Patent Publication Number: US-7216946-B2

Title: Ink feeding rate control method and data correcting method for a printing machine

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   This invention relates to an ink feeding rate control method for controlling an ink feeding rate for each area corresponding to an ink key of an ink feeder in a printing machine based on measurement information on detecting patches printed on prints. Further, the invention relates to a data correcting method for a printing machine for correcting data such as measurement information on detecting patches in order to control an ink feeding rate and/or a dampening water feeding rate. 
   2. Description of the Related Art 
   Such a printing machine includes ink feeders for adjusting the rates of feeding inks to ink rollers. Each ink feeder has a plurality of ink keys juxtaposed in a direction perpendicular to a direction for transporting printing paper in time of printing. The rate of feeding ink to the ink rollers is adjusted by varying the opening degree of each ink key. In this way, the rate of feeding ink ultimately to a printing plate is adjusted. 
   The printing plate has regions called detecting patches or control strips formed in positions corresponding to the respective ink keys. The opening degree of each ink key is adjusted by measuring, with a densitometer, the color density of a corresponding detecting patch actually printed on printing paper (see Japanese Unexamined Patent Publication No. 2002-355950, for example). 
     FIG. 11  is an explanatory view schematically showing detecting patches P 1 , P 2 , P 3  and P 4  printed on printing paper S acting as a print. 
   Each of areas E 1 , E 2  and so on of the printing paper S corresponding to the respective ink keys of ink feeders has printed therein, for example, a detecting patch P 1  corresponding to cyan ink, a detecting patch P 2  corresponding to magenta ink, a detecting patch P 3  corresponding to yellow ink, and a detecting patch P 4  corresponding to black ink. 
   Specifically, as shown in  FIG. 11 , where a plurality of cyan images I are printed on the printing paper S, the area E 1  has no images I in an area e 1  aligned with the detecting patch P 1  in the printing direction (i.e. a rectangular area having substantially the same width as the detecting patch and extending in the printing direction). On the other hand, the area E 2  has a plurality of images I arranged in an area e 2  aligned with the detecting patch P 1  in the printing direction (i.e. a rectangular area having substantially the same width as the detecting patch and extending in the printing direction). 
   With such prints, cyan ink is little consumed in an image area of a printing plate corresponding to the area e 1 . The detecting patch P 1  in the area E 1  has a relatively high color density of cyan ink. Thus, the ink feeder is controlled to reduce the feeding rate of cyan ink for the area E 1 . Conversely, cyan ink is consumed in a relatively large quantity in an image area of the printing plate corresponding to the area e 2 . The detecting patch P 1  in the area E 2  has a relatively low color density of cyan ink. Thus, the ink feeder is controlled to increase the feeding rate of cyan ink for the area E 2 . 
   There can be a difference between an image area of areas e 1 , e 2  and so on aligned with the detecting patches P 1 , P 2 , P 3  and P 4  in the printing direction (i.e. rectangular areas having substantially the same width as the detecting patches and extending in the printing direction) and an average image area of the areas E 1 , E 2  and so on having these detecting patches P 1 , P 2 , P 3  and P 4  arranged therein. In such a case, the ink feeding rate cannot be controlled accurately. Such a drawback constitutes a serious problem particularly where each ink feeder of the printing machine has a small number of ink rollers with a low effect of ink distribution. 
   SUMMARY OF THE INVENTION 
   The object of this invention, therefore, is to provide an ink feeding rate control method and a data correcting method for a printing machine capable of accurately controlling an ink feeding rate regardless of images to be printed. 
   The above object is fulfilled, according to this invention, by an ink feeding rate control method for controlling a feeding rate of ink for each of areas corresponding to ink keys of an ink feeder in a printing machine, by comparing measurement information and reference information on detecting patches printed on prints, the method comprising the steps of determining an average of image area ratios of images in the areas on the prints corresponding to the ink keys, determining an average of image area ratios of images in positions aligned in a printing direction with the detecting patches printed in the areas on the prints corresponding to the ink keys, and correcting one of the reference information and the measurement information based on the average of image area ratios of the images in the areas on the prints corresponding to the ink keys, and the average of image area ratios of the images in the positions aligned in the printing direction with the detecting patches printed on the prints. 
   This ink feeding rate control method can control the ink feeding rate accurately and easily regardless of the images to be printed. It is thus possible to avoid the drawback noted hereinbefore that the ink feeding rate cannot be controlled accurately. 
   In a preferred embodiment of the invention, the measurement information on the detecting patches comprises densities of the detecting patches, and the reference information comprises reference densities. 
