INK JET MAINTENANCE SPIT PATTERN

Abstract

A method of ink jet printing using an ink jet imaging device including a plurality of nozzles arranged for expelling droplets of an ink by actuation of an ink channel includes the steps of: a) providing a bit map of a spit pattern comprising an arrangement of refresh dots to be printed by each of the plurality of nozzles; and d) printing the bitmap of the spit pattern. The spit pattern includes a plurality of clusters of refresh dots, each cluster including at least two sequential refresh dots expelled from a single nozzle. A spit pattern for use in such a method is disclosed.

Claims

1. A method of ink jet printing using an ink jet imaging device comprising a plurality of nozzles arranged for expelling droplets of an ink by actuation of an ink channel, the method comprising the steps of: a) providing a bit map of a spit pattern comprising an arrangement of refresh dots to be printed by each of the plurality of nozzles; and d) printing the bitmap of the spit pattern, wherein the spit pattern comprises a plurality of clusters of refresh dots, each cluster comprising at least two sequential refresh dots expelled from a single nozzle.

2. The method according to claim 1, wherein the at least two sequential refresh dots of a cluster are printed at a time interval of between 0.5 ms and 100 ms.

3. The method according to claim 1, wherein the spit pattern comprises a first cluster of at least two sequential refresh dots and a second cluster of at least two sequential refresh dots, the first cluster and the second cluster are printed at a time interval of at least 125 ms.

4. The method of ink jet printing according to claim 1, wherein the method comprises the steps of: a) providing a bit map of a spit pattern comprising an arrangement of refresh dots to be printed by each of the plurality of nozzles; b) providing a bitmap of an image to be printed; c) superimposing the bitmap of the spit pattern onto the bitmap of the image to be printed and hence creating an execution bitmap; and d) printing the execution bitmap, wherein the spit pattern comprises a plurality of clusters of refresh dots, each cluster comprising at least two sequential refresh dots expelled from a single nozzle.

5. A spit pattern for use in the method according to claim 1, wherein the spit pattern comprises a plurality of clusters of refresh dots, each cluster comprising at least two sequential refresh dots expelled from a single nozzle.

6. The spit pattern according to claim 5, wherein the at least two sequential refresh dots are arranged at a distance of one another such that on a time scale the sequential refresh dots are printed at a time interval of between 0.5 ms and 100 ms.

7. The spit pattern according to claim 5, wherein a first cluster of at least two sequential refresh dots and a second cluster of at least two sequential refresh dots are arranged at a distance of one another such that the first cluster and the second cluster are printed at a time interval of at least 125 ms.

8. The spit pattern according to claim 5, wherein the pixels are printed at a frequency of 32 kHz and wherein a first cluster of eight sequential refresh dots and a second cluster of eight sequential refresh dots are arranged at a distance of 16000 pixels, wherein the distance between each of the eight pixels of each cluster is 50 pixels.

9. The method according to claim 2, wherein the spit pattern comprises a first cluster of at least two sequential refresh dots and a second cluster of at least two sequential refresh dots, the first cluster and the second cluster are printed at a time interval of at least 125 ms.

10. The method of ink jet printing according to claim 2, wherein the method comprises the steps of: a) providing a bit map of a spit pattern comprising an arrangement of refresh dots to be printed by each of the plurality of nozzles; b) providing a bitmap of an image to be printed; c) superimposing the bitmap of the spit pattern onto the bitmap of the image to be printed and hence creating an execution bitmap; and d) printing the execution bitmap, wherein the spit pattern comprises a plurality of clusters of refresh dots, each cluster comprising at least two sequential refresh dots expelled from a single nozzle.

11. The method of ink jet printing according to claim 3, wherein the method comprises the steps of: a) providing a bit map of a spit pattern comprising an arrangement of refresh dots to be printed by each of the plurality of nozzles; b) providing a bitmap of an image to be printed; c) superimposing the bitmap of the spit pattern onto the bitmap of the image to be printed and hence creating an execution bitmap; and d) printing the execution bitmap, wherein the spit pattern comprises a plurality of clusters of refresh dots, each cluster comprising at least two sequential refresh dots expelled from a single nozzle.

12. A spit pattern for use in the method according to claim 2, wherein the spit pattern comprises a plurality of clusters of refresh dots, each cluster comprising at least two sequential refresh dots expelled from a single nozzle.

