An inkjet printing method

Abstract

A method for printing with an inkjet printing device comprising transferring a plurality of consecutive pixel rows of a digital monochromatic image to a piezoelectric printhead; and jetting droplets onto a print receiver moving relative with a predetermined print speed to the piezoelectric printhead according to each transferred pixel row; according to a predetermined waveform and with variation of print periods selected from a predetermined plurality of print periods.

Description

1 / 48 GN25004 W02DescriptionTitle of Invention

[0001] An inkjet printing methodTechnical Field

[0002] The present invention relates generally to inkjet printing technology. More particularly, to a method for improving print quality at high print speeds (above 75 meters per minute).Background Art

[0003] Inkjet printing technology has become increasingly prevalent in various industries due to its ability to produce high-quality prints with precision and efficiency. This technology utilizes a printhead containing numerous tiny nozzles in one or more rows that eject droplets of ink onto a print receiver to create images or text. The process involves the controlled formation and ejection of ink droplets, which are typically generated through thermal or piezoelectric mechanisms. As the printhead moves relatively across the print receiver, it deposits ink droplets at specific locations at a certain print speed to form the desired output, such as a printed digital image, with a) a first print resolution; which is measured in a direction perpendicular to the one or more rows of nozzles; and b) a second print resolution; which is measured in a direction parallel to the one or more rows of nozzles.

[0004] In recent years, there has been a growing demand for higher print speeds with piezoelectric printheads to improve productivity and meet increasing production requirements. However, as print speeds increase, particularly beyond 75 meters per minute, maintaining print quality becomes a significant challenge. At such high speeds, the jetting frequencies required to achieve the desired output can lead to issues with print quality. The rapid succession of droplet ejection can result in inconsistencies in droplet formation, placement, and overall print uniformity.

[0005] One of the primary challenges associated with high-speed inkjet printing is the variation in ink droplet characteristics. As the print speed increases, factors such as droplet size, velocity, and trajectory can be affected, leading to inconsistencies in the final print output. These variations can manifest as streaking, banding, or other artifacts that compromise the overall print quality. Additionally, the increased print speed can impact the interaction between the ink and the print receiver, potentially affecting ink absorption, drying time, and color accuracy. Especially variations in print speed — caused by upstream or downstream processes such as drying, labelling, foil stamping,2 / 48 GN25004 W02impregnation, coating, or gluing — can introduce timing errors that degrade image fidelity, leading to artifacts such as density variation, dot placement or satellite droplets.

[0006] The management of ink flow and pressure within the printhead system also becomes more challenging at higher print speeds. Maintaining consistent ink supply and pressure across all nozzles is crucial for uniform droplet formation and ejection. However, as the print speed increases, the system's ability to replenish ink and maintain stable pressure may be compromised, leading to variations in droplet characteristics and ultimately affecting print quality.

[0007] Conventional approaches to addressing these challenges have included optimizing ink formulations to improve their performance at high print speeds, enhancing printhead design to better withstand the demands of high-speed printing, and implementing advanced control systems to manage ink flow and pressure. Some have explored the use of multiple printheads or nozzle arrays to distribute the printing load and reduce the print speed demands on individual nozzles.

[0008] A person skilled in the art of inkjet printing with piezoelectric printheads may utilize a system that determines the jetting frequency based on print receiver transport speed; measured by an encoder signal, having a plurality of movement pulses; and uses each movement pulse of the encoder signal for forming drops with a waveform wherein the certain formed drops having a certain drop characteristic.

[0009] US 2020 / 0298561 A1 (Ricoh Co Ltd) discloses an inkjet printing apparatus comprising a conveying drum fortransporting sheet material and a plurality of discharge units arranged around the drum. It discloses that the apparatus employs a dual-encoder architecture for discharge timing control: a first encoder outputs rotational amount detection signals used to determine the discharge start timing, while a second encoder - a linear encoder mounted on the conveying drum circumference - outputs conveying amount detection signals (output pulses proportional to the movement of the sheet surface) used to generate subsequent discharge timing signals.

[0010] There is a need to overcome the problems discussed above by developing a method that can maintain consistent print quality across a range of print speeds, especially at very high print speeds exceeding 75 and more meters per minute so variations in speeds becomes possible and still have a high print quality. Such a solution should address the challenges of droplet formation, placement accuracy, and overall print uniformity while considering the practical limitations of printhead mechanics and ink behavior at high print speeds.3 / 48 GN25004 W02Summary of Invention

[0011] In order to overcome the problems described above, preferred embodiments of the present invention have been realized with a high-speed inkjet printing method as defined in claim 1 and an inkjet printing device as defined in claim 9.

[0012] A primary objective of the present invention is to improve inkjet printing technology by employing selected, discrete print periods that are optimized for stable droplet formation, thereby enhancing print quality and operational efficiency.

[0013] Another objective of the invention is to introduce a novel control approach that enables dynamic switching or alternation between predetermined print periods during the printing process, even at a constant print speed, to avoid problematic frequencies and maintain consistent output.

[0014] A further objective is to enable high-quality printing across a range of print speeds by utilizing combinations of discrete print periods tailored to different speed conditions. When the print speed varies, the ratio between the selected print periods may be adjusted accordingly to preserve print quality and minimize artifacts such as satellite droplets or dot misplacement.Brief Description of Drawings

[0015] [Fig.1] illustrates graph represents an example of print frequency response curve with on the x-axis the print period in ps and on the y-axis the drop speed in m / s.

[0016] [Fig. 2] illustrates graph represents an example of print frequency response curve.

[0017] [Fig. 3] is similar as Fig.2 with marked measurements visible as open circles.

[0018] [Fig. 4] is similar as Fig.3 with marked measurements visible as open circles.

[0019] [Fig. 5] is similar as Fig.4 with marked measurements visible as open circles.

[0020] Fig. 6A], [Fig. 6B], [Fig. 6C] and [Fig. 6D] illustrate a timeline with a plurality of signals according to a preferred embodiment.

[0021] [Fig. 7] illustrates a preferred embodiment of a print control system and inkjet printing device which is able to perform the presented method for printing with drop-on-demand technology.

[0022] [Fig. 8] illustrates a preferred embodiment of a manufacturing line.4 / 48 GN25004 W02Description of Embodiments

[0023] According to one objective of the present invention, a method for printing a plurality of consecutive pixel rows during a relative movement between a print receiver and a piezoelectric printhead at a print speed is provided. The method comprises transferring each pixel row of the plurality of consecutive pixel rows to the piezoelectric print head and jetting droplets with a predetermined waveform to the relative moving print receiver with a predetermined print speed according to each transferred pixel row and according to a print period selected from a plurality of different print periods.

[0024] In the state of the art a single print period is used with a predetermined print speed to result in a print resolution which is measured in a direction perpendicular to the one or more rows of nozzles. Said print resolution is called the vertical print resolution.

[0025] In a nutshell the present invention is a method for printing, especially for printing with drop-on-demand technology, comprising: transferring the plurality of consecutive pixel rows of a digital monochromatic image to a piezoelectric printhead; jetting droplets with a predetermined waveform onto a print receiver moving relative with a predetermined print speed to the piezoelectric printhead according to each transferred pixel row; with variation of print periods selected from a predetermined plurality of print periods.Wherein preferably at least two and more preferably maximum five of the predetermined plurality of print periods are selected during the jetting. The selection can be varying between the plurality of print periods, preferably according to a predetermined print speed, which is preferably maintained during the printing of the plurality of consecutive pixel rows.The print periods are effectively mixed during the printing of the pixel rows preferably at a predetermined print speed.Consequently, the printing of the plurality of consecutive pixel rows involves the jetting of droplets with a predetermined waveform onto a print receiver that moves at a predetermined speed relative to the piezoelectric printhead, in accordance with each transferred pixel row.The core inventive concept is thus using a variation of print periods selected from a predetermined plurality of print periods for drop-on-demand inkjet printing with a piezoelectric printhead

[0026] To vary the print period, the plurality of print periods comprises preferably all different print periods. The number of selections of a print period from the plurality of different print periods preferably depends on the predetermined print speed and more preferably on the number of consecutive pixel rows.5 / 48 GN25004 W02

[0027] Another objective of the present invention is that the plurality of different print periods includes at least a first and a second print period. Each of these periods has a minimum of one selection, ensuring a mix of print periods and thus variations during the printing process.

[0028] Each print period of the plurality of print periods is preferably capable of forming droplets with a drop speed and / or drop mass and / or satellite behavior that is equal or within one or more predetermined thresholds for each print period of the plurality of different print periods. This ensures consistency in droplet formation across the print periods.

[0029] The difference between the maximum value and minimum value of the predetermined threshold for drop speed is lower than 2 m / s, preferably lower than 1 m / s. This narrow range of drop speeds contributes to uniform print quality across different print periods.

[0030] An effective implementation for varying between the different print periods, that preferably is used in a single pass inkjet printing method, involves using a fire pulse controller having a sampling clock that has discrete delays corresponding to the predetermined plurality of print periods. When an movement pulse from the print receiver movement driver is detected, the fire pulse controller is armed (= ready for sending fire pulses to the piezoelectric printhead). Upon the next discrete delay on the sampling clock, a fire pulse is generated for printing a transferred pixel row, and the sampling clock is then restarted, and the fire pulse controller is disarmed (= not ready for sending fire pulses to the piezoelectric printhead). The fire pulse controller can e.g., be implemented in a field-programmable gate array.

[0031] Said fire pulse controller; sampling clock are preferably part of a print control system.

[0032] In a preferred embodiment of the method according to the present invention, the jetting of droplets is triggered by a fire pulse generated by a fire pulse controller. The fire pulse controller comprises in said preferred embodiment a sampling clock having discrete delays corresponding to the predetermined plurality of print periods, such that each selectable print period is represented by a respective discrete delay value on the sampling clock. The fire pulse controller operates in an event-driven manner: it is armed upon receipt of a movement pulse from a print receiver movement driver, thereby synchronizing the firing event with the actual physical displacement of the print receiver relative to the piezoelectric printhead. Once armed, the fire pulse controller generates a fire pulse according to the first discrete pulse on the sampling clock that corresponds to the selected print period, and this fire pulse is transmitted to the piezoelectric printhead6 / 48 GN25004 W02to trigger droplet ejection. Upon receipt of the fire pulse by the piezoelectric printhead, the fire pulse controller is disarmed, sometimes called unarmed, and the sampling clock is simultaneously restarted, thereby resetting the control cycle in preparation for the next firing event based on the predetermined plurality of print periods. This event-driven architecture ensures that each droplet is ejected at a precisely determined moment relative to the movement of the print receiver, enabling accurate dot placement and stable droplet formation across the predetermined plurality of print periods, even under the high-speed printing conditions.

