CN110733247B

CN110733247B - Method for avoiding crosstalk in an inkjet printer

Info

Publication number: CN110733247B
Application number: CN201910653536.6A
Authority: CN
Inventors: S·内布; N·R·诺瑞克; A·亨; J·福歇; J·魏格特; D·基希霍夫; B·施特里泽尔
Original assignee: Heidelberger Druckmaschinen AG
Current assignee: Heidelberger Druckmaschinen AG
Priority date: 2018-07-19
Filing date: 2019-07-19
Publication date: 2021-03-09
Anticipated expiration: 2039-07-19
Also published as: DE102018212063B3; CN110733247A

Abstract

A method for avoiding crosstalk in a printing process of an inkjet printer (7) by means of a computer (6), wherein the inkjet printer (7) has a print head (5) with the following printing nozzles (13): the printing nozzles are arranged in rows and columns, each having a defined number of printing nozzles (13), and the printing nozzles (13) of each row share in each case one ink supply channel (12), and the computer (6) carries out a screening process for carrying out the printing process, characterized in that, during the screening process, the computer (6) is adapted to the image data (15) to be generated in such a way that the printing nozzles (13) of a row do not exceed a limit value (8) for a defined ink extraction frequency.

Description

Method for avoiding crosstalk in an inkjet printer

Technical Field

The invention relates to a method for avoiding crosstalk (Cross-Talk) in an inkjet printer.

The invention belongs to the technical field of ink jet printing.

Background

In modern inkjet print heads, the printing nozzles are arranged in a plurality of rows in order to achieve the required high resolution. Also internally, each adjacent printing nozzle (with respect to the ink supply channel) is connected to each other hydraulically, (with respect to the piezoelectric material of the nozzle plate) mechanically and electrically. Such a connection has an effect on the performance of adjacent printing nozzles, for example when printing nozzles which are commonly attached to one supply channel are simultaneously printing. This is called so-called crosstalk. This effect leads to: the portions of the ink drops produced may deviate significantly from their desired size and desired velocity. This can cause locally disturbed printed images, thereby reducing print quality.

This means in particular that the size and position of the printed dots on the substrate deviate from their desired value. Cross talk also causes instability in ejection, which can be perceived by adding formed ink droplets to the nozzle plate. These drops are increasingly large and may eventually lead to slant ejection and failure of the printing nozzle.

Various solutions for solving the crosstalk problem are known from the prior art. Hardware solutions are very common, where the print heads have been designed such that cross talk is minimized as much as possible. Print head manufacturers, for example, attempt to reduce crosstalk by adding certain design elements (e.g., constrictions and disruptions) to the ink supply. However, such constrictions and interruptions increase the flow resistance, which ultimately leads to a possible reduction in printing speed.

From us patent 7,661,793B2 a print head for an inkjet printer is known, in which print head each print nozzle has the following chambers: the chamber has a separate ejection mechanism for ink droplets and a separate ink inlet. Furthermore, a plurality of ink supply channels are exclusively connected to the inlets of the printing nozzles. This is achieved by: inlets are etched in one side of the base wafer, on which the printing nozzles are constructed, and ink supply channels are etched in the other side of the wafer, respectively. By this construction with correspondingly long ink supply channels, cross-talk between adjacent printing nozzles is relatively well minimized.

However, a disadvantage of this hardware solution is that it is a very special solution. Not every print head has this particular structure. If it is not possible to use such a print head specifically designed for crosstalk avoidance for design-dependent, legal or economic reasons, further possibilities for crosstalk avoidance have to be found.

Disclosure of Invention

The object of the present invention is therefore to find a method for avoiding crosstalk in an inkjet printer which is independent of the structure of the inkjet print head used.

