Patent Publication Number: US-8524327-B2

Title: Liquid adhesive dispensing system

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This patent application is a divisional of U.S. patent application Ser. No. 11/153,265, filed Jun. 15, 2005, now U.S. Pat. No. 7,717,059. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to the manufacture and processing of laminated sheet material, and more particularly, to a system for dispensing liquid adhesive onto a moving ply or sheet substrate in the manufacture of multi-ply laminant materials, such as bathroom tissue, facial tissue, napkins, paper towels, non-woven sheet material, and the like. 
     BACKGROUND OF THE INVENTION 
     Various techniques have been used and proposed for bonding layers of laminated sheet material. These techniques have included mechanically forcing the layers together to physically interlock the laminated layers, applying hot melt adhesives to the sheet material for adhesively bonding the laminated layers, and applying water-based adhesives to the sheets. The systems for carrying out these techniques have suffered various drawbacks, including necessitating equipment that was expensive in construction and difficult to maintain, creating mechanical or adhesive bondings of the laminated layers that were inconsistent or inadequate, being difficult to reliably control during changes in processing speeds and conditions, and resulting in over application, waste, slow drying, and bleed through of the applied liquid adhesives. Efforts to facilitate application of the liquid adhesives through pressurized air atomization of the liquid adhesive also have been the subject of problems which detract from the uniform or reliable application of the adhesive. Since atomizing air pressure can create a back pressure in the liquid adhesive supplied to a spray or dispensing nozzle, changes in the atomizing air pressure, such as during a processing change, can alter the flow rate of liquid through the spray nozzle. Hence, it has been difficult to accurately control processing parameters when modifying liquid adhesive and/or atomizing air pressures for different product requirements. Moreover, spraying adhesive with such atomization systems is relatively dirty and inefficient due to low transfer efficiency, blow off, misting, and build up of adhesive on the machinery components. 
     OBJECTS AND SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a liquid adhesive dispensing system for laminating sheet material that is adapted for more uniformly applying liquid adhesives notwithstanding changes in processing conditions. 
     Another object is to provide a liquid adhesive dispensing system as characterized above which is operable for generating a predetermined uniformly controlled, fine bubble foam of liquid adhesive prior to dispensing onto moving sheet material. 
     A further object is to provide a liquid adhesive dispensing system of the above kind in which pressurized air foaming and/or atomization of the liquid adhesive can be uniformly effected and controlled, notwithstanding changes in the line speed of the moving substrate material, changes in the liquid adhesive flow rate, or changes in atomizing air pressure. 
     Yet another object is to provide a liquid adhesive dispensing system of such type that permits selective control and changes in foam density and/or application rates as required during different sheet lamination processing. 
     Still another object is to provide such dispensing system that is effective for generating and applying a water based liquid adhesive in the manner that facilitates faster drying and minimizes damaging bleed through of the tissue substrate. 
     Another object is to provide a liquid adhesive dispensing system of the foregoing type which includes a plurality of liquid adhesive dispensing nozzles disposed across the width of a moving ply of sheet material for enabling selected patterns and/or concentrations of adhesive to be applied to the moving sheet material. 
     A further object is to provide such a liquid adhesive dispensing system that is adapted for relatively economical construction and easy maintenance. A related object is to provide such an adhesive dispensing system that enables automated cleaning of adhesive dispensing nozzles and associated liquid adhesive supply components. 