[0033] The step of jetting droplets is preferably performed with a print head voltage setting selected from a group of print head voltage settings, more preferably a group of different print head voltage settings. In a preferred embodiment the step of jetting droplets is performed with a print head voltage setting selected from a plurality of print head voltage settings according to a selected print period. This allows for optimization of droplet formation for different print periods.Drop-on-demand technology

[0034] The drop-on-demand technology (= DOD technology) employed in this invention offers numerous advantages over other printing technologies. DOD technology releases liquid droplets only when needed, resulting in precise and controlled printing, such as inkjet printing technology. It allows for non-contact printing, which is beneficial for delicate print receivers and enables printing on various surface types and textures. The digital nature of inkjet printing enables on-demand and variable data printing, making it suitable for customized and short-run print jobs. The method of varying between (workable) print periods during the jetting droplets according to the plurality of consecutive pixel rows of a digital monochromatic image enhance the versatility and performance of the drop-on-demand device, such as an inkjet printing device.Advantages of this approach include improved print quality, increased print speed, and enhanced reliability of the printing process across a wide range of operating conditions, such as print speed.

[0035] The method of varying between (workable) print periods can be applied in various drop-on-demand applications e.g., by an inkjet printing device. In commercial printing, this technology can enable high-speed, high-quality printing of marketing materials, packaging, and other promotional items. The ability to maintain consistent print quality at high speeds can significantly increase productivity in large-scale printing operations.

[0036] In a preferred embodiment a single pass inkjet printing method is used. Single pass inkjet printing is normally used as a high-speed printing method where e.g., a print7 / 48 GN25004 W02receiver (such as paper, fabric, or other materials) passes under the printhead only once to receive a liquid such as ink, primer or varnish.

[0037] In textile printing, the invention can be used to improve the quality and efficiency of fabric printing processes. By optimizing droplet formation and placement, the technology can enable more precise and vibrant prints on a wide range of fabric types, from delicate silks to durable polyesters at a high print speed. This can be particularly beneficial in the fashion and home decor industries, where print quality and color accuracy are crucial. An example of an inkjet printing device for decorating textile is Reggiani Digital Printer of manufacturer EFI™ ■.

[0038] The technology can also be applied in industrial inkjet printing applications, such as manufacturing decorated cardboards such as IDERA of manufacturer Xeikon™ or manufacturing decorated laminates such as RotaJet of manufacturer KBATMor manufacturing decorated labels such as Tau 330 RSC of manufacturer DURST™. The ability to maintain consistent print quality at high print speeds can significantly improve the efficiency and reliability of these manufacturing processes.

[0039] Particularly in industrial inkjet printing applications where the drop-on-demand technology is integrated into a manufacturing process, such as the production of laminates, the manufacturing speed can vary according to the process phase. For example, the process may include a startup phase with a first print speed, a maintenance phase with a second print speed, a production phase with a third print speed, and a stop phase with a fourth print speed. Regardless of the phase, print quality must be maintained. Additionally, changing the print speed between these phases should ensure consistent print quality. Said print speeds may e.g., not match with the optimal (workable) print periods of the piezoelectric printhead. That is why the present invention achieves a big advantage over the current state-of-the art.

[0040] Similar in web printing, where the linear speed of the web may be dictated by nonprinting processes such as drying, coating, and gluing, changes in speed often need to be applied gradually to ensure optimal performance of these processes.

[0041] The predetermined print speed is preferably above 75 m / min; more preferably above 150 m / min. An operator of the inkjet printing device may have the ability to configure the inkjet printing device for printing consecutive pixel rows of a digital monochromatic images at said pre-determined print speed and / or pre-determined waveform.

[0042] Preferably a single predetermined waveform is used during the printing of the plurality of consecutive pixel rows.8 / 48 GN25004 W02Inkjet printing device

[0043] The present invention comprises also a drop-on-demand device, such as an inkjet printing device, comprising:a piezoelectric printhead configured to receive plurality of consecutive pixel rows of a digital monochromatic image; which is preferably part of a raster image; derived from a halftoned color separation of digital colored continuous tone image; anda mechanism for moving a print receiver relative to the piezoelectric printhead at a predetermined print speed; anda control system for jetting droplets with a waveform onto the print receiver according to each transferred pixel row and with variation of print periods selected from a predetermined plurality of print periods. The jetted droplets are droplets of a liquid, preferably an ink. The ink is supplied to the piezoelectric printheads with an ink supply system. Detailed information on inkjet technologies and the construction of inkjet printing devices is disclosed in ‘Inkjet Technology and Product Development Strategies’ by Stephen F. Pond, Torrey Pines, 2000.

[0044] Preferably it is an inkjet printing device wherein at least two and a maximum of six of the predetermined plurality of print periods are selected during the jetting.

[0045] Preferably it is an inkjet printing device wherein each print period of the predetermined plurality of print periods is capable of forming droplets with an equal drop speed or a drop speed within a predetermined threshold, wherein the difference between the maximum value and minimum value of the predetermined threshold is lower than 2 m / s, preferably lower than 1 m / s.

[0046] Preferably it is an inkjet printing device wherein the piezoelectric printhead operates with a print head voltage setting selected from a plurality of print head voltage settings according to a selected print period.

[0047] The plurality of printed consecutive pixel rows has a) a first print resolution; which is measured in a direction perpendicular to one or more rows of nozzles of the piezoelectric printhead; and b) a second print resolution; which is measured in a direction parallel to the one or more rows of nozzles of the piezoelectric printhead. The first print resolution is sometimes called the vertical print resolution and the second print resolution is sometimes called the horizontal print resolution.

[0048] If a single pass inkjet printing technology is used than the inkjet printing device is called a single pass inkjet printing device.9 / 48 GN25004 W02Print control system

[0049] The print control system of the present invention is designed to coordinate the ejection of ink droplets from a printhead with the movement of a print receiver in highspeed inkjet printing environments. It employs an event-driven control architecture which comprises a fire pulse controller (170), a sampling clock (150), and an input interface (160) for receiving movement pulses. This system ensures that each droplet is fired at the optimal moment to maintain accurate dot placement and consistent print quality, even under variable speed conditions.

[0050] At the core of the system is the fire pulse controller (170), which governs the timing of droplet ejection. Upon receiving a movement pulse — indicating a defined increment of print receiver displacement — the fire pulse controller is armed. However, it does not immediately trigger a droplet ejection. Instead, it waits for the next discrete pulse from the sampling clock, which operates based on a predetermined plurality of print periods. When this pulse occurs, the fire pulse is generated, and the controller is disarmed. This mechanism introduces a controlled delay that aligns droplet ejection with the actual position of the print receiver, enhancing temporal precision.To know if the print control system is armed or disarmed; this can preferably be got from a memory, accessible by the print control system, (e.g., FALSE or TRUE)

[0051] The invention ensures consistent print quality even during acceleration / deceleration.

[0052] The sampling clock (150) is a critical component that generates a sequence of discrete timing signals corresponding to the selected print periods. After each fire pulse is generated, the sampling clock is restarted, ensuring that each droplet is timed independently based on real-time movement data. This design eliminates the need for complex buffering or predictive algorithms and allows for event-driven control, which is particularly advantageous in manufacturing lines where speed fluctuations are common.

[0053] The input interface (160) receives movement pulses from a speed sensor e.g., an encoder device, which may be located on the inkjet printing device itself or on an upstream or downstream process unit. This flexibility allows the print control system to adapt to a wide range of manufacturing configurations. For example, in a line where the print receiver is influenced by a drying unit or a cutting station, the system can still maintain precise synchronization between print receiver movement and droplet ejection.

[0054] By enabling dynamic selection from a plurality of (workable) print periods, the print control system can optimize droplet formation for different print speeds, ink types, and print receiver characteristics. This adaptability ensures high-quality output across avariety of industrial applications, including decorative laminates, labels, textiles, and packaging. The system may also incorporate additional parameters such as printhead voltage settings (VAA), Digital-to-Analog Converter (DAC) scaling setting and waveform selection, further enhancing its ability to maintain consistent droplet speed, mass, and satellite behavior.

[0055] The number of print periods in the plurality of print periods is preferably between 2 and 30; more preferably between 2 and 10. During the printing of the print layer preferably at least two and a maximum of six of the predetermined plurality of print periods are used during the jetting.

[0056] In a preferred embodiment if the printhead is a piezoelectric printhead capably of jetting by using a waveform, the print control system may select a waveform from a plurality of waveforms. Said plurality of waveforms are stored in a memory which is accessible by the print control system. The selection of the waveform may be chosen depending on a print speed and / or on a measurement of at least one fluid property of a liquid used for forming a droplet by the printhead. The predetermined plurality of print periods may be depending on said selected waveform. This adaptive waveform selection ensures that each droplet is formed with optimal energy and timing, maintaining consistent drop speed, mass, and satellite behavior across a wide range of ink types and operating conditions which results in improved print quality, reduced satellite formation, and enhanced system flexibility, particularly in multi-ink or multi-print receiver printing environments.

[0057] In a preferred embodiment, the inkjet printing device is configured to accommodate a plurality of print receivers or types of print receivers. The names or identifiers of these various print receivers can be stored in a memory accessible by the print control system. The predetermined plurality of print periods may be selected based on the chosen print receiver or print receiver type. This selection can account for differences in print quality tolerance — for example, certain print receivers or types of print receivers may be more forgiving due to higher ink absorption, which helps minimize issues such as smudging or ink pooling, compared to less absorbent print receivers or types of print receivers.

