WO2025032206A1

WO2025032206A1 - Method and system of setting the printhead driving voltage in an inkjet printer

Info

Publication number: WO2025032206A1
Application number: PCT/EP2024/072546
Authority: WO
Inventors: Petrus Leonardus Johannes Welten; Johannes Maria VAN DEN HEUVEL
Original assignee: SPGPrints BV
Current assignee: SPGPrints BV
Priority date: 2023-08-09
Filing date: 2024-08-09
Publication date: 2025-02-13
Anticipated expiration: 2026-02-09
Also published as: KR20260043108A; CN121488246A; IL325414A; NL2035569B1

Abstract

A method of setting the printhead driving voltage of a printhead (40) of a multi-colour inkjet printer (12) of the bidirectional scanning type for printing an image on a printing medium comprises printing a first test pattern (28; 30) by ejecting first droplets (44) and second droplets from the nozzles (42) of each printhead (40) at different printhead driving voltages during movement of the carriage (14) in a first stroke and opposite second stroke of the scanning direction at predetermined positions of the printhead. From a test pattern scan the jet velocity is calculated, which is compared to a reference jet velocity. Based on this comparison a corrected printhead driving voltage is determined which can be used in actual printing.

Description

METHOD AND SYSTEM OF SETTING THE PRINTHEAD DRIVING VOLTAGE IN AN INKJET PRINTER

Technical Field

The present invention relates to a method and system of setting the printhead driving voltage of a printhead in an inkjet printer, as well as to an inkjet printing method.

Technical Background

Inkjet printing is printing an image by ejecting ink droplets from one or more printheads through nozzles thereof.

In a scanning inkjet printer the printheads, such as piezo printheads, mounted on print colour bars carry out a reciprocal scanning movement in the width direction of a non-moving printing medium, e.g. the printheads are carried by a driven carriage, during which an image or a part thereof is printed on the printing medium. During a stroke of the printhead in the width direction a nozzle prints at controlled positions along a line in the width direction of the printing medium. The printing medium is intermittently conveyed in the length direction in order to position it for the next stroke of the printhead. In a multi pass mode of a scanning printer an image part is printed on the printing medium during multiple reciprocating strokes wherein the printing medium is moved intermittently between strokes. In a single pass mode the image part is printed during one stroke. The print colour bars, that have a motion by means of the driven carriage in the scanning direction perpendicular to the conveyance direction of the printing medium, are considerably smaller than the maximum printing medium width.

In inkjet printing the quality of a printed image is dependent on a number of factors, among which is also the performance of a printhead and its nozzles. Printheads are supplied by the manufacturer thereof with an advised reference driving voltage for firing a reference ink from its nozzles. In the printer where these printheads are installed, the actual inkjet ink may differ. Furthermore, the electronics as well as the design of the printer may have its effect on the jetting of ink. Slight misalignment of the printheads may also occur. One phenomenon that should typically corrected is the occurrence of misting, wherein tiny ‘satellite’ droplets, typically present in a droplet jet following larger main droplets, are scattered over the image, thereby affecting the image quality.

Generally in industrial practice the occurrence of misting is corrected by adjusting the settings of the printhead. E.g. an appropriate waveform for firing droplets from the nozzles of a printhead is selected and then the printhead driving voltage is varied and a correction of the reference printhead driving voltage is determined from printed test images, at which correction the occurrence of misting is minimized or absent.

For example, misting and uniformity of jet velocity are often evaluated using an USB microscope or high quality magnifier by the operator based on a number of test images printed at several printhead driving voltages for each printhead or series of printheads of the same printing colour. The criteria for evaluating these test images are not unambiguously clear and leave some room for personal interpretation. As a rule of thumb if the test image shows misting, then the printhead voltage at which the image was printed, is too high and if, due to incorrect jet velocity a test image shows curved lines, then the printhead voltage is too low. Therefore the judgement of both misting and jet velocity is somewhat subjective. A correction of the printhead voltage as proposed by an operator based on his own judgement and personal skills and experience is a subjective adjustment and hard to implement in industry.

