Patent Grant
9352556
The present invention relates to electrostatic inkjet print technologies and, more particularly, to printheads and printers of the type such as described in WO 93/11866 and related patent specifications.
Electrostatic printers of this type eject charged solid particles dispersed in a chemically inert, insulating carrier fluid by using an applied electric field to first concentrate and then eject the solid particles. Concentration occurs because the applied electric field causes electrophoresis and the charged particles move in the electric field towards the substrate until they encounter the surface of the ink. Ejection occurs when the applied electric field creates an electrophoretic force that is large enough to overcome the surface tension. The electric field is generated by creating a potential difference between the ejection location and the substrate; this is achieved by applying voltages to electrodes at and/or surrounding the ejection location. One particular advantage of this type of print technology over that of conventional drop-on-demand (DOD) printers is the ability to print using greyscale, something which is not possible with conventional DOD printers.
The location from which ejection occurs is determined by the printhead geometry and the position and shape of the electrodes that create the electric field. Typically, a printhead consists of one or more protrusions from the body of the printhead and these protrusions (also known as ejection upstands) have electrodes on their surface. The polarity of the bias applied to the electrodes is the same as the polarity of the charged particle so that the direction of the electrophoretic force is towards the substrate. Further, the overall geometry of the printhead structure and the position of the electrodes are designed such that concentration and then ejection occurs at a highly localised region around the tip of the protrusions.
To operate reliably, the ink must flow past the ejection location continuously in order to replenish the particles that have been ejected. To enable this flow the ink must be of a low viscosity, typically a few centipoise. The material that is ejected is more viscous because of the concentration of particles; as a result, the technology can be used to print onto non-absorbing substrates because the material will not spread significantly upon impact.
Various printhead designs have been described in the prior art, such as those in WO 93/11866, WO 97/27058, WO 97/27056, WO 98/32609, WO 01/30576 and WO 03/101741, all of which relate to the so-called Tonejet® method described in WO 93/11866.
The solid arrow 11 shows the ejection direction and again points in the direction of the substrate. In
In operation, it is usual to hold the substrate at ground (0 V), and apply a voltage, VIE, between the intermediate electrode 10 and the substrate. A further potential difference of VB is applied between the intermediate electrode 10 and the electrodes 7 on the ejection upstand 2 and the cheeks 3, such that the potential of these electrodes is VIE+VB. The magnitude of VB is chosen such that an electric field is generated at the ejection location 6 that concentrates the particles, but does not eject the particles. Ejection spontaneously occurs at applied biases of VB above a certain threshold voltage, VS, corresponding to the electric field strength at which the electrophoretic force on the particles exactly balances the surface tension of the ink. It is therefore always the case that VB is selected to be less than VS. Upon application of VB, the ink meniscus moves forwards to cover more of the ejection upstand 2. To eject the concentrated particles, a further voltage pulse of amplitude VP is applied to the ejection upstand 2, such that the potential difference between the ejection upstand 2 and the intermediate electrode 10 is VB+VP. Ejection will continue for the duration of the voltage pulse. Typical values for these biases are VIE=500 volts, VB=1000 V and VP=300 volts.
The voltages actually applied in use may be derived from the bit values of the individual pixels of a bit-mapped image to be printed. The bit-mapped image is created or processed using conventional design graphics software such as Adobe Photoshop and saved to memory from where the data can be output by a number of methods (parallel port, USB port, purpose-made data transfer hardware) to the printhead drive electronics, where the voltage pulses which are applied to the ejection electrodes of the printhead are generated.
One of the advantages of electrostatic printers of this type is that greyscale printing can be achieved by modulating either the duration or the amplitude of the voltage pulse. The voltage pulses may be generated such that the amplitude of individual pulses are derived from the bitmap data, or such that the pulse duration is derived from the bitmap data, or using a combination of both techniques.