[0058] The print control system may also incorporate a feedback loop using real-time measurements of drop speed, drop mass, or satellite behavior. These measurements can be obtained using high-speed imaging systems or laser-based droplet analyzers. Based on this feedback, the system can adjust the selected print period, voltage setting, Digital-to-Analog Converter (DAC) scaling setting or waveform parameters to maintain11 / 48 GN25004 W02optimal print quality. This closed-loop control enhances the robustness of the printing process, particularly in high-speed or variable-speed manufacturing environments.

[0059] In a further preferred embodiment, the print control system is implemented using a field-programmable gate array (FPGA) or a real-time digital signal processor (DSP). This hardware-based implementation allows for ultra-fast response times and precise synchronization between the movement pulses, sampling clock, and fire pulse generation. The FPGA may also manage the restart of the sampling clock after each fire pulse, ensuring that each droplet is timed independently and accurately.

[0060] Additionally, the print control system may support adaptive print period selection algorithms. These algorithms analyze the characteristics of the digital image to be printed — such as density, complexity, or edge sharpness — and adjust the print period selection strategy accordingly. For example, regions of the image with fine detail may use shorter print periods to increase resolution, while uniform areas may use longer periods to optimize throughput.

[0061] In yet another embodiment, the print control system is integrated with a manufacturing execution system (MES) or industrial control network, allowing it to receive real-time updates on line speed, print receiver type, and / or environmental conditions. This integration enables the print control system to proactively adjust its parameters in anticipation of changes in the production environment, further enhancing print quality and system efficiency.

[0062] The print control system interfaces with the inkjet printing device and may include a graphical user interface (GUI) for configuring print modes, monitoring system status, logging performance metrics, adapting the plurality of print periods.

[0063] The print control system may also handle the digital image for transferring the data to the one or more printheads of the inkjet printing device. The print layer is a representation of said digital image. The digital image may be a colour digital image or a digital monochromatic image such as a separation of a colour digital image.

[0064] In a preferred embodiment, the print control system includes logic for managing transitions between different print speed regimes, such as from a low-print-speed mode to a high-print-speed mode. The transition may be initiated manually by an operator, automatically by the print control system based on speed thresholds, or through external control signals. During transitions, the system may adjust the number of print periods of the plurality of print periods, waveform parameters, Digital-to-Analog Converter (DAC) scaling setting and / or voltage settings to maintain print quality. Safeguards such as12 / 48 GN25004 W02alarms, interlocks, or fallback modes may be implemented to ensure safe and stable operation during rapid speed changes.

[0065] In a preferred embodiment, the print control system analyzes the content of the digital image intended to form the print layer in order to determine an appropriate print period strategy. For example, if the image indicates a low ink load, the system may select a different set of print periods optimized for such conditions.

[0066] Preferred embodiments of the print control system are- wherein the memory storing the predetermined plurality of print periods comprises an indexed lookup table, each index corresponding to one print period of the plurality; or - wherein the sampling clock comprises a fresh-sequence generator configured to reload the predetermined plurality of print periods from memory upon each restart; or - wherein the input interface comprises an encoder-compensation unit configured to correct movement pulses for mechanical slippage, backlash, or non-linear transport behavior; or- wherein the print control system is implemented on a field-programmable gate array (FPGA) or a real-time digital signal processor (DSP) configured to execute the arming, disarming, and sampling-clock restart functions with microsecond-level latency.Single pass inkjet printing device

[0067] A single pass inkjet printing device for performing the present invention may include a transport mechanism for providing a print receiver to a plurality of piezoelectric printheads, arranged in one or more lines. Each line arrangement is called a print bar.

[0068] The width of the one or more print bars of the preferred single pass inkjet printing device is preferably at least 1.2 m, 1.6 m, 2.8 m, and even up to 3.5 m for printing large print receivers. The width of the print receiver and the print bar are determined in the same direction as the one or more nozzle rows (250) of the piezoelectric printheads.

[0069] The transport mechanism of the preferred single pass inkjet printing device for the relative movement of the print receiver is not particularly limited but preferably includes a rotatable conveyor belt, a rotatable vacuum conveyor belt, or a composition of one or more transport rollers, with at least one driven by a motor. Providing a print receiver as a web-based print receiver is preferably done by a transport mechanism comprising a rotatable impression cylinder or transport rollers, with at least one driven by a motor. An13 / 48 GN25004 W02example of a single pass inkjet printing device with rotatable impression cylinder is Truepress PAC 520P of manufacturer SCREEN™ Graphic Solutions Co.

[0070] To achieve high print quality with the printed consecutive pixel rows the nozzle resolution of the piezoelectric printhead of the preferred single pass inkjet printing device should preferably be 225 nozzles per inch or more, more preferably 500 nozzles per inch or more, and most preferably 1000 nozzles per inch or more. The nozzles may be arranged in one or more nozzle rows (250). An example of a nozzle row arrangement is disclosed in US20180370234A1 (Fujifilm Dimatix Inc) By a high nozzle resolution, a larger second print resolution can be achieved (> higher the print resolution, better the print quality).

[0071] To achieve a high second print resolution and higher printing speed and thus a better print quality at high speed (> higher the print resolution, better the print quality) with said single inkjet printing device preferably all two printed dots of the same liquid on the print receiver which are neighboring each other, aligned along the one or more nozzle rows (250) is achieved by jetting each printed dot of said two printed dots with a different piezoelectric printhead. EP2633998A1 (AGFA) discloses such single pass inkjet printing device limited to UV inkjet printing.Piezoelectric printhead

[0072] The piezoelectric printhead utilizes piezoelectric materials that deform when a waveform is applied, creating pressure waves in the liquid chamber to eject droplets. The printhead can be designed with various configurations, such as bend-mode, push-mode, or shear-mode actuators, to optimize droplet formation and ejection. This technology offers precise control over droplet size and placement, making it suitable for high-quality printing applications. Particularly suitable piezoelectric inkjet printheads are Gen5 and Gen5S from RICOH™, KJ4B from KYOCERA™, Samba G3L and G5L from FUJIFILM DIMATIX™ and the 5601 printhead from XAAR™.

[0073] To eject droplets according to a transferred pixel row, a liquid is supplied to the piezoelectric printhead towards the liquid chamber wherein the ejection is caused when a fire pulse is received from a fire pulse controller which preferably in turn receives an encoder signal, having a plurality of movement pulses from a print receiver movement driver.

[0074] If the print speed is constant the period between said movement pulses is constant. If the print speed becomes faster I slower the period between consecutive movement pulses becomes smaller / bigger.14 / 48 GN25004 W02

[0075] Higher nozzle resolution allows for finer detail and higher quality prints, as more nozzles can deposit smaller and more precise droplets of ink. For example, a printhead with a nozzle resolution of 1200 npi (nozzles ger inch) can produce very detailed and sharp images.

[0076] Nozzles of a piezoelectric printhead are preferably organized in a nozzle row (250) or preferably in a plurality of nozzle rows to enhance the nozzle resolution of the piezoelectric printhead.

[0077] In a preferred embodiment the piezoelectric printhead is a piezoelectric through-flow printhead wherein the liquid flows continuously via a liquid inlet through the liquid channel and exits the nozzle only when required, otherwise the liquid exits the liquid channel via a liquid outlet of the printhead. The liquid flows via the liquid channel and over the inner nozzle aperture of the nozzle. The liquid is recirculated, as it were, through the printhead. W02009109342 A 1 (AGFA NV) discloses a liquid supply wherein liquid is recirculated through the printhead.

[0078] The waveform refers to the electrical signal applied to the piezoelectric elements in the printhead. It is characterized by its shape, amplitude, and duration, which directly influence the droplet formation process. Waveforms can be designed with multiple phases, including expansion, hold, and contraction phases, to optimize droplet ejection for different liquid properties and printing requirements. The properties of the waveform, such as rise time, fall time, and pulse width, can be adjusted to control droplet volume, velocity, and satellite formation. Advantages of using optimized waveforms include improved print quality, reduced liquid consumption, and enhanced reliability of the printing process.

[0079] Preferably during the printing of the plurality of consecutive pixel rows a single predetermined waveform is used.

[0080] Waveforms can be determined to optimize droplet formation for different liquid types, nozzle geometries, and printing conditions. Methods for waveform determination include empirical testing, computational fluid dynamics simulations, and real-time optimization techniques. Factors considered in waveform design include droplet volume control, satellite droplet suppression, and meniscus stability. Types of waveforms may include single-pulse, multi-pulse, or complex waveforms with multiple phases to achieve specific droplet formation characteristics. Advantages of optimized waveforms include improved print quality, reduced liquid consumption, and enhanced reliability of the printing process.15 / 48 GN25004 W02

[0081] Typically, a determined waveform comprises one or more print pulses and / or cancel pulse. A print pulse may comprise:rise segment: an increase in voltage that causes the piezoelectric element to expand, pushing liquid towards the nozzle;dwell segment: a period of constant voltage where the element remains expanded to allow liquid to form a meniscus at the nozzle;fall segment: a decrease in voltage that causes the element to contract, creating a pressure that ejects liquid from the nozzle.

[0082] The amount of the ejected liquid and the velocity of the ejected liquid are determined by the slope and the amplitude of the print pulse. Typically, the amount of ejected liquid grows if the number of print pulses is higher.

[0083] A cancel pulse, sometimes called dampening pulse, may also comprise several similar segments, but a cancel pulse is causing to dampen any oscillations and to stabilize the printhead for the next ejection of liquid. A cancel pulse results mainly in minimizing satellites and mist formation.

[0084] Other type of pulses may be comprised in the determined waveform such as a prepulse that come shortly before the first print pulse. It can help the liquid moving so that the print pulse has less energy to overcome. They have typically a low amplitude.

[0085] Preferably the determined waveform comprises one or more print pulses but preferably less than five. After the one or more print pulses the waveform comprises preferably a cancel pulse.

[0086] In a preferred embodiment the piezoelectric printhead is configured to jet droplets of varying sizes based on the determined waveform during the variation between printing periods.

[0087] The determined waveform can be stored in a memory e.g. a non-transitory computer-readable storage medium so it can be used by an inkjet printing method as pre-determined waveform. The transfer of a waveform to the piezoelectric printhead can be handled by a waveform controller.

