Abstract:
A system for pressing lignocellulosic and/or cellulosic materials to produce composite products such as medium density fiberboard subjects a mat of material to alternating or vibratory forces applied to a face surface of the mat. The frequency of the alternating forces may be between 0.2 Hz and 2 Hz. This vibratory pressing action causes heat to be transferred more quickly to the core of the mat from the surface, and reduces the pressing time required for thermosetting resins within the mat to cure. The system comprises a press for accepting the mat; a sensor for sensing a parameter indicative of heat in the core of the mat; and a control system for controlling the application of vibratory and non-vibratory forces to the mat.

Description:
RELATED APPLICATIONS  
       [0001]    This application is a continuation-in-part of U.S. patent application Ser. No. 10/429,722 filed 6 May 2003 and claims the benefit of the filing date thereof. 
     
    
     
       TECHNICAL FIELD  
         [0002]    The present invention relates to pressing of materials to form pressed sheets of material, and more particularly to methods and systems for pressing lignocellulosic materials to produce composite wood products such as medium density fiberboard, oriented strand board and particle board.  
         BACKGROUND  
         [0003]    There are many types of composite wood products formed from lignocellulosic material. Some examples include medium density fiberboard (“MDF”), oriented strand board (“OSB”), and particleboard. Others include laminated veneer lumber (“LVL”), laminated structural lumber (known in the art as “Glulam”) and plywood. These composite products are generally made by adhering together wood materials such as lumber, veneers, strands, flakes, particles, and fibers with adhesives. Thermosetting adhesives are commonly used, which include phenol-formaldehyde (PF) resins, phenol-resorcinol-formaldehyde (PRF) resins, resorcinol-formaldehyde (RF) resins, urea formaldehyde (UF) resins, melamine urea formaldehyde (MUF) resin and isocyanate (MDI) resin.  
           [0004]    Such composite wood products are typically produced by adding the adhesive to the lignocellulosic material and forming a mat of the combination. The mat is heated to a temperature range within which the rate of curing of the adhesive is significant, and is pressed for a period of time, allowing the resin to cure completely, or nearly completely. This process is well known in the art as “hot pressing”. The range of temperature within which the rate of curing of adhesive is significant is referred to throughout this document as the “curing temperature range”. Those skilled in the art of thermosetting adhesives will appreciate that some adhesive curing may occur below the curing temperature range, but that the rate of curing increases dramatically when the adhesive reaches its curing temperature range. The curing temperature range is different for different types of adhesives and may also depend on other factors, such as the type of cellulosic material in the mat, the moisture content of the mat and the vapour pressure within the mat.  
           [0005]    For best hot pressing results, it is desirable for the resins within the mat to reach the curing temperature range in order to cure properly. Preferably, the entire volume of the mat will reach the curing temperature range to allow all of the resin in the mat to cure. When heat is applied to the mat in a typical hot pressing operation, however, it is only applied to one or both face surfaces of the mat. In a typical system, the conveyor belt on which the mat rides is heated and supplies heat directly to a surface of the mat. The heat is expected to be conducted from the surface of the mat to the core of the mat. Since wood is not a good conductor of heat, however, this conduction of heat inwardly from the surface of the mat is an inefficient way of heating the core of the mat, and while the resin near the surface of the mat may reach the curing temperature range relatively quickly, it takes much longer for the core of the mat to reach the curing temperature range.  
           [0006]    It is desirable to attempt to heat the core of the mat more quickly to save time and production cost in the manufacture of composite wood products. Attempts have been made in the past to add more heat to pressing systems by way of steam injection, and also by way of subjecting the mat to microwave and radio frequency radiation. These techniques are not desirable, however, because they require great amounts of energy, and, especially in the case of radiation, require significant protective mechanisms which are cost-prohibitive.  
           [0007]    Accordingly, there is a desire for a system for heating and pressing a mat of lignocellulosic material which permits shorter pressing times.  
         SUMMARY OF INVENTION  
         [0008]    A first aspect of the present invention provides a system for heating and pressing a mat of material having at least one face surface and comprising a thermosetting adhesive. The system comprises a press for accepting the mat; at least one sensor for sensing a parameter indicative of heat within a core of the mat; and a control system connected to accept a signal from the sensor. The control system is configured to cause the press to apply vibratory forces to the face surface when the signal indicates that the parameter is below a predetermined reference level and to cause the press to apply a non-vibratory force to the face surface when the parameter is above the predetermined reference level.  