Now US10,576,736B2 has disclosed a head voltage correcting method for an inkjet printer, comprising the steps of printing a testing chart corresponding to each different drive voltage by applying, to the head, drive voltages shifted at predetermined steps from a reference voltage for dispensing the ink droplets with predetermined density; acquiring a testing chart image for each testing chart by reading the testing chart; determining presence or absence of satellite droplets for each drive voltage, with regard to each testing chart image, based on a luminance distribution of main droplets and satellite droplets; obtaining distances between the main droplets and the satellite droplets for each drive voltage based on the luminance distribution, from the testing chart image for the drive voltage providing the satellite droplets; obtaining a distance reference drive voltage as a drive voltage satisfying a distance threshold, from a relationship of the distances for each drive voltage and the distance threshold which is a maximum of such distances between the main droplets and the satellite droplets that are, in the acquiring step, short enough to regard each pair of the main droplets and the satellite droplets as one ink droplet; obtaining ink droplet sizes for each drive voltage, with regard to each testing chart image, based on the luminance distribution of the main droplets and the satellite droplets; obtaining a size reference drive voltage as a drive voltage satisfying a size threshold, from a relationship of the sizes for each drive voltage and the size threshold which is an area providing a specified density; and comparing the distance reference drive voltage and the size reference drive voltage, and making correction by adopting a larger one as the reference voltage. According to this US patent the accuracy of voltage correction can be performed with the use of a scanner of relatively low resolution by extracting characteristic points of satellite droplets based on a luminance distribution of testing chart images, and taking also the droplet size into consideration for determining droplet density satisfying a certain level. A disadvantage of this known method is that because the satellite droplets are tiny and therefore not intense, as well as their positions are unpredictable, e.g. downstream and/or upstream of the printed image or in the printed image itself, detection of misting is difficult, even with the use of a high resolution scanner. Moreover, a powerful processor for executing all calculation steps in the algorithm is required.

It is to be noted that US6669324B1 has disclosed a method and apparatus for optimizing a relationship between fire energy and drop velocity in an imaging device with the purpose to avoid overdriving the heater elements of the nozzles of the printheads used for bubble generation and drop ejection, resulting according to this patent document in inter alia an increased lifetime of the heater elements, higher productivity and compensation of manufacturing variations of the printhead. The method includes printing a test pattern on a print media by selectively supplying energy distribution signals to a plurality of actuators of the printhead, typically wherein the signals have distinct energy profiles. The thus printed test pattern is scanned to obtain offset values, wherein each offset value represents a distance between at least two corresponding portions of the test patterns. Next drop velocities are calculated from the offset values. Finally an energy distribution signal is selected that corresponds with an optimal one of the drop velocities for actual printing. The optimal energy profile is determined by using the drop velocities to determine when an incremental change in energy corresponds with a disproportionate change in drop velocity. In particular, the optimal energy profile comprises the duration of the fire energy in terms of pre-fire pulse, delay and fire pulse. Thus this known method and apparatus aim at setting an optimal energy profile in view of lifetime of the printhead.

An object of the present invention is to obtain a method for operating an inkjet printer that allows the printer to be more versatile while ensuring consistent print quality.

Particularly, the present invention aims at providing a method and system of setting the printhead driving voltage in order to minimize the occurrence of misting at least to an acceptable degree, particularly to determine the printhead driving voltage at which no misting occurs, which method can be performed in an easy way with minimum subjective judgement of the operator.

More particularly it is an object of the present invention to provide a method and system of setting the printhead driving voltage of a printhead in an inkjet printer of the bidirectional scanning type for printing an image on a printing medium, or at least to provide an alternative method for this purpose.

Summary of the Invention

According to a first aspect of the invention a method of setting the printhead driving voltage of a printhead in a multi-colour inkjet printer of the bidirectional scanning type for printing an image on a printing medium that is movable with respect to the inkjet printer in a conveying direction (C), the multi-colour inkjet printer having a plurality of printheads, the inkjet printer having at least one printhead for each printing colour, each printhead having an array of nozzles configured for ejecting ink droplets of a printing colour, the plurality of printheads being mounted on a carriage that can reciprocate in a scanning direction perpendicular to the conveying direction (C) of the printing medium and in a matrix configuration of columns and rows, wherein the printheads (40) of the same printing colour are arranged in a column of the matrix configuration extending in a direction parallel to the conveying direction (C) and the printheads (40) for different printing colours are arranged in rows extending in a direction perpendicular to the conveying direction (C), wherein the method comprises the steps of: a) printing a first test pattern by ejecting first droplets from the nozzles of each printhead of each colour at different printhead driving voltages during movement of the carriage in a first stroke of the scanning direction at at least one predetermined first position of the printhead, and by ejecting second droplets from the nozzles of each printhead at the same printhead driving voltages during movement of the carriage in a second stroke of the scanning direction opposite to the first direction at at least one predetermined second position of the printhead; b) scanning the test pattern printed in step a), thereby obtaining a test pattern scan; c) determining a distance (D) between first droplets and second droplets ejected at the same driving voltage for each of the different driving voltages for each printhead for each printing colour in the test pattern scan; d) calculating the jet velocity (Vd) for each of the different driving voltages for each printhead for each printing colour from the determined distance (D); e) comparing the calculated jet velocity (Vd) for each of the different driving voltages for each printhead for each printing colour with a reference jet velocity, which reference jet velocity is the same for each printhead of the plurality of printheads (40) ; f) for each printhead for each printing colour of the plurality of printheads selecting the printhead driving voltage where for each of the plurality of printheads the jet velocity is equal to the reference jet velocity as corrected printhead driving voltage.

