Patent Description:
Image formation is a procedure whereby a digital image is printed on a medium e.g. by jetting droplets of liquid or another type of print fluid onto said medium, such as paper, plastic, a substrate for 3D printing.

Said image formation is commonly employed in apparatuses, such as printers (e.g. inkjet printer) but also facsimile machines, copying machines, plotting machines, multifunction peripherals, etc. The core of a typical jetting apparatus or image forming apparatus is one or more liquid droplet generators (also known as "printheads") having nozzles that discharge liquid droplets, a mechanism for moving the liquid droplet generator and/or the medium in relation to one another, and a controller that controls how liquid is discharged from the individual nozzles of the liquid droplet generator onto the medium in the form of pixels.

A typical liquid droplet generator includes a plurality of nozzles aligned in one or more rows (called "nozzlerows") along a discharge surface of the liquid droplet generator. Each nozzle is part of a "jetting channel", which includes the nozzle, a pressure chamber, and an actuator, such as a piezoelectric actuator. A liquid droplet generator also includes a drive circuit that controls when each individual jetting channel fires based on image data. To jet from a jetting channel, the drive circuit provides a jetting pulse to the actuator, which causes the actuator to deform a wall of the pressure chamber. The deformation of the pressure chamber creates pressure waves within the pressure chamber that eject a droplet of print fluid (e.g., liquid) out of the nozzle.

Drop on Demand (DoD) printing is moving towards higher productivity and quality, which requires small droplet sizes ejected at high jetting frequencies. The print quality delivered by a liquid droplet generator depends on ejection or jetting characteristics, such as droplet velocity, droplet mass (or volume/diameter), jetting direction, etc. Unfortunately, liquid-sedimentation inside the liquid droplet generator, especially in the manifold, negatively affects the jetting characteristics and thus the image quality especially when said liquid droplet generator has a manifold with two inlets at both sides of said manifold. The liquid-sedimentation in the manifold occurs e.g. when said liquid droplet generator, filled with liquid, is not used for a long period. Such a liquid droplet generator is described in <CIT>. It is found that liquid-sedimentation issue in the manifold of such type of head results in failing nozzles and non-equal densities between the droplets of nozzles which results in annoying density profile of the liquid droplet generator.

Document <CIT> discloses a droplet ejection control apparatus that has a droplet ejection amount control unit configured to change droplet ejection amounts for each raster in a region where the overlap printing is performed so that a total sum of the droplet ejection amounts for each raster in the region where the overlap printing is performed sequentially changes to be equal to or greater than a reference amount that is defined by a total sum of droplet ejection amounts in a normal region and to be equal to or smaller than the above reference amount.

Problems of liquid sedimentation lead to frequent maintenance of liquid droplet generators using so-called "flushing" or "washing" liquids for unclogging of nozzles and cleaning the nozzle plate of said liquid droplet generators. For example, <CIT> discloses a maintenance liquid for inkjet printers comprising at least one of glycol ethers and glycol esters and <NUM> to <NUM>/L of dissolved oxygen. <CIT> discloses a washing solution for washing a cationically UV curable inkjet ink inkjet printer head, which contains not less than <NUM> parts by weight of a polymerizable compound selected from the at least two kinds of polymerizable compounds included in the ink and having the lowest viscosity among the at least two kinds of polymerizable compounds, or not less than <NUM> parts by weight of a polymerizable compound having a viscosity of <NUM> mPa·s or less at ordinary temperature.

During research it is found that due the collision of the liquid from the first and second inlet at the middle of the manifold, a so-called dead zone, the liquid flow is dropped. Now, if the liquid droplet generator is not used for a certain period, liquid-sedimentation is formed at the bottom of the manifold (= which is the side closest towards the nozzles). But when the manifold is (re)filled via the first and second inlet, the liquid-sedimentation in the manifold is accumulated at said dead-zone. The liquid-sedimentation has here a higher (volumetric mass) density than the liquid on other places in the manifold. It is found that during printing, failing nozzles occur in the nozzles nearby said dead zone and/or a large print density difference occur at the nozzles nearby said dead zone. The operator has thereby to maintain the liquid droplet generator by flushing and/or cleaning or has to replace the expensive liquid droplet generator so the production time drops. Therefore, there is a need in an efficient printing method for a liquid droplet generator that has not used for a certain period and having liquid-sedimentation in the manifold without expensive time cost for maintenance procedures.

