Abstract:
A hybrid double backer for the formation of a double face corrugated paperboard web combines an upstream driven holddown belt section and a downstream static porous mesh belt holddown section that provides optimum curing and drying of the paperboard web.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    The present invention pertains to the formation of a double face corrugated paperboard web under the application of heat and pressure and, more particularly, to a hybrid double backer construction utilizing a combination of a driven holddown belt and a stationary porous holddown belt.  
           [0002]    Double backer apparatus for the production of double face corrugated paperboard has been well developed in the art. The use of starch-based adhesives to form the glue lines on the corrugated medium flute tips to which a liner web is applied requires the application of heat and pressure. Typically, a single face corrugated web, comprising a corrugated medium to which a single liner web has been glued, is formed upstream of the double backer in one of several types of single facer machines. The exposed flute tips on the single face web are then coated with glue lines and brought into contact with a second liner web just prior to entry into the double backer.  
           [0003]    In virtually all double backers in use today, the freshly glued double face web is moved over a hot plate section with the newly applied liner web lying against the hot plates. The entire heated surface of the double backer may be 40 feet (about 12 m) in length and be comprised of 20 individually heated hot plates mounted in abutting relation. All prior art double backers also include some means for applying a load to the upper surface of the double face web to enhance the heat exchanging contact between the hot plates and the web. As the web is moved through the heating section, between the hot plates and some type of overlying holddown means, the starch adhesive is first gelatinized and then moves into the so-called “green bond stage” where the glue lines cure and gain strength by dehydration. As the web moves further toward the downstream outfeed end, the heat from the hot plates drives moisture from the web and causes it to dry.  
           [0004]    For many years, double backers have been constructed with a driven holddown belt extending over the entire heating section and positioned in contact with the double face web, so that the holddown belt performs the dual function of holding the freshly glued web in intimate contact with the hot plates and moving the web through the heating section by direct frictional contact with the web. In addition, the horizontal run of the holddown belt in contact with the web is typically provided with a supplemental ballast load to enhance the engagement of the web by the belt. Many variations of ballast loading devices have been used, including ballast rollers, air pressure nozzles, inflatable bladders, and pressure plates. Means to vary the magnitude of the ballast load applied over the length of the heating section have been used with all of the various ballast load devices.  
           [0005]    However, holddown belts that extend the full length of the heating section of a double backer are cumbersome to operate and to maintain and in addition, as the speed of corrugator lines has increased, it has been found difficult to adjust the speed through the double backer to accommodate higher line speeds and yet assure that the double face web exiting the double backer is thoroughly dried. For example, the impervious holddown belt has been found to inhibit the passage of steam and moisture from the web as it dries, particularly in the downstream section of the double backer. As a result, double face web exiting the double backer that is not sufficiently dried, may not be suitably conditioned for the cutting and slitting operations that follow immediately in the dry end of the corrugator.  
           [0006]    More recently, attempts have been made to eliminate the driven holddown belt with static holddown devices that lie atop the single face web which web must then be pulled through the double backer by a downstream web drive means, such as a pair of drive belts between which the web travels or a vacuum conveyor device. Such static holddown devices have provided some significant benefits in controlling web drying. One such device comprises a flexible holddown mat, the length of which in contact with the moving double face web is controlled by a downstream lift device that allows more or less of the length of the holddown mat to be maintained in contact with the web. Other static holddown devices, such as plates, rollers, and inflatable bladders have also been used with varying degrees of success. One problem common to most of such static holddown devices that are placed in direct contact with the web is damage to the freshly glued web, particularly at the upstream infeed of the double backer. In particular, rupture of the moist and relatively weak double face web, particularly when running lighter weight papers, has become a serious problem. Web rupture may be caused either by friction or blowouts from poorly vented steam. In addition, so-called beltless double backers do not provide the initial heat retention at the upstream end of the double backer that facilitates rapid green bond formation in the fresh glue lines.  
           [0007]    Thus, the more recently developed beltless double backers, though having solved some of the problems associated with the prior use of driven holddown belts, have resulted in the creation of new problems that have not been satisfactorily addressed and resolved.  
