NL2022767B1

NL2022767B1 - A Method of Ink Jet Printing

Info

Publication number: NL2022767B1
Application number: NL2022767A
Authority: NL
Inventors: J M Lejeune Guido; Dey Arghya; A C M Goosen- Van Den Heuvel Desie
Original assignee: Canon Production Printing Holding Bv
Priority date: 2019-03-19
Filing date: 2019-03-19
Publication date: 2020-09-28

Abstract

P4102NL01 14 Abstract A method of printing with an inkjet printer is disclosed. This printer has a nozzle connected to a pressure chamber, and an actuator arranged to excite a pressure wave 5 in an ink in the pressure chamber in order to jet-out a droplet of ink from the nozzle. A print head comprising the nozzle and the actuator scans a recording medium on which an image is to be printed. The nozzle starts jetting when it reaches a transition from a background area to an image area on the recording medium and stops jetting when it reaches a transition from an image area to a background area. When the nozzle 10 reaches a pixel immediately before a transition from background to image, or a pixel immediately after a transition from image to background, the actuator is energized with a prefire pulse that is sufficient for agitating the ink in the pressure chamber but insufficient for forming a droplet.

Description

P4102NL01 1 A Method of Ink Jet Printing

BACKGROUND OF THE INVENTION

1. Field of the invention The invention relates to a method of printing with an ink jet printer having a nozzle connected to a pressure chamber, and an actuator arranged to excite a pressure wave in an ink in the pressure chamber in order to jet-out a droplet of ink from the nozzle, in which method a print head comprising the nozzle and the actuator scans a recording medium on which an image is to be printed, and the nozzle starts jetting when it reaches a transition from a background area to an image area on the recording medium and stops jetting when it reaches a transition from an image area to a background area. More particularly, the invention relates to a printing method in which the ink that is used for printing includes particles, such as pigment particles or magnetic particles. An example of an application of the invention is printing MICR (Magnetic Ink Character Recognition) code.

2. Description of the Related Art MICR is a technology used mainly in the banking industry for making the processing and clearance of checks easier and more reliable. In order for the check reader to validate the check, an MICR character line printed on the check needs to have a Magnetic Signal Strength (MSS) above a certain target. However, voids and satellites in the printed image tend to change the MSS, so that the characters are more difficult to recognize. Voids are pixel locations on the recording medium, e.g. the check, where a pixel should have been printed but has not been printed due a failure of the corresponding nozzle. Satellites are pixel locations on the background area of the image, i.e. locations where no pixel should be printed, but where an ink dot has nevertheless been formed because extra droplets have been generated in the jetting process. Both effects are particularly likely to occur in inks that contain relatively large particles and are therefore relatively inhomogeneous. In general, such voids and satellites tend to blur the boundaries between image areas and background areas and thereby tend to reduce the contrast of the image. It is an object of the invention to provide a method that permits to improve the contrast

P4102NL01 2 in ink jet printing, particularly in ink jet printing with inks that contain particles.

SUMMARY OF THE INVENTION In the method according to the invention, in order to achieve this object, when the nozzle reaches a pixel immediately before a transition from background to image, or a pixel immediately after a transition from image to background, the actuator is energized with a prefire pulse that is sufficient for agitating the ink in the pressure chamber but insufficient for forming a droplet.

It has been observed that voids occur mainly at transitions from background to image, where the nozzle starts jetting. The prefire pulse has the effect that the ink in the pressure chamber is agitated and the mobility of the ink is increased, so that, although no droplet is jetted out at the present pixel position, a droplet will more reliably jetted out when the next jetting pulse arrives and the nozzle passes the first pixel of the image area.

Conversely, satellites are mainly produced at transitions from image to background where the nozzle stops jetting. In such a situation, although the actuator is no longer exited by pressure pulses, the ink has been agitated by the previous jetting pulses and a decaying pressure wave is still present in the pressure chamber, so that, in some cases, a satellite droplet may be formed right after the droplet that has been jetted out for printing the last pixel in the image area, and the satellite droplet will then be deposited in the background area of the recording medium. In such cases the prefire pulse may have the effect to quench the decaying pressure wave in the ink, so that the likelihood of satellites is reduced.