   In another preferred embodiment, the reference information or measurement information is corrected by using a correction factor obtained empirically. 
   Preferably, a corrected value of the reference information or measurement information is stored from time to time, and the reference information or measurement information is corrected in time of subsequent printing processes by using the corrected value stored. 
   In another aspect of this invention, an ink feeding rate control method is provided for a printing machine having an image recorder for recording images on a printing plate based on image data, for controlling a feeding rate of ink for each of areas corresponding to ink keys of the ink feeder by comparing measurement information and reference information on detecting patches printed on prints, the method comprising the steps of determining, from the image data, an average of image area ratios of images in the areas on the prints corresponding to the ink keys, determining, from the image data, an average of image area ratios of images in positions aligned in a printing direction with the detecting patches printed in the areas on the prints corresponding to the ink keys, and correcting one of the reference information and the measurement information based on the average of image area ratios of the images in the areas on the prints corresponding to the ink keys, and the average of image area ratios of the images in the positions aligned in the printing direction with the detecting patches printed on the prints. 
   In a further aspect of this invention, a data correcting method for a printing machine is provided for correcting one of measurement information and predetermined reference information when controlling the printing machine by comparing the measurement information and the reference information, the measurement information being obtained by measuring detecting patches printed on prints and corresponding to ink keys of the printing machine, the method comprising the steps of determining an average of image area ratios of images in the areas on the prints corresponding to the ink keys, determining an average of image area ratios of images in positions aligned in a printing direction with the detecting patches printed in the areas on the prints corresponding to the ink keys, and correcting one of the reference information and the measurement information based on the average of image area ratios of the images in the areas on the prints corresponding to the ink keys, and the average of image area ratios of the images in the positions aligned in the printing direction with the detecting patches printed on the prints. 
   Other features and advantages of the invention will be apparent from the following detailed description of the embodiments of the invention. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For the purpose of illustrating the invention, there are shown in the drawings several forms which are presently preferred, it being understood, however, that the invention is not limited to the precise arrangement and instrumentalities shown. 
       FIG. 1  is a schematic side view of a printing machine according to this invention; 
       FIG. 2A  is an explanatory view showing an arrangement of image areas on a printing plate, one for printing in black ink and the other for printing in magenta ink; 
       FIG. 2B  is an explanatory view showing an arrangement of image areas on a printing plate, one for printing in cyan ink and the other for printing in yellow ink; 
       FIG. 3  is a schematic side view of an ink source; 
       FIG. 4  is a plan view of the ink source; 
       FIG. 5  is a schematic side view of a dampening water feeder; 
       FIG. 6  is a schematic side view of an image pickup station shown with chains; 
       FIG. 7  is a flow chart of prepress and printing operations of the printing machine; 
       FIG. 8  is a flow chart of a prepress process; 
       FIG. 9  is a block diagram of a principal electrical structure of the printing machine for implementing an ink feeding rate control method according to this invention; 
       FIG. 10  is a flow chart showing steps in the ink feeding rate control method according to this invention; and 
       FIG. 11  is an explanatory view schematically showing detecting patches printed on printing paper. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   An embodiment of the invention will be described hereinafter with reference to the drawings. 
   &lt;Construction of Printing Machine&gt; 
   A construction of a printing machine according to this invention will be described first.  FIG. 1  is a schematic view of the printing machine according to this invention. 
   This printing machine records images on blank plates mounted on first and second plate cylinders  11  and  12 , feeds inks to the plates having the images recorded thereon, and transfers the inks from the plates through first and second blanket cylinders  13  and  14  to printing paper held on an impression cylinder  15 , thereby printing the images on the printing paper. 
   The first plate cylinder  11  is movable between a first printing position shown in a solid line and an image recording position shown in a two-dot chain line in  FIG. 1 . The second plate cylinder  12  is movable between a second printing position shown in a solid line in  FIG. 1  and the same image recording position. 
   Around the first plate cylinder  11  in the first printing position are an ink feeder  20   a  for feeding an ink of black (K), for example, to the plate, an ink feeder  20   b  for feeding an ink of magenta (M), for example, to the plate, and dampening water feeders  21   a  and  21   b  for feeding dampening water to the plate. Around the second plate cylinder  12  in the second printing position are an ink feeder  20   c  for feeding an ink of cyan (C), for example, to the plate, an ink feeder  20   d  for feeding an ink of yellow (Y), for example, to the plate, and dampening water feeders  21   c  and  21   d  for feeding dampening water to the plate. Further, around the first or second plate cylinder  11  or  12  in the image recording position are a plate feeder  23 , a plate remover  24 , an image recorder  25  and a developing device  26 . 
