Patent Publication Number: US-7595086-B2

Title: Method for producing corrugated cardboard

Description:
BACKGROUND OF THE INVENTION 
     The present invention generally relates to the production of corrugated cardboard, and more particularly to a novel and improved method for accurately applying an adhesive to the flutes of corrugated board centered on the flute crests, so that the flutes can be bonded to a face. 
     Typically, corrugated cardboard is formed by producing a corrugated sheet which is initially bonded along one side to a single face. Adhesive is then applied to the crests of the flutes remote from the single face by an applicator roll of a glue machine. Thereafter, a second face is applied to the adhesive on the flutes to produce a composite structure in which corrugations extend between and are bonded to spaced-apart faces. 
     In some instances, multiple-layer cardboard is produced in which more than one corrugated sheet is adhesively attached to additional faces so that, for example, a central flat face is bonded to a corrugated sheet on each side thereof, and outer flat faces are bonded to the sides of the two corrugated sheets remote from the central face. 
     The corrugated sheet is typically passed between a rider roll and an applicator roll to apply the adhesive to the flutes. The rider roll typically applies sufficient downward pressure to force the flute tips into contact with the applicator roll. This downward pressure causes compression or deformation of the flutes. The flutes enter the adhesive layer prior to being crushed against the applicator and often become overly wetted or saturated with adhesive due to the long dwell time. As a result, the flutes do not return to their original shape after being crushed. This permanent deformation of the flutes reduces the strength of the final cardboard. 
     It has been known in the art that glue machines can be run with the applicator roll operating at a lower speed than the web speed (speed at which the corrugated sheet passes between the applicator roll and the rider roll) in order to adjust glue weight. U.S. Pat. No. 6,602,546, incorporated herein by reference, discloses a method for operating a glue machine such that the applicator roll can be operated at speed lower than the web speed while still applying a uniformly thick glue line at the flute crests, minimizing glue application along the leading or trailing faces of the flutes. 
     Previously, it was believed that the applicator roll must always be operated at a lower speed than the web speed. However, it has been discovered, surprisingly and unexpectedly, that excellent glue weight control and reproducibility also can be achieved when the applicator roll is operated at a speed (surface linear speed) greater than the web speed, e.g. at least 105% of the web speed 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention provides a method and apparatus for uniformly and accurately applying adhesive to the crests of the flutes of corrugated sheets with little or no (or substantially no) adhesive being applied to either the leading or trailing sloped faces of the flutes. In accordance with the present invention, higher line speeds can be achieved, tighter performance specifications exceeding the capability of the industry&#39;s standard machines are possible, and a significant reduction in the amount of glue used is achieved. In addition, accurately centering the adhesive onto the crests of the flutes provides stronger bond strength between the corrugated sheet and the adhered-to face sheet. Directional differences in strength are minimized or substantially eliminated, and surface smoothness of the face sheets is improved (washboarding reduced). Because the adhesive is very accurately deposited only to the flute crests, it is possible to reduce the adhesive weight deposition rate about 10-70% of that required in conventional machines while delivering the same or comparable bond and crush strength. Furthermore, in accordance with the present invention, smoother and more printable boards with greatly reduced warpage and improved surface finish are produced. 
     A method of applying adhesive to flutes of a corrugated sheet is provided, wherein each flute has a leading sloped face, a trailing sloped face and a crest. The method includes the following steps: a) applying a layer of adhesive on an outer surface of an applicator roll and rotating the applicator roll in a first direction; and b) moving the corrugated sheet along a path adjacent the outer surface of the applicator roll to apply adhesive to the flutes from the layer of adhesive; wherein a roll speed ratio is defined as the ratio of the surface linear velocity of the applicator roll to the speed at which the corrugated sheet is moving, and the roll speed ratio is greater than 1. 
     A further method of applying adhesive to flutes of a corrugated sheet is provided, wherein each flute has a leading sloped face, a trailing sloped face and a crest therebetween. The method includes the following steps: a) applying a layer of adhesive on an outer surface of an applicator roll and rotating the applicator roll in a first direction; b) moving the corrugated sheet along a path adjacent the outer surface of the applicator roll to apply adhesive to the flutes from the layer of adhesive; and c) initially contacting each of the flutes with the applicator roll such that the crest of each of the flutes is compressed in a forward direction relative to the direction in which the corrugated sheet is moving along the path adjacent the outer surface of the applicator roll. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
