Method to improve the system stability of inkjet printing systems

A method for stabilizing a print quality in an inkjet printing system is described. The inkjet printing system can include a nozzle arrangement that may be activated with a number of control signals that can be used to fire ink droplets with corresponding different droplet sizes onto a recording medium. In the method for stabilizing a print quality in an inkjet printing system, a rastered image for an image template can be created. The rastered image can be printable by the inkjet printing system using a subset of the different droplet sizes. Further, the nozzle arrangement can be activated with a control signal for an unused droplet size of the different droplet sizes to induce the nozzle arrangement to generate a prefire pulse.

CROSS REFERENCE TO RELATED APPLICATIONS

This patent application claims the benefit of German Patent Application No. 102015103102.7, filed Mar. 4, 2015, which is incorporated herein by reference in its entirety.

BACKGROUND

The embodiments described herein generally relate to devices and corresponding methods to stabilize the print quality of an inkjet printing system, including stabilizing the print quality when using inks with a high color density.

Inkjet printing systems may be used to print to recording media (such as paper, for example). For this, a plurality of nozzles may be used in order to fire or push ink droplets onto the recording medium, and thus in order to generate a desired print image on the recording medium.

During printing, print quality problems (for example an incorrect positioning of an ink droplet or a nozzle failure) may occur depending on the type of ink that is used and/or depending on the print speed and/or depending on the ejected droplet size per nozzle. These print quality problems typically arise due to the increase of the viscosity of the ink in the nozzle and/or due to waveforms for the drop generation that are not optimally adapted to the type or to the properties of the ink that is used. The waveform for activation of a nozzle or of a nozzle arrangement that is used for the ejection of an ink droplet typically depends on the properties of the ink and on the print speed. For specific combinations of inks/print speeds, it may be problematic to provide waveforms for different droplet sizes that lead to reproducible results over the duration of the printing operation.

The present document deals with the technical object to provide inkjet printing systems that deliver a print quality that is high and stable over an optimally long time period given use of different combinations of inks/print speeds.

DETAILED DESCRIPTION

According to one aspect, a method is described for stabilization of the print quality in an inkjet printing system. The inkjet printing system comprises a nozzle arrangement that may be activated with a limited number M of control signals in order to fire ink droplets with accordingly M different droplet sizes towards a recording medium. M is thereby typically greater than 1 (for example M=3). The method includes the creation of a rastered image for an image template that should be printed by the inkjet printing system using a subset of the M different droplet sizes, i.e. using fewer than M droplet sizes. Moreover, the method includes the activation of the nozzle arrangement with a control signal for an unused droplet size of the M droplet sizes in order to induce the nozzle arrangement to generate a prefire pulse (also designated as a service pulse).

According to a further aspect, a method is described for stabilization of the print quality in an inkjet printing system. The inkjet printing system comprises a nozzle arrangement that may be activated in order to generate a prefire pulse or in order to fire ink droplets with one or more different droplet sizes towards a recording medium. The method includes the creation of a rastered image for an image template that should be printed by the inkjet printing system. Moreover, the method includes the determination—on the basis of the rastered image—of a dead time between two ink droplets that directly follow one another chronologically, which ink droplets should be fired from the nozzle arrangement to print the rastered image. Furthermore, the method includes the determination—on the basis of the dead time—of whether the nozzle arrangement should generate a prefire pulse or not during the dead time. If it is determined that the nozzle arrangement should generate a prefire pulse during the dead time, the rastered image may be modified in order to induce the nozzle arrangement to generate one or more prefire pulses during the dead time.

According to a further aspect, a controller (which includes processor circuitry) is described that is set up to execute a method described in this document.

According to a further aspect, an inkjet printing system is described that comprises a controller described in this document.

