Method of multi-color pagewide printing with optimized nozzle hydration

Abstract

A method of multi-color printing in a single pass using a fixed pagewide array of monochrome inkjet printheads. The printheads are aligned with each other in a media feed direction and the method includes the steps of: feeding print media along the media feed direction past a first printhead positioned furthest upstream in the array, a third printhead downstream of the first printhead and a second printhead positioned furthest downstream in the array; printing a yellow ink onto the print media using the first printhead; printing a black ink onto the print media using the third printhead; and printing a cyan or magenta ink onto the print media using the second printhead.

Claims

1. A method of multi-color printing in a single pass using a fixed pagewide array of monochrome inkjet printheads, the printheads being aligned with each other in a media feed direction, the method comprising the steps of: feeding print media along the media feed direction past a first printhead positioned furthest upstream in the array, a third printhead downstream of the first printhead and a second printhead positioned furthest downstream in the array; printing a yellow ink onto the print media using the first printhead; printing a black ink onto the print media using the third printhead; and printing a cyan or magenta ink onto the print media using the second printhead, wherein: each of the first, second and third printheads prints a respective keep-wet pattern onto print media; each keep-wet pattern is defined by a plurality of dots printed at a frequency sufficient to maintain hydration of each nozzle of a respective printhead; all nozzles of the first printhead print a first keep-wet pattern at a first average frequency, all nozzles of the second printhead print a second keep-wet pattern at a second average frequency, and all nozzles of the third printhead print a third keep-wet pattern at a third average frequency; and the first average frequency is higher than the third average frequency.

2. The method of claim 1, further comprising the steps of: feeding the print media past a fourth printhead positioned between the first and second printheads; and printing a magenta or cyan ink onto the print media using said fourth printhead.

3. The method of claim 1, wherein the first average frequency is higher than the second average frequency.

4. The method of claim 1, wherein the print media is fed past the array at a speed of greater than 0.2 meters per second.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Embodiments of the present invention will now be described by way of example only with reference to the accompanying drawings, in which:

(2) FIG. 1 shows data flow between a computer system and a printer;

(3) FIG. 2 shows data between a print engine controller chip (PEC) and a printhead;

(4) FIG. 3 shows schematically a page tiled with a keep-wet pattern based on a unit cell;

(5) FIG. 4 is a schematic side view of a printhead having upstream and downstream ink planes; and

(6) FIG. 5 is a schematic plan view of a printer comprising multiple aligned monochrome printheads.

DETAILED DESCRIPTION OF THE INVENTION

(7) Tailored Keep-Wet Pattern Per Ink Plane

(8) Referring to FIG. 1, there is shown schematically a printing system having a specific architecture for implementing the method described in connection with the first aspect.

(9) A computer system 2 communicates with a printer 4 via a suitable communications link, such as a wired or wireless connection. The computer system 2 comprises a raster image processor (RIP) 6 which receives a compressed image file from a suitable application 8 generating images to be printed. The compressed image file may be in any suitable image file format, such as PDF, JPEG, TIFF, GIF etc or any suitable page description language, such as a PostScript, PDL etc. The RIP 6 processes the compressed image data and sends bitmap image data to a printer driver 10. The printer driver 10 sends the bitmap image data together with keep-wet pattern data (keep-wet data) for each ink plane of a printhead 20 to a print engine controller chip (PEC) 12 of the printer 4. Determination of the appropriate keep-wet pattern data for each ink plane will be described in further detail below.

(10) In an alternative architecture, the application 8 may send a compressed image file directly to the printer driver 10, which sends compressed image data to the PEC 12. In this alternative architecture, the PEC 12 decompresses the compressed image data to generate bitmap image data.

(11) In a still further alternative architecture, the printer driver 10 may send a pattern identifier for each ink plane to the PEC 12 instead of actual keep-wet pattern data. In this alternative architecture, the PEC 12 retrieves keep-wet pattern data corresponding to each pattern identifier from a memory of the printer 4 (e.g. a memory in the PEC 12), which stores a plurality of different keep-wet pattern data, each being indexed with a respective pattern identifier.

(12) In a still further alternative architecture, the printer driver 10 sends only image data to the PEC 12. In this alternative architecture, the PEC 12 (rather than the printer driver 10) determines appropriate keep-wet pattern data for each ink plane and retrieves these data from a memory.

