METHOD AND APPARATUS FOR MULTI-COLOR PRINTING

Abstract: In a method of multi-colof offset printing each of plurality of blankets is contacted with respective printing plates corresponding to each color of the blankets to transfer color ink of same color thereof to each of the blankets, then each of the blankets is contacted with respective preceding printing plates color each of which is printed prior to that of each of the blankets for transferring color ink of the respective preceding color printing plates to each of the blankets, and after then each of the blankets is pressed in the predetermined order onto a sheet of printing paper to transfer each color ink to the printing paper after one another.

Description: The present invention relates to a multi-color offset printing method and a printing press for printing a multi-colored image pattern by using a plurality of color inks, and particularly to a method and an apparatus which can shorten the rising time necessary for stabilizing printing conditions from the beginning of printing. The offset printing press having been generally used for multi-color offset printing is operated fundamentally with the following processes. (1) Each of the printing plates is damped. (2) A plurality of color printing inks are supplied to an image area of each of the corresponding printing plates by inking arrangements. (3) An ink (hereinafter referred to as "a color ink pattern" supplied to an image area of each plate is transferred to respective corresponding blankets. (4) The color pattern ink transferred to the blankets in a predetermined order is overlapped on a sheet of paper so that each of the color images may be properly aligned to obtain a desired color print. As described the above, the offset printing is a printing method in which inks are not transferred directly to a paper from the plates, but transferred thereto through a blanket. Hereinafter, outlines of mechanism and operation of several kinds of multi-color offset presses which carry out offset printing are described. Fig. 10 is a sectional view of a four colors offset proofing press for carrying out continuous printing operation on a sheet paper (hereinafter the press of this type is referred to a rotary press), which has been described in US-A-3,536,006 and US-A-3,347,160. The apparatus comprises a plate cylinder (103) on the outer circumference of which printing plates (101a) (101b) (101c) and (101d) for four colors are provided in a required order, a blanket cylinder (107) having the same diameter as that of the plate cylinder (103) and on the outer circumference of which four blankets (105a) (105b) (105c) and (105d) corresponding to each of the colors are provided in a required order, and a printing cylinder (109) having a diameter of one fourth of those of the plate cylinder (103) and the blanket cylinder (107) and on the outer circumference of which papers are to be set. Here, each of the suffixes attached to each reference number (a, b, c, d) represents a respective color of printing ink which is applied to a corresponding part, and the printing order, and a reference number to which no suffix is attached indicates a part as such. The apparatus shown in Fig. 10 is so arranged that, during one revolution of the plate cylinder (103) in the direction shown by an arrow mark, water is supplied to each of the plates (101a) (101b) (101c) and (101d) from respective dampening devices (113a) (113b) (113c) and (113d) to dampen the plates. Then, from the inking arrangements (111a) (111b) (111c) and (111d) corresponding to respective colors a color ink is supplied to each of the plates to form a color image on the respective plates. During one revolution of the blanket cylinder (107) which rotates in contact with the plate cylinder (103), the color images on the plates are transferred to the corresponding blankets (105a) (105b) (105c) and (105d), respectively. On the other hand paper is fed from a paper feeder (115) to the outer circumference of the printing cylinder (109), and during one revolution of the blanket cylinder (107), the printing cylinder (109) contacting with the blanket cylinder (107) rotates four turns, and inks of the color images on the blankets are overlapped on the paper and transferred thereto. The paper on which four color inks are printed is fed to a paper receiving tray (117). An apparatus shown in Fig. 11 is another rotary type four-color offset proofing press and is the same as described on pp.47 - 50 in a publication titled "Deutscher Drucker Nr. 33/18-10-1984". This apparatus comprises four plate cylinders (121a) (121b) (121c) and (121d) each of which has one of four printing plates on its outer circumference, four blanket cylinders (123a) (123b) (123c) and (123d) having the same diameter as the plate cylinders, a blanket being provided on each of the outer circumferences of them and each of them being rotated in contact with the corresponding plate cylinder, and a printing cylinder (125) having a diameter of about three times that of the plate cylinders and the blanket cylinders. To each of the plate cylinders (121a) (121b) (121c) and (121d) there is attached an inking arrangement and a dampening device. This apparatus is an apparatus of the same type as that of shown in Fig. 10 which forms color images on the printing plates by feeding an ink from each of inking arrangements to the respective corresponding plates, and the apparatus prints each of the color ink patterns on the same area of a paper through each of the blankets. According to rotation of the printing cylinder (125), a sheet of paper fed from a paper feeder (127) passes through points at each of which each of the blanket cylinders (123a) (123b) (123c) and (123d) contacts with the printing cylinder (125) in order, and on the paper color images of the respective color inks are overlapped and thus printed. The paper on which a printing of four colors is printed is fed out to a receiving device (129). Fig. 12 shows a four color offset proofing press disclosed in British Patent Laid-Open Publication No. 2164295A. The apparatus disclosed in this publication is one of types which are different from the afore-described two rotary type apparatus, and is a flat table type in which printing plates are loaded planely. The proofing press of this type is provided in a frame (146) with tables or beds (141) (142) (143) and (144) on which printing plates of each color is to be loaded, a set of dampening devices (166) and inking arrangements (181) (182) (183) and (184) for each of the colors. In addition, in a carriage (148) which travels on the frame (146) there are provided a blanket cylinder (150) on the outer circumference of which four blankets (151) (152) (153) and (154)are loaded, a printing cylinder (158) having diameter of one fourth of that of the blanket cylinder (150), water supplying rollers (160) for supplying dampening water to a water distributing pate (168), dampening rollers (161) (162) (163) and (164) for each of the colors, and inking rollers (171) (172) (173) and (174) for each of the colors. When the carriage (148) is driven from the right side to the left side, the blanket cylinder (150) is raised to separate from the surfaces of the tables or beds (141) - (144), and the dampening rollers (161) - (164) and the inking rollers (171) - (174) touch in order with the corresponding printing plates so that the dampening water and the color inks are supplied to the plates. Next, when the carriage (148) is driven from the left side to the right side, each of the color ink patterns formed on each of the plates is transferred to the blankets (151) (152) (153) and (154) loaded on the blanket cylinder (150). A paper to be printed is loaded on the outer circumference of the printing cylinder (158) and during one revolution of the blanket cylinder (150), the printing cylinder (158) which contacts with the blanket cylinder (150) revolves four times, and the color ink patterns on the blankets (151) (152) (1530 and (154) are transferred in order to the paper so as to print a four color image thereon. Fig. 13 shows a two color offset proofing press of a flat table type disclosed in GB-A-20241051. In a frame (118) of this apparatus there are provided two plate tables (112) and (114) on which printing plates are to be loaded, a paper table (116) on which sheets of paper to be printed are loaded, a dampening device (132) and two sets of inking arrangements (134) and (136). Further, in a carriage (120) which travels on the frame (118) there are provided two blanket cylinders (122) and (124) on each of the outer circumferences of which a blanket is loaded, a set of damping rollers (126) and two sets of inking rollers (128) and (130). When the carriage (120) is driven from the right side to the left side, similar as with the apparatus shown in Fig. 12, damping water is supplied from the damping roller (126) to the plates loaded on the tables (112) and (114), and a color ink is supplied from the inking rollers (128) and (130) to the corresponding plate(s). On the other hand when the carriage (120) is driven inversely, that is, driven from the left side to the right side, color pattern inks on the plates are transferred to the corresponding blanket of the blanket cylinder (122) or (124), and then they are transferred in order onto a paper on the paper table (116). Thus, a two-color image is printed. In those afore-described various types of multi-color offset proofing presses blankets for each color are pressed in order onto a paper to transfer each color ink pattern thereon, and a four or two color image is printed. In this case because the first color ink printed on the paper from the blanket of the first color contacts with the surfaces of the second blanket and those of the following ones, a phenomenon in which inks on the paper are transferred back to the surfaces of the following blankets, so called "back trapping", occurs, so that the ink quality on the paper is remarkably deteriorated, so that the desired result of obtaining an excellent ink quality cannot be achieved. In each of the afore-described apparatus immediately before starting the printing operation only one color ink corresponding to each of the blankets is to be transferred, and all these blankets are contacted in order with the same paper. For example, observing the first color ink, after the first color ink has been transferred from the first color blanket to the paper, the second blanket contacts with the paper, while the paper is still in wet condition, so that a part of the first coloring on the paper is transferred to the second color blanket. Thus, the so-called the "back trapping" phenomenon occurs. In the case of the paper contacting with the third and the fourth color blankets, in the same manner as afore-described, the first color ink on the paper is also transferred to the following blankets, so that when the four color image is printed, quantity of the first color on the paper becomes considerably insufficient to that necessary for obtaining a desired printing effect. When four color printing to the first paper is finished, on the