FIG. 23 shows the main part of an inker (ink supply device) in a printing unit of each color of a web offset printing press. Referring to FIG. 23, reference numeral 1 denotes an ink fountain; 2, ink stored in the ink fountain 1; 3, an ink fountain roller; 4 (4-1 to 4-n), a plurality of ink fountain keys juxtaposed in the axial direction of the ink fountain roller 3; 5, an ink ductor roller; 6, an ink roller group; 7, a printing plate; 8, a plate cylinder on which the printing plate 7 is mounted; 9, a blanket cylinder; and 10, an impression cylinder.
This ink supply device supplies the ink 2 in the ink fountain 1 to the ink fountain roller 3 by adjusting the opening ratios of the ink fountain keys 4-1 to 4-n, and supplies, via the ink roller group 6, the ink supplied to the ink fountain roller 3 to the printing plate 7 by the feed operation of the ink ductor roller 5.
An image is printed on the printing plate 7. The ink supplied to the printing plate 7 is received by the blanket cylinder 9. The ink received by the blanket cylinder 9 is transferred to printing paper (target printing material) 11 conveyed between the blanket cylinder 9 and the impression cylinder 10.
Note that ink form rollers 6-1 to 6-4 in contact with the printing plate 7 are provided at the end of the ink flow path of the ink roller group 6. Together with the ink supplied via the ink form rollers 6-1 to 6-4, dampening water stored in a fountain pan 13 is supplied to the printing plate 7 via a dampening form roller 12.
In this ink supply device, when a print job is switched, that is, when the printing plate 7 of a preceding print job is exchanged with a printing plate 7′ of the next print job, the opening ratios of the ink fountain keys 4-1 to 4-n and the rotation amount of the ink fountain roller 3 are changed to values corresponding to the image of the printing plate 7′ of the next print job, and the ink 2 in the ink fountain 1 is supplied to the exchanged printing plate 7′ via the ink roller group 6. In this case, test printing is performed before final printing, and the ink supply amount is adjusted, thereby obtaining a satisfactory color tone. A desired ink film thickness distribution (the gradient of an ink film thickness) is thus formed on the ink roller group 6, the plate cylinder 8, and the blanket cylinder 9.
However, in this ink supply device, when exchanging the printing plate 7 with the printing plate 7′ and executing the next print job, the ink film thickness distribution for the printing plate 7 of the preceding print job still remains on the ink roller group 6. In this case, the ink film thickness distribution for the printing plate 7 of the preceding print job needs to be gradually changed to the ink film thickness distribution for the printing plate 7′ of the next print job. Excessive ink supply amount adjustment and test printing are needed until a satisfactory color tone is obtained. This poses problems such as “an increase in the preparation time before printing”, “an increase in working load”, “a waste of printing materials”, “a decrease in production efficiency”, and “an increase in cost”.
Hence, aiming at decreasing the numbers of times of ink supply amount adjustment and test printing until a satisfactory color tone is obtained, “ink film thickness control methods” described in patent literatures 1 and 2 have been proposed.
[Patent Literature 1 (Ink Decrease+Preinking 2)]
In the ink film thickness control method described in patent literature 1, when switching a print job, the feed operation of the ink ductor roller 5 is turned off. In a state in which the printing plate 7 of the preceding print job is kept mounted, the printing press is operated to print a predetermined number of sheets (blank sheet printing). The ink in the ink supply device is thus decreased (ink decrease), and a minimum ink film thickness distribution Ma (see FIG. 24A) that is needed during printing and becomes thinner from the upstream to the downstream, that is, the reference ink film thickness distribution Ma corresponding to a portion (a portion where the image area ratio is zero) of the printing plate 7 without any image is left on the ink roller group 6 (ink removing).
Next, the opening ratios of the ink fountain keys 4-1 to 4-n, the rotation amount of the ink fountain roller 3, and the like are set to values corresponding to the image of the printing plate 7′ of the next print job. Then, in a state in which the ink form rollers 6-1 to 6-4 are thrown off, the printing press is operated to cause the ink ductor roller 5 to perform the feed operation a predetermined number of times, thereby superimposing an ink film thickness distribution Mb (see FIG. 24B) corresponding to the image of the printing plate 7′ of the next print job on the reference ink film thickness distribution Ma remaining on the ink roller group 6 (preinking 2).
