METHOD FOR COMPENSATING FOR FAILED PRINTING NOZZLES IN AN INKJET PRINTING MACHINE BY USING DEVIATING NOZZLES

Abstract

A method for compensating for failed printing nozzles in an inkjet printing machine uses a computer to provide an increased ink drop volume of adjacent nozzles. All existing printing nozzles are measured regarding print failure and deviation of a printed dot beyond a specific threshold, so that the printing nozzles are marked either as functioning, failed, or printing a dot deviating in a direction transverse to the printing direction, and the results are saved on the computer. The computer hides all functioning printing nozzles and marks all remaining isolated printing nozzles as failed nozzles. The computer groups adjacent remaining printing nozzles by marking nozzles printing a dot deviating in a direction transverse to the printing direction as functioning printing nozzles contributing to the compensation or as failed printing nozzles in accordance with a rule, and the computer compensates for all printing nozzles having been marked as failed.

Claims

1. A method for compensating for failed printing nozzles in an inkjet printing machine, the method comprising the following steps: measuring all existing printing nozzles regarding print failure and deviation of a printed dot beyond a defined threshold, marking the printing nozzles as functioning, failed, or printing a dot deviating in a direction transverse to a printing direction, and saving measuring results on a computer; using the computer to hide all functioning printing nozzles; using the computer to mark all remaining isolated printing nozzles as failed printing nozzles; using the computer to group adjacent remaining printing nozzles, marking printing nozzles printing a dot deviating in a direction transverse to the printing direction as functioning printing nozzles contributing to a compensation or as failed printing nozzles in accordance with a rule; and using the computer to compensate for all printing nozzles having been marked as failed by producing an increased ink drop volume of adjacent printing nozzles.

2. The method according to claim 1, which further comprises using printing nozzles printing a deviating print dot and printing at an angle along the printing direction to compensate for printing nozzles being adjacent along the printing direction and having been marked as failed printing nozzles.

3. The method according to claim 1, which further comprises grouping the adjacent remaining printing nozzles in accordance with a fixed table saved on the computer and used as a grouping rule.

4. The method according to claim 3, which further comprises providing the fixed table used as the rule with all possible combinations for groups of both two and three printing nozzles printing at an angle transverse to and along the printing direction and printing nozzles having failed.

5. The method according to claim 4, which further comprises when groups of printing nozzles occur containing more than three printing nozzles printing at an angle in a direction transverse to or along the printing direction and printing nozzles having failed, subdividing the groups into smaller groups of two or three printing nozzles.

6. The method according to claim 4, which further comprises when groups of printing nozzles occur containing more than three printing nozzles printing at an angle in a direction transverse to or along the printing direction and printing nozzles having failed, creating and using new fixed tables for all possibly occurring combinations.

7. The method according to claim 1, which further comprises grouping the adjacent remaining printing nozzles in accordance with an algorithm implemented on the computer and used as a grouping rule.

8. The method according to claim 3, which further comprises carrying out the step of measuring all existing printing nozzles by determining a value of a numerically established obliqueness, factoring the value in the fixed table or algorithm as the grouping rule and using the value to contribute to a decision made by the computer on how to group the adjacent remaining printing nozzles.

9. The method according to claim 8, which further comprises using the computer to vary a dot size of the printing nozzles as a function of a value of the numerically established obliqueness of the printing nozzles.

Description

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

[0032] FIG. 1 is a diagrammatic, longitudinal-sectional view of an inkjet printing machine;

[0033] FIG. 2 is a plan view of a sheet with a representation of a white line resulting from a failed nozzle; and

[0034] FIG. 3 is a flow chart of the method of the invention.

DETAILED DESCRIPTION OF THE INVENTION

[0035] Referring now in detail to the figures of the drawings, in which mutually corresponding elements have the same reference symbols. and first, particularly, to FIG. 1 thereof, there is seen an illustration of an example of the structure of a sheet-fed printing machine 10. When this printing machine 10 is in operation, individual nozzles in inkjet print heads 4 in a printing unit 2 may fail as described above. Such a failure results in white lines 13 or, in the case of multicolor printing, in distorted color values. An example of such a white line 13 in a printed image 12 is shown in FIG. 2.