second, the third and the fourth blankets there is still remaining the first color ink which was transferred back thereto from the first paper, although the quantity of the ink is small, and the later the printing order becomes, the smaller the quantity of the ink on the blanket becomes. Accordingly, even in the printing operation to the second paper the first color ink is transferred to the second blanket and the following ones, so that an insufficiency of the first color again occurs in the printed image. Regarding the second color ink or the third color ink, the conditions are quite the same as those for the first color ink, that is, the second color ink is transferred back to the third and the fourth blankets, and the third color ink is transferred back to the fourth blanket , so that the quantities of these color inks are insufficient in a finished printing. Such phenomena as they occurred in the afore-described cases in which quantities of color inks of the preceding print become insufficient do not occur after a number of sheets of paper have been printed and the quantities of color inks of the preceding order have saturated on the following blankets, so that, thereafter, printing conditions are stabilized. However, a considerably large number of papers must be printed to reach the stabilizing conditions. Usually the number of color printings required for an offset proofing press is relatively small, however, in the preparing step, until the offset proofing press reaches the stabilized conditions, too many sheets of paper and a large quantity of the inks are needed, and further too much troublesome work is required. GB-A-7 382 and GB-A-10 451 describe three-color offset printing presses having three plate cylinders arranged around one blanket cylinder having three blankets provided thereon which are sequentially brought into contact with each of the printing plates provided on the plate cylinders. It is an object of the present invention is to provide a method and an apparatus for multi-color offset printing which can improve disadvantages of the afore-described prior art. It is another object of the present invention to provide a method and an apparatus for shortening rising time, that is, time for arriving at conditions for stabilizing multi-color printing operation from start of the working. It is further object to provide in the offset proof printing a method and an apparatus for saving materials and labels by directly supplying a color ink for preceding printing from a preceding printing plate to the following blanket, and by having been previously saturated the color ink so that the color ink may be prevented from being transferred back to the blanket from a paper. The invention is as claimed in claims 1 and 5. Other advantages and objects of the present invention will become more apparent as the following descriptions are considered with reference to the accompanying drawings, in which: Fig. 1 is a view showing a schematic construction of an embodiment of a rotary type offset proofing press according to the present invention; Figs. 2A and 2B show an embodiment of a plate cylinder shifting device; Figs. 3A and 3B show another embodiment of a plate cylinder shifting device; Fig. 4 is a block diagram of a control circuit for shifting the plate cylinder; Fig. 5 is a block diagram of a control of a flat table type offset proofing press as another embodiment of the present invention; Figs. 6 and 7 are time charts of operations of the plate cylinders of the rotary type offset proofing press shown in Fig. 1; Figs. 8 and 9 are time charts of operations of the flat table type offset proofing press shown in Fig. 5; and Figs. 10, 11, 12 and 13 are schemata of the prior arts as afore-described. The rotary type offset proofing press shown in Fig. 1 is composed of a printing section (P), a feeder section (F) for feeding papers to be printed and a delivery section (D) for feeding out and piling up the printed papers. The printing section (P) comprises a blanket cylinder (8) having loaded four blankets (6a) (6b) (6c) and (6d) on the outer circumference thereof by retainers (2) and stretchers (4), four plate cylinders (12a) (12b) (12c) and (12d) each of which is loaded by one of vises (9) on each of their outer circumferences with a printing plate among printing plates (10a) (10b) (10c) and (10d) respectively, and each of said four plate cylinders has the same diameter which is one fourth of that of the blanket cylinder (8), and a printing cylinder (14) having the same diameter as the plate cylinders (12a) - (12d). Here, it is defined that the wording "diameter of the blanket cylinder (8)" indicates fundamentally a diameter at the loaded portions (6a) - (6d) of the blanket cylinder (8), each of the diameters of the plate cylinders (12a) - (12d) indicates respective diameters including the thickness of each of the printing plates (10a) - (10d), and the diameter of the printing cylinder (14) indicates a diameter including the thickness of a paper. Dimensions of these diameters, as well as those having been practiced generally in the art of printing press, can be varied in ratio a little according to printing conditions, therefore, it should be understood that ratios of diameters of the cylinders include these slight variation. Since the apparatus shown in Fig. 1 is a four color printing press, the ratio of diameters between the blanket cylinder (8) and the plate cylinders (12a) (12b) (12c) (12d) and the printing cylinder (14) is defined as 4 : 1, however, of course, if the apparatus is a six color printing