   The first blanket cylinder  13  is contactable with the first plate cylinder  11 , while the second blanket cylinder  14  is contactable with the second plate cylinder  12 . The impression cylinder  15  is contactable with the first and second blanket cylinders  13  and  14  in different positions. The machine further includes a paper feed cylinder  16  for transferring printing paper supplied from a paper storage  27  to the impression cylinder  15 , a paper discharge cylinder  17  with chains  19  wound thereon for discharging printed paper from the impression cylinder  15  to a paper discharge station  28 , an image pickup station  40  for measuring color densities of detecting patches printed on the printing paper, and a blanket cleaning unit  29 . 
   Each of the first and second plate cylinders  11  and  12  is coupled to a plate cylinder moving mechanism not shown, and driven by this moving mechanism to reciprocate between the first or second printing position and the image recording position. In the first printing position, the first plate cylinder  11  is driven by a motor not shown to rotate synchronously with the first blanket cylinder  13 . In the second printing position, the second plate cylinder  12  is rotatable synchronously with the second blanket cylinder  14 . Adjacent the image recording position is a plate cylinder rotating mechanism, not shown, for rotating the first or second plate cylinder  11  or  12  whichever is in the image recording position. 
   The plate feeder  23  and plate remover  24  are arranged around the first or second plate cylinder  11  or  12  in the image recording position. 
   The plate feeder  23  includes a supply cassette  63  storing a roll of elongate blank plate in light-shielded state, a guide member  64  and guide rollers  65  for guiding a forward end of the plate drawn from the cassette  63  to the surface of the first or second plate cylinder  11  or  12 , and a cutter  66  for cutting the elongate plate into sheet plates. Each of the first and second plate cylinders  11  and  12  has a pair of clamping jaws, not shown, for clamping the forward and rear ends of the plate fed from the plate feeder  23 . 
   The plate remover  24  has a blade mechanism  73  for separating a plate from the first or second plate cylinder  11  or  12  after a printing operation, a discharge cassette  68 , and a conveyor mechanism  69  for transporting the plate separated by the blade mechanism  73  to the discharge cassette  68 . 
   The forward end of the plate drawn from the feeder cassette  63  is guided by the guide rollers  65  and guide member  64 , and clamped by one of the clamping jaws on the first or second plate cylinder  11  or  12 . Then, the first or second plate cylinder  11  or  12  is rotated by the plate cylinder rotating mechanism not shown, whereby the plate is wrapped around the first or second plate cylinder  11  or  12 . The rear end of the plate cut by the cutter  66  is clamped by the other clamping jaw. While, in this state, the first or second plate cylinder  11  or  12  is rotated at low speed, the image recorder  25  irradiates the surface of the plate mounted peripherally of the first or second plate cylinder  11  or  12  with a modulated laser beam for recording images thereon. 
   On the plate P mounted peripherally of the first plate cylinder  11 , the image recorder  25 , as shown in  FIG. 2A , records an image area  67   a  to be printed with black ink, and an image area  67   b  to be printed with magenta ink. On the plate P mounted peripherally of the second plate cylinder  12 , the image recorder  25 , as shown in  FIG. 2B , records an image area  67   c  to be printed with cyan ink, and an image area  67   d  to be printed with yellow ink. The image areas  67   a  and  67   b  are recorded in evenly separated positions, i.e. in positions separated from each other by 180 degrees, on the plate P mounted peripherally of the first plate cylinder  11 . Similarly, the image areas  67   c  and  67   d  are recorded in evenly separated positions, i.e. in positions separated from each other by 180 degrees, on the plate P mounted peripherally of the second plate cylinder  12 . 
   Referring again to  FIG. 1 , the ink feeders  20   a  and  20   b  are arranged around the first plate cylinder  11  in the first printing position, while the ink feeders  20   c  and  20   d  are arranged around the second plate cylinder  12  in the second printing position, as described hereinbefore. Each of these ink feeders  20   a ,  20   b ,  20   c  and  20   d  (which may be referred to collectively as “ink feeders  20 ”) includes a plurality of ink rollers  71  and an ink source  72 . 
