Patent Publication Number: US-8973638-B2

Title: Systems and methods for manufacturing composite wood products to reduce bowing

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a divisional of and is entitled to and claims the benefit of priority from U.S. Nonprovisional patent application Ser. No. 12/819,929 filed Jun. 21, 2010, now U.S. Pat. No. 8,414,720, and titled “Systems and Methods for Manufacturing Composite Wood Products to Reduce Bowing,” the contents of which are incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The present disclosure is directed generally to systems and methods for manufacturing composite wood products in order to reduce bowing and other undesirable effects from manufacturing stresses. 
     BACKGROUND 
     Composite wood products are manufactured by binding together wood elements (e.g., veneers, strands, flakes, chips, particles, fiber, solid pieces of wood, etc.) with suitable adhesives. Such products are typically used in construction applications as beams, headers, columns, joists, rafters, studs, or other structural components. To be suitable for this type of use, composite wood products should exhibit a certain degree of stiffness and dimensional stability. 
     Parallel strand lumber (PSL) is a high-strength composite wood product marketed under the name Parallam™ from iLevel by Weyerhaeuser Company.  FIG. 1  is a schematic showing a conventional method for manufacturing PSL. Typically, wood elements (indicated by arrow  102 ) and an adhesive (indicated by arrow  104 ) are combined in an adhesive application station  106 . The coated wood elements then enter a forming station  108  where they are laid up in a substantially parallel relationship to form a mat. The mat is consolidated in a press assembly  110  to form a billet  200  (shown in  FIG. 2 ) having four faces: a top face  202 , a bottom face  204 , and two side faces  206  and  208 . The thickness T of the billet  200  is substantially greater than its width W. Referring back to  FIG. 1 , the resulting billet is typically cut into two or three beams with a cutting assembly  112 .  FIG. 3  illustrates how the billet  200  may be cut into three beams along dotted lines  302  and  304  according to conventional methods. 
     In some applications, PSL produced according to methods described in  FIG. 1  exhibit bulging on the top face  202  and the bottom face  204 . Because the top and bottom faces of the billet are typically the coolest after exiting the press, they are more prone to blows and fissures. As a remedy, some PSL manufacturing companies have added a top/bottom preheat step to the PSL manufacturing process. As illustrated in  FIG. 4 , the mat is exposed to a top/bottom preheat assembly  402  prior to entering the press assembly.  FIG. 5  shows an example of a top/bottom preheat assembly  402  for use with the method described in  FIG. 4 . A mat  502  is shown passing through a preheat assembly  402  and a press assembly  110 . The preheat assembly  402  includes an upper wave guide  504  and a lower wave guide  506 . Microwave energy represented by arrows  508  and  510  is delivered to the top and bottom of the mat before pressing. 
     In all conventional PSL manufacturing methods, the process of cutting the billet into beams releases stresses induced by the manufacturing process. As shown in  FIG. 6 , this can result in a beam  600  that is not completely straight. Beams manufactured according to conventional methods tend to exhibit bowing (indicated by reference character  602 ) and often revert to a shape similar to but less severe than the shape of the platen radius in the press. This shape is undesirable and can result in the product being unsuitable for use in a particular application due to its dimensional instability. Although the systems and methods shown in  FIGS. 4 and 5  may be beneficial for reducing bulging, they have also been shown to exacerbate the problem associated with bowing in PSL beams. Bowing effects are well documented in PSL manufacturing and similar problems occur during the manufacturing other types of composite wood products. 
     Wood product manufacturing companies have taken numerous steps to reduce bowing and other defects associated with composite wood product manufacturing. One method involves modification of the shape of platen used in pressing operations. If the press infeed is reshaped to have a substantially larger radius, bowing effects can be mitigated. One drawback of this approach is that retrofitting presses can be extremely expensive. In addition, retrofitting is not possible in some facilities due to space constraints. Another method for mitigating bowing in PSL involves reducing the line speed for PSL manufacturing processes. Although this approach has been shown to reduce bowing to some extent, imposing line speed restrictions can ultimately lead to significant financial losses due to reductions in the amount of product that can be produced. 
     Accordingly, there is a need to develop new systems and methods for manufacturing PSL and other composite wood products to reduce and/or eliminate bowing and other undesirable effects from manufacturing stresses. Ideally, a process capable of reducing bow in PSL beams (when compared to conventional methods) without reducing throughput would be extremely useful in the composite wood products manufacturing industry. 