       These and further features of the present invention will be apparent with reference to the following description and drawings, wherein: 
         FIG. 1  is a schematic elevational view of a machine for producing cardboard in accordance with the present invention; 
         FIG. 2A  is an enlarged elevation view of a single face corrugated sheet; 
         FIG. 2B  is an enlarged elevational view of the single face corrugated sheet of  FIG. 2A  with adhesive applied to the crests of the flutes; 
         FIG. 2C  is an elevational view of the single face corrugated sheet of  FIG. 2A  with a second face secured thereto; 
         FIG. 3  is an enlarged fragmentary view, partially in cross-section, showing a portion of the machine of  FIG. 1  at a glue mechanism for applying adhesive to crests of a single faced corrugation assembly; 
         FIG. 4  is an enlarged fragmentary view, partially in cross-section, showing a portion of the glue mechanism of  FIG. 3  at an interface between an applicator roll and a vacuum rider roll; 
         FIG. 4A  is an enlarged view as in  FIG. 4 , showing glue being applied to the flute crests of a corrugated sheet along a path between the applicator roll and the rider roll according to a preferred method of the invention; 
         FIG. 5  is a fragmentary side view, partially in cross-section, of the applicator roll of  FIGS. 3 and 4 ; 
         FIG. 6  is an enlarged fragmentary side view of an isobar metering device of the glue mechanism of  FIG. 2 ; 
         FIG. 6A  is an enlarged fragmentary view at an interface between the isobar metering device and the applicator roll; 
         FIG. 7  is an enlarged fragmentary side view, similar to  FIG. 6 , of an alternative isobar metering device which may be used with the glue mechanism of  FIG. 3 ; 
         FIG. 8  is a cross-sectional end view of the vacuum rider roll of  FIGS. 3 and 4 ; 
         FIG. 9  is a schematic view of a drive system for driving the applicator roll and for controlling the speed thereof; 
         FIG. 10  is a fragmentary elevational view, partially in cross-section, similar to  FIG. 2  but viewed from the opposite side and showing additional features of a rider system of the glue mechanism; 
         FIG. 11  is a fragmentary elevational view, partially in cross-section, similar to  FIG. 10  but showing an alternative embodiment of the rider system; and 
         FIG. 12  is a fragmentary elevational view, partially in cross-section, similar to  FIGS. 10 and 11  but showing another alternative embodiment of the rider system. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     As used herein, the terms ‘glue’ and ‘adhesive’ are used interchangeably, and refer to the adhesive that is applied to the flute crests of a corrugated sheet  18  according to the invention as hereinafter described. Also as used herein, the term ‘web’ refers to the corrugated sheet  18  traveling through a glue machine  38 , and particularly as it travels past an applicator roll  48  for applying adhesive thereto as will be further described. In the description that follows, and from the drawings, it will be apparent that the web speed is controlled, at least in part, by the rotational speed of the rider roll  52 . 
       FIG. 1  schematically illustrates a machine  10  for producing single-corrugated cardboard sheet  12 . As best shown in  FIGS. 2A ,  2 B, and  2 C, the single-corrugated cardboard sheet  12  is produced by joining a web of single-face corrugation assembly  14  with a face sheet  16 . The single-face corrugation assembly  14  includes a corrugated sheet  18  having a plurality of flutes  20  and a first face sheet  22  bonded to the crests or tips of the flutes  20  on a first side of the corrugated sheet  18 . The crests or tips of the flutes  20  on the second side of the corrugated sheet  18 , remote from the attached first face sheet  22 , are exposed. 
     It should be understood that the illustrated machine  10  is shown only by way of example and that the present invention can be applied to many different types of machines. For example, the present invention can be easily utilized with machines for producing double-corrugated cardboard or triple corrugated cardboard, as well as for applying the corrugated sheet  18  to the first face sheet  22 . 
     The machine  10  preferably includes a source  24  of the single-face corrugation assembly  14 , a source  26  of the second face sheet  16 , a coating station  28  for the second face sheet  16 , a pre-heating station  30  for heating the corrugation assembly  14  and the second face sheet  16 , a gluing station  32  for applying glue to the corrugation assembly  14 , a curing station  34  for joining the corrugation assembly  14  and the second face sheet  16 , and a traction station  36  for pulling the finished corrugated cardboard sheet  12  through the machine  10 . 
     The web of the single-face corrugation assembly  14  is supplied to the machine  10  from a source  24  such as, for example, a single facing machine. The source  24  of the corrugation assembly  14  can be of any conventional type. The second face sheet  16  is supplied from a source  26  such as, for example, a supply roll. 