   The ink rollers  71  of the ink feeders  20   a  and  20   b  are swingable by action of cams or the like not shown. With the swinging movement, the ink rollers  71  of the ink feeder  20   a  or  20   b  come into contact with one of the two image areas  67   a  and  67   b  formed on the plate P mounted peripherally of the first plate cylinder  11 . Thus, the ink is fed only to an intended one of the image areas  67   a  and  67   b . Similarly, the ink rollers  71  of the ink feeders  20   c  and  20   d  are swingable by action of cams or the like not shown. With the swinging movement, the ink rollers  71  of the ink feeder  20   c  or  20   d  come into contact with one of the two image areas  67   c  and  67   d  formed on the plate P mounted peripherally of the second plate cylinder  12 . Thus, the ink is fed only to an intended one of the image areas  67   c  and  67   d.    
     FIG. 3  is a schematic side view of the ink source  72  noted above.  FIG. 4  is a plan view thereof. Ink  3  is omitted from  FIG. 4 . 
   The ink source  72  includes an ink fountain roller  1  having an axis thereof extending in a direction of width of printed matter (i.e. perpendicular to a printing direction of the printing machine), and ink keys  2  ( 1 ),  2  ( 2 ) . . .  2  (L) arranged in the direction of width of the printed matter. (In this specification, these ink keys may be collectively called “ink keys  2 ”.) The ink keys  2  correspond in number to the number L of areas divided in the direction of width of the printed matter. Each of the ink keys  2  has an adjustable opening degree with respect to the outer periphery of the ink fountain roller  1 . The ink fountain roller  1  and ink keys  2  define an ink well for storing ink  3 . 
   Eccentric cams  4 , L in number, are arranged under the respective ink keys  2  for pressing the ink keys  2  toward the surface of ink fountain roller  1  to vary the opening degree of each ink key  2  with respect to the ink fountain roller  1 . The eccentric cams  4  are connected through shafts  5  to pulse motors  6 , L in number, for rotating the eccentric cams  4 , respectively. 
   Each pulse motor  6 , in response to an ink key drive pulse applied thereto, rotates the eccentric cam  4  about the shaft  5  to vary a pressure applied to the ink key  2 . The opening degree of the ink key  2  with respect to the ink fountain roller  1  is thereby varied to vary the rate of ink fed to the printing plate. 
   Referring again to  FIG. 1 , the dampening water feeders  21   a ,  21   b ,  21   c  and  21   d  (which may be referred to collectively as “dampening water feeders  21 ”) feed dampening water to the plates P before the ink feeders  20  feed the inks thereto. Of the dampening water feeders  21 , the water feeder  21   a  feeds dampening water to the image area  67   a  on the plate P, the water feeder  21   b  feeds dampening water to the image area  67   b  on the plate P, the water feeder  21   c  feeds dampening water to the image area  67   c  on the plate P, and the water feeder  21   d  feeds dampening water to the image area  67   d  on the plate P. 
     FIG. 5  is a schematic side view of the dampening water feeder  21   b.    
   The dampening water feeder  21   b  includes a water source having a water vessel  31  for storing dampening water and a water fountain roller  32  rotatable by a motor, not shown, and two water rollers  33  and  34  for transferring dampening water from the fountain roller  32  to the surface of the plate mounted peripherally of the first plate cylinder  11 . This dampening water feeder is capable of adjusting the rate of feeding dampening water to the surface of the plate by varying the rotating rate of fountain roller  32 . 
   The three other water feeders  21   a ,  21   c  and  21   d  have the same construction as the water feeder  21   b.    
   Referring again to  FIG. 1 , the developing device  26  is disposed under the first plate cylinder  11  or second plate cylinder  12  in the image recording position. This developing device  26  includes a developing unit, a fixing unit and a squeezing unit, which are vertically movable between a standby position shown in two-dot chain lines and a developing position shown in solid lines in  FIG. 1 . 
   In developing the images recorded on the plate P by the image recorder  25 , the developing unit, fixing unit and squeezing unit are successively brought into contact with the plate P rotated with the first or second plate cylinder  11  or  12 . 
   The first and second blanket cylinders  13  and  14  movable into contact with the first and second plate cylinders  11  and  12  have the same diameter as the first and second plate cylinders  11  and  12 , and have ink transfer blankets mounted peripherally thereof. Each of the first and second blanket cylinders  13  and  14  is movable into and out of contact with the first or second plate cylinder  11  or  12  and the impression cylinder  15  by a contact mechanism not shown. 
   The blanket cleaning unit  29  disposed between the first and second blanket cylinders  13  and  14  cleans the surfaces of the first and second blanket cylinders  13  and  14  by feeding a cleaning solution to an elongate cleaning cloth extending from a delivery roll to a take-up roll through a plurality of pressure rollers, and sliding the cleaning cloth in contact with the first and second blanket cylinders  13  and  14 . 