Printheads comprising any number of ejectors can be constructed by fabricating numerous cells 5 of the type shown in
However, stitch lines frequently result from the use of overlapped printheads or from overlapping on multiple passes and therefore it is known to use interleaving techniques (printing alternate single or groups of pixels from adjacent printheads or from different passes of the same or a different printhead) to distribute and hide the edge effects of the print swathes resulting from the overlapping ends of the printheads. It is generally recognised that a stitching strategy is necessary to obtain good print quality across a join between printed swathes. The known techniques rely on the use of a binary interleaving strategy i.e. a given pixel is printed by one printhead or the other. For example, alternate pixels along the x-axis are printed from adjacent overlapping printheads. Alternatively, a gradual blend from one swathe to the next can be used, by gradually decreasing the numbers of adjacent pixels printed from one printhead while increasing the numbers of adjacent pixels printed from the other printhead. This latter technique can be expanded by dithering the print in the y-axis direction. Another known technique is the use of a saw tooth or sinusoidal ‘stitch’ to disrupt any visible stitch line.
These techniques all represent different ways in which printing can be alternated between the nozzles of two overlapping printheads and the success of them depends on the droplet placement accuracy and registration of the two printheads, and is particularly sensitive to factors like substrate wander between lines of printheads. This can be mitigated by the dispersion and deliberate movement of the stitch to break up visible lines and disperse the errors over the width of the overlapping regions of the adjacent printed swathes.
According to the present invention there is provided a method of printing a two-dimensional bit-mapped image having a number of pixels per row for printing using a plurality of overlapping printheads or a printhead or printheads indexed through overlapping positions, the or each printhead having a row of ejection channels, each ejection channel having associated ejection electrodes to which a voltage is applied in use sufficient to cause particulate concentrations to be formed from within a body of printing fluid, and wherein, in order to cause volumes of charged particulate concentrations of one of a number of predetermined volume sizes to be ejected as printed droplets from selected ejection channels of the overlapping printheads, voltage pulses of respective predetermined amplitude and duration, as determined by respective image pixel bit values, are applied to the electrodes of the selected ejection channels, characterised in that
This technique provides an alternative strategy to those known in the art, which creates each printed pixel in the overlap region of printheads from a contribution from both printheads in the overlap region, i.e. an ejection from one printhead plus an ejection from the overlapping printhead, which together give a pixel of the required size and/or density. The relative contributions from the two printheads change to create a progressive fade-out from the one printhead with an overlapping fade-in to the other printhead across the overlap region. This is less sensitive to dot placement errors and substrate wander, because such errors are less inclined to produce white space between dots.
This fading technique involves reducing the pulse lengths (or else the amplitude) of the ejection voltage pulses to vary the volume of the droplets providing the pixels printed in the overlap region so that one printhead fades out as the other fades in, the sum of the print from the two heads producing the required optical density uniformly across the overlap.
The technique is not usable by other greyscale inkjet technologies, whose ejection is limited to a fixed set of droplet sizes as it requires a high level of variable droplet size control. The Tonejet® method as referred to above, by contrast, has the feature that the ejection volume is continuously, addressably, variable through the mechanism of pulse length control. In the Tonejet® method, for a given pixel level, a continuous-tone pulse value can be assigned to produce the desired dot size. Such calibrations are not possible for a conventional drop-on-demand (DOD) printhead whose drop volumes are quantised by chamber volume, nozzle size, etc.
Similar issues arise and the same solution can be used whether the printheads carry out printing in a single pass, printing the required pixels from multiple (interleaved) printheads closely spaced one behind another, or if the pixels are printed from multiple passes of the same or different printheads. The printhead(s) may be indexed multiple times.
In order to provide the required ‘fade’, a fading function for each printhead or swathe of print is used to define the profile of the fade across the overlap region. It is usual to restrict droplet volumes in printheads of the Tonejet® type to a number of predetermined sizes to simplify computations. In the method of the invention it is advantageous to provide a different fading function for different droplet volumes. This arises from the fact that the additive print density of pixels printed by two droplets follows a function which is non-linear with droplet volume.
The invention also includes apparatus for printing a two-dimensional bit-mapped image having a number of pixels per row, said apparatus having a plurality of overlapping printheads or a printhead or printheads indexed through overlapping positions, the or each printhead having a row of ejection channels, each ejection channel having associated ejection electrodes to which a voltage is applied in use sufficient to cause particulate concentrations to be formed from within a body of printing fluid, and wherein, in order to cause volumes of charged particulate concentrations of one of a number of predetermined volume sizes to be ejected as printed droplets from selected ejection channels of the overlapping printheads, voltage pulses of respective predetermined amplitude and duration, as determined by respective image pixel bit values, are applied to the electrodes of the selected ejection channels, characterised in that
The plurality of overlapping printheads may be fixed in position relative to one another in use.