press, then ratio of diameters is 6 : 1, and in the case of an eight color printing press, the ratio becomes 8 : 1, thus, any design is possible so that the ratio may become n : 1 in accordance with the number of colors (n). To each of the plate cylinders (12a) (12b) (12c) and (12d) respective color inking arrangements (16a) (16b) (16c) (16d) and dampening devices (18a) (18b) (18c) (18d) are attached. The inking arrangements (16) and the dampening devices (18) are adapted so that they may always contact with the corresponding plate cylinders (12), respectively, or may be adapted so that they may contact with the corresponding plate cylinders (12) respectively, only the time when the plate cylinder (12) is separated from the blanket (8). As shown in Figs. 2A and 2B, each of the plate cylinders (12) is supported at both ends of its shaft with eccentric bearings (20), and the eccentric bearings (20) are connected with a rod of a pneumatic cylinder (24) mounted on a side frame (22) so that the plate cylinder (12) is shifted to a position at which it contacts with the outer circumference of the blanket cylinder (8) and another position at which it separates from the blanket cylinder (8), according to rotation of the eccentric bearings (20) by basing on movement of the rod. Further, at one end of the shaft of the blanket cylinder (8) there is provided a rotary encoder (25) for detecting a rotating angle of the blanket cylinder (8). Fig. 4 is a block diagram of a circuit for controlling shifting of the plate cylinder (12). The control circuit is composed of a keyboard (54) which is a data input device, a computer (56), a pneumatic cylinder driving means (58), an interface (60) etc. In order to contact each of the plate cylinders (12) with the blanket cylinder (8) at a desired angular position thereof, at first a program for transferring a color ink pattern ink for the preceding printing to the blanket of a later color print and a printing program of a usual regular method are input to a RAM (62) of the computer (56) by the keyboard (54), or in the case of applying a program previously stored in a ROM (64) of the computer (56), the keyboard (54) selects it. When the blanket cylinder (8) starts rotating, a pulse signal from the rotary encoder (25) is input to the computer (56) through an interface (60), and an angular position of the blanket cylinder (8) or the angular positions of each of the blankets are calculated. By basing on data of the calculated angular positions, a control signal is input to the pneumatic cylinder driving means (54) at the position where the plate cylinders should contact with the blanket cylinder and at the position where the former should separate from the latter. The pneumatic cylinder drive means (54) drives the pneumatic cylinder (24) by the control signal from the computer (56), and by rotating the eccentric bearings (20) at a required angle basing on the movement of the rod, each of the plate cylinders (12a) - (12d) is shifted at each of their required positions between the position at which it comes into contact with the blanket cylinder (8) and the position at which it separates from the blanket cylinder (8). Figs. 3A and 3B show another embodiment in which limit switches are used instead of the rotary encoder to shift the plate cylinders. Each of four limit switches (26) is provided at respective positions of the side frame (22) which are relatively identical to the respective plate cylinders (12a) (12b) (12c) and (12d) only differing by each of axial positions, and at each of relatively identical positions of the end surface of the blanket cylinder (8) to the respective blankets (6a) (6b) (6c) and (6d) actuators (28) which are to engage with the respective limit switches (26) are provided. To each of the limit switches (26) two actuators (28) are provided, and their axial positions are aligned with the limit switches to be engaged with, respectively. Signals from the limit switches (26) are input to the computer (56) instead of the signals from the rotary encoder (25) of the embodiment shown in Figs. 2A and 2B. By each signal relating to the preceding actuator among the respective pairs of actuators (28) the respective plate cylinders (12a) - (12d) corresponding thereto is contacted with the blanket cylinder (8), and by a signal relating to the following actuator the plate cylinder is separated from the blanket cylinder (8). The feeder section (F) is composed of a paper storage (30), an endless belt (32), a conveyer (36) provided with a plurality of rollers (34), a suctioning means (38) for sending out papers from the paper storage (30) to the conveyer (36), and a swing gripper (40). The suctioning means (38) has two sets of suckers (42) and (44). It operates as follows. At first the uppermost paper among papers piled in the paper storage (30) is held by the suckers (42) and raised, then the raised paper is advanced by the suckers (44) till it comes to be inserted between the endless belt (32) and the first roller (34) so that it may be transported by the conveyer (36). The transported paper is, as well as in the case of the ordinary printing press, delivered to grips (46) of the printing cylinder (14) by the swing gripper (40) provided at one end of the conveyer (36). The delivery section (D) is composed of a delivery cylinder (15) which comes into contact with the printing cylinder (14) and rotates in synchronization with the printing cylinder (14) and an endless chain (50) provided with delivery grips (48) each having a constant pitch therebetween. The pitch of