[0036] The printing operation is monitored by a control computer 21. A sheet 11 is transported in a direction of transport T from a feeder 1 over a transfer cylinder 5 to a printing cylinder or jetting cylinder 7 and to the print heads 4 which are formed of one or more rows of printing nozzles. The printing cylinder or jetting cylinder 7 has sheet-holding regions 8 and gaps 9 therebetween with grippers for holding the sheets 11. The cylinders 5, 7 are driven by one or more drives 6. The printing nozzles in the print heads 4 apply the ink to the sheets 11 to create the printed image 12. The sheet 11 is moved onwards, dried, and transported over the transfer cylinder 5 to a delivery 3.

[0037] FIG. 3 is a flow chart of the method of the invention. In the method of the invention, the creation of a printing nozzle list is introduced between the traditional steps of detecting failed printing nozzles and compensating for the failed nozzles. A measuring system takes measurements of all nozzles in a print head 4 to detect the failed printing nozzles. For every printing nozzle, known prior art measuring systems indicate a deviation in μm from an ideal position and a value indicating the continuity of a line that has been printed. In addition, printing nozzles are marked as totally failed if they print below a defined threshold of visibility or not at all. The invention preferably makes use of data-optical measuring systems. A nozzle list 14 that has been created in this way is then subjected to the method of the invention for nozzle list processing. The processing of the nozzle list represents the method of the invention. Once the nozzle list has been processed, an updated remaining nozzle list 19 is used as a list of printing nozzles 20 to be compensated for in order to compensate the corresponding failed printing nozzles. Thus, the method of the invention is largely independent of the way in which the measured values are established and of the way in which the compensation is effected although it is to be understood that the compensation method needs to be based on the fundamental principle of compensation by adjacent printing nozzles.

[0038] In the method of the invention, which is carried out by the control computer 21 in a fully computerized way, the first step is to filter all defective printing nozzles out of the list of all measured printing nozzles 14. This means that all nozzles that have been found to exceed a defined threshold are deemed to be defective. In this case, “defective” may refer to a nozzle that has completely failed or to a nozzle that prints in a deviating way. All functioning printing nozzles are removed from the list. In a next step, all remaining printing nozzles in the list 15 are examined to find out whether the respective nozzle still has directly adjacent printing nozzles. If this is not the case, the nozzle is an individual or isolated defective nozzle which needs to be compensated for in any case, irrespective of whether it has failed or only deviates from the desired print result. In this case the nozzle in question remains on the list 15 and the next printing nozzle is examined. If the printing nozzle has immediate neighbors, however, it belongs to a group of defective printing nozzles. Such a group is formed, in accordance with a grouping rule in the form of a fixed table 16, resulting in a printing nozzle list 17 that includes grouped adjacent defective printing nozzles in addition to the isolated defective printing nozzles. The fixed table 16 shown below includes all possible combinations that may occur between failed printing nozzles and printing nozzles that print in a deviating way to the left and to the right.

[0039] 2 Defective Printing Nozzles—1 Missing Nozzle and 1 Nozzle Spraying at an Angle

TABLE-US-00001 Case input Output 1.1 M.fwdarw. X.fwdarw. 1.2 .fwdarw.M .fwdarw.X 1.3 M← X← 1.4 ←M ←X

[0040] 2 Defective Printing Nozzles—2 Nozzles that Spray at an Angle

TABLE-US-00002 Case input Output 2.1 .fwdarw..fwdarw. .fwdarw.X 2.2 .fwdarw.← X← 2.3 ←.fwdarw. X.fwdarw. 2.4 ←← X←