FIG. 1shows a block diagram of an example of an inkjet printing system100according to an exemplary embodiment of the present disclosure. The printing system100presented inFIG. 1is designed for printing to a web-shaped recording medium120(also designated as a “continuous feed”). However, the aspects described in this document are also applicable to printing systems100that are set up in order to print to sheet-shaped recording media120. A web-shaped recording medium120is typically unspooled from a roll (the take-off) and then supplied to the print group of the printing system100. A print image is applied to the recording medium120via the print group, and after fixing/drying of the print image the printed recording medium120is taken up again on an additional roll (the take-up) again or cut into sheets. InFIG. 1, the movement direction of the recording medium120is represented by an arrow. The recording medium120may be produced from paper, paperboard, cardboard, metal, plastic, textiles and/or other suitable and printable materials.

In the depicted example, the print group of the printing system100comprises four print head arrangements102(that are also respectively designated as print bars). The different print head arrangements102may be used for printing with inks of different colors (for example black, cyan, magenta and/or yellow). The print group may comprise still further print head arrangements102for printing with additional colors or additional inks (for example, Magnetic Ink Character Recognition (MICR) ink).

A print head arrangement102comprises one or more print heads103. In the shown example, a print head arrangement102comprises five respective print heads103. Each print head103may in turn be subdivided into a plurality of print head segments104, wherein each print head segment104typically comprises a plurality of nozzles or, respectively, nozzle arrangements.

A fitting position/orientation of a print head103within a print head arrangement102may depend on the type of print head103. Each print head103comprises multiple nozzles or nozzle arrangements that may be arranged in different segments104, wherein each nozzle is set up to fire or spray ink droplets onto the recording medium120. For example, a print head103may comprise 2558 effectively utilized nozzles that are arranged along one or more rows transversal to the travel direction of the recording medium120. The nozzles in the individual rows may be arranged offset from one another. A respective line on the recording medium120may be printed transversal to the travel direction by means of the nozzles of a print head103. An increased resolution may be provided via the use of a plurality of rows with (transversally offset) nozzles. In total, 12790 droplets may thus be sprayed onto the recording medium120along a transversal line by a print head arrangement102depicted inFIG. 1. Each print head arrangement100may thus be set up to print a transversal line of a defined color on the recording medium120at a defined point in time.

The printing system100furthermore comprises a controller101(for example an activation hardware and/or a controller) that may be configured to activate the actuators of the individual nozzle arrangements of the individual print heads103in order to apply a print image onto the recording medium120depending on print data. In an exemplary embodiment, the controller101includes processor circuitry that is configured to perform one or more operations of the controller101, including, for example, activating the actuators of the individual nozzle arrangements of the individual print heads103in order to apply a print image onto the recording medium120depending on print data.

FIG. 2shows an example design of a nozzle arrangement200of a print head103according to an exemplary embodiment of the present disclosure. In an exemplary embodiment, the nozzle arrangement200comprises walls202which, together with an actuator220and a nozzle201, form a receptacle or chamber212to receive ink. An ink droplet may be sprayed onto the recording medium120via the nozzle201of the nozzle arrangement200. The ink forms what is known as a meniscus210at the nozzle201. Furthermore, the nozzle arrangement200comprises an actuator220(for example a piezoelectric element) that is set up to vary the volume of the chamber212to receive ink or, respectively, to vary the pressure in the chamber212of the nozzle arrangement200. In particular, the volume of the chamber212may be reduced and the pressure in the chamber212increased by the actuator220as a result of a deflection222, and thus an ink droplet may be pushed out of the nozzle arrangement200via the nozzle201.FIG. 2shows a corresponding deflection222of the actuator220(dotted line). Moreover, the volume of the chamber212may be increased via the actuator220(see deflection221) in order to draw new ink into the receptacle or chamber212via an inlet.

The ink212within the nozzle arrangement200may thus be moved, and the chamber212may be put under pressure, via a deflection221,222of the actuator220. A defined movement of the actuator220thereby produces a correspondingly defined movement of the ink. The defined movement of the actuator220is typically produced via a corresponding waveform or a corresponding specific pulse of an activation signal of the actuator220. In particular, via a fire pulse to activate the actuator220it may be produced that the nozzle arrangement200ejects an ink droplet via the nozzle201. Different ink droplets may be ejected via different activation signals to the actuator220. In particular, the ink droplets may thus be ejected with different droplet size (for example 5 pl, 7 pl or 12 pl). Furthermore via a prefire pulse for activation of the actuator220it may be brought about that, although the nozzle arrangement200produces a movement of the ink and an oscillation of the meniscus210, no ink droplet is thereby ejected via the nozzle201.