The plurality of overlapping printheads may comprise a first printhead printing on a first pass over the print substrate and the same or another printhead printing on a later pass over the print substrate and overlapping in position with the position of the first printhead. The first printhead can be indexed between passes over the substrate by a distance equal to the width of the row of channels of the printhead less the desired overlap.
The printhead may be one of a number of identical printheads disposed in a module parallel to one another and offset by a proportion of the distance between adjacent ejection channels whereby the printed image has a resolution greater than the distance between adjacent ejection channels. A plurality of said modules can be overlapped one with another to enable a print width greater than the width of an individual module. Alternatively, the module can be indexed between passes over the substrate by a distance equal to the width of the row of channels of a printhead less the desired overlap.
In the case of a single printhead, the printhead may be indexed by a proportion of the distance between adjacent ejection channels whereby the printed image has a resolution greater than the distance between adjacent ejection channels.
Preferably, the values of the voltage pulses to be applied to the overlapping printheads may be determined from a predetermined fading function dependent on the level of the predetermined volume sizes of the pixels to be printed in the overlapped region of the printheads.
The pixel bit values may be adjusted in dependence on the position of the pixel within an overlapped region of the printheads and in dependence on the predetermined volume size of the pixel, prior to conversion of the pixel values into voltage pulses of respective predetermined amplitude and duration to cause printing.
Alternatively, the pixel bit values of the image may be provided to printhead drive electronics which converts the values into voltage pulses, and the voltage pulse values are therein determined in dependence on the position of the pixel within an overlapped region of the printheads and in dependence on the predetermined volume size of the pixel, prior to being applied to the ejection electrodes of the printhead.
In a particular method, fading functions of the following form can be used to define the profile of the fade across the overlap region of two printheads/swathes of print A and B:
ƒA(x)=ƒmin+(1−ƒmin)(1−x)α
ƒB(x)=ƒmin+(1−ƒmin)·xα
Where
In colour printers the printheads of each colour may be provided with different fading functions. The overlap position between printheads of the different colours may also be different.
The fading function may additionally be adjusted, either randomly or according to a suitable waveform function, so as to move the centre point of the fade around within the area of overlap to ‘dither’, effectively, the stitching between the print swathes to still further reduce the observable artifacts.
The fading functions may be applied at one of a number of stages in the processing of the image for printing, for example:
The fading functions may be applied to the pixel value data in the form of a mathematical function in software, or in the form of a look-up table stored in the memory of the controlling computer, the data feed electronics or the pulse generation electronics.
Examples of methods and apparatus according to the present invention will now be described with reference to the accompanying drawings, in which:
The examples illustrated with reference to
In a variation (not shown) a single one of the modules as per
A further example of printhead (not shown) may utilise a single printhead indexed by substantially a quarter of the printhead width between passes to (a) provide (say) 600 dpi printing from a 150 dpi printhead, and (b) an overall print width much greater than the printhead width (the number of indexing motions and hence passes being determined by the desired overall print width. In this case, swathes of 150 dpi print from each pass are interleaved to create 600 dpi print. The overlap between 150 dpi swathes occurs between the first, fifth, ninth, etc. passes/indexations and stitching of the swathes correspondingly occurs between opposite ends of the (single) printhead on the first, fifth, ninth, etc. passes/indexations; similarly, overlap and stitching of 150 dpi swathes occurs between the second, sixth, tenth, etc. passes, between the third, seventh, eleventh, etc. passes and between the fourth, eighth, twelfth, etc. passes.
In all examples, a substrate position synchronisation signal (originating from, for example, a shaft encoder 216 (see
Before describing an example of the method according to the invention, it may be useful to describe the two methods generally usable to control the volume of droplets printed (or ejected) using the Tonejet® method.
The value of Pn of the bitmap pixel to be printed corresponds to a duty cycle (of the ejection pulse) between 0% and 100%. Typically, when printing at a resolution of 600 dpi and with relative motion between the print substrate and the printhead being at a speed of 1 ms−1, this equates to a pulse length of between 0 and 42 μm on a 42 μm pulse repetition period.
Of these alternative techniques, in practice it is simpler to modulate the duration of the pulse, but either technique may be appropriate in given circumstances and both may be used together.