     SUMMARY 
     The following summary is provided for the benefit of the reader only and is not intended to limit in any way the invention as set forth by the claims. The present disclosure is directed generally towards systems and methods for manufacturing composite wood products in order to reduce bowing and other undesirable effects from manufacturing stresses. 
     In one embodiment, the disclosure includes a method for manufacturing a composite wood product. The method may include the steps of forming a mat from a plurality of wood elements and an adhesive, the mat having a width W measured substantially perpendicular to a longitudinal axis of the mat and an initial thickness T 1 . The mat is then moved in a machine direction, the machine direction being substantially parallel to the longitudinal axis and the temperature across the width W of the mat is adjusted. The mat is then compressed into a billet having a final thickness T 2 . 
     Further aspects are directed towards systems for manufacturing composite wood products. In some embodiments, the system includes a continuous movement mechanism, a side preheat assembly, and a press assembly. The continuous movement mechanism may be configured to move the mat in a machine direction, the machine direction being substantially parallel to the longitudinal axis. 
     The side preheat assembly may include one or more side energy generator assemblies configured to apply energy to one or more sides of the mat. The energy may be being applied in one or more horizontal directions, the horizontal directions being substantially perpendicular to the longitudinal axis of the mat. The press assembly may be configured to compress the mat to a billet having a final thickness T 2 . 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present disclosure is better understood by reading the following description of non-limitative embodiments with reference to the attached drawings wherein like parts of each of the figures are identified by the same reference characters, and are briefly described as follows: 
         FIG. 1  is a flow chart schematically showing a first conventional method for manufacturing a composite wood product; 
         FIG. 2  is a perspective view of a billet produced according to conventional methods; 
         FIG. 3  is a perspective view of a billet being cut into one or more beams according to conventional methods; 
         FIG. 4  is a flow chart schematically showing a second conventional method for manufacturing a composite wood product; 
         FIG. 5  is a side view of a conventional preheat assembly; 
         FIG. 6  is a perspective viewing of a composite wood product manufactured according to conventional methods, which exhibits bowing; 
         FIG. 7  is a flow chart schematically showing methods for manufacturing composite wood products according to embodiments of the disclosure; 
         FIG. 8  is a perspective view of a mat formed according to embodiments of the disclosure; 
         FIG. 9  is perspective view of a mat according to embodiments of the disclosure having one boundary plane; 
         FIG. 10  is perspective view of a mat according to embodiments of the disclosure having two boundary planes; 
         FIG. 11  is a side view of a system for manufacturing a composite wood product according to embodiments of the disclosure; 
         FIG. 12  is a top view of a system for manufacturing a composite wood product according to embodiments of the disclosure; 
         FIG. 13  is a perspective view of a billet cut into two beams; 
         FIG. 14  is a perspective view of a billet cut into three beams; 
         FIG. 15  is a perspective view of a side preheat assembly according to embodiments of the disclosure; 
         FIG. 16  is a perspective view of another side preheat assembly according to embodiments of the disclosure; 
         FIG. 17  is a perspective view of a conventional top/bottom preheat assembly; 
         FIG. 18  is a diagram depicting the heating pattern for the top/bottom preheat assembly in  FIG. 17 ; 
         FIG. 19  is a diagram depicting the expected heating pattern for the side preheat assembly in  FIG. 15 ; 
         FIGS. 20-22  are flow charts schematically showing methods for manufacturing composite wood products according to embodiments of the disclosure; 
         FIG. 23  is a side view of a system for manufacturing a composite wood product according to embodiments of the disclosure; 
         FIG. 24  is a top view of a system for manufacturing a composite wood product according to embodiments of the disclosure; 
         FIG. 25  is a schematic showing how billets were cut into beams in a comparative experiment; 
         FIG. 26  is a plot showing deflection of beams made according to conventional methods compared to beams made using systems and methods according to the disclosure; 
         FIGS. 27 and 28  are a plots showing deflection of beams made using systems and methods according to the disclosure; and 
         FIG. 29  is a plot showing deflection of beams made using systems and methods according to the disclosure over time. 
     
    
    
     DETAILED DESCRIPTION 
     The present disclosure describes systems and methods for manufacturing composite wood products in order to reduce bowing and other undesirable effects from manufacturing stresses. Certain specific details are set forth in the following description and  FIGS. 7-16  and  18 - 29  to provide a thorough understanding of various embodiments of the disclosure. Well-known structures, systems, and methods often associated with such systems have not been shown or described in details to avoid unnecessarily obscuring the description of various embodiments of the disclosure. In addition, those of ordinary skill in the relevant art will understand that additional embodiments of the disclosure may be practiced without several of the details described below. 