[0088] In a preferred embodiment the piezoelectric printhead includes (i) a nozzle ejecting the liquid, (ii) a piezoelectric element generating energy for jetting droplets of the liquid, and (iii) first and second flow paths that communicate between the nozzle and the piezoelectric element and in which the liquid flows, the method comprises: a flow step of16 / 48 GN25004 W02flowing the liquid in the first flow path to the second flow path separately from the step of jetting droplets.

[0089] The term “acoustic resonance period” (ARP) of a piezoelectric print head is the time period that refers to the time that it takes for a pressure wave to travel both ways in an ink chamber. It is a characteristic for each print head type and can differ depending on the ink used. The period is namely related to the speed of sound in the ink. The acoustic resonance period can be measured by changing the timing between two ceramic movements in a single drop wave form.

[0090] Another term that describes resonant phenomena in a piezoelectric printhead is “resonance frequency”. This frequency can be determined e.g., through methods like fast sine sweep and white noise excitation. “Acoustic Phenomena in a Demand-Mode Piezoelectric Ink-Jet Printer” of Bogdan V. Antohe & co (2001; NIP & Digital Fabrication Conference 17: publisher: Society of Imaging Science and Technology) discloses such method. The resonance frequency can be affected by the length of the channels in the printhead and the feedback from the fluid to the piezoelectric structure.Liquid

[0091] The liquid is preferably an ink, a so-called inkjet-ink.

[0092] The inkjet inks that are printed on the print receiver preferably contain a colorant. The colorant may be a dye but is preferably a pigment. A colour pigment is less susceptible to light fading.

[0093] To be suitable for inkjet printing, the liquid must be jettable, which depends on the specifications of the printhead technology used. Printhead manufacturers typically provide guidelines for jettable fluids, including recommended ranges for viscosity, surface tension, and particle size.

[0094] The pigmented inkjet inks that are inkjet printed may be selected from aqueous pigmented inkjet inks, solvent based pigmented inkjet inks and radiation curable pigmented inkjet inks such as UV curable inkjet inks. An aqueous pigment inkjet ink may be a latex ink which contains polymer particles (latex) dispersed in water, along with pigments and other additives.

[0095] The pigmented inkjet inks preferably contain organic colour pigments as they allow for obtaining a high colour gamut on the print receiver. Carbon black and titanium dioxide are inorganic pigments, which can be advantageously used in the present invention for composing respectively black and white pigmented inkjet inks.17 / 48 GN25004 W02

[0096] An organic colour pigment may be chosen from those disclosed by HERBST, Willy, etal. Industrial Organic Pigments, Production, Properties, Applications. 3rd edition. Wiley - VCH , 2004. ISBN 3527305769. Suitable colour pigments are disclosed in paragraphs

[0128] to

[0138] of WO 2008 / 074548 (AGFA GRAPHICS).

[0097] Pigment particles in inkjet inks should be sufficiently small to permit free flow of the ink through the inkjet printing device, especially at the ejecting nozzles. It is also desirable to use small particles for maximum colour strength and to slow down sedimentation.

[0098] The numeric average pigment particle size of an organic colour pigment and an inorganic black pigment is preferably between 0.050 and 1 pm, more preferably between 0.070 and 0.300 pm and most preferably between 0.080 and 0.200 pm. The determination of the numeric average particle diameter is best performed by photon correlation spectroscopy at a wavelength of 633 nm with a 4mW HeNe laser on a diluted sample of the pigmented inkjet ink. A suitable particle size analyzer is a Malvern™ nano- S available from Goffin-Meyvis. A sample can, for example, be prepared by addition of one drop of ink to a cuvette containing 1.5 mL ethyl acetate and mixed until a homogenous sample was obtained. The measured particle size is the average value of 3 consecutive measurements consisting of 6 runs of 20 seconds.

[0099] A white pigment preferably has a numeric average pigment particle size larger than 180 nm in order to have a strong opacifying capability. Suitable white pigments are given by Table 2 in

[0116] of WO 2008 / 074548 (AGFA GRAPHICS) . The white pigment is preferably a pigment with a refractive index greater than 1.60. The white pigments may be employed singly or in combination. Preferably titanium dioxide is used as pigment with a refractive index greater than 1.60. Suitable titanium dioxide pigments are those disclosed in

[0117] and in

[0118] of WO 2008 / 074548 (AGFA GRAPHICS) .

[0100] The one or more pigmented inkjet inks may contain further additives such as surfactants, polymerization inhibitors and dispersants for the colour pigments.

[0101] If the inkjet printing device is integrated in an electronics manufacturing line which produces printed electronic circuits or flexible hybrid electronics the liquid may be a (semi-)conductive ink (silver nanoparticle ink; copper nanoparticle ink; or carbon-based ink silver), or a dielectric ink or functional ink used for specialized roles like sensing, energy storage, or light emission.

[0102] In a preferred embodiment, if the inkjet printing device includes additional printheads dedicated to jetting primers or varnishes. Said additional printheads does not to be from18 / 48 GN25004 W02the same printhead-type. These printheads may operate with a separate set of predetermined print periods optimized for the rheological properties of the primer or varnish. The print control system may coordinate the timing of these functional layers with the main image printing to ensure proper adhesion, gloss, or durability. This embodiment is particularly useful in packaging, label, and decorative laminate applications where multi-layer printing is required.Monochromatic image & Pixel row

[0103] To form an inkjet printed image by a piezoelectric printhead there is need for a transfer of pixel rows of a digital monochromatic image to the piezoelectric printhead. This can be handled by a pixel row transfer controller, preferably embedded in the piezoelectric printhead. This involves the communication of digital monochromatic image data from the print controller to the printhead driver electronics. Preferably the transfer is using one or more high-speed data channels. Methods for pixel row transfer may include serial data transmission, parallel data buses, and / or high-speed digital interfaces such as PCI Express, JESD204B / C, FiberOptic Communication or LVDS (Low-Voltage Differential Signaling) to support high data rates. The system of transferring of pixel rows is preferably supporting data rates of 100 Gbps (Giga bits ger second) and beyond.

[0104] The timing and synchronization of pixel data transfer with the piezoelectric printhead firing and print receiver movement are critical for accurate image reproduction but well known by a skilled person. Advantages of efficient pixel data transfer systems include reduced latency, improved print speed, and enhanced image quality through precise droplet placement.

[0105] The plurality of consecutive pixel rows that are printed by drop-on-demand technology during the relative movement between the print receiver and the printhead form the basis of an inkjet printed image. Each pixel row corresponds to a line of the digital monochromatic image to be printed. The generation and processing of pixel row data involve techniques such as image rasterization, color separation, and halftoning. These steps can be integrated into workflow software like ASANTI from AGFA™. The digital monochromatic image is preferably part of a raster image derived from a halftoned color separation of a digital colored continuous-tone image, such as scanned or digitally taken photo.

[0106] The plurality of consecutive pixel rows, halftoned color separation, digital colored continuous-tone image can be stored in a memory e.g. a non-transitory computer- readable storage medium. Transferred consecutive pixel rows towards the piezoelectric19 / 48 GN25004 W02printhead can also be stored in a memory which is preferably part of the piezoelectric printhead.Print receiver

[0107] The print receiver used in the printing process can vary widely depending on the application. It may include paper, plastic films, textiles, ceramics, or other materials suitable for inkjet printing. The properties of the print receiver, such as surface energy, porosity, and thermal characteristics, can significantly influence the printing process and the selection of appropriate print periods.

[0108] Methods for optimizing print periods for different print receiver types may include surface treatment techniques, print receiver-specific waveform designs, and adaptive print period selection algorithms.

[0109] Types of print receivers that can benefit from this printing method include both porous and non-porous materials, coated and uncoated surfaces, and flexible and rigid print receivers.

[0110] The relative movement with a predetermined print speed between the print receiver and the piezoelectric printhead can be achieved through various mechanisms, such as a moving printhead carriage, a moving print receiver, or a combination of both. The speed of this relative movement, referred to as the print speed, is a parameter that affects the overall printing process. In the present invention is the predetermined print speed during the printing of the pixel rows preferably constant. Methods for controlling print speed may include stepper motor systems, linear encoders, and servo-driven mechanisms. The print speed can be adjusted based on factors such as desired print quality, print receiver characteristics, and ink drying requirements. Types of print speed control may include constant speed, variable speed, or acceleration / deceleration profiles to optimize print quality and throughput. The print speed of the print receiver may be measured by an encoder signal, having plurality of movement pulses, derived from a print receiver movement driver which moves the print receiver. For example, EP2996878A1 (AGFA NV) discloses an accurate encoder signal for a belt step conveyor system, especially useful in multi-pass inkjet printing. Other ways of measuring print speed in an inkjet printing device are:Linear Encoders: These measure the linear position of the print receiver directly. They can be optical, magnetic, or capacitive. Linear encoders provide high precision and are often used in applications requiring accurate positioning;Rotary encoders: These are attached to a rotating shaft and measure the angular position or motion. They can be either incremental or absolute encoders. Incremental encoders generate a series of pulses as the shaft rotates, which can be counted to determine print speed;Measuring wheels: These are often used in conjunction with rotary encoders. The wheel is in contact with the moving print receiver, and as it rotates, the encoder attached to it generates pulses proportional to the distance traveled.

[0111] The print receiver, sometimes called substrate, is preferably paper print receiver, having a certain absorbability towards the jetted liquid.

[0112] Methods for optimizing print quality on print receiver types may include surface treatment techniques, substrate-specific waveform designs, and adaptive print period selection algorithms.

[0113] Types of print receivers that can benefit from the invention include both porous and non-porous materials, coated and uncoated surfaces, and flexible and rigid substrates. Quantitatively, a porous print receiver indicates a print receiver having a porosity, measured using a Gurley air permeability meter, of equal to 100 seconds or less', also expressed as a Gurley porosity below 100 seconds. A Gurley air permeability meter measures the airflow between the paper substrate, clamped between a flat glass plate and a circular metal head. A suitable Air Permeability Tester measuring the air permeability of a paper substrate is a Bendtsen Porosity Tester (RL-BRAPT-A) from Rycolab. The Gurley porosity is calculated from the measured values.