     From the source  26 , the second face sheet  16  passes to the coating station  28 . The coating station  28  includes a coating machine for applying a coating to one side of the second face sheet  16 . The coating station  28  is not essential to the present invention and is merely illustrated as one available processing apparatus that can be incorporated into the machine  10 , particularly where at least one side of the cardboard sheet  12  is to be provided with printing and/or a decorative finish. 
     Next, the corrugation assembly  14  and the second face sheet  16  both pass through the pre-heating station  30 . The pre-heating station  30  includes a heating machine for pre-heating the corrugation assembly  14  and the second face sheet  16 . The pre-heating station  30  also is optional depending upon the type of adhesive being applied to the corrugation assembly  14  to join the second face sheet  16 . 
     From the pre-heating station  30 , the single-face corrugation assembly  14  passes to the gluing station  32 . The gluing station  32  includes a precision glue machine  38  in accordance with the present invention. The glue machine  38  applies an accurately controlled amount of adhesive  40  (best shown in  FIG. 2B ) to the crests of the flutes  20  as described in more detail hereinafter. 
     Next, the corrugation assembly  14  and the second face sheet  16  both pass through the curing station  34 . The curing station  34  includes a “double facer” which brings the single-face corrugation assembly  14  and the second face sheet  16  together. The double facer can be of any conventional type. Once brought together, the single-face corrugation assembly  14  and the second face sheet  16  pass between guide plates  42  which maintain the assembly flat and straight as the adhesive  40  cures. Additionally, heat can be applied to the plate to aid in the curing of the adhesive. 
     From the curing station  34 , the glued and dried cardboard sheet  12 , including the two face sheets  16 ,  22  bonded to opposite sides of the corrugated sheet  18 , passes to the traction station  36 . The traction station  36  includes drive and traction rollers  44  which pull the cardboard sheet  12  from the machine  10 . 
     As best shown in  FIG. 3 , the glue machine  38  includes a glue tray  46 , a glue applicator roll  48 , an isobar assembly  50 , and a rider roll  52 . The glue tray  46  is a container having an open top which when filled with glue provides a source or supply of adhesive. The glue tray  46  is located directly below the applicator roll  48  and extends below at least a portion of each of the isobar assembly  50  and the rider roll  52 . 
     The applicator roll  48  is journaled for rotation about a horizontal and transverse rotational axis  54  in the direction indicated by the arrow (clockwise as viewed in  FIG. 3 ). The applicator roll  48  is located above the glue tray  46  and positioned so the lower portion of the applicator roll  48  is immersed in the adhesive within the glue tray  46  at a coating position of the roll  48 . As the applicator roll  48  rotates, a coating of adhesive is applied to the periphery of the applicator roll  48  at the coating position. As the surface of the applicator roll  48  emerges from the adhesive within the glue tray  46 , a coating of adhesive exceeding the desired final coating or film thickness adheres to the outer peripheral surface of the roll  48 . 
     As best shown in  FIGS. 4 and 5 , the applicator roll  48  preferably has an outer shell  56 , a pair of end plates  58 , and a pair of support shafts  60 . The outer shell  56  is cylindrically-shaped and formed from a suitable metal. The end plates  58  are secured to opposite ends of the shell  56  in any suitable manner such as, for example, by welding. The support shafts  60  are secured to the end plates  58  at the rotational axis  54  so that the end plates  58  connect the support shafts  60  to the outer shell  56 . The shafts  60  are secured to the end plates  58  in any suitable manner such as, for example, by welding. A coating  62  can be applied to the outer peripheral surface of the cylindrical shell  56  and provides a smooth peripheral contact surface of the applicator roll  48 . The coating  62  is of any suitable material such as, for example rubber and preferably has a hardness in the range of 0 to 5 P &amp; J hardness. The coating  62  is preferably provided with an extremely smooth surface finish. 
     The isobar assembly  50  is mounted adjacent to the periphery of the applicator roll  48  and removes excess adhesive from the outer peripheral surface of the applicator roll  48  to provide an adhesive coating  41  having precise uniform thickness on the outer peripheral surface of the applicator roll  48  after it has rotated past the isobar assembly  50 . The most preferred thickness of the adhesive coating  41  depends on the size of the flutes to which the glue is to be applied. Table 1 below shows the most preferred adhesive coating  41  thicknesses for different size flutes. The A, B, C, and E flutes listed in table 1 refer to standard flute sizes well known in the corrugated board art by their respective letter designations. Alternatively, the adhesive coating thickness is preferably at least 0.002, 0.003, 0.004, 0.005, or 0.006, inches. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Preferred thickness of adhesive coating on outer surface of applicator 
               