In an exemplary embodiment, the controller101of the printing system100can be configured to determine a waveform or a pulse for each pixel of a print image that is to be printed, with which waveform or pulse the actuator220of the nozzle arrangement200should be activated in order to produce an ink firing from the nozzle201and in order to thus print a pixel on the recording medium120. The waveform for the pixel to be printed may include a fire pulse via which the ink firing is produced. For example, the waveform may depend on the color and/or the color brightness of the pixel to be printed. For the printing of continuous tones, different droplet sizes (for example 5 pl, 7 pl or 12 pl) may be used depending on brightness. The ejection of ink droplets of different droplet sizes may be produced via different waveforms (for example via fire pulses of different strength, or of modified fire pulses) of the actuator220. Furthermore, the waveform may depend on the print speed and/or on the properties (for example on the viscosity) of the ink.

As presented above, in specific situations—in particular given the use of inks with a relatively high viscosity and/or at relatively low print speeds—it may be problematic to determine waveforms for the ink ejection that may ensure a high print quality over a long time period. In particular, incorrect positioning of ink droplets and/or nozzle failures may occur in such situations.

The reduction of print quality may typically be ascribed to an increase of the viscosity of the ink within individual nozzle arrangements200due to evaporation effects. One possibility in order to counteract such an increase in viscosity is the printing of non-imaging information, for example the printing of refresh dots and/or of refresh lines. Refresh dots thereby comprise additional ink droplets that are printed in the background of a print image such that the print image is only slightly negatively affected by this. Refresh lines comprise one or more dedicated printed lines that must be cut out at the end of the printing process. These measures thus lead to an increased consumption of printing materials (such as ink and/or paper).

An additional possibility in order to counteract an increase in viscosity of the ink is the use of prefire pulses. Via a prefire pulse, the actuator220of a nozzle arrangement200is induced to move the ink within the nozzle arrangement200, and to bring the meniscus210at the nozzle201into oscillation, such that, although a mixing of the ink within the chamber212of the nozzle arrangement200occurs, an ejection of ink does not. A prefire pulse thus enables the viscosity of the ink within the nozzle arrangement200to be reduced without printing a “non-white” pixel.

For every individual nozzle arrangement200of the printing system100, the controller101may be set up to determine—on the basis of the print data (in particular on the basis of a rastered image)—whether a “white” pixel or a “non-white” pixel should be printed at a specific point in time. If it is determined that a “non-white” pixel should be printed at the specific point in time, the controller101may determine the droplet size to be printed on the basis of the print data. If it is determined that a “white” pixel should be printed at the specific point in time, the controller101may thus determine (on the basis of the print data) whether a prefire pulse should take place at the specific point in time in order to reduce the viscosity of the ink in the nozzle arrangement200. It is thereby typically advantageous to keep the number of prefire pulses as low as possible in order to reduce a loading of and the danger of overheating the nozzle arrangement200.

For a specific pixel of a rastered image, it may thus be determined whethera) a droplet of a specific size should be ejected from the nozzle arrangement200(in order to print a “non-white” pixel);b) the actuator220of the nozzle arrangement200should be activated with a prefire pulse (in order to print a “white” pixel, and in order to reduce the viscosity of the ink in the nozzle arrangement); orc) no activation of the actuator220of the nozzle arrangement200should take place (in order to “print” a “white” pixel).

In an exemplary embodiment, this information may be transmitted from the controller101to a controller105of the print bar102in encoded form (for example as an N-bit value, wherein N=2, for example), in which print bar102the activated nozzle arrangement200is located. In an exemplary embodiment, the controller105is configured to select a suitable waveform for activation of the actuator220of the nozzle arrangement200depending on the received information, and activate the actuator220according to the selected waveform. In an exemplary embodiment, the controller105includes processor circuitry configured to perform one or more operations of the controller105, including, for example, the selection of the suitable waveform and the activation of the actuator220.