[0114] Absorbing paper print receivers include paper, coated paper, coated paper having an ink receiving layer on at least one surface, offset paper, office paper, newspaper paper, cardboard, white lined chipboard, folding board.

[0115] For packaging applications absorbing paper print receivers include cardboard, paper lines, and paper. The paper can be a single layer of a multilayer paper. The paper liner may be brown kraft liner, top kraft liner, white coated kraft liner or bleached paper board.

[0116] The absorbing paper print receiver may be manufactured from chemical, wood, or recycled fibre.

[0117] The print receiver is preferably a porous paper print receiver which indicates a "paper print receiver having a porosity, measured using a Gurley air permeability meter, of equal to 100 seconds or less', also expressed as a Gurley porosity below 100 seconds. A Gurley air permeability meter measures the airflow between the paper print receiver, clamped between a flat glass plate and a circular metal head. A suitable Air Permeability21 / 48 GN25004 W02Tester measuring the air permeability of a paper print receiver is a Bendtsen Porosity Tester (RL-BRAPT-A) from Rycolab. The Gurley porosity is calculated from the measured values.

[0118] The method according to the invention is useful for paper print receiver web width of 1.0 m, 1.6 m, 2.8 m and even up to 3.5 m. Especially, corrugators require a paper print receiver web width of 2.8 m.

[0119] The paper print receiver may also be separate sheets instead of web-based paper.Boards used for boxes and other packaging applications are also suitable. Most preferably, the paper print receiver is an uncoated paper liner. The absence of a coating further improves penetration of the ink vehicle while keeping at least a part of the pigments at the surface.

[0120] In a preferred embodiment the print speed during the printing of the plurality of pixel rows is maintained whereby a constant pre-determined print speed is used during the printing of the plurality of pixel rows with a variation of print periods.

[0121] In a preferred embodiment the print receiver is preferably a web-based print receiver for web printing such as roll-to-roll printing or roll-to-sheet printing where a flexible print receiver (such as paper, plastic, or foil) is fed continuously from one roll to another or from one roll to sheet. This method is widely used in various industries for high-speed and high-volume production. Preferably the plurality of pixel rows is transferred to the piezoelectric printhead in a sequential manner to ensure web printing.Manufacturing line

[0122] In a preferred embodiment, the invention is preferably implemented within a manufacturing line designed for producing an object, wherein the line comprises at least two process devices arranged sequentially along a manufacturing path. The present method for printing with drop-on-demand technology becomes then preferably part of the method of manufacturing an object.One of these two process devices is an inkjet printing device configured to apply a print layer onto a print receiver, while the other device is responsible for either preparing the print receiver or converting it — after printing — into the final object or an intermediate product thereof. This configuration enables seamless integration of high-quality inkjet printing into complex, multi-step manufacturing workflows.

[0123] In a nutshell, the other process device processes a material before printing (e.g., shaping, cleaning, coating) whereby the material becomes the print receiver, or processes the printed substrate further (e.g., curing, cutting, assembling).22 / 48 GN25004 W02

[0124] [Fig. 8] illustrates a preferred manufacturing line (500) for producing a product, comprising a second process device (510), an inkjet printing device (300), a first process device (520), and a speed sensor (600). The inkjet printing device (300) comprises a print control system (100), an inkjet printhead (200), and a substrate (400), as described with reference to [Fig. 7],The manufacturing line (500) defines a manufacturing path along which the substrate (400) is transported in the direction of the manufacturing movement (750). Along this manufacturing path, a second process device (510) performs an upstream process (310), such as impregnating the substrate (400), prior to printing. The inkjet printing device (300) performs the printing process (315) at an intermediate position along the manufacturing path, between the upstream process (310) and the downstream process (320). The first process device (520) performs the downstream process (320), such as drying the printed impregnated substrate, after printing.The speed sensor (600) is positioned at the upstream process (310) and measures the movement of the substrate (400) at that location. The speed sensor (600) generates movement pulses corresponding to the manufacturing speed and provides these movement pulses to the input interface (160) of the print control system (100). The print control system (100) receives said movement pulses via its input interface, not shown, and uses them to synchronize the generation of fire pulses (175) to the inkjet printhead (200), thereby ensuring droplet placement onto the substrate (400) in accordance with the manufacturing speed, even when said manufacturing speed varies due to operations performed by the second process device (510) or the first process device (520).

[0125] The manufacturing speed — defined as the speed at which the print receiver moves through the line — may vary depending on the operational requirements of upstream or downstream processes. For example, a resin impregnation unit may require slower speeds to ensure proper saturation of a printed decor paper, while a cutting or profiling station may introduce speed fluctuations for precision alignment or tool changes. The print control system of the present invention is designed to accommodate such variations by dynamically adjusting the print periods based on movement pulses derived from speed sensors positioned along the line.

[0126] Examples of manufacturing lines that benefit from this invention include:A decorative laminate production line, where a printed paper substrate is impregnated with melamine resin and later pressed onto a core board to form flooring panels.A label production line, where a plastic substrate is coated, printed with branding or product information, and then die-cut into individual labels.23 / 48 GN25004 W02A packaging line, where cardboard or folding board is printed, varnished, and folded into boxes or cartons.A textile printing line, where fabric is printed with decorative patterns and subsequently dried, fixed, or laminated;An electronics manufacturing line which produces printed electronic circuits or flexible hybrid electronics. It integrates material preparation, inkjet printing, and post-processing in a continuous, synchronized flow. Typical processes in such line are cleaning (e.g., plasma or solvent); surface treatment (e.g., corona or UV) and pre-coating (e.g., dielectric or adhesion layer).

[0127] In the manufacturing of laminate flooring, a plurality of material layers — such as decorative paper, kraft paper, and wear-resistant overlays — are laminated together under the application of heat and pressure. These layers are typically supplied from continuous rolls (webs), necessitating uninterrupted operation of the production line to maintain process efficiency and product quality.

[0128] To facilitate such continuous operation, a web accumulator is sometimes in manufacturing line employed. The web accumulator functions to temporarily store web material during roll changes or brief interruptions, thereby preventing disruption of downstream processes. Additionally, it serves to maintain consistent web tension across the various layers, which is critical to avoiding defects such as wrinkling, misalignment, or delamination. This functionality is particularly advantageous in high-speed production environments, where even minimal downtime can result in significant material waste and operational inefficiencies.

[0129] Although web accumulators are commonly utilized in industries such as packaging, printing, and textiles, their application in laminate flooring production provides similar benefits, particularly in processes requiring precise web handling and continuous lamination.

[0130] In each of these examples, the inkjet printing device must operate in harmony with other process devices, despite differences in speed, acceleration / deceleration, or material handling requirements. The invention’s ability to synchronize droplet ejection with real-time movement pulses ensures that print quality is maintained across all phases of production — from startup and ramp-up to full-speed operation and controlled shutdown.

[0131] Furthermore, the modular nature of the system allows for scalability and customization. Additional process devices such as corona treaters, priming stations, embossing units, or inspection systems can be integrated into the line without24 / 48 GN25004 W02compromising the synchronization between printhead firing and substrate movement. This makes the invention highly adaptable to a wide range of industrial applications where precision printing is essential.Print periods

[0132] The print period is the time interval between the printing of two consecutive pixel rows.

[0133] In the state of the art only a single print period is used for a predetermined print speed to achieve a certain first print resolution which can be measured by the distance between printed neighboring pixel rows. If the predetermined print speed becomes faster the print period shall be adapted to a smaller print period to have the same distance between the printed neighboring pixel rows.

[0134] The present invention uses a plurality of print periods instead that can be stored in a memory e.g. a non-transitory computer-readable storage medium.

[0135] Preferably the print periods of the plurality of print periods are below 100 ps (=microseconds) and more preferably above 4 ps. This means that a nozzle can eject between 10000 and 250000 droplets per second. If the print speed is low and there is need for a print period above 100 ps there is no real need for varying the print periods during the printing of the plurality of pixel rows. Above 100 ps is the drop speed normally stable and consistent enough for having a good print quality.

[0136] In the present invention, the variation of print periods is preferably chosen from a predetermined set of workable print periods. These periods are selected to compensate for discrepancies between the achieved vertical print resolution and the predetermined vertical print resolution. The absolute difference between the average vertical print resolution over the printed pixel rows and the predetermined vertical print resolution is preferably less than 80 pm, more preferably less than 50 pm, and most preferably less than 30 pm.

[0137] The predetermined plurality of print periods is preferably determined based on a predetermined frequency response curve of the piezoelectric printhead, wherein print periods are selected where the drop speed is stable and consistent.The invention ensures that there are never negative delays, eliminating the need for buffering transferred pixel rows and preventing the accumulation of errors. Additionally, this approach enhances the precision and reliability of the printing process, ensuring consistent droplet formation and placement. This also simplifies the overall design and reduces the complexity.25 / 48 GN25004 W02[Table 1]ti t2 ts t4 ts tsSampling clock 1 0 0 1 0Movement signal 0 1 0 0 0Armed signal 0 1 1 0 0Fire signal 0 0 0 1 038]Table 1 demonstrates the principle of the preferred embodiment. At time t2, a movement pulse is received, indicated by a T in the "Movement Signal" row. This pulse arms the fire pulse controller, shown by a 'T in the "Armed Signal" row at times ts and t4. At time t4, a discrete delay is determined, marked by a 'T in the "Sampling clock" row, resulting in forming a fire pulse, indicated by a 'T in the "Fire Signal" row at time t4. The fire pulse controller is then disarmed, shown by a 'O' in the "Armed Signal" row at time t4, and the sampling clock restarts at time ts (discrete delays according to the plurality of print periods are not shown).At time ti , there was a discrete delay, indicated by a T in the "Sampling clock" row, but it did not result in forming a fire pulse, as shown by a 'O' in the "Fire Signal" row at time ti , because the fire pulse controller was disarmed, indicated by a 'O' at time ti. As demonstrated, the movement pulse does not directly trigger a fire pulse but causes a delay in the fire pulse according to the discrete delays in the sampling clock.So, the controller enters a ready state ("armed") when it detects movement, preparing to fire. Once a droplet is ejected (fire pulse is generated), it returns to a non-ready state ("disarmed").