   The impression cylinder  15  contactable by the first and second blanket cylinders  13  and  14  has half the diameter of the first and second plate cylinders  11  and  12  and the first and second blanket cylinders  13  and  14 , as noted hereinbefore. Further, the impression cylinder  15  has a gripper, not shown, for holding and transporting the forward end of printing paper. 
   The paper feed cylinder  16  disposed adjacent the impression cylinder  15  has the same diameter as the impression cylinder  15 . The paper feed cylinder  16  has a gripper, not shown, for holding and transporting the forward end of each sheet of printing paper fed from the paper storage  27  by a reciprocating suction board  74 . When the printing paper is transferred from the feed cylinder  16  to the impression cylinder  15 , the gripper of the impression cylinder  15  holds the forward end of the printing paper which has been held by the gripper of the feed cylinder  16 . 
   The paper discharge cylinder  17  disposed adjacent the impression cylinder  15  has the same diameter as the impression cylinder  15 . The discharge cylinder  17  has a pair of chains  19  wound around opposite ends thereof. The chains  19  are interconnected by coupling members, not shown, having a plurality of grippers  41  arranged thereon. When the impression cylinder  15  transfers the printing paper to the discharge cylinder  17 , one of the grippers  41  of the discharge cylinder  17  holds the forward end of the printing paper having been held by the gripper of the impression cylinder  15 . With movement of the chains  19 , color densities of the detecting patches printed on the printing paper are measured at the image pickup station  40 . Thereafter the printing paper is transported to the paper discharge station  28  to be discharged thereon. 
   The paper feed cylinder  16  is connected to a drive motor through a belt not shown. The paper feed cylinder  16 , impression cylinder  15 , paper discharge cylinder  17  and the first and second blanket cylinders  13  and  14  are coupled to one another by gears mounted on end portions thereof, respectively. Further, the first and second blanket cylinders  13  and  14  are coupled to the first and second plate cylinders  11  and  12  in the first and second printing positions, respectively, by gears mounted on end portions thereof. Thus, a motor, not shown, is operable to rotate the paper feed cylinder  16 , impression cylinder  15 , paper discharge cylinder  17 , the first and second blanket cylinders  13  and  14  and the first and second plate cylinders  11  and  12  synchronously with one another. 
     FIG. 6  is a schematic side view of the image pickup station  40  for measuring color densities of the detecting patches printed on the printing paper, which is shown with the chains  19 . 
   The pair of chains  19  are endlessly wound around the opposite ends of the paper discharge cylinder  17  shown in  FIG. 1  and a pair of large sprockets  18 . As noted hereinbefore, the chains  19  are interconnected by coupling members, not shown, having a plurality of grippers  41  arranged thereon each for gripping a forward end of printing paper S transported. 
   The pair of chains  19  have a length corresponding to a multiple of the circumference of paper discharge cylinder  17 . The grippers  41  are arranged on the chains  19  at intervals each corresponding to the circumference of paper discharge cylinder  17 . Each gripper  41  is opened and closed by a cam mechanism, not shown, synchronously with the gripper on the paper discharge cylinder  17 . Thus, each gripper  41  receives printing paper S from the paper discharge cylinder  17 , transports the printing paper S with rotation of the chains  19 , and discharges the paper S to the paper discharge station  28 . 
   The printing paper S is transported with only the forward end thereof held by one of the grippers  41 , the rear end of printing paper S not being fixed. Consequently, the printing paper S could flap during transport, which impairs an operation, to be described hereinafter, of the image pickup station  40  to measure densities of the detecting patches. To avoid such an inconvenience, this printing machine provides a suction roller  43  disposed upstream of the paper discharge station  28  for stabilizing the printing paper S transported. 
   The suction roller  43  is in the form of a hollow roller having a surface defining minute suction bores, with the hollow interior thereof connected to a vacuum pump not shown. The suction roller  43  is disposed to have an axis thereof extending parallel to the grippers  41  bridging the pair of chains  19 , a top portion of the suction roller  43  being substantially at the same height as a lower run of the chains  19 . 
   The suction roller  43  is driven to rotate or freely rotatable in a matching relationship with a moving speed of the grippers  41 . Thus, the printing paper S is drawn to the surface of the suction roller  43 , thereby being held against flapping when passing over the suction roller  43 . In place of the suction roller  43 , a suction plate may be used to suck the printing paper S two-dimensionally. 
