Incremental printing of symbolic information – Ink jet – Controller
Reexamination Certificate
1998-09-11
2003-05-27
Fuller, Benjamin R. (Department: 2855)
Incremental printing of symbolic information
Ink jet
Controller
Reexamination Certificate
active
06568779
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to methods of operation of droplet deposition apparatus, particularly inkjet printheads, comprising a chamber supplied with droplet fluid and communicating with a nozzle for ejection of droplets therefrom; and means actuable by electrical signals to vary the volume of said chamber, volume variation sufficient to effect droplet ejection being effected in accordance with droplet ejection input data.
DESCRIPTION OF RELATED ART
Apparatus of this kind is well known in the art. EP-A-0 364 136 shows a printhead formed with a number of ink channels bounded on both sides by piezoelectric side walls which deflect in the direction of an electric field applied by electrodes on the wall surfaces, thereby to reduce the volume of the ink channel and eject a droplet from an associated nozzle.
Unlike ‘thermal’ printheads in which each ink channel is provided with a heater that can be actuated so as to generate a bubble of vapour which pushes ink out of the channel via an associated nozzle, there is no need for ‘variable volume chamber’ printheads of the kind described above to