Instead of determining misting, the presence of satellite droplets, directly and adjusting the printhead driving voltage, the method according to the invention determines the jet velocity and adjusts the printhead driving voltage per printhead based on the determined jet velocity with respect to a reference jet velocity. This reference jet velocity being equal for all printheads can be set based on the jet velocity recommended by the manufacturer for a reference and/or collected experimental data and is typically within a certain window. The higher the jet velocity, the better the positioning of the droplets on the printing medium and the less susceptible to variations in the printing process. In the method according to the invention the printhead driving voltage of a printhead is calculated from the scan of the printed test image on a test substrate and applied to the controller of the inkjet printer automatically. The corrected printhead driving voltage is used as control parameter for actual printing an image.

In another aspect the invention relates to a system for setting the printhead driving voltage of a printhead of a multi-colour inkjet printer of the bidirectional scanning type for printing an image on a printing medium that is movable with respect to the inkjet printer in a conveying direction (C), comprising a multi-colour inkjet printer having the multi-colour inkjet printer having a plurality of printheads comprising at least one printhead for each printing colour, each printhead having an array of nozzles configured for ejecting ink droplets of a printing colour, the plurality of printheads being mounted on a carriage that can reciprocate in a scanning direction (S) perpendicular to the conveying direction (C) of the printing medium and in a matrix configuration of columns and rows, wherein the printheads (40) of the same printing colour are arranged in a column of the matrix configuration extending in a direction parallel to the conveying direction (C) and the printheads (40) for different printing colours are arranged in rows extending in a direction perpendicular to the conveying direction (C); the inkjet printer being provided with a controller configured for controlling the nozzles of the at least one printhead; a scanner for scanning a test pattern printed by the inkjet printer; a computer configured to set the printhead driving voltage according to the method according to the first aspect of the invention.

In yet another aspect of the method an inkjet printing method of inkjet printing an image on a printing medium comprises jetting inkjet ink from a multi-colour inkjet printhead of an inkjet printer having a controller, which jetting is controlled by the controller using the corrected printhead driving voltage obtained by the method according to the first aspect of the invention. In another aspect the invention relates to a computer program comprising instructions which, when the program is executed by a computing device, cause the computing device to carry out the method according to the first aspect of the invention.

Brief Description of the Drawings

Fig. 1 shows a flow diagram of an embodiment of method of setting the printhead driving voltage of a printhead of a multi-colour inkjet printer of the bidirectional scanning type according to the first aspect of the invention;

Fig. 2 schematically shows an embodiment of a system according to the second aspect of the invention comprising a multi-colour inkjet printer of the bidirectional scanning type; Fig. 3 shows an embodiment of a matrix configuration of a printhead arrangement of a multicolour inkjet printer of the bidirectional scanning type.

Fig. 4 schematically shows an embodiment of a test pattern as used in an embodiment of the method according to the invention for a multi-colour inkjet printer of the bidirectional scanning type;

Fig. 5 shows a detail of the test pattern embodiment of Fig. 4;

Fig. 6 is drawing illustrating how the jet velocity can be derived from a printed test pattern for a certain printhead;

Fig. 7 is a graph representing calculated jet velocity as function of the printhead driving voltage for differently sized droplets;

Fig. 8 is a graph representing calculated jet velocities as function of the printhead driving voltage for a plurality of printheads of one colour and comparison with a reference jet velocity; Fig. 9 shows an embodiment of a test pattern for evaluating the occurrence of misting using the determined printhead driving voltage; and Fig. 10 shows a detail of Fig. 9.

Detailed Description of the Invention

The method according to the first aspect of the invention is applicable to a multi-colour inkjet printer of the bidirectional scanning type for printing an image on a printing medium. In such an inkjet printer the printing medium is movable relative to the inkjet printer in a conveying direction. The inkjet printer comprises at least one printhead for each printing colour. The printheads are mounted on a driven movable carriage. The inkjet printer is a multi-colour inkjet printer having a number of printheads for each printing colour, wherein the plurality of printheads are mounted on a carriage in a matrix configuration of columns and rows, wherein the printheads of the same printing colour are arranged in a column of the matrix configuration extending in a direction parallel to the conveying direction C and the printheads for different printing colours are arranged in rows extending in a direction perpendicular to the conveying direction C. The carriage can reciprocate in a scanning direction S that is perpendicular to the conveying direction. In the context of this application the movement of the carriage in a first direction of the scanning direction is called a first (or forward) stroke, while the movement of the carriage in the opposite second direction of the scanning direction is called the second (or backward) stroke. Typically the printing colours comprise at least black (K), cyan (C), magenta (M) and yellow (Y) and optionally one or more additional colours, e.g. blue (Bl) and orange (O). The number of printheads for a printing colour is not limited, but typically in the range of 4-12, such as 6 or 8, in an inkjet printer of the scanning type. In another frequently encountered embodiment a multicolour inkjet printer of the scanning type has one printhead per colour. The method according to the invention can be equally applied to such an embodiment.

