Abstract:
A device for manufacturing a corrugated cardboard sheet includes a conveyor which conveys a corrugated medium and a liner, a starch applicator which is arranged along a conveying path of the corrugated medium and the liner and applies liquid starch to flute tops of the corrugated medium, a bonding unit which bonds the liner and the corrugated medium by placing the liner on the corrugated medium to which the liquid starch is applied and by heating and pressing the corrugated medium and the liner to gelatinize the liquid starch; and a spraying unit which is arranged on an upstream side of the bonding unit and sprays moist air on the flute tops of the corrugated medium to which the liquid starch is applied.

Description:
[0001]    The present Application is the U.S. National Stage of International Application No. PCT/JP2010/055689, filed on Mar. 30, 2010, published in English; which Application claims priority benefit of Japanese Application No. 2009-095327, filed Apr. 9, 2009. 
     
    
     TECHNICAL FIELD 
       [0002]    The present invention relates to a method and device for preheating a flute top of a corrugated medium prior to a step of gluing the corrugated medium and a liner in a process of manufacturing a corrugated cardboard sheet by means of a single facer or a double facer. 
       BACKGROUND ART 
       [0003]    A corrugated cardboard sheet is manufactured by transferring a containerboard such as a corrugated medium, a backside liner and a surface liner between a variety of rollers and belts. First, the corrugated medium is formed by traveling through a pair of corrugated rolls with corrugated surfaces in a single facer. A liquid starch made by mixing dry starch with water, is applied to a flute top of the corrugated medium. The corrugated medium and the backside liner are bonded together and then heated and pressed to form a single faced corrugated cardboard sheet. 
         [0004]    Next, in a double face located on a downstream side of the single facer in a conveying direction of the containerboard, the liquid starch is applied to a flute top of the corrugated medium of the single faced corrugated cardboard sheet. The single faced corrugated cardboard sheet and the surface liner are bonded together and then heated and pressed from both sides thereof to form a double faced corrugated cardboard sheet. To manufacture a double wall corrugated cardboard sheet, at least two single faced corrugated cardboard sheets and a surface liner are glued and bonded together at a time. 
         [0005]    To improve the bonding of the liners and the flute top of the corrugated medium, it is necessary to balance water contained in the liquid starch and a thermal dose applied to the liquid starch. Specifically, it is necessary to gelatinize the liquid starch by heating the liquid starch to the gelatinization temperature after allowing the liquid starch to sufficiently penetrate in the bonding area of the flute top of the corrugated medium and the liner. The starch is gelatinized to generate adherence property. After the starch becomes adhesive, the gelatinized liquid starch is dried. 
         [0006]    If the heating temperature is too high, the liquid starch becomes gelatinized before sufficiently penetrating through the corrugated medium and the liner in the bonding area. This results in adhesion bonding failure, which causes the medium and the liner to peel off from each other due to insufficient gelatinization of the liquid starch. 
         [0007]    To improve the production efficiency of the corrugated cardboard sheet, the transferring speed of each containerboard is increased. The transferring speed is increased up to 350 to 400 m/min for a single faced corrugated cardboard sheet and 300 to 350 m/min for a double wall corrugated cardboard sheet. 
         [0008]    Therefore, it is now difficult to control the heating and pressuring of each containerboard during the production process. 
         [0009]    In the double facer, prior to bonding the single faced corrugated cardboard sheet and the surface liner, the single faced corrugated cardboard sheet and the surface liner are winded around a preheat roll for preheating. Next, the liquid starch is applied to the single faced corrugated cardboard sheet and the surface liner. The single faced corrugated cardboard sheet and the surface liner are put on top of each other and then heated by steam and pressed between a flat heat plate heated by steam and a pressure belt while being transferred. The heating and pressing of the heat plate gelatinizes the liquid starch and dries the bonding area. Saturated steam of normally 1.0 to 1.1 Mpa is supplied to the inside of the heat plate. The top surface of the heat plate is heated to 170 to 180° C. 