     Other objects and advantages of the invention will become apparent upon reading the following detailed description and upon reference to the drawings, in which: 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective of a spray header of a liquid adhesive dispensing system in accordance with the invention shown directing a liquid adhesive foam onto a passing ply, such as a web of sheet material to be used in the manufacture of a laminated product; 
         FIG. 2  is a vertical section of the illustrated spray header taken in the plane of line  2 - 2  in  FIG. 1 ; 
         FIG. 3  is a vertical section, similar to  FIG. 2 , but showing the spray header in a closed self-cleaning condition; 
         FIG. 4  is a schematic of a liquid adhesive dispensing system according to the invention utilizing a spray header such as shown in  FIG. 1 ; 
         FIG. 5  is an enlarged vertical section of one of the liquid adhesive dispensing guns of the illustrated header; 
         FIG. 5A  is an enlarged fragmentary section of a nozzle insert included in the adhesive dispensing gun shown in  FIG. 5 ; 
         FIG. 6  is a fragmentary section of an alternative embodiment of spray gun for use in the liquid dispensing system of the present invention; 
         FIG. 6A  is an enlarged fragmentary section of the spray nozzle of the spray gun shown in  FIG. 6 ; 
         FIG. 7  is a diagrammatic depiction particularly showing of the liquid adhesive delivery control system for the illustrated dispensing system; 
         FIG. 7A  is an enlarged fragmentary section of one of the positive displacement pumps, taken in the plane of line  7 A in  FIG. 7 ; 
         FIG. 8  is a perspective of a pumping apparatus used in the illustrated liquid adhesive delivery control system for directing liquid adhesive from a liquid adhesive supply to the spray header; 
         FIGS. 9 and 10  are side elevational and end views, respectfully, of the pumping apparatus shown in  FIG. 8 ; 
         FIGS. 11 and 12  are more detailed schematics of the liquid direction control system for the illustrated dispensing system; and 
         FIG. 13  is a diagrammatic depiction of an alternative embodiment of a liquid adhesive control system for the illustrated dispensing system. 
     
    
    
     While the invention is susceptible of various modifications and alternative constructions, a certain illustrated embodiment thereof has been shown in the drawings and will be described below in detail. It should be understood, however, that there is no intention to limit the invention to the specific form disclosed, but on the contrary, the intention is to cover all modifications, alternative constructions and equivalents falling within the spirit and scope of the invention. 
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENT 
     Referring now more particularly to the drawings, there is shown an illustrative liquid adhesive dispensing system  10  in accordance with the invention operable for directing water based liquid adhesive onto a moving ply or sheet substrate  11 , such as in the manufacture of laminated sheet materials, including bathroom tissue, facial tissue, napkins, paper towels and the like. The illustrated adhesive dispensing system  10  basically includes a spray header  12  ( FIGS. 1-6 ), a liquid adhesive supply  14  ( FIGS. 7 and 11 ), and a liquid adhesive delivery control system  15  ( FIGS. 7 ,  11  and  12 ) for controlling the delivery of liquid adhesive from the liquid supply  14  to the spray header  12 . It will be understood by one skilled in the art that following the dispensing of adhesive onto the moving substrate  11 , the substrate can be joined to another moving ply in a known manner to form a multiple ply laminate. Moreover, while the invention has particular utility for dispensing water based adhesives in the manufacture of laminated products, it will be understood that the liquid dispensing system  10  can be used for dispensing other types of liquids in other applications. 
     The spray header  12  in this case includes a plurality of spray guns or nozzle assemblies  20  disposed in transversely spaced relation across the width of the moving substrate  11 . The spray guns  20  are supported on a common cross beam  21 , which in turn is supported at opposite ends by rods  22 . The spray guns  20  each are bolted onto the crossbeam  21  in parallel relation to each other, and the support rods  22  preferably are mounted for selective pivotal movement for enabling the desired direction of discharging adhesives from the guns in predetermined angular relation to the moving substrate. The illustrated spray header  12  has a rectangular longitudinally extending enclosure or housing  24  mounted in surrounding relation to the spray guns  20 , with the housing  24  having an open end  25  from which adhesive is discharged from the spray guns  20 . As depicted in  FIG. 2 , and as will become apparent, fluid supply lines for the spray guns  20  extend along and are protectively contained within the housing  24 . It will be understood that the number of spray guns may vary depending upon a particular spray application. 
     In carrying out one aspect of the invention, the spray guns  20  each comprise internal mix air atomizing spray nozzle adapted for generating a fine adhesive foam within the nozzle which can be dispensed in a controlled manner over a predetermined lateral segment or zone of the moving substrate. The illustrated spray guns  20 , as depicted in  FIG. 5 , each include a main body or housing  26 , a rear housing cap  28  threadedly engageable with the body  26 , a nozzle  30  threadedly engaged in a downstream end of the body  26 , and an air cap  31  mounted in overlying surrounding relation to the nozzle  30  and retained on the main housing body  26  by a retaining nut  32 . The nozzle body  26  has a liquid adhesive inlet port  34 , a cylinder air inlet port  35 , a foaming/atomizing air inlet port  36 , and a fan air inlet port  38 . Liquid adhesive supplied to the inlet port  34  from an appropriate supply line  40  ( FIGS. 4 ,  7  and  11 ) communicates with a central longitudinal passageway  41  in the nozzle  30 , and in turn, with a liquid flow passage  42  in the nozzle  30  prior to discharge through a foam discharge orifice  44  in the air cap  31  ( FIGS. 5 and 6 ). The nozzle flow passageway  42  in this case is defined by an upstream cylindrical inlet section  45 , a tapered entry and valve seating section  46 , a small diameter nozzling section  48 , and a downstream, large diameter, mixing chamber  49  ( FIGS. 5 and 6 ). 
