This invention relates to a method and an arrangement for pre- -pressing a mat formed of disintegrated . lignocellύlose- containing fiber material prior to its final pressing at the continuous manufacture of board, such as fiberboard and particle board.

Fiberboard normally is manufactured in the form of MDF (Medium Density Fiberboard) , which is a board product based on wood and which in recent years has been used to a rapidly increasing extent. MDF is to be understood here as fiberboard with varying density manufactured according to the dry method. MDF is made of wood fibers, which in known manner are dried, glued, formed and pressed in a hot press. Almost without exception the fiber web or mat formed is a so-called single-layer board, i.e. it has a substantially homogenous structure with uniform fiber distribution, uniform moisture content and uniform glue addition across the thickness. The relatively dry surface layer of fluff fibers is hereby exposed to radiation heat and contact heat in the hot inlet to the hot press, which normally is a continuous press.

Particle board is manufactured in a similar way and is now mostly built up as so-called triple-layer board, i.e. it comprises a central layer of coarse chips and two surface layers of fine chips. These layers are manufactured separately and,therefore, it is also possible to select different moisture content and glue content in the layers. At the manufacture of particle board the surface layer is also exposed to the press heat, but is not dried out with the same intensity, because of the higher moisture content in the surface layers and of the more compact chip material.

At the manufacture of board of the said and similar kind, as mentioned, a web or mat is formed which is pre-pressed and possibly pre-heated prior to its feed to the hot press where the pressing

is carried out at controlled surface pressure and/or thickness at a temperature of 150-230°C, ^an d where conventional urea formaldehyde glues are used. (Other glues also are used, espec¬ ially at high board densities). In order to bring about necess¬ ary board properties, a continuous press is required which is flexible, and at which a.o. a high surface pressure can be applied at an early stage in the press. This implies at the same time, that the thickness of the mat already at this early stage is very close to the final pressed thickness, i.e. the mat thickness must be reduced very substantially in the press inlet.

In order to ensure that such a reduction in thickness takes place without destroying or attenuating the surface layers of the mat, the inlet portion must be long and preferably wedge- shaped, as this provides the time required for air enclosed in the mat to be transported out of the mat in a gentle way. Such an inlet, however, causes the surface layer to be heated substantially and dried-out in a position where the surface pressures required for compressing the mat still are very low. At this method, therefore, the surface particles are dried out, whereby also the glue dries out and is inactiv¬ ated, which results in an unsatisfactory hardness and strength of the surface layer. The surface layer obtained as a result thereof often is called pre-hardening layer, because the glue there has hardened and/or been dried out before sufficient surface pressures bringing about good contact between fibers or particles have arisen. This surface layer must be ground in a later production step and, thus, constitutes a substantial loss of raw material and handling.

It should also be mentioned in this connection that the press temperature at the beginning of the press cycle, i.e. in the inlet portion (compression portion) of the press should be as high as possible in order to soften the surface layer as rapidly as possible when surface pressure has been applied, and in order to obtain the highest possible heat penetration rate in the board. This desire, thus, is in direct conflict with

the problem complex of pre-hardening.

Another factor promoting the increase of pre-hardening is the fact that known continuous presses are provided with endless conveying belts of steel, and that these belts require large radii of curvature, of the magnitude 800-1000 mm, which renders heating in the inlet unavoidable.

When instead pre-hardening is to be minimized, the aim must be to compress the mat as quickly as possible to sufficient surface pressure for good bonding. With known designs of continuous presses it is possible to reduce pre-hardening to some extent, but there is instead the risk, of attenuations and surface cracks arising in the surface material, because enclosed air, which must be pressed out rapidly, causes over¬ pressure in the mat. Such faults may be discovered only at a much later date. Such surface cracks, for example, often can pass unnoticed through the board production, and first when the board is to be painted at the customer the surface cracks are found to have caused variations in the surface density, resulting in varying paint suction and thereby varying glaze. This gives without fail rise to complaints.

The method and arrangement according to the invention solve the aforesaid problems and simultaneously yield additional advant¬ ages. The said pre-hardening layer, for example, can be minimized or prevented and the evacuation of air takes place more gently.

The characterizing features of the invention are defined in the attached claims.

The invention, thus, implies that a successive re-compression of the fiber mat compressed and expanded in connection with the forming is carried out without the addition of heat. There¬ after the mat is introduced as far as practically possible into the inlet portion of the hot press where the imat is trans¬ ferred to the hot surfaces in the hot press, whereby high surface pressures can be applied immediately at a minimum pre-hardening of the surface layers.

