EP1894728A1 - Controlling the working condition of inkjet printheads in digital printing machines - Google Patents

Abstract

The method involves receiving a defined print (20), which contains a point that is printed by each inkjet-nozzle of inkjet-print heads (6, 7) by individual very highly resolving electronic cameras (9, 12). The point is processed in an image evaluating unit (16) and the optical appearance of individual print points are individually and statistically examined with respect to contrast, magnitude, shape and/or position. Conclusions on the function or malfunctioning of the individual inkjet-nozzles are made from the examination. An independent claim is also included for a device installed in a digital printing machine.

Description

In digital printing machines, paper speeds of more than 300 m / min are already possible today. Therefore, printhead malfunctions (such as single printhead nozzle failure) will result in significant material loss until the error can finally be detected and resolved.

In order to avoid this, there are automatic monitoring methods which make pressure density measurements on the printed product or form sealant values on the printed pages column by column and compare them with the associated nominal values. In another method, 100% full frame control, each print page is completely scanned and compared to a stored reference page. However, as the resolution of inkjet printheads in commercial printing at up to 300dpi (dots per inch) is very high and on sheetfed or web presses even multiple double-sided benefits are printed side-by-side, all of which must be inspected, such a full-frame inspection is very expensive.

Disadvantages of the previous methods are, firstly, the relatively low spatial resolution, which detects disturbances of the inkjet nozzles only when malfunctions have already significantly affected the print image, and second, that during production by sporadic printing and paper errors and error rates of Test systems themselves ("false rejects") unintentionally result in high reject rates. In production, it is therefore more important to monitor the correct function of the printing press than the 100% accuracy of each individual product.

Object of the present invention is to propose an optical test method, with the nozzle failures and other emerging Identify incorrect behavior of individual printhead nozzles or the entire printhead at an early stage.

The invention relates to a method and a device for automatic quality monitoring of inkjet printheads in digital printing machines by the high-resolution detection and evaluation of defined printing patterns using an image processing system. For example, in a typical application of the method in a digital web press, the image processing system consists essentially of two high resolution cameras, one for the front and one for the back of the paper web, transversely to the direction of transport across the entire width by means of a mechanical shuttle for a few seconds the paper web are moved back and forth. In each case, they take up small sections of a test pattern strip with microscope-like resolution. A connected image evaluation unit evaluates the image data at high speed and checks whether all the inkjet nozzles used are working properly. For this, the inkjet dots of a defined printed test strip are examined for their presence, their contrast, their size, their shape and their position. It also detects the intensity of satellite droplet formation and smearing. By statistically evaluating the data obtained, tendencies deviations from preset target values are recognized in good time and compared with empirical data of earlier malfunctions of the same printheads. This makes it possible to replace printheads in time, possibly even before it comes to a total failure or other gross malfunction of individual nozzles or the entire printhead. If adjustable limit values are exceeded, warning signals and error messages can be output and, if necessary, the machine is also stopped.

In order to be able to examine the pressure points in terms of size, position and shape exactly, it requires a very high camera resolution, which must be at a multiple of the inkjet resolution. With an inkjet print dot resolution of 300 dpi (dots per inch), the optical camera resolution is on the order of at least 1200 to 2400 ppi (pixels per inch). Because of the high transport speed of the substrate matrix cameras are used. In order to keep the motion blur very small during image acquisition at such high transport speeds of the objects to be detected, a combination of a short electronic shutter (shutter) of the camera and very short and bright strobe flashes are used. The required short exposure time is on the order of down to 2 μsec. The required sufficient field illumination is achieved by very bright flashes of light.

The "defined print pattern" is either a special test strip, which is located in the sheet edge area to be cut off later anyway, or specially selected, known cutouts in the used print field. Especially suitable are horizontal lines and line sections, where the behavior of the nozzles can be observed, especially at the edges. "Defined Print Pattern" means the expected print pattern to be captured by the camera at a particular location. This is defined as a desired pressure pattern by a teach-in process or by the pressure-underlying record. Whether the defined print pattern is selected from the printed work area or is a test strip printed specifically for this purpose, in both cases the defined print pattern should meet some basic conditions. Each print dot position within the image window must be clearly assignable to a particular nozzle of the print head and printed at least once and printed at least once. The printed dots (Ink dots) should occur as isolated as possible in order to minimize the overlay by the influence of several adjacent pressure points on a specific point of the print pattern.