13. A spit pattern for use in the method according to claim 3, wherein the spit pattern comprises a plurality of clusters of refresh dots, each cluster comprising at least two sequential refresh dots expelled from a single nozzle.

14. A spit pattern for use in the method according to claim 4, wherein the spit pattern comprises a plurality of clusters of refresh dots, each cluster comprising at least two sequential refresh dots expelled from a single nozzle.

15. The spit pattern according to claim 6, wherein a first cluster of at least two sequential refresh dots and a second cluster of at least two sequential refresh dots are arranged at a distance of one another such that the first cluster and the second cluster are printed at a time interval of at least 125 ms.

16. The spit pattern according to claim 6, wherein the pixels are printed at a frequency of 32 kHz and wherein a first cluster of eight sequential refresh dots and a second cluster of eight sequential refresh dots are arranged at a distance of 16000 pixels, wherein the distance between each of the eight pixels of each cluster is 50 pixels.

17. The spit pattern according to claim 7, wherein the pixels are printed at a frequency of 32 kHz and wherein a first cluster of eight sequential refresh dots and a second cluster of eight sequential refresh dots are arranged at a distance of 16000 pixels, wherein the distance between each of the eight pixels of each cluster is 50 pixels.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0024] For a more complete understanding of the invention and the advantages thereof, exemplary embodiments of the invention are explained in more detail in the following description with reference to the accompanying drawing figures, in which like reference characters designate like parts and in which:

[0025] FIG. 1 schematic representation of a normal spit pattern (prior art): 1 dot in frequency 1 on 1000 pixels;

[0026] FIG. 2 schematic representation of a spit pattern comprising less dots: 1 dot in frequency 1 on 2000 pixels;

[0027] FIG. 3 schematic representation of a spit pattern according to the present invention: cluster of 2 sequential dots spaced 50 pixels apart in frequency 1 (cluster) on 4000 pixels.

DETAILED DESCRIPTION

[0028] In ink jet printing systems, in particular in printing systems using page wide image formation, refresh dots are printed to deal with evaporating ink by jetting away the deteriorated ink out of idle nozzles and hence at least partially refreshing the ink (or other functional liquid) in functional parts of the jetting device. The amount of refresh dots should be as low as possible. Too many refresh dots may create a haze in the background, i.e. in case of black ink a grey background.

[0029] In any printing method a 1 dot in x pixels spit pattern that is on the boundary of visibility can be empirically determined, in the current examples 1 dot in 2000 pixels as shown in FIG. 2. When clustering n (n=2, 3, 4, etc.) dots originating from a single nozzle, the pixel distance between dot clusters increases with a factor f=n.Math.x (present example n=2, f=2.2000=4000). Essential for staying below the visibility limit is that the dots are evenly spaced. Therefore, the minimum distance between two adjacent dots in a cluster is determined in accordance with the minimum distance between dots in a 1 dot in x pixels spit pattern, which is on or below the visibility limit. The minimum distance between dots is calculated by taking the square root of the 1 dot in x pixels distance, because the dots closest to one another originate from different nozzles: d=√(x). For the examples, the 1 dot in 2000 pixels pattern, the minimum distance is √(2000)=44.7 pixels. Hence a pixel distance between two adjacent dots of 50 pixels is sufficient to stay below the visibility limit. Maintaining a certain distance between two subsequently ejected droplets also prevents coagulation of droplets and/or creating large ink blobs in the image which would impart visibility significantly.

[0030] Typically, the allowed refresh rate is 1 on 2000 pixels for a 1200 dpi system in order to stay below the visibility limit. Therefore, every nozzle jets a drop every 2000 pixels, additional to the intended bitmap. These refresh dots are typically placed in a regular pattern, because this leads to the lowest visibility.

[0031] It has been found that evaporation in and in the vicinity of nozzle openings of an ink jet imaging device (print head) slows down after approximately 200 ms after ejection of an ink droplet from the nozzle. In the interval 0-200 ms after ejection of an ink droplet the amount of evaporated water is significantly higher than thereafter (e.g. in the interval from 200 ms-400 ms).

[0032] Further, it has been found that a single refresh dot (one droplet ejection) is not capable of perfectly restoring the jetting stability to its original state.