         SUMMARY OF THE INVENTION  
         [0008]    In accordance with the present invention, a hybrid double backer utilizes the best features of a driven holddown belt and a porous static holddown device to provide optimum curing and drying of the double face web while significantly reducing tear outs and other web damage.  
           [0009]    A double backer in accordance with the present invention utilizes a conventional heating section that includes a plurality of hot plates defining a generally horizontal and substantially continuous heated surface to support and heat the freshly glued web. A driven holddown belt section overlies an upstream portion of the heated surface beginning at the web infeed end of the double backer. The holddown belt section presses the freshly glued web against the heated surface and assists in moving the web over the surface. A horizontally stationary and vertically flexible porous belt section overlies the web along the remaining portion of the heated surface downstream from the driven belt section. The porous belt section maintains the web in contact with the heated surface and permits moisture to escape from the web. The porous belt section is provided with a lift device on the downstream end to permit the length of the flexible belt section that is contact with the web to be selectively varied. Immediately downstream from the outfeed end of the heating section, a main driven traction section engages the web and pulls the web over the surface and through the heating section.  
           [0010]    In the presently preferred embodiment, the holddown belt section extends over less than one-half the length of the continuous heated surface. The driven holddown belt section may have a width in the cross machine direction approximately equal to the width of the hot plates. The driven holddown belt section further comprises a continuous generally impervious flexible belt that is entrained around horizontally spaced upstream and downstream pulleys to present a lower web-engaging belt face to the web moving over the heated surface. In addition, a belt pressure device overlies a portion of the holddown belt adjacent the upstream end. The belt pressure device may comprise an inflatable air bag.  
           [0011]    The porous belt section preferably extends over the remaining portion of the heated surface not covered by the driven holddown belt section, thereby extending over more than one-half the length of the heated surface. The porous belt section has a width comparable to the width of the driven holddown belt section and thus has a width in the cross machine direction approximately equal to the width of the hot plates. In the preferred embodiment, the porous belt section comprises a flexible mesh belt that includes a series of generally parallel holddown strips that extend over the web in the direction of web travel between upstream and downstream mat end supports. The mesh belt also preferably includes a series of generally parallel flexible tie strips that extend generally perpendicular to, overlie and interconnect the holddown strips. The mesh is constructed and arranged to apply a uniformly distributed load to the double face web as it moves under the mesh. The lift device comprises a downstream cross support that is movable in a generally vertical direction. The lift device also preferably includes an upstream cross support that is operable with the downstream cross support to lift the entire porous belt section vertically from the web.  
           [0012]    The invention also includes a method for bonding and drying a freshly glued single face corrugated paperboard web by utilizing the steps of (1) moving the freshly glued web over a heated surface between an upstream web infeed and a downstream web outfeed end, (2) forming a green bond in the adhesive by applying a combined holddown load and traction force to the web with a driven holddown belt section that overlies the web along an upstream portion of the heated surface, (3) drying the web by pressing it against a remaining portion of the heated surface downstream from the holddown belt section by use of a flexible porous belt section, and (4) pulling the web over the heated surface with a main driven traction section downstream from the outfeed end.  
           [0013]    The forming step preferably includes applying a supplemental downward load on the holddown belt section and the web that is moving thereunder. The drying step preferably includes adjusting the vertical distance of the downstream end of the porous belt section form the web to selectively vary the length of said porous belt section in contact with the web. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]    [0014]FIG. 1 is a side elevation view of a double backer of the present invention positioned between an upstream web delivery apparatus and a downstream web traction section.  
         [0015]    [0015]FIG. 2 is an enlarged elevation view of a portion of FIG. 10 showing the web infeed and the upstream portion of the double backer.  
         [0016]    [0016]FIG. 3 is a further enlarged elevation detail of FIG. 2 showing the web infeed and double backer interface.  
         [0017]    [0017]FIG. 4 is an enlarged elevation detail of a portion of FIG. 3.  