[Patent Literature 2 (Ink return+Preinking 1)]
In the ink film thickness control method described in patent literature 2, when switching a print job, the opening amounts of the ink fountain keys 4-1 to 4-n are set to zero. In this state, the ink ductor roller 5 is caused to perform the feed operation a predetermined number of times, thereby wholly returning the ink on the ink roller group 6 to the ink fountain 1 (ink return). A state in which each roller in the ink roller group 6 does not hold ink is thus attained.
Next, the opening ratios of the ink fountain keys 4-1 to 4-n are set to a predetermined opening ratio (for example, 50%). In addition, the rotation amount of the ink fountain roller 3 is set to a predetermined amount (for example, 50%). Then, the ink ductor roller 5 is caused to perform the feed operation a predetermined number of times, thereby forming the minimum ink film thickness distribution (reference ink film thickness distribution) Ma (see FIG. 24A) needed during printing on the ink roller group 6 (the first step of preinking 1).
The opening ratios of the ink fountain keys 4-1 to 4-n, the rotation amount of the ink fountain roller 3, and the like are set to values corresponding to the image of the printing plate 7′ of the next print job. Then, in a state in which the ink form rollers 6-1 to 6-4 are thrown off, the printing press is operated to cause the ink ductor roller 5 to perform the feed operation a predetermined number of times, thereby superimposing the ink film thickness distribution Mb (see FIG. 24B) corresponding to the image of the printing plate 7′ of the next print job on the reference ink film thickness distribution Ma formed on the ink roller group 6 (the second step of preinking 1).
FIG. 25 shows a printed product printed by the printing press including the ink supply device. A strip-shaped color bar 11-2 is printed in a margin portion except an image region 11-1 on the printed product (target printing material) 11. In general four-color printing, the color bar 11-2 is formed from density measurement patches (density measurement solid color patches of a percent dot area of 100%) 11a1, 11a2, 11a3, and 11a4 of cyan, magenta, yellow, and black. Regions S1 to Sn correspond to the key zones of the ink fountain keys 4-1 to 4-n in the printing units of the respective colors of the printing press.
[Color Matching]
A reference density is set in advance for the printing unit of each color. That is, a reference density value is set in advance for each of cyan, magenta, yellow, and black. When printing the printed product 11, the opening ratios of the ink fountain keys 4-1 to 4-n in the printing unit of each color are adjusted so as to make the density value of each color match the reference density value. The adjustment of the opening ratios of the ink fountain keys 4-1 to 4-n in the printing unit of each color is performed by an ink supply amount adjustment device (not shown) based on the densities of the density measurement patches 11a (11a1, 11a2, 11a3, and 11a4) of the respective colors printed on the printed product 11.
For example, the region S1 in the printed product 11 will representatively be described. The density values of the density measurement patches 11a of the respective colors on the printed product 11 obtained by test printing or final printing are measured. The density difference between the measured density value of each color and the preset reference density value of the color is obtained. The adjustment value of the opening amount of the ink fountain key 4-1 (the adjustment value of the ink supply amount to the region S1) in the printing unit of each color is obtained from the density difference of the color. The obtained adjustment value (reference adjustment value) is multiplied by a unique coefficient (control ratio) to obtain a correction value. The opening amount of the ink fountain key 4-1 in the printing unit of each color is adjusted using the correction value as a feedback amount.
For the regions S2 to Sn as well, the adjustment values of the opening amounts of the ink fountain keys 4-2 to 4-n (the adjustment values of the ink supply amounts to the regions S2 to Sn) in the printing unit of each color are obtained in a similar manner. The obtained adjustment values (reference adjustment values) are multiplied by a unique coefficient (control ratio) to obtain correction values. The opening amounts of the ink fountain keys 4-2 to 4-n in the printing unit of each color are adjusted using the correction values as feedback amounts.