the delivery grips (48) is the same as that of the adjacent blankets (6), that is, it is adapted to coincide with circumferential lengths of the printing cylinder (14) and the plate cylinder (12). Thus, the delivery grips (48) receive the printed papers from the printing cylinder (14) and transports them to a receiving stand (52). Hereinafter, operation of the afore-mentioned apparatus is described by dividing into two processes, one of them is to describe with respect to the regular printing process and the other is for the preparing process. In the regular printing process the four plate cylinders (12a) (12b) (12c) (12d) and the printing cylinder (14) rotate in synchronization with the blanket cylinder (8), and the chain (50) is driven to pass through the outer circumference of the delivery cylinder (15). To each of the printing plates (10) loaded on the respective plate cylinders (12) dampening water is supplied from each of the dampening devices (18), and each color ink is supplied to the respective printing plates from each of the inking arrangements (16). When each of the leading edges of the blankets (6a) - (6d) arrives at each of the contact positions of the respective corresponding plate cylinders (12a) - (12d), a pneumatic cylinder (24) is actuated by a pulse signal of a rotary encoder (25) input to a computer (56) through an interface (60), and the eccentric bearings (20) begin to rotate to let the plate cylinder (12) contact with the blanket cylinder (8). Thus, "setting on" is performed. Fig. 1 shows a case in which the plate cylinder (12a) of the first color is in this situation. The blanket cylinder (8) rotates further, and the trailing edge of the blanket (6) arrives at the contact position with the plate cylinder (12), then the pneumatic cylinder (24) is actuated through the interface (60), and then the pneumatic cylinder (24) is returned to the original position, and the plate cylinder (12) separates from the blanket cylinder (8). Fig. 6(a) shows time charts representing motions of the four plate cylinders (12) in one rotating cycle of the blanket cylinder (8), in which lines of level indicated by an index A represent periods for the plate cylinders (12) separating from the blanket cylinder (8), and the lower lines represent periods for the plate cylinders (12) coming into contact with the blanket cylinder (8). Further, the time chart shown in Fig. 6(a) is a time chart assuming a case in which four plate cylinders are arranged around the blanket cylinder and separated from each another by an angular interval of 90 and by an equi-angular phase, respectively. In fact in the arrangement of the plate cylinders shown in Fig. 1, lines of the plate cylinders of the second color to the fourth color are shown such as shifted to the left side, respectively. The printing cylinder (14) always contacts with the blanket cylinder (8), and it rotates four times for one rotation of the blanket cylinder (8). In regular printing process one sheet of paper to be printed is fed from the paper storage (30) to the printing cylinder (14) every one rotation of the blanket cylinder (8), and loaded on the outer circumference of the printing cylinder (14). Thus, according to four turns of the printing cylinder (14), four of the blankets (6) contact with the fed paper in sequence to make up an image print of four colors. Next, the paper is held by the delivery grips (48) on the endless chain (50), and fed to the receiving stand (52). In this case since the delivery grips (48) are arranged on the endless chain (50) with the same pitch as the circumferential length of the printing cylinder (14), one of the four grips (48) is used. Next, descriptions are given to the process according to the present invention for supplying inks to the blankets for the later printing with the plates for the preceding printing in the afore-mentioned four color offset proofing rotary press. Fig. 6(b) is a time chart showing the fundamental way of the present invention, which indicates timing of contact and separation between each of the plate cylinders (12) and the blanket cylinder (8) at preparing steps prior to the beginning of printing operation. To facilitate understanding the situation is shown in Fig. 6(a) for four plate cylinders arranged at equal intervals and with 90 degrees phase difference between each another. During one revolution of the blanket cylinder (8), the plate cylinder (12a) for the first color is set on the blankets (6b) (6c) and (6d) for the second, the third and the fourth colors, the plate cylinder (12b) for the second color is set on the blankets (6c) and (6d) for the third and the fourth colors, and the plate cylinder (12c) for the third color is set on the blanket (6d) for the fourth color. As described above, each of the color ink patterns on the respective plates is supplied to each of the blankets required for the subsequent printing operation. If the required quantity of ink is supplied to the blanket for the later printing, and if print is carried out with the same process as the afore-described regular method, even at the beginning of the multi-color printing, conditions very similar to those of a case in which the printing operation has already been carried out certain times, that is, conditions can be obtained similar to those of the case in which an ink for the preceding printing has been transferred back onto the blanket for the later printing and saturated thereat. Thereby the ink(s) is (are) prevented from being transferred back to the blanket for the subsequent printing, and the printing