           [0009]    Another aspect of the present invention provides a system for heating and pressing a mat of material having at least one face surface and comprising a thermosetting adhesive. The system comprises a press for accepting the mat; at least one sensor for sensing a parameter indicative of heat within a core of the mat; and a control system connected to accept a signal from the sensor. The control system is configured to cause the press to apply vibratory forces to the face surface when the signal indicates that the parameter has reached a first predetermined reference level and to cause the press to apply a non-vibratory force to the face surface when the parameter has reached a second predetermined reference level.  
           [0010]    Yet another aspect of the invention provides a system for pressing a mat of material having at least one face surface and comprising a thermosetting adhesive. The system comprises a press for accepting the mat; at least one temperature sensor for sensing the temperature of the core of the mat; and a control system for accepting a signal from the temperature sensor. In response to the signal, the control system causes the press to apply vibratory forces to the face surface when the temperature of the core of the mat is below a predetermined temperature. The control system is also programmed to cause the press to apply a non-vibratory force to the face surface when the temperature of the core of the mat is above the predetermined temperature.  
           [0011]    Still another aspect of the invention provides a system for heating and pressing a mat of material having at least one face surface and comprising a thermosetting adhesive. The system comprises a press for accepting the mat; at least one temperature sensor for sensing a temperature of the core of the mat; at least one vapour pressure sensor for sensing a vapour pressure within a core of the mat; and a control system for accepting signals from the temperature sensor and the vapour pressure sensor. The control system is configured to cause the press to apply vibratory forces to the face surface when the signal from the vapour pressure sensor indicates that the vapour pressure within the core of the mat has reached a predetermined vapour pressure level and to cause the press to apply a non-vibratory force to the face surface when the signal from the temperature sensor indicates that the temperature of the core of the mat has reached a predetermined temperature level.  
           [0012]    The parameter indicative of heat within a core of the mat may comprise a temperature of the core of the mat, a vapour pressure within the core of the mat, ultrasonic emissions from the mat and/or radiation emission from the mat.  
           [0013]    The system may comprise a pressing face for applying the vibratory and non-vibratory forces to the mat. The pressing face may apply the vibratory and non-vibratory forces to the mat in a direction that is normal to the face surface. The pressing face may provide heat to the mat by conduction from the pressing face.  
           [0014]    The system may comprise a conveyor for transporting the mat and for providing heat to the mat by conduction from the conveyor. The system may also comprise a source of steam for providing heat to the mat.  
           [0015]    The vibratory forces may have a frequency in a range of 0.2-2.0 Hz. The vibratory forces may be applied by reciprocatably moving the pressing face over a distance in a range of 0.2-3.0 mm.  
           [0016]    The predetermined temperature, predetermined vapour pressure and/or predetermined reference level(s) may correspond with a temperature that is below a curing temperature range of the adhesive. The onset of the curing temperature range may be approximately 60-110° C.  
           [0017]    Prior to being accepted on the press, the temperature of the mat is preferably below a curing temperature range of the adhesive.  
           [0018]    Another aspect of the invention provides a system for pressing a mat of material having at least one face surface and comprising a thermosetting adhesive. The system comprises a press for accepting the mat; at least one sensor for monitoring a parameter indicative of heat within a core of the mat, the sensor providing a first trigger signal when the parameter reaches a first predetermined reference level and a second trigger signal when the parameter reaches a second predetermined reference level; and a control system connected to receive the first and second trigger signals. The control system is configured to cause the press to apply vibratory forces to the face surface in response to the first trigger signal and to cause the press to apply a non-vibratory force to the face surface in response to the second trigger signal.  
           [0019]    Yet another aspect of the invention provides a system for pressing a mat of material having a thermosetting adhesive having a temperature range at which the adhesive cures. The mat has a face surface and typically has an initial temperature below the temperature range at which the adhesive cures. The system comprises a press for accepting the mat; a temperature sensor for sensing the temperature of the core of the mat; and a control system for accepting a first signal from the temperature sensor and, in response to the signal, causing the press to apply vibratory forces to the mat normal to the face surface when the core of the mat is below a predetermined temperature. The control system is also programmed to cause the press to apply a non-vibratory force to the mat normal to the face surface when the core of the mat is above the predetermined temperature.  
           [0020]    Another aspect of the invention provides a system for pressing a mat of material having a thermosetting adhesive having a temperature range at which the adhesive cures. The mat has a face surface and typically has an initial temperature below the temperature range at which the adhesive cures. The system comprises a press for accepting the mat; a vapour pressure sensor for sensing a vapour pressure within a core of the mat; and a control system for accepting a first signal from the vapour pressure sensor and, in response to the first signal, causing the press to apply vibratory forces to the mat normal to the face surface when the core of the mat is below a predetermined vapour pressure. The control system is also programmed to cause the press to apply a non-vibratory force to the mat normal to the face surface when the core of the mat is above the predetermined vapour pressure.  