         [0010]    Patent Document 1 (JP2007-30171A) discloses a method of heating in the double facer. According to the heating method, superheated steam is applied to the flute top of the corrugated medium after applying liquid starch and before heating and pressing the corrugated medium with the liner. The sprayed superheated steam gelatinizes the liquid starch and thus, the heating step by means of the heat plate is no longer necessary. 
         [0011]    In the single face, prior to bonding the corrugated medium and the backside liner, the corrugated medium and the backside liner are winded around the preheat roll to be preheated. Then the steam is supplied into the corrugating roll so as to heat the corrugated medium while being bonded with the backside liner. The steam supplied to the corrugating roller is saturated steam that has the same pressure and temperature as the heat plate. 
         [0012]    Patent Document 2 (JP2000-25131A) discloses a method of heating the single faced corrugated cardboard sheet in the single facer. According to the heating method, auxiliary heating is performed to meet the increased transferring speed of the containerboard. The auxiliary heat is performed in such a manner that the steam is sprayed to the flute top of the corrugated medium after applying the liquid starch so as to promote the gelatinization of the liquid starch. 
       Citation List 
     [Patent Document] 
     [PATENT DOCUMENT 1] JP2007-30171A 
     [PATENT DOCUMENT 2] JP2000-25131A 
     SUMMARY OF INVENTION 
     Technical Problem 
       [0013]    The gelatinization temperature of the liquid starch is around 60° C., normally in the range of 57 to 62° C. The liquid starch needs to be heated to the temperature range to be gelatinized. 
         [0014]    Meanwhile, the production of the corrugated cardboard sheet is mainly small lot production. The small lot production requires frequent changing of production conditions such as paper quality of a containerboard and a transferring speed (production speed). Accordingly, in the single facer and the double facer, it is necessary to change the following conditions every few minutes such as the amount of each containerboard to be wrapped around the preheat roll and a heating temperature of the corrugating roll and the heat plate. 
         [0015]    To avoid wasting paper during the process of changing the production conditions, it is necessary to adjust the heating temperature setting to a temperature of updated production conditions. However, in the heating methods disclosed in Patent Document 1 and Patent Document 2, it is difficult to promptly change the production conditions. 
         [0016]    The steam used in the above methods is 100° C. under atmospheric pressure and thus, the liquid starch is likely to be heated beyond the gelatinization temperature. Heating the liquid starch too high causes the liquid starch to be dry before sufficiently penetrating in the containerboards. This results in the adhesion bonding failure mentioned above. 
         [0017]    If the steam is sprayed under reduced pressure, the temperature is below 100° C. However, this requires a pressure reducing device and thus, the cardboard production device becomes bigger and it is practically impossible to implement such device. 
         [0018]    When the steam is used in the device, condensation tends to occur and mechanical parts tend to gather rust. The dew drop falls on the corrugated cardboard sheet, thereby causing the sheet to wrinkle and producing an inferior product. 
         [0019]    In view of the problems above, an object of the present invention is to achieve a heating device which heats the liquid starch supplimentarily without using steam and also controls a temperature of the liquid starch to the gelatinization temperature without forming water drops during the production of the corrugated cardboard sheet. 
       Solution to Problem 
       [0020]    To achieve the object of the present invention, a method for manufacturing a corrugated cardboard sheet, may include, but not limited to, the steps of: applying liquid starch to flute tops of a corrugated medium; bonding the corrugated medium and a liner by placing the liner on the corrugated medium to which the liquid starch is applied and by heating and pressing the corrugated medium and the liner to gelatinize the liquid starch; and prior to the step of bonding, preheating the flute tops of the corrugated medium to which the liquid starch is applied by spraying moist air to the flute tops. 