               
                 roll for different sized flutes 
               
            
           
           
               
               
               
            
               
                   
                 Flute Size 
                 Adhesive Coating Thickness (inches) 
               
               
                   
                   
               
               
                   
                 A 
                 0.008 or less, preferably 0.006-0.008 
               
               
                   
                 B 
                 0.006 or less, preferably 0.003-0.006 
               
               
                   
                 C 
                 0.006 or less, preferably 0.003-0.006 
               
               
                   
                 E 
                 0.006 or less, preferably 0.003-0.006 
               
               
                   
                 Smaller than E 
                 0.004 or less, preferably 0.001-0.003 
               
               
                   
                   
               
            
           
         
       
     
     Preferably, the isobar assembly  50  is located at the rear side of the applicator roll  48  (opposite the rider roll  52 ) and at the same height as the rotational axis  54  of the applicator roll  48 , that is, the isobar assembly  50  is located at a 9-o&#39;clock position relative to applicator roll  48  (as best shown in  FIG. 3 ). 
     The illustrated embodiment of the isobar assembly  50  includes a frame member  64  and first and second metering rod assemblies  66 ,  68 . The frame member  64  is relatively stiff and is mounted on the glue machine  38  for rotation about a central axis  70  over at least 180 degrees. Therefore, the frame member  64  can be rotated from the position illustrated to a position of opposite orientation. The metering rod assemblies  66 ,  68  are mounted on opposite sides of the frame member with the first assembly on the side facing the applicator roll  48  and the second assembly on the side facing away from the applicator roll  48 . It can be seen that when the frame member  64  is rotated 180 degrees, the position of the assemblies  66 ,  68  is reversed; that is, with the second assembly  68  on the side facing the applicator roll  48  and the first assembly  66  on the side facing away from the applicator roll  48 . Optionally, isobar assembly  50  can have additional metering rod assemblies, e.g. spaced on all four sides of the frame member  64  offset by 90-degree angles (not shown). In this embodiment, it will be understood that the frame member  64  would rotate about axis  70  in 90-degree intervals to sequentially place the respective metering rod assemblies in the operative position adjacent the applicator roll  48 . 
     In instances where it is necessary to use two (or more) different types or thicknesses of adhesives which require different isobar structures, the first and second assemblies  66 ,  68  (and third, fourth, etc. if provided) are each selected to be suitable with one of the two (or several) adhesives. When the adhesive is changed, requiring a different isobar structure, the isobar assembly  50  is rotated to place the appropriate metering rod assembly in the operative position as described above. 
     In instances where it is not necessary to change adhesives, the additional assemblies (e.g. assembly  68  in  FIG. 3 ) can be spare or backup assemblies. In the event that the first assembly  66  wears or becomes unsatisfactory for any reason, the isobar assembly  50  is rotated 180 degrees so that the second assembly  68  is pivoted into the operative position without delay. 
     The metering rod assemblies  66 ,  68  are substantially identical in structure, and each includes a channel member  72 , a holder  74 , a tubular pressure-tight bladder  76 , and an isobar or metering rod  78 . The channel member  72  is secured to the side of the frame member  64  and forms a longitudinally extending channel. The holder  74  has a projection on an inner side and a groove on an outer side. The projection is sized and shaped to extend into the channel so that the holder  74  is moveable toward and away from the frame member  64  within the channel member  72 . The groove is sized and shaped for receiving the metering rod  78  so that the metering rod  78  is mounted in and supported by the holder  74 . 
     The bladder  76  is positioned between the holder  74  and the channel member  72  within the channel of the member  72 . Fluid pressure, preferably air pressure, is applied to the bladder  76  of the active metering rod assembly which is the assembly in the operative position adjacent the applicator roll  48  (assembly  66  in  FIG. 3 ). The fluid pressure within the bladder  76  produces a force urging the holder  74  and the associated metering rod  78  toward the outer peripheral surface of the applicator roll  48 . It should be noted that the force produced by the bladder  76  is uniform along the entire length of the metering rod  78 . 
     It is important for the metering rod  78  to be supported such that the metering rod  78  is not deflected up or down with respect to the applicator roll  48  as a result of the hydraulic pressure; i.e. the metering rod  78  is urged toward the applicator roll  48  such that the metering rod axis  79  and the applicator roll axis  54  remain substantially coplanar in a horizontal plane during operation as shown in  FIG. 3 . The hydraulic pressure is a function of applicator roll speed and adhesive viscosity, among other things. The metering rod  78  and the holder  74  are sized such that they are flexible under the hydraulic forces encountered and therefore are not displaced from hydraulic pressure. Because the pressure supplied from the bladder  76  establishes a uniform force along the entire length of the metering rod, however, there is no change in spacing between the outer peripheral surface of the applicator roll  48  and the metering rod  78  along its entire length. Therefore, the metering rod  78  is positioned to produce a uniform thickness or coating of adhesive on the outer peripheral surface of the applicator roll  48  along its entire length. This is true even if the frame member  64  deflects to some degree under loading from hydraulic pressure. 
     In conjunction with the isobar assembly  50  as above described, it is possible to use a glue with very high solids content, preferably at least 25, more preferably 27, most preferably 30, weight percent solids, balance water, compared to other conventional glue machines that do not use an isobar assembly as described. This enables application of a very thin, uniform glue coating  41  on the surface of the applicator roll  48  that will not saturate the flutes of a corrugated sheet  18  as they come into contact with the glue layer as described in detail below. 