           [0021]    Further features and applications of specific embodiments of the invention are described below.  
       
    
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0022]    In the accompanying drawings which illustrate specific embodiments of the invention, but which should not be construed as restricting the spirit or scope of the invention in any way:  
         [0023]    [0023]FIG. 1 is schematic view of a hot pressing system in accordance with a particular embodiment of the invention.  
     
    
     DESCRIPTION  
       [0024]    Throughout the following description, specific details are set forth in order to provide a more thorough understanding of the invention. However, the invention may be practised without these particulars. In other instances, well known elements have not been shown or described in detail to avoid unnecessarily obscuring the invention. Accordingly, the specification and drawings are to be regarded in an illustrative, rather than a restrictive, sense.  
         [0025]    [0025]FIG. 1 is a schematic depiction of a hot pressing system according to a particular embodiment of the invention. Referring to FIG. 1, the hot pressing system of the present invention comprises a press  20  for pressing a mat of material  10 . In a preferred embodiment of the invention, the material in mat  10  is a lignocellulosic material such as wood fibres used to produce MDF, in combination with a thermosetting adhesive such as a UF resin, although the invention contemplates use with any particulate materials which require hot pressing and which do not conduct heat well.  
         [0026]    In the illustrated embodiment of FIG. 1, press  20  may be provided in an MDF mill. Such a mill typically has a conveyor system  30  for conveying mat  10  to the pressing face  25  of press  20 . This system may be provided in a continuous press, a multi-daylight press, or some other type of press known in the art. Press  20  is controlled by control system  50 . When mat  10  is in place within press  20 , press  20  is instructed by control system  50  to press mat  10  by forcing pressing face  25  against the top surface of mat  10  for a predetermined period of time. The time required for pressing mat  10  in known systems is widely variable, and may depend upon the type of cellulosic material in mat  10 , the type of adhesive therein, the moisture content of the mat, the ambient temperature of the system, the initial temperature within mat  10  and/or the vapour pressure within mat  10 .  
         [0027]    The system of the present invention comprises at least one sensor  40  for measuring one or more parameters indicative of the heat within the core of mat  10 . Such parameter(s) may generally comprise any detectable indicator of heat energy with the core of mat  10  or any combination of such indicators. In preferred embodiments, the parameter comprises the core temperature of mat  10 , the vapour pressure within the core of mat  10 , ultrasonic emission from mat  10  and/or radiation emitted from mat  10 . The particular nature of sensor  40  is not in and of itself important to the operation of the invention, and variations of such sensors may be or become known to those skilled in the art. Sensor  40  is able to sense one or more parameters indicative of the heat within mat  10  while mat  10  is being pressed by press  20 .  
         [0028]    In operation, following the preparation of mat  10  (ie. the addition of a thermosetting adhesive to some cellulosic material), mat  10  is introduced to press  20  by conveyor  30 . Press  20  preferably comprises a means for providing heat to mat  10 . Heat may be provided to mat  10  using any of a variety of heating apparatus and techniques. For example, mat  10  may be heated on its bottom surface by conduction of heat from conveyor  30 , and may be further heated by conduction of heat from pressing face  25 . Mat  10  may also be heated in some other manner, such as by the introduction of steam and/or radiation (not shown). In any case, heat may be provided to mat  10  when in place in press  20 . In a typical pressing system, mat  10  is pressed by pressing press face  25  against one surface of mat  10  with some original pressing force. Preferably, the original pressing force is applied in a direction normal to the top surface of the mat.  
         [0029]    For ease of explanation, it is assumed for the purpose of describing the system of the present invention that sensor  40  is a temperature sensor and that temperature is the parameter indicative of the heat within mat  10 . As discussed above, temperature sensor  40  may generally comprise any sensor(s) for measuring one or more parameters indicative of the heat with mat  10 . Those skilled in the art will appreciate that pressing systems incorporating different sensors will function in a similar manner to the pressing systems described herein.  
         [0030]    In a particular embodiment of the present system, while heat is supplied to mat  10 , sensor  40  monitors the temperature of the core of mat  10  via signal  35  and provides information about the temperature of the core of mat  10  to control system  50  via signal  45 . Control system  50  is programmed or otherwise configured to monitor signal  45  from sensor  40 . In response to signal  45  from sensor  40  indicating that the temperature of the core of mat  10  has reached a first predetermined reference level, control system  50  provides signal  55  to press  20 , causing press face  25  to provide, alternatingly, a first force and a second, lesser force to mat  10  in a vibratory fashion. In some embodiments, control system  50  causes pressing face  25  to move in a reciprocating manner so as to provide a vibratory force normal to a face surface of mat  10 . In the illustrated embodiment of FIG. 1, control system  50  causes pressing face  25  to be lowered and raised, providing a vibratory force normal to the top face of mat  10 . The second force provided by press face  25  need not be zero; rather it needs only be less than the first force. The first or second alternating force may be the same as the original pressing force, or may both be different.  