More specific optional features of the invention are indicated in the dependent claims. The method according to the invention is particularly advantageous in printing applications where the inks contain particles, such as in MICR printing, where the inks contain magnetic particles, or in printing with white ink which also contains relatively large white pigment particles.

In a conventional black and white print process, the print data are supplied to the printer

P4102NL01 3 in the form of bitmap data that specify for each pixel position that is reached by a nozzle of the print head whether or not a droplet shall be jetted out. Thus, in the conventional bitmap, each pixel may only assume one of the two values “image”, which means a jetting pulse is required, and “background” which means that the actuator shall not be energized at all. In a method according to the invention, the pixels in the bitmap may assume one of three different values, namely “image”, “background” or “prefire”, which means that a non-jetting prefire pulse shall be applied. Then, a raster image processing operation in which the bitmap is formed, may comprise a step of surrounding each image area with a seam of prefire pixels, so that a prefire pulse will be jetted at each transition from background to image and also at each transition from image to background. The seam of prefire pixels may be one or more pixels wide.

In general, it would be sufficient to provide the seam of prefire pixels only at the transitions that are crossed by the nozzles of the print head during the scan movement in the main scanning direction. When a transition between image and background extends in parallel with the main scanning direction, a nozzle moving along that transition will be either on the image side of the transition and will be jetting constantly, or it will be on the background side of the transition and will not jet at all, so that neither voids nor satellites would be created, anyway. However, since it may not be known at the time when the print job is created, which direction will be the main scanning direction of the printer, and since the prefire pixels are not distinguishable from background pixels in the printed image, it is preferred to surround each image area by a complete seam of prefire pixels, even at the transitions that extend in parallel with the main scanning direction.

BRIEF DESCRIPTION OF THE DRAWINGS Embodiment examples will now be described in conjunction with the drawings, wherein: Fig. 1 is a schematic cross-sectional view of a single jetting device of an ink jet print head in a print process according to the invention; Fig. 2 shows the jetting device according to the Fig. 1 together with an

P4102NL01 4 electronic control and measurement circuit; Fig. 3 is a time chart showing waveforms of energizing pulses to be applied to an actuator of the jetting device; Fig. 4 is a view of a MICR character with voids and satellites produced in a conventional printing method; Fig. 5 is a bitmap for printing the same MICR character as in Fig. 5 by means of a method according to the invention; and Fig. 6 is a flow diagram showing essential steps of a method according to the invention.

DETAILED DESCRIPTION OF EMBODIMENTS A single jetting unit of an ink jet print head has been shown in Fig. 1. The device comprises a wafer 10 and a support member 12 that are bonded to opposite sides of a thin flexible membrane 14.

Arecess that forms an ink duct 16 is formed in the face of the wafer 10 that engages the membrane 14, i.e. the bottom face in Fig. 1. The ink duct 16 has an essentially rectangular shape. An end portion on the left side in Fig. 1 is connected to an ink supply line 18 that passes through the wafer 10 in thickness direction of the wafer and serves for supplying liquid ink to the ink duct 16.

An opposite end of the ink duct 16, on the right side in Fig. 1, is connected, through an opening in the membrane 14, to a chamber 20 that is formed in the support member 12 and opens out into a nozzle 22 that is formed in a nozzle face 24 constituting the bottom face of the support member.

Adjacent to the membrane 14 and separated from the chamber 20, the support member 12 forms another cavity 26 accommodating a piezoelectric actuator 28 that is bonded to the membrane 14.

P4102NL01 An ink supply system which has not been shown here keeps the pressure of the liquid ink in the ink duct 18 slightly below the atmospheric pressure, so as to prevent the ink from leaking out through the nozzle 22.