     As best shown in  FIGS. 6 and 6A , the isobar or metering rod  78  preferably includes a cylindrical rod  80  and spiral wound wire  82  thereon. The rod  80  extends the length of the applicator roll  48  and has a uniform diameter such as, for example about ⅝ of an inch. The wire  82  has a relatively small diameter such as, for example, of about 0.060 inches. The wire  82  is tightly spiral wound around the rod  80  in abutting contact along the length of the rod  80  to provide an outer surface, best illustrated in  FIG. 6A , which forms small concave symmetrical cavities between the contact points of adjacent loops of wire  82 . These small concave cavities  84  provide spaces with respect to the smooth outer surface of the applicator roll  48  so that small ridges of adhesive remain on the surface of the applicator roll  48  as the surface passes the metering rod  78 . 
     It should be noted that even though adhesive on the outer surface of the applicator roll  48  tends to be in the form of ridges after it passes the metering rod  78 , the adhesive tends to flow laterally and assume a uniform, flat and thin coating layer via cohesion. Of course, the viscosity of the adhesive in relation to the cohesion thereof determines the extent to which the adhesive coating becomes completely smooth. Preferably, the adhesive is a high-solids content adhesive as described above, having a viscosity of 15-55 Stein-Hall seconds. 
     The position of the isobar assembly  50  is adjustable toward and away from the applicator roll  48  to precisely set the gap therebetween (as indicated in  FIG. 3 ). When the isobar assembly  50  is adjusted so that metering rod  78  is in virtual contact with the outer surface of the applicator roll  48 , essentially all of the adhesive except that passing through the concave cavities between adjacent turnings of the wire  82 , is removed from the outer surface of the applicator roll  48 . On the other hand, when the metering rod  78  is spaced slightly away from the outer surface of the applicator roll  48  by reducing the pressure within the associated bladder  76 , a coating of adhesive having greater thickness remains on the outer surface of the applicator roll  48 . As indicated above, in a preferred embodiment the isobar assembly  50  is positioned with respect to the applicator roll  48  to provide a uniform adhesive coating on the outer surface having the preferred thickness for the desired flute size as explained above. It will be understood that the optimal position for the isobar assembly  50  will depend upon the viscosity, the solids content, and the surface tension of the adhesive being used, as well as the size of the flutes (e.g. A, B, C, E, etc.). 
     As best shown in  FIG. 3 , the metering rod  78  is mounted in and supported by the outer groove of holder  74  for rotation therein about its central axis  79 . In operation, the metering rod  78  is rotated at a relatively high speed in the same direction as the rotation of the applicator roll  48  so that the metering rod  78  remains clean by throwing off excess adhesive. By rotating in the same direction as the applicator roll  48 , excess adhesive is thrown in a downward direction back into the glue tray  46 . 
     As best shown in  FIG. 7 , the metering rod  78  can alternatively be a solid rod that has been machined to provide a grooved outer surface rather than having wire wound thereon. The machined outer surface preferably has inwardly extending cavities or grooves  86  which function similarly to the concave cavities  84  formed by the wire  82 . The illustrated grooves  86  are axially spaced along the length of the metering rod  78  to provide a narrow flat section between the grooves  86 . This variation of the metering rod  78  tends to remove a greater amount of adhesive and is typically used in applications where very thin coatings are required. Here again, the rod  78  is rotated to keep it from accumulating excess adhesive. 
     As best shown in  FIG. 3 , the rider roll  52  is journaled for rotation about a horizontal and transverse axis  87  in the direction opposite that of the applicator roll  48  and indicated by the arrow (counterclockwise) as viewed in  FIG. 3 . Preferably, the rider roll  52  is located at the forward or downstream side of the applicator roll  48  and with the axis  87  at the same height as the axis  54  of applicator roll  48 ; that is, the rider roll  52  is located at a 3-o&#39;clock position relative to applicator roll  48  (as best shown in  FIG. 3 ) opposite the isobar assembly  50 . As such, the metering rod  78 , the applicator roll  48 , and the rider roll  52  are positioned at substantially the same elevation with the axes  79 ,  54 , and  87  of the metering rod  78 , the applicator roll  48 , and the rider roll  52  respectively being substantially in the same horizontal plane (best shown in  FIG. 3 ). Additionally, a vertically extending gap or space  88  is formed between the applicator roll  48  and the rider roll  52  for passage of the corrugation assembly  14  therethrough. 
     As best shown in  FIG. 4 , the position of the rider roll  52  is adjustable directly toward and away from the applicator roll  48  so that the width of the gap  88  can be precisely adjusted to control the degree to which the flutes  20  of the corrugation assembly  14  are compressed against the applicator roll  48  as they pass through gap  88 . The degree of flute compression can be controlled to a high degree of accuracy because the rider roll  52  is linearly adjustable; that is, the rotational axis  87  of the rider roll  52  is movable directly toward and away from the rotational axis  54  of the applicator roll  48 . Additionally, flexure of the rolls  48 ,  52  due to gravity does not affect the gap  88  because the gap  88  is vertical. 
     The gap  88  is preferably precisely closed and opened by a closed loop system including a motor and a linear transducer that moves the rider roll  52  toward and away from the applicator roll  48 . Preferably, a pair of air cylinders can also open the gap between the rider roll  52  and the applicator roll  48  to a relatively large distance, such as about 4 inches, to meet various safety requirements. 
     Side to side accuracy of the precise gap, that is along the length of the rider roll  52 , is maintained with two adjustment jacks and a cross-connecting shaft. The shaft transversely extends the length of the rider roll  52  and the adjustment jacks are located at or near the ends of the shaft so that the rider roll outer surface can be adjusted to be precisely parallel to the applicator roll outer surface. The cross-connecting shaft of the illustrated embodiment is a central shaft  89  of an idler roll  90  (best shown in  FIG. 10 ) discussed in more detail below. It is noted, however, that the cross-connecting shaft could alternately be a central shaft in the rider roll  52 . 
