Patent Publication Number: US-6698353-B2

Title: Apparatus and method for lithographic printing utilizing a precision emulsion ink feeding mechanism

Description:
RELATED APPLICATIONS 
     This application is a continuation filed U.S. application Ser. No. 08/923, 010, filed Sep. 3, 1997 now U.S. Pat. No. 6,318,259. 
    
    
     FIELD OF THE INVENTION 
     The present invention is generally related to a method and apparatus for lithographic printing using emulsion ink and, more particularly, a method and apparatus for feeding emulsion ink to a plate cylinder of a lithographic printing press. 
     BACKGROUND OF THE INVENTION 
     In a conventional lithographic printing process, an inking system is used to feed ink to the image areas of the printing plate and a separate dampening system is used to dampen the non-image areas of the printing plate. The water provided for dampening is more or less uniform across the press, while the ink input is regulated according to the image coverage of each printing zone and hence varies across the press. Such conventional processes have numerous drawbacks. The print quality is highly sensitive to the quality of the dampening systems, which are complex, expensive, difficult to maintain and take up valuable space. Great skill is required of the press operators to ensure that the proper ink/water ratio (i.e., ink/water balance) is maintained across the press during printing. 
     A relatively long start-up time is required before the ink/water balance reaches a steady state, and print quality varies during the start-up time. The time for the press to reach a steady state after a change in the ink feed rate is inversely proportional to the image coverage of each printing zone. Press operators commonly adjust the ink feed rate before the press has reached a steady state condition and hence end up chasing after a target print density constantly throughout an entire press run. This also accounts for inconsistent print quality. When the optical print density is lower than the target value, it could be caused by either insufficient ink supply or too much water supply. It requires a skilled crew to make the correct adjustment. Failure to do so may eventually result in tremendous print waste. 
     Ink input requirements vary across the press, which adds complexity to the printing process control, especially for a large newspaper press which may have as many as a thousand ink keys that need to be adjusted. 
     The aforementioned difficulties associated with presses having separate ink supply and dampening systems have prompted the development of systems using a single fluid for both inking and dampening: emulsion ink. Emulsion inks used in lithography are made from an emulsion of an oil-based ink and a water-based fountain solution. The emulsion ink is applied to a printing plate (typically mounted on a plate cylinder) having distinct image areas and non-image areas. The image areas have an oleophilic material, such as an oleophilic polymer, disposed on the surface thereof, so that the oil-based ink will adhere thereto for subsequent transfer to a printing substrate, such as a paper web. The non-image areas have a hydrophilic material, such as an aluminum oxide, disposed on the surface thereof, so that the water-based fountain solution will adhere thereto, thereby forming a protective film over the non-image areas, to prevent ink from adhering thereto. A principal advantage of the use of emulsion inks is that emulsion inks can eliminate the need for a separate system to dampen the printing plate and hence eliminates printing problems associated with keeping the ink/water properly in balance. Also, using emulsion inks simplifies the printing process by eliminating the need for many ink keys that would otherwise be required in presses using separate dampening and inking systems, i.e., to account for variations in image density. 
     However, a major drawback of the use of emulsion inks is that emulsion inks are often unstable (i.e. the oil-based ink and water-based fountain solution separate into distinct liquid layers). Such instability is undesirable because it interferes with ink transfer. For example, if the emulsion ink is not stable enough, the oil-based ink and water-based fountain solution will separate prematurely, before reaching the printing plate, resulting in scumming and wash marks, as water released from the emulsion ink will interfere with ink transfer by (a) reducing the amount of emulsion ink fed to the printing plate and (b) flushing across image areas of the printing plate. However, if the emulsion ink is overly stable, it will not release a sufficient amount of water to the printing plate to keep the non-image areas of the printing plate free of ink. Accordingly, the emulsion ink is formulated to have a stability that is within a “window” between being too stable and too unstable for satisfactory litho-graphic printing. It has been found that suitable emulsion inks have a water content of at least 35% by weight. 