In operation, in one example according to the invention, as shown in
Given the known number of strips or swathes of print which are required to be laid down, greyscale data for each primary colour is then stripped 213 into data sets—in this case two data sets 302A, 302B for one pair of overlapped print swathes or printheads 300A/300B to represent pixel values for each column of the individual printhead widths (number of pixels across the print substrate provided by a single printhead). These data sets provide bitmaps which correspond to the ejection channels 301 of the individual printheads 300A, 300B used to print the final image.
The pulse data 304A, 304B is then transferred in step 305A/305B, according to the relative position of the print substrate and the printheads (as determined by the shaft encoder 216), to the driver cards (pulse generator electronics) 306A, 306B in which the data is utilised to determine the length of the drive pulses applied to the individual printhead ejection channels 301 as required and in which voltage pulses of predetermined duration and/or amplitude are generated according to the pulse data for each pixel. The data is transferred in time-dependency on the substrate position and offset of the ejection channels 301 of one printhead 300A from those of the adjacent overlapping printhead 300B.
A process of generating and applying the fading functions will now be described in an example which uses four passes of two 150 channel per inch printheads overlapped to print a cylindrical substrate with the two overlapped heads spanning the width of the substrate, and the substrate being spun four times to achieve full coverage at 600 dpi. The fading technique described is directly applicable to the overlapped portions of multiple or single printheads making one or more passes over the substrate.
An overlap of 10 printhead channels (40 pixels) is used in the specific example described. However, the width of the overlap region will affect the visibility of the join: generally, the larger the overlap, the more the errors can be dispersed and the less visible the join. This has to be balanced with the desire for the smallest overlap to maximise the print width.
In order to prepare the required fading functions a series of test images were prepared using single printheads and printed with a selection of fading functions to experimentally determine the most effective. The image used was a benchmark test image that contains a full range of print levels. The image was screened using a standard 4-level error diffusion method, rendering the image in dot sizes of 0%, 50%, 75% and 100% of the maximum dot size that gives the required maximum optical density of print. Initial function parameters were estimated and then iterated twice until the print quality looked acceptable. The parameters were then determined to be as follows:
For information, the pulse length curves corresponding to the last iteration of the parameters are shown plotted in
As mentioned above, in this example, for each droplet volume size level, fading functions of the following form are used to define the profile of the fade across the overlap region of two printheads/swathes 300A, 300B of print A and B:
ƒA(x)=ƒmin+(1−ƒmin)(1−x)α
ƒB(x)=ƒmin+(1−ƒmin)·xα
Where
Examples of the fading functions are shown plotted in
The fading functions are applied to the image data by multiplying with the image pixel values. This is applied to the image data after screening, i.e. after the pixel values have otherwise been calculated, and may be applied in Raster Image Processing on a controlling computer or in the printhead drive electronics. As the fading function is dependent on the grey level/droplet volume size, the function to apply for a given pixel is chosen according the screened value of that pixel. For example, a 50% level pixel will be multiplied by the fading function for the 50% level, etc. A family of fading functions therefore exists that contains as many curves as there are non-zero droplet sizes in the screened image (e.g. 3 to a 4-level image; 7 for an 8-level image).
The pixel values that result from multiplying an image pixel of level PL by the fading function for that level are derived from the following:
Taking the generic fading function for one side (B):
ƒ(x)=ƒmin+(1−ƒmin)·xα
For each pixel level L in the screened image there is a fading function ƒL(x):
ƒL(x)=ƒmin
A pixel of level L in position x across the image is faded by multiplying its value PL by the fading function for its level:
P(x)=PL·ƒL(x)
P(x)=PL{ƒmin
P(x)=Pmin
where
PminL is a minimum desired pixel value, which is approximately the same whatever the original value PL of a pixel.
Hence, the pixel values that result from multiplying an image pixel of level PL by the fading function for that level are:
PA(x)=Pmin
PB(x)=Pmin
Where
| Number | Date | Country | Kind |
|---|---|---|---|
| 12169098 | May 2012 | EP | regional |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2013/063494 | 6/27/2013 | WO | 00 |
| Publishing Document | Publishing Date | Country | Kind |
|---|---|---|---|
| WO2013/175024 | 11/28/2013 | WO | A |
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