     Referring to  FIG. 4A , a preferred method for applying adhesive to the crests of the flutes of a corrugated sheet  18  is shown. In this method, the position of the rider roll  52  is set to adjust the gap  88  between the rider roll  52  and the applicator roll  48  so that the flutes are compressed 3-30, preferably 5-15 or 5-10, percent of their initial flute height upon contact with the applicator roll  48 . In other words, the flutes are compressed down to 70-97, preferably 85-95 or 90-95, percent of their initial flute height. As shown in  FIG. 4A , a characteristic flute  150  has a leading sloped face  151 , a trailing sloped face  152 , and a crest  153 . (Flute  150  in  FIG. 4A  is simply a characteristic flute  20  as it passes through the gap  88 . The reference numeral  150  is used here instead of  20  merely for clarity to indicate a flute as it passes through the gap  88 ). In  FIG. 4A , the notation a/b/c refers to the relative position of the characteristic flute  150 ; i.e.  150   a  refers to a position of the flute  150  on contact with the applicator roll  48 ,  150   b  refers to a position at the nip point in contact with the applicator roll  48  for wiping adhesive onto the flute  150 , and  150   c  refers to a position following contact with the applicator roll  48 . This a/b/c notation is used consistently in the following description with reference to  FIG. 4A . 
     According to the present method, the web of the single-face corrugation assembly  14  carrying the flute  150  in  FIG. 4  is traveling at a linear web speed, S, over the rider roll  52  in the gap  88  between that roll and the applicator roll  48 . The applicator roll is rotated at an angular velocity (RPMs) such that based on the RPMs and the diameter of the applicator roll  52 , the surface linear velocity of the applicator roll during operation of the glue machine is greater than the web speed, S, to provide glue to the exposed flute crests of the flutes  150  as the web traverses the gap  88 . The surface linear velocity of the applicator roll  48  refers to the linear speed of the outer surface of the applicator roll  48 , measurable in feet per minute. The surface linear velocity is related to the angular velocity (i.e. rotations per minute or RPMs) by the relation v=2πrΩ; where v is the surface linear velocity in feet/min, r is the radius of the applicator roll  48  in feet, and Ω is the angular velocity of the applicator roll  48  in RPMs. The ratio of the surface linear velocity of the applicator roll  48  to the speed of the web carrying the flute  150  is referred to as the roll speed ratio. It will be appreciated that according to the invention, the roll speed ratio is greater than 100%; i.e. greater than 1. For all roll speed ratios herein, the surface linear velocity of the applicator roll  48  is greater than the web speed, S. 
     In one embodiment, the roll speed ratio is 105% to 200%. More preferably, the upper limit of that ratio is 150%, more preferably 140% or 130%. Alternatively, the upper limit can be 125%, 120%, 115% or 110%. By controlling the roll speed ratio, e.g. in the range of 105% to 130%, it has been found, surprisingly and unexpectedly, that substantially uniform glue application to the crests of flutes  150  can be achieved. Alternatively, roll speed ratios in the range of 130% to 150% also are preferred. 
     Without wishing to be bound by any particular theory, it is believed that uniform glue application is achieved for one or more of the following reasons, described with reference to  FIG. 4A  discussed above. As already noted, the position of the rider roll  52  can be set to adjust the gap  88  so the flutes are compressed a desired degree on contact with the applicator roll  48 . As seen in  FIG. 4A , the characteristic flute  150  is initially compressed on contact with the applicator roll  48  at position  150   a . Because the applicator roll surface linear velocity is greater than the speed of the web that is carrying the flute, the surface of the applicator roll  48  engages the crest of flute  150   a  and drives it (compresses it) in a forward direction relative to the direction of travel of the web through the gap  88 . At position  150   a , the applicator roll  48  also wipes glue coated onto against the flute crest. Because the surface linear velocity of the roll  48  is greater than the speed of the web (and therefore the flute  150   a ), glue is wiped onto the flute  150   a  such that it pools in a region defined between the applicator roll surface and the trailing sloped face  152   a  of the flute  150   a , near the crest  153   a . As the flute  150   a  progresses to an intermediate position,  150   b , the flute is even further compressed based on the width of the gap  88  and the above-noted pooling action continues to result in glue accumulation. Also at this point a quantity of glue from the pooled portion or from the surface of the applicator roll  48  (or both) is accumulated at the flute crest  153   b , more adjacent the leading sloped face  151   b  of the flute  150   b . Subsequently, as the flute  150   b  proceeds further and begins to emerge from contact and compression against the applicator roll  48  surface (shown in phantom in  FIG. 4A ), the greater velocity of the roll  48  surface tends to drag glue previously pooled adjacent the trailing sloped face up over the crest. 
     Essentially, by operating at roll speed ratios greater than 1, as the flute emerges from contact with the applicator roll  48  to proceed from  150   b  to  150   c , glue from the trailing sloped face  151   b  is actually dragged forward by cohesive forces in a direction toward the crest  153   b . Thus, at  150   c  there is substantially no glue remaining on the trailing sloped face  151   c  and all of the glue has been piled onto the crest  153   c . Furthermore, because little or no glue is deposited to the leading sloped face at  152   b  due to its being compressed downward away from the applicator roll, none is present at  152   c . The result is a flute at  150   c  following application of the adhesive that has glue only on the crest  153   c , and substantially none on either the leading or trailing sloped faces  151   c  or  152   c.    
     On emergence of the flute at  150   c , the glue is substantially uniformly applied at the flute crest  153   c . It will be appreciated that the glue&#39;s adhesive properties (adherence to the applicator roll surface) and cohesive properties (adherence to itself) probably play a roll in producing the above-noted dragging effect that results in a uniformly applied glue bead on the flute crest  153   c . Both these properties may be enhanced compared to more conventional glue compositions based on the relatively high solids content that can be used for glue compositions according to the present invention, using the isobar assembly  50  to very precisely meter a thin glue film on the applicator roll surface. 