In step (a) of the method according to the invention a first part of the test pattern for a given driving voltage is printed in the first stroke for each printhead of each colour. This first part, typically a line perpendicular to the scanning direction (thus parallel to the conveying direction), is composed of first droplets ejected by the nozzles of the respective printhead at predetermined first positions of the printhead with respect to the test substrate. A second part of the test pattern is printed during the second stroke in the opposite direction by ejecting second droplets from the nozzles of the respective printhead at the predetermined second positions of the printhead, thus typically also a line perpendicular to the scanning direction, using the same driving voltage. The first and second positions of the printhead are known. Preferably the predetermined first and second positions of the printhead are the same. Due to the movement of the carriage in opposite directions during the first and second stroke the second test pattern part is printed spaced apart from the first part of the test pattern. The test pattern comprises such first and second parts for each of the printhead driving voltages to be tested, which first and second parts are printed by the nozzles of the printhead in the first and second stroke respectively. The test pattern thus printed in step a), typically on a test substrate having a receiving layer that is adapted to the nature of the inkjet printing ink, preferably a rigid, white test substrate, is scanned in a next step b). In the scan of the test pattern thus obtained in step b) the distance between the printed first and second parts of the test pattern for each driving voltage is determined in step c), and the jet velocity is calculated therefrom in step d), for example, as explained in more detail hereinbelow regarding Fig. 6. The thus calculated jet velocity as function of the driving voltage tested may be plotted in a graph, typically as the difference between the tested printhead driving voltage and the reference driving voltage recommended by the manufacturer of the printhead. In steps e) and f) the corrected printhead driving voltage or the driving voltage difference can be found at the position where the curve in the graph intersects the reference jet velocity. Thus a corrected printhead driving voltage or a difference is obtained for each printhead of each printing colour at the same jet velocity. For verification purposes, one or more second test patterns designed for evaluating misting and/or rectilinearity of lines can be printed using the corrected printhead driving voltage and evaluated for the presence of misting and/or rectilinearity in optional additional steps g) printing a second test pattern using the corrected printhead driving voltage; h) evaluating the printed second test pattern for the occurrence of misting.

Thus the second test pattern is printed using the corrected printhead driving voltage, obtained in step f) for each printhead for each printing colour. The test pattern is evaluated for the occurrence of misting by any printhead of the plurality of printheads. If misting is determined for any of the printheads, then an additionally corrected printhead driving voltage is selected at a lower jet velocity for all the printheads and these steps g) and h) are repeated. If no longer misting is detected then the corrected printhead driving voltages or additionally corrected printhead driving voltages as determined for each printhead of the plurality of printheads are used in actual printing of an image. The driving voltage for actual printing is referred to as printing printhead driving voltage.

Thus in a multicolour bidirectional scanning type inkjet printer the corrected printhead driving voltage is determined for each printhead separately. Upon actual printing each printhead is controlled by its own corrected printhead driving voltage.

Optionally, step c) may comprise determining the presence or absence of the droplets for each driving voltage of the different driving voltages in the printed test pattern.

In an embodiment the test pattern comprises a line to be printed in both strokes by differently sized droplets at each driving voltage of the different voltages, such as small droplets, medium sized droplets and large droplets. As in general the large size droplets are used most frequently in actual printing an image, the jet velocity is calculated from the speed of the large droplets. Typically the smaller droplets have a somewhat slower speed than the larger ones.

Using a single reference jet velocity for all printheads for all colours (in other words the jet velocity is not colour dependent) has the advantage that the alignment in a stroke (unidirectional printing) is independent from the printhead height (standoff) above the printing medium, although some of the colour printheads can eject at a higher speed without causing misting. For example, in a fast printing mode (78 m/min printhead speed in scanning direction) and 3 mm standoff the distance between ejection position and landing point on the printing medium would be 600 micrometres for droplets having a velocity of 6.5 m/s and 678 micrometres for droplets having a velocity of 5.75 m/s. This difference would be compensated in the alignment settings. However, changing the standoff to 4 mm would increase this distance to 800 micrometres for droplets having a speed of 6.5 m/s and to 904 micrometres for droplets at a speed of 5.75 m/s. If the alignment was set perfectly for a standoff of 3 mm, it would deviate by 26 micrometres for droplets at a speed of 6.5 m/s versus droplets at a speed of 5.75 m/s. Setting a single reference jet velocity for all printheads of all colours for determination of the associated (additionally) corrected printhead driving voltage according to the invention, which are to be used as printing printhead driving voltages in actual printing, results in each printhead having the same jet velocity in actual printing which thus allows a single adjustment to adapt to a thickness of the substrate to be printed, which may differ from substrate to substrate and/or different speed of the carriage on which the printheads are mounted. Thus a versatile and easy operation of the multi-colour inkjet printer is achieved. In a second aspect of the invention a system for setting the printhead driving voltage of a printhead of a multi-colour inkjet printer of the bidirectional scanning type is provided as defined in claim 7. The system comprises -briefly said- a multi-colour inkjet printer having a plurality of printheads comprising at least one printhead for each colour with a plurality of nozzles which plurality of printheads is carried by a carriage that can move to-and-from in the scanning direction S, a controller configured for controlling the multi-colour inkjet printer, a scanner for scanning a test pattern printed by the multi-colour inkjet printer and a computer that is configured for performing the method according to the first aspect of the invention. This system comprising a multi-colour inkjet printer, scanner, computer and controller offers the same advantages as the above method of setting the printhead driving voltage according to the first aspect of the invention. The embodiments of the method are equally applicable to this system of the second aspect of the invention.