     For controlling the discharge of liquid adhesive from the spray gun  20 , a valve needle  50  coaxially extends through the housing body  26  for reciprocating movement between a valve closing position in seated engagement with the tapered entry section  46  of the nozzle passage  42  and unseated valve open position. The valve needle  50  in this case has a tapered seating section, preferably formed by two conical sections which define a sealing edge  51  engageable with the tapered entry section  46  of the nozzle  30 , and an axially extending clean out nose portion  52  that is positionable into the nozzling section  48  of the valve passage  42  when in a closed position for maintaining the passage free of adhesive buildup during usage. 
     For operating the valve needle  15 , as is known in the art and disclosed in more detail in U.S. Pat. No. 6,776,360 assigned to Spraying Systems Company, one of the co-assignees of the present application, the disclosure of which is incorporated herein by reference, the valve needle  50  has a piston assembly  53  at an upstream end which is biased in a valve closing direction by a compression spring  54  interposed between the piston assembly  53  and the upstream housing cap  28 . The piston assembly  53  includes a piston head portion  55  and a resilient annular cup shaped sealing ring  55   a  in sealing engagement with a cylindrical bore  56  in the housing body  26 . The compression spring  54  biases the piston assembly  53 , and hence the valve needle  50 , forwardly to a fully seated, i.e., valve closed position, depicted in  FIG. 5 . The valve needle  50  is movable axially in the opposite direction (to the right in  FIG. 5 ) against the force of the spring  54  by pressurized air (hereinafter “cylinder air”) selectively directed into the cylinder air inlet port  35  from the pressurized air supply line  58  ( FIGS. 4 ,  11 ,  12 ) which communicates through the housing body  26  with an air chamber  57  on the downstream side of the piston assembly  53 . 
     In carrying out the invention, the nozzle mixing chamber  49  is designed for enhancing atomization and foaming of the adhesive liquid within the spray gun for generating a fine bubble foam that can be discharged onto the moving substrate  11  in a controlled fashion for effective adhesion of laminated plies of sheet material without undesirable bleed through in the substrate. To this end, the mixing chamber  49  of the nozzle  30  includes an outwardly tapered pressurized air interacting section  60  that communicates between the nozzling section  48  and a downstream cylindrical expansion chamber  61  ( FIG. 5A ). For directing pressurized air into the tapered air interaction section  60 , the nozzle  30  is formed with a plurality of radial air passageways  62  communicating through the tapered side wall surface of the air interacting section  60  at a location adjacent the downstream end of the nozzling passage section  48 . 
     The radial air passages  62 , which in this case are disposed at 90° circumferential spacing to each other, communicate with an annular air chamber  64  defined between the nozzle  30  and the air cap  31 , which in turn communicates with the foaming/atomizing air inlet port  36  through a passageway  65  in the nozzle body  26 . The nozzle  30  and air cap  31  have tapered surfaces  66  in contacting relation to each other about the air cap foam discharge orifice  44 , and to facilitate an air tight connection, a suitable O-ring may be provided on an inner side of that juncture. The nozzle expansion chamber  61  preferably has a diameter of at least three times the diameter of the nozzling passage section  48  and at least twice the diameter of the air cap foam discharge orifice  44 . More preferably, the expansion chamber  61  has a diameter about five times the diameter of the nozzling passage section  48 , and the air cap foam discharge orifice  44  has a diameter of about twice the diameter of the nozzling passage section  48 . While the theory of operation is not completely understood, it is believed that intersection of the air inlet passages  62  with the tapered air interaction section  60  of the nozzle  30  creates a relatively large orifice area in close proximity to the nozzling section  48  such that liquid entering the interaction section  60  cannot escape the effect of the incoming pressurized air streams, such as by closely following wall surfaces of the liquid flow passage  42 . Hence, it has been found that when liquid adhesive is directed through the nozzle  30  the plurality of circumferentially spaced radial atomizing air streams directed into the tapered air interacting section  60  effect thorough agitation, atomization, and fine bubble foamation of the adhesive, which thereupon expands into the expansion chamber  61  prior to further atomization of the foam by the pressurized air as foam is emitted from the discharges through the relatively smaller diameter air cap foam discharge orifice  44 . 