This object is achieved by a method for avoiding crosstalk in a printing process of an inkjet printer by means of a computer, wherein the inkjet printer has a printing head with printing nozzles which are arranged in rows and columns, each having a defined number of printing nozzles, and the printing nozzles of each row share an ink supply channel, and the computer executes a screening process (rasterprocess) for executing the printing process, characterized in that, during the screening process, the computer adapts the image data to be generated in such a way that the printing nozzles of a row do not exceed a defined limit value for the ink extraction frequency. Since crosstalk is caused by "adjacent printing nozzles belonging to the same ink supply channel affect each other when they print simultaneously and thus extract ink from the same ink supply channel", the best solution is: this simultaneous ink extraction of adjacent printing nozzles belonging to the same ink supply channel is minimized. Since this obviously has to be done depending on the print image data to be produced, since the use of printing nozzles in the print head cannot be simply and freely inhibited without adversely affecting the print quality that can be achieved by the inkjet printer, the most desirable solution is: the following screening process is correspondingly effected: the screening process generates print image data for an inkjet printer. Since all the necessary information for generating the print image data (for example, about the print image resolution to be achieved) is known during the screening process carried out by the computer, the raster image can be manipulated in this respect in such a way that adjacent printing nozzles belonging to the same ink supply channel are used as little as possible simultaneously. To ensure this, a certain ink extraction frequency should not be exceeded for these relevant supply channels and all the printing nozzles attached thereto. As long as the limit value of the limit frequency or ink extraction frequency is followed, the crosstalk is kept correspondingly to a minimum. Advantageous and therefore preferred embodiments of the method result from the following description and the drawings.

In a preferred embodiment of the method according to the invention, the limit value for the ink extraction frequency is derived from the screen frequency in the machine direction and a factor n for a certain number of time steps in the printing direction. This describes the relationship already explained. The screening frequency in the longitudinal direction is no different from the description of how frequently the respective printing nozzles, which are assigned to a common ink supply channel, are printed. Here, the factor n is decisive. Because this factor accounts for: the nozzles of the common supply channel are allowed to print more frequently within a certain time step, without exceeding the respective limit frequency. Thus, the factor n is a real limiting factor defining the limit frequency or limit value of the ink extraction frequency.

In a further preferred embodiment of the method according to the invention, the screening frequency in the machine direction is determined by the printing speed of the inkjet printer and the resolution in the printing direction. These data are derived on the one hand from the print image data (i.e. in this case from the print resolution), which are predefined by the print job, and on the other hand from the parameters of the inkjet printer (i.e. in this case mainly from the corresponding printing speed).

A further preferred embodiment of the method according to the invention is that the limit value for the ink extraction frequency of a row of printing nozzles of the printing head is determined by measurement or is predefined by the printing head manufacturer. On the one hand, the maximum possible ink extraction frequency of the printing nozzles associated with the same ink supply channel can be determined by measurement or, in many cases, can be directly predetermined by the print head manufacturer. In this case, the limit value must be complied with according to the method. As already stated, the factor n, which is a real limiting factor for the implementation of the screening process, accounts for the corresponding time step in which the simultaneous use of printing nozzles of the same ink supply channel is not permitted.

In order to avoid crosstalk, the computer is preferably adapted to the screen in accordance with the limit value of the ink extraction frequency in such a way that the printing nozzles of the same row of the printing head are not used a number of times within the factor n. According to the invention, the limiting factor n can be calculated from the known limit value of the limiting frequency or ink extraction frequency, which then has to be taken into account by the computer during the screening process. This is done by: the computer is adapted to screen such that printing nozzles belonging to the same ink supply channel are not permitted to be used several times within a certain number of time steps (which is predefined by a factor n). Thus, in the same row of printing nozzles belonging to the same ink supply channel, only at most one printing nozzle is always printing during n time steps.