[0139] Figures 6A, 6B, 6C and 6D illustrate the principle of the preferred embodiment with a timeline showing sampling clock (1000), movement signal (2000), armed signal (3000), and fire signal (4000). The sampling clock experiences multiple delays corresponding to three print periods (1001, 1002, 1003). Upon receiving a movement pulse (2001), the fire pulse controller, not shown, is armed (3001), as indicated by the armed signal (3000). In Fig. 61 A, the first print delay (1001) on the sampling clock results in the forming of a fire pulse (4001). Fig. 6B shows that after the fire pulse (4001), the sampling clock is restarted (1050) with its multiple delays according to three print periods (1001, 1002, 1003). Fig. 60 depicts the previous movement, armed, and fire pulses (2011, 3011, 4011) shaded, and a new movement pulse (2001) leading to new arming (3001) of the26 / 48 GN25004 W02fire pulse controller. The second print delay (1002) results in the forming of a fire pulse (4001). Fig. 6D shows that after the fire pulse (4001), the sampling clock is restarted (1050) with its multiple delays according to two print periods (1001, 1002), with the third delay for the third print period not shown.

[0140] [Fig. 7] illustrates a preferred embodiment of an inkjet printing device (300), not visible, comprising a print control system (100), an inkjet printhead (200), a substrate (400), and a speed sensor (600). The print control system (100) comprises a memory (130), a sampling clock (150), a fire pulse controller (170), and an input interface (160). The memory (130) stores a predetermined plurality of print periods. Upon being started or restarted of the sampling clock (150) , the sampling clock (150) loads (153) the predetermined plurality of print periods from the memory (130) and generates a sequence of discrete pulses corresponding to said print periods. The discrete pulses are provided (155) to the fire pulse controller (170).The speed sensor (600) measures the relative movement (700) of the substrate (400) with respect to the inkjet printhead (200) and generates movement pulses accordingly. These movement pulses are received by the input interface (160), which forwards them to the fire pulse controller (170), thereby arming (165) said fire pulse controller (170). The armed state of the fire pulse controller (170) is indicated by the arming signal, represented as a circular symbol in [Fig. 7],Upon receiving the first discrete pulse (155) from the sampling clock (150) while in the armed state, the fire pulse controller (170) generates a fire pulse and sends it (175) to the inkjet printhead (200), which ejects one or more droplets of a liquid onto the substrate (400), thereby forming a print layer thereon. After generating the fire pulse, the fire pulse controller (170) is disarmed and the sampling clock (150) is restarted by a reset signal (172), whereupon the sampling clock (150) reloads the predetermined plurality of print periods from the memory (130) and generates a fresh sequence of discrete pulses.

[0141] The determination of (workable) print periods for the plurality of print periods can be based on various factors, such as the characteristics of the digital image being printed, the print receiver properties, and the desired print quality. The determination of (workable) print periods involve analyzing the printhead's performance characteristics, ink properties, and desired print quality outcomes. Methods for determining workable print periods may include empirical testing, computational modeling of droplet formation, and real-time analysis of print quality. Factors considered in this determination include droplet formation stability, satellite droplet suppression, and consistency of droplet placement. Types of analysis techniques may include high-speed imaging of droplet27 / 48 GN25004 W02formation, acoustic measurements of printhead performance, and statistical analysis of print quality metrics across different operating conditions.

[0142] The plurality of different print periods comprises at least a first print period and a second print period, with the number of selections for each print period being preferably minimum one. This approach allows for dynamic adjustment of the printing process to optimize performance under various conditions such as the pre-determined print speed which is received by the movement pulses.

[0143] Preferably a print period is selected from the plurality of print periods that provides a similar drop speed or similar drop speed range for each print period of the plurality of print periods. The consistency in drop speed is important for maintaining print quality and accuracy.

[0144] The drop speed can be measured using various techniques, such as high-speed imaging or laser-based droplet velocity measurement systems e.g., JetXpert Dropwatcher of manufacturer imageXpert™ Factors affecting drop speed include ink viscosity, surface tension, nozzle geometry, and applied waveform characteristics. Maintaining consistent drop speeds across different print periods of the plurality of print periods helps ensure uniform dot placement and line quality in the printed output.

[0145] The drop speed range is preferably a predetermined threshold for drop speed variation. The difference between the maximum and minimum values of the predetermined threshold is lower than 2 m / s, preferably lower than 1 m / s. This tight control over drop speed variation helps maintain uniform droplet spacing and overlap, which is essential for achieving high-quality prints.

[0146] Preferably a print period is selected for the plurality of print periods that provides a similar drop mass or similar drop mass range for each print period of the plurality of print periods. The consistency in drop mass is important for maintaining print quality and accuracy.

[0147] The drop mass can be measured using various techniques, such as high-speed imaging or laser-based droplet velocity measurement systems or high precision scales e.g., JetXpert Dropwatcher of manufacturer imageXpert™. Drop mass is typically measured in picoliters (pL). Factors affecting drop mass include ink viscosity, surface tension, nozzle geometry, and applied waveform characteristics. Maintaining consistent drop mass across different print periods of the plurality of print periods helps ensure uniform density in the printed output.28 / 48 GN25004 W02

[0148] The drop mass range is preferably a predetermined threshold for drop mass variation.The difference between the maximum and minimum values of the predetermined threshold is lower than 1 pL (= picoliter), preferably lower than 0.5 pL (picoliter). This tight control over drop mass variation helps maintain uniform density, which is essential for achieving high-quality prints.

[0149] Preferably a print period is selected for the plurality of print periods that provides a similar satellite behavior or similar satellite behavior range for each print period of the plurality of print periods. The consistency in satellite behavior is important for maintaining print quality and accuracy. Similar satellite behavior is defined as a satellite behavior within a predetermined threshold wherein the difference between the maximum and minimum number of satellite droplets per primary droplet is lower than 3, preferably lower than 2, and the distance between satellite and primary droplet is below 10 pm.

[0150] The satellite behavior can be measured using various techniques, such as highspeed imaging or laser-based droplet velocity measurement systems e.g., JetXpert Dropwatcher of manufacturer imageXpert™ Satellite behavior is typically measured by counting the number of satellite droplets formed per primary droplet and / or measuring the distance between the primary droplet and its satellites. Factors affecting satellite behavior include ink viscosity, surface tension, nozzle geometry, and applied waveform characteristics. Maintaining consistent satellite behavior different print periods of the plurality of print periods helps ensure uniform density in the printed output.

[0151] The satellite behavior range is preferably a predetermined threshold for satellite behavior variation. The difference between the maximum and minimum values of the predetermined threshold is lower than 3 satellites next to the primary droplet, preferably lower than 2 satellites next to the primary droplet wherein the distance between satellite and primary droplet is below 10 pm. This tight control over satellite behavior variation helps maintain uniform density and drop accuracy, which is essential for achieving high- quality prints.

[0152] Preferably each print period of the predetermined plurality of print periods is capable of forming droplets with an equal drop speed or a drop speed within a predetermined threshold, wherein the difference between the maximum value and minimum value of the predetermined threshold is lower than 2 m / s, preferably lower than 1 m / s.

[0153] The print quality can be measured by comparing densities of patches and / or a plurality of linewidth variations if the variation of print periods according to the present invention are used or only a single print period is used at several print speeds.29 / 48 GN25004 W02

[0154] If the plurality of print periods is determined the determined print periods are best validated through additional testing and fine-tuning. Ensure that the determined print periods provide the desired print quality and meet the requirements of your specific application.

[0155] Methods for achieving this level of consistency preferably comprise precise temperature control of the printhead and / or ink, optimization of ink formulations.Advantages of maintaining a narrow drop speed range include improved colour uniformity, reduced banding artifacts, and enhanced overall print quality such as similar dot placement between the several used print periods.

[0156] The invention's approach of using a plurality of different print periods offers significant advantages in terms of print quality and process flexibility. By varying between different print periods, the printing system can compensate for various factors that may affect droplet formation and placement. This can include variations in ink viscosity due to temperature fluctuations, changes in meniscus conditions at the nozzle, and differences in print receiver properties. The method of selecting and varying between (workable) print periods allow for fine-tuning of the printing process to achieve optimal results under varying conditions. This approach can lead to improved print quality, increased print speed, and enhanced reliability of the printing process across a wide range of operating conditions and print receiver types.

[0157] The implementation of the invention requires preferably sophisticated control systems and computer implemented methods to manage the variation of print periods, as well as the synchronization with other printing parameters such as printhead voltage settings I Digital-to-Analog Converter (DAC) scaling setting and print receiver movement. Methods for implementing this control may include digital signal processing techniques, real-time feedback systems, and adaptive control algorithms. The control system may also incorporate predictive models to anticipate and compensate for changes in printing conditions. Types of control architectures may include centralized control systems, distributed control systems, or hybrid approaches that combine local and global optimization strategies. Advantages of advanced control implementations include improved print quality consistency, enhanced system reliability, and increased flexibility to accommodate different printing requirements and conditions.

[0158] Other ways of determining (workable) print periods are:- Machine learning-based optimization e.g., by using historical print data and real-time sensor feedback to train models that predict optimal print periods based on: Printhead type and waveform; liquid properties (e.g., viscosity, surface tension); substratecharacteristics; environmental conditions (temperature, humidity). The benefit is that it is an adaptive and predictive control that improves over time.- (Real-time) spectral analysis of printhead vibrations, e.g.: by monitoring acoustic or mechanical vibrations of the printhead during operation to detect resonance or instability whereby e.g., fast fourier transform (FFT) is used to identify problematic frequencies. The print periods may be adjusted to avoid these frequencies. The benefit is preventing resonance-related artifacts and improves droplet stability.- Ink rheology profiling, e.g.: by measuring ink behavior under shear and thermal conditions using rheometers whereby e.g., ink viscosity changes are determined with temperature and shear rate. Use this data to adjust print periods, eventually dynamical. The benefit is ensuring consistent droplet formation across varying ink conditions.- Substrate-specific calibration routines, e.g., by running calibration tests on each new substrate type such as printing test patterns at various print periods at certain print speeds whereby e.g.: dot gain, edge sharpness, and colour density are analyzed. The print periods may be adjusted to optimal print periods based on said results. The benefit is that tailors print periods to substrate properties are used for best quality.