         [0021]    In the above method, prior to the step of bonding, the flute tops of the corrugated medium to which the liquid starch is applied is preheated by spraying the moist air thereto. In the case where the flute tops of the corrugated medium where the liquid starch is applied has a temperature not greater than the dew-point temperature, the moist air condenses in contact with the flute tops of the corrugated medium while releasing the latent heat of condensation onto the liquid starch, thereby heating the liquid starch. In contrast, in the case where the flute tops of the corrugated medium to which the liquid starch is applied has a temperature not less than the dew-point temperature, the vapor in the moist air does not condense and thus, the latent heat of condensation is not released onto the liquid starch. 
         [0022]    The latent heat of condensation has greater amount of heat than sensible heat of the moist air. The latent heat of condensation is applied to the liquid starch, thereby enhancing heating effect. Meanwhile, the liquid starch is not heated beyond the dew-point temperature of the moist air. Thus, in comparison to the case of using steam, the overheating of the liquid starch is prevented and the quality deterioration of the product such as the adhesion bonding failure is solved. 
         [0023]    The liquid starch is not excessively heated, which allows the heating time to be set with plenty of leeway. Thus, even when the conveying speed of the paper web is reduced, the heating can be performed accurately. 
         [0024]    In the above method, it is preferable that the moist air has a dew point temperature that is within ±5° C. of a gelatinization temperature of the liquid starch. In this case, the moist air has the dew point near the gelatinization temperature. Thus, if the temperature of the flute tops of the corrugated medium to which the liquid starch is applied is not greater than the dew-point temperature, the moist air condenses in contact with the flute tops of the corrugated medium while releasing the latent heat of condensation onto the liquid starch g, thereby heating the liquid starch g. 
         [0025]    In the case where the flute tops of the corrugated medium to which the liquid starch is applied has a temperature not less than the dew-point temperature, the vapor in the moist air does not condense and thus, the latent heat of condensation is not applied to the liquid starch g. 
         [0026]    As a result, the liquid starch is heated accurately to the gelatinization temperature by the moist air and not beyond approximately the gelatinization temperature. Thus, the overheating of the liquid starch is prevented and the quality deterioration of the product such as the adhesion bonding failure is solved. 
         [0027]    In the above method, it is preferable that the moist air has a temperature that is 10° C. to 100° C. higher than a gelatinization temperature of the liquid starch. Even when the temperature of the moist air decreases to some extent, no water drop is formed. Thus, there is no concern for inferior products which are generated due to formation of rust in the mechanical parts or the dew drops falling on the double wall corrugated cardboard sheet. 
         [0028]    In the present invention, it is also preferable that the corrugated cardboard sheet includes one of a single faced corrugated cardboard sheet made at a single facer, a double faced corrugated cardboard sheet made at a double facer and a double wall corrugated cardboard sheet made at the double facer. In the production process of the double wall corrugated cardboard sheet, it is hard to conduct the heat from the heat plates to an upper corrugated cardboard sheet of the double wall corrugated cardboard sheet. In the present invention, the upper corrugated cardboard sheet is directed heated by the moist air prior to the bonding. Thus, the upper corrugated cardboard sheet can be sufficiently heated. 
         [0029]    The present invention also provides a device for manufacturing a corrugated cardboard sheet. The device may include, but is not limited to: a conveyor which conveys a corrugated medium and a liner; a starch applicator which is arranged along a conveying path of the corrugated medium and the liner and applies liquid starch to flute tops of the corrugated medium; a bonding unit which bonds the liner and the corrugated medium by placing the liner on the corrugated medium to which the liquid starch is applied and by heating and pressing the corrugated medium and the liner to gelatinize the liquid starch; and a spraying unit which is arranged on an upstream side of the bonding unit and sprays moist air on the flute tops of the corrugated medium to which the liquid starch is applied. 
         [0030]    In the above device of the present invention, the spraying unit sprays the moist air on the flute tops of the corrugated medium to which the liquid starch is applied. Similar to the aforementioned method of the present invention, it is possible to enhance the heating effect and prevent the overheating of the liquid starch in comparison of using the steam. As a result, the quality deterioration of the product such as the adhesion bonding failure is solved. 