FIG. 3shows a workflow diagram of a method300to stabilize the print quality of an inkjet printing system100. In particular, the method300is designed to determine a number of prefire pulses that should be used for a nozzle arrangement200upon printing of a print image in order to stably keep the print quality of the nozzle arrangement200at a high level. The method300may be executed by the controller101, for example.

The processing of an image321to be printed begins in step301, and the method300thereupon has the status311, “Data processing has begun.” The image321to be printed may already be present in a rastered form, meaning that the image321to be printed may comprise a plurality of pixels (for example a matrix of pixels), wherein each pixel is printed in a print bar102of the printing system100via precisely one nozzle arrangement200of the inkjet printing system100. In other words: the rastered image321comprises a plurality of pixels, wherein each pixel includes control instructions (for example in the aforementioned encoded form) for respectively precisely one nozzle arrangement200of a print bar102of the printing system100. In particular, the pixels of a line of the rastered image321are printed by the corresponding nozzle arrangements200of a print bar102. This process repeats for the following lines of the rastered image321. The pixels of a specific column of the rastered image321are thereby printed by a specific nozzle arrangement200of a specific print bar102. Each pixel typically includes control instructions for a plurality of print bars102of the printing system100that are used. The rastered image321may have been created in a rastering and screening process on the basis of an image template (a PDF file, for example) to be printed.

The image321typically comprises a plurality of image layers322, wherein each image layer322is typically printed by a different print bar102of the printing system100. For example, the different image layers322may correspond to different color components of the image321. In step302, the image321is divided up into one or more image layers322so that the method300thereupon has the status312, “Print image divided up.” An image layer322then comprises the control instructions for the nozzle arrangements200of a print bar102of the printing system100.

For a nozzle device200of a print bar102of the printing system100, the method300additionally includes the determination303of a dead time325—NPT (Non-Printing Time)—between two successive “non-white” pixels to be printed. The dead time NPT325is determined on the basis of the print data of the image layer322for the print bar102. As presented above, the image layer322may comprise a matrix of pixels to be printed, wherein each column of the matrix is to be printed by a respective nozzle device200of the print bar102. The dead time NPT325can thus be determined on the basis of the column of the matrix that should be printed by the respective nozzle device200. Furthermore, the dead time325depends on the print speed323. In particular, the dead time NPT325is typically inversely proportional to the print speed323. After determination of the dead time NPT325, the method300is in the “NPT determined” state313.

If a dead time NPT325between two “non-white” pixels to be printed that reaches or exceeds a specific dead time threshold324has been determined for a nozzle arrangement200, this may lead to a viscosity increase of the ink within the nozzle arrangement200, due to which a reduction of the print quality may be caused. The dead time threshold324may thereby depend on the plurality of factors. The method300therefore includes a step305to determine the dead time threshold324. The dead time threshold324may in particular depend on the ink326that is used (in particular on a property of the ink326that is used), on a climatic condition327(for example on the temperature and/or the humidity) in the environment of the nozzle arrangement200and/or on a requirement328for the print quality (for example on an acceptable offset of pixels).

It may then be determined304whether the dead time NPT325is greater than or equal to the dead time threshold324. If the dead time NPT325is less than or equal to the dead time threshold324(state315), the image layer may322remain unchanged. In other words, in this case it may be arranged for that no activation of the nozzle arrangement200with a prefire pulse (as provided by the print data of the image layer322) takes place during the dead time NPT325.

If it is determined that the dead time NPT325is greater than the dead time threshold324(state314), it may be arranged for that the nozzle arrangement200is charged with one or more prefire pulses during the dead time NPT325(step306). In other words, a prefire pulse sequence for the dead time NPT325may be inserted into the print data of the image layer322. It may thus be achieved that the viscosity of the ink in the nozzle arrangement200is sufficiently reduced so that a high print quality is maintained, even given a (chronologically speaking) relatively long non-use of the nozzle arrangement200.