conditions are stabilized so that good results of prints may be obtained. Hereinafter, concrete and preferred operation of the present invention is described. Prior to supplying an ink from the plate for preceding printing to the blanket for the subsequent printing, according to the regular process shown in Fig. 6(a), the respective color image inks are supplied to the respective corresponding blankets (6a) - (6d) by the respective printing plates (10a) - (10d) loaded on the respective plate cylinders (12a) - (12d). Ink supplying work according to the regular process is performed with the following reason. That is, in the case of a halftone image composed of screen dots, particularly in an area of large dot percentage, dots of one color are printed by partly overlapping with dots of other colors. In this case it is considered that even on the blankets there must be parts with overlapping plural color inks. However, in fact, an ink(s) subsequently supplied is repelled by the pre-existing ink(s), and cannot adhere to the blanket, so that no overlapping condition occurs. Accordingly, since a proper quantity of each of color ink to be printed by the blankets is supplied onto the respective blankets, each color ink pattern is previously supplied to the respective blankets (6a) - (6d) by the regular process. The ink for the preceding printing can also be supplied to the blanket for a subsequent printing by an operation as shown in Figs. 7(a), 7(b) and 7(c) during three turns of the blanket cylinder (8). To facilitate understanding, time charts shown in Figs. 7(a), 7(b) and 7(c) are, as well as those shown in Figs. 6(a) and 6(b), for an arrangement in which the plate cylinders are provided on the outer circumference of the blanket cylinder with an equal distance from each other and in 90 degree phase difference. At the first rotating period of the blanket cylinder (8), as shown in Fig. 7(a), the first color plate (10a) is set on the second color blanket (6b), the second color plate (10b) is set on the third color blanket (6c) and the third color plate (10c) is set on the fourth color blanket (6d). At the second rotating period of the blanket cylinder (8), as shown in Fig. 7(b) the first color plate (10a) is set on the third color blanket (6c) and the second color plate (10b) is set on the fourth color blanket (6d), respectively. At the third rotating period of the blanket cylinder (8), as shown in Fig. 7(c), the first color plate (10a) is set on the fourth color blanket (6d). In order to supply the ink from the plate for the preceding printing to the blankets for the subsequent printing, the process described already by referring to Fig. 6(b) may be applied, however, by the following two reasons it is preferable to apply a method comprising three steps as shown in Figs. 7(a), 7(b) and 7(c). The first reason lies in supplying sufficient quantity of ink. For example, in the process shown in Figs. 6(b), the first color plate (10a) supplies the ink three times from the second to the fourth blankets continuously, so that to the later blanket a smaller quantity of the ink is supplied, which results in insufficiency ink quantity. The same holds also for the second plate (10b). The second reason relates to the dampening water. If the process shown in Fig. 6(b) is applied, for example, to the fourth color blanket (6d) ink is supplied continuously by three plates, i.e., the first, the second and the third plate (10a) (10b) and (10c). However, from these plates also dampening water is supplied to it together with the ink. That is, in the lithography printing, to prevent portions other than image areas from being inked, the plate is dampened by water, so that the dampening water is transferred to the blankets together with the ink. The image areas to be inked change according to ink colors, therefore, to areas among the image areas to be inked by an ink for the subsequent printing water supplied from the plate(s) for the preceding printing is retained. In the continuous operation shown in Fig. 6(b), the ink for the subsequent printing is continued to be supplied, before the water supplied to the blanket from the plate(s) for the preceding printing could evaporate, so that transferring of the ink for the subsequent printing becomes insufficient, which results in an insufficient quantity of the ink. Such conditions are found not only for of the fourth blanket (6d) but also for of the third blanket (6c). Then, as shown in Figs. 7(a), 7(b) and 7(c), by defining the first color as reference, during three times revolution period of the blanket cylinder (8), each ink is supplied to each of the plates at every revolution of the blanket cylinder (8), and by adapting to transfer a color image ink to any one of the subsequent color blankets, an equal quantity of ink can be supplied to all the blankets (6a), (6b), (6c) and (6d). Further, the dampening water supplied to the blankets can be evaporated, as that in common lithographic printing operation, during one revolution period of the blanket cylinder (8), so that troubles resulting from insufficiency of ink supply for the subsequent printing can be solved. After having completed the afore-mentioned preparing steps, actual multi-color printing operation is started. At this time, as described the above, during one revolution of the blanket cylinder (8), the color inks are transferred to the blankets (6a), (6b), (6c) and (6d) from the respectively corresponding plates (10a), (10b), (10c) and (10d), and then the process in which the color image inks on the blankets (6a) - (6d) are