METHOD FOR FORMING OF A FIBER MAT

Abstract: Method for continuous forming of a fiber mat for manufacturing fiberboard according to the dry method, including metering fiber material and depositing the material on a surface of a moving transport means and forming the mat by means of adjustable rotating forming rolls (1). The forming rolls (1) are working in the deposited material whereby the upper surface of the mat is defined by the last forming roll (5). A bulb (4) of material is formed in front of the last forming roll (5) and the size of the bulb is indicated and used for control of the vertical position of the last forming roll (5).

Description: Method for forming of a fiber mat This invention concerns a method for continuous forming of a fiber mat for manufacturing fiberboard according to the dry method. When producing fiberboard according to the dry method (MDF, HDF, LDF etc) a fiber mat is formed on a moving transport means and subsequently pressed in a hot pressing operation. The mat can be formed as a single, homogenous, layer or in multiple layers, with different characteristics for fiber, resin, moisture etc, and such a fiber mat can also be formed on top of other types of mats, such as OSB, wafer board, particle board etc, to create a smooth surface layer. Forming of the mat is usually performed in a passive way, i.e. by merely spreading the fibers onto a moving belt or wire and then adjusting the thickness of the mat by removing fibers from the upper surface of the mat. For instance, the forming can be performed by carrying the fibers in an airstream from a fiber bin and depositing them onto a wire screen while sucking the air through the mat and screen, or by purely mechanical deposition of the fibers, after being etered from a dosing bin, onto a number of spreader rolls, which spread the fibers down onto a forming belt, forming a mat on the belt. There are also systems available which include combinations of pneumatical and mechanical systems. The mat so formed is in all cases adjusted by means of a scalper roll system to get an even upper surface and more important a correct mat weight. This scalper operation means that a certain amount of the fibers already formed are pneumatically transported back to the fiber bin. The amount of fibers so recirculated amounts to 20-40% of the final fiber mat for a pneumatic system and 5-15% for a mechanical one. This operation thus increases the load onto the former itself, requires a pneumatic system and electric energy for the transport and will finally degrade the fibers, due to mechanical treatment when passing the transport fan, aging of the resin due to the longer dwell time until hot pressing and uncontrolled change of the moisture content of the material when recirculated. The distribution of fibers crosswise when using the pneumatic type of former is made by means of a mechanical nozzle or impulse air pushing the falling fibers to one side or the other. This method is very sensitive to changes in the pneu atical balances and needs frequent observations and adjustments . When using the mechanical type of former, most of the distribution crosswise is determined by the spreading of fibers into the dosing bin, little can be done when the mat is formed. A conventional mechanical former is disclosed in US Patent 5,496,570. This patent shows a bed of forming rolls working above the mat for spreading the fibers. This patent also shows a typical scalper roll used for adjusting the mat surface. Patent application WO 96/16776 shows a device for levelling a particulate material web or mat. This device consists of elliptical disks used to even out the upper surface of a formed mat with the aim to diminish the scalping. Our copending SE patent application 9800209-0 discloses a method and an apparatus for improving the forming operation and eliminating the scalping off operation by means of a set of rotating forming rolls with forming blades working in the deposited material. It has found to be essential to control the position of the last forming roll as described in the copending application. It is an object of this invention to present a method for controlling the last roll in the set of rolls as described in the above mentioned SE patent application. This is done by means of a bulb of material which is formed in front of the last roll. Control of the vertical position of the last roll can be made by controlling the size of the bulb, directly by indicating the height or volume of the bulb of fiber material or, alternatively, indirectly by indicating the difference in height between the last roll and the mat after the last roll. The indication in the form of a signal can be used for closed loop control of the vertical movement of the last roll. Thus, it has been found that the volume of the fiber bulb in front of the last roll represents a buffer volume of fiber material. If fiber material are lacking in the incoming mat to the last roll, i.e. if there is a "hole" or indentation in the mat, then this buffer can be used to fill this "hole", as long as the buffer volume is sufficient. Also the fact that the roll is throwing material in both directions will help to supply material to sections of the mat with lack of material. If a section of the mat with locally too high mat thickness approaches the last roll, this excessive material is absorbed by the bulb, and is also thrown sideways so that it is spread over the width. This is valid until the bulb reaches such a height, that excessive fibers are thrown over the roll on top of the outgoing mat. Thus, it has shown to be essential for a good forming according to the present invention to keep good control of the height of this fiber bulb. Too low bulb hight, i.e. a too high roll setting, will result in "holes" in the formed mat, while too high bulb hight, i.e. a too low roll setting, will result in that fibers are thrown over the last roll, thereby causing an uneven mat . Further, it is also critical to adjust the position of the last roll continuously in relation to the mat density. This can also automatically be achieved by control of the bulb. The alternative way of controlling the level of the last forming roll is to use the difference in height between the last roll and the mat height after the last roll. This difference is also related to the size of the bulb before the last roll. If the last roll is set too high, then the difference is large and at the same time the fibers pass under the last roll without a bulb being created. If the last roll is set too low, then the difference is small or even negative and a large bulb is created resulting in fibers being thrown over the last roll, thereby adding to the mat height. Thus, also by means of this indirect indication of the size of bulb it is possible to maintain a suitable bulb size in order to control the vertical position of the last forming roll and thereby obtain the equalizing effect on the mat forming as described above . The invention is further described and explained in the following text and figures, which show some embodiments of the invention. Figure 1 shows a set of vertically movable forming rolls according to the invention. Figure 2 shows alternative indicating devices (a - e) based on contact indication. Figure 3 shows alternative indicating devices (a, b) based on contact free indication. Figure 4 shows an alternative indication method. Figure 5 shows further alternative indicating devices (a, b) based on contact indication. Figure 6 shows further alternative indicating devices (a, b) based contact free indication. Figure 1 shows a general view of the forming head area, where fibers are metered down from a dosing bin onto forming rolls 1 in the right part of the figure, and the formed mat is transported to the left in the figure. The rolls 1 as shown in SE patent application 9800209-0 are movable vertically as required by means of jacks 2 and 3 to get best forming, and the height of the last roll 5 is set so that an even mat is produced. Other alternatives of the forming head than shown in figure 1 can be learned from the above mentioned SE patent application. The height or volume of the fiber bulb 4 before the last roll 5 is measured and the signal is used for closed loop control 6 of the lifting device, for example a jack 3. If the jack is not centrally located above the roll, an algoritm for this deviation must be introduced in the closed loop. The signal from the bulb measuring device may be obtained in different ways, and Figures 2-4 show some solutions. In Figure 2 some mechanical devices for contact measuring of the top of the bulb are shown. A swinging arm 7 or movable line 8 can have a potentiometer, a pulse encoder or similar for measuring of the position of the arm or line. As shown in figure 2e a fluidistor 9 can be used for the measuring. The signal from this measuring to be used for the closed loop 6. Figure 3 shows some solutions where a contact-free measuring is made. In Figure 3a an analog signal from a device 10 is used to measure from above, in Figure 3b a number of on/off photocells 11 or similar are used, reading from outside a glass sidewall. Figure 4 shows a principle where a motor 12 is used for rotating the last roll 5. A suitable signal from the motor 12 (such as rpm, motor power, motor current, motor torque, cos φ etc) is used for determining the bulb height. A preferred solution is to use the signal for the motor current, which is treated in a load transmitter. In such a transmitter, both the active and reactive current are measured. When working from an idling situation when the reactive current is rather large, to a situation when some load is applied to the motor from the fiber bulb, and when the reactive current diminishes, i.e. the motor efficiency increases (cos φ increases) , this actually used span in motor current can be used, and the outfeed signal can then be set to 4-20 mA for this span. Thus a reliable control of the roll height can be achieved without external measuring devices. Figure 5 and 6 show some embodiments which can be used if the difference between the height of the last forming roll 5 and the mat height after this last roll is used for control. Figure 5 shows contact indication principles. Figure 5a shows a structure 13 located on the inside of the forming wall. A measuring sensor 14, such as an angular sensor, is attached to this structure. The sensor 14 has some sort of sliding or rolling member 15 in contact with the mat. Figure 5b shows a similar structure 16 located on the outside of the former area with a vertical member 17 arranged at a suitable position along the width of the mat where a measuring sensor 18 with a sliding member 19 is attached. Figure 6 shows contact free indication principles. Figure 6a shows a fixed structural beam 20 on top, where measuring sensors 21, 22 for the vertical position of the last roll 5 and the mat height, respectively, are attached. The difference between the values in the form of signals from these two sensors 21, 22 is used for the control. Figure 6b shows a structure 23 which is movable vertically with the last roll 5. Thereby only one measuring sensor 24 is required. Surprisingly it has been found that the bulb 4 can be maintained at a certain size if the difference as described above is kept at a constant measure within the range of -15 to +25 mm, and in most cases within the range of -5 to +15 mm. The setpoint is slightly influenced by the thickness of the mat, the level of the first roll (the first jack 2), fiber characteristics and fiber moiture. However, these conditions can be compensated for by adding recipes for the difference in the main control system. The invention is not limited to the solutions described but can be varied within the scope of the claims.