     Also, because the viscosity of lithographic inks is relatively high, about 10 to a few hundred poises, lithographic inks generally do not flow freely. As water is dispersed into a matrix of lithographic ink to produce emulsion inks, the flow properties further deteriorate, making the formation of a suitably stable emulsion ink difficult. 
     Accordingly, when using emulsion ink, it is often necessary to adjust the ink/water balance during operation of the printing press. With existing press configurations, the adjustment will not take effect until the emulsion ink needing adjustment is substantially used up. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic diagrammatic view of a printing press incorporating a precision emulsion ink feeding mechanism in accordance with the present invention; 
     FIG. 2 is a schematic diagrammatic view of the emulsion ink feeding mechanism; 
     FIG. 3 is a side elevational view, partially in cross-section, of a liquid mixing and dispersing apparatus forming part of the emulsion ink feeding mechanism; 
     FIG. 4 is a plan view showing a rotor, an inner stator member, and an outer stator member forming part of the liquid mixing and dispersing apparatus; 
     FIG. 5 is a fragmentary side elevational view showing the rotor, the inner stator member, and the outer stator member, partially in cross-section taken generally along lines  5 — 5  of FIG. 4; 
     FIG. 6 is an elevational view, taken from below, showing the rotor; 
     FIG. 7 is a schematic diagrammatic view of a control system for regulating the emulsion ink composition for the precision emulsion ink feeding mechanism; 
     FIG. 8 is a schematic diagrammatic view of an ink distribution rail and fountain roller assembly forming part of the precision emulsion ink feeding mechanism; 
     FIG. 9 is a plan view, partially in cross-section, of the ink distribution rail and fountain roller of FIG. 8; 
     FIG. 10 is a cross-sectional view, taken generally along lines  10 — 10  of FIG. 9, of the ink distribution rail; and 
     FIG. 11 is a schematic diagrammatic view of an alternative ink distribution rail and fountain roller assembly. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The embodiments described herein are not intended to be exhaustive or to limit the scope of the invention to the precise form or forms disclosed. Instead, the following embodiments have been described in order to best explain the principles of the invention and to enable others skilled in the art to follow its teachings. 
     In the illustrations given, and with reference first to FIG. 1, there is shown a printing press generally designated  10  for printing an image on a paper web  12 . The press  10  has a printing unit  14  for printing ink on the web  12 . Although not shown, the press  10  may include one or more additional printing units that may each be used, for example, for printing a different color of ink on the web  12 . 
     The printing unit  14  has a plate cylinder  16  associated with a blanket cylinder  18 . During printing by the press  10 , an image of the ink is transferred from the plate cylinder  16  to the blanket cylinder  18  to print the image on one surface of the web  12 . An emulsion ink, made up of an oil-based ink and a water-based fountain solution, is fed to the plate cylinder  16  from a digitally-controlled gear pump ink injector unit  20  through a plurality of distribution rollers  22 , including a fountain roller  23 , an auxiliary vibrator drum  27 , a vibrator/scraper drum  24 , and a pair of form rollers  25   a  and  25   b . A smoothing blade  21  is mounted to the gear pump injector unit  20  and contacts the surface of the fountain roller  23  in order to evenly spread the emulsion ink onto the fountain roller  23 . The surface of the fountain roller  23  is covered with a brush surface made from a material available commercially as Part No. 2A3 from Kanebo USA Inc., 693 5th Avenue, 17th Floor, New York, N.Y. 10022. This material is similar to the “loop” portion of so-called “hook and loop” fasteners, such as VELCRO®. The vibrator/scraper drum  24  has a wear-resistant, oleophilic/hydrophobic surface that collects excess emulsion ink that is in turn scraped off of the vibrator/scraper drum  24  by a doctor blade  26  and collected for re-use by an auger and scraper assembly  28 , forming part of an ink feed and recirculation system  29 , shown schematically in FIG.  2 . The auxiliary vibrator drum  27  and the vibrator/scraper drum  24  oscillate in an axial direction (i.e., in a direction perpendicular to the plane of the paper in FIG. 1) to help ensure that a uniform emulsion ink film is supplied to the plate cylinder  16  and to prevent the formation of ridges on the emulsion ink film. 