In an embodiment the system also comprises a substrate having a receiving layer adapted to the nature of the inkjet ink.

In an embodiment the system also comprises a memory readable by the controller and configured for storing the corrected printhead driving voltage for a printhead.

In an embodiment the controller is configured to print a test pattern on the test substrate, by jetting inkjet ink from an inkjet printhead of the multi-colour inkjet printer and configured to print an image on a printing medium by jetting inkjet ink from an inkjet printhead of the multicolour inkjet printer using the corrected printhead driving voltage.

In an embodiment for verification the system further comprises a magnifying means for evaluating the occurrence of misting in a test pattern printed on a test substrate (or a scan thereof), such as an USB microscope or magnifier.

A multi-colour inkjet printer has at least one, typically a plurality of printheads for each printing colour, the plurality of printheads is mounted on a movable carriage in a matrix configuration of columns and rows, wherein the printheads of the same printing colour are arranged in a column of the matrix configuration extending in a direction parallel to the conveying direction and the printheads for different printing colours are arranged in rows extending in a direction perpendicular to the conveying direction.

Detailed Description of the Drawing

In Fig. 1 a flow diagram of an embodiment of a method of setting the printhead driving voltage according to the invention, applicable to an inkjet printer of the bidirectional scanning type, is shown. This embodiment concerns the implementation of the method in a multicolour inkjet printer of the bidirectional scanning type, and comprises a step a), wherein a test pattern is printed by the nozzles of each printhead of each printing colour on a test substrate at a number of printhead driving voltages shifted from a reference printhead driving voltage as recommended by the printhead manufacturer. For each printhead driving voltage the test pattern comprises test pattern sections for each printhead of each printing colour. The test pattern sections for each printhead mounted on the carriage comprises e.g. a first line part to be printed in the first (e.g. forward) stroke of the printheads and a second line part to be printed in the opposite second (e.g. backward) stroke at known positions of the printhead in those strokes, preferably at the same position of the printhead . A line part may be printed by a consecutive series of differently sized droplets. The test substrate may have a receiving layer that is compatible with the inkjet ink used. In step b) the test substrate having the test pattern printed thereon is scanned by a scanner, thereby obtaining a digital scan of the printed test pattern. In step c) the obtained scan is analysed and the relative positions of the printed first and second test pattern parts is determined for each printhead for each driving voltage, and if applicable for each different drop size. From the relative positions of the printed first and second line parts the relative distance are determined in step c). In step d) the jet velocity of each printhead is calculated from the determined distance. The jet velocity thus calculated is compared to the reference jet velocity in step e), e.g. the jet velocities of all printheads for a given colour as function of the respective printhead driving voltage are plotted in a graph and curves are drawn through the calculated jet velocities resulting in a curve for each printhead of that colour. In step f) a corrected printhead driving voltage is selected based on the comparison of step e), e.g. per printhead the intersection of the desired jet velocity with its curve is taken as the corrected printhead driving voltage for that printhead of that colour. Thus to each printhead its dedicated own correction (corrected printhead driving voltage or shift from reference printhead driving voltage from manufacturer) is assigned. For optional verification, in step g) a second test pattern is printed with the corrected printhead driving voltage and in step h) the thus printed test pattern is evaluated for the presence of misting (scattered satellite drops) for example using an USB microscope or other magnifying means. If misting still occurs for one or more colours, the method steps g) and h) could be repeated typically using a setting of the so called additionally corrected printhead driving voltages corresponding to the desired lower value of the jet velocity. The (additionally) corrected printhead driving voltages as determined can be used for a print job of actual printing an image on a printing medium as represented in step i).

The method, of which an embodiment is shown in Fig. 1, is typically performed upon the initial start when one or more (defective) printheads are replaced, usually together with other test methods for determining other potential anomalies, such as malfunctioning nozzles, misalignment, colour uniformity, such as disclosed e.g. in W02020239820A1 in the name of the present applicant.