5 Asis shown in Fig. 2, the piezoelectric actuator 28 has electrodes 34 that are connected to an electronic circuit that has been shown in the lower part of Fig. 2. In the example shown, one electrode of the actuator is grounded via a line 36 and a resistor 38. Another electrode of the actuator is connected to an output of an amplifier 40 that is feedback-controlled via a feedback network 42, so that a voltage V applied to the actuator will be proportional to a signal on an input line 44 of the amplifier. The signal on the input line 44 is generated by a D/A-converter 46 that receives a digital input from a local digital controller 48. The controller 48 is connected to a processor 50. When an ink droplet is to be expelled from the nozzle 22, the processor 50 sends a command to the controller 48 which outputs a digital signal that causes the D/A- converter 46 and the amplifier 40 to apply a voltage pulse to the actuator 26. This voltage pulse causes the actuator to deform in a bending mode. More specifically, the actuator 28 is caused to flex downward, so that the membrane 14 which is bonded to the actuator 28 will also flex downward, thereby to increase the volume of the ink duct

16. As a consequence, additional ink will be sucked-in via the supply line 18. Then, when the voltage pulse falls off again, the membrane 14 will flex back into the original state, so that a positive acoustic pressure wave is generated in the liquid ink in the duct

16. This pressure wave propagates to the nozzle 22 and causes an ink droplet to be expelled.

The electrodes 34 of the actuator 28 are also connected to an A/D converter 52 which measures a voltage drop across the actuator and also a voltage drop across the resistor 38 and thereby implicitly the current flowing through the actuator. Corresponding digital signals are forwarded to the controller 48 which can derive the impedance of the actuator 28 from these signals. The measured electric response (current, voltage, impedance, etc.) is signaled to the processor 50 where the electric response is processed further. The circuit described above permits to monitor the response of the ink to the energizing

P4102NL01 6 pulses applied to the actuator 28, so that the energizing pulses may be configured precisely for achieving the desired effects. In particular, as will be described by reference to Fig. 3, the processor 50 and the controller 48 may be used not only for applying jetting pulses to the actuator in order to jet out ink droplets, but also for applying prefire pulses which only agitate the ink in the ink duct 16 and the chamber 20 but are not strong enough for expelling an ink droplet. Such pressure pulses may be used on the one hand for preparing the jetting unit for droplet ejection processes after it has been inactive for a while. On the other hand, the prefire pulses may also be used for reducing the tendency to form satellite ink droplets when the jetting unit is deactivated after it has been active for a while.

Fig. 3 shows the voltage V (in arbitrary units) applied to the actuator 28 as a function of the time t.

When an ink droplet is to be expelled from the nozzle, the actuator 28 is energized with a jetting pulse 54 which comprises an actuation pulse 54a with positive polarity, followed, after a certain delay time, by a quench pulse 54b which has negative polarity and a somewhat smaller amplitude. The actuation pulse 54a has a rising flank 58 with a height A1, and a descending flank 60 with a height A2. During the rising flank 58 of the actuation pulse, the membrane 14 is flexed downwardly, so that fresh ink is drawn in from the ink supply line 18. Then, during the descending flank 60, the membrane 14 moves upwards again, so that the volume of the ink duct 16 is reduced and a pressure wave is excited in the liquid ink. This pressure wave will propagate to the nozzle 22 and will cause an ink droplet to be expelled. While the droplet is being jetted out, the pressure wave is reflected (with phase reversal) at the nozzle 22 and will propagate back into the ink duct 16 at the end of which it will be reflected again, so that the ink in the ink duct 16 undergoes periodic pressure fluctuations which gradually decay in the course of time, before a next droplet is to be ejected.

The quench pulse 54b is timed and dimensioned so as to attenuate the pressure fluctuations by destructive interference, so that the fluctuations may be reduced before the next ink droplet is to be ejected.

Alternatively, the actuator 28 may be energized with a prefire pulse 56 comprising an

P4102NL01 7 actuation pulse 56a and a quench pulse 56b.

In this example, the actuation pulse 56a is asymmetric in the sense that the height B1 of the rising flank is smaller than the height B2 of the descending flank or, in other words, the flank ratio B1/B2 is smaller than 1. The overall amplitude is so small that no droplet will be expelled.