     The fountain roller  23  rotates at a maximum speed of about 60 revolutions per minute, and proportionally slower as press speed is slowed. The rotation of the fountain roller  23  is thus quite slow in comparison to the rotation of the auxiliary vibrator drum  27 , which typically rotates at a speed of about 1,000 revolutions per minute when the press  10  is operating. 
     The inking rollers (i.e. the fountain roller  23 , the form rollers  25   a ,  25   b , and those rollers therebetween) are driven by a separate motor so that the inking rollers may be driven while the plate cylinder  16  and the blanket cylinder  18  are stationary. Therefore, the inking rollers may be driven at press startup until an acceptable emulsion ink has been formed by the ink feed and recirculation system  29 , thereby minimizing print waste during press startup. 
     With reference to FIG. 2, the collected excess ink is transported by the auger and scraper assembly  28  to a conduit  30  which feeds a mixing and dispersing apparatus  56 . Fresh ink is fed to the mixing and dispersing apparatus  56  from an ink supply reservoir  34  through a conduit  36 . The flow of fresh ink through the conduit  36  is controlled by a new ink valve  38  that is responsive to a liquid level sensor  40  that senses the level of liquid in the mixing and dispersing apparatus  56 . If the liquid level sensor  40  determines an overflow level of liquid, emulsion ink via conduit  62  is diverted to a conduit  61  and into an auxiliary reservoir  65 . Liquid from the auxiliary reservoir  65  may be used again by feeding it back to the mixing and dispersing apparatus  56  via a conduit  67 . Liquid discharge into or out of the auxiliary reservoir  65  is controlled via solenoid valves  69   a ,  69   b , and  69   c  and by air depressurizing (discharge into) or air pressurizing (discharge out of) the auxiliary reservoir  65 . The solenoid valve  69 c can connect the auxiliary reservoir  65  to either a shop air system  71 , providing air pressure of from about 40 psi (about 276 kPa) to about 70 psi (about 483 kPa), for pressurizing the auxiliary reservoir  65 , or an ambient air source  73 , providing air at an ambient pressure of about 14.7 psi (about 101 kPa), for depressurizing the auxiliary reservoir  65 . If desired, a vacuum source may be substituted for the ambient air source  73 , in order to provide air at an even lower pressure. 
     Fresh fountain solution (or clean water, as the case may be) is fed to the mixing and dispersing apparatus  56  from a fountain solution supply reservoir  44  through a conduit  46 . The flow of fresh fountain solution through the conduit  46  is controlled by a valve  48  that is responsive to a water content sensor  50  that senses the percentage of water flowing out of the mixing and dispersing apparatus  56  in an outlet conduit  52 . The emulsion ink is fed to an ink distribution rail  60  via a conduit  59 . The ink distribution rail  60  in turn feeds the digitally-controlled gear pump ink injector unit  20 . Unused emulsion ink is continuously recirculated to the mixing and dispersing apparatus  56  via the return conduit  62 . This continuous recirculation of unused emulsion ink via the return conduit  62  is in addition to the ink scraped off of the vibrator/scraper drum  24  and returned to the mixing and dispersing apparatus  56  via the conduit  30 . The continuous recirculation of emulsion ink ensures that the emulsion ink remains stable and makes it possible to reformulate the emulsion ink, if necessary (i.e. if the water content thereof is too low or too high), without having to wait for the exhaustion of all of the emulsion ink that is in need of reformulation. This dramatically reduces the amount of print waste due to poorly formulated emulsion ink. 
     A restriction valve  51  ensures that the pressure in the conduit  62  is between about 10 psi (about 69 kPa) and about 20 psi (about 138 kPa). The restriction valve  51  ensures that there is adequate pressure in the ink distribution rail  60  and adequate pressure for filling the auxiliary reservoir  65 , when necessary. 
     With reference to FIGS. 3 and 5, the mixing and dispersing apparatus  56  includes a vessel  63  comprising a first circular horizontal wall  64 , and a cylindrically-shaped upper vertical wall  66  having a height of about 21.0 cm and an inner diameter of about 17.8 cm, that together define a cylindrically-shaped upper chamber  68 . 