The (additionally) corrected printhead driving voltages for each printhead (or the deviations with respect to the reference printhead driving voltage as recommended by the printhead manufacturer) are usually stored in a memory, for example in the form of a chart or table. Typically the value of the printhead driving voltages are rounded to 1/10 or 1/20 V. Fig. 2 shows diagrammatically an embodiment of a system for printhead driving voltage setting in an inkjet printer of the bidirectional scanning type. The system 10 comprises an inkjet printer 12 of the bidirectional scanning type wherein the printheads (see Fig. 3) are mounted on a carriage 14, that is reciprocating movably arranged on rails 16 in the width direction (scanning direction; see arrow S) of the test substrate 18, that is intermittently conveyed in a conveyance direction C from an infeed 20 to an outlet 22, e.g. temporarily adhered to a conveyor 24 such as an endless belt. Movement of the carriage 14 in a back- and-forth motion perpendicular to the conveyor direction between positions A and B along the edge of the conveyor 24 and firing by the printheads is controlled by a controller 26, also synchronizing these actions with the movement of the conveyor 24 in order to print a test pattern 28 having test pattern sections 30 on the test substrate 18, temporarily adhesively attached to the conveyor 20. A scanner 32 is configured to scan the test substrate 18 that has been printed with the test pattern 28. The digital scan of the test pattern is processed by a computer 34 having a processor which is configured to identify the positions of first and second test pattern parts from the digital scan of the test pattern 26, to determine the distance D between them, calculate the jet velocities of each printhead for each tested printhead driving voltage, compare the calculated jet velocities with a set desired jet velocity and selecting a corrected printhead driving voltage from this comparison. The system may comprise means for inputting the reference jet velocity, the series of printhead driving voltage to be tested, such as a keyboard connected to the computer and a display such as a monitor. These corrected printhead driving voltages or if applicable additionally corrected printhead driving voltages are recorded e.g. in a memory 36 of computer 34 and subsequently used by the controller 26 for printing one or more further test patterns and/or performing an actual print job.

Fig. 3 shows an embodiment of the printheads 40 of an inkjet printer 12. In the embodiment shown, the printheads 40 are arranged in a matrix configuration of columns, in this embodiment representing the printheads of a single printing colour Clr, and rows representing the number n of the printhead for a printing colour. The position of a printhead in the matrix configuration is indicated by (Clr, n). In the embodiment shown in Fig. 3, the number of printheads for each printing colour is six (n=6). In this embodiment shown the printing colours are black (K), cyan (C), magenta (M), yellow (Y), blue (Bl) and orange (O). A particular printhead can be identified by its position. The position of the first black printhead is indicated as (K, 1) and the last one as (K, 6). Similar indications are used for the other printing colours. The printheads for one colour are arranged adjacent to one another in the conveyor movement direction C, typically on a print colour bar that is releasably mounted on the carriage. The printheads of differing colours are arranged in the width direction of the conveyor (i.e. in the scanning direction S, perpendicular to the conveyor movement direction C). Each printhead 40 comprises an array of nozzles 42. The (piezo) printheads are configured for jetting ink droplets (typical droplet size e.g. 2-10 picolitres) onto a printing medium conveyed by the conveyor. Differently sized drops may be printed by a different number of droplets (of the same volume).

Fig. 4 shows an embodiment of a printed test pattern 28 (90 ° turned) having test pattern sections 30 for each printhead. In this case there are eight (numbered 1-8) printheads of each printing colour Black (K), Cyan (C), Magenta (M), Yellow (Y), Orange (O) and Blue (Bl).

Fig. 5 shows a scan of the printed test pattern sections for one specific printhead of one colour, e.g. M, in detail. The upper broad strip is a strip printed during start-up to clean the nozzles from partially dried ink and the like. In this embodiment the test pattern comprises six test sections 30 (shown by broken line), one for each printhead driving voltage, indicated by - 4, -2, -1, 0, +1 , +2, wherein 0 indicates a reference printhead driving voltage as recommended by the printhead manufacturer and the other indicate the shift from this reference printhead driving voltage. For each driving voltage the test section is the same and comprises a line part 30a printed in three pieces by small (S), medium (M) and large (L) droplets respectively in the first stroke and a line part 30b printed in three pieces by small (S), medium (M) and large (L) droplets respectively in the opposite stroke at the same position of the printhead. Due to the movement of the carriage and thus the printhead the line parts 30a, 30b are printed a distance D apart. Thus the test pattern section 30 for each driving voltage tested comprises two spaced apart line parts 30a, 30b, wherein each line part itself comprises three pieces, printed by the small, medium, respectively large droplets. It appears that for -4V the small and medium droplets are scattered over the test substrate and that the large droplets hardly make it. At +2V the droplets of all sizes are printed at the correct positions and the distance D between the line parts is the smallest.