[0033] Without wanting to be bound to any theory, it is believed that at least two sequentially expelled droplets provide an improved refreshment of the ink in an individual nozzle, to such an extent that the time interval between subsequent refreshment cycles (i.e. clusters) can be increased. Overall this leads to maintenance of jetting stability with less refresh dots, in practice a reduction of required refresh dots may be a factor 2. It has been found that up to 8 sequentially expelled droplets (and maybe even beyond that number) still provides improvement.

[0034] The Figures show schematic representations of spit patterns in tabular form. The rows (horizontal) represent a nozzle array. In a single row actuated nozzles are indicated with black squares. In a column (vertical) it can be seen when a single nozzle is actuated to print a refresh dot. The paper feed direction is indicated with arrow 1. The examples shown in the Figures and described hereafter are based on printing with a drop on demand frequency (DoD) of 32 kHz, which implies a droplet cycle time of 31.25 μs. For examples, the time between two dots printed in a 1 dot in 1 on 1000 spit pattern (FIG. 1) is 31.25 ms.

[0035] FIG. 1 shows a schematic representation of a normal spit pattern according to the prior art. The distance, indicated with double arrow 2, between two fire moments of the same nozzle is 1 in 1000 pixels (so a single droplet is fired, i.e. 1 refresh dot, every 1000 pixels). In practice such spit pattern provides sufficient refreshment of the ink in the nozzles. The average distance between neighboring refresh dots (in all directions) is equal to √1000≈32 pixels, indicated with double arrow 6 (not on scale). However, as indicated above, in for a 1200 dpi system, a 1 refresh dot in a 1 on 2000 frequency is required in order to stay below the visibility limit. Such a pattern is shown in FIG. 2.

[0036] FIG. 2 shows a schematic representation of a sparser spit pattern, meeting the required visibility limit for a 1200 dpi printing system. Again the same nozzle array is shown. Each individual nozzle now spits once every 2000 pixels, which is indicated with double arrow 3. The distance between two sequential refresh dots from the same nozzle has doubled compared to the pattern shown in FIG. 1. The average distance between neighboring refresh dots (in all directions) is equal to √2000≈45 pixels, indicated with double arrow 7 (not on scale). In practice such spit pattern has proven to provide insufficient refreshment of the ink in the nozzles to maintain the jet stability on a desired level. The print quality significantly decreases.

[0037] FIG. 3 shows a spit pattern used in a method according to the present invention. At first glance it can be seen that the number of refresh dots is comparable to the pattern shown in FIG. 2. However, in the spit pattern shown in FIG. 3, the refresh dots are clustered (clusters of two dots) and the refresh dot distance between subsequent refresh dots in a cluster of refresh dots is 50 pixels, as indicated with double arrow 5. The repeat frequency of a cluster of refresh dots has doubled compared to the pattern shown in FIG. 2 (2 dots; 1 on 4000 nozzles), as indicated with double arrow 4. The average distance between clusters is √4000≈63 pixels and the distance between subsequent refresh dots in a cluster is 50 pixels (double arrow 5), both larger than the average distance in a 1 dot in 2000 pixels pattern as shown in FIG. 2, which distance is approximately 45 pixels. Therefore, the spit pattern shown in FIG. 3 is well below the visibility limit. It is to be noted that FIG. 3 (the figures in general) is not on scale. In the present example double arrow 5 represents 50 pixels and double arrow 4 represents 4000 pixels, hence in practice double arrow 4 is a factor 80 longer than double arrow 5. To indicate this scale discrepancy double arrow 4 is interrupted as indicated with 6. In the horizontal direction (nozzle array) the average distance between refresh dots is also 50 pixels as indicated with arrow 8 (not on scale) In practice this pattern has proven to provide sufficient refreshment of ink in the nozzles to maintain the jet stability on a desired level. Without wanting to be bound to any theory, it is believed that multiple refresh dots in a relative short time interval improves the quality of the refreshment of the ink in the nozzles significantly, which has the effect that the clusters can be repeated at a lower frequency.

EXAMPLES

[0038] Spit patterns as described in Table 1 below were applied to a 1200 dpi printing system using an in-house developed piezo based MEMS print head printing an in-house developed water-based, pigmented latex ink with high solid load. The ink compositions used comprised 20 wt % glycerol, 10 wt % solid particles (in total) and 70% water. It is noted that the present invention will work with any print head and ink combination. Droplet size was 2 pl, Drop on Demand (DoD) frequency was 32 kHz.