     For forming and directing the foam into a flat fan spray pattern for wider lateral application onto the moving substrate  11 , each spray gun  20  is operable for impinging pressurized air (i.e., “fan air”) on opposite sides of the foam following discharge from the air cap discharge orifice  44 . In the illustrated embodiment, pressurized air is communicated to the fan air inlet port  38  of the spray gun from a pressurized air supply line  67  ( FIGS. 4 ,  11 ,  12 ), which in turn communicates through the nozzle body  26  with an annular chamber  68  defined between axial ends of the nozzle body  26  and air cap  31 . The annular chamber  68  communicates pressurized air to a pair of longitudinal passages  69 , which terminate in opposed angled discharge passages  69   a  ( FIG. 5 ) that direct pressurized air streams at an acute angle on opposite sides of the discharging liquid adhesive foam for spreading the foam into a relatively flat narrow spray pattern transverse to the direction of movement of the substrate upon which it is directed. It will be appreciated that the width of the flat spray fan spray, and hence the width of the application zone on the substrate can be controlled by the fan air pressure. 
     Referring to  FIGS. 6 and 6A , there is shown an alternative embodiment of a spray gun that can be used in the illustrated liquid adhesive dispensing system, wherein items similar to those described above have been given similar reference numerals. The spray gun in this case has an alternative form of spray nozzle design which utilizes a combination internal/external air atomization technique in generating and atomizing fine bubble liquid adhesive foam. The spray gun  20  again comprises a housing body  26 , a nozzle  30  threadedly engaging a discharge into the body  26 , and an air cap  31  disposed in surrounding relation to the nozzle  30  and retained on the housing body  26  by a retaining nut  32 . The nozzle  30  in this case has a relatively small diameter forwardly extending nose portion  33  which defines a liquid discharge orifice  33   a  in coaxial relation to the air cap foam discharge orifice  44 . The nozzle  30  and air cap  31  in this instance define foaming/atomizing air passages  37  communicating between an annular air supply chamber  37   a , which in turn communicates with the foaming/atomizing air supply passage  65 . 
     In carrying out the invention, the nozzle nose portion  33  is disposed in recessed relation to the air cap discharge orifice  44  for defining a liquid adhesive mixing and atomizing chamber  43  immediately downstream of the nozzle discharge orifice  33   a  adapted for effectively foaming and atomizing the liquid adhesive flow stream both prior to and as an incident to discharge from the spray gun. To this end, in the illustrated embodiment, the downstream end of the nozzle nose portion  33  is recessed a distance d from the downstream side of the central air cap orifice  44  for defining a mixing chamber  47  immediately about the downstream end of the nozzle nose portion  33 . The nozzle nose portion  33  preferably has an outer diameter d 1  slightly less than the diameter d 2  of the air cap discharge orifice  44 , and the downstream end of the nose portion  33  extends a relatively small distance d 3  into the air cap orifice  44 . The downstream end of the nozzle nose portion defines a sharp annular corner, which together with a sharp annular corner defined by an inside edge of the air cap orifice  44 , defines an angled passageway  63  communicating with the mixing chamber  47 . 
     In practice, it has been unexpectedly found that the angled passage  63  defined between the sharp corners of the nozzle nose portion  33  and air cap discharge orifice  44  create eddy currents and turbulence in the pressurized air directed into the mixing chamber  47 , which enhances foaming and atomization of the liquid adhesive within the mixing chamber  47  prior to the discharge from the spray gun. The turbulence further has been found to more effectively maintain the discharge orifices  33   a ,  44  of the nozzle and air cap free of significant buildup which could impede efficient performance. The recessed distance d of the nozzle nose portion  33  from the downstream side of the air cap discharge orifice  44  preferably is less than the diameter “d 4 ” of the nose portion liquid discharge orifice  33   a . In practice, good operating results have been obtained when the diameter d 4  of the liquid discharge orifice  33   a  is 0.025 inches, the recessed distance d of the nozzle nose portion from the air cap end face is 0.013 inches, the distance d 1  is 0.05 inches, the distance d 2  is 0.067 inches, the distance d 3  is 0.001 inches, and the distance d 5  is 0.008 inches. 
