Prior Art During the production of panels of lignocellulose-based material, such as particleboard and fibreboard manufactured using dry processes (LDF, MDF, HDF), the material is formed to suitable particles or fibres that are coated with adhesive and transported to a pressing line with former, transport conveyor, pre-press, hot press and equipment for handling the panels produced in this manner. It is normal in these production lines to have a weigh scale in order to control the material that is dispensed from a dosing bin to a forming head. Previously, the most common type of equipment has been of the mechanical type with a dosing bin with a bottom belt that feeds material at a controlled speed into a forming head that forms and produces an even mat, which is then transported onwards by a forming belt into a pre-press. It is important for an accurate control of the longitudinal weight along the belt to keep the dead time, that is, the delay between feed of the material into the former (up on the forming belt) and the passage of the material past the downstream weigh scale, short. This is due to the fact that no change in the speed of feed, that is, the speed of the said bottom belt, can be made before the dead time has passed. This is most obvious during the production of medium thick panels and of thick panels, which correspond to low forming speeds.

The forming head can in some applications be followed by a scalper roller.

This scalps off the surface of the formed mat in order to even out further the weight of the mat. This is commonly found when dry fibreboard mats are produced. When a scalper roll is used, the weigh scale can be located directly after the scalper roll, giving a short dead time. The use of a scalper roll, however, has other disadvantages. It requires pneumatic systems for the balancing of air and return

transport of excess material to a storage bin or to the dosing bin. Not only is this extra transport expensive, both as an investment and with respect to the cost of energy, but it also has a detrimental effect on the fibres. The use of scalping also means that the complete dosing bin and the forming system must be designed for the greater flow of material, including scalper material, through it.

Another common method for controlling the weight of the mat is to place the weigh scale after the former itself, even if no scalper roll is used. This construction is normally used for manufacturing lines manufacturing particleboard. It is also used according to SE, C2,511 259, which describes the manufacture of a fibre mat without the use of a scalper roll and with a weigh scale placed downstream (page 3, lines 18-20). The delay time is longer in these cases since the material must also pass the forming head. This means that this requires a more stable supply from the dosing bin. On the other hand, if rapid variations occur with respect to the material that arrives at the bin, depending on variations in humidity or variations in the stock, it may be difficult to control in time.

Other methods of locating the weighing equipment are also known. One method is to install a separate conveyer between the forming bin and the forming head. This gives a short dead time, but it is expensive and it requires a greater building height. It also produces new problems when the intermediate mat produced in this way is to be fed into the forming head.

Another method is to extend the bottom belt in the dosing bin in order to create space for a weigh scale. New problems are also created in this case, since this mat is to be fed to the forming head.

A further method is to install an impact weigh plate into the flow of material that emerges from the dosing bin, see EP, B1, 109 456. This then disturbs the distribution of material, and it cannot either satisfy the desired reliability. The three latter methods described above have only achieved marginal acceptance on the market.

Summary of the Invention The aim of the present invention is to achieve a mat without scalping and without using extra equipment, while at the same time still being able to achieve a short dead time for regulation of the weight of the mat.

The aim is achieved through a device for forming a mat of preferably adhesive-coated wood fibres on a running forming belt whereby the device comprises at least one dosing bin with at least one bottom belt that transports fibres to one end of the said forming belt and allows the fibres to fall down onto this in a first region, at least one weigh scale, at least one forming head and at least one pre- press. The said weigh scale is placed under an upper section of the said forming belt and directly after the said first region in the direction of motion of the forming belt, but before the said forming head.

Control equipment is used for the reception of a signal from the said weigh scale, which signal may be used to regulate the speed of the said bottom belt. The said control equipment may be arranged in such a manner that the said signal can be used for the said purpose only after a certain part of the said mat has fully passed the said weigh scale, whereby the speed of the bottom belt can be increased and/or the speed of the forming belt can be decreased if the weigh scale registers a weight that is too low relative to a desired value, and the speed of the bottom belt can be decreased and/or the speed of the forming belt can be increased if the weigh scale registers a weight that is too high relative to the said desired value. The said signal may be an electrical signal.

The said forming head may comprise at least one forming roll for evening out the mat that has been formed on the said forming belt. The said at least one forming roll may be comprised of at least two active forming rolls that have the ability to distribute material transversely across the said forming belt alternately to one side and to the other side. The said at least one forming roll may form a bed of material.

At least two side-walls may be present in order to limit the extent of the said mat in the transverse direction on the said forming belt. At least one hot press may be located after the said pre-press in the direction of motion of the forming belt.

The invention thus also comprises a method for forming a mat of preferably adhesive-coated wood fibres on a running forming belt whereby the method comprises the steps of transporting wood fibres to one end of the said forming belt with the aid of one dosing bin with one bottom belt, and allowing the fibres to fall down onto this in a first region, transporting the wood fibres onwards on the said forming belt to a forming head comprising at least one forming roll for evening out the mat formed on the said forming belt, and transporting the mat that has been formed

in the said forming head onwards to a pre-press for an initial compression of the mat.

The method also comprises the step of continuously weighing a part of the mat that is located above a weigh scale with the aid of the said weigh scale placed under an upper section of the said forming belt and directly after the first region in the direction of motion of the forming belt, but before the said forming head.