A further preferred embodiment of the method according to the invention is to increase the achievable areal density

The rows of the print head are divided into a plurality of partial areas and the "inhibiting of multiple use of the printing nozzles in the same row" applies only to said partial areas. If areas with very high areal density (e.g. solid areas) need to be printed in order to complete a print job, it may happen that: due to the limitation of the printing by means of the adjacency, which is predefined by the limit value or factor n, the printing nozzles cannot print these solid regions with sufficient resolution. In this case, the rows of printing nozzles belonging to the same ink supply channel can be divided into smaller part-areas. Therefore, the limit values taking into account the ink extraction frequency are respectively applied only to these partial regions. These partial regions can be configured, for example, such that each partial region has four nozzles adjacent to one another. It is clear that crosstalk cannot be completely prevented thereby, but a significant improvement in reducing such disturbing effects can still be achieved. Finally, the print quality must be weighed between the requirements of the print job to be achieved and the degradation of the print quality caused by the occurring crosstalk.

A further preferred embodiment of the method according to the invention is to reduce the number of time steps by a factor n in order to further increase the achievable areal density. If the division of the rows of printing nozzles belonging to a common ink supply channel into partial areas is still insufficient to achieve the desired areal density, the number of time steps by the factor n can be reduced in a further step. In this case, it makes sense to change the combination of the parameters and the size of the partial region. Which value combination of the size of the part area and the number of time steps is most advantageous depends on the respective case. However, the number of time steps by the factor n should not be easily reduced in particular before the relevant row of printing nozzles is divided into partial regions. This measure should only be taken if the desired areal density can no longer be achieved by limiting the ink extraction frequency of the method according to the invention.

In a further preferred embodiment of the method according to the invention, the computer calculates an error term during the course of the screening process, which error term describes the deviation from the original image, the computer carries out the screening process in such a way that it is minimized, and the computer takes into account the limit value of the ink extraction frequency during the calculation of the error term. This process is an aid to configure the screening process such that it outputs the following screened images: the screened image is positioned as much as possible in the original image known from the print job data and reflects the original image as faithfully as possible. The calculation of the error term can be very well associated with the method according to the invention. The ink extraction frequency or the limit value of the limiting factor n can be well integrated into the calculation of the error term, so that the resulting minimum error term takes into account the influence of crosstalk at the same time.

A further preferred embodiment of the method according to the invention provides that the computer always uses the maximum printing speed of the inkjet printer to calculate the factor n. From this fact, it follows that, in order to take into account and reduce crosstalk, no screening is performed in relation to the printing speed. Since the limit frequency is known anyway (since it is determined at the time of waveform design or is already predefined by the print head manufacturer), the variables of the method according to the invention are primarily the limiting factor n and, of course, also the printing resolution predefined by the print job data.

A further preferred embodiment of the method according to the invention is that the method is carried out for a print head as follows: the printing nozzles of the print head are arranged in a front Bank and a rear Bank, which are respectively composed of 16 rows and 64 columns. The print head used for which the method according to the invention was developed has 2048 printing nozzles which are staggered in rows and columns. Here, the printing nozzles are divided into a front block and a rear block, which are respectively composed of 16 rows and 64 columns of printing nozzles. Here, the printing nozzles of 16 rows are each assigned to an ink supply channel and are therefore susceptible to crosstalk effects. When using such a print head, for example, a limiting factor n of two time steps in the printing direction is obtained. This means that reuse of printing nozzles within 16 rows is not allowed for two time steps in the printing direction. However, the method according to the invention can of course also be used for other print heads having a different structure (with differently arranged printing nozzles) and is not limited to this particular configuration.

Drawings

The invention and its structurally and/or functionally advantageous embodiments are further described below with reference to the attached drawings in accordance with at least one preferred embodiment. In the drawings, mutually corresponding elements are provided with the same reference numerals, respectively.

The figures show:

fig. 1 shows an example of the structure of a sheet inkjet printer;

fig. 2 schematically shows crosstalk from a limit frequency;

FIG. 3 shows a schematic arrangement of printing nozzles on a print head;

fig. 4 shows a schematic flow of the method according to the invention.