     The liquid adhesive supply  14  in this case includes a closed pressure vessel  70  ( FIGS. 7 and 11 ) into which liquid adhesive is pumped from an appropriate supply source through inlet supply line  71  having a control valve  72 , and exits through a delivery line  74  communicating from near the bottom of the pressure vessel  70 . The vessel  70  is pressurized by a pressurized air supply line  75  communicating with the pressurized air source under the control of a pressure regulator  76 . 
     For automatically maintaining a level of liquid adhesive in the illustrated pressure vessel  70 , a level sensor  78  of a known type is provided which includes a level monitoring float  79 . When the liquid adhesive level is lowered to a predetermined level, the fill control valve  72  can be actuated in response to a signal from the sensor  78  to cause additional liquid tube pumped into the vessel  70 . When the liquid adhesive reaches a predetermined upward level, the level sensor  78  will cause closure of the valve  72 . 
     A wide variety of liquid adhesives may be used with the adhesive dispensing system of the present invention, including the water based liquid adhesives disclosed in U.S. application Ser. No. 10/654,335 filed Sep. 5, 2003, assigned to the H. B. Fuller Company, one of the co-assignees of the present invention, the disclosure of which is incorporated herein by reference. Representative aqueous adhesive compositions may include one or more monomeric, oligomeric and/or polymeric components, dispersed, suspended, emulsified, dissolved, or the like, in an aqueous medium. The adhesive composition may include at least one resin that is water-soluble or water-dispersible at a temperature in the range of from about 20° C. to about 90° C. A wide variety of different resin(s) and/or monomer ingredients thereof may be used. Representative examples of suitable resin types include one or more of acrylic, styrene-acrylic, styrene-butadiene, vinyl acetate, polyvinyl alcohol, urethane, chloroprene, phenolic, polyamide, polyether, polyester, polysaccharides (including starch, dextrin, cellulose, gums, or the like), combinations of these, and the like. Particularly useful resin(s) are acrylic, vinyl acetate, polyvinyl alcohol, dextrin, starch, and the like. The composition may be supplied as a solution, latex, emulsion, dispersion, or the like. In addition to the resins and monomer ingredients, the adhesive compositions may include lubricants, emollients, rheology modifying agents, antimisting additives, fillers, extenders, foaming agents, or the like. 
     Examples of adhesive compositions include the following:
         1. One part of Laponite RDS is dispersed in water for 20 minutes; 20 parts of a low-molecular polyvinyl alcohol resin (Celvol 205) is added and blended until a smooth mixture is obtained. Then the blend is heated to 190-200° F. for 30 minutes under a gentile agitation. The solution is then cooled to 100-120 and a biocide is added and the viscosity adjusted between 250-300 cP at room temperature (72 F). The resulting composition can be used in the illustrated dispensing system to produce a foam of fine beads or bubbles for effectively bonding layers of multiple ply tissue and the like.   2. A product obtained from the polymerization of vinyl acetate monomer (30 parts) in an aqueous solution of dextrin (40 parts dextrin and 30 parts water) is diluted to a viscosity range of 250 to 300 cP at 72 F, to yield a solution containing about 50% solids. A diluted solution can then be generated into a fine bubble foam by the illustrated dispensing system for effectively bonding laminated sheet material.       

     Heretofore as indicated above, it has been not only difficult to generate suitable finely atomized foam from liquid adhesives, but even more difficult to control the uniform application of the foam onto a moving substrate during start-up operations in which the movement of a substrate is accelerating and during changes in processing conditions. Moreover, when pressurized air atomization has been used to assist in atomization and foaming of the adhesive, changes in air atomizing pressure create changes in back pressure to the liquid supply which can impede the liquid supply, affect the desired density and makeup of the foam, and hinder reliable processing control. 