     In this disclosure, the term “wood” is used to refer to any organic material produced from trees, shrubs, bushes, grasses or the like. The disclosure is not intended to be limited to a particular species or type of wood. The term “wood elements” is used to refer to discrete pieces derived from wood that are used to form a wood composite product. Wood elements in this disclosure include, but are not limited to veneers, strands, flakes, chips, particles, fiber, solid pieces of wood, or other similar materials. The term “composite wood product” is used to refer to a range of derivative wood products which are manufactured by binding together wood elements with adhesives. Examples of composite wood products include but are not limited to parallel strand lumber, oriented strand board, oriented strand lumber, and laminated strand lumber. Although many examples and descriptions in this disclosure refer to embodiments for manufacturing parallel strand lumber, the disclosure is intended to apply to a wide range of wood composite products. 
     Various terminology is used within the wood product manufacturing industry to refer to composite wood products in different stages of the manufacturing process. In this disclosure, the term “mat” is used to refer to a plurality of wood elements and an adhesive formed in a layup prior to consolidation or pressing. The term “billet” is used to describe the wood elements and adhesive after consolidation or pressing. The term “beam” is used to refer to one or more sections which have been cut from the billet after consolidation or pressing. 
       FIG. 7  is a flow chart schematically showing methods for manufacturing composite wood products according to embodiments of the disclosure. According to  FIG. 7 , a plurality of wood elements (indicated by arrow  702 ) and an adhesive (indicated by arrow  704 ) are combined in an adhesive application station  706 . The coated wood elements then enter a forming station  708  where they are laid up in a substantially parallel relationship to form a mat. The mat is then treated in a side preheat assembly  710  and consolidated in a press assembly  712 . After pressing, the resulting billet is typically cut into two or three beams with a cutting assembly  714 . Systems and methods for resin application, lay up, and pressing are described in the following patent documents, all of which are hereby incorporated by reference: U.S. Pat. Nos. 4,020,311, 6,201,224, 7,048,825. Systems and methods according to the disclosure for the various steps in  FIG. 7  will now be described in detail. 
     Referring to  FIG. 8 , the first step involved in manufacturing composite wood products according to the disclosure includes forming a mat  800  from a plurality of wood elements and an adhesive. The wood elements may be, for example, veneers, strands, flakes, chips, particles, fiber, solid pieces of wood, or other similar materials. Adhesives suitable for use with methods according to the disclosure include phenol formaldehyde resins, isocyanate resins, thermosetting resins, or other suitable binders known to a person of ordinary skill in the art. In some embodiments, the wood elements are arranged in a layup in a parallel relationship to one another. In some embodiments, the mat  800  has a width W measured substantially perpendicular to a longitudinal axis  802  of the mat  800 , a length L, and an initial thickness T 1 . The thickness T 1  may be substantially greater than the width W. Exemplary dimensions for embodiments according to the disclosure include a width W ranging from about 6 inches to about 48 inches, a length L ranging from about 20 feet to about 200 feet, and an initial thickness T 1  ranging from about 6 inches to about 48 inches. 
     In some embodiments, the mat  800  may be thought of as divided into two or more sections defined by one or more boundary planes. The one or more boundary planes may be substantially parallel to the longitudinal axis  802  of the mat  800 . In  FIG. 9 , the mat  800  is shown divided into a first section  902  and a second section  904  defined by a first boundary plane  906 . The mat has a top face  908 , a bottom face  910 , a first side face  912 , a second side face  914 , a front face  916 , and a back face  918 . In  FIG. 10 , the mat is shown divided into a first section  1002 , a second section  1004 , and a third section  1006 . The first section  1002  and the second section  1004  are defined by the first boundary plane  1008 . The second section  1004  and the third section  1006  are defined by a second boundary plane  1010 . The boundary planes roughly correlate to the beams that will be cut from the mat  800  after it has been formed into a billet. A person of ordinary skill in the art will appreciate that composite wood products according to the disclosure may include any number of sections greater than two and any number of boundary planes greater than one. 
     In some embodiments, the next step in the process includes forming the mat  800  into a billet by substantially reducing the initial thickness T 1  of the mat  800 . In some embodiments, this may be accomplished using systems for manufacturing a composite wood product. Examples of such systems are shown in  FIG. 11  (side view) and  FIG. 12  (top view). Embodiments of the system  1100  include a continuous movement mechanism  1102 , a side preheat assembly  1104 , and a press assembly  1106 . 