Detailed Description

The field of application of the preferred embodiment variant is an ink jet printer 7. An example of the basic structure of such a machine 7 is shown in fig. 1, which consists of a feeder 1 for supplying a printing substrate 2, which is printed in a printing mechanism by a print head 5, to a receiver 3. In this case, a sheet-fed ink-jet printer 7 is provided, which is controlled by a control computer and whose data are obtained from a computer 6 of a Raster Image Processor (RIP). During operation of the printing press 7, as already described, crosstalk effects occur due to the influence of the common ink supply 12 of certain adjacent printing nozzles 13.

The method according to the invention provides that, when preparing the printed material (i.e. producing a grid), the defined dot arrangement in the grid is disadvantageous, which leads to an unfavorable occupancy of the printing nozzles 13 in terms of crosstalk effects. This is performed by a computer, preferably a computer 6 which also controls the screening process.

The print heads 5 used preferably each have 2048 printing nozzles 13 which are arranged staggered in rows and columns in order to achieve the necessary resolution transversely to the printing direction. Fig. 3 schematically shows a schematic arrangement of the printing nozzles 13 on such a print head 5. Here, the dot pitch or pixel pitch transverse to the printing direction is 1200dpi (i.e., 21.667 μm). The printing nozzles 13 are divided into front and rear blocks consisting of 16 rows and 64 columns, respectively. The printing nozzles 13 of the 16 rows are assigned to one supply channel 12 and are therefore coupled to one another. Two successive printing nozzles 13 of 16 rows have a pitch of four pixels transverse to the printing direction. In order to print two adjacent dots, one printing nozzle 13 from the front block and one from the rear block is always required. The next printing nozzle 13 of the 16 rows prints only after 4 pixels in the transverse direction to the printing direction.

Furthermore, the following alternative structures are known: in an alternative configuration, the printing nozzles 13 in the 16 rows are always arranged offset by 14 pixels in the printing direction. The distance between the printing nozzles 13 is known and is derived by the Pythagoras theorem and the nozzle grid with ax and ay. The printing wave propagation time can be calculated with the known wave propagation velocity c:

t_i＝a_i/c

the printing wave propagation time is not allowed to coincide with the firing time, which is derived from the arrangement of the printing nozzles 13 in the y-direction and the printing speed:

t_jetting＝y_i/v_druck

now, the crosstalk effect is a disturbance of the printing characteristics of the ink within the supply channel 12. When a certain ink extraction frequency (i.e. the limit frequency 8) is exceeded, resonance effects occur and a volumetrically accurate extraction cannot be ensured. This is schematically illustrated in fig. 2. It is well evident from the measurement curve 11 that the resonance effect is not controlled when the limit frequency 8 is exceeded, and the ratio 9 of the drop velocity to the drop size deviates more and more as the ejection frequency 10 increases.

In order to prevent cross-talk, i.e. to prevent two printing nozzles 13 in a row 16 from being used simultaneously at a time above the limit frequency 8, the following equation must be finally followed:

t_{extreme limit}<n*t_Spraying

Here, t_{Extreme limit}Is the limiting frequency f_{Extreme limit}Reciprocal, i.e. t_{Extreme limit}Corresponding to a time of 33ps in the order of 1/30 kHz.

Here, t_SprayingIs the inverse of the screening frequency in the machine direction, which depends on the printing speed v_{Machine with a rotatable shaft}And resolution s in the printing direction_Screening：

t_Spraying＝s_Screening/v_{Machine with a rotatable shaft}

Example of calculation:

v_{machine with a rotatable shaft}＝0.7m/s

s_Screening21.667 μm (corresponding to 1200dpi)

f_{Extreme limit}30kHz (known from measurements on the print head 5)

This gives n-2. Here, n must be an integer and always rounded (aufrunden). This means that reuse of 16 rows of printing nozzles 13 within 2 time steps in the printing direction is not allowed. In the print heads 5 used so far, the ink supply channels 12 for the nozzle rows are again separated by a constriction. Such a constriction acts as a damper for the printing wave, so that in a preferred embodiment the above-mentioned provisions (i.e. "not allowing the reuse of 16 rows of printing nozzles 13") can still be improved according to the following criteria:

1. in the ideal case, 16 rows of printing nozzles 13 are not used.