         [0018]    [0018]FIG. 5 is a side elevation detail of the downstream end of the double backer portion shown in FIG. 2.  
         [0019]    [0019]FIG. 6 is an enlarged end elevation view of the upstream portion of the double backer taken on line  6 - 6  of FIG. 2 with portions of the holddown belt removed to show details of the belt lift system.  
         [0020]    [0020]FIG. 7 is a top plan view of the belt lift system shown in FIG. 6, also with the holddown belt removed.  
         [0021]    [0021]FIG. 8 is a detail taken on line  8 - 8  of FIG. 7.  
         [0022]    [0022]FIG. 9 is a top plan view of the framework supporting the upstream section of the double backer with many parts eliminated to show particular details of one feature.  
         [0023]    [0023]FIG. 10 is a top plan view of the upstream belt section of the double facer with portions of mesh ballast mat and lift devices removed.  
         [0024]    [0024]FIG. 11 is an enlarged elevation view taken on line  10 - 10  of FIG. 1 with the mesh belt section removed.  
         [0025]    [0025]FIG. 12 is a side elevation detail of the belt lift device on the upstream end of the mesh belt section of the double backer. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0026]    Referring initially to FIGS. 1 and 2, a double backer  10  receives one or more single face webs  11  to the flute tips of which a starch-based adhesive has been applied by a glue machine  13  and joins the glued single face web to a liner web  12  delivered from beneath the single face web  11 . The liner face of the single face web  11  passes over a web preheater prior to entry into the double backer. Similarly, the liner web  12  also passes over a liner preheater station  15  before being joined with the single face web  11  (or multiple webs if running multi-wall board) at a double backer infeed  16  at the upstream end of the double backer  10 . To facilitate joinder of the component webs, the infeed  16  includes a lower liner infeed roll  17  and an upper single face infeed roll  18  that form a low pressure nip to initially form the freshly glued double face web  20 , or, if multiple single face webs are joined, a multi-wall board  19  as shown schematically in FIG. 3.  
         [0027]    The double backer  10  includes a lower heating section  21  over which the double face web  20  travels in contact with a generally horizontal heated surface  22 . The surface is defined by a series of hot plates  23  mounted in abutting edge-to-edge relation along the full length of the heating section  21 . The hot plates  23  are typically constructed of cast iron or steel and are internally heated by steam to a temperature high enough to heat the double face web  20  to a temperature of at least 100° C. Typically, each hot plate  23  extends the full width of the double backer (in the cross machine direction) and thus has a width of about 9 feet (about 2.75 m). In the machine direction (direction of movement of the double face web  20 ), each hot plate  23  is substantially narrower and may have a length of about 2 feet (about 600 mm). A typical double backer may have a substantially continuous heated surface  22  of about 40 feet (about 12 m) in length, comprising twenty hot plates  23 .  
         [0028]    To facilitate curing of the starch adhesive and drying of the double face web  20 , the web is pressed from the top against the hot plates  23  by the combination of an upstream driven holddown belt section  24  and an immediately adjoining downstream porous belt section  25  that is horizontally stationary but vertically flexible.  
         [0029]    The upstream driven holddown belt section  24  includes a continuous impervious belt  26  which may typically be made of a fiber or fabric material, such as woven polyester. The belt is entrained around a driven head pulley  27  and an idler tail pulley  28  which preferably acts with the belt  26  as the single face infeed roll  18 . A motor and belt reduction arrangement  30  are mounted at the top of the holddown belt section  24  to drive the head pulley  27  and belt  26 . The belt  26  is positioned to operate in direct contact with the double face web  20  and thus acts to press the freshly glued web against the underlying heated surface  22  and assists in moving the web through the double backer. The primary web drive is located downstream from the heating section  21  and will be described in greater detail below. However, it has been found that the drive assistance provided by the driven belt section  24  is particularly important in preventing tear out in the freshly glued double face web which, at this point, is still quite moist and has significantly less strength than after it has cured and dried in the downstream portion of the double backer. The driven holddown belt section  24  is also very useful in initial web thread-up and, in this manner, operates like a prior art double backer.  