     Referring to  FIG. 11  and  FIG. 12 , the mat  800  may be moved on the continuous movement mechanism  1102  in a machine direction indicated by arrow  1108 . As shown in  FIGS. 11 and 12 , the machine direction  1108  is substantially parallel to the longitudinal axis  802 . The continuous movement mechanism  1102  may be a conveyor belt or any other device for continuously moving an article in a manufacturing process known to a person of ordinary skill in the art. 
     According to some embodiments, the mat  800  is moved via the continuous movement mechanism  1102  through the side preheat assembly  1104 . The side preheat assembly  1104  is configured to adjust the temperature across the width W of the mat  800 . In some embodiments, temperature adjustment is accomplished by applying energy to the mat  800  in one or more horizontal directions shown by arrows  1202  and  1204  in  FIG. 12 . Accordingly, the energy is applied to the first side face  912  of the mat  800  and the second side face  914  of the mat  800  in one or more directions that are perpendicular to the longitudinal axis  802 . 
     In some embodiments, the side preheat assembly  1104  includes one or more side energy generator assemblies.  FIG. 12 , for example, shows a first side energy generator assembly  1206  and a second side energy generator assembly  1208 . Side energy generators suitable for use with embodiments of the disclosure may include microwave generator assemblies, hot air generator assemblies, steam generator assemblies, or any other suitable mechanism for adjusting the temperature across the width W of the mat  800 . The number and overall arrangement of the side energy generator assemblies shown in the Figures is not intended to limit embodiments of the disclosure. A person of ordinary skill in the art will appreciate that although  FIG. 12  shows two side energy generator assemblies, embodiments according to the disclosure may include a single side energy generator assembly or more than two side energy generator assemblies. Furthermore, side energy generator assemblies according to embodiments of the disclosure may be arranged in different configurations or arrangements other than those explicitly shown. 
     Microwave energy may be employed as the energy generating mechanism for the side preheat assembly  1104  according to embodiments of the disclosure. In the embodiment depicted in  FIG. 12 , the first side energy generator assembly  1206  includes a first microwave generator  1210  and a first microwave window  1212 . The first side energy generator assembly  1206  is positioned adjacent to the first side face  912  of the mat  800  and is configured to direct microwave energy (in a horizontal direction shown by the arrow  1202 ) from the first microwave generator  1210  through the first microwave window  1212  and into the mat  800  as it passes through a zone in which it is either not compressed at all or is lightly compressed prior to entering the press assembly  1106 . The second side energy generator assembly  1208  includes a second microwave generator  1214  and a second microwave window  1216 . The second side energy generator assembly  1208  is positioned adjacent to the second side face  914  of the mat  800  and is configured to direct microwave energy (in a horizontal direction shown by the arrow  1204 ) from the second microwave generator  1214  through the second microwave window  1216  and into the mat  800 . 
     Microwave generators according to embodiments of the disclosure may be configured to produce microwave energy at a frequency ranging from about 200 MHz to about 10 GHz. Microwave generators according to embodiments of the disclosure may be magnetrons having a power output of about 1 kW to about 500 kW. Alternatively in some embodiments, klystron, or other microwave generators known to a person of ordinary skill in the art may be used. Microwave windows according to embodiments of the disclosure may be constructed from ceramics, Teflon, glass, or any combination of similar materials. 
     After the mat  800  is preheated, it is passed through the press assembly  1106  where it is compressed into a billet  1110 . Continuous belt-type presses suitable for use with embodiments of the disclosure are well known in the art and not described in detail. Referring back to  FIG. 11 , the press assembly  1106  may include a first press section  1112  and a second press section  1114 , which are spaced apart to form a press passage  1116  through which the mat  800  may be passed. Some embodiments of press assemblies  1106  according to the disclosure may also include an in-press microwave heating mechanism such as those described in U.S. Pat. No. 7,070,676, which is hereby incorporated by reference. As the mat is moved through the press assembly  1106 , the first press section  1112  and the second press section  1114  compress the mat  800  into a billet  1110  having a final thickness T 2 . In some embodiments, the length L and width W of the mat  800  do not substantially change. In embodiments according to the disclosure, the initial thickness T 1  may range from about 1 foot to about 6 feet and the final thickness T 2  may range from about 1 inch to about 2 feet. 