These conditions are best met by a specially defined and printed test strip. Here is an example of such a test strip definition:

The test strip consists of 9 pressure point lines, of which only 5 are printed. Each nozzle prints 1 to 4 dark dots. Lines 1, 3, 5 and 9 are always empty (---------...). In line 2 is a test pattern, where only every second point is printed, in line 4, the inverted pattern. In line 6 there is a position code. The position code consists of a sequence of data blocks with 21 dot positions each. Each data block consists of an always identical start code (-xxx-) and an incremental 8-bit binary counter: 00000001 b, 00000010b, 00000011b, 00000100b, 00000101b ... The binary digits "0" and "1" are entered by a Dot pair represented, either "- x" (white + black for information bit "0") or "x -" (black + white for information bit "1"). The 8-bit sample counter of the position code theoretically increments up to a maximum of 255, but practically less, depending on the width of the print head or the print side. At 300dpi each data block width is about 1.8mm (21 dots). In line 7 is only the start code, the counter points are empty. In line 8 is the start code followed by the inverted position code, in which each dot is black, blackened in line 6, and conversely, each dot is white, which is white in line 6.

For a better understanding below the beginning of the test strip. The character "-" stands for unprinted, white dot, the character "x" for a printed dot. -------------------------------------------------- -------------------------------- ... empty line xxxxxxxxxxxxxxxxxxxxx-xxxxxxxxxxx ... test pressure -------------------------------------------------- ------------------------------- ... empty line -xxxxxxxxxxxxxxxxxxxxx-xxxxxxxxxx- ... Inv.Testdruck -------------------------------------------------- -------------------------------- ... empty line -xxx - xxxxxx xxxxx - xxxxx-xxx-xxx - xxxxx-xx-x ... position code -xxx ------------------ ------------------ xxx xxx ---------- --------------------- ... start code -xxx-xxxxxxxx - xxxxxxxx xxx - xxx-xxxxxxxx ... Inv.Pos.Code -------------------------------------------------- --------------------------------- ... empty line Counter = 1 Counter = 2 Counter = 3

Counter Code:

Decimal Binary Hex Dot pattern (- = white, x = black) 0 00000000b 00H -x-x-x-x-x-x-x-x 1 00000001b 01H -x-x-x-x-x-x-xx 2 00000010b 02H -x-xx-xx-xx - x 3 00000011B 03H -x-xx-xx-xx-x 4 00000100b 04H -x-xx-x-xx - xx 5 00000101b 05H -x-xx-x-xx - xx 6 00000110b 06H -x-xx-x-xx-xx 7 00000111b 07H -x-xx-x-xx-x-x- 8th 00001000b 08H -x-xx-xx - xx-x 9 00001001b 09H -x-xx-xx - x-xx 10 00001010b OAH -x-xx-xx - xx - x 11 00001011b OBH -x-xx-xx - xx-x 12 00001100b OCH -x-xx-xx-xx-x 13 00001101b ODH -x-xx-xx-x - xx 14 00001110b OEH -x-xx-xx-xx - x 15 00001111b OFH -x-xx-xx-xx-x 16 00010000b 10H -x-x-xx - xx-xx 17 00010001b 11H -x-x-xx - xx-xx ............. 97 01100001b 61H -xx-x - x-x-x-xx (0 * 128 + 1 * 64 + 1 * 32 + 0 * 16 + 0 * 8 + 0 * 4 + 0 * 2 + 1 * 1) .............

During transport from the printhead to the camera, the paper may shift sideways, it may slightly buckle, warp, twist, stretch, or it may shrink slightly. Therefore, the exact position of the image section in the print pattern itself must be included, because even a very precise absolute position sensor on the camera positioning device can not measure exactly where the camera image field in the test strip and thus could not be determined which pressure point through which inkjet Nozzle has been printed. That is, without additional information in the image itself, it would not be possible to determine which pressure point is to be assigned exactly to which nozzle. It is less the exact absolute position of the pressure points on the paper important, but the relative position to the adjacent pressure points. The assignment of which pressure point originates from which pressure nozzle or which of the successively recorded pressure point images correspond with each other is only possible if the constellation of printed dots in the camera image field in the vicinity of each pressure point is unique and unique, and thus the pressure points can be unambiguously identified. To insure this, the test strip includes such - just a few millimeters wide - individual dot sequences with a position code (e.g., a counter incrementing over the printhead width).

