Method for alternately operating an inkjet printing machine

Abstract

A method for alternately operating an inkjet printing machine applying a plurality of inks to a printing material in accordance with a print image, includes conveying the printing material at a conveying speed. A switch is made between the following two modes, a 1.sup.st mode of printing an n-color first print job at a first conveying speed and at a print resolution using n inks, i.e. using black ink and n1 different chromatic inks; and a 2.sup.nd mode of printing a reduced-color second print job at a second conveying speed greater than the first conveying speed and at the same print resolution using the n inks. The printing speed and thus the number of prints produced per unit of time in a machine can therefore advantageously be increased.

Claims

1. A method for alternately operating an inkjet printing machine applying a plurality of inks to a printing material in accordance with a print image, the method comprising the following steps: conveying the printing material at a conveying speed; operating the inkjet printing machine in a first mode or a second mode in dependence on a print job to be produced as follows: in the first mode: printing an n-color first print job at a first conveying speed and at a print resolution using n inks including black ink and n1 different chromatic inks; in the second mode: printing a reduced-color second print job at a second conveying speed greater than the first conveying speed and at an identical print resolution using the n inks: printing a black-and-white or a grayscale print job in the second mode; printing a proportion of black or gray elements of the print by using the n1 different chromatic inks, where n=7, K and CMY OGV inks are applied, K is black, C is cyan, M is magenta, Y is yellow, O is orange, G is green, and V is violet; and printing three quarters of the black or gray elements of the print by using OC, GM, or VY inks.

2. The method according to claim 1, wherein the second conveying speed approximately equals four times the first conveying speed.

Description

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

(1) FIG. 1 is a diagrammatic, longitudinal-sectional view of a machine implementing a method of the invention;

(2) FIG. 2A is a plan view of a section of a seven-color print printed in the 1.sup.st mode;

(3) FIG. 2B is a plan view of a section of a four-color print printed in the 1.sup.st mode;

(4) FIG. 3A is a plan view of a section of a seven-color print printed in the 2.sup.nd mode;

(5) FIG. 3B is a plan view of a section of a seven-color print printed in the 2.sup.nd mode;

(6) FIG. 3C is a plan view of a section of a seven-color print printed in the 2.sup.nd mode; and

(7) FIG. 4 is a plan view of a section of a four-color print printed in the 2.sup.nd mode.

DETAILED DESCRIPTION OF THE INVENTION

(8) Referring now in detail to the figures of the drawings, in which corresponding elements have the same reference symbols, and first, particularly, to FIG. 1 thereof, there is seen an industrial production inkjet printing machine 1 processing sheets to produce monochrome or multicolor printed products in an inkjet printing operation. The machine 1 is shown as it is implementing the method of the invention. The prints are produced at a specified print resolution of 1,2001,200 dpi.

(9) The machine 1 includes a stack feeder 2, a pre-coating unit 3 for applying a precoat, an inkjet printing unit 4 for printing on sheets 8 in accordance with an image, one or more driers 5, a varnishing unit 6, and a stack delivery 7. The sheets 8 are conveyed through the machine by cylinders and are preferably made of paper, alternatively of cardboard.

(10) The printing unit 4 includes seven printing stations 4a to 4g that succeed one another and transfer ink drops, preferably in process colors K CMY OGV (black, cyan, magenta, yellow, orange, green, and violet). Every station includes an assembly of print heads that is oriented to be substantially perpendicular to a conveying direction. The print heads are stationary during the printing operation and include a plurality of individually controllable printing nozzles for generating the drops (drop on demand, DOD).

(11) The stations 4a to 4g allow one side of a sheet to be printed on over its entire width in a so-called single pass operation, i.e. the sheet sides are moved into the effective region of the units only once. The inks are water-based inks having drops which react with the precoat to create print dots of high dot sharpness. They contain pigments as colorants.

(12) The driers 5 are preferably thermal driers with infrared lamps, preferably LED lamps, that act to evaporate humidity from the applied ink, precoat and varnish and from the printing material.

(13) The sheets 8 receive the print while they are being conveyed through the machine 1 in a direction 9a at a conveying speed.

(14) The machine 1 further includes a machine control unit 14 including a computer and a control program running on the computer. The computer may be part of a non-illustrated control console. A device for supplying inks to the printing stations and a device for cleaning the print heads are likewise provided but not illustrated.

(15) In accordance with the invention, the machine 1 may alternately be operated in a 1.sup.st mode and in a 2.sup.nd mode. The switch may be implemented by the control unit 14.

(16) FIG. 2A illustrates an exemplary embodiment of the invention, in this case a section of two times four elements 13 of a print 12 printed in accordance with the invention. The print is created on the printing material 8, in the present case paper sheets, by applying ink 10 in the form of drops 11 of a specific size. The same applies to FIGS. 2B, 3A to 3C, and 4.