(13) From the foregoing, it will be appreciated that various alternative architectures will be readily apparent to the skilled person for implementing the present invention. The particular architecture shown in FIG. 1 is not limiting and has been shown for illustrative purposes only.

(14) Referring now to FIG. 2, the PEC 12 generates print data for each ink plane of the printhead 20. In this case, the printhead 20 has five ink planes, although it will be appreciated that the printhead may have any number of ink planes. The keep-wet data for each of the five ink planes, received from the printer driver 10, is loaded into a first writable memory 22 (e.g. RAM) of the PEC 12 while the image data is loaded into a second writable memory 24, which may be a same or different memory unit of the PEC. The image data is separated into the different ink planes and processed in the PEC to generate first print data for each ink plane. The first print data for each ink plane is merged (OR'd) with corresponding keep-wet data for that ink plane (by retrieving the corresponding keep-wet data from the first writable memory 22) to generate second print data. Finally, print data is sent to the printhead 20 for each ink plane. The second print data resulting from the merging step is usually processed further in the PEC 12 to generate third print data before being sent to the printhead 20. It will, of course be appreciated that FIG. 2 represents a simplified scheme for PEC processing and that some processing steps for generating print data have been omitted for clarity.

(15) The keep-wet pattern data represents a pseudo random pattern of dots which is superimposed on the printed image. The keep-wet pattern ensures that each nozzle of the printhead 20 is fired within a predetermined period of time, which is generally less than the decap time of that nozzle. The keep-wet pattern therefore ensures that each nozzle of the printhead stays properly hydrated during a print job, even if the printed image does not demand firing of that nozzle and there has been no maintenance intervention.

(16) The pseudo random pattern of dots in the keep-wet pattern of each ink plane may be based on a unit cell (e.g. a rectangular tile), which is repeated both across and down the print media. For example, and referring to FIG. 3, each unit cell of the keep-wet pattern for a particular ink channel may be comprised of a m?n rectangular cell 26, which is tiled over a page 27. The number of rows n (representing the height of the cell) may be in the range of 200 to 100,000 lines of print and the number of columns m (representing the width of the cell) may be in the range of 100 to 5,000 nozzles. In FIG. 3, the lines of print are schematically represented as lines 28, while the nozzles are schematically represented as arrows 29 (only two shown for clarity).

(17) It will be appreciated the unit cell 26 may have any suitable shape (e.g. hexagonal, triangular etc) or dimension. However, relatively larger cells 26 provide a greater degree of pseudo randomness in the keep-wet pattern and lower overall visibility.

(18) In order to randomize the keep-wet pattern further, a different offset may be applied to the keep-wet pattern on sequential pages so that the same keep-wet pattern is not tiled across each printed page in a sequence. The offset helps to remove repetition artifacts which may be visible in collated documents e.g. a dot appearing at the same position at an edge of every page. The offset is typically applied by the PEC 12 before merging the keep-wet pattern data with the first print data. The offset may be a simple instruction to advance the keep-wet pattern by p row(s) and/or q column(s) for every printed page, where p<n and q<m. Typically, p and q are each independently integers of 1 to 50.

(19) Self-evidently, a drawback of printing the keep-wet pattern is a loss of print quality and it is, therefore, important to ensure that the visibility of the keep-wet pattern is minimized as far as possible.

(20) The first aspect of the present invention enables the keep-wet pattern for each ink plane of the printhead to be tailored to a particular print job. Typically, the printer driver 10 determines a keep-wet pattern suitable for each ink plane based on one or more input parameters and sends appropriate keep-wet pattern data to the PEC 12. The printer driver 10 typically has an algorithm for determining the most appropriate combination of keep-wet patterns for the ink planes by weighting the various input parameters accordingly. As described above, in an alternative system architecture, determination of the keep-wet pattern data may be performed entirely by the PEC 12 in the printer 4.