2. If a greater number of dots/areas are required due to the desired screening tone, care should be taken that at least the printing nozzles 13 in 8 rows are not used during these 2 time steps.

3. At higher printing densities, the number of time steps can be reduced to one following the specification (in which the other seven printing nozzles 13 are not used).

Furthermore, the method according to the invention can be further extended. Therefore, during the calculation of the grid, an error term 14 is usually calculated, which accounts for deviations from the original image. The goal of the grid method is always to minimize this error term 14. Now, the error term 14 can be extended such that it causes the above-mentioned undesirable properties. According to the invention, the grid is thus configured by the computer 6 in the RIP in such a way that crosstalk is avoided to the greatest possible extent. Furthermore, the error term 14 can also be configured such that a weighting is achieved between the image quality and the print quality.

With the limit frequency 8 and print resolution being the same, the problem becomes more severe at higher print speeds. In the case of 2.7 m/s (this corresponds to 10000 prints/hour), n-5 has been found.

In this case, the factor n for the respective limiting frequency 8 must always be determined at the maximum printing speed during the screen printing. This means that no screening in relation to the printing speed is required. Such screening in relation to the printing speed, while possible in principle, is very complex. The limit frequency 8 is known anyway, since this is either sought when designing the waveform or is predetermined by the print head manufacturer.

As already mentioned, crosstalk also leads to instability in ejection, which can be perceived by the increased formation of ink drops on the printing nozzle plate. These droplets are increasingly large and eventually lead to slant ejection and failure of the printing nozzle 13. Furthermore, due to the negative meniscus pressure and the very different surface tension of the printing nozzle plate surface to the inner surface of the printing nozzle 13, a non-printing nozzle accumulates such ink on the printing nozzle plate if a droplet is close enough to such a printing nozzle 13 or contacts the nozzle edge.

Thus, another condition for the raster image 15 now arises (especially when printing very high areal coverage): such conditions are taken into account in a further preferred embodiment variant of the process according to the invention:

the raster image 15 must be formed by the computer 6 in such a way that not all printing nozzles 13 print simultaneously, but the printing nozzles 13 are switched off alternately at a defined repetition frequency.

Furthermore, further embodiment variants exist which supplement the method according to the invention.

Alternatively, there may be a database with undesired patterns, for example. These patterns depend on the geometry of the print head (i.e. the arrangement of the printing nozzles 13 and the position of the supply channels 12). Here, it must be known for each printing nozzle 13: which other printing nozzles 13 in the printing head 5 are connected by the supply channel 12 and how far from these other printing nozzles 13. The pattern of the database is typically dependent on the printing speed. In order to calculate the grid by means of the computer 6, the error weights of the pattern are now stored in a database. Now, the error terms 14 calculated during grid generation can be extended with these error weights when the pattern stored in the database is present. In this way, the probability of the occurrence of a pattern stored in the database can be minimized. The individual patterns may also be weighted and thus the degree of undesirability may still be taken into account.

Such a database of undesired patterns may also be generated manually or by subjective evaluation. For this purpose, different patterns are printed and the degree of uneasiness of the grid surface to the human eye is subjectively evaluated. The weighting of the patterns is then performed according to a subjective evaluation.

Another preferred embodiment variant of the screening according to the invention comprises the following alternatives:

alternative 1:

in the process performed after the screening, an undesired pattern may be searched for. Such a search is performed by the computer 6 either by means of a known solution or based on a database of patterns. If such a pattern is found, it may be attempted to modify the grid at that location in such a way as to obtain a pattern that does not produce crosstalk. This modification may be achieved, for example, by moving the affected pixels to one of their adjacent positions. Furthermore, such a modification can also be made only with a probability of hue value correlation, since the frequency of these patterns increases sharply with increasing hue value.

Alternative 2:

the following possibilities also exist: the computer 6 generates the raster image 15 based on a look-up table of threshold matrices or grating lobes (Rasterkacheln) calculated from the beginning. All the previously described methods for reducing crosstalk can also be used when calculating such a threshold matrix or such a look-up table.