   The image pickup station  40  includes an illuminating unit  44  for illuminating the printing paper S transported, and an image pickup unit  45  for picking up images of the detecting patches on the printing paper S illuminated by the illuminating unit  44  and measuring color densities of the patches. The illuminating unit  44  is disposed between the upper and lower runs of chains  19  to extend along the suction roller  43 , and has a plurality of linear light sources for illuminating the printing paper S over the suction roller  43 . 
   The image pickup unit  45  includes a light-shielding and dustproof case  46 , and a mirror  49 , a lens  48  and a CCD line sensor  47  arranged inside the case  46 . The image pickup unit  45  picks up the image of printing paper S over the suction roller  43  through slits of the illuminating unit  44 . Incident light of the image reflected by the mirror  49  passes through the lens  48  to be received by the CCD line sensor  47 . 
   The prepress and printing operations of the printing machine will be described next.  FIG. 7  is a flow chart showing an outline of the prepress and printing operations of the printing machine. These prepress and printing operations are directed to multicolor printing of printing paper S with the four color inks of yellow, magenta, cyan and black. 
   First, the printing machine executes a prepress process for recording and developing images on the plates P mounted on the first and second plate cylinders  11  and  12  (step S 1 ). This prepress process follows the steps constituting a subroutine as shown in the flow chart of  FIG. 8 . 
   The first plate cylinder  11  is first moved to the image recording position shown in the two-dot chain line in  FIG. 1 . (step S 11 ). 
   Next, a plate P is fed to the outer periphery of the first plate cylinder  11  (step S 12 ). To achieve the feeding of the plate P, the pair of clamping jaws, not shown, clamp the forward end of plate P drawn from the supply cassette  63 , and the rear end of plate P cut by the cutter  66 . 
   Then, an image is recorded on the plate P mounted peripherally of the first plate cylinder  11  (step S 13 ). For recording the image, the image recorder  25  irradiates the plate P mounted peripherally of the first plate cylinder  11  with a modulated laser beam while the first plate cylinder  11  is rotated at low speed. 
   Next, the image recorded on the plate P is developed (step S 14 ). The developing step is executed by raising the developing device  26  from the standby position shown in two-dot chain lines to the developing position shown in solid lines in  FIG. 1  and thereafter successively moving the developing unit, fixing unit and squeezing unit into contact with the plate P rotating with the first plate cylinder  11 . 
   Upon completion of the developing step, the first plate cylinder  11  is moved to the first printing position shown in the solid line in  FIG. 1  (step S 15 ). 
   Subsequently, the printing machine carries out an operation similar to steps S 11  to S 15  by way of a prepress process for the plate P mounted peripherally of the second plate cylinder  12  (steps S 16  to S 20 ). Completion of the prepress steps for the plates P mounted peripherally of the first and second plate cylinders  11  and  12  brings the prepress process to an end. 
   Referring again to  FIG. 7 , the prepress process is followed by a printing process for printing the printing paper S with the plates P mounted on the first and second plate cylinders  11  and  12  (step S 2 ). This printing process is carried out as follows. 
   First, each dampening water feeder  21  and each ink feeder  20  are placed in contact with only a corresponding one of the image areas on the plates P mounted on the first and second plate cylinders  11  and  12 . Consequently, dampening water and inks are fed to the image areas  67   a ,  67   b ,  67   c  and  67   d  from the corresponding water feeders  21  and ink feeders  20 , respectively. These inks are transferred from the plates P to the corresponding regions of the first and second blanket cylinders  13  and  14 , respectively. 
   Then, the printing paper S is fed to the paper feed cylinder  16 . The printing paper S is subsequently passed from the paper feed cylinder  16  to the impression cylinder  15 . The impression cylinder  15  continues to rotate in this state. Since the impression cylinder  15  has half the diameter of the first and second plate cylinders  11  and  12  and the first and second blanket cylinders  13  and  14 , the black and cyan inks are transferred to the printing paper S wrapped around the impression cylinder  15  in its first rotation, and the magenta and yellow inks in its second rotation. 
   The forward end of the printing paper S printed in the four colors is passed from the impression cylinder  15  to the paper discharge cylinder  17 . This printing paper S is transported by the pair of chains  19  toward the paper discharge station  28 . After the color densities of the detecting patches are measured at the image pickup station  40 , the printing paper S is discharged to the paper discharge station  28 . 