     After the billet  1110  is formed, it is moved through a cutting assembly to be cut into beams for commercial sale. Cutting assemblies suitable for use with embodiments of the disclosure are well known in the art and are therefore not described in detail. Referring to  FIGS. 13 and 14 , the billet  1110  is shown being cut into two and three beams respectively. In  FIG. 13 , the billet  1110  may be cut along a cutting plane  1302  that is substantially parallel with the length L of the billet into two beams: a first beam  1302  and a second beam  1304 . The cutting plane  1302  may be located at substantially the same position as first boundary plane  906  from  FIG. 9  or may be located at a slightly different position on the mat  800 . The first beam  1302  and the second beam each have a thickness T 2 /2 which is substantially equal to half of the final thickness T 2 . In embodiments according to the disclosure, the T 2 /2 may be approximately 5 inches. 
     Referring to  FIG. 14 , the billet  1110  may be cut along a first cutting plane  1402  and a second cutting plane  1404  into three beams: a first beam  1406 , a second beam  1408 , and a third beam  1410 . The first cutting plane  1402  may be located at substantially the same position as the first boundary plane  1008  from  FIG. 10  or may be located at a slightly different position on the mat  800 . Likewise the locations of the second cutting plane  1404  may also correlate to the location of the second boundary plane  1010 . The first beam  1406 , the second beam  1408 , and the third beam  1410  each have a thickness T 2 /3 which is substantially equal to one third of the final thickness T 2 . In embodiments according to the disclosure, the T 2 /3 may be approximately 3 inches. 
     A person of ordinary skill in the art will appreciate that billets made using systems and methods according to the disclosure may be cut into any number of beams greater than one. For example, a billet may be cut into two, three, four, five, or more beams. Accordingly, billets may have varying dimensions, thereby yielding beams of varying widths, thicknesses, and lengths. Systems and methods according to the disclosure may be effective to reduce bowing in beams cut from billets by preheating a well defined area of the mat prior to compression. The details of systems methods for preheating according to embodiments of the disclosure will now be described in further detail. Although such methods are described with respect to microwave energy systems, a person of ordinary skill in the art will understand that the same principles may be applied to other mechanism for generating energy suitable for adjusting temperature in a composite wood product. 
       FIG. 15  is a perspective view of the mat  800  moving through a side preheat assembly according to embodiments of the disclosure. In  FIG. 15 , the mat  800  is shown divided by a first boundary plane  1008  and a second boundary plane  1010  as initially presented in  FIG. 10 . As shown, the boundary planes are substantially parallel to the top face  908  of the mat  800 . The longitudinal axis  802  is oriented in a direction pointing out of the page. As the mat  800  moves in the machine direction  1108 , it passes by a side preheat assembly including: a first side energy generator assembly  1502  and a second side energy generator assembly  1504 . In the embodiment shown in  FIG. 15 , the first side energy generator assembly  1502  includes a first microwave window  1506  and a second microwave window  1508 . The second side energy generator assembly  1504  includes a third microwave window  1510  and a fourth microwave window  1512 . Each microwave window is connected to one or more microwave generators (not shown) so that microwave energy may be applied to the mat  800 . 
     In embodiments according to the disclosure, the first microwave window  1506  and the third microwave window  1510  are positioned adjacent to the first boundary plane  1008 . Likewise, the second microwave window  1508  and the fourth microwave window  1512  are positioned adjacent to the second boundary plane  1010 . Referring back to  FIG. 14 , the locations of the boundary planes are determined by estimating the locations of the cutting planes  1402  and  1404 . Accordingly, some embodiments of the disclosure endeavor to heat the portion of the mat  800  that will become the top and/or bottom face of the first beam  1406 , the second beam  1408 , and/or the third beam  1410  after formation into a billet and cutting. The heating pattern from side preheat systems and methods according to embodiments of the disclosure is expected to minimize bowing by focusing heat on the bottom of the first beam  1406  (pulling it downward), at the top and bottom of the second beam  1408 , and at the top of the third beam  1410  (pulling it upward). Preheating according to embodiments of the disclosure may induce a lengthening of wood elements opposite to the top/bottom pre-heat and consequently creating cupping in the beam that will counterbalance the cupping induced by the pressing geometry. In some embodiments, this may be effective to ensure there is a straightening effect from the induced thermal elongation before pressure is applied to the mat  800  in the press assembly  1106 . 