         [0031]    In the above device, it is preferable that the moist air has a dew point temperature that is within ±5° C. of a gelatinization temperature of the liquid starch. 
         [0032]    By using the moist air, the liquid starch is accurately heated to the gelatinization temperature and the overheating of the liquid starch is prevented by not heating the liquid starch beyond the gelatinization temperature. As a result, the quality deterioration of the product such as the adhesion bonding failure is solved. 
         [0033]    In the above device, the spray unit may include, but not limited to: a blower which supplies air to a steam heater; the steam heater which heats the supplied air by a steam; a mixing chamber which mixes the heated air and a steam to produce the moist air; a discharge part which is arranged to face a conveying path of the corrugated medium and through which the moist air is sprayed to the flute tops of the corrugated medium; a sensor unit comprising a pressure meter, a temperature meter and one of a dew-point meter and a steam partial pressure meter which are arranged in a passage for the moist air between the mixing chamber and the discharge part; and a controller which controls a flow amount of the air and the steam to the steam heater and the mixing chamber by inputting values detected by the sensor unit. 
         [0034]    With the above structure, the air can be supplied to the steam heater by the air delivery fan instead of compressing the air and pumping it to the steam heater. Thus, it is not necessary to make component devices and pipes pressure-proof and the structure can be simplified. 
         [0035]    Further, the moist air having a desired dew-point temperature can be accurately produced with use of the controller. 
       Effect of the Invention 
       [0036]    According to the method of the present invention for manufacturing the corrugated cardboard sheet, the method may include, but not limited to, the steps of: applying liquid starch to flute tops of a corrugated medium; bonding the corrugated medium and a liner by placing the liner on the corrugated medium to which the liquid starch is applied and by heating and pressing the corrugated medium and the liner to gelatinize the liquid starch; and prior to the step of bonding, preheating the flute tops of the corrugated medium to which the liquid starch is applied by spraying moist air to the flute tops. The liquid starch is heated with use of the latent heat of condensation of the vapor contained in the moist air and thus, it is possible to improve the heating effect and to prevent the overheating of the liquid starch in comparison to using steam by not heating the liquid starch beyond the dew-point of the moist air. 
         [0037]    Therefore, the quality deterioration of the product such as the adhesion bonding failure is solved. The heating time can be set with plenty of leeway. Thus, even when the conveying speed of the paper web is reduced, the heating accuracy can be improved. 
         [0038]    According to the device of the present invention for manufacturing the corrugated cardboard sheet, the device may include, but is not limited to: a conveyor which conveys a corrugated medium and a liner; a starch applicator which is arranged along a conveying path of the corrugated medium and the liner and applies liquid starch to flute tops of the corrugated medium; a bonding unit which bonds the liner and the corrugated medium by placing the liner on the corrugated medium to which the liquid starch is applied and by heating and pressing the corrugated medium and the liner to gelatinize the liquid starch; and a spraying unit which is arranged on an upstream side of the bonding unit and sprays moist air on the flute tops of the corrugated medium to which the liquid starch is applied. As a result, it is possible to achieve the same operational effects as the method of the present invention. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0039]      FIG. 1  is a partial front view of a double facer in relation to a first preferred embodiment of the present invention. 
           [0040]      FIG. 1  is a block diagram of a moist air spraying unit of the first preferred embodiment of the present invention. 
           [0041]      FIG. 1  is a front view of a single facer in relation to a second preferred embodiment of the present invention. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0042]    A preferred embodiment of the present invention will now be described in detail with reference to the accompanying drawings. It is intended, however, that unless particularly specified, dimensions, materials, shape, its relative positions and the like shall be interpreted as illustrative only and not limitative of the scope of the present invention. 
       First Preferred Embodiment 
       [0043]    A first preferred embodiment of the present invention, which is applied to a production process of a double wall corrugated cardboard sheet made at a double facer, is explained in reference to  FIG. 1  and  FIG. 2 . 