The invention is described in greater detail in the following, with reference to the accompanying drawings showing an embodiment of the invention. Fig. 1 is a lateral view of an arrangement according to the invention, Fig. 2 is an enlarged section of the feed area in the hot press according to a conventional design, Fig. 3 is an enlarged section of the feed area in the hot press comprising an arrangement according to the invention. The inlet portion of the continuous hot press shown in the Figures is designed in known manner with front guide rollers 1 and heating plates 2, which are formed with an inlet radius of the same magn¬ itude as the radius of the guide rollers 1, and thereafter trans¬ form to a substantially parallel portion 4. The distance between the heating plates 2 can only to a small extent be varied in relation to the distance at the inlet radius. A steel belt 3 is stretched over guide rollers and drive rollers and slides or rolls in known manner to the heating plates.' The transition between the inlet portion and parallel portion 4 is marked by the centre line 5.

In said inlet portion the pre-pressing arrangement according to the invention is located. This arrangement comprises three main parts: a converging inlet and compression portion 7, one or several nip roll pairs 8, and a slightly diverging delivery portion 9. The inlet opening 10 of the inlet portion is adjustable in some suitable way, automatically or manually, to be adapted to the height of the incoming mat 11. Hereby, in combination with a suitable length of the inlet portion 7, the pressing of air out of the mat 11 takes place in a gentle way without risk of damages. To the upper nip roll 8 movable in vertical direction a suitable load is applied so that the desired compression of the mat is effected. It is expedient here to compress to a density close to and preferably immediately below the density achieved at the previous compression after the forming operation. Such a re-compression requires a relatively moderate load. The load is applied prefer-

ably by air cylinders, hydraulic cylinders or the like. The end of the inlet portion 7 closest to the upper nip roll 8 preferably is coupled together mechanically with the nip roll so as to follow the vertical movement of the nip roll.

In the subsequent diverging delivery portion 9 the mat expands slightly, of the magnitude 5-15%, which reduces substantially the power required for holding the mat compressed.Hereby this portion can be dimensioned moderately. The ends of the delivery portion 9 closest to the nip rolls 8 also are coupled together mechanically with their respective nip roll.

The mat 11 is transported through the arrangement between two end¬ less belts 12, which can be solid, air permeable or formed as wires. In the inlet portion 7 the belts are supported on rolls and/or sliding surfaces. At the outlet end of the delivery portion 9 the belts are broken over a small radius 13 formed as sliding nose or rollers.

The belts 12 are driven and guided in known manner. If deemed suitable in view of forces acting on the belts in the inlet portion 7 and nip 8, stronger inner belts can be used.

EXAMPLE

Conventional design (Fig. 2): At the manufacture of net 19 mm board a normally pre-pressed fiber mat for the hot press is assumed to be about 160 mm thick. Without the proposed invention, the mat there is compressed from 160 mm to about 25 mm in the inlet nip of the hot press. The surface pressure at A is assumed having in¬ creased to a level, at which good bonds between fibers and particles can be obtained at the hardening of the glue. The thickness here is assumed to be about 50% greater than in the inlet nip. The distance from the mat contact to A is designated by .

Design according to the invention (Fig. 3): In this case the mat has been re-compressed to high density in the inlet and expands slightly before being passed into the inlet of the hot press. The distance from mat contact to A is Fig. 3 is designated by b. At the

entrance into the inlet of the hot press the thickness of the mat preferably is 1,3-2 times the thickness of the finished board. At normal designs of the inlet portion in the hot press, the distance b_ can be reduced by the invention to a fraction of • Typical values are of the magnitude 10-30%. The pre-hardening decreases thereby approximately correspondingly. It is hereby also possible to increase the steel belt temperature. Additionally, the mat is not exposed to radiation heat from the hot upper belt before the mat contact, whereby pre-hardening also is reduced.

The angle at which the steel belt meets the belt is a measure of the compression rate of the mat. It is understood from the Figs. 2 and 3 that the angle <λ is more than twice the size of the angle B, which should be smaller than 15°, preferably smaller than 10°. It is also easily understood that the amount of air to be evacuated is 3-4 times greater in Fig. 2 compared with Fig. 3. The result of all this is that the risk of surface cracks and attenuation has been reduced substantially by the invention in Fig. 3.

At the manufacture of board with normal thicknesses in continuous presses, for example at thickness 19 mm, often thickness tolerances after the finishing press of about +/- 0,15 mm are obtained, which normally is sufficient for selling the board unground. Owing to the pre-hardening of the surfaces, however, the pressing must take place at a nominal thickness of about 20,2 mm. This excess in size, therefore, must be reduced by grinding, i.e. the result is a direct production loss of raw material (wood raw material, glue, wax), drying energy, grinding energy, of 6-7% plus the total cost of the grinding operation. Elimination of pre-hardening, thus, implies an essential saving, which pays for the investment of the invention in a short time. Another essential advantage of the invention is that the surface layers are hard and less paint sucking while maintaining a bright surface. (In contrast to when water is sprayed through nozzles on the surfaces prior to pressing) .

The invention, of course, is not restricted to the embodiment shown, but can be varied within the scope of the invention idea.