Fig. 6 shows how the jet velocity can be calculated from the determined distance D. The printhead 40 mounted on the carriage travels over the test substrate with a constant velocity Vh in both strokes. A first droplet 44 (droplets represented by small dots) ejected at a predetermined first position from a nozzle of the printhead 40 positioned at a height h (also known as standoff) above the test substrate 28 will move to the paper at a jet velocity Vd, but also continues to travel in the direction of the moving carriage (first direction). A second droplet 46 is ejected in the opposite second stroke at the same predetermined position (or at a known shifted position with respect to the predetermined first position) from a nozzle of the printhead 40 positioned at a height h. Therefore the droplets printed in the first stroke and the opposite second stroke are printed a distance D apart. Assuming that the jet velocity Vd of the jetted droplet is constant in both strokes the time for a droplet to travel from the printhead to the test substrate is given by the formula t = h/Vd. The distance Dh travelled in the direction of movement of the carriage over the test substrate during this time t is given by the formula Dh = Vh*t. Furthermore, the distance D between the droplets printed in the first stroke and the second stroke respectively is D = Dh*2. Therefore the jet velocity Vd of a droplet can be calculated by the formula Vd = h/(Dh/ h) = 2*h*Vh/D.

The thus calculated jet velocity as function of the driving voltage used can be plotted in a graph, typically as the difference between the set printhead driving voltage and the printhead driving voltage recommended by the manufacturer of the printhead (WAC).

Fig. 7 represents the resulting graph for the test pattern sections 30 shown in Fig. 5. As can be seen for this printhead the small droplets (dropletSize 1) are slower than the large droplets (dropletSize 3), which is typically the case for most printheads, although not for all printheads. As the large droplets are the most frequently used ones, the jet velocity is calculated for the large droplets.

Fig. 8 shows the calculated jet velocities for the large droplets (dropletSize 3) for all (eight) printheads of magenta (M) at a certain shift from WAC. The corrected printhead driving voltage is determined by calculating where the jet velocity curve of a printhead crosses the reference jet velocity (line parallel to x-axis), e.g. at 5.75 m/s. Thus in this case eight corrected printhead driving voltages are obtained., one for each magenta printhead.

Example

In an experiment it was attempted to achieve a jet velocity of 6.5 m/s in a multicolour inkjet printer of the bidirectional scanning type having eight printheads (n=8) per colour (K, M, C, Y, O, B). After printing a test pattern as shown in Fig. 4 and 5 the corrected printhead driving voltages were determined and applied to the printheads. With these corrected printhead driving voltages a second test pattern was printed and evaluated for the occurrence of misting and rectilinearity of lines in view of jet velocity. An embodiment of the second test pattern used is shown in Fig. 9 and comprises broad printed strips for each colour. Such a second test image is printed using corrected printhead driving voltages for each printhead. A detail thereof is depicted in Fig. 10, showing misting in the form of tiny droplets scattered over the image.

In particular the results at 6.5 m/s showed that good jetting behaviour (mist free and sufficient speed) was achieved for black, cyan, yellow and blue. The magenta printheads showed some misting, while the orange printheads showed severe misting. The level of misting did not vary strongly between the printheads of the same colour, both for magenta and orange. These results indicate that misting behaviour is dependent on the colour. In view of these results showing misting behaviour for magenta and orange at jet velocity of 6.5 m/s the reference jet velocity was reduced to 6 m/s and the experiment was repeated using the corrected printhead driving voltages corresponding to the reduced reference jet velocity. The results showed that all colours were mist free, except for orange which still showed some misting. The jet velocity was further lowered to 5.75 m/s and the experiment repeated with the additionally corrected printhead driving voltages. From the results it appears that all colours were free of misting, while still sufficient speed was achieved. Thus the additionally corrected driving voltages are suitable as printing printhead driving voltages in an actual print job of printing an image.

Claims

1. Method of setting the printhead driving voltage of printheads (40) of a multi-colour inkjet printer (12) of the bidirectional scanning type for printing an image on a printing medium that is movable with respect to the inkjet printer (12) in a conveying direction (C), the multicolour inkjet printer (12) having a plurality of printheads (40), the multi-colour inkjet printer (12) having at least one printhead (40) for each printing colour, each printhead (40) having an array of nozzles (42) configured for ejecting ink droplets of a printing colour, the plurality of printheads (40) being mounted on a carriage (14) that can reciprocate in a scanning direction (S) perpendicular to the conveying direction (C) of the printing medium and in a matrix configuration of columns and rows, wherein the printheads (40) of the same printing colour are arranged in a column of the matrix configuration extending in a direction parallel to the conveying direction (C) and the printheads (40) for different printing colours are arranged in rows extending in a direction perpendicular to the conveying direction (C), wherein the method comprises the steps of: a) printing a first test pattern (28; 30) by ejecting first droplets (44) from the nozzles (42) of each printhead (40) of each printing colour at different printhead driving voltages during movement of the carriage (14) in a first stroke of the scanning direction at at least one predetermined first position of the printhead, and by ejecting second droplets (46) from the nozzles of each printhead at the same printhead driving voltages during movement of the carriage (14) in a second stroke of the scanning direction opposite to the first direction at at least one predetermined second position of the printhead; b) scanning the test pattern (28; 30) printed in step a), thereby obtaining a test pattern scan; c) determining a distance (D) between first droplets (44) and second droplets (46) ejected at the same driving voltage for each of the different driving voltages for each printhead (40) for each printing colour in the test pattern scan; d) calculating the jet velocity (Vd), for each of the different driving voltages for each printhead (40) for each printing colour, from the determined distance (D); e) comparing the calculated jet velocity (Vd) for each of the different driving voltages for each printhead (40) for each printing colour with a reference jet velocity, which reference jet velocity is the same for each printhead of the plurality of printheads; f) for each printhead (40) for each printing colour of the plurality of printheads, selecting the printhead driving voltage where the jet velocity is equal to the reference jet velocity as corrected printhead driving voltage;