[0041] 3 Defective Printing Nozzles

TABLE-US-00003 case input output 3.1 .fwdarw..fwdarw..fwdarw. .fwdarw..fwdarw.X 3.2 .fwdarw..fwdarw.← .fwdarw.X← 3.3 .fwdarw.←.fwdarw. X←.fwdarw. 3.4 .fwdarw.←← ←X← 3.5 ←.fwdarw..fwdarw. ←.fwdarw.X 3.6 ←.fwdarw.← ←.fwdarw.X 3.7 ←←.fwdarw. X←.fwdarw. 3.8 ←←← X←← 3.9 M.fwdarw..fwdarw. X.fwdarw.X 3.10 M.fwdarw.← X.fwdarw.X 3.11 M←.fwdarw. X←X 3.12 M←← X←X 3.14 .fwdarw.M.fwdarw. .fwdarw.X.fwdarw. 3.15 .fwdarw.M← .fwdarw.X← 3.16 ←M.fwdarw. ←X.fwdarw. 3.17 ←M← ←X← 3.18 .fwdarw..fwdarw.M X.fwdarw.X 3.19 .fwdarw.←M X←X 3.20 ←.fwdarw.M X.fwdarw.X 3.21 ←←M X←X 3.22 MM.fwdarw. XX.fwdarw. 3.23 MM← XX← 3.24 M.fwdarw.M X.fwdarw.X 3.25 M←M X←X 3.26 .fwdarw.MM .fwdarw.XX 3.27 ←MM ←XX Explanation: M—missing; .fwdarw.—nozzle sprays to the right; ←—nozzle sprays to the left; X—nozzle is compensated for

[0042] The table 16 encompasses both the cases in which two defective printing nozzles are disposed next to one another and the case in which three defective printing nozzles are present. Then a suitable compensation rule is proposed as an output for every possible combination as an input variable. Possible output values for the individual printing nozzles are values that indicate that the printing nozzle needs to be compensated for or that it is used for compensation purposes, printing to the left or to the right. If a group of defective printing nozzles includes more than three defective printing nozzles, the group is subdivided into smaller sub-groups, each sub-group including two or three defective printing nozzles, and is then processed in a corresponding recursive way. In a case in which the group only is formed of failed printing nozzles and no printing nozzles that spray to the left or to the right, clearly no grouping rule is required. Instead, all failed nozzles are compensated for in the traditional way suggested by the prior art. Once this step has been completed, a printing nozzle list 18 includes grouped adjacent printing nozzles, which have been marked as to be compensated for or marked as functioning in accordance with the initial value of the fixed table 16.

[0043] If the grouping was done in accordance with the grouping rule, the printing nozzles that have not been marked as to be compensated for, i.e. the nozzles that are functioning printing nozzles, are removed. Thus, the printing nozzle list 19 now only contains printing nozzles that are too defective and need to be compensated for, i.e. the remaining isolated ones and the grouped printing nozzles. The resultant reduced printing nozzle list 20 will then be used for compensation. Since it contains many fewer printing nozzles to be compensated for than without the method of the invention and since the number has been reduced in particular in those difficult areas that include multiple defective nozzles located next to one another, the method of the invention results in a much more efficient compensation than the method that has previously been known in the art.

[0044] 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

[0045] T direction of transport [0046] 1 feeder [0047] 2 printing unit [0048] 3 delivery [0049] 4 inkjet heads [0050] 5 transfer cylinder [0051] 6 drive [0052] 7 printing cylinder (jetting cylinder) [0053] 8 sheet-holding region [0054] 9 gap [0055] 10 sheet-fed printing machine [0056] 11 sheet [0057] 12 printed image [0058] 13 white line [0059] 14 measured, complete printing nozzle list [0060] 15 printing nozzle list of defective printing nozzles [0061] 16 grouping rule—fixed table [0062] 17 printing nozzle list of grouped adjacent printing nozzles [0063] 18 printing nozzle list of marked adjacent printing nozzles [0064] 19 printing nozzle list of filtered compensating printing nozzles [0065] 20 complete printing nozzle list for compensation [0066] 21 control computer