(21) Some of the parameters that may be used for determining the keep-wet pattern for each ink plane are discussed in detail below:

(22) (1) Position of Ink Plane in Printhead

(23) The position of the ink plane in the printhead determines, to a large extent, the local dehydrating environment of the ink plane and, therefore, the frequency of keep-wet ejections required. Typically, the ink plane furthest upstream in the printhead is in the most dehydrating environment as a result of the airflow experienced by the printhead and, therefore, requires a more frequent keep-wet pattern than the downstream ink planes. This is discussed in more detail below.

(24) (2) Print Speed

(25) The print speed is directly related to the speed of airflow experienced by the printhead. With higher print speeds, the speed of the airflow generated by the moving print media is higher and this has a greater dehydrating effect on the nozzles.

(26) (3) Type of Ink

(27) The color of ink is an important factor in determining an appropriate keep-wet pattern. For example, the keep-wet pattern is most visible with high luminance inks, such as black and least visible with low luminance inks, such as yellow. Therefore, a higher frequency keep-wet pattern is usually more tolerable in a yellow ink plane than a black ink plane. Indeed, yellow keep-wet patterns are virtually invisible, even at relatively high keep-wet frequencies.

(28) Furthermore, some inks intrinsically have different dehydration characteristics than other inks and this is a fundamental criterion for determining an appropriate keep-wet pattern for a particular ink plane. For example, inks having a relatively high colorant loading tend to suffer more from dehydration effects than inks having a relatively low colorant loading. Of course, in a monochrome printhead, where all ink planes eject the same ink, the intrinsic dehydration characteristics of the ink will be the same in each ink plane of the printhead.

(29) (4) Type of Print Media

(30) Keep-wet patterns are usually less visible when printed on plain print media and more visible when printed on glossy print media.

(31) (5) Length of Print Job

(32) Dehydrating effects tend to increase over time, rather than reach a point of equilibration. Therefore, the length of the print job is an important parameter for determining an appropriate keep-wet pattern. Generally, it is undesirable for a long print run to have varying print quality, so the keep-wet pattern should be determined based on the greatest anticipated dehydrating environment, which will usually be at the end of the print run.

(33) (6) Ambient Humidity

(34) Ambient humidity may be measured using an appropriate humidity sensor on the printer and feeding back ambient humidity data to the printer driver. If the printer is positioned in a relatively humid environment, then a less frequent keep-wet pattern will be required compared to a relatively dry environment.

(35) (7) Ambient Temperature

(36) Ambient temperature may be measured using a temperature sensor on the printer and feeding back ambient temperature data to the printer driver. If the printer is positioned in a relatively cool environment, then a less frequent keep-wet pattern will be required compared to a relatively warm environment.

(37) (8) Image Content

(38) Ideally, the keep-wet dots should be coincident with the image, as far as possible, so that they have minimal effect on print quality. Likewise, printing high luminance (black) keep-wet dots on areas of low luminance in the image should be avoided as far as possible. Accordingly, the determination of the most appropriate keep-wet pattern for each ink plane may take into account the image data. For example, if the image contains regularly repeating blocks of color, then a keep-wet pattern coincident with these repeating blocks of color may be most appropriate.

(39) (9) Optical Interference

(40) Some or all of the ink planes of the printhead typically eject different keep-wet patterns. Visibility of the combined keep-wet patterns may be inadvertently increased if there are any optical interference effects (e.g. Moir? interference effects) between the various keep-wet patterns. Therefore, the selected keep-wet patterns for the ink planes of the printhead should preferably be orthogonal in the sense that they produce minimal optical interference effects when printed together on the print media. Usually, the keep-wet patterns are selected to minimize any dot-on-dot printing from the different keep-wet patterns.

(41) (10) Minimum Print Quality Threshold

(42) Each print job may have a minimum print quality threshold which is set by the end user. Although maximizing print quality is paramount, some end uses may have different print quality criteria to others. This, in turn, affects the keep-wet patterns available for use. In some circumstances, it may be necessary to change other print parameters (e.g. print speed or length of print job) so that the keep-wet pattern can be incorporated within acceptable print quality limits.

(43) From the foregoing, it will be appreciated that the keep-wet pattern for each ink plane of the printhead 4 may be tailored to provide an overall printed keep-wet pattern, which has minimum visibility.

