The invention relates to a method for the production of boards of ligneous material and to a steam press for the practice of the method. 
In a known method according to EP 0 339 952 a preliminary compression is at first applied to the mat of raw material and steam is introduced into the press chamber alternately from the top down and from the bottom up.  This has the advantage that the heating of the mat by the transfer of heat from the steam to the mat material takes place rapidly and the cycling time can be reduced by as much as 75%.  Sealing from the atmosphere is provided by a frame which is tightly anchored to the margin of the upper press platen.  Two pressure bars of different thickness, 6 and 12 mm, are used for the frame.  In the case of a changeover of production, the frame is replaced.  This procedure takes about two days, and in addition to installation personnel it requires that the press be shut down with the attendant loss of production, since the frame is composed of several bars.  The force applied by the piston-and-cylinder system is divided into a force acting on the actual mat of raw material and a force acting on the material under the sealing bars.  Assuming that a uniform depth of material is applied over the entire width of the press, a higher pressure on the material is established under the sealing bars.  As the state of compression advances, the ratio of force Fpressure bar to Fchipboard shifts toward ever increasing values.  This effect can be seen in exponential compression curves (layer depth over press pressure). 
The disadvantages of a geometrically and permanently anchored frame on the press platen are:
During the preliminary compression of the actual raw material mat to establish a permeability suitable for the mass flow of the steam, a certain material density is established under the bar by the compression curve, according to the height of the bar selected, to seal it off from the atmosphere, but depending on the steam pressure it may not be sufficient to prevent the leakage of steam; in other words, the material density under the pressure bars might not suffice to prevent the escape of steam from the press area. 
During the main compression, the actual board is to be brought down to the desired thickness.  The force applied by the main cylinder is here again divided between the force on the pressure bars and the force on the platen. 
Due to the pressure which increases exponentially as compression increases, the pressure under the pressure bars increases considerably more than it does on the actual product.  For this reason, for the production of greater final board thicknesses it is essential to change the pressure bars to different thicknesses.  The press pressure on the board itself is limited, the pressure under the pressure bars increases rapidly, and this can also lead to material damages at the upper and lower press platen.  When the press is opened, the ligneous material board still has an elevated temperature level and the residual water contained in the board evaporates.  The raising of the upper press platen is simultaneous with the raising of the pressure bar and the expansion steam escapes to the sides.  This results in considerable soiling of the steam press, especially the cylinder surfaces.  To avoid blowouts in the board, before raising the upper press platen a relief period of about one minute must be observed.  This steam expansion process can be abbreviated in some cases by the use of a vacuum. 
The lateral escape of the expanded steam is a disadvantage, because it leads to considerable soiling of the steam press as well as its surroundings, and long expansion periods are necessary before the steam press is opened so as to prevent blowouts. 
The invention is addressed to the problem of devising a method in which the escape of steam from the press space is prevented in every phase of the compression of the mat, in which no damage or contamination of the steam press occurs, and in which the described disadvantages do not appear, and also the problem of creating a steam press for the practice of the method. 
The solution of this problem includes a separation of the press platen applying the pressure from the frame sealing the mat at its outer margin, so that the press pressure is applied independently of the pressure necessary for the edge sealing. 
A steam press for the practice of the method of the invention is characterized by the fact that the frame is configured as a sealing frame in contact with the lateral walls of the moving press platen, can be raised and lowered thereon by actuators, and grips the mat at its outer margin, and that the sealing frame can be controlled as to pressure, time and motion, independently of the application of pressure to the moving press platen. 
The separation of the actual pressing surface and sealing frame by hydraulic actuators for control of stroke and pressure independent of the press movement results in the advantage that the press pressure on the mat can be optimized through the entire processing time as required.  The sealing pressure is independent of the press pressure and can be kept constant or adapted to requirements throughout the pressing period, independently of the actual board thickness. 
In the preliminary compression phase the preliminary pressure on the platen can be variable, that is to say, it can be adjusted regardless of the preliminary compression under the sealing frame.  An optimum setting of the permeability of the mat can be established by optimizing the sealing pressure and material density under the sealing frame.  This sealing pressure can be sustained throughout the process. 
In the main compression, a defined and measurable pressure on the mat can be established as required.  Due to the defined pressure conditions uncontrolled mechanical deformation or destruction of the steam press can no longer occur. 
Before the press is opened, the upper press platen can be set for a compression pressure of from xe2x89xa64 N/mm2 to an expansion or steam relief pressure of 0.5 N/mm2, while the sealing frame can be held down on the compressed board.  The expansion or steam relief pressure is equal to or greater than the transverse tensile strength in the press platen corresponding to the glue setting that has taken place up to this moment of the pressing process.  The expanding steam can then be relieved into the space between the board and the upper press platen and be carried out through the nozzles.  The expansion counter-pressure of the upper press platen is controlled over a distance/pressure gradient of from xe2x89xa70.5 N/mm2 toward 0 N/mm2.  After that, the press is opened.  It is especially advantageous that the expansion steam can no longer escape laterally and thus contamination of the press and its surroundings can be prevented by the controlled removal of the steam.  Also, blowouts in the middle and cover layers are prevented, and the formerly necessary steam relief period of one minute has been saved, without the need to use the vacuum systems of the former state of the art. 
Additional advantages of the invention are protection of the equipment by the sealing provided by the controlled sealing frame at all board thicknesses, reduction of the steaming time by controlling permeability and thus increasing the throughput, shortening of the steam escape period which can be further optimized by adapting the distance/pressure control gradients of the upper press platen to larger steam carrying cross sections, the loss of production and elimination of the need for personnel for product changes (changing the pressure bars) because a flying or automated change has become possible, production of LDF (low-density fiberboard) with a density lower than 500 kg/m3 on account of the independently controllable pressure and distances between the upper press platen and the sealing frame, optimization of the pressing process for MDF (medium-density fiberboard), the use of higher steam pressures ( greater than 16 bar) is possible resulting in a reduction of the consumption of industrial binding agent by activating the lignin contained in wood as adhesive, and creation of defined and desired pressure conditions on the platen and the sealing frame in every phase of the production process. 
To let off the expanding steam using the pressure gradient from  greater than 0.5 N/mm2 to 0 N/mm2, the cross sections of the steam-bearing lines in the upper and lower press platens and their feed lines and removal lines as well as the control valves can be adapted to the greater volume of the diminishing steam pressure as well as the short expansion periods best for the process.