The prefire pulse sequence that is inserted between two successive “non-white” pixels to be printed (if the dead time NPT325is greater than the dead time threshold324) may be described by a plurality of prefire parameters331. The prefire parameters331include one or more of:a number of prefire pulses in the prefire pulse sequence; and/ora chronological placement of the one or more prefire pulses during the dead time NPT325.

The method300includes the determination308of the prefire parameters331. The prefire parameters331may be determined depending on a plurality of state data, for example depending on the ink326that is used (in particular on the property of the ink326that is used), on a climatic condition327(for example on the temperature and/or the humidity) in the environment of the nozzle arrangement200, on the dead time NPT325and/or on a requirement328for the print quality (for example on an acceptable offset of pixels). Furthermore, predefined rules329,332with regard to the prefire parameters331(for example in the form of lookup tables) may be used in order to determine the prefire parameters331(and therefore the prefire pulse sequence). The predefined rules329,332may associate different prefire parameters331with different combinations of state data. The predefined rules329,332may be determined experimentally, for example.

The prefire pulse sequence corresponding to the prefire parameters331is inserted into the print data of the image layer322(step306), such that the nozzle arrangement200is charged with one or more prefire pulses according to the prefire pulse sequence between the successive “non-white” pixels. This method300may be implemented for all nozzle arrangements200of a print bar102(state316) and for all image layers322, i.e. for all print bars102that are used (state318). If the print data for all print bars102and all nozzle arrangements200have been processed (state317), the (modified) image layers322may be combined with one another again (step307) and the processing of the print data may be concluded (step309).

The controller101transmits the print data for a (modified) image layer322to the controller105of the corresponding print bar102. For each pixel, the print data of the image layer322indicate whether a droplet ejection should take place, and if applicable in which droplet size a droplet ejection should take place. If no droplet ejection should take place for a pixel, the print data show whether the corresponding nozzle arrangement200should be activated with a prefire pulse or not.

In an example printing system100, the number of bits of the print data (which may be transmitted from the controller101to the controller105for each pixel) may be limited to N control bits (for example N=2). In other words: the number of control signals that may be transferred from the controller101to the controller105per pixel may be limited. With 2 control bits, for example, it may be indicated whetherno droplet ejection should take place (“white” pixel);a droplet ejection should take place with 7 pl;a droplet ejection should take place with 9 pl; ora droplet ejection should take place with 12 pl.

In order to enable the controller101to indicate to the controller105that a prefire pulse should take place without the number of transferred control bits/pixels being thereby increased, a reassignment of the available N (for example 2) control bits may take place. For example, the instruction “droplet ejection with 7 pl) may be replaced with the instruction “prefire pulse”, such that with 2 control bits it may be indicated whetherno droplet ejection should take place (“white” pixel);a prefire pulse should take place;a droplet ejection should take place with 9 pl; ora droplet ejection should take place with 12 pl.

Alternatively, a different droplet size (for example 12 pl or 9 pl) may be used for the instruction to generate a prefire pulse.

Within the scope of the rastering of an image template to be printed, the image template to be printed is divided up into a plurality of template layers, wherein each template layer corresponds to a different color that is printed by a different print bar102of the printing system100. The individual template layers typically include regions with different inking levels of the respective color (for example inking levels from 0% to 100%). In order to be able to print the regions with different inking levels, different distributions—in particular different densities—of ink droplets and/or different droplet sizes are typically used. Within the scope of the rastering, a region of a template layer with a defined inking level may be transformed into a corresponding region of the rastered image layer322using what are known as screening sets or, respectively, screens, wherein the region of the image layer322includes a plurality of image points or pixels that indicate whether and possibly in what size an ink droplet should be printed at the respective image points.

The reduction of the number of available droplet sizes as described above thus typically requires a modified rastering of an image template which should be printed by the printing system100. In particular, different screening sets or, respectively, screens which take into account that only a limited number of droplet sizes is available (for example that the 7 pl droplet size is not available) are used for the determination of an image layer322from a template layer. A reduction of the print image quality due to the reduced number of available droplet sizes may be at least partially avoided via the consideration of the reduced number of droplet sizes in the rastering of the image templates to be printed. On the other hand, the reduction of the print image quality may be limited via the modified rastering of the image template with adapted screens.