The amplitude of the quench pulse 56b is also reduced, so that a residual pressure wave will prevail and will assist in the formation of an ink droplet in the next cycle.

On the other hand, the amplitude and phase of the prefire pulse 56 are selected such that a residual pressure wave that may have been left over from a previous jetting cycle, will be suppressed further, so that the formation of satellite ink droplets can be prevented mare reliably.

In the example shown, prefire pulses 56 with identical waveforms are used for both purposes, i.e. agitating the ink at the start of a jetting sequence and suppressing satellites at the end of a jetting sequence.

In a modified embodiment, prefire pulses with different wave forms may be used for these two purposes in order to optimize the waveform in view of the respective purpose.

In any case, the pressure waves in the ink can be monitored by the circuit shown in Fig. 2, so that the waveforms of the jetting pulses 54 and the prefire pulses 56 may be corrected, if necessary.

In Fig. 1, the jetting device is shown in a situation in which the print head moves in a main scanning direction x relative to a recording medium 62, and the nozzle 22 has just reached a transition between an image area 64 and a background area 66 of an image to be printed onto the recording medium.

The nozzle 22 is ejecting an ink droplet 68 in order to print a pixel 70 which is the last pixel in a sequence of image pixels 72 that have been printed earlier.

In the next jetting cycle, the nozzle 22 could eject another dropletin order to print another pixel 74. However, since the position of this pixel 74 is already in the background area 66, no jetting pulse is applied to the actuator 28, so that no droplet will be ejected.

Instead, in order to prevent the undesired creation of a satellite droplet, the actuator is energized with a prefire pulse.

P4102NL01 8 Earlier in the print process, the actuator 28 had been energized with another prefire pulse at the time when the nozzle 22 was above the position of a pixel 76 at a transition from the background area 66 to the image area 64. In this case, the prefire pulse had the function to agitate the ink in the ink duct 18 and the chamber 20 in order to increase the mobility of the ink, so as to assure that ink droplets are actually ejected when the nozzle 22 passes over the image area 64. In other words, the prefire pulse aims in this case at avoiding the formation of voids in the image area.

In a practical application, the print head having the jetting device shown in Figs. 1 and 2, and actually a large number of such jetting devices aligned in the direction normal to the plane of the drawing in Fig. 1, may be utilized for printing a MICR character 78 an example of which has been shown in Fig. 4. However, Fig. 4 shows how the character 78 (“0”) would appear if the character had been printed by a conventional printing method, with the print head moving from left to right in Fig. 4. In that case, voids 80 may be produced at the edges of the character that correspond to transitions from the background area to the image area, and satellites 82 may be produced at the edges that correspond to a transition from an image area to a background area.

In contrast, in the method according to the invention, the print instructions for the character 78 are encoded in a bitmap 84 as shown in Fig. 5. Here, a seam 86 of prefire pixels (such as the pixels 74 and 76 in Fig. 1) has been provided along all edges of the character 78 that separate the background area 66 from the image area 64. The prefire pulses applied for each of the pixels in the seam 86 will reliably prevent the formation of both, voids 80 and satellites 82, regardless of the direction in which the recording medium is scanned.

Essential steps of a printing method according to the invention will now be summarized by reference to the flow diagram shown in Fig. 6.

Step S1 is a step of reading image data for the image to be printed. In step S2, the image is segmented into background areas 66 and image areas 64. Then, in step S3, the pixels in the background areas 66 are marked as background pixels, which means that no energizing pulse whatsoever will be applied to an actuator of the print head when the associated nozzle passes over the pixel.

P4102NL01 9 Similarly, in step S4, all pixels in in the image areas 64 are marked as image pixels, which means that a jetting pulse will be applied whenever a nozzle moves over such a pixel.

Step S5 is a step of searching for pixels at the boundary between background areas 66 and image areas 64, e.g. the pixels forming the seam 86 in Fig. 5. Then, in step S6, these pixels will be marked as prefire pixels, which means that a prefire pulse 56 will be applied whenever a nozzle passes over such a pixel.