     The first horizontal wall  64  has a circular opening  70  therein having a diameter of about 6.4 cm. The vessel  63  also includes a cylindrically-shaped lower vertical wall  72  having an inner diameter of about 13.8 cm, that is disposed directly below the first horizontal wall  64 . The first horizontal wall  64 , the cylindrically-shaped lower vertical wall  72 , and a second circular horizontal wall  74 , together define a cylindrically-shaped lower chamber  76 . The second circular horizontal wall  74  has a substantially square-shaped opening  78  therein, having dimensions of about 8.0 by 8.0 cm, that leads to a gear pump  80 , driven by a motor  81  (FIG.  2 ), that pumps emulsion ink out of the lower chamber  76 . 
     A cup-shaped outer stator  82  is fixedly attached to the first horizontal wall  64  and is perforated by twenty four vertical slots  84  evenly distributed about an outer stator cylindrical wall  86 , having a wall thickness of about 4.8 mm. A cup-shaped inner stator  88  is fixedly attached to the outer stator  82  and is perforated by sixteen vertical slots  90  evenly distributed about an inner stator cylindrical wall  92 , having a wall thickness of about 4.0 mm. Each of the slots  84  and  90  has a height of about 15.9 mm and a width of about 3.4 mm. 
     A high-speed electric motor  94  is disposed above the upper chamber  68  and drives a motor shaft  96  in a clockwise direction as viewed from above, as indicated by an arrow  98 . A propeller  100  is mounted to the motor shaft  96  for rotation therewith and comprises three propeller blades  102  equally angularly spaced apart from one another by 120 degrees and each pitched by an angle of about 20 degrees with respect to the horizontal such that a leading edge  104  of each propeller blade  102  is above a respective trailing edge  106  of each propeller blade  102 . The propeller  100  has a diameter of about 12.7 cm and is mounted to the motor shaft  96  in the upper chamber  68  at a location that is preferably between one half to one full propeller diameter above the first horizontal wall  64 . 
     A rotor  108  (best seen in FIGS. 4 and 6) is mounted to the lower end of the motor shaft  96  for rotation therewith. The rotor  108  includes three horizontal blades  110  that are equally angularly spaced apart from one another by 120 degrees. Each blade  110  includes a downwardly extending inner tooth  112  and a downwardly extending outer tooth  114 . Each inner tooth  112  is disposed radially inwardly of the inner stator wall  92  and each outer tooth  114  is disposed between the inner stator wall  92  and the outer stator wall  86 . A relatively close clearance of about 0.4 mm is provided between the teeth  112 ,  114  and the stator walls  86 ,  92 . 
     In operation, the motor  94  is rotated at a speed of between about 500 and about 4,000 revolutions per minute, and the motor shaft  96 , the rotor  108 , and the propeller  100  rotate at the same speed as the motor  94 . Due to the pitch of the propeller blades  102 , the rotation of the propeller  100  causes the ink and fountain solution in the upper chamber  68  to mix together and to flow downwardly toward the rotor  108 . The rotation of the rotor  108  shears the ink and fountain solution between the rotor teeth  112 ,  114  and the inner and outer stator walls  92  and  86 . This shearing causes the formation of a fine emulsion ink that is dispersed through the slots  90  and  84  in the inner and outer stator walls  92  and  86  into the lower chamber  76 . The emulsion ink is then pumped by the gear pump  80  to the conduit  57  (FIG.  2 ). 
     The propeller  100  pre-mixes the ink and fountain solution together and ensures that the fountain solution added to the upper chamber  68  does not simply sit on top of the ink surface and fail to mix with the ink matrix to form an emulsion ink having the desired water content. The propeller  100  also prevents a cavity from forming above the rotor  108 , that would inhibit ink and fountain solution from flowing into the lower chamber  76 . 
     With reference to FIG. 7, an electronic control system  116 , for regulating the emulsion ink composition for the precision emulsion ink feeding mechanism, comprises a new ink controller  118  and a water content controller  120 . 