         [0030]    Means are provided to impose a static ballast load on the belt. This supplemental holddown may be provided in a number of ways, including a vertically flexible holddown mat providing a uniform load of about 25 lbs. per square foot (about 1200 Pa). The supplemental holddown load could also be provided by a series of rollers, plates or semi-rigid bars or strips extending in the cross machine direction between the side frame members  32 . In the embodiment shown, the supplemental ballast load is provided by an open mesh mat  29  that lies atop the belt  26  between the head and tail pulleys  27  and  28 . The mat  29  provides the needed uniform load on the belt  26  and the double face web  20  on which the belt rests. The open mesh ballast mat  29  is preferably of the type shown in U.S. Pat. No. 5,853,527, the disclosure of which is incorporated herein by reference, and includes closely spaced machine direction stainless steel strips  44  joined by weighted cross-tie strips  45 , as best seen in FIGS. 4 and 9. The ballast mat is disposed with the machine direction stainless steel strips  44  lying directly atop the board-contacting run of the belt  26 . The upstream ends of the steel strips  44  are attached to a common cross machine direction mounting plate  46 . The mounting plate  46  is curved upwardly to keep the bolted connections for the strips out of contact with the belt  26  as it travels downwardly around the tail pulley  28  and under the ballast mat  29 .  
         [0031]    The downstream end of the ballast mat  29  remains unattached and simply lies on the inside surface of the driven belt  26 . However, just upstream from the downstream end of the ballast mat  29  and at two additional equally spaced points upstream therefrom, the ballast mat is connected to cross machine direction angle members  47  with bolted connectors  48  that permit limited vertical movement of the ballast mat, but help raise the mat when the entire holddown belt section  24  is lifted from the hot plates  23  as will be described hereinafter.  
         [0032]    It has been found to be important to bring the freshly glued double face web  20  between the belt  26  and hot plates  23  as quickly as possible. The water in the starch-based adhesive reaches the boiling point very rapidly and steam is evolved almost immediately in the double backer. In addition and as indicated above, the freshly glued double face (or multiple wall) web  20  is still quite moist and has significantly less strength than after it is fully cured. If the double face web is not captured and held between the belt and the hot plates quite quickly, steam pressure within the flutes of the web may tend to blow out and rupture the web, particularly in relatively weak areas. On the other hand, if the double face web is sandwiched securely and uniformly between the belt and the hot plates, the steam will migrate laterally along the flutes in the cross machine direction and exit from the edges of the board.  
         [0033]    In order to help assure rapid and uniform holddown of the double face web  20  as it enters the double backer, air bags  31  are positioned at the upstream and downstream ends of the lower run of the holddown belt  26  and connected to impose a further downward load on the ballast mat  29 , the belt  26 , and the double face web  20  traveling thereunder. The air bags are positioned as close as practicable to the upstream tail pulley  28  to remove some of the natural catenary curve in the belt and cause it to be pressed into contact with the double face web more closely to the upstream infeed. Correspondingly, the downstream air bag  31  maintains the belt in contact with the double face web for a slightly longer period of time, again by eliminating some of the natural catenary as the belt lifts off the hot plate section to proceed around the head pulley  27 . On the upstream end, two closely spaced air bags are utilized which extend in parallel across the entire width of the belt. Each air bag is mounted beneath a cross machine direction box beam  50  having a series of support plates  54  attached to the underside which provide an upper vertical restraint. The lower faces of the air bags lie directly atop the ballast mat  29  such that, when inflated, the bags are pressed downwardly against the mat, underlying belt and double face web. On the downstream end of the belt section  24 , a single air bag  31  is mounted beneath a box beam  50 , but otherwise operates in the same manner as the pair of upstream air bags.  