         [0044]    In  FIG. 1 , two single faced corrugated cardboard sheets K 1  and K 2  and a surface liner L 3  are transferred in the direction of the arrow a. The single faced corrugated cardboard sheet K 1  or K 2  is manufactured at a single-facer (unshown) arranged on an upstream side in the conveying direction by gluing a backside liner L 1  or L 2  to flute tops of the corrugated medium N 1  or N 2 . 
         [0045]    The single faced corrugated cardboard sheets K 1  and K 2  and the surface liner L 3  are respectively winded around preheat rolls  12   a  to  12   c  by guide rolls  10   a  to  10   c  and  14   a  to  14   c  to preheat the sheets. The preheat rolls  12   a  to  12   c  have a steam generator  56  inside as a heat source. The steam generator supplies steam to the preheat rolls  12   a  to  12   c.  The details of the steam generator  56  are described later. 
         [0046]    The single faced corrugated cardboard sheets K 1  and K 2  and the surface liner L 3  having been heated by the preheat rolls  12   a  to  12   c  are transferred to a gluing unit  16 . 
         [0047]    The gluing unit  16  has applicator rolls  20   a    20   b  and glue trays  18   a  and  18   b  along the conveying path of the single faced corrugated cardboard sheets K 1  and K 2 . The glue trays  18   a  and  18   b  stores liquid starch g inside. Lower parts of the applicator rolls  20   a  and  20   b  are immersed in the liquid starch. The top surfaces of the applicator rolls  20   a  and  20   b  are in contact with the flute tops of the single faced corrugated cardboard sheet K 1  and K 2 . Thus, the liquid starch g is applied to the flute tops of the corrugated medium of the single faced corrugated cardboard sheets K 1  and K 2  by rotating the applicator rolls  20   a  and  20   b.    
         [0048]    The gluing unit  16  also includes doctor rolls  22   a  and  22   b  which are in contact with the applicator rolls  20   a  and  20   b.  The doctor rolls  22   a  and  22   b  rotate in the same direction with the applicator rolls  20   a  and  20   b  respectively so as to adjust the amount of the liquid starch to be applied to the applicator rolls  20   a  and  20   b.    
         [0049]    The surface liner L 3  in the gluing unit  16  is guided to a double facer  30  by a guide roll  24 . 
         [0050]    Moist air discharge parts  41   a  and  41   b  are arranged along the conveying path of the single faced corrugated cardboard sheets K 1  and K 2  between the double facer  90  and the gluing unit  16 . The moist air is discharged from the discharge parts  41   a  and  41   b  which constitute a part of a moist air spraying unit  40  shown in  FIG. 2 . 
         [0051]    The moist air discharge parts  41   a  and  41   b  are shaped into a container. In the discharge part  41   a  and  41   b , supplied is moist air whose absolute humidity and enthalpy are adjusted so that the dew point temperature is near the gelatinization temperature of the liquid starch g. 
         [0052]    The moist air whose dew-point temperature is adjusted to the gelatinization temperature of the liquid starch g is sprayed to the flute tops of the corrugated media of the single faced corrugated cardboard sheets K 1  and K 2  from the moist air discharge parts  41   a  and  41   b.  The liquid starch g applied to the flute tops are gelatinized. The bonding part of the flute tops of the corrugated media N 1  and N 2  and the backside liners L 1  and L 2  is in an uncured-bonding state. 
         [0053]    Next, the single faced corrugated cardboard sheets K 1  and K 2  and the surface liner L 3  are conveyed to the double facer  90 . The double facer  90  has a guide roll  92  at an inlet thereof. A belt conveyor  94  is installed about the guide roll  92  and extends in the conveying direction of each paper web, i.e. the direction indicated with the arrow a. A plurality of heat plates  96  are arranged below the belt conveyor  94  along the conveying direction. A top surface of each heat plate  96  is formed flat. The paper web is conveyed over the flat surfaces of the heat plates  96 . 