         [0031]    These vibratory forces are provided to mat  10  until signal  45  from sensor  40  indicates that the temperature of the core of mat  10  has reached a second predetermined reference level. In preferred embodiments, the temperature of the core of mat  10  at the second predetermined reference level is below the curing temperature range of the adhesive in mat  10 . When control system  50  determines that signal  45  from sensor  40  indicates that the temperature of the core of mat  10  has reached the second predetermined reference level, control system  50  provides signal  55  to press  20 , causing press face  25  to press the face surface of mat  10  with a constant, non-vibrating force. In preferred embodiments, this non-vibratory force is applied by press face  25  in a direction that is normal to the face surface of mat  10 .  
         [0032]    In alternative embodiments of the invention, it is not necessary to have a precise predetermined temperature at which vibratory forces are applied to mat  10 . For example, vibratory forces may automatically be applied to mat  10  when mat  10  is received in press  20  or vibratory forces may be applied to mat  10  after mat  10  has been received in press  20  for a period of time. Control system  50  may monitor signal  45  from sensor  40 . When signal  45  indicates that the temperature of the core of mat  10  is below a predetermined reference level, then control system  50  may cause vibratory forces to be applied to mat  10 . When signal  45  indicates that the temperature of the core of mat  10  is above the predetermined reference level, then control system  50  may cause a non-vibratory force to be applied to mat  10 .  
         [0033]    In general, the vibratory forces may be applied at any time when the temperature of the core of mat  10  is below the predetermined reference level and the non-vibratory force may be applied at any time when the temperature of the core of mat  10  is above the predetermined reference level. Preferably, however, the predetermined temperature is at or below the curing temperature range of the adhesive used in mat  10 .  
         [0034]    Essentially, the vibrating forces applied to mat  10  are thought to serve to assist the transfer of heat from the surfaces of the mat to the core by way of convection. The inventors have determined that vibrations falling in the frequency range of 0.2-2.0 Hz are suitably employed in the invention. The inventors have also determined that for a typical MDF mat, the press face can be reciprocatively moved over a distance in a range of 0.2-3 mm to provide the vibratory forces. In experimental testing, the system has been found to reduce pressing time in the manufacture of MDF by 12%, with no degradation in the quality of the finished MDF panel.  
         [0035]    As will be apparent to those skilled in the art in the light of the foregoing disclosure, many alterations and modifications are possible in the practice of this invention without departing from the spirit or scope thereof. For example:  
         [0036]    It is convenient but not essential that temperature of the core of mat  10  be monitored. As discussed above, sensor  40  may also sense other parameter(s) of mat  10  which are indicative of the amount of heat within the core of mat  10 , such as vapour pressure, ultrasonic emission and/or radiation emission for example. In such embodiments, control system  50  may use vapour pressure, ultrasonic emission and/or radiation emission reference level(s) to control the application of vibratory and/or non-vibratory forces to mat  10 .  
         [0037]    In other alternative embodiments, sensor  40  may comprise circuitry and/or logic which allows it to determine when the measured parameter has reached its predetermined reference level(s). In such embodiments, control system  50  need not constantly monitor sensor  40  and may be free to perform other tasks. Sensor  40  may only provide signal  45  to control system  50  when the measured parameter has reached its first predetermined reference level and/or when the measured parameter has reached its second predetermined reference level. In such embodiments, the signal  45  from sensor  40  comprises trigger signals, which trigger control system  50  to apply vibrating or non-vibratory forces to mat  10 .  
         [0038]    Those skilled in the art will appreciate that control system  50  may be implemented by a wide variety of apparatus. By way of non-limiting example, control system  50  may comprise a microprocessor, a programmable logic circuit, a personal computer, a digital signal processor, an off-the-shelf programmable controller or the like. Control system  50  may also comprise any combination of these types of apparatus.  
         [0039]    As discussed above, it is preferable that vibratory forces are applied only when the temperature of the core of the mat is below the curing temperature range of the adhesive. In preferred embodiments, the onset (i.e. the low end) of the curing temperature range is approximately 80-85° C. In other embodiments, however, the onset of the curing temperature range may be as low as approximately 60° C. or as high as approximately 110° C.  
         [0040]    Accordingly, the scope of the invention is to be construed in accordance with the substance defined by the following claims.