       FIG. 16  is a perspective view of the mat  800  moving through a side preheat assembly according to embodiments of the disclosure. In contrast to  FIG. 15 , which depicted an embodiment of a side preheat assembly comprising a total of four microwave windows, the embodiment shown in  FIG. 16  comprises two microwave windows: a first microwave window  1212  and a second microwave window  1214 . In  FIG. 16 , the mat  800  is shown divided by the boundary plane  906  initially presented in  FIG. 9 , the boundary plane  906  being substantially parallel to the top face  908  of the mat  800 . Referring back to  FIG. 13 , positioning of the first microwave window  1212  and positioning of the second microwave window  1216  may be determined by estimating the locations of the cutting plane  1302 . Accordingly, some embodiments of the disclosure endeavor to heat the portion of the mat  800  that will become the top and/or bottom face of the first beam  1304  and the second beam  1306  after formation into a billet and cutting. 
     Embodiments of the disclosure are envisioned to encompass arrangements of side preheat assemblies other than those explicitly shown in  FIGS. 15 and 16 . Based on the application, any number of boundary planes may be used to divide the mat  800  into any number of sections. Accordingly, many different numbers and configurations of microwave windows may be used. 
       FIGS. 17-19  illustrate expected heating patterns for embodiments of the disclosure compared with heating patterns from conventional top/bottom preheat assemblies.  FIG. 17  is a perspective view of a conventional top/bottom preheat assembly. In  FIG. 17 , the mat  800  is shown divided by a first boundary plane  1008  and a second boundary plane  1010  into a first section  1002 , a second section  1004 , and a third section  1006  (see  FIG. 10 ). The mat modeled has a width W of approximately 12.5 inches and an initial thickness T 1  of approximately 36 inches. A top/bottom preheat assembly is positioned adjacent to the top face  908  and the bottom face  910  of the mat  800 . The top/bottom preheat assembly includes a first microwave window  1702  configured to apply energy in a first direction indicated by arrow  1704  and a second microwave window  1706  configured to apply energy in a second direction indicated by arrow  1708 . In this example, both windows are configured to apply microwave energy having a frequency of about 915 MHz. 
       FIG. 18  is a diagram depicting the heating pattern for the top/bottom preheat assembly in  FIG. 17 . The heating pattern (measured in Watts per cubic meter) indicates hot spots near the top face  908  and the bottom face  910 . Accordingly, if the mat is cut into three beams, the beam made from the first section  1002  and the beam made from the third section  1006  may be expected to exhibit bowing. 
       FIG. 19  is a diagram depicting the expected heating pattern for side preheating assemblies according to embodiments of the disclosure. The heating pattern in  FIG. 19  was created based on a mat having the same dimensions as the mat modeled in  FIG. 18 . In  FIG. 19 , the model is based on the mat passing through the side preheating assembly from  FIG. 15 . The diagram in  FIG. 19  indicates a heating pattern that is substantially symmetrical about each of the boundary planes  1008  and  1010 . Accordingly, if the mat is cut into three beams, the energy supplied by the side preheat assembly is expected to help remediate bowing in beams cut from the first section  1002  and the third section  1006 . Preheating according to embodiments of the disclosure is expected to be effective to counterbalance any bowing in the beam cut from the second section  1004 . 
     In some embodiments, the disclosure contemplates using top/bottom preheat systems and methods in combination with the side preheat systems and methods discussed above. This type of arrangement may be effective to provide symmetric heat to the middle beam (see e.g., second beam  1408  in  FIG. 14 ) thereby reducing bowing.  FIGS. 20-22  are flow charts schematically showing methods for manufacturing composite wood products according to embodiments of the disclosure. According to  FIG. 20 , a plurality of wood elements  702  and an adhesive  704  are combined in an adhesive application station  706 . The coated wood elements then enter a forming station  708  where they are laid up in a substantially parallel relationship to form a mat. The mat is then treated in a side preheat assembly  710  and a top/bottom preheat assembly  2002  before it is consolidated in a press assembly  712  and cut by a cutting assembly  714 . As shown in  FIG. 21 , in some embodiments, top/bottom preheating may be performed before side preheating. Alternatively, a combination top/bottom and side preheat assembly may be used to apply microwave energy to the top, bottom, and side faces of the mat simultaneously. 