         [0054]    The heat plates  96  are arranged over a certain distance, e.g. 7 to 8 meters. Saturated steam s of 1.0 to 1.1 Mpa is supplied to the heat plates  96  and the top surfaces of the heat plates  96  are heated to 170 to 180° C. A plurality of pressure rolls  98  are arranged on a rear side of the belt conveyor  94  along the conveying direction. The pressure rolls  98  presses the paper web being conveyed over the heat plates  96 . 
         [0055]    The single faced corrugated cardboard sheets K 1  and K 2  and the surface liner L 3  are conveyed over the heat plates in a state of being stacked together and pressed and heated. 
         [0056]    The liquid starch g is applied to the flute tops of the corrugated media of the single faced corrugated cardboard sheets K 1  and K 2 . The liquid starch g is heated by the moist air sprayed from the moist air discharge parts  41   a  and  41   b  and reaches the gelatinization temperature and is in the uncured-bonding state at the inlet of the double facer  90 . Then, the liquid starch g is further heated by traveling over the heat plates  76  in the double facer  90  to bond together and dry the single faced corrugated cardboard sheets K 1  and K 2  and the surface liner L. 
         [0057]    The configuration of the moist air spraying unit  40  is explained in reference to  FIG. 2 . As shown in  FIG. 2 , the air is introduced to a pipe line  43  by a air delivery fan (or a blower) via a filter  44  which filters foreign substances of the air. The air delivery fan  46  is driven by a drive motor  48 . An inverter  50  controls the rotation of the drive motor  48  to control the atmospheric pressure in the pipe line  43 . The inverter  50  is controlled by a controller  62 . A damper  52  is installed in the pipe line  43  to adjust the air flow. 
         [0058]    The controller  62  controls the inverter  50 , a temperature regulating valve  59  and a moisture regulating valve  72  according to operation conditions such as paper types (basic weight (weight of the paper per 1m 2 ) and the conveying speed of the paper web which are stored in a production control unit  60 . 
         [0059]    A steam heater  54  is installed on a downstream side of the damper  52 . A steam pipe  58  is connected to the steam heater  54  to supply the steam generated in the steam generator to the steam heater  54  via the steam pipe  58 . The temperature regulating valve  59  is installed in the steam pipe  58  to adjust the temperature of the air in the steam heater  54  by adjusting the flow rate of the steam in the steam pipe  58 . The steam generator  56  also supplies steam to the preheat rolls  12   a  to  12   c  and the heat plates  96 . 
         [0060]    The steam pipe  58  is connected to a meandering pipe  64  inside the steam heater  54 . The steam streams in the meandering pipe  64  and heat the air. The meandering pipe  64  is connected to a steam trap  66  outside the steam heater  54 . The steam is condensed in the steam trap  66  to discharge water d and is discharged outside. 
         [0061]    The heated air is sent to a mixing chamber  68 . The steam generator  56  and the mixing chamber  68  are connected via the steam pipe  65 . The mixing chamber  68  has a plurality of injection nozzles connected to the steam pipe  65 . The injection nozzles  70  are decentrally-disposed inside the mixing chamber  68 . The steam is injected from the injection nozzles  70  in the mixing chamber  68  so as to produce the most air. The moisture regulating valve  72  is installed in the steam pipe  65  so as to adjust the partial pressure of the steam in the moist air by regulating the flow rate of the steam in the steam pipe  65 . 
         [0062]    A pressure meter  76 , a temperature meter and one of a dew-point meter and a steam partial pressure meter  80  are installed in a discharge pipe  74  of the mixing chamber  68 . These measuring devices measure the pressure, temperature and steam partial pressure of the moist air streaming in the discharge pipe  74 . The measured values are inputted to the controller  62 . The controller  62  controls the rotation of the drive motor  48  by means of the inverter  50  and the opening of the temperature regulating valve  59  or the moisture regulating valve  72  based on the measured values. 