   Upon completion of the printing process, the plates P used in the printing are removed (step S 3 ). To remove the plates P, the first plate cylinder  11  is first moved to the image recording position shown in the two-dot chain line in  FIG. 1 . Then, while the first plate cylinder  11  is rotated counterclockwise, the blade mechanism  73  separates an end of the plate P from the first plate cylinder  11 . The plate P separated is guided by the conveyor mechanism  69  into the discharge cassette  68 . After returning the first plate cylinder  11  to the first printing position, the second plate cylinder  12  is moved from the second printing position to the image recording position to undergo an operation similar to the above, thereby having the plate P removed from the second plate cylinder  12  for discharge into the discharge cassette  68 . 
   Upon completion of the plate removing step, the first and second blanket cylinders  13  and  14  are cleaned by the blanket cleaning unit  29  (step S 4 ). 
   After completing the cleaning of the first and second blanket cylinders  13  and  14 , the printing machine determines whether or not a further image is to be printed (step S 5 ). If a further printing operation is required, the machine repeats steps S 1  to S 4 . 
   If the printing operation is ended, the printing machine cleans the inks (step S 6 ). For cleaning the inks, an ink cleaning device, not shown, provided for each ink feeder  20  removes the ink adhering to the ink rollers  71  and ink source  72  of each ink feeder  20 . 
   With completion of the ink cleaning step, the printing machine ends the entire process. 
   &lt;Ink Feeding Rate Control Method According to This Invention&gt; 
   An ink feeding rate control method according to this invention will be described next.  FIG. 9  is a block diagram of a principal electrical structure of the above printing machine for implementing the ink feeding rate control method according to this invention. 
   The above printing machine includes a control unit  140  for controlling the entire machine. The control unit  140  is connected to the ink feeders  20  and image recorder  25 . The control unit  140  is connected also to the image pickup station  40  through an image processing unit  141 . The control unit  140  is arranged to receive OK sheet data such as density levels obtained by measuring an OK sheet corresponding to a target print. The control unit  140  and image recorder  25  receive image data corresponding to images to be recorded by the image recorder  25 . The image data inputted to the control unit  140  may be relatively coarse image data such as PPF data. The image data inputted to the image recorder  25  is high-density image data passed through a RIP (raster image processor). 
     FIG. 10  is a flow chart showing steps in the ink feeding rate control method according to this invention. 
   In performing the ink feeding rate control method according to this invention, a reference density is first set for each of the detecting patches P 1 , P 2 , P 3  and P 4  (step S 21 ). This reference density is set for each color based, for example, on the type of printing paper (whether coated paper or wood free paper). 
   Next, an average of image area ratios in the areas E 1 , E 2  and so on corresponding to the respective ink keys  2  shown in  FIGS. 3 and 4  is calculated for each color of magenta, yellow, cyan and black (step S 22 ). The image area ratios are calculated by using the image data inputted to the control unit  140 . 
   Next, an average of image area ratios of images in positions aligned in the printing direction with the detecting patches P 1 , P 2 , P 3  and P 4 , shown in  FIG. 11 , printed in the areas E 1 , E 2  and so on of the printing paper S corresponding to the respective ink keys  2 , is calculated for each color of magenta, yellow, cyan and black (step S 23 ). For cyan, for example, an average is calculated of image area ratios in the areas e 1 , e 2  and so on, each aligned with the detecting patch P 1  in the printing direction indicated by an arrow in  FIG. 11  (i.e. rectangular areas having substantially the same width as the detecting patches and extending in the printing direction). These image area ratios also are calculated by using the image data inputted to the control unit  140 . 
   Then, a value of reference density set beforehand for each of the detecting patches P 1 , P 2 , P 3  and P 4  is corrected based on the average of image area ratios in the areas E 1 , E 2  and so on corresponding to the respective ink keys  2 , and the average of image area ratios of images in the positions aligned in the printing direction with the detecting patches P 1 , P 2 , P 3  and P 4  printed on the printing paper S (step S 24 ). 
   Linear approximation, for example, is used in determining a corrected value for the reference density. That is, a corrected value is derived from the equation set out below, where Dp is a corrected reference density of each of the detecting patches P 1 , P 2 , P 3  and P 4 , Dt is a target density (reference density of ink) of each of the detecting patches P 1 , P 2 , P 3  and P 4  determined from the type of ink or paper, So is an average of image area ratios of each color in the areas E 1 , E 2  and so on corresponding to the respective ink keys  2 , Sp is an average of image area ratios of images in the positions aligned in the printing direction with the detecting patches P 1 , P 2 , P 3  and P 4  in the areas E 1 , E 2  and so on, and “a” is a correction factor. 