     Examples of systems according to the disclosure that incorporate top/bottom preheating are shown in  FIG. 23  (side view) and  FIG. 24  (top view). Embodiments of the system  2300  include a continuous movement mechanism  1102 , a top/bottom preheat assembly  2102 , a side preheat assembly  1104 , and a press assembly  1106 . The continuous movement mechanism  1102 , side preheat assembly  1104 , and press assembly  1106  may incorporate substantially the same components as the system described in  FIGS. 11 and 12 . The top/bottom preheat assembly  1104  is configured to adjust the temperature across the thickness T 1  of the mat  800 . In some embodiments, temperature adjustment is accomplished by applying energy to the mat  800  in one or more vertical directions shown by arrows  2302  and  2304  in  FIG. 23 . Accordingly, the energy is applied to the top face  908  of the mat  800  and the bottom face  910  of the mat. 
     In some embodiments, the top/bottom preheat assembly  2002  includes one or more top/bottom energy generator assemblies.  FIG. 23 , for example, shows a top energy generator assembly  2306  and a bottom energy generator assembly  2308 . Energy generators suitable for use with embodiments of the disclosure may include microwave generator assemblies, hot air generator assemblies, steam generator assemblies, or any other suitable mechanism for adjusting the temperature across the thickness T 1  of the mat  800 . The number and overall arrangement of the top/bottom energy generator assemblies shown in the Figures is not intended to limit embodiments of the disclosure. A person of ordinary skill in the art will appreciate that although  FIG. 23  shows two top/bottom energy generator assemblies (e.g., a top and a bottom), embodiments according to the disclosure may include a single top energy generator assembly, a single bottom energy generator assembly, or more than two top/bottom energy generator assemblies. Furthermore, top/bottom energy generator assemblies according to embodiments of the disclosure may be arranged in different configurations or arrangements than explicitly shown. 
     Microwave energy may be employed as part of the top/bottom preheat assembly  2002  according to embodiments of the disclosure. In the embodiment depicted in  FIG. 23 , the top energy generator assembly  2306  includes a first microwave generator  2310  and a first microwave window  2312 . The top energy generator assembly  2306  is positioned adjacent to the top face  908  of the mat  800  and is configured to direct microwave energy (in a vertical direction shown by the arrow  2302 ) from the first microwave generator  2310  through the first microwave window  2312  and into the mat  800 . The bottom energy generator assembly  2308  includes a second microwave generator  2314  and a second microwave window  2316 . The bottom energy generator assembly  2308  is positioned adjacent to the bottom face  910  of the mat  800  and is configured to direct microwave energy (in a vertical direction shown by the arrow  2304 ) from the second microwave generator  2314  through the second microwave window  2316  and into the mat  800 . 
     From the foregoing, it will be appreciated that the specific embodiments of the disclosure have been described herein for purposes of illustration, but that various modifications may be made without deviating from the disclosure. For example, the configuration and number of preheat assemblies shown in the Figures may be modified without departing from the spirit of the disclosure. Additionally systems and methods according to the disclosure may be used to manufacture wood products other than those explicitly described. 
     Aspects of the disclosure described in the context of particular embodiments may be combined or eliminated in other embodiments. For example, aspects of the disclosure described with respect to embodiments involving two beams may be applied to embodiments involving more than two beams. Features described with respect to embodiments involving microwave energy generators may also be applied to embodiments utilizing other types of energy generation. 
     Further, while advantages associated with certain embodiments of the disclosure may have been described in the context of those embodiments, other embodiments may also exhibit such advantages, and not all embodiments need necessarily exhibit such advantages to fall within the scope of the disclosure. Accordingly, the invention is not limited except as by the appended claims. 
     The following examples will serve to illustrate aspects of the present disclosure. The examples are intended only as a means of illustration and should not be construed to limit the scope of the disclosure in any way. Those skilled in the art will recognize many variations that may be made without departing from the spirit of the disclosure. 
     EXAMPLE 1 
     Deflection in Composite Wood Products Made Using Conventional Methods and Methods According to Embodiments of the Disclosure 
     In a comparative experiment, beams of composite wood products were produced using conventional methods and systems and methods according to embodiments of the disclosure. Three groups of billets were produced in order to evaluate bowing in the resultant beams. The first group (the control group) was produced using conventional systems and methods. The second and third groups were produced using systems and methods according to embodiments of the disclosure. 
     The beams in each of the three groups were produced from mats made from wood elements and an adhesive (shown schematically in  FIG. 25  by reference character  2500 ). The wood elements were strands made from a mix of yellow pine and poplar. The adhesive was a phenoyl formaldehyde resin. The mats were laid up using conventional methods. Each mat had a length L of approximately 48 feet, a width W of about 12 inches, and an initial thickness T 1  of about 36 inches. 