overlapped on a paper loaded on the printing cylinder (14) is repeated. If the printing plate is a dry offset type which requires for no dampening water, and an inking arrangement can supply sufficient quantity of ink continuously, the method shown in Fig. 6(b) may be applied for supplying an ink for the preceding printing to the blanket(s) for the subsequent printing, or as shown in Fig. 6(c), proper color inks and the preceding color ink for each of the blankets may be supplied in parallel during one revolution period of the blanket cylinder (8). The quantity of the ink supplied to the blanket for the subsequent printing may be a level sufficient for preventing the ink from transferring from the paper back to the blanket. According to this method, a smaller quantity of ink compared with that which is supplied to the blanket in the regular printing operation may be sufficient. Accordingly, in the steps shown in Figs. 7(a), 7(b) and 7(c), the ink for the preceding printing is supplied to the blanket for the subsequent printing only one time, while proper color ink is supplied twice, one time at the preparing step and the other time at the printing step. In case of necessity, prior to the printing step, by supplying the ink several times, the quantity of the proper ink is changed to those of other color inks. That is, contacting times of the printing plate with the blanket for the proper color are made larger than those of the printing plate with each of the blankets of other colors to make the quantity of the proper color ink to be supplied to the blanket larger than those of the other inks. The above-mentioned controlling of the ink quantity is not limited to the contacting times of the above-described plate with the blanket, but any similar methods which have been applied in a usual printing press, for example, an adjusting method of contacting times of the inking rollers with the printing plate, or that of controlling quantity of ink to be supplied to the inking roller from a ink fountain etc., may be applied thereto. These methods for controlling ink supplying to the blanket can be applied to a multi-color offset proofing press of flat table type which will be described hereinafter. Fig. 5 is a schematic sectional elevation of a four color offset proofing press of flat table type for practicing the present invention. Four plate tables (21a) (21b) (21c) (21d) and a paper table (23) are mounted on the frame (25) in a line, and a carriage (27) is driven along the line of the tables. On the carriage (27) there are provided four blanket cylinders (29a) (29b) (29c) and (29d) corresponding to four colors respectively, four sets of inking rollers (31a) (31b) (31c) (31d), and a set of dampening rollers (33) which can be commonly used for each of the colors so that they may be traveled together with the carriage (27). At the left end of the frame (25) a damping means (35) for supplying damping water to the damping rollers (33) is provided, and at the right side of the frame (25) four sets of inking arrangements (37a) (37b) (37c) and (37d) which correspond to the four colors are provided with the same pitch as that of the inking rollers (31a) (31b) (31c) and (31d). The flat table type offset proofing press shown in Fig. 5 is operated in proof printing in the regular process as follows. Firstly, the printing plates (41a) - (41d) for respective proper colors are loaded on the four plate tables (21a) - (21d), and paper to be printed is loaded on the paper table (23). When the carriage (27) travels along the tables, at the left end of its stroke the damping rollers (33) engage with the damping means (35) to supply water, and at the right end of the stroke each of four sets of the inking rollers (31a) - (31d) engages with the respective inking arrangements (37a) - (37d) to supply the proper color ink. According to the travel of the carriage (27), the plates (41a) - (41d) loaded on the plate tables (21a) - (21d) respectively are damped by the damping rollers (33), and by the corresponding inking rollers (31a) - (31d), the proper color inks are supplied. When the carriage (27) returns, that is, when the carriage (27) travels from the left side to the right side, the inks on the respective plates (41a) - (41d) are transferred onto the outer circumferences of the corresponding blanket cylinders (29a) - (29d), and printed on a paper loaded on the paper table (23) to make up a four color print. Fig. 8(a) is a time chart showing vertical movements of the blanket cylinders (29a) - (29d) in this regular process, when the carriage (27) travels from the left side to the right side. Each of the blanket cylinders is set on at the area of the plate tables corresponding to that blanket cylinder, and to each of them an ink of color image of the plate loaded on the table is transferred, and then at an area of the paper table it is set on to be printed on the paper table. In this regular printing process as a mechanism for moving the blanket cylinders vertically within a required area such a mechanism as having been used in the flat table type offset proofing press, for example, such as described in GB-A-2024105, may be applied, so that detailed description is abbreviated. Practical operating system of the offset proofing press shown in Fig. 5 according to the present invention is as follows. Fig. 8(b) shows the basic process of the present invention in which when the carriage (27) travels from the left side to the right side as shown in Fig. 5, the fourth blanket cylinder (29d) is set on the third, the second and the first