10, wherein there is a channellike, narrowing air inlet gap between the outer circumference of the air guide roller and the outer circumference of a drive roller of the metering belt.

1 2. A device according to claim 11, wherein an air guide wall directed toward the forming belt is disposed in the region of the air guide roller and the bottom side of the metering belt such that an air guide channel which tapers in the direction of rotation of the air guide roller is created for the inlet of the separating air stream.

1 3. A device according to any one of claims 6 to 12, wherein disposed in the rear of the particle stream entraining the coarse particles is a baffle wall which in use deflects said coarse particles downwardly to the forming belt.

14. A device according to claim 13, wherein the baffle wall has a form such as to interrupt the trajectory of the coarse particles which fly farthest, deflecting the latter downwardly to the forming belt.

1 5. A device according to claim 1 3 or claim 14, wherein the longitudinal distance between the baffle wall and the spreading roller is adjustable.

16. A device according to any one of claims 1 3 to 15, wherein the baffle wall runs in an arch from the spreading roller as far as the vicinity of the surface of the preform which is being spread.

1 7. A device according to claim 12 and any one of claims 1 3 to 16, wherein the baffle wall, the spreading roller, the front section of the metering belt and the air guide wall form a space which is substantially enclosed from the point of view of fluid mechanics and whose bottom boundary is formed by the forming belt.

18. A device according to any one of claims 1 3 to 17, wherein the baffle wall has a substantially parabolic form.

19. A device according to any one of claims 1 3 to 17, wherein the baffle wall has a substantially elliptical form.

20. A device as defined in any one of claims 1 3 to 19, wherein the baffle wall extends at least as far as the downstream end of the wedge-shaped spreading region of the preform.

21. A method of spreading bulk material substantially as hereinbefore described.

22. A device for implementing the method of claim 21 substantially as hereinbefore described with reference to and as illustrated in the accompanying drawing.