The device and the method will be described below in more detail in association with embodiments of the invention, and with reference to~the attached drawing.

Schedule of Drawings Figure 1 shows a schematic side view of a forming station for wood fibre panels that includes the device according to the invention.

Figure 2 shows a schematic side view of an alternative forming station for wood fibre panels that includes the device according to the invention.

Description of Embodiments The forming station for wood fibre panels shown in Figure 1 comprises a dosing bin 1, shown schematically, for adhesive-coated wood fibres. The dosing bin is equipped with a bottom belt for transport of fibres to one end of a forming belt 3, whereby the fibres from the bottom belt 2 fall down onto the forming belt 3 into a first region 3a of this belt. Thus it can be envisaged that an upper part of the bottom belt 2, which runs in an endless loop, moves to the left in the figure, as does an upper part of the bottom belt 3, which also runs in an endless loop, and thus the material is transported from right to left in the figure. The material that falls down onto the forming belt 3 constitutes a somewhat uneven mat, but this mat is sufficiently even in order for it to be possible with the aid of a weigh scale 4 to obtain an estimate of the weight of the mat per unit area. When the mat passes over the weigh scale 4 on the belt, it passes between two side-walls 8 that thus limit the transverse extent of the mat on the belt. When the mat has passed the weigh scale 4 it is further transported into a forming head 5 where the mat is processed by six forming rolls 6 that actively even out the mat that has been formed on the forming belt 3. The mat continues on

continues on the forming belt 3 between a further two side-walls 8 onwards to a pre- press 7 that presses the mat in a first step on its path to a finished panel.

Since the weigh scale 4 measures the weight of the mat over a certain area of the extent of the mat, small variations with respect to the weight of the mat along its length are evened out. Thus the weigh scale does not measure the weight in a point- based manner or in a line-based manner under the belt, but measure the weight of a surface of the belt and of the mat that lies on top of it. Thus, the signai from the weigh scale cannot, in principle, be used for adjusting the speed of the bottom belt 2 until a certain point on the mat has passed over the complete extent of the weigh scale, that is, over the complete area of the transport belt that is sensed by the weigh scale. Only then can the small variation in weight of the mat along its length be calculated and this small variation be compared with a desired target value and its tolerances. Calculation and comparison of this type take place in control equipment that is not shown. If the weigh scale records a weight that is too low relative to the desired target value, and the value recorded by the weigh scale thus lies outside of the tolerance limits, this means that the mat has a weight that is too low and the amount of adhesive-coated wood fibres that is dispensed to the forming belt 3 via the bottom belt 2 must be increased, whereby the speed of the bottom belt 2 must therefore be increased. Alternatively or simultaneously, the speed of the forming belt 3 can be decreased. If, on the other hand, the opposite situation occurs and the weigh scale records a weight that is too high relative to the said desired target value, and the value lies outside of the tolerance limits, the speed of the bottom belt 2 must be decreased and/or the speed of the forming belt 3 must be increased.

The dead time is minimized with respect to regulating the speed of the bottom belt 2 and/or the forming belt 3 through the weigh scale being placed directly after the said first region 3a of the forming belt 3 onto which the fibres fall. The system thus has sufficient time to regulate the large variations. that can be envisaged to occur during the supply of wood fibres to the forming belt 3.

One condition that is required for the present invention to function is the availability of what are known as"active forming rolls"that actively process the mat produced and even out both transverse and longitudinal irregularities. It is in this way possible to produce a stable mat with high precision in weight without using scalper rolls.

The dosing bin 1 is equipped in a conventional manner with equipment for spreading the incoming material in a transverse direction, equipment for return transport and delivery rolls for delivery of the fibre material.

Further equipment concerning handling of the mat, such as a hot press, may be present downstream of the equipment shown in the figure.

The fibre material may, as an alternative to the dosing bin shown in Figure 1, be transported with the aid of a pneumatic system. Such a system may comprise a preparation unit in which the wood fibres are dispersed in a carrier gas, at least one channel, at least one fan with at least one motor, at least one distribution means for distribution of wood fibres arriving from the said preparation unit over at least the major part of the width of the said forming belt 3, and at least one separation means for separating the fibres from the carrier gas. Such an equipment may also comprise a retardation means, located at or beside at least one forming box, that reduces the speed of motion of the fibres when these have exited from the said separation means, but before they reach the said forming belt 3 for transport onwards to the said weigh scale and forming head.

The rolls 9 shown in Figure 2 may optionally be used in order to even out immediately the greatest irregularities in the mat that is formed after the deposition from the dosing bin, and they can in this way ensure a more even weighing signal.

The aim of the invention, to achieve a short dead time for the regulation of a weigh scale, is achieved, since the number of rolls 9 is limited. The rolls 6 are used in this case solely for a fine adjustment of the mat. The rolls 9 may be active rolls as has been described previously, or they may be more conventional rolls such as spiked rolls or similar. The vertical position of the rolls 9 can be adjusted to the relevant measuring height in the same way as has been previously described for the active forming rolls 6. The flow of fibres from the dosing bin can, alternatively, pass over the rolls 9 and from there fall down onto the forming belt, something that also provides an evening out effect.

The invention is not limited to the embodiments shown here, but may be varied within the framework of the attached claims.