     In accordance with an important aspect of the invention, the liquid adhesive delivery control system  15  is operable for generating and dispensing foam with desired properties during a full range of operation of the dispensing machine, as well as during machine start up and changes in processing parameters, including changes in liquid and/or air atomizing pressures. To this end, the liquid dispensing system includes a plurality of positive displacement pumps  80  which each are dedicated to a respective one of the spray guns  20  for directing predetermined metered quantities of liquid to the spray guns  20  for consistent and uniform application onto a moving substrate  11 , notwithstanding changes in processing speeds or conditions. The illustrated positive displacement pumps  80  are gear-type pumps which each comprise a pair of intermeshing gears  81 , one of which is power driven from a drive shaft  82 . ( FIGS. 7 and 7A ) As is known in the art, as one of the gears  81  is driven, the two gears rotate and mesh to force a specific quantity of liquid from the inlet to the outlet side of the pump  80  in a positive manner during each revolution of the gears. Such positive displacement gear pumps are commercially available, such as Brown &amp; Sharp Model 700 Series gear pumps offered by BSM Pump Corporation, North Kingstown, R.I. It has been found that such positive displacement pumps  80  effectively act as a liquid metering device for each spray gun  20  such that the supply of liquid adhesive to the spray guns  20  can be precisely controlled and changed through control of the operating speed of the pumps  80 . It will be understood that while gear pumps are disclosed in the illustrated embodiment, other types of positive displacement pumps may be used in the liquid adhesive delivery system, such as progressive cavity displacement pumps of a known type. 
     In carrying out the invention, the positive displacement pumps  80  in the illustrated embodiment are driven from a common power source such that the pumps  80  uniformly deliver similar quantities of liquid adhesive to the respective spray guns  20 . In the illustrated embodiment, as depicted in  FIGS. 8-10 , the pumps are mounted on a frame  85  and are driven by a common drive motor  86 , such as a selectively controllable variable frequency drive motor of a conventional type. The illustrated frame  85  has a rectangular construction which supports a first plurality of pumps  80  in a first row along a bottom of the frame  85  and a second plurality of pumps  80  in a second row along a top of the frame  85 . The drive shafts  82  of each pump  80  carry a respective drive sprocket  88 , and the drive motor  86  in this case has a gear box  89  with an output drive shaft  90  that carries a pair of drive sprockets. One of the drive motor sprockets is operatively coupled to and drives the first row of pumps  80  via a first endless belt or chain  94  trained about the drive sprockets  88  for the pumps  80  in the first row and drive sprockets  95 . The other drive motor sprocket is coupled to and drives the pumps  80  of the second row via a belt or chain  96  trained about the drive sprockets  88  for the pumps  80  of the second row and drive sprockets  98 . Hence, selected operation of the drive motor  86  will simultaneously operate the positive displacement pumps  80  of both rows, causing the pumps  80  to direct substantially similar quantities of adhesive to the respective spray guns  20  based upon the operating speed of the pumps  80 . Although the common drive for the multiplicity of positive displacement pumps  80  provides economy in design and manufacture of the dispensing system, alternatively it will be understood that individual drive motors could be used to permit independent flow control for each spray gun. 
     In further carrying out this aspect of the invention, the liquid delivery control system  15  is operable for controlling the speed of the positive displacement pumps, and hence the quantity of adhesive liquid directed to the spray guns  20 , proportional to the speed of the moving substrate  11  such that a constant quantity of adhesive may be applied to the substrate within a full range of operating web speeds. For this purpose, the delivery control system  15  includes a tachometer  99  of a known type for sensing the speed of the moving substrate  11  and a main controller  100  for the dispensing system responsive to signals from the tachometer  99  for proportionally controlling the operating speed of the positive displacement pumps  80 . Hence, it can be seen that the desired adhesive application rate can be set in the controller either prior to or during operation, and the delivery control system  15  will automatically compensate for changes in line speed by adjusting the operating speed of the pumps  80 . Hence, a preprogrammed foam application rate can be set in the controller  100  and the system will automatically begin spraying at the programmed rate. During ramp-up, this rate will be maintained up through the maximum operating speed without further operator intervention. Moreover, since the positive displacement pumps  80  effectively meter the liquid delivery, the application rate is unaffected by other changes in processing parameters, including changes in atomizing air pressure, as will become apparent. 