The rastered images321used in method300may be rastered or, respectively, may have been re-rastered under consideration of the reduced number of droplet sizes. The controller101may thus be enabled to transmit the “prefire pulse” instruction to the controller105within the scope of the available number N of control bits. In other words, a stable print quality may be achieved.

Given a typical rastering/screening method, an image template to be printed with M=3 different droplet sizes (for example 5 pl, 7 pl, 12 pl) may thus be prepared. Given the rastering method described in this document, in a deviation from this an image template to be printed may be prepared with only (M−1) different droplet sizes (for example 5 pl, 12 pl), such that one droplet size remains unused and is available for control signals with regard to a prefire pulse. The number of imaging droplet sizes is thus reduced via the modified rastering method, such that a reduced number of stable waveforms for the ejection of the reduced number of imaging droplet sizes may be used in order to generate the print image on the recording medium120. The screening process and the rastering may thereby be modified such that the print quality—i.e. the reproduction of the image template to be printed—is not (substantially) reduced with regard to tonal value scale and detail sharpness.

Via the reduction of the imaging droplet sizes, the possibility is thus achieved to integrate a non-imaging maintenance pulse (i.e. a prefire pulse) into the rastered image given an unmodified data set. The integration of one or more prefire pulses into the print data may take place with the method300depicted inFIG. 3. The method300determines the necessary number and/or placement of prefire pulses depending on the non-printing time (NPT or, respectively, dead time)325and inserts this into the image321.

FIG. 4shows a workflow diagram of an example of a method400for stabilization of the print quality in an inkjet printing system100. The inkjet printing system100comprises (at least) a nozzle arrangement200that may be activated with a limited number M of control signals in order to fire or eject ink droplets with corresponding M different droplet sizes onto a recording medium120. In other words, the inkjet printing system100is set up such that the ejection of ink droplets with M different droplet sizes may be produced using M different control signals. This means that the M different control signals may be used by the printing system100in order to induce a nozzle arrangement200of the printing system100to eject ink droplets with M different droplet sizes. The inkjet printing system100typically comprises a plurality of nozzle arrangements100that are arranged in a print bar102, and that are set up to print a line of a rastered image321or to print rastered print data.

The nozzle arrangement200or the print bar102thus has a limitation to the effect that only M control signals may be used for the activation of a nozzle arrangement200(for example due to a limitation of the transfer rate or of the transfer protocol between a controller101of the inkjet printing system100and a controller105of the print bar102or of the nozzle arrangement200). For example, the nozzle arrangement200and/or the print bar102may be limited such that the nozzle arrangement200may be activated with only M=3 control signals in order to fire or eject ink droplets with accordingly M different droplet sizes onto the recording medium120. The M droplet sizes may include droplet sizes that are greater than 0 pl (picoliter), for example a droplet size of 7 pl, a droplet size of 9 pl and/or a droplet size of 12 pl. The M control signals may be encoded with a predetermined number N of control bits. The nozzle arrangement200typically may be controlled with an additional control signal in order to “print” a “white” pixel on the recording medium120, i.e. to produce no droplet ejection for an image point of a rastered image321. In particular, with a specific combination of control bits the nozzle arrangement200may be informed that no droplet ejection should take place at a specific point in time.

For example, control signals for printing a line of the rastered image321at the nozzle arrangements200may be transmitted to a print bar102with a specific frequency. The frequency with which control signals are transmitted to the nozzle arrangements200thereby depends on the print speed (i.e. on the number of printed lines per time unit). For each line, the control signals may indicate to the individual nozzle arrangements200whether an image point should be printed, and possibly with what droplet size the image point should be printed. Given use of M different droplet sizes, for each nozzle arrangement200this information may be communicated via one of M+1 different control signals (for example via one of M+1 predefined combinations of control bits). For each line of the rastered image321, a specific control signal (for example a specific combination of control bits) per nozzle arrangement200may be sent to the respective nozzle arrangement200. The number of different control signals (for example the number of different combinations of control bits) that may be transmitted to a nozzle arrangement200for a line may thereby be limited to M+1.