The steps shown in Fig. 6 may form part of a raster image processing routine in which a bitmap is created for the image to be printed. In another embodiment, the seam 86 may be defined already in an earlier stage of the creation of the print job, i.e. prior to raster image processing.

P4102NL01 10 Embodiments

1. A method of printing with an ink jet printer having a nozzle (22) connected to a pressure chamber (16, 20), and an actuator (28) arranged to excite a pressure wave in an ink in the pressure chamber (16, 20) in order to jet-out a droplet (68) of ink from the nozzle (22), in which method a print head comprising the nozzle (22} and the actuator (28) scans a recording medium (62) on which an image is to be printed, and the nozzle (22) starts jetting when it reaches a transition from a background area (66) to an image area (64) on the recording medium and stops jetting when it reaches a transition from an image area (64) to a background area (66), characterized in that, when the nozzle (22) reaches a pixel (76) immediately before a transition from background to image, or a pixel (74) immediately after a transition from image to background, the actuator (28) is energized with a prefire pulse (56) sufficient for agitating the ink in the pressure chamber (16, 20) but insufficient for forming a droplet.

2. The method according to claim 1, wherein the prefire pulses (56) applied at the transition from background to image and the prefire pulses applied at the transition from image to background have identical waveforms.

3. The method according to any of the preceding claims, comprising a step of identifying, in print data to be sent to the printer, pixels (74, 76), to which prefire pulses shall be applied.

4. The method according to claim 3, wherein the step of identifying prefire pixels (74, 76) is integrated in a raster image processing step.

5. The method according to claim 3 or 4, comprising a step of image processing in which, for each image area (64) that is surrounded by a background area (66), a seam (86) of prefire pixels (74, 78) is defined so as to entirely surround the image area (64), and, for each background area (66) that is surrounded by an image area (64), a seam (86) of prefire pixels (74, 78) is defined so as to entirely surround the background area (66).

6. The method according to any of the preceding claims, wherein an ink that

P4102NL01 11 contains particulate matter is used for printing.

7. The method according to claim 6, wherein an image to be printed includes MICR characters (78).

Claims

P4102EN01 12 Conclusions

A method of printing with an inkjet printer having a nozzle (22) connected to a pressure chamber (18, 20), and an actuator (28) arranged to excite a pressure wave in an ink in the pressure chamber (16, 20) to generate a droplet. (68) eject ink from the nozzle (22), in which method a print head comprising the nozzle (22) and the actuator (28) scans a recording medium (62) on which an image is to be printed, and the nozzle (22) starts spraying when it reaches a transition from a background area (66) to an image area (64) on the recording medium and stops spraying when it reaches a transition from an image area (64) to a background area (66), characterized in that when the nozzle (22) reaches a pixel (76) immediately before a background to image transition, or a pixel (74) immediately after an image to background transition, the actuator (28) is energized with a pre-pulse (56) sufficient to sliding the ink into the pressure chamber (16, 20) dden, but insufficient to form a drop.

The method of claim 1, wherein the pre-pulses (58) used in the background to picture transition and the pre-pulses used in the picture to background transition have identical waveforms.

A method according to any preceding claim including a step of identifying, in print data to be sent to the printer, pixels (74, 76) to which pre-pulses are to be applied.

The method of claim 3, wherein the step of identifying front pixels (74, 78) is integrated into a raster image processing step.

A method according to claim 3 or 4, comprising an image processing step wherein, for each image area (64) surrounded by a background area (66), a layer (86) of front pixels (74, 76) is defined such that the image area (64) is completely surrounded, and, for each background area (66) surrounded by an image area (64), a layer (86) of front pixels (74, 76) is defined such that the background area (66) is completely surrounded.

P4102NL01 13

A method according to any preceding claim, wherein an ink containing particulate matter is used for printing.

The method of claim 6, wherein an image to be printed includes MICR characters (78).