     Regardless of the actual mechanism, the ability to provide such a well metered glue bead at the flute crests using an applicator roll surface linear velocity greater than the web speed was a highly unexpected and surprising result. This is because based on the conventional wisdom, it was believed that relatively lower applicator roll surface velocities were necessary to achieve adequate wiping of glue from the applicator roll surface to the flute crests. Otherwise, it was believed accurate metering of glue could not be achieved because as the ratio of the two speeds (surface linear speed and web speed) approached unity, the only, known parameter for regulating glue application thickness (relative speed) was neutralized. Also, when using an isobar assembly  50  to meter the glue layer thickness on the applicator roll, the cavities  84  between adjacent turns of the wire  82  on the metering rod result in circumferential glue lines or ridges on the applicator roll surface. As noted above, the glue will tend to flow laterally, but complete lateral leveling often is not achieved by the time the applicator roll  48  has rotated 50% of its circumference based on operational speeds. As a result, the glue film still can have non-uniform peak and valley (ridge-like) characteristics along the length of the applicator roll surface. Glue wiping onto the flute crests based on a relatively slow applicator roll surface linear speed to solve the above problems was deemed necessary also to prevent applying glue “points” along the lengths of flute crests, as opposed to a uniform glue bead. 
     Operating the applicator roller at a higher surface linear velocity than the web speed was not considered an option in the conventional art. It was believed that doing so would result in excessive pooling of adhesive against the trailing faces of the flutes, which it was thought would lead to undesirable washboarding effects as described above. That a ratio of applicator roll surface linear velocity to web speed greater than 1 actually produces uniformly thick and well metered glue beads that can be applied with precision to the crests of flutes  150  was an extremely surprising and unexpected result. 
     It is noted that when operating at certain roll speed ratios (such as at least 125% or at least 130%), each subsequent flute passes over at least a portion of the preceding flute&#39;s path against the roll  48 . The result is that the applicator roll  48  is wiped substantially clean of all of the glue thereon. This in turn results in a substantially linear relationship between roll speed and glue weight applied to the flute tips, with the glue weight being substantially uniform among the flute tips. This means that the amount of glue applied to the flute tips can be reliably and reproducibly controlled as a function of roll speed, particularly at relatively higher roll speed ratios, meaning ratios in the range of, say 125% to 150% or 160%. Much above these ratios, for example at roll speed ratios higher than about 160 to 180 percent, some glue from the applicator roll may tend to be dragged onto some of the flutes as they emerge from the gap  88  due to adhesive and cohesive forces, glue surface tension effects, and glue absorbency in the flute material. In that event, the applied glue weight may vary unpredictably and uncontrollably from flute to flute. For this reason it is contemplated that roll speed ratios much above these, for example greater than 200%, may be impractical. 
     In one example, it has been found that an adhesive coating  41  thickness on the outer surface of applicator roll  48  less than about 0.006 inches combined with a roll speed ratio of about 130% together result in the flutes being able to accept and absorb more glue, and the entire surface of the roll  48  being substantially wiped clean. Under these conditions, excellent glue weight control and reproducibility can be achieved. 
     Following contact with the applicator roll  48 , the flute  150   c  rebounds to substantially its initial dimensions (height). Preferably, the flute  150   c  rebounds to at least 80, preferably at least 85, preferably at least 90, preferably at least 95, preferably at least 96, preferably at least 98, percent of its initial height. Near complete rebound is possible in the present invention because of the very thin, high-solids content adhesive coating  41  on the outer surface of applicator roll  48 . Such a coating is achieved via the isobar assembly  50  as described above, and results in the flutes not becoming saturated with or absorbing a significant amount of water as they come into contact with the adhesive coating  41  on the surface of applicator roll  48 . 
     The combination of a glue machine  38  as described above having an isobar assembly  50 , and the described method of applying glue only to the crests of the flutes of a corrugated sheet  18 , provides precise control of glue weight over a wide range while ensuring proper placement only on the flute crests. The adhesive has an even thickness and is symmetric about the crest  153   c  of the flute  150   c  with sharply defined edges resulting in both a reduction in the amount of adhesive used and a maximum bonding strength. 
     The result is a finished corrugated cardboard product having superior surface appearance with substantially no washboarding, and superior and uniform impact and crush strength independent of direction. 
     The size of the rider roll  52  is preferably minimized to as small as practically possible. The minimized size of the rider roll  52  reduces the number of the flutes  20  of the corrugation assembly  14  that are in contact with the adhesive coating at one time, and thus reduces the dwell time in which the flutes  20  are in contact with the adhesive coating as discussed below in more detail. 
       FIG. 9  schematically illustrates a drive system for the applicator roll  48 . A variable speed motor  108  is connected to the applicator roll  48  and provides power to rotate the applicator roll  48  during the operation of the machine  10 . An electronic control  110  is connected to the motor  108  and adjustably controls the rotational speed of the applicator roll  48 . This ability to control the speed of the roll  48  is an important feature of the present invention because it allows adjustment of the applicator roll surface linear velocity relative to the velocity of the corrugation assembly  14  (and therefore corrugated sheet  18 ) as described above. This provides the very precise control of the transfer of adhesive from the applicator roll  48  to the flutes  20  of the corrugation assembly  14 . 
     Because the gap  88  between the applicator roll  48  and the rider roll  52  is vertical, gravity pulls straight down on the glue layer at the nip point of the gap  88  so that the amount of glue applied is directly proportional to the rotational speed of the applicator roll  48 . Therefore, changes in glue coating  41  thickness on the applicator roll  48  are no longer necessary for controlling the amount of glue applied to the corrugation assembly  14  or coating weight control. The coating weight can be automatically controlled by connecting a glue weight sensor  112  to the motor controller  110  so that the controller  110  automatically adjusts the speed of the applicator roll  48  until the weight detected by the sensor  112  is equal to a desired amount. Furthermore, by using a high-solids content glue and compressing the flutes 3-30 (preferably 5-15) percent of their initial height as above-described, it is now possible to adjust the applicator roll  48  speed across a far greater range than was previously possible while still providing glue only on the crests of the flutes  20 . 