[0039] Table 1 shows the results of this printing experiments, wherein the judgement NOK/OK is based on whether or not the nozzles fail due to drying in of ink in or in the vicinity of a nozzle opening. Furthermore, the judgement “OK” was only awarded when no visible print artefacts known to be caused by drying-in of ink in or in the vicinity of the nozzles, such as OD (optical density) variations or line raggedness, were detected (in a visual inspection of the prints).

TABLE-US-00001 TABLE 1 results of print experiments Nozzle Refresh # Refresh Spit pattern stability Overall Example rate dots/m.sup.2 visible? OK? judgement CE 1 1 × 1 on 2.2 * 10.sup.6 yes yes NOK 1000 CE 2 1 × 1 on 1.9 * 10.sup.6 not available no NOK 1200 1 8 × 1 on 1.1 * 10.sup.6 no yes OK 16000

[0040] In comparative example 1 (CE 1), the print test was carried out for 1 hour during which the nozzle stability remained OK. The spit pattern was however visible in the prints.

[0041] In comparative example 2 (CE 2), some nozzles started failing after 1 minute of printing and significant loss of droplet volume and speed for all nozzles was detected after several minutes, indicating a failing droplet ejection stability.

[0042] In example 1 a cluster of 8 refresh dots at a cluster repeat frequency of 1 in 16000 pixels was applied. The total number of refresh dots was reduced by a factor 2 compared to CE 1. The distance between the dots in the cluster was 50 pixels, corresponding to the average distance between the dots in a regular 1 dot in 1 on 2000 pixels frequency. The 50 pixel distance is selected to prevent subsequent droplets forming a large ink blob in the image which disturbs the visibility. In the 1 dot in 1 on 2000 pixels, the average distance between two adjacent dots (originating from different nozzles) is √(2000) (square root)=44.7 pixels. Therefore, a distance of 50 pixels does not impart visibility. Therefore, the visibility of the pattern used in Example 1 (8 refresh dots in 1 on 16000 frequency) is similar to the visibility of a 1×1 on 2000 pattern.

[0043] Without wanting to be bound to any theory, the mechanism behind the effect of the present invention is based on decreasing evaporation rate of water (or other liquid components) from ink present inside or in the vicinity of the nozzle openings: models of water evaporation from a nozzle show that water evaporation slows down after 100 ms-200 ms. The time between two subsequent spit droplets in a 1×1 on 2000 spit pattern at 32 kHz printing is 62.5 ms. The time interval between subsequent clusters in a 8×1 on 16000 spit pattern is 500 ms. The amount of water loss in 500 ms using the 8×1 on 16000 spit pattern is far less than 8 times the amount of water loss in 62.5 ms when using the 1×1 on 2000 spit pattern (the time between the first and the eighth dot is 7 times 2000 pixels, which is 437.5 ms). Most of the evaporation occurs in the 100 ms-200 ms after droplet ejection (i.e. starting with fresh ink in the nozzle). Due to slowing down of evaporation, most of the damage caused by evaporation has already been done in the first 100 ms-200 ms. Furthermore, a single refresh dot is not sufficient to bring the jetting stability of the nozzle back to the initial state. These combined effects lead to the fact that the jet stability of a nozzle is better reset to its initial state by jetting 8 droplet in a row (each 50 pixels apart, distance between first and eighth dot is 350 pixels, which in the present example equals 10.9 ms) repeated every 500 ms, instead of 1 droplet repeated every 62.5 ms.

[0044] In conclusion, by clustering the refresh dots printed by a single nozzle in groups of at least 2 subsequently expelled droplets (in Example 1 a cluster of 8 subsequently expelled droplets) with relatively short time interval (in the present example 50 pixels) enables reducing the total number of refresh dots required to maintain jet stability and hence the print quality on a desired level.

[0045] The optimum number of refresh dots in a cluster may be higher than represented by FIG. 3 or even higher than described above for Example 1 (8×1 on 16000) and may be dependent on the type of print head and ink used, the design of air refreshment in a printing device and environmental conditions. Inventors have found that clusters of up to 8 droplets still provided improvement.