[0159] In a preferred embodiment, the selection of print periods is dynamically adapted based on real-time measurements or predefined parameters, including ink viscosity, ink or printhead temperature, ink type, pigment load, substrate type, and substrate temperature. The system may apply non-linear scaling factors or coefficients (e.g., a variable k) to adjust print periods in response to environmental or material changes.

[0160] In another preferred embodiment, each print period of the predetermined plurality of print periods is adaptable, preferably real-time, by adding a value and / or multiplying a factor based on a measurement of at least one fluid property of a liquid, such as a rheological property of the liquid, used for forming one or more droplets by the printhead. Said one or more droplets is formed after receiving the fire pulse.

[0161] By varying the print period within the plurality of (workable) print periods, the system can ensure that the average first print resolution over the printed plurality of consecutive pixel rows matches the predetermined first print resolution, also called vertical resolution, more closely.

[0162] In a preferred embodiment, the variation of print periods is chosen from a predetermined set of workable print periods. These periods are selected to compensate for discrepancies between the achieved vertical print resolution and the predetermined vertical print resolution. The absolute difference between the average vertical print31 / 48 GN25004 W02resolution over the printed pixel rows and the predetermined vertical print resolution is preferably less than 80 pm, more preferably less than 50 pm, and most preferably less than 30 pm.

[0163] In piezoelectric inkjet printheads, the print frequency response curve helps to understand how efficiently the printhead can eject droplets at various frequencies or print periods. It is crucial because the printhead’s ability to maintain consistent drop size and drop speed across different print frequencies or print periods directly impacts print quality. The curve can reveal the optimal operating frequencies where the printhead performs best and highlight any print frequencies or print periods where performance may degrade due to factors like crosstalk or resonance.

[0164] More information on frequency response curves is disclosed in “A Simple Model for DoD Inkjet Frequency Response”, by Stephen D. Hoath in Digital Fabrication and Digital Printing: NIP31 Technical Program and Proceedings (2015). Also chapter 3.6.3.1, especially Figure 3.22 (b) “Inkjet Printheads - Residual Oscillations” of “Fundamentals of Inkjet Printing: The Science of Inkjet and Droplets” - Naoki Morita, Amol A. Khalate, Arend M.van Buul, Herman Wijshoff - Book Editor: Stephen D. Hoath - First published by Wiley-VCH on 18 December 2015, especially Figure 3.22 (b) discloses a frequency response curves as DoD curve (= Drop-on-Demand curve). Said curve is influenced by residual oscillations. After a drop has been ejected from the nozzle the pressure waves responsible still run through the ink channel. These oscillations are called residual oscillations. The time required to damp the residual oscillations depends mainly on the geometry and material properties of the ink channel and the viscosity of the ink. Said residual oscillations can be measured using a piezo self-sensing mechanism.

[0165] The determination of (workable) print periods for the plurality of print periods can be based on the pre-determined frequency response curve wherein preferably print periods on the frequency response curve are selected for the plurality of print periods where the drop speed is stable and consistent ([Fig. 5]).

[0166] Preferably a print period is selected from the plurality of print periods that provides a similar drop speed or similar drop speed range for each print period of the plurality of print periods ([Fig. 5]). The consistency in drop speed is important for maintaining print quality and accuracy.

[0167] Preferably a print period is selected from the plurality of print periods which has a period of (N + k) times the resonance frequency of the piezoelectric printhead whereby N is an integer including zero and k is selected in the range between 0 and 0.10, between 0.15 and 0.35, between 0.40 and 0.60, between 0.65 and 0.85 or between 0.9 and 0.99.32 / 48 GN25004 W02The range is preferably between 0.60 and 0.90 or between 0.10 and 0.40; more preferably k is selected in the range between 0.65 and 0.85 or between 0.15 and 0.35; most preferably k is 0.75, 0.25 or in the range between 0.70 and 0.80 or in the range between 0.20 and 0.30. This approach helps in better control over droplet formation and ejection; thus, maintaining consistent print quality.

[0168] The jetting of droplets is performed with a predetermined waveform and preferable with a print head voltage setting selected from a group of print head voltage settings comprising a first print head voltage setting and a second print head voltage setting, which is different from the first print head voltage setting. An example of a printhead voltage setting is a Voltage Amplitude Adjustment (VAA) which determines the amplitude of the driving waveform applied to the piezoelectric actuator in the printhead. The print head voltage settings directly influence the deformation of the piezoelectric elements and, consequently, the droplet ejection characteristics. The plurality of print head voltage settings can be stored in a memory e.g., a non-transitory computer-readable storage medium.

[0169] Methods for determining optimal voltage settings may be empirical testing, computational fluid dynamics simulations, and / or real-time feedback systems. Different voltage settings can be used to compensate for variations in ink properties, nozzle conditions, and desired droplet characteristics.

[0170] It has been observed that printing a plurality of consecutive pixel rows at a predetermined print speed, with variations in print period according to the present invention may result in non-uniform density across the printed output. Specifically, density measurements indicate that the density of pixel rows printed using one print period may differ from the density of pixel rows printed using another print period. To compensate the differences, selection of print head voltage settings from a group of print head voltage settings can be beneficial to solve the non-uniform density across the printed output. The selection is preferably dependable on the selected print period of the plurality of print periods.

[0171] The number of selections for the first print head voltage setting is preferably the same as the number of selections for the first print period, and / or the number of selections for the second print head voltage setting is preferably the same as the number of selections for the second print period. This correlation between voltage settings and print periods allows for optimized droplet formation across different operating conditions.

[0172] The method of matching voltage settings to print periods can be implemented through lookup tables stored in e.g., non-transitory computer-readable storage medium,33 / 48 GN25004 W02real-time calculations, or adaptive algorithms that adjust based on print quality feedback. Advantages of this approach include improved consistency in droplet characteristics.

[0173] The relationship between the number of selections of a print period of the plurality of print periods for varying the print periods and the number of pixel rows and predetermined print speed may allow for dynamic adjustment of the printing process to maintain optimal performance across during print of the plurality of pixel rows at a predetermined print speed. Methods for determining the optimal period selection pattern may include statistical analysis of print quality outcomes, machine learning algorithms that adapt to changing printing conditions, or predictive models based on theoretical and empirical data. Advantages of this approach include improved print quality consistency across different print jobs, enhanced adaptability to varying print receiver and environmental conditions, and optimized balance between predetermined print speed and quality.

[0174] The invention's approach of using a plurality of different print periods offers significant advantages in terms of print quality and process flexibility. By varying between different print periods, the printing system can compensate for various factors that may affect droplet formation and placement. This can include variations in ink viscosity due to temperature fluctuations, changes in meniscus conditions at the nozzle, and differences in print receiver properties. The method of selecting and varying between (workable) print periods allow for fine-tuning of the printing process to achieve optimal results under varying conditions. This approach can lead to improved print quality, increased print speed, and enhanced reliability of the printing process across a wide range of operating conditions and print receiver types.

[0175] The implementation of the invention requires preferably sophisticated control systems and computer implemented methods to manage the variation of print periods, as well as the synchronization with other printing parameters such as printhead voltage settings and print receiver movement. Methods for implementing this control may include digital signal processing techniques, real-time feedback systems, and adaptive control algorithms. The control system may also incorporate predictive models to anticipate and compensate for changes in printing conditions. Types of control architectures may include centralized control systems, distributed control systems, or hybrid approaches that combine local and global optimization strategies. Advantages of advanced control implementations include improved print quality consistency, enhanced system reliability, and increased flexibility to accommodate different printing requirements and conditions.34 / 48 GN25004 W02

[0176] The invention's approach to inkjet printing technology, which involves varying between (workable) print periods, represents a significant advancement in the field. This method addresses many of the challenges associated with maintaining consistent print quality across different operating conditions and print receiver types, even at predetermined print speeds above 150 m / min which is challenging with the state-of-the- art printheads. By optimizing the printing process through careful print period variation, the invention enables improved droplet formation and placement accuracy during the printing of the plurality of pixel rows at high print speed. This can lead to enhanced print quality, increased print speed, and greater versatility in terms of the types of print receivers and inks that can be used effectively. The novel feature of mixing print periods opens new possibilities for fine-tuning the printing process to achieve optimal results under varying conditions, potentially expanding the applications and capabilities of inkjet printing technology, especially at high print speed.Movement pulses

[0177] In the context of the present invention, movement pulses are discrete signals that represent the relative motion between the printhead and the print receiver. These pulses are typically generated by a speed sensor e.g., encoder device and serve as a fundamental input to the print control system. Each movement pulse corresponds to a specific increment of displacement or velocity change, allowing the system to synchronize droplet ejection with the actual position or speed of the print receiver.

[0178] The input interface of the print control system receives signals (movement pulses) e.g., from an encoder device or motion sensor that tracks the movement of the printhead or pr.int receiver. Synchronizing droplet ejection with physical movement, ensuring that droplets are placed relative to the print receiver’s position.

[0179] If the inkjet printing device is part of a manufacturing line for producing an object as process device, the pulses may come from another process device. In this case, the inkjet printing device operates as a slave, while the process device functions as the master and the print speed may be called manufacturing speed. So, the movement pulses may originate from various sources depending on the configuration of the manufacturing line.

[0180] In one embodiment, the pulses are generated by an encoder device mounted on the inkjet printing device itself. Alternatively, the encoder device may be positioned on an upstream or downstream process device, such as a drying unit, coating station, or impregnation line. This flexibility allows the print control system to adapt to dynamic35 / 48 GN25004 W02changes in manufacturing speed caused by non-printing processes, ensuring consistent print quality even under variable-speed conditions.

[0181] Upon receiving a movement pulse, the fire pulse controller is armed, indicating that the system is ready to generate a fire pulse for droplet ejection. However, the actual firing is delayed until the next discrete pulse of the sampling clock, which operates based on a predetermined plurality of print periods. This design introduces a controlled delay between movement detection and droplet ejection, enabling precise timing and alignment of printed dots with the substrate's position.