   Specifically, to control the feeding rate of cyan ink, a corrected value is derived from the equation set out hereunder by using the corrected reference density Dp of the detecting patches P 1 , target density Dt, average values So and Sp of the image areas for to the color of cyan, and the correction factor “a”. Similarly, to control the feeding rate of magenta ink, a corrected value is derived from the equation set out hereunder by using the corrected reference density Dp of the detecting patches P 2 , target density Dt, average values So and Sp of the image areas for the color of magenta, and the correction factor “a”. To control the feeding rate of yellow ink, a corrected value is derived from the equation set out hereunder by using the corrected reference density Dp of the detecting patches P 3 , target density Dt, average values So and Sp of the image areas for the color of yellow, and the correction factor “a”. Further, to control the feeding rate of black ink, a corrected value is derived from the equation set out hereunder by using the corrected reference density Dp of the detecting patches P 4 , target density Dt, average values So and Sp of the image areas for the color of black, and the correction factor “a”.
 
 Dp=a ·( So−Sp )+ Dt 
 
   The correction factor “a” is empirically determined beforehand by using print samples of image area ratios of each color arranged at a plurality of stages. Curve approximation may be used instead of the linear approximation noted above, or corrected values of reference density may be determined based on data stored in a look-up table. 
   Once corrected values of reference density are determined, ink feeding rates are controlled in time of printing by using the corrected values reference density (step S 25 ). That is, the printing machine starts a printing operation. Then, density levels of the detecting patches P 1 , P 2 , P 3  and P 4  on the printing paper S immediately after printing are measured at the image pickup station  40 . The density levels measured are compared with the reference density corrected in step S 24 . Based on results of the comparison, ink feeding rates are adjusted by driving the pulse motor  6  of the ink source  72  in each ink feeder  20  shown in  FIG. 3 . 
   When required prints have been made (step S 26 ), it is determined whether the correction of reference density made at this time should be reflected on subsequent printing operations (step S 27 ). That is, the correction factor “a” and other data are stored in the memory of the control unit  140  or the like when it is decided that the data concerning the corrections, such as the correction factor “a”, are stored at appropriate times for use in optimizing the reference density for subsequent printing operations (step S 28 ). On the other hand, when it is decided not to use the latest correction of reference density in subsequent printing operations, the current printing operation is terminated with no further action. 
   In the foregoing embodiment, density levels of the detecting patches P 1 , P 2 , P 3  and P 4  are used as measurement information on the detecting patches P 1 , P 2 , P 3  and P 4 . Instead, color information other than density may be used. Further, the detecting patches P 1 , P 2 , P 3  and P 4  may be not only solid patches but also dot patches or line patches. 
   In the foregoing embodiment, ink feeding rates are automatically controlled by controlling the ink feeders  20  based on collected values of reference density. Instead, the operator may adjust the ink feeding rates from the ink sources  72  directly by using the corrected values of reference density. 
   &lt;Other Embodiments&gt; 
   Reference density is corrected in the foregoing embodiment. Instead of correcting reference density, the density measured of each detecting patch may be corrected. Corrected values may be derived from the approximation set out hereunder, for example, where Nt is a density level measured of each detecting patch, Np is a corrected density level of each detecting patch, Mo is an average of image area ratios of each color in the areas E 1 , E 2  and so on corresponding to the respective ink keys  2 , Mp is an average of image area ratios of images in the positions aligned in the printing direction with the detecting patches P 1 , P 2 , P 3  and P 4  (i.e. rectangular areas having substantially the same width as the detecting patches and extending in the printing direction) in the areas E 1 , E 2  and so on, and “b” is a correction factor.
 
 Np=b ·( Mp−Mo )+ Nt 
 
   Of course, curve approximation or a look-up table may be used instead of linear approximation. 
   In the foregoing embodiment, ink feeding rates are controlled based on density levels of the detecting patches. This invention is applicable not only to the control of the ink feeding rates but also to the control of dampening water feeding rates. To control the dampening water feeding rate in the above printing machine, the rate of feeding dampening water to the surface of a printing plate may be adjusted by varying the rotating rate of each fountain roller  32  shown in  FIG. 5 . A method of controlling the feeding rate of dampening water based on measured density of detecting patches is described in Japanese Unexamined Patent Publication No. 2002-355950, and will not particularly be described herein. 
   This invention may be embodied in other specific forms without departing from the spirit or essential attributes thereof and, accordingly, reference should be made to the appended claims, rather than to the foregoing specification, as indicating the scope of the invention. 
   This application claims priority benefit under 35 U.S.C. Section 119 of Japanese Patent Application No. 2003-69790 filed in the Japanese Patent Office on Mar. 14, 2003, the entire disclosure of which is incorporated herein by reference.