         [0034]    As indicated above, it is most helpful, particularly in the upstreammost portion of the double backer to permit evolving steam to escape laterally through the open flutes of the corrugated double face web  20 . However, if a narrower web is being run than the typical maximum of 96 inches (about 2400 mm), the lateral outer edges of the driven belt  26  will tend to be forced by the air bags  31  against the surfaces of the hot plates  23  where there is no double face web present. This not only causes undue wear to the belt, but it tends to close off the flute ends and inhibit the escape of steam. This, of course, may increase the possibility of blowout and rupture of one of the liner webs, particularly the upper single face liner. To alleviate this potential problem, a bag end lift device  51  is provided on each end of the pair of upstream air bags  31  and on each end of the downstream single air bag. Referring particularly to the upstream lift device  51  shown in FIGS. 3, 4 and  9 , four air cylinders  52  are attached to the downstreammost of the two box beams  50  with their rod ends attached to flexible straps  53  encircling the lower face of the air bag and secured to support plates  54  on the opposite side of the upstream most box beam  50 . The air cylinders  52  are operated in pairs of two such that, as a narrower web is being processed, the pairs of outermost cylinders  52  are actuated to retract and cause the flexible strips  53  to squeeze and slightly flatten the ends of the air bags  31 . As webs that are narrower yet are run, the inside pairs of cylinders  52  on both ends of the air bags are also actuated to relieve direct air bag pressure from the belt  26 , particularly when there is no double face web running immediately thereunder. On the downstream end of the driven belt section  24 , the bag end lift device  51  is substantially the same, except that only a single air bag  31  is used and the flexible straps  55  are correspondingly shorter.  
         [0035]    Referring to FIGS.  6 - 8 , it is also desirable to lift the entire holddown belt section  24  vertically to facilitate cleaning and for clearing jams. The entire belt section is supported by a pair of lateral side frame members  32  which support a belt section lift device  39 . The lift device  39  comprises four worm screw actuators  56 , a pair of which are mounted to the outside face of each side frame member  32 . The actuator screws  57  are positioned to bear against a horizontal side flange  58  of the lower heating section  21 . The actuators  56  are connected to run in synchronization and, as the screws  57  are driven downwardly, the ends bear on the side flanges  58 , causing the entire belt section  24  to lift vertically from the hot plates  23 . To maintain the synchronized operation of the screw actuators  56 , two of the actuators on one side are connected to right angle gear boxes  60  which are, in turn, tied together with a timing shaft  61  and from each of which a driveshaft  62  extends to the opposite side and into driving engagement with an actuator  56 . The entire synchronized arrangement is driven by a single electric motor  63  operatively connected to one of the screw actuators  56 .  
         [0036]    The driven holddown belt section  24  preferably has a length slightly less than half the length of the heating section  21  and, thus, may extend over approximately the first eight hot plates or about 16 feet (about 5 m).  
         [0037]    Overlying the remainder of the heating section  21 , downstream from the driven belt section  24 , the porous mesh belt section  25  extends between upstream and downstream cross supports  35  and  36 , respectively. The cross supports, in turn, are mounted between upstream and downstream pairs of vertical supports  64  and  65 , respectively. The porous belt section  25  preferably comprises an open mesh belt  37  similar to or the same as ballast mat  29 , such as described in U.S. Pat. No. 5,853,527. Thus, closely spaced machine direction flexible stainless steel strips  44  are joined by weighted cross-tie strips  45 . The downstream cross support  36  is provided with a lift mechanism  38  permitting the downstream end of the mesh belt  37  to be lifted vertically such that more or less of the total length of the belt is permitted to rest on the double face web  20  traveling over the heated surface of the hot plates  23 . As is best seen in FIGS. 11 and 12, the downstream lift mechanism  38  comprises a pair of lead screws  66  attached to the downstream vertical supports  65  and to which are operatively attached respective screw followers  67  mounted on each end of the downstream cross support  36 . The upper ends of the lead screws  66  are driven from right angle gear boxes  68  which are interconnected with a timing shaft  70 . One of the gear boxes  68  is driven by a motor  71 . The porous mat  37  is constructed to provide a uniform holddown load on the double face web. The uniform load may be similar to that provided by ballast mat  29  in the driven belt section  24 , but the porous mesh belt  37  may be constructed to provide a higher or lower holddown load, as desired. A uniform load of 13 lbs. per square foot (about 620 Pa) has been found to work well. The upstream cross support  35  is also provided with a lift mechanism  40  so that, for cleaning, thread-up and the like, the porous mesh belt  37  may be lifted completely from contact with the hot plates or a double face web traveling through the heating section  21 . The upstream lift mechanism  40  operates in the same manner as the downstream lift mechanism  30  described above. Thus, it includes a pair of lead screws  66  attached to the upstream vertical supports  64  and driven to cause the screw followers  67  attached to the upstream cross support  35  to move vertically and carry the upstream end of the mesh belt  37  with it. The porous mesh belt section  25  is positioned immediately adjacent the upstream driven belt section  24  and, thus, preferably covers approximately the last  12  hot plates  23 . In the example described above, the porous belt section  25  would have a length of about 24 feet (about 7 m).  