2. Method according to claim 1, further comprising the steps of g) printing a second test pattern using the corrected printhead driving voltage for each printhead (40) for each printing colour, selected in step f); and h) evaluating the printed second test pattern for the occurrence of misting for each printhead (40) for each printing colour for the selected corrected printhead driving voltage

3. Method according to claim 2, further comprising the steps of: if misting is determined for any one of the plurality of printheads (40), setting an additionally corrected printhead driving voltage corresponding to a lower jet velocity which lower jet velocity is equal for the plurality of printheads (40), and repeating steps g)-h) with the additionally corrected printhead driving voltage; or if misting is not determined setting the selected corrected printhead driving voltage or selected additionally corrected printhead driving voltage for each printhead (40) of each printing colour, as printing printhead driving voltage.

4. Method according to any one of the preceding claims, wherein the at least one predetermined first position of the printhead is the same as the at least one predetermined second position of the printhead.

5. Method according to any one of the preceding claims, wherein step c) also comprises determining the presence or absence of the droplets (44, 46) for each driving voltage of the different driving voltages in the printed test pattern.

6. Method according to any one of the preceding claims, wherein the test pattern (28; 30) comprises a line (30a; 30b) to be printed by differently sized droplets (S, M, L) at each printhead driving voltage of the different driving voltages in both strokes; and preferably the corrected printhead driving voltage is set where the jet velocity of the largest sized droplets is equal to the reference jet velocity.

7. System (10) for setting the printhead driving voltage of a printhead (40) of a multicolour inkjet printer (12) of the bidirectional scanning type for printing an image on a printing medium that is movable with respect to the inkjet printer (12) in a conveying direction (C), the multi-colour inkjet printer (12) having a plurality of printheads (4), comprising at least one printhead (40) for each printing colour, each printhead (40) having an array of nozzles (42) configured for ejecting ink droplets of a printing colour, the plurality of printheads (40), being mounted on a carriage (14) that can reciprocate in a scanning direction (S) perpendicular to the conveying direction (C) of the printing medium and in a matrix configuration of columns and rows on the carriage (14), wherein the printheads (40) of the same printing colour are arranged in a column of the matrix configuration extending in a direction parallel to the conveying direction (C) and the printheads (40) for different printing colours are arranged in rows extending in a direction perpendicular to the conveying direction (C). the multi-colour inkjet printer (12) being provided with a controller (26) configured for controlling the nozzles (42) of the at least one inkjet printhead (40); a scanner (32) for scanning a test pattern printed by the inkjet printer; a computer (34) configured for setting the printhead driving voltage of a printhead (40) of according to the method of any one of the preceding claims 1-6.

8. System according to claim 7, further comprising a test substrate (18) having a receiving layer adapted to the nature of the inkjet ink.

9. System according to claim 7 or claim 8, further comprising a memory (36) readable by the controller (26) and configured for storing the corrected printhead driving voltage of a printhead (40).

10. System according to any one of the claims 7-9, further comprising a magnifying means for evaluating the occurrence of misting in a test pattern printed on a test substrate.

11. System according to any one of claims 7-10, wherein the controller (26) is configured to print a test pattern (28;30) on the test substrate (18), by jetting inkjet ink from an inkjet printhead (40) of the multi-colour inkjet printer (12) and configured to print an image on a printing medium by jetting inkjet ink from an inkjet printhead 40) of the multi-colour inkjet printer (12) using the corrected printhead driving voltage and/or additionally corrected printhead driving voltage.

12. Inkjet printing method of inkjet printing an image on a printing medium, comprising jetting inkjet ink from an inkjet printhead (40) of a multi-colour inkjet printer (12) having a controller (26), which jetting is controlled by the controller using the printing printhead driving voltage obtained by the method according to any one of the preceding claims 1-4.

13. A computer program comprising instructions which, when the program is executed by a computing device (34), cause the computing device (34) to carry out the method according to any one of the preceding claims 1-6.