Incremental printing of symbolic information – Ink jet – Ejector mechanism
Reexamination Certificate
1999-11-15
2001-08-28
Barlow, John (Department: 2853)
Incremental printing of symbolic information
Ink jet
Ejector mechanism
C347S007000, C347S011000
Reexamination Certificate
active
06281913
ABSTRACT:
The present invention relates to methods of operating pulsed droplet deposition apparatus, in particular an inkjet printhead, comprising an array of parallel channels disposed side-by-side, a series of nozzles which communicate respectively with said channels for ejection of droplets therefrom; connection means for connecting the channels with a source of droplet fluid; and electrically actuable means for ejecting a droplet from a selected channel.
Such apparatus is known, for example, from W095/25011, U.S. Pat. No. 5,227,813 and EP-A-0 422 870 (all incorporated herein by reference) and in which the channels are separated one from the next by side walls which extend in the lengthwise direction of the channels and which can be displaced in response to the actuating signal. The electrically actuable means typically comprise piezoelectric material in at least some of the side walls.
The last of the aforementioned documents discloses the concept of “multipulse greyscale printing”: firing a variable number of ink droplets from a single channel within a short period of time, the droplets merging (in flight and/or on the paper) to form a correspondingly variable size printed dot on the paper.
FIG. 1
is taken from the aforementioned EP-A-0 422 870 and illustrates diagrammatically droplet ejection from ten neighbouring printhead channels ejecting varying numbers (64,60,55,40,etc.) of droplets. The regular spacing of successive droplets ejected from any one channel indicates that the ejection velocity of successive droplets is constant. It will also be noted that this spacing is the same for channels ejecting a high number of droplets as for channels ejecting a low number of droplets.
In the course of experiment, several deviations from the behaviour described in EP-A-0 422 870 have been discovered.
The first finding is that the first droplet to be ejected from a given channel is slowed by air resistance and may find itself hit from behind by subsequently ejected droplets travelling in its slipstream and therefore subject to less air drag. First and subsequent droplets may then merge to form a single, large drop.
The second finding is that the velocity of such a single, large drop will vary depending on the total number of droplets ejected in one go from a given channel. This is not a desirable condition: as is generally known, variation in drop velocity leads to dot placement errors.
A third finding relates to three-cycle operation of the printhead—described, for example in EP-A-0 376 532—in which successive channels in a printhead are alternately assigned to one of three groups. Each group is enabled in turn, with enabled channels ejecting one or more droplets in accordance with incoming print data as described above. It has been discovered that the velocity of the single, large drop formed by the merging of such droplets will vary depending on whether the adjacent channel in the same group is also being operated (i.e. 1 in 3 channels) or whether only the next-but-one channel in the same group is being operated (i.e. 1 in 6 channels).
These findings are illustrated in
FIG. 2
which shows the velocity U of the first drop to hit the paper (which may be a single droplet or a large drop made up of several merged droplets) against the total duration T of a draw-reinforce-release (DRR) actuating waveform. Such a waveform—well known in the art—is illustrated in
FIG. 3
a
and places a printhead channel initially in an expanded condition (a “draw” as at E), subsequently switches to a contracted condition (a “reinforce” as at RF) and then “releases” (as at RL) the channel back to its original condition. As shown in
FIG. 3
a
, the draw and reinforce periods of the waveform used to obtain
FIG. 2
are equal and have a peak-to-peak amplitude of 40V (this need not necessarily be the case, however). Each repetition of the waveform results in the ejection of one droplet and, as shown in
FIG. 3
b
, the waveform may be repeated several times in immediate succession so as to eject several droplets (“droplets per dot” or “dpd”) and form a correspondingly sized dot on the paper. It will be appreciated that this step is repeated for each channel every time the group to which it belongs is enabled and the incoming print data is such that it is required to print a dot. In the experiment used to obtain the data shown in
FIG. 2
, channels were repeatedly enabled—and dots were printed—at a frequency of 60Hz.
It will be seen that the application of a single DRR waveform of around 4.5 &mgr;s duration (to eject a single droplet i.e. 1 dpd) will result in a velocity of approximately to 12 m/s per second if only alternate channels in a group are fired (1 in 6 operation) whereas a velocity of around 14 m/s results if every channel in a group is fired (1 in 3 operation). The velocity is that measured shortly before the drop hits the paper and after any merging has taken place. However, applying the same waveform seven times in immediate succession (7 dpd) so as to eject seven droplets results in a velocity of around 37 m/s when operated “1 in 3” and a velocity of around 25 m/s when operated “1 in 6”.
Such wide variations in velocity could give rise to significant dot placement errors. The present invention at least in its preferred embodiments has as an objective the avoidance of such dot placement errors when generated by the newly-discovered phenomenon described above.
FIG. 1
is a diagram that illustrates prior art droplet ejection from ten neighboring printhead channels ejecting varying numbers of droplets.
FIG. 2
is a chart plotting first drop velocity against total time duration of a draw-reinforced-release (DRR) actuating waveform.
FIG. 3
a
illustrates a prior art DRR waveform as represented in FIG.
2
.
FIG. 3
b
illustrates the waveform shown in
FIG. 3A
repeated a number of times in immediate succession so as to eject several droplets per printed dot.
FIG. 4
is a chart of data obtained utilizing a DDR waveform shown in FIG.
3
A.
FIG. 5
is a chart plotting first and second ejected droplet velocity against total waveform duration for a printhead of the type used to obtain the data shown in FIG.
2
.
FIG. 6
is a diagram that illustrates the variation of peak-to-peak waveform amplitude or voltage against increasing contraction duration (DR) to achieve a droplet ejection velocity of 5 m/s.
FIG. 7
illustrates an actuating waveform utilized to obtain the diagram shown in
FIG. 6
with actuating voltage indicated on the ordinate and normalized time on the abscissa.
FIG. 8
is a chart plotting a variation in droplet ejection velocity against peak-to-peak amplitude or voltage for a particular droplet ejection regime.
FIG. 9
illustrates a prior art non-ejecting waveform shape.
FIG. 10
a
illustrates an example of ejecting non-actuation waveforms that can be applied to three neighboring channels of three successively-enabled channel groups A, B, and C.
FIG. 10
b
illustrates a corresponding voltage waveform applied to the channel electrodes of the three neighboring channels to generate the actuating waveforms shown in
FIG. 10
a.
FIG. 11
is a chart plotting droplet ejection velocity against peak-to-peak amplitude or voltage and shows the effect of varying an offset “P” for the voltage pulse applied to a channel to an enabled channel group relative to a voltage pulse applied to neighboring channels belonging to non-enabled groups.
FIG. 12
is a diagram that illustrates performance of a printhead used to obtain the chart data of
FIG. 8
when operated using a non-ejecting waveform having an offset P=0.35.
FIG. 13
illustrates an enlarged view of part of the diagram of
FIG. 12
showing an operating window of approximately 3.6 volts.
Accordingly, the present invention consists in a first aspect in a method of operating an inkjet printhead for printing on a substrate, the printhead having an array of channels; a series of nozzles which communicate respectively with said channels for ejection of droplets therefrom; connection means for connecting the channels with a source of ink; and electrically a
Barlow John
Marshall O'Toole Gerstein Murray & Borun
Stewart Jr. Charles W.
Xaar Technology Limited
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