         [0038]    As mentioned above, the upstream driven belt section  24  provides a region of concentrated heating applied immediately to the freshly glued double face web  20  to cause rapid gelatinization and substantial completion of the green bond which occurs by dehydration of the starch-based adhesive. The open construction provided by the mesh belt  37  in the downstream porous belt section  25  permits completion of the green bond cure while allowing moisture to dissipate thereby promoting rapid drying of the entire web  20 .  
         [0039]    Referring to FIG. 12, the upstream cross support  35  for the porous mesh belt  37  comprises a generally cylindrical drum  72  to which the ends of the machine direction stainless steel strips  44  of the belt  37  are attached. The lower portion of the drum  72  is cut out to provide a full cross machine direction slot  73  within which is mounted an air bag  74  similar to the air bags  36  used on the upstream driven belt section  24 . In normal operation, the upstream lift mechanism is operated to lower the upstream cross support  35  to bring the upstream end of porous mesh belt  37  down into direct contact with the double face web  20  that is running over the hot plates  23 . Inflation of the air bag  74  helps eliminate some portion of the natural catenary in the mesh belt  37  leading from its connection to the drum  72 . It is also desirable to provide the air bag  74  with a bag end lift device to eliminate a portion of the downward load applied by the inflated bag near the lateral edges thereof when running narrower webs. The lift device may be constructed and operated in the same manner as the lift devices  51  utilized on the upstream driven holddown belt section  24 .  
         [0040]    Immediately downstream from the heating section  21  is a main driven traction section  41  providing the main drive for pulling the double face web  20  through the heating section. In the embodiment shown in FIG. 1 of the drawings, the traction section  41  comprises a vacuum conveyor  42 . However, other types of belt drives well known in the industry, such as a pair of driven sandwich belts, could be used. One type of suitable vacuum conveyor web drive is shown in U.S. Pat. No. 5,706,994, the disclosure of which is incorporated herein by reference.  
         [0041]    In operation, beginning with machine startup, freshly glued double face web  20  can be threaded into the double backer by the machine operator in the same manner as a conventional prior art double backer. By the time the single face web leaves the driven belt section  24 , the starch adhesive will have reached a significant green bond strength level such that the web is rigid enough to allow it to be pushed over the remaining portion of the heating section  21  until the lead end is engaged by the main vacuum traction conveyor  42 . The porous mesh belt section  25  may then be lowered into contact with the web for full operation. As with conventional double backers, hot plate temperatures may be individually controlled or controlled in a number of zones along the heating section  21 . The driven holddown belt section  24  aids in driving the web, but is primarily used to reduce friction and enhance initial cure. The system has been found to work well in handling lighter weight double face webs and at line speeds well in excess of 1000 fpm (300 m per minute). For normal operation, the upstream driven holddown belt  26  is operated in a torque limited mode with respect to the main traction section  41 . The belt  26  is driven at or slightly greater than the speed of the main traction section  41  such that the belt  26  will assist in moving the web, but will not be driven with enough torque to move the web independently of main vacuum traction conveyor  42 .