[0182] The use of movement pulses also allows the system to operate in a real-time, event- driven manner, reducing the need for complex buffering or predictive algorithms. By restarting the sampling clock after each fire pulse, the system ensures that each droplet is ejected in response to actual movement, rather than relying on estimated or interpolated positions. This approach enhances the temporal precision of the printing process and minimizes artifacts such as banding, misalignment, or density variation.

[0183] Furthermore, the granularity and frequency of the movement pulses can be tailored to the specific application. For example, high-resolution encoder devices may be used in applications requiring fine detail and high-speed printing, while lower-resolution encoder devices may suffice for less demanding tasks. The system may also incorporate filtering or interpolation algorithms to smooth out noise or jitter in the movement signal, further improving print stability.

[0184] The print speed may be measured by a movement signal, having plurality of movement pulses, derived from the relative movement between the print receiver driver and the printhead. For example, EP2996878A1 (AGFA NV) discloses a movement signal for a belt step conveyor system, especially useful in multi-pass inkjet printing. Other ways of measuring print speed in an inkjet printing device (100) are a) linear encoder devices which measure the linear position of the print receiver directly. They can be optical, magnetic, or capacitive. Linear encoder devices provide high precision and are often used in applications requiring accurate positioning; b) rotary encoder devices: These are attached to a rotating shaft and measure the angular position or motion. They can be either incremental or absolute encoder devices. Incremental encoder devices generate a series of pulses as the shaft rotates, which can be counted to determine print speed; c) measuring wheels: These are often used in conjunction with rotary encoder devices. The wheel is in contact with the moving substrate, and as it rotates, the encoder device attached to it generates pulses proportional to the distance traveled.36 / 48 GN25004 W02

[0185] If the print speed is constant the period between said movement pulses is constant. If the print speed becomes faster I slower the period between consecutive movement pulses becomes smaller / bigger.

[0186] In certain cases, such as when the print receiver moves slightly faster than expected due to roller slippage, the encoder may report fewer pulses than the actual displacement. Without compensation, this discrepancy can cause the printhead to fire too late, resulting in misalignment of the printed image. To address this, compensation factors can be applied to adjust the movement signal, ensuring that the fire pulse remains synchronized with the true physical position of the substrate. In inkjet printing devices, this process — commonly referred to as movement or encoder compensation — is essential for maintaining precise droplet placement, particularly at variable speeds, and for preserving consistent print resolution across the substrate. Compensation may be implemented during a calibration phase or dynamically during real-time operation. It may be required to correct for mechanical inaccuracies (such as gear backlash or slippage), limitations in encoder resolution, non-linear motion behavior, or thermal expansion and contraction of the print receiver. The movement pulses are modified using compensation data, which may include scaling factors or offset corrections. Some systems employ closed-loop control mechanisms to continuously refine compensation during operation, further enhancing print accuracy and reliability.Other embodiments

[0187] The present invention comprises also the embodiment: use of more than one print period, selected from a predetermined plurality of print periods, for a predetermined print speed and predetermined vertical resolution during the inkjet printing of a plurality of consecutive pixel rows of a digital monochromatic image with a piezoelectric printing head. Preferably the predetermined print speed is above 75 m / min and more preferably the predetermined vertical resolution is higher or equal than 300 dots per inch and most preferred the nozzle resolution of the piezoelectric printing head is equal or above 300 nozzles per inch. The preferred embodiments of the embodiment with the printing method with drop-on-demand technology are also preferred embodiments of the embodiment with the use of more than one print period.

[0188] The present invention comprises also the embodiment: a method for printing a digital monochromatic image, having a plurality of consecutive pixel rows, with drop-on-demand technology comprising:- transferring a plurality of consecutive pixel rows to a piezoelectric printhead;- jetting droplets onto a print receiver moving relative with a predetermined print speed to37 / 48 GN25004 W02the piezoelectric printhead according to each transferred pixel row; according to a predetermined waveform and with variable print periods selected from a predetermined plurality of print periods. The preferred embodiments of the embodiment with the printing method with drop-on-demand technology are also preferred embodiments of the embodiment with the method for printing a digital monochromatic image.Industrial Applicability

[0189] An efficient printing method ensures a good print quality at a high speed. So, the present invention offers significant benefits that align with the needs of industrial printing operations, making it highly applicable in the industry.Patent Literature

[0190] PTL1: US20180370234A1 (Fujifilm Dimatix Inc)

[0191] PLT2: EP2633998A1 (AGFA)

[0192] PLT3: US 2020 / 0298561 A1 (Ricoh Co Ltd)Non Patent Literature

[0193] NPL1: A Simple Model for DoD Inkjet Frequency Response”, by Stephen D Hoath in Digital Fabrication and Digital Printing: NIP31 Technical Program and Proceedings (2015)

[0194] NPL2: ‘Inkjet Technology and Product Development Strategies’ by Stephen F. Pond, Torrey Pines, 2000

[0195] NPL3: Chapter 3.6.3.1 “Inkjet Printheads - Residual Oscillations” of “Fundamentals of Inkjet Printing: The Science of Inkjet and Droplets” - Naoki Morita, Amol A. Khalate, Arend M.van Buul, Herman Wijshoff - Book Editor: Stephen D. Hoath - First published by Wiley-VCH on 18 December 2015

[0196] NPL4: “Structure- and fluid-dynamics in piezo inkjet printheads" by Herman Wijshoff (2008), specifically on page 69 (chapter 3.4. Residual Vibrations)

[0197] NPL5: “Acoustic Phenomena in a Demand-Mode Piezoelectric Ink-Jet Printer” of Bogdan V. Antohe & co (2001) out NIP & Digital Fabrication Conference 17 from publisher: Society of Imaging Science and Technology ]

Claims

Claims

1. |A method for printing with drop-on-demand technology comprising:- transferring a plurality of consecutive pixel rows of a digital monochromatic image to a piezoelectric printhead;- jetting droplets, according to a predetermined waveform, onto a print receiver moving relative with a predetermined print speed to the piezoelectric printhead according to each transferred pixel row with variation of print periods selected from a predetermined plurality of print periods.

2. The method of claim 1 , wherein at least two of the predetermined plurality of print periods are selected during the jetting.

3. A method according to claim 2 wherein each print period of the predetermined plurality of print periods are capable of forming droplets- with an equal drop speed or a drop speed within a first predetermined threshold wherein difference of maximum value and minimum value of the first predetermined threshold is lower than 2 m / s; preferably lower than 1 m / s; and / or - with an equal drop mass or a drop mass within a second predetermined threshold wherein difference of maximum value and minimum value of the second predetermined threshold is lower than 1 pL; preferably lower than 0.5 pL; and / or- with a print period that is equal to (N + k) times a resonance frequency of the piezoelectric printhead whereby N is an integer and k is selected in the range between 0 and 0.10, between 0.15 and 0.35, between 0.40 and 0.60, between 0.65 and 0.85 or between 0.9 and 0.99; and / or- having a satellite behavior within a predetermined threshold wherein the difference between the maximum and minimum number of satellite droplets per primary droplet is lower than 3, preferably lower than 2, and the distance between satellite and primary droplet is below 10 pm.

4. A method according to claim 3 wherein the step of jetting droplets is performed with a print head voltage setting selected from a plurality of print head voltage settings according to a selected print period.

5. A method according to any of the previous claims wherein the droplets are jetted when the piezoelectric printhead receives a fire pulse from a fire pulse controller, having a sampling clock with discrete delays according to the predetermined plurality of print periods,which is armed when receiving an movement pulse from a print receivermovement driver; andwhich is disarmed after the fire pulse is received by the piezoelectric printhead; wherein the sampling clock is restarted after the fire pulse is received by the piezoelectric printhead; andwherein the fire pulse is formed according by the first discrete delay on the sampling clock when the fire pulse controller is armed.

6. A method according to any of the previous claims wherein the selected print speed is above 75 m / min, preferably above 100 m / min.

7. A method according to any of the previous claims wherein the piezoelectric printhead is a piezoelectric through-flow printhead.

8. A method according to any of the previous claims wherein the method is a single pass inkjet printing method.

9. A method according to any of the previous claims wherein the print receiver is a web-based print receiver, and the method is a web printing method.

10. A method according to any of the previous claims wherein the variation of print periods selected from the predetermined plurality of print periods are selected to compensate for discrepancies between the achieved vertical print resolution, and a predetermined vertical print resolution, whereby the absolute difference of an average vertical print resolution over the printed plurality of consecutive pixel rows and the predetermined vertical print resolution is smaller than 80 pm.

11. An inkjet printing device for printing, comprising:a piezoelectric printhead configured to receive a plurality of consecutive pixel rows of a digital monochromatic image; anda mechanism for moving at a predetermined print speed a print receiver relative to the piezoelectric printhead; anda control system for jetting droplets with a waveform onto the print receiver according to each transferred pixel row, with variation of print periods selected from a predetermined plurality of print periods.

12. The inkjet printing device of claim 11 , wherein at least two of the predetermined plurality of print periods are selected during the jetting.

13. The inkjet printing device of claim 12, wherein each print period of the predetermined plurality of print periods is capable of forming droplets with an equal drop speed or a drop speed within a predetermined threshold, wherein thedifference between the maximum value and minimum value of the predetermined threshold is lower than 2 m / s, preferably lower than 1 m / s; and / or with an equal drop mass or a drop mass within a second predetermined threshold wherein difference of maximum value and minimum value of the second predetermined threshold is lower than 1 pL; preferably lower than 0.5 pL; and / or- with a print period that is equal to (N + k) times a resonance frequency of the piezoelectric printhead whereby N is an integer and k is selected in the range between 0 and 0.10, between 0.15 and 0.35, between 0.40 and 0.60, between 0.65 and 0.85 or between 0.9 and 0.99; and / or- having a satellite behavior within a predetermined threshold wherein the difference between the maximum and minimum number of satellite droplets per primary droplet is lower than 3, preferably lower than 2, and the distance between satellite and primary droplet is below 10 pm.

14. The inkjet printing device of claim 13, wherein the piezoelectric printhead operates with a print head voltage setting selected from a plurality of print head voltage settings according to a selected print period.

15. The inkjet printing device of any of the claims from 11 to 14, wherein the plurality of consecutive pixel rows is transferred to the piezoelectric printhead in a sequential manner to ensure continuous printing.]

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