The advantages of the method according to the invention are, inter alia: the determination of the factor n can be easily and quickly integrated into the grid algorithm. All that is required is that there is a maximum print speed and limit frequency 8 for the corresponding ink/waveform combination. When new print heads 5 or other inks or the like are introduced, the database formulated or generated by a large number of printing tests must be updated accordingly. The second alternative is preferably used in the screening, since the current screening is based on a look-up table.

List of reference numerals

1 feeder

2 Current printing substrate/Current printing sheet

3 material collector

4 ink-jet printing mechanism

5 ink jet print head

6 computer

7 ink jet printer

8 limit frequency

9 ratio of ink drop velocity/ink drop size

10 jet frequency

11 ink drop velocity/size measurement curve with respect to ejection frequency

12 ink supply channel

13 ink jet printing nozzle

14 calculated error term

15 raster image

Claims

1. A method for avoiding crosstalk in a printing process of an inkjet printer (7) by means of a computer (6), wherein the inkjet printer (7) has a print head (5) with printing nozzles (13) which are arranged in rows and columns, each having a defined number of printing nozzles (13), and the printing nozzles (13) of each row share in each case one ink supply channel (12), and the computer (6) carries out a screening process for carrying out the printing process,

it is characterized in that the preparation method is characterized in that,

during the screening process, the computer (6) adapts the image data to be generated in such a way that the printing nozzles (13) of a row do not exceed a limit value (8) for a defined ink extraction frequency,

wherein the limit value (8) for the ink extraction frequency is derived from the screen frequency in the machine direction and a factor n for a defined number of time steps in the printing direction, which factor n specifies the corresponding time step in which the use of printing nozzles of the same ink supply channel at the same time is not permitted.

2. The method of claim 1, wherein the first and second light sources are selected from the group consisting of,

it is characterized in that the preparation method is characterized in that,

the screening frequency in the machine direction is determined by the printing speed of the inkjet printer (7) and the resolution in the printing direction.

3. The method according to claim 1 or 2,

it is characterized in that the preparation method is characterized in that,

the limit value (8) for the ink extraction frequency of the printing nozzles (13) of a row of the printing head (5) is determined by measuring the ratio of the ink drop velocity to the ink drop size, or is predefined by the printing head manufacturer.

4. The method according to claim 1 or 2,

it is characterized in that the preparation method is characterized in that,

in order to avoid crosstalk, the computer (6) adapts the grid in accordance with the limit value (8) of the ink extraction frequency in such a way that the printing nozzles of the same row of the printing head (5) are not used a plurality of times within the factor n.

5. The method of claim 4, wherein the first and second light sources are selected from the group consisting of,

it is characterized in that the preparation method is characterized in that,

in order to increase the areal density that can be achieved during printing, the rows of the print heads (5) are divided into partial regions, and the inhibition of the multiple use of the same rows of printing nozzles (13) is applied only to the partial regions.

6. The method of claim 5, wherein the first and second light sources are selected from the group consisting of,

it is characterized in that the preparation method is characterized in that,

to further increase the achievable areal density, the number of time steps by a factor n is reduced.

7. The method of claim 4, wherein the first and second light sources are selected from the group consisting of,

it is characterized in that the preparation method is characterized in that,

the computer (6) calculates an error term (14) which describes the deviation from the original image during the course of the screening process, the computer carrying out the screening process in such a way that the error term (14) is minimized, and the computer taking into account the limit value (8) for the ink extraction frequency when calculating the error term (14).

8. The method according to claim 1 or 2,

it is characterized in that the preparation method is characterized in that,

the computer (6) always calculates the factor n using the maximum printing speed of the inkjet printer (7).

9. The method according to claim 1 or 2,

it is characterized in that the preparation method is characterized in that,

the method is performed for a print head (5) as follows: the printing nozzles (13) of the print head are arranged in a front block and a rear block, respectively consisting of 16 rows and 64 columns.