Abstract:
In a continuous twin-belt prepress for the production of particle board panels with pressure rollers suspended in pairs, the upper pressure rollers are suspended resiliently so that they can be pressed upwardly counter to the spring force by a catch member of a metal support screen bearing the chipboard cake when that member passes through. This prevents damage to the press and especially its forming belts as a result of increased pressure during pressing.

Description:
FIELD OF THE INVENTION 
     This invention relates to a method of continuously pressing a material particle mat in the production of wooden material panels or the like such as particle boards (chipboards) and MDF (medium density fiber) panels, as well as a twin-belt press suitable for this purpose. 
     BACKGROUND 
     In the production of chipboard and fiber panels there is a requirement for precompression of the blanks by continuously working prepresses. Twin-belt presses in which the particle mat (“cake”) of chips mixed with bonding glue deposited on a support is pulled off from the support before it enters the two forming belts of the press are known per se in the continuous production of wooden chipboard with precompression (DE 4441017 A1). 
     In other systems, on the other hand, known as “FLEXOPLAN” systems, the particle mat is formed by scattering the “glued chips” onto a flexible metal support screen on which they remain during precompression. At the head end of the support screen, a catch strip member is attached, 25 mm high, for example, which is engaged by chains that draw the support screen to the press. In these systems, it has not been possible to date to use a continuously working prepress because, in the passage of the catch strip member through the roller pairs and pressing segments of the known twin-belt presses, an inadmissibly high pressing pressure results and damages the forming belts and/or other parts of the press. 
     Systems in use to date do not allow continuous precompression of the panel material before hot pressing. Therefore, it has been necessary to provide a relatively large press plate spacing of about 25 cm, into which the chipboard cake was fed and then pressed relatively slowly, so that air can escape from the material. Precompressed panels are much thinner, so that a press plate spacing of 15 cm is sufficient and, for a press of the same height, it is possible to arrange 15 units one above the other instead of only ten with a larger plate spacing. Continuous precompression of the panel material is consequently very desirable. Not only because of the greater capacity of the hot press, but also because of the shorter pressing time since the precompressed material can also be pressed faster as a result of the smaller air content. In this way, productivity can be increased by about 50%. 
     OBJECT OF THE INVENTION 
     Accordingly, an object of the invention is to provide a method and press that enables continuous pressing, preferably precompression, without the risk of damage to the press through the presence of foreign bodies, especially the above mentioned catch of a support screen, that do not belong in the particle mat or chip cake or the like. 
     Other objects and advantages of the invention will become apparent to those skilled in the art and from the drawings, the detailed description and the appended claims. 
     SUMMARY OF THE INVENTION 
     In accordance with aspects of the invention, pressure rollers can be pushed away or apart by a passing catch of a support screen or other foreign bodies because of their sprung or resilient mounting and the entire pressure roller with its rotary axis is shifted or tilted and, subsequently, spring back automatically to their operating position to enable a continuous pressing operation without degradation by the foreign bodies. 
     At least one but preferably at least between two and four sprung pressure rollers are possible, although a much greater number may be appropriate for the purpose, e.g., up to about 20 sprung pressure rollers. As each of them is separately sprung and, therefore, individually shifted or tilted by the foreign body, the remaining other pressure rollers can during the same time stay in their normal position as necessary for the desired compression forming operation. The spring pressure of the pressure rollers can be generated hydraulically, pneumatically or also mechanically, e.g., by steel springs, and should preferably be adjustable, and/or controllable in a closed control loop as a function of setpoint values to maintain stability. In the case of a plurality of sprung pressure rollers, their spring pressure, depending on system requirements, can be the same or be set differently from one pressure roller to the next. 
     The spring pressure, for matching to the usually adjustable operating line pressure of the pressure rollers, can be adjustable between a minimum figure and a figure that amounts to at least about two times but, preferably, more than about five times this minimum figure. For example, the line pressure (the “punctiform” pressure applied by a roller across its width upon a surface) can be selected for normal operation depending on the required level of precompression between about 30 and about 200 kg/cm and, as the pressure rollers should not be pressed away until this nominal pressure is exceeded, the spring pressure can be set to figures corresponding to a line pressure of about 30 to about 200 kg/cm. 
     In the case of continuous prepresses with pressure rollers opposite one another, the upper pressure roller will preferably be sprung, while the other, i.e., the lower pressure roller, may serve as a rigid counter-pressure roller. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The drawing is a schematic side elevational view of a twin-belt press according to aspects of the invention serving as a continuous prepress to produce chipboards. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     It will be appreciate that the following description is intended to refer to specific embodiments of the invention selected for illustration in the drawing and is not intended to define or limit the invention, other than in the appended claims. 