Without the unique assignment of the individual pressure point positions to points in the camera image, it would not be possible to make statistical statements about the behavior of individual nozzles by comparing entire image sequences. Because out Due to the disturbing influence of the substrate surface (eg paper structure, absorbency), it is only possible to make statements about nozzle behavior in some cases. This is achieved to a limited extent by recognizing the surface structure from the shadow and then determining and compensating for its influence on the visible properties of the pressure points. In order to be able to filter out the disturbance variables better, image sequences are evaluated. However, since these images are taken at different times at different locations on the paper web or sheet, it is necessary to have recognizable patterns close to the print point positions to be examined, which do not repeat over a distance width of the possible lateral position variations of the paper.

Hereinafter, a basic embodiment will be described with reference to FIG. This example shows an embodiment for double-sided inkjet monitoring in a web press. In the printing machine, the paper web (1) passes through the front-side printing roll (2) with the front-side inkjet printing head (6) or with a number of such print heads. On the platen roller (3), the back of the paper web is printed by means of one or a series of inkjet printheads (7). Each inkjet print head (6,7) has a large number of parallel inkjet nozzles. At any point on the machine after each printing or after both printing operations, the inline inkjet inspection is done for the front of the paper web (4,8,9,10) and for the back (5,11,12,13), if it is double-sided printing. The inspection has in each case a camera movement unit (10, 13) transversely to the transport direction of the paper web. The movement units (10, 13) may be e.g. have a linear motor and the sensor units (8,9 and 11,12) over the entire width of the paper web. The sensor units consist here of high-resolution, fast, triggerable matrix cameras (9,12) including macro lenses, as well as LED flash units (8,11), which are preferably equipped with monochrome black printing with infrared LEDs. There is also a triggering and synchronizing unit for cameras and flashlights with e.g. Print mark sensors (14,15) and, if necessary, an encoder. The image processing electronics (16) moves, positions and triggers the sensor units (8,9 and 11,12), generates the pulses for the flash lights (8,11) and has a communication interface to the printing press.

Claims (13)

Method for the automatic quality control of the function of inkjet printheads (6,7) in digital printing machines. The method is characterized in that a defined print pattern (20) containing at least one dot printed by each used nozzle of the inkjet print head (6, 7) is received by at least one single very high resolution electronic camera (9, 12) and is processed in an image evaluation unit (16) in such a way that the visual appearance of the individual printing dots in terms of contrast, size, shape and / or position is examined individually and / or statistically and conclusions are drawn about the function or malfunction of the individual inkjet nozzles , A method according to claim 1, characterized in that the camera or cameras (9,12) thereby moved transversely to the transport direction (10,13) are. Method according to claims 1 and 2, characterized in that the defined printing pattern is a horizontal test strip (20) designed and printed for this purpose. Method according to claim 3, characterized in that this test strip contains a position identifier (20). A method according to claims 1 and 2, characterized in that the printed useful range of the arc cut-outs are removed as defined printing pattern. The method of claim 1-5, wherein the inspection system warns the user in case of failure optically and / or acoustically. The method of claim 1-6, wherein the inspection system stops in case of failure, the printing press. The method of claim 1-7, wherein the inspection system marks and / or rejects defective printed matter or groups of defective printed matter. The method of claim 1-8, wherein the test result of the inspection system is fed back to the printing device to readjust accordingly. Apparatus according to claim 1-9 mounted on a web-fed printing press (fig. 1). Apparatus according to claim 1-9 mounted on a sheet-fed printing machine. Apparatus according to claim 10 and 11 with inspection camera (s) (9 or 12) on only one side of the printed material web (1) or sheet. Apparatus according to claim 10 - 12, wherein a camera or a plurality of cameras (9,12) are moved by a linear drive over the web or sheet width.

Images