     It is noted that as the speed of the applicator roll  48  is increased relative to the rider roll  52 , the amount of glue applied to the corrugation assembly  14  (flute crests) is increased. As described above, large roll speed ratios enable the flutes  20 ,  150  to receive a more controlled and smaller amount of adhesive and enables the flutes  20 ,  150  to remove virtually all of the adhesive from the applicator roll  48  to reduce over spray. 
     As best shown in  FIG. 10 , the idler roll  90  is arranged so that the corrugation assembly  14  is substantially tightly wrapped around the circumference of the rider roll  52 , particularly, in the area of the gap  88  between the applicator roll  48  and the rider roll  52 . Such an arrangement reduces the number of flutes  20  in contact with the adhesive layer and thus the dwell time during which the flutes  20  of the corrugation assembly  14  are in contact with the adhesive layer as discussed in more detail below. The corrugation assembly  14  preferably wraps around at least 30 percent of the periphery of the rider roll  52 , and more preferably wraps around about 50 percent, that is about 180 degrees, of the periphery of the rider roll  52 . In the illustrated embodiment, the idler roll  90  is positioned on the forward side of the rider roll  52  so that the corrugation assembly moves in a generally S-shaped pathway around the idler roll  90  and the rider roll  52 . 
     The idler roll  90  is preferably carried by an arm assembly that moves the rider roll  52  so that the idler roll  90  and rider roll  52  are rigidly connected together. As a result, the idler roll  90  moves with rider roll  52  as the rider roll  52  is moved to adjust the precisely controlled gap  88 . Therefore, there is no change in the length of the web path if the width of gap  88  is changed or the position of the glue machine  38  is moved. 
     As best shown in  FIGS. 11 and 12 , alternate embodiments of the rider system can be utilized within the scope of the present invention to even further reduce the number of flutes  20  in contact with the glue layer and thus the dwell time. As shown in  FIG. 11 , the rider system can be a relatively small diameter rod  112  supported by a rod holder  114 . The rod holder  114  can have a structure similar to the metering rod assemblies described above in detail. The rod  112  is preferably positioned between a pair of idler rolls  116  arranged to direct the corrugation assembly  14  to and from the rod  112 . The rod  112  is an extremely small sized rider roll which operates as described in detail hereinabove with regard to the rider roll  52  of the first embodiment. The rod  112 , however, provides an extremely small diameter compared to typical rider rolls. The rod  112  can have a diameter of less than 3 inches, for example 1.5 inches. 
     As shown in  FIG. 12 , the rider system can alternatively include three of the relatively small rods  112  supported by three of the rod holders  114 . The two additional rods  112  function as and replace the idler rolls  116  discussed above with regard to the embodiment of  FIG. 11 . 
     During operation of the glue machine  10 , the applicator roll  48  rotates and picks-up adhesive from the glue pan  46  onto the smooth peripheral outer surface of the applicator roll  48 . As the adhesive rotates past the isobar assembly  50 , the metering rod  78  removes excess adhesive from the outer surface of the applicator roll  48  and leaves a precisely controlled extremely thin layer of adhesive coating  41  on the outer surface of the applicator roll  48 . As the applicator roll  48  continues to rotate, the precisely controlled adhesive coating  41  travels from the isobar assembly  50  to a position adjacent the gap  88 ; that is, the location where the flutes  20  of the corrugation assembly engage the applicator roll  48  as previously described. 
     The rider roll  52  rotates in a direction opposite the applicator roll  48 . The first face sheet  22  smoothly engages the outer surface of the rider roll  52  and is held substantially against slippage relative thereto. 
     As the flutes  20  of the corrugation assembly  14  pass through the nip point of the precisely controlled vertical gap  88  between the applicator roll  48  and the rider roll  52 , the flutes come into contact with the thin coating  41  of adhesive and/or the applicator roll  48  as described above. 
     Because the corrugation assembly  14  is substantially wrapped around the rider roll  52  and/or the size of the rider system is minimized, the flutes  20  contact the adhesive coating  41  and/or the applicator roll  52  only at the nip point of the gap  88  so that they are wetted with adhesive and compressed at essentially the same time. Preferably, only 1 to 2 flutes  20  are in contact with the adhesive and/or the applicator roll  48  at any given time. No presoaking or post soaking of the flutes  20  occurs; that is, the flutes  20  do not touch the adhesive before reaching the nip point or after leaving the nip point. Therefore the dwell time, the time for which the flutes  20  are in contact with the adhesive and/or the applicator roll  48 , is minimized so that the flutes  20  remain as resilient as possible. 
     As the flutes  20  pass through the nip point of the vertical gap  88 , the thin coating  41  of adhesive on the applicator roll  48  is transferred to the crests of the flutes  20 . Any spray of adhesive generated at the nip point is downwardly directed without a horizontal velocity component. Therefore, no adhesive is sprayed outside the glue tray  46 , which is located directly below the nip point, even at high speeds. Additionally, gravity eliminates any pooling problems of the adhesive because gravity pulls the adhesive straight down at the nip point. 
     The combination of a) metering a very thin layer of adhesive on the applicator roll  48 , b) maintaining a precise and adjustable vertical gap  88  between the applicator roll  48  and the rider roll  52 , c) eliminating pre-nip and post-nip soaking of the flutes  20  in the thin layer of adhesive, and d) compressing the flutes 3-30 (most preferably 5-15) percent of their initial height at the nip point, allows the surface linear velocity of the applicator roll  48  to be greater than the web speed, S, for example roll speed ratio greater than 105% or at least 130%, with no discernable snow plow effects, and without applying glue to the leading or trailing sloped faces of the flutes  20 . Additionally, the amount of glue consumed is dramatically reduced because of minimized spray and stringing of the adhesive. Furthermore, the glue is precisely positioned on the tip of the flutes so that the final product has a maximum caliper and an extremely smooth outer surface finish. 
     With the present invention it is possible to efficiently apply virtually any type of hot or cold adhesive and obtain maximum strength in the finished product while applying substantially less adhesive per unit of area of the finished product. 
     Although particular embodiments of the invention have been described in detail, it will be understood that the invention is not limited correspondingly in scope, but includes all changes and modifications coming within the spirit and terms of the claims appended hereto.