It has been found that the problems described above while using a liquid droplet generator (<NUM>) having a manifold with two inlets at both side of the manifold can be overcome by a method of printing an halftone image wherein a determined part of the halftone image is printed by a plurality of nozzles, arranged in one or more rows, of the liquid droplet generator, where the liquid droplet generator has at a first location in a manifold of the liquid droplet generator a liquid-sedimentation with highest (volumetric mass) density and wherein the manifold is.

whereby <NUM> >= N1 > N2 > N3 >= <NUM>. The manifold is preferably filled via the first and second inlet simultaneously.

By the displacement of the liquid-sedimentation nozzle failures and (print) density profiles caused by liquid-sedimentation in the manifold is solved whereby no long during maintenance (e.g. cleaning, flushing,. ) of the liquid droplet generator is needed and the life time of the liquid droplet generator is enlarged.

Further advantages and embodiments of the present invention will become apparent from the following description.

<FIG> is a cross-sectional view of liquid droplet generator (<NUM>) in an illustrative embodiment as illustrated in <FIG>. Through this cross-sectional view, the elements of (supply) manifold (<NUM>) are visible. The manifold <NUM> is formed by elongated body (<NUM>) of main body (<NUM>) that extends (left to right in <FIG>) between ends (<NUM>,<NUM>). The manifold (<NUM>) is also formed by a first inlet (<NUM>) and second inlet (<NUM>) (<NUM>) that fluidly couple supply ports (<NUM>) to said opposing ends (<NUM>, <NUM>) of elongated body (<NUM>). Thus, print fluid is supplied to the elongated body (<NUM>) at opposing ends (<NUM>, <NUM>) in this embodiment. The elongated body (<NUM>) is a conduit for the print fluid to flow. The bottom portion of elongated body (<NUM>) and thus manifold is open to the jetting channels connected to nozzles in a nozzle row (<NUM>). Thus, elongated body (<NUM>) is the portion of (supply) manifold (<NUM>) that delivers the print fluid to the jetting channels.

<FIG> illustrates as embodiment a nozzle row (<NUM>) of a liquid droplet generator (<NUM>) with <NUM> nozzles (illustrated as circles). The selection of nozzles for jetting are determined by a (repeatable) printmask (<NUM>) for two-pass printing wherein said printmask (<NUM>) has a width of <NUM> and height of <NUM> and the nozzle row is divided in three sequential zones (<NUM>, <NUM>, <NUM>). "<NUM>" in said printmask (<NUM>) is used for the first pass; "<NUM>" in said printmask (<NUM>) is used for the second pass. It is clear that in the first pass nozzles in the first zone (<NUM>) shall be used more than the nozzles in the second zone (<NUM>) and even more than the nozzles in the third zone (<NUM>).

The liquid droplet generator (<NUM>) in the present embodiment and preferred embodiments has one or more rows of nozzles for jetting droplets of liquid, also called drop-on-demand printheads. Preferably said drop ejection is performed by using piezoelectric effects. Such printheads are nowadays dominating in home and office inkjet print systems but also in industrial applications for either printing or digital fabrication. Several actuation mechanisms are possible such as push-mode actuators or squeeze-mode actuators.

The liquid droplet generator (<NUM>) in the present embodiment has an essential manifold elongated in parallel to a nozzle row; and configured to provide a liquid path for the liquid to the plurality of nozzles. The liquid is provided by a first inlet (<NUM>) and a second inlet (<NUM>) which are connected with said manifold each at both side of said manifold. Preferably no outlet is provided as known in reflow liquid droplet generators. The speed flow of the liquid at both inlets determines the liquid flow in said manifold. Preferably the speed flow in the first inlet (<NUM>) and second inlet (<NUM>) are the same.

The liquid droplet generator (<NUM>) may have a master liquid inlet which is split in said first inlet (<NUM>) and said second inlet (<NUM>). The liquid supply is then connected to said master liquid inlet where from the liquid is further provided in said liquid droplet generator (<NUM>) via said first and second inlet (<NUM>, <NUM>).

As already described around the middle of such manifold the liquid flow is dropped due the collision of the liquid from the first and second inlet, a so-called dead zone. Due said dead zone, liquid-sedimentation in the manifold is accumulated with a high density at said dead zone whereby during printing failing nozzles occur in the nozzles nearby said dead zone and/or a large density difference occur at the nozzles nearby said dead zone.