     While the positive displacement pumps  80 , and particularly the illustrated gear pumps, function as an effective liquid metering devices, it has been found that a high differential pressure build-up across the pumps can result in liquid being forced under pressure through the pumps by virtue of manufacturing tolerances between the gears and the pump housings. This phenomena, sometimes referred to as liquid slippage, can augment the throughput affected by rotary operation of the gears and alter uniformity of the generated foam. 
     In carrying out the invention, in order to prevent liquid slippage through the pumps  80  and enhance reliable control in the delivery of liquid adhesive to the spray guns  20 , the delivery control system  15  is operable for balancing the inlet and outlet pressures for each of the positive displacement pumps  80  to prevent pressure induced liquid slippage through the pumps. For this purpose, in the illustrated embodiment, a nozzle pressure transmitter  104  is provided in the outlet line  40  of each pump  80  (in this case the inlet line  40  to each spray gun  20 ) and a manifold pressure transmitter  105  is provided in a manifold supply line  106  that feeds the inlets to each of the pumps  80  ( FIG. 11 ). In a typical operation of the dispensing system, for a programmed operating speed for the pumps  80 , the nozzle pressure transmitter  104  will sense a pressure in the outlet line commiserate with the programmed flow rate. When the manifold pressure transmitter  105  senses a different pressure, the air regulator  76  to the liquid supply pressure vessel  70  is operated by pneumatic pilot signal from an I/P converter  107  under the control of the controller  100  to adjust the pressure in the pressure vessel  70 , and hence, the liquid pressure in the manifold line  106  to equalize the inlet and outlet pressures across the pumps  80 . 
     In keeping with still a further feature of the invention, the foaming/atomizing air and fan air to the spray guns  20  also can be selectively controlled for generating and applying foam with the desired characteristics. For controlling foaming/atomizing air, a foaming/atomizing air regulator  110  is provided in a foaming/atomizing air manifold line  111  that communicates with each of the spray guns  20  and which can be controlled by an I/P converter  112  via the controller  100 . Fan air is communicated to each of the spray guns  20  via the fan air supply line  67 , the pressure of which is controlled by a fan air regulator  114  via an I/P converter  115 . Preferably through programming of the controller  100 , uniform density of the foam can be achieved by automatically increasing foaming/atomizing air pressure proportionate to the operating speed of the positive displacement pumps  80 . Alternatively, both foaming/atomizing air and fan air can be selectively controlled by the controller  100  independently of the liquid adhesive flow rates for a particular application. This can be particularly desirable when there is a need to increase the concentration of the adhesive, such as at the beginning or ending of a roll strip. This can be effected by reducing the foaming/atomizing air pressure, which will reduce atomization and permit the application of a more concentrated liquid adhesive. Likewise, reducing fan air pressure will result in a narrower, more concentrated, adhesive application. 
     From the foregoing, it can be seen that the liquid adhesive delivery control system  15  is effective for enabling precise control of both the adhesive delivery rate and the foam characteristics over a wide range of operating line speeds. In a typical operation of the liquid dispensing system  10 , the substrate  11  can be moved at line speeds of up to 2,500 feet per minute with constant foam characteristics and uniform adhesive application rates. The adhesive application rates can vary between about 15 and 200 mg/ft 2  depending upon the desired bond strength. The foaming/atomizing air pressure preferably may be between 10-20 psi, with fan air pressures of 10 psi or less. The spray guns may be located between 6-12 inches from the moving web and dispense foam with transverse widths of about 5 to 6 inches. The foaming/atomizing air generates an adhesive foam within the nozzles, as described above, which is further atomized as the pressurized discharge emits from the nozzles. The fine bubble foamation of the adhesive and its atomized discharge substantially eliminates bleed through in even highly porous substrate tissue materials. The foam may have average bubble sizes of 100 microns or less, depending on the particular application and drying requirements. By appropriate control of the fan air, the system is operable for applying adhesive in either strips or 100% coverage. Tissue ply strength and other characteristics of the tissue, such as hand feel, smoothness, cushion, drape, emboss definition, bulk, absorbency, color, also are maintained. 