The method400includes the creation401of a rastered image321or of rastered image data for an image template that should be printed by the inkjet printing system100. The rastered image321is thereby created using a subset of the M different droplet sizes. In other words, not all droplet sizes which could in principle be fired from the nozzle arrangement are considered in the rastering and/or screening. A negative effect on the print quality provided by the inkjet printing system100may be reduced via the consideration of a reduced number of available droplet sizes directly in the creation of the rastered image321.

The method400additionally includes the activation402of the nozzle arrangement200with a control signal for an unused droplet size of the M droplet sizes in order to induce the nozzle arrangement200to generate a prefire pulse. For example, the unused droplet size may correspond to the smallest droplet size or a middle droplet size (for example 7 pl) of the M droplet sizes.

Via the reduction of the number of droplet sizes that are used, at least one control signal is available which is not used for the printing of the rastered image321. This control signal may now be used to generate one or more prefire pulses with the nozzle arrangement200as needed. Given a prefire pulse, an ink meniscus210is typically set into oscillation at a nozzle201of the nozzle arrangement200, and no ejection of ink326from the nozzle arrangement200takes place. A reduction of the viscosity of the ink326within the nozzle arrangement200may be counteracted via a prefire pulse, and thus a uniform (i.e. stable) high print quality may be ensured.

The method may additionally include the determination303—on the basis of the rastered image321—of a dead time325between two ink droplets in direct chronological succession, which ink droplets should be fired or ejected from the nozzle arrangement200to print the rastered image321. The dead time325is thereby typically already determined in advance, i.e. before the ejection of the two ink droplets in direct chronological succession via the nozzle arrangement200. Moreover, the method400includes the determination304—on the basis of the dead time325—of whether the nozzle arrangement200should generate one or more prefire pulses or not during the dead time325.

If it is determined that the nozzle arrangement200should generate one or more prefire pulses during the dead time325, the nozzle arrangement200may be activated with the available control signal during the printing of the rastered image321in order to print the one or more prefire pulses between the two ink droplets in direct chronological succession, i.e. during the dead time325, i.e. in order to print “white” pixels with excitation of the ink meniscus210. On the other hand, if it is determined that the nozzle arrangement200should generate no prefire pulse during the dead time325, the nozzle arrangement200may be activated between the two ink droplets in direct chronological succession in order to print “white” pixels without excitation of the ink meniscus210of the nozzle arrangement200. In particular, a control signal may be transmitted to the nozzle arrangement200, via which it is indicated that no pixel should be printed in a specific line of the rastered image321.

Via the determination of the dead time325, it may be ensured that prefire pulses are only generated as needed, and the nozzle arrangement200may otherwise recover. An overheating of the nozzle arrangement200may thus be avoided.

The method may additionally include the determination305of a dead time threshold324. The dead time threshold324may, for example, be determined depending on one or more of the following state data: a property of the ink326used by the nozzle arrangement200; a climatic condition327in an environment of the nozzle arrangement200; and/or a requirement328for the print quality of the inkjet printing system.

The determination304may include the comparison of the dead time325with the dead time threshold324. It may then be determined that the nozzle arrangement200should generate one or more prefire pulses during the dead time325if the dead time325is greater than the dead time threshold324.

The method400may additionally include the determination308of one or more prefire parameters331, wherein the one or more prefire parameters331indicates a number and/or a time distribution of prefire pulses that should be generated by the nozzle arrangement200during the dead time325. The one or more prefire parameters331may be determined depending on one or more of the following state data: a property of the ink326used by the nozzle arrangement200; a climatic condition327in an environment of the nozzle arrangement200; a requirement328for the print quality of the inkjet printing system100; and/or the dead time325(in particular the duration of the dead time325). The number and/or the distribution of prefire pulses to be generated may thus be adapted in order to achieve an optimally high degree of stabilization of the print quality.