plate tables, the third blanket cylinder (29c) is set on the second and the first plate tables, and the second blanket cylinder (29b) is set on the first plate table. Thereby, ink of color image for desired preceding printing is transferred. A more preferable operation process in the flat table type offset proofing press shown in Fig. 5 will be described hereinafter. As well as the afore-mentioned process for the rotary type offset proofing press shown in Fig. 1, in this flat table type offset proofing press an ink of color image of a proper color is transferred to the respective blanket cylinder (29a) - (29d) by the corresponding plate (41a) - (41d), respectively. Vertical movements of the blanket cylinders in this step are the same as those of the case shown in Fig. 8(a), However, at the area of the paper table no setting on is brought about. Next, the ink is transferred to the blanket cylinders for the subsequent printing from the plates of the preceding printing. These steps are, like the afore-described preferable operation in the rotary type offset proofing press, preferably to be practiced by three steps as shown in Figs. 9(a) - 9(c). In the first traveling of the carriage (27) from the left side to the right side of the frame (25), as shown in Fig. 9(a), an ink on the first plate (41a) is transferred to the second blanket cylinder (29b), an ink on the second plate (41b) is transferred to the third blanket cylinder (29c), and an ink on the third plate (41c) is transferred to the fourth blanket cylinder (29d). In the second traveling, as shown in Fig. 9(b), the ink on the first plate (41a) is transferred to the third blanket cylinder (29c) and the ink on the second plate (41b) is transferred to the fourth blanket cylinder (29d). Further, in the third traveling, as shown in Fig. 9(c), the ink on the first plate (41a) is transferred to the fourth blanket cylinder (29d). By the afore-described three operation steps, after transferring the ink for the preceding printing to the blanket for required subsequent printing, according to the operation shown in Fig. 8(a), by carrying out multi-color printing, desired multi-color proofing image can be printed.

Claims: A method of multi-color offset printing comprising a step (a) of bringing into contact each of a plurality of blankets (6a - 6d) with a corresponding printing plate (10a - 10d) to transfer ink of a corresponding color to each of the blankets, a step (b) of pressing said blankets (6a - 6d) in a predetermined order onto a printing paper to transfer the inks of different colors one after the other and in an overlapping relationship to the paper, characterized by a step (c), preceding step (b), of bringing into contact each of the blankets (6a - 6d) with the printing plate or plates (10a - 10d) corresponding to the blanket or blankets (6a - 6d) which are to be pressed onto the paper prior to said each of the blankets. A method according to claim 1, characterized in that said step (c) is carried out subsequent or previous to step (a). A method according to claim 1 or 2, characterized in that the number of colors is four and step (c) is carried out by contacting the first color printing plate (10a) with the second, third and fourth color blanket (6b - 6d), the second color printing plate (10b) with the third and fourth color blanket (6c, 6d), and the third color printing plate (10c) with the fourth color blanket (6d). A method according to anyone of the preceding claims, characterized in that in the step (c) ink is supplied to each printing plate (10a - 10d) whenever ink is transferred from the same to a blanket (6a - 6d). A multi-color offset printing apparatus in which multi-color printing is carried out on one printing sheet, comprising: a blanket cylinder (8) being provided with n blankets (6a - 6d) at its outer periphery; n plate cylinders (12a - 12d) disposed to respectively oppose said blanket cylinder (8) and each having a printing plate (10a - 10d) on its outer periphery and having an outer diameter of 1/n of that of said blanket cylinder (8) and rotating in synchronism with said blanket cylinder (8); a printing cylinder (14) having a diameter of 1/n of that of said blanket cylinder (8), disposed at a position opposing to said plate cylinders (12a - 12d), holding a printing sheet at its outer periphery and rotating in synchronism with said blanket cylinder (8); n inking arrangements (16a - 16d) which supply ink separately to each of said plate cylinders (12a - 12d); a detecting means (25) for detecting the angular rotational position of each of said plate cylinders (12a - 12d); and control means (56), operating on the basis of signals from said detecting means, for causing (a) each of said plurality of blankets (6a - 6d) to be brought into contact with a corresponding printing plate (10a - 10d) to transfer ink of the corresponding color to the respective blanket (6a - 6d), (b) pressing said blankets (6a - 6d) in a predetermined order onto a printing paper to transfer the inks of different colors one after the other and in an overlapping relationship to the paper, and characterized by the control means (56) causing, (c), preceding to (b), each of the blankets (6a - 6d) to be brought into contact with the printing plate or plates (10a -10d) corresponding to the blanket or blankets (6a - 6d) which are to be pressed onto the paper prior to said each of the blankets.

Inventors: ODA, OSAMU DAINIPPON SCREEN MFG. CO. LTD. PAT DIV.
Applicants: DAINIPPON SCREEN MFG. CO., LTD.
Application year: 1987
Main classification: B41F7
IPC codes: B41F7