However, in the ink film thickness control method described in patent literature 1 (ink decrease+preinking 2), since blank sheet printing is performed when leaving the ink film thickness distribution Ma on the ink roller group 6, paper is wasted.
In the ink film thickness control method described in patent literature 2 (ink return+preinking 1), since the ink on the ink roller group 6 is wholly returned to the ink fountain 1, and a corrected ink film thickness distribution (Ma+Mb) is formed from zero, a long time is needed. Additionally, in this method, since emulsified ink (ink blended with dampening water) is returned to the ink fountain 1, a printing failure occurs, and printing materials are wasted.
The present applicant considers an ink supply method that enables to divide the ink roller group 6 in the ink supply device into the upstream side and the downstream side, provides an ink scraper blade configured to remove ink remaining on the roller group on the upstream side and removes the ink remaining on the roller group on the upstream side in a state in which the ink roller group 6 is separated into the upstream side and the downstream side to decrease the ink, connects the roller group on the upstream side and the roller group on the downstream side after that, sets the opening amounts of the ink fountain keys to opening amounts corresponding to the image of the printing plate 7′ of the next print job, and performs the feed operation of the ink ductor roller 5 a predetermined number of times in that state to superimpose an ink film thickness distribution corresponding to the image of the printing plate 7′ of the next print job, thereby enabling to immediately print a normal printed product.
In this ink supply method, however, even if the ink remaining on the roller group on the upstream side is scraped by the ink scraper blade, the difference in the ink film thickness caused by the difference in the image area ratio in the area corresponding to the ink fountain keys cannot completely be eliminated. The adverse effect remains in the ink film thickness distribution corresponding to the image of the next print job, and a normal printed product cannot be printed immediately. Hence, the printing materials are wasted.
This problem, that is, the problem that even if the ink remaining on the roller group on the upstream side is scraped by the ink scraper blade, the difference in the ink film thickness caused by the difference in the image area ratio in areas corresponding to the ink fountain keys cannot completely be eliminated will be described with reference to FIGS. 26, 27A, and 27B.
FIG. 26 is a view showing the images of printing plates of a preceding print job (job A) and a current print job (job B) and ink film thicknesses (printed states) in the ink supply device corresponding to the images. The view on the upper side of FIG. 26 shows the images of the printing plates of the jobs A and B, and the view on the lower side shows the ink film thicknesses in the ink supply device corresponding to the images of the printing plates of the jobs A and B.
In the job A, the image area ratio in the area corresponding to the ink fountain keys increases (the ink film thickness increases) from left to right. In the job B, the image area ratio in the area corresponding to the ink fountain keys increases (the ink film thickness increases) from right to left, contrary to the job A. Note that t1 is the reference ink film thickness, t2 is the image film thickness, and W is the width of the printing plate. Each of the reference ink film thickness t1 and the image film thickness t2 indicates the average value of ink film thicknesses that decrease from the upstream side to the downstream side on the ink roller group 6.
FIG. 27A shows an ink film thickness after ink is decreased by the method (blank sheet printing) described in patent literature 1 (a film thickness after removing) when switching the print job from the job A to the job B. In this case, since the ink is decreased by blank sheet printing, the image film thickness t2 of the printing plate of the job A is removed, and only the reference ink film thickness t1 is left. Hence, when the opening amounts of the ink fountain keys are set to opening amounts corresponding to the image of the printing plate of the job B, the image film thickness t2 of the job B is superimposed on the reference ink film thickness t1.
FIG. 27B shows an ink film thickness after ink is scraped by the ink scraper blade (a film thickness after removing) when switching the print job from the job A to the job B. In this case, since the ink scraper blade 15 scrapes the ink evenly in the roller width direction, the influence of the unevenness of the ink film thickness caused by the difference in the image area ratio in the area corresponding to the ink fountain keys cannot be eliminated. In addition, the ink is removed while cutting into the reference ink film thickness t1. For this reason, even when the opening amounts of the ink fountain keys are set to opening amounts corresponding to the image of the printing plate of the job B, an ink film thickness including the image film thickness t2 of the job B superimposed on the reference ink film thickness t1 cannot be obtained.