     The control system  116  ensures that the liquid level in the mixing and dispersing apparatus  56  is maintained at an acceptable level. The control system  116  comprises the liquid level sensor  40  that includes an overflow sensor  122  and a minimum liquid level sensor  124 , both of which are shown schematically in FIG.  2 . New ink and/or fountain solution is added to the mixing and dispersing apparatus  56  from the ink supply reservoir  34 , the fountain solution supply reservoir  44  and/or the auxiliary reservoir  65  if the liquid level in the mixing and dispersing apparatus  56  is too low, and emulsion ink is discharged from the mixing and dispersing apparatus  56  to the auxiliary reservoir  65  if the liquid level in the mixing and dispersing apparatus  56  is too high. Emulsion ink may also be discharged from the mixing and dispensing apparatus  56  to the auxiliary reservoir  65  if the water content of the emulsion ink is out of tolerance, so that either fresh ink or fountain solution can be added to the mixing and dispersing apparatus  56  from the ink supply reservoir  34  or the fountain solution supply reservoir  44 , respectively, to quickly reformulate the emulsion ink without overflowing the mixing and dispersing apparatus  56 . 
     The water content controller  120  uses both feedback and feedforward control strategies. The feedback control strategy is based on the difference between the desired water content value and the actual water content value, as sensed by the water content sensor  50 , generating an error signal, E, which is input into a PID (proportional, integration, differentiation) controller  121 . 
     The feedforward control strategy includes an input K n , based on the status of the new ink valve  38 , an input K s , based on the press speed, and a water offset input that accounts for evaporation. The input K n  accounts for the fact that, when the new ink valve  38  is open, fresh ink is being fed to the mixing and dispersing apparatus  56 . The input K s  accounts for the fact that, as press speed goes up, more scraped ink will be fed to the mixing and dispersing apparatus  56 . The feedforward control strategy therefore anticipates the expected requirements for fountain solution and minimizes the error, E, that needs to be resolved by the PID controller  121 . Accordingly, response time for necessary adjustments to the emulsion ink is minimized. 
     The operation of the water content controller  120  is also regulated by the new ink controller  118 . For example, when the ink level is low, fresh ink will be pumped into the mixing and dispersing apparatus  56 . This action will lower the water content of the emulsion ink in the ink feed and recirculation system  29 . Hence, more water has to be pumped into the ink feed and recirculation system  29  to maintain a constant level of water content. 
     With reference to FIGS. 8-10, the digitally-controlled gear pump ink injector unit  20  comprises a plurality of positive displacement gear pumps  126  mounted within the ink distribution rail  60 . Each of the positive displacement gear pumps  126  may be any suitable positive displacement pump, such as a diaphragm pump, a reciprocating piston pump, a moving vane pump, or a lobe pump. The positive displacement gear pumps  126  are driven by a single electric motor  128  by means of a single drive shaft  130 . The positive displacement gear pumps  126  can be precisely controlled electronically, thereby providing optimal coverage of the fountain roller  23  with emulsion ink and providing the capability, for example, to proportionally control the flow rate through the positive displacement gear pumps  126  based on press speed. This makes it unnecessary to rely on a metering roll to achieve optimal coverage of the plate cylinder  16 , and makes the printing press  10  reach a steady state fairly quickly. 
     In an alternative embodiment, shown schematically in FIG. 11, a proportionally-controlled positive displacement pump ink injector unit  220  comprises a plurality of positive displacement pumps  226  mounted within the ink distribution rail  60 . Each of the positive displacement pumps  226  is independently driven by a separate digitally-controlled electric motor  228  by means of a separate drive shaft  230 , in order to allow a marginal differentiation of the ink feed rate of the positive displacement pumps  226 , for example, to compensate for the effects of what are commonly known as starvation and ghosting. 
     Numerous modifications and alternative embodiments of the invention will be apparent to those skilled in the art in view of the foregoing description. Accordingly, this description is to be construed as illustrative only and is for the purpose of teaching those skilled in the art the best mode of carrying out the invention. The details of the structure may be varied substantially without departing from the spirit of the invention, and the exclusive use of all modifications which come within the scope of the appended claims is reserved.