     Turning now to the Drawing, the prepress shown on a foundation has two forming belts  1  and  2 , driven in a known manner not shown, of which the upper belt  1  runs over a front guide or deflection drum  4 , two guide or deflection rollers  5  and  5 ′ are arranged above and a rear guide or deflection drum  6 , is positioned below the forming belt  2  and runs at the same speed through front and rear guide or deflection drums  8  and  8 ′. The front guide drums  4  and  8  are parallel to the working direction indicated by the arrow  10 , are suspended so that they can be shifted horizontally, and have tensioners  12  attached, while the rear guide drums  6  and  8 ′ may be suspended so that they are substantially horizontally immobile. 
     Vertically, the front guide drums  4  and  8  and the rear bottom guide drum  8 ′ are suspended so that they are substantially vertically immobile (fixed). 
     Between the horizontal lower part of the upper forming belt  1  and the likewise horizontal upper part of the lower forming belt  2  is the pressing segment for compressing the particle mat or cake (not shown), which is deposited in a manner known per se on a flexible, metallic support screen and, on this screen, is drawn in the direction of the arrow  10  into the gap formed between the guide drums  4  and  8  and then continues through the pressing segment. The support screen (not shown) can be drawn into the prepress by chains engaging in a catch member at the front end of the screen, for example, the driving system of the prepress being synchronized with the motion of the chains. 
     In this example, to compress the particle cake in the pressing segment, there are three pairs of pressure rollers, these rollers being distributed at substantially uniform mutual horizontal spacings between the guide drums of the forming belts and engaging the inner side of the upper and lower forming belt to form the pressing segment. Also, the rear guide drums  6  and  8 ′ serve to compress the cake. 
     The foremost pair of pressure rollers in the working direction consists of an upper, vertically sprung pressure roller  14  and a lower, rigidly mounted counter-pressure roller  15 . As already explained, the pressure roller  14  is adapted to be pressed away simply resiliently transversely of the plane of the mat formed by the particle cake, i.e., upwards in this case, by a foreign body passing through the pair of rollers, especially the catch of the support screen of the particle cake, and then to spring back to its operating position. For this purpose the pressure roller  14  with its rotary axis is mounted on a rocking member  20 , which in turn is rotatable at  21  about a fixed swivel axis horizontally offset from the rotary axis of the pressure roller, and on its horizontally opposite side, referred to as the rotary axis of the press, has an arm  22  by which it engages in a compression spring  23  compressible transverse to the plane of the mat, i.e., upwardly in a substantially vertical direction. 
     The compression spring  23  in this example can be a hydraulic or pneumatic spring on whose vertically mobile plunger the rocking member arm  22  is pivoted, while its cylinder is pivoted on the upper part  26  of the supporting construction of the press. The upward and downward swing of the entire pressure roller  14  against the force of the compression spring  23  as effected by a foreign body is indicated by the arrow  24 . The spring pressure of the pressure roller is preferably regulated by a closed loop control. 
     The two following pairs of pressure rollers in the working direction, arranged before the rear guide drums  6  and  8 ′, with sprung pressure rollers  14 ′ and counter-pressure rollers  15 ′ suspended substantially vertically rigidly in the bottom part  28  of the supporting construction of the press, correspond to the pair of rollers  14 ,  15 . 
     In the example the rear guide drum  6  of the upper forming belt  1 , which is horizontally aligned with the pressure rollers  14  and  14 ′, serves as a sprung pressure roller with adjustable spring pressure and is consequently pivoted to be swiveled like pressure roller  14 . The only difference is that the pivot axis  21 ′ of the rotating shaft  20 ′ of the guide drum  6  is on the rear side, in the working direction, of the rotary axis of the guide drum  6  that forms the sprung pressure roller. The guide drum  8 ′ serves as a vertically rigid counter-pressure roller for the guide drum  6 . 
     The height of the previously mentioned catch, usually between about 25 and about 40 mm, for example, results in the necessary capacity for excursion of the pressure rollers, i.e., their compliance or the amount by which the sprung pressure rollers can be pressed away counter to the spring force and transversely to the plane of the mat. This distance, limited by the compressibility of the compression spring, may be between about 10 and about 40 mm in the example described, allowing for the fact that the excursion can usually be somewhat less than the height of the foreign body. Preferably, the capacity for excursion of the pressure rollers is about 30 mm. 
     The two parts  26  and  28  of the supporting construction in which the guide drums  4  and  6  and the pressure rollers  14  and  14 ′ are mounted or suspended in the top and the guide drums  8  and  8 ′ and the counter-pressure rollers  15  and  15 ′ in the bottom, respectively, can be moved vertically to one another for adjustment or matching to particle cakes of different thicknesses. 
     After leaving the prepress the precompressed panel blank is fed to another press for a main pressing operation, for example, in a conventional clock-operated box or multiplaten press or a continuous press, in which the chipboard is ready pressed by applying heat to it. The panel blank may also remain on the support screen mentioned above during the main pressing operation.