         [0063]    By this, the dew-point temperature of the moist air produced in the mixing chamber  68  becomes near the gelatinization temperature of the liquid starch g. The temperature of the moist air is controlled to be 10° C. to 100° C. higher than the dew-point temperature. 
         [0064]    The moist air produced in the mixing chamber  68  is supplied to the moist air discharge parts  41   a  and  41   b . The moist air discharge parts  41   a  and  41   b  have discharge panels  42   a  and  42   b  respectively. The discharge panels  42   a  and  42   b  are made of punching metal having slit-like holes which are arranged to face the corrugated medium of the single faced corrugated cardboard sheets K 1  and K 2  respectively. The moist air is sprayed at the flute tops of the corrugated media through the discharge panels  42   a  and  42   b.    
         [0065]    In the above structure, the dew-point temperature of the moist air sprayed through the discharge panels  42   a  and  42   b  is set near the gelatinization temperature of the liquid starch g. Thus, in the case where the flute tops of the corrugated medium where the liquid starch is applied has a temperature not greater than the dew-point temperature, the moist air condenses in contact with the flute tops of the corrugated media while releasing the latent heat of condensation onto the liquid starch g, thereby heating the liquid starch g. In contrast, in the case where the flute tops of the corrugated medium to which the liquid starch is applied has a temperature not less than the dew-point temperature, the vapor in the moist air does not condense and thus, the latent heat of condensation is not released onto the liquid starch g. 
         [0066]    In this manner, the liquid starch g is heated to approximately the gelatinization temperature on the upstream side of the double facer  90  and gelatinized to be in the uncured-bonding state. Next, the webs are transferred over the heat plates  96  in the double facer  90  where the webs are heated and pressed. Then, the corrugated media N and the backside liner L are bonded and dried and the double wall corrugated cardboard sheet D is produced. 
         [0067]    According to the preferred embodiment, it is possible to heat the liquid starch g accurately to near the gelatinization temperature by the moist air. It is also possible to avoid overheating of the liquid starch g as the liquid starch g is not heated beyond the gelatinization temperature. Therefore, quality deterioration of the product such as the adhesion bonding failure is solved. 
         [0068]    The latent heat of the condensation of the vapor contained in the moist air is released onto the liquid starch g so as to heat the liquid starch g. Thus, the heating effect is improved. 
         [0069]    As described above, the liquid starch g is not heated beyond the gelatinization temperature, which allows the heating time to be set with plenty of leeway. Thus, even when the conveying speed of the paper web is reduced, the heating accuracy can still be improved. 
         [0070]    The moist air has a temperature that is 10° C. to 100° C. higher than the gelatinization temperature of the liquid starch. Even when the temperature of the moist air decreases to some extent, no water drop is formed. Thus, there is no concern for inferior products which are generated due to formation of rust in the mechanical parts or the dew drops falling on the double wall corrugated cardboard sheet. 
         [0071]    The moist water discharge parts  41   a  and  41   b  are provided so as to lessen the required heating power of the preheat roll  12   a  to  12   c  or the heat plates  96 . 
         [0072]    The moist air spraying unit  40  of  FIG. 2  is provided. Thus, the air is supplied to the pipe line  43  by the air delivery fan  46  (or an air blower) instead of compressing the air and pumping it to the pipe line. Therefore, it is not necessary to make component devices and pipes pressure-proof and the structure can be simplified. 
         [0073]    Further, the moist air having a desired dew-point temperature can be accurately produced with use of the controller  62 . 
       Second Preferred Embodiment 
       [0074]    A second preferred embodiment of the present invention, which is applied to a production process of a single faced corrugated cardboard sheet made at a single facer, is explained in reference to  FIG. 3 . 
         [0075]    In  FIG. 3 , the media N is reeled out from a paper roll unshown in the drawing. The media N is winded around the preheat roll  104  by the guide roll  102  so as to be preheated. The preheated medium N is carried to a engagement part P between a lower corrugating roll  106  and an upper corrugating roll  108  to produce the corrugated media N. The lower and upper corrugating rolls  106  and  108  have corrugating surfaces to engage with each other in the engagement part P. A gluing unit  110  is provided on a downstream side of the engagement part between the upper and lower rolls. 