Claims: Claims 1. Method for continuous forming of a fiber mat for manufacturing fiberboard according to the dry method, including metering fiber material and depositing the material on a surface of a moving transport means and forming the mat by means of adjustable rotating forming rolls (1) , c h a r a c t e r i z e d i n that the forming rolls (1) are working in the deposited material, the upper surface of the mat being defined by the last forming roll (5), that a bulb (4) of material is formed in front of the last forming roll (5) and that the size of the bulb is indicated and used for control of the vertical position of the last forming roll (5) . 2. Method according to claim 1, c h a r a c t e r i z e d i n that the size of the bulb (4) is measured with means (7, 8, 9) making mechanical contact with the bulb surface. 3. Method according to claim 1, c h a r a c t e r i z e d i n that the size of the bulb (4) is measured with contact-free measuring devices (10, 11). 4. Method according to claims 1, c h a r a c t e r i z e d i n that a signal from a motor (12) for rotating the last forming roll (5) is used for detection of the size of the bulb (4). 5. Method according to claim 4, c h a r a c t e r i z e d i n that the signal from the motor (12) is the motor current, which is treated in a load transmitter. 6. Method according to claim 1, c h a r a c t e r i z e d i n that the size of the bulb (4) is indicated indirectly by means of indicating the difference in height between the last roll (5) and the mat after the last roll. 7. Method according to claim 6, c h a r a c t e r i z e d i n that the difference in height between the last roll (5) and the mat after the last roll is measured with means (15, 19) making mechanical contact with the mat surface. 8. Method according to claim 6, c h a r a c t e r i z e d i n that the difference in height between the last roll (5) and the mat after the last roll is measured with contact-free measuring devices (21, 22, 24).

Inventors: MATHIASSON MIKAEL, THORBJOERNSSON SVEN-INGVAR, MATHIASSON, MIKAEL, THORBJOERNSSON, SVEN-INGVAR
Applicants: MATHIASSON MIKAEL, SUNDS DEFIBRATOR IND AB, THORBJOERNSSON SVEN INGVAR, MATHIASSON, MIKAEL, SUNDS DEFIBRATOR INDUSTRIES AB, THORBJOERNSSON, SVEN-INGVAR
Application year: 1999
Main classification: B27N3
IPC codes: B27N3