(17) The elements 13 in the rows of the illustrated section (i.e. in the lateral direction 9b) are generated by adjacent nozzles of a print head (or multiple print heads for different colors succeeding one another in the conveying direction 9a). Every element is formed of one or more colors, i.e. of ink drops 11 of the corresponding color, which generate print dots of a corresponding color on the sheet 8. The print dots may be located next to one another or there may be a partial or total overlap between them. The elements 13 in the columns of the illustrated section (i.e. in the conveying direction 9a) are generated in accordance with the cycle of the print head or of the multiple print heads. Again, the same applies to FIGS. 2B, 3A to 3C, and 4.

(18) FIG. 2A illustrates a section of a print 12 generated in a first mode M1 of the method of the invention: every element (i.e. every halftone dot) may be composed of multiple colors, i.e. of the corresponding inks 10. In the illustrated example, there are seven colors K CMY OGV. Thus, by way of example, a seven-color print is produced. The (average) conveying speed of the printing material attained in this process will be referred to as the standard speed in the following paragraphs. The printing operation is carried out on the machine 1 shown in FIG. 1, for instance.

(19) In a way similar to FIG. 2A, FIG. 2B illustrates a section of a color print that has been printed in the first mode 1, but in the illustrated example only using the four colors K CMY. They are printed by a machine 1 corresponding to the one in FIG. 1, for example, but with only four printing stations 4a to 4d and at a standard speed attainable with this type of four-color printing machine.

(20) The following FIGS. 3A to 3C and 4 illustrate sections of prints 12 that have been printed in the second mode M2 of the method of the invention. In a way corresponding to the print of FIG. 2A, the prints 12 of FIGS. 3A to 3C are printed by a seven-color printing machine, and the print shown in FIG. 4 is printed by a four-color printing machine in a way corresponding to the print of FIG. 2B.

(21) FIG. 3A shows that compared to the conveying speed of the embodiment of FIG. 2A, i.e. to the standard speed of the latter embodiment, the conveying speed of FIG. 3A has been increased to the fourfold, for lines of the same color, e.g. black/K, repeat every four lines, i.e. three more lines are printed in between. Thus, the length of the sheet that is printed per unit of time or in accordance with the cycle of the print heads is four times longer. For this purpose, the sheet is conveyed at an substantially fourfold conveying speed. The same applies to the embodiments shown in FIGS. 3B and 3C.

(22) The respective reduced-color print 12 shown in FIGS. 3B and 3C is a black or black-and-white or grayscale print. The elements 13 of the respective print 12 are either black/K or a combination of OC, GM, or YV. The latter three combinations of complementary colors ideally also create black or, more realistically, a grayscale. In the context of the invention, instead of the OGV colors, it is also possible and even advantageous to use special adapted colors F1, F2, and F3 thatin the combinations F1 plus C (F1C), F2 plus M (F2M) and F3 plus Y (F3Y)get as close as possible to black or the ideal grayscales and are as little off-color to black or the ideal grayscales as possible. Instead of orange, a red color might be used, for instance.

(23) A comparison between FIGS. 3A to 3C shows that although the print 12 is composed of the same elements 13, they are disposed in different ways in the various embodiments, i.e. they form different patterns. FIG. 3A shows that identical elements 13 are disposed in the rows, namely elements K, OC, GM, and VY. Such cross-lines may be visible to the human eye and are therefore undesired. FIG. 3B shows diagonal lines that might also be visible. FIG. 3C finally shows elements 13 that are disposed in an alternating way so that no continuous lines may be discerned, which means that the last embodiment is the preferred embodiment. Alternatively, a statistical configuration of the cells may be selected.

(24) The preferred embodiment shown in FIG. 3C indicates that on average, for instance in a field of 4 times 8 elements (4 in the lateral direction, 8 in the conveying direction), the black elements 13 and the chromatic cells located therebetween result in a grayscale value.

(25) In order to obtain the grayscale value to be achieved at the respective location of the print in accordance with the print 12, the halftone and/or dot size are adapted in a known way at the location in question.

(26) FIG. 4 shows that the conveying speed has been doubled compared to the standard speed of the embodiment shown in FIG. 2B: One line is printed using black K, the next line is printed using CMY, and so on. The print generated in this embodiment 12 likewise has reduced colors, in particular black or black-and-white or a grayscale print.

(27) In the embodiments of FIGS. 3A to 3C and 4, the drop sizes 11 are different, in particular larger than in the embodiments of FIGS. 2A and 2B, i.e. in the second mode M2 they are different from the drop sizes in the first mode M1. Since only three colors are mixed to form black or gray in the embodiment of FIG. 4, the color density achievable at a small drop size may be too low.

(28) Consequently, in this case, the use of larger drop sizes may be expedient. The following is a summary list of reference numerals and the corresponding structure used in the above description of the invention.

LIST OF REFERENCE SYMBOLS

(29) 1 inkjet printing machine 2 feeder 3 pre-coating unit 4 printing unit 4a-4f K CMY OGV printing stations 5 drier 6 varnishing unit 7 delivery 8 sheet of printing material 9a conveying direction/conveying speed 9b lateral direction 10 inks 11 ink drops/size thereof 12 print 13 elements of the printed image 14 machine control unit M1 first mode M2 second mode