The method400may additionally include the modification of an image point of the rastered image321or the modification of the rastered image data. The modification may be made in order to induce—by means of the modified image point—the nozzle arrangement to generate a prefire pulse upon printing of the image point. The modified image point is thereby an image point of the rastered image321that should be printed by the nozzle arrangement200. Furthermore, the modified image point corresponds to the point in time at which the nozzle arrangement200should generate the prefire pulse, and the modified image point indicates that the nozzle arrangement should generate the prefire pulse. For example, the image point may include the control signal which induces the nozzle arrangement to generate a prefire pulse.

As discussed above, in an exemplary embodiment, the inkjet printing system100may comprise a plurality of nozzle arrangements200. In an exemplary embodiment, for the plurality of nozzle arrangements200, the control signal is used for the unused droplet size can be used in order to induce the respective nozzle arrangement200to generate a prefire pulse. Moreover, when a prefire pulse should be generated can be determined based on the rastered image321for one or more (e.g. each) nozzle arrangements200of the plurality of nozzle arrangements200.

The aforementioned method for stabilization of the print quality in an inkjet printing system100may also be used for an inkjet printing system100that does not have the aforementioned limitation with regard to the number M of control signals for activation of a nozzle arrangement200. In particular, the method may be applied to an inkjet printing system100which comprises (at least) a nozzle arrangement200that may be activated in order to generate a prefire pulse or in order to fire ink droplets with one or more different droplet sizes into a recording medium120.

The method for stabilization of the print quality in an inkjet printing system100may in this case include the creation401of a rastered image321for an image template that should be printed by the inkjet printing system100. Moreover, the method may include the determination303—on the basis of the rastered image321—of a dead time325between two ink droplets in direct chronological succession, which ink droplets should be fired or ejected by the nozzle arrangement200to print the rastered image321. On the basis of the dead time325, it may then be determined304whether the nozzle arrangement200should generate a prefire pulse during the dead time325. If it is determined that the nozzle arrangement200should generate a prefire pulse during the dead time325, the rastered image321may be modified at a corresponding image point in order to induce the nozzle arrangement200to generate a prefire pulse during the dead time325. Via the selective insertion of one or more prefire pulses, the print quality may be stabilized over a longer duration, and at the same time a loading of the nozzle arrangement200may be minimized.

An increase of the stability of a printing system100with regard to the print quality and the reliability of the printing system100is achieved via the method described in this document. A maintenance pulse (i.e. a prefire pulse) may thereby also be used with bar driving boards (BDB)105that exhibit a limitation with regard to the number N of control bits. This enables the use of novel inks (for example inks with high color density that dry relatively quickly) in such limited print bars102. Furthermore, the droplet positioning may be improved given rapidly drying inks or given inks with a relatively small operating window and/or a relatively low stability (for example a relatively low viscosity).

Moreover, the effort for the creation and testing of waveforms for the individual droplet sizes is reduced via a reduction of the number of droplet sizes to be printed. Print image flaws (in particular due to refresh dots) may be reduced via the use of non-imaging maintenance pulses. Moreover, the amount of ink consumed may be reduced via a reduced number of refresh dots.

CONCLUSION

For the purposes of this discussion, the term “processor circuitry” shall be understood to be circuit(s), processor(s), logic, or a combination thereof. For example, a circuit can include an analog circuit, a digital circuit, state machine logic, other structural electronic hardware, or a combination thereof. A processor can include a microprocessor, a digital signal processor (DSP), or other hardware processor. In one or more exemplary embodiments, the processor can include a memory, and the processor can be “hard-coded” with instructions to perform corresponding function(s) according to embodiments described herein. In these examples, the hard-coded instructions can be stored on the memory. Alternatively or additionally, the processor can access an internal and/or external memory to retrieve instructions stored in the internal and/or external memory, which when executed by the processor, perform the corresponding function(s) associated with the processor, and/or one or more functions and/or operations related to the operation of a component having the processor included therein.

REFERENCE LIST