     In accordance with still a further feature of the invention, an automatically operable cleaning system is provided for cleaning the both exterior and interior surfaces of the spray guns  20 . In the illustrated embodiment, the spray header housing  24  has a cover  120  which is normally disposed in an open position, as depicted in  FIG. 2 , during adhesive dispensing operations. To initiate a cleaning operation, the controller  100  can be programmed to actuate an air cylinder  121  which causes the cover  120  to pivot to a closed position, as depicted in  FIG. 3 , enclosing the spray guns  20  within the housing  24  so that all sprays and purge water are captured. 
     For cleaning external surfaces of the spray guns  20 , the housing cover  120  serves as a header for two rows of water spray nozzles  122 , which may be conventional full cone spray nozzles, with pairs of the nozzles  122  being located adjacent the ends of respective of the spray guns  20  when the cover  120  is closed. Through actuation of an air operated flow valve  123 , water can be directed to a water manifold line  124 , which in turn communicates with the exterior water spray nozzles  122  ( FIGS. 3 and 12 ). Check valves, designated CK in  FIG. 12 , are provided in the inlet water supply lines to prevent back flow and dripping. 
     For effecting internal cleaning of the spray guns, again either manually or through automatic programming of the controller  100 , an adhesive supply line control valve  126  is first closed and an adhesive purge valve  128  is opened to permit purging of liquid adhesive remaining in the liquid supply lines. Actuation of the control valve  130  to a purge line  131  permits communication of the purging water from the liquid adhesive manifold  132  and liquid passageways of the respective spray guns  20 . In addition, actuation of control valves  135  effects the transmission of a water supply from line  136  through the foaming/atomizing air and fan air lines  111 ,  67  respectively, for cleaning the foaming/atomizing air and fan air passageways of the spray guns  20 . Check valves, again designated “CK” in  FIGS. 5 and 12 , are provided for preventing air from entering the water supply lines and water from entering the air supply lines. 
     During a cleaning cycle purge water is collected within the housing  24 , which preferably has sufficient pitch to allow gravity to carry the purge water to a discharge drain  129  ( FIG. 3 ). For preventing the escape of purge water during a cleaning cycle, the cover  120  and main housing  24  have a dual wall construction to permit interfitting of inner and out panels  120   a ,  120   b  of the cover and inner and outer panels  24   a ,  24   b , of the housing for preventing of the escape of the purge water without the necessity for resilient seals or precision inter-engagement of the cover and housing. 
     Referring now to  FIG. 13 , there is shown an alternative liquid supply control systems that may be used in connection with the liquid adhesive delivery system of the present invention, wherein liquid flow is metered and compared with a theoretical value for compensating for and preventing liquid slippage through the positive displacement pumps. Again, items similar to those described above have been similar reference numerals. In this case, liquid adhesive is delivered under pressure to an inlet port  140  of a flow meter  141 . Web speed is detected by a tachometer  99  and the positive displacement pump  80  is operated by the controller  100  at a speed to provide the necessary adhesive delivery rate to the spray gun  20 . Pressure transmitters  104 ,  105  detect the pressure differential across the pump  80  and control the inlet pressure to the pump  80  by an automatic liquid regulator  142  to control and minimize liquid slippage at the pump  80 . The actual liquid flow rate, as measured by the flow meter  141 , is compared by the controller  100  to a theoretical flow rate and the speed of the pump  80  is adjusted to compensate for any differences between the theoretical flow rate and actual flow rate. The automatic air pressure regulators  110 ,  114  again control foaming/atomizing and fan air pressures to the spray gun  20 . As described previously, individual pumps  80  supply adhesive to each additional spray gun  20  and foaming/atomizing and fan air ports  144 ,  145  respectively supply the additional spray guns. Air regulators are supplied by common air supply line and control signals from the regulators  110 ,  114  and  142  are supplied by current to pressure converters as described previously. 
     From the foregoing, it can be seen that the adhesive dispensing system of the present invention is adapted for more uniformly applying liquid adhesives onto moving substrates, notwithstanding changes in line speed, adhesive liquid flow rates, or air atomizing pressures. The liquid dispensing system is effective for generating and applying a water based liquid adhesive foam in a manner that augments adhesive bonds of the laminated plies, facilitates faster drying, and minimizing damaging bleed through of the substrate. The liquid adhesive dispensing system is relatively economical in construction and is adapted for efficient automated control. The system further includes an automatically operable cleaning system for easy maintenance.