         [0076]    The gluing unit  110  has glue trays  112  in which the liquid starch g is stored, an applicator roll  114  whose lower part is immersed in the liquid starch g, a doctor roll  116  which is in contact with the applicator roll  114  and rotates in the opposite direction to the applicator roll  114  to adjust the amount of the liquid starch applied to the applicator roll  114 , and a scraper  117  which scrapes the liquid starch g off from the doctor roll  116 . The liquid starch g is applied to the flute tops of the corrugated media N by the applicator roll  114 . 
         [0077]    A moist air discharge part  118  is disposed to face the upper corrugated roll  108  on a downstream side of the gluing unit  110 . The moist air discharge part  118  has the same structure as the moist air discharge parts  41   a  and  41   b  of the first preferred embodiment shown in  FIG. 1  and has a discharge panel  120  which is arranged to face the upper corrugating roll  108 . The discharge panel  120  is made of punching metal or the like. The moist air is supplied to the discharge part from a moist air spraying unit not shown. The moist air spraying unit has the same structure as the moist air spraying unit  40  shown in  FIG. 2 . In the same manner to the first preferred embodiment, the discharge panel  120  has slit-like holes along the direction of the flute tops of the corrugated medium N. The moist air having the dew-point temperature near the gelatinization temperature of the liquid starch g is sprayed through the slit-like holes to the flute tops of the corrugated medium N. 
         [0078]    On a downstream side of the moist air discharge part  118 , a pressure belt  126  is installed around a pair of pressure rolls  122  and  124 . The pressure belt  126  is pressed against the upper corrugating roll  108  while moving in the direction of the arrow by the rotation of the pressure rolls  122  and  124 . The downward force of the pressure belt  126  against the upper corrugating roll  108  is adjustable by changing a tensile force of the pressure belt  126 . The tensile force of the pressure belt  126  can be changed by changing the distance between the pressure rolls  122  and  124 . 
         [0079]    The backside liner L is guided by a guide roll  128  and winded around a preheat roll  130 . The backside liner L is preheated by the preheat roll  130  and placed on the pressure belt  126  installed around the pressure rolls and  124 . Then, the backside liner L enters a nip area Q between the upper corrugating roll  108  and the pressure belt  126 . 
         [0080]    A steam generator unshown in the drawing supplies steam to the preheat rolls  104  and  130 . The steam generator supplies steam to the lower corrugating roll  106  to preheat the corrugated medium N. The steam having the same temperature and pressure as the steam s supplied to the heat plates  96  of  FIG. 1  is supplied to the upper corrugating roll  108  by the steam generator. 
         [0081]    The moist air is sprayed to the flute tops of the corrugated medium N by the moist air discharge part  118  and the liquid starch g thereon is gelatinized into the uncured bonding state. The corrugating medium N in the uncured bonding state is fed to the nip area Q to be placed on the backside liner and pressed and heated together in the nip area Q. IN this manner, the corrugated medium N and the backside liner L are bonded and the bonding part is dried. 
         [0082]    According to the second preferred embodiment, the liquid starch g is heated by the moist air having the dew-point temperature near the gelatinization temperature of the liquid starch g prior to the bonding of the corrugated medium N and the backside liner L. By this, the liquid starch g can be heated accurately to the gelatinization temperature but not beyond the gelatinization temperature. As a result, it is possible to achieve the same operation and effect as the first preferred embodiment, such as prevention of overheating of the liquid starch g. 
       INDUSTRIAL APPLICABILITY 
       [0083]    According to the present invention, it is possible in the production process of the corrugated cardboard sheet to heat the liquid starch accurately to the gelatinization temperature prior to the bonding of the corrugated cardboard sheet without forming water drops.