Methods of manufacturing board from raw materials based on lignocellulose are well-known, and are now widely in use. These methods of manufacture generally comprise the following steps: disintegration of the raw material into particles and/or fibers of a suitable size, drying to a definite moisture ratio and glue-coating the material prior to or after the drying step, forming the glue-coated material into a mat, which can be built-up from several layers, possibly cold pre-pressing, pre-heating, nozzle-spraying of the faces, as well as hot pressing during the simultaneous application of pressure and heat in a discontinuous or continuous press into the form of a finished board.
During conventional hot pressing the pressed material is normally heated substantially only by thermal conduction from adjacent heating plates or steel belts, which have a temperature of between about 150 and 250.degree. C., depending on the type of pressed product being utilized, the type of glue being used, the desired capacity, etc. In this manner, the moisture of the material closest to the heat sources is vaporized, whereby a dry layer develops, and a steam front moves successively inwardly to the core of the board from each side as the pressing process proceeds. When the dry layer develops, a temperature of at least 100.degree. C. prevails in this layer, which initiates the curing of normal glues. When the steam front has arrived at the core, a temperature of at least 100.degree. C. has been reached at the core, and the board also begins to harden at this point, whereafter the pressing can be finished within seconds. This applies to the use of conventional urea-formaldehyde glues (UF) and the like, such as melamine-reinforced (MUF) glues. When using other types of glues with higher curing temperatures, a higher temperature and a higher steam pressure must be developed in the board before hardening can take place.
In order to provide desired board properties, a press must be capable of applying high face pressures at high temperatures. This is not a problem on its face for a discontinuous press which, however, has other disadvantages, such as inferior thickness tolerances, etc. In continuous presses the requirement of high face pressures with simultaneously high temperatures has created the necessity for expensive precision solutions as regards the roller bed between a steel belt and the underlying heating plate.
The method of introducing heat to the board by means of thermal conduction also makes it necessary for the heating to a relatively long time, which results in long press lengths (i.e., large press surfaces). Presses up to a length of about 40 meters have thus been utilized. Besides, during the use of known continuous presses it is practically impossible to make the heating plates sufficiently flexible, so that density profiles cannot be formed as freely as is the use with discontinuous pressing.
Another method of board manufacture which is based on the introduction of steam between the heating plates in a discontinuous press has also been used to a limited extent. Since during the supply of steam the material is heated in a matter of seconds, the heating time is radically shortened. Furthermore, the compression resistance of the material is reduced considerably when steam has been supplied. This is a positive feature, which implies that the press could be designed with less press power and with a substantially shorter length (i.e., smaller press surface). In order to obtain the desired properties of a board manufactured according to this method, however, it has been necessary to apply conventional pressing techniques with high surface pressures and thermal conduction from conventional heating plates at the beginning of the press cycle, whereby after a long period of heating a face layer with a high density has been obtained. It was initially possible to thereafter blow in steam in order to heat the core portion of the board. This, however, has involved problems, because the steam had to be blown through the newly formed high density face layer, and because the pressing time has been extended considerably during the period of high pressure and thermal conduction. Due to all this, a steam press operating according to this concept has a substantially lower capacity, and alternatively requires a larger press surface and higher press power than would be necessary when uniform density is desired.
During all of the manufacturing methods mentioned above, a soft face layer with lower strength, unacceptable paintability, etc., is obtained, which requires this layer to be ground off. The resulting material loss amounts to about 5 to 15%, depending on board type, its thickness, etc.
It is an object of the present invention to provide a novel manufacturing process for the continuous pressing of board based on lignocellulose material, by which method it is possible to utilize the advantages of steam heating, which implies that the equipment can thus be designed with a substantially smaller press surface and lower press power, i.e.., it is less expensive. Furthermore, it can now be designed without heating plates, which renders the equipment still less expensive, and thereby producing a board with a substantially uniform density profile, which can be used in that state, or be further refined.