(44) Keep-Wet Frequency Highest in Upstream Ink Plane

(45) A printhead employed in connection with the present disclosure typically comprises a plurality of ink planes. Each ink plane comprises one or more nozzle rows, with each nozzle in one ink plane being supplied with the same ink. For example, a Memjet? printhead comprises a pair of nozzle rows per ink plane, which are supplied with the same inkone nozzle row prints even dots and the other nozzle row prints odd dots to make up a line of print for one ink plane.

(46) The plurality of ink planes may be supplied with the same ink, all different inks, or at least one same ink and at least one different ink. For example, in a printhead having five ink planes, all five ink planes may be supplied with the same ink to provide a monochrome printhead (e.g. CCCCC, MMMMM, YYYYY, KKKKK etc.). Alternatively, only some of the ink planes may be supplied with the same ink (e.g. CMYKK, CCMMY etc). Alternatively, each ink plane may be supplied with a different ink (e.g. CMYK(IR) or CMYKS, where IR is an infrared ink and S is a spot color, such as khaki, orange, green, metallic inks etc).

(47) With a fixed or stationary inkjet printhead, each ink plane of the printhead is positioned relatively upstream or downstream with respect to the media feed direction. The present inventors have found that the relative positioning of each ink plane in a fixed inkjet printhead has a marked effect on the local humidity of that ink plane relative to the other ink planes in the printhead during printing. Generally, the ink plane positioned furthest upstream with respect to the media feed direction is observed to be a in a relatively more dehydrating environment (i.e. less humid) than other ink planes in the printhead.

(48) Referring to FIG. 4, there is shown schematically a side view of the inkjet printhead 20 comprising five ink planes (32, 34, 36, 38 and 40), each comprising a pair of nozzle rows (32A & 32B, 34A & 34B, 36A & 36B, 38A & 38B and 40A & 40B). The ink planes are separated from each by a distance in the range of 50 to 100 microns.

(49) A print medium 45 is fed in a media feed direction (right to left as shown in FIG. 4) by a media feed mechanism 47, which may take the form of a pair of opposed rollers gripping the print medium in a nip defined therebetween. The media feed direction therefore defines an upstream side and a downstream side of the printhead 20.

(50) The motion of the print medium 45 in the media feed direction generates an airflow in a corresponding direction, as shown in FIG. 4. The speed of this airflow depends on the speed of the print medium, and to some extent, the type of print medium. For example, a continuous web will tend to generate a higher airflow than printing onto discrete sheets of print media.

(51) As a consequence of this airflow, the ink plane 32 furthest upstream in the printhead 20 is positioned in the relatively most dehydrating environment compared to the other ink planes 34, 36, 38 and 40. The ink plane 32 is most exposed to the airflow, whereas the downstream ink planes 34, 36, 38 and 40 enjoy a degree of shielding from this dehydrating airflow by virtue of a stream of ink droplets ejected from nozzle rows 32A and 32B.

(52) It is desirable for the printhead 20 to eject the minimum required frequency of keep-wet drops in order to maintain each nozzle of the printhead sufficiently hydrated during a print job. Any keep-wet drops which are excess to requirements are not only wasteful of ink, but more importantly, reduce print quality unnecessarily.

(53) From the foregoing, it will be apparent that the minimum keep-wet frequency required for ink plane 32 will be higher than the minimum keep-wet frequency required for the other ink planes 34, 36, 38 and 40. This observation may be used in both monochrome and multicolor printheads to minimize the overall visibility of keep-wet patterns by ensuring only a minimum required keep-wet frequency for each ink plane.

(54) Moreover, in a multicolor printhead, supplying a low luminance color, such as yellow, to the furthest upstream ink plane 32 advantageously minimizes the visibility of the relatively high frequency keep-wet pattern ejected from this ink plane. In a typical CMYK ink set, yellow has by far the lowest luminance compared to other colors. (The nominal luminances of CMYK inks on white paper are as follows: C (30%), M (59%), Y (11%) and K (100%)). Therefore, by supplying yellow ink to the furthest upstream ink plane 32, the perceived visibility of the overall keep-wet pattern ejected by all color planes can be significantly reduced.