     After mat formation, the mats in the first group were pressed into billets without being subjected to any preheating treatment.  FIG. 1  schematically shows the process used to manufacture the first group. The line speed for the first group was approximately 7 feet per minute. 
     As described above, the mats in the second and third groups were subjected to preheating according to embodiments of the disclosure. The second group was produced using systems and methods according to the disclosure such as those shown in  FIGS. 7 and 15 . Four 15 kW microwave windows (having a total power of 60 kW) were used to apply side preheating microwave energy at a frequency of approximately 915 MHz. The microwave windows were positioned adjacent to the boundary planes  2502  and  2504 . The line speed for the second billet group was approximately 7 feet per minute. 
     The third group was produced using systems and methods according to the disclosure such as those shown in  FIGS. 7 and 15 . Four 30 kW microwave windows (having a total power of 120 kW) were used to apply side preheating microwave energy at a frequency of approximately 915 MHz. The microwave windows were positioned adjacent to the boundary planes  2502  and  2504 . The line speed for the second billet group was approximately 7 feet per minute. 
     After pressing each mat  2500  into a billet  2506  having a final thickness T 2  of approximately 11.1 inches, each billet  2506  was cut along cutting planes  2508  and  2510  into three equally sized beams: beam A (reference character  2512 ), beam B (reference character  2514 ), and beam C (reference character  2516 ). Accordingly, each group consisted of 15 beams, each having a thickness of approximately 3.5 inches. 
     After each billet was cut into three beams, deflection (in inches) was measured over the length of each beam. Deflection was measured using laser scanning and manual techniques that are well known to a person of ordinary skill in the art.  FIG. 26  is a plot showing average deflection of beam A, beam B, and beam C in each of the three groups. As demonstrated, systems and methods according to the disclosure may be able to reduce bowing effects in beam A and beam C when compared to conventional methods. 
     EXAMPLE 2 
     Deflection in Composite Wood Products Made Using Systems and Methods According to the Disclosure 
       FIGS. 27 and 28  are a plots showing deflection of beams made using systems and methods according to the disclosure. The beams were cut from a billet in the same manner as reference in Example 1 (see  FIG. 25 ). After being cut, deflection was measured in the same manner as described in Example 1. 
     The beams representing the data shown in  FIG. 27  were produced using systems and methods according to the disclosure such as those shown in  FIGS. 7 and 15 . Four 15 kW microwave windows (having a total power of 60 kW) were used to apply side preheating microwave energy at a frequency of approximately 915 MHz. The microwave windows were positioned adjacent to the boundary planes  2502  and  2504 . The line speed for the second billet group was approximately 7 feet per minute.  FIG. 27  shows deflection measured in beam A, beam B, and beam C. 
     The beams representing the data shown in  FIG. 28  were produced using systems and methods according to the disclosure such as those shown in  FIGS. 7 and 15 . Four 15 kW microwave windows (having a total power of 60 kW) were used to apply side preheating microwave energy at a frequency of approximately 915 MHz. The microwave windows were positioned adjacent to the boundary planes  2502  and  2504 . The line speed for the second billet group was approximately 7 feet per minute.  FIG. 28  shows deflection measured in beam A, beam B, and beam C. 
     EXAMPLE 3 
     Deflection Over Time in Composite Wood Products Made Using Systems and Methods According to the Disclosure 
     In a series of experiments, deflection over time was measured in composite wood products made using systems and methods according to the disclosure. Fifteen beams were produced using systems and methods according to the disclosure such as those shown in  FIGS. 7 and 15 . Beams were cut from a billet in the same manner as reference in Example 1 (see  FIG. 25 ). Four 15 kW microwave windows (having a total power of 60 kW) were used to apply side preheating microwave energy at a frequency of approximately 915 MHz. The microwave windows were positioned adjacent to the boundary planes  2502  and  2504 . The line speed for the second billet group was approximately 7 feet per minute. 
     After cutting, the deflection of the beams was measured using techniques described in the previous examples. The beams were then placed in a warehouse where they were stored under consistent temperature and humidity conditions for 14 days. The deflection of the beams was measured again after 14 days and the beams were stored for a total of 45 days. After 45 days, the deflection was measured for a third time.  FIG. 29  shows the measured in beam A, beam B, and beam C at the following time periods: 1 day, 14 days, and 45 days. As demonstrated by the data, the reduced bowing effect remains relatively stable over time.