But by using a printmask (<NUM>) according the present embodiment and preferred embodiments the liquid-sedimentation in the manifold can be displaced whereby the liquid droplet generator (<NUM>) can directly be used for printing without the necessary maintenance so an operator can directly start the production of printed articles.

The droplet generator is part of a multi-pass inkjet printer wherein the halftone image is printed in multiple passes and the part is jetted in a single pass. The printmask (<NUM>) defines then the jetting of nozzles in each pass.

In a preferred embodiment, the determined part of the halftone image is additional printed by another plurality of nozzles, arranged in one or more rows, of the liquid droplet generator (<NUM>),
where the liquid droplet generator (<NUM>) has at a third location in another manifold a liquid-sedimentation with highest density and wherein the other manifold is.

The liquid droplet generator (<NUM>) is preferably part of a printing system, such as an inkjet printer. The printing system is preferably for printing on flat ink receiving media but may also be a three-dimensional printing system.

In the printing system an image is formed by one or more of said liquid droplet generators which are capable of printing one ink or more inks.

The printing system is preferably configured to move the liquid droplet generator (<NUM>) for printing parts of an image, such as multi-pass inkjet printing system. A multi-pass inkjet printing method is used in the Jeti Tauro™ manufactured by AGFA NV with a maximum printable width of <NUM> and which can accommodate for example rigid media up to <NUM> in length.

The printing system may also be hybrid printing device wherein conventional printing technologies and liquid droplet technology are combined in a printing system.

The idea of displacement of the liquid-sedimentation in the manifold was tested by printing large rectangular triangles, having a base and height with a white ink after a period of non-printing. The height is oriented parallel to the nozzle row (and print direction) and the base is oriented parallel to the scan-direction of the liquid droplet generator). By adapting the base / height of the single colored triangle and/or mirroring the triangle, the displacement of the liquid-sedimentation in the manifold could be detected.

The use of printmasks is well known in the technical field of printing with liquid jetting technology, mainly in multi-pass inkjet printing technology. Print masks are stored in a memory of the printing system, e.g. in a hardware or software printer driver, and control the signals applied to liquid droplet generator (<NUM>). The actual implementation of the print mask in either hardware or software formats is considered to be within the skill of those who have knowledge of the inkjet printer arts, when this skill is applied in view of the teachings herein.

The printmask (<NUM>) in the present embodiment has a width and height, so called two-dimensional printmask (<NUM>). The width and height are larger than <NUM>.

The number of sequential zones may be <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM> or <NUM> or equal to the width of the printmask (<NUM>) or number of nozzles.

Preferably the printmask (<NUM>) represents a blue noise halftoned gradient from nearest nozzle of the plurality of nozzles at the first inlet (<NUM>) to nearest nozzle of the plurality of nozzles at the second inlet (<NUM>); respectively from dark to light.

In a preferred embodiment the printmask (<NUM>) divides for another selected printing period following the selected printing period the plurality of nozzles in other sequential zones between the first inlet (<NUM>) and second inlet (<NUM>) comprising minimal for printing the determined part:.

In a preferred embodiment the part is determined by using a printmask (<NUM>) which divides for a determined printing period the plurality of nozzles in sequential zones between said first and second inlet (<NUM>, <NUM>) comprising minimal.

The liquid is preferably an ink but it may also be a varnish, primer, a coating, a cleaning liquid, a top-protection coating, pre-treatment liquid, post-treatment liquid.

Liquid sedimentation results in clogging of the liquid droplet generator (<NUM>) and poor storage stability of the liquid. They are mainly some particles of the liquid having a specific gravity between said particles and the liquid medium.

The liquid is preferably pigmented inkjet inks as the use of colour pigments provide higher light stability to the decorative laminate panels than dyes. It may be a pigmented aqueous inkjet ink or an UV curable inkjet ink. Said pigments are mainly found in said liquid sedimentation.

An aqueous inkjet ink preferably includes at least a colour pigment and water, more preferably completed with one or more organic solvents such as humectants, and a dispersant if the colour pigment is not a self-dispersible colour pigment.

An UV curable inkjet ink preferably includes at least a colour pigment, a polymeric dispersant, a photoinitiator and a polymerizable compound, such as a monomer or oligomer.