(55) As discussed above, the furthest upstream ink plane 32 is positioned in a locally most dehydrating environment of the printhead 20, because it does not benefit from any shielding from the airflow. Aside from the shielding effect of upstream ink plane(s), a secondary factor determining local humidity of a particular ink plane is the number of neighboring ink planes. For example, in FIG. 4, ink planes 34, 36 and 38 each have a pair of neighboring ink planes, whereas ink planes 32 and 40 only have one neighboring ink plane. Neighboring ink planes tend to increase the local humidity of an ink plane sandwiched therebetween. Accordingly, ink plane 40 positioned furthest downstream in printhead 20 is positioned in a relatively more dehydrating environment than ink planes 34, 36 and 38, but in a relatively less dehydrating environment than ink plane 32. Consequently, the relative minimum keep-wet frequencies of the ink planes for the printhead 20 may be in the order: ink plane 32>ink plane 40>ink planes 34, 36 and 40

(56) Since ink planes 34, 36 and 40 are positioned in the least dehydrating local environment, it is advantageous to supply the highest luminance ink(s) (typically black) to these middle ink planes in order to minimize visibility of keep-wet patterns.

(57) In light of the foregoing, in a Memjet? printhead having five ink planes supplied with CMYK inks, an advantageous plumbing arrangement may be Y-K-M-K-C or Y-K-C-K-M, with yellow (Y) furthest upstream and black (K) occupying middle ink planes.

(58) Multiple Aligned Monochrome Printheads

(59) The principles discussed above in connection with ink planes of a single printhead 20, may be applied in a printer comprised of a plurality of monochrome printheads aligned in a media feed direction.

(60) FIG. 5 shows schematically in plan view a high-speed web printer 50 comprised of five fixed inkjet printheads 52, 54, 56, 58 and 60, which are aligned with each other in a media feed direction. The printheads are spaced apart from each by a distance in the range of 3 to 20 cm. Each printhead is a monochrome printhead, which ejects a single color of ink from a plurality of ink planes. For example, the five monochrome printheads 52, 54, 56, 58 and 60 may eject CMYK inks (e.g. CMYKK) or CMYKS inks.

(61) A web of print media 62 is fed past each of the printheads in the media feed direction as shown using a suitable media feed mechanism. This type of printer, which is described in more detail in US 2012/0092403 (incorporated herein by reference), is capable of printing at very high speeds, such as speeds greater than 0.2 meters per second, greater than 0.5 meters per second, or greater than 1 meter per second.

(62) By extension of the principles discussed above in connection with FIG. 4, the printhead 52 positioned furthest upstream with respect to the media feed direction is in the relatively most dehydrating environment compared to the other printheads 54, 56, 58 and 60. Therefore, the printhead 52 generally requires a higher average keep-wet frequency than the other printheads. (Note that individual ink planes in each printhead may have different keep-wet frequencies, but the average minimum keep-wet frequency across all ink planes in printhead 52 is higher than the average minimum keep-wet frequency for the other printheads 54, 56, 58 and 60). Furthermore, it is advantageous to supply printhead 52 with the lowest luminance ink (usually yellow) so that the keep-wet pattern ejected from printhead 52 has minimal visibilitythe lower luminance of yellow ink effectively compensates for the higher average keep-wet frequency required in printhead 52.

(63) Similarly, it is advantageous to supply the highest luminance ink to one or more of the middle printheads 54, 56 and 58. These printheads benefit, at least to some extent, from the upstream shielding effect of printhead 52 as well as the humidifying effect of two neighboring printheads.

(64) Since the printhead spacing in the printer 50 is of the order of centimeters, as opposed to the micron-scale separation of ink planes within the printhead 20, the local humidifying effects in the printer 50 will be less pronounced than those described above in connection with FIG. 4. Nevertheless, there is a demonstrable advantage in positioning the yellow printhead 52 furthest upstream in the printer 50 and this has a direct effect in improving print quality via minimization of keep-wet frequencies. Keep-wet patterns are virtually inevitable for maintaining adequate hydration in inkjet web printers, where there is no opportunity for between-page spitting and less opportunity for maintenance interventions compared to desktop sheet-fed printers. Accordingly, the present invention is most advantageous when employed in connection with inkjet web printers, such as the printer 50 shown in FIG. 5.

(65) It will, of course, be appreciated that the present invention has been described by way of example only and that modifications of detail may be made within the scope of the invention, which is defined in the accompanying claims.