Preferably the jetting viscosity of the liquid is between <NUM> mPa·s and <NUM> mPa·s and the jetting temperature of the liquid is between <NUM> and <NUM>° C degrees. The jetting viscosity is measured by measuring the viscosity of the liquid at the jetting temperature. The jetting viscosity may be measured with various types of viscometers such as a Brookfield DV-II+ viscometer at jetting temperature and at <NUM> rotations per minute (RPM) using a CPE <NUM> spindle which corresponds to a shear rate of <NUM>-<NUM> or with the HAAKE Rotovisco <NUM> Rheometer with sensor C60/<NUM> Ti at a shear rate of <NUM>-<NUM>. In a preferred embodiment the jetting viscosity is from <NUM> mPa·s to <NUM> mPa· s more preferably from <NUM> mPa·s to <NUM> mPa·s and most preferably from <NUM> mPa·s to <NUM> mPa·s. The jetting temperature may be measured with various types of thermometers. The jetting temperature of jetted liquid is measured at the exit of a nozzle in the liquid droplet generator (<NUM>) while jetting or it may be measured by measuring the temperature of the liquid in the liquid channels or nozzle while jetting through the nozzle. In a preferred embodiment the jetting temperature is from <NUM>° C to <NUM>° C more preferably from <NUM>° C to <NUM>° C and most preferably from <NUM>° C to <NUM>° C.

The liquid has preferably a pigment an average particle size larger than <NUM> more preferably <NUM> or has a pigment in an amount of more than <NUM> wt% based on the total weight of the liquid. Such kind of liquids are known to have issues in liquid-sedimentation in a manifold of a liquid droplet generator (<NUM>). Said pigment is preferably an inorganic pigment and most preferably a white pigment as colorant. The determination of the numeric average particle diameter is best performed by photon correlation spectroscopy at a wavelength of <NUM> with a 4mW HeNe laser on a diluted sample of the pigmented inkjet ink. A suitable particle size analyzer used was 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 <NUM> ethyl acetate and mixed until a homogenous sample was obtained. The measured particle size is the average value of <NUM> consecutive measurements consisting of <NUM> runs of <NUM> seconds.

A white pigment preferably has a numeric average pigment particle size larger than <NUM> in order to have a strong opacifying capability. Suitable white pigments are given by Table <NUM> in [<NUM>] of <CIT>. The white pigment is preferably a pigment with a refractive index greater than <NUM>. The white pigments may be employed singly or in combination. Preferably titanium dioxide is used as pigment with a refractive index greater than <NUM>. Suitable titanium dioxide pigments are those disclosed in [<NUM>] and in [<NUM>] of <CIT>.

Claim 1:
A method of printing a halftone image wherein a determined part of the halftone image is printed by a plurality of nozzles, arranged in one or more rows, of a liquid droplet generator (<NUM>),
where the liquid droplet generator (<NUM>) has at a first location in a manifold of the liquid droplet generator (<NUM>) a liquid-sedimentation with highest density and wherein the manifold is
- having a first inlet (<NUM>) and second inlet (<NUM>) for filling the manifold with a liquid;
- elongated in parallel to the plurality of nozzles between the first inlet (<NUM>) and second inlet (<NUM>); and
- configured to provide a liquid path for the liquid to the plurality of nozzles;
wherein the part is determined for displacing the liquid-sedimentation to a second location in the manifold by using a printmask (<NUM>), having a width and height, which divides for a selected printing period the plurality of nozzles in minimum three sequential zones between the first inlet (<NUM>) and second inlet (<NUM>) comprising minimal for printing the determined part:
- a first zone (<NUM>) at the first inlet (<NUM>) wherein the printmask (<NUM>) determines that the nozzles of the first zone (<NUM>) are used for printing N1% of the part; and
- a second zone (<NUM>) at the middle of the first inlet (<NUM>) and second inlet (<NUM>) wherein the printmask (<NUM>) determines that the nozzles of the second zone (<NUM>) are used for printing N2% of the part; and
; and
- a third zone (<NUM>) at the second inlet (<NUM>) wherein the printmask (<NUM>) determines that the nozzles of the third zone (<NUM>) are used for printing N3% of the part;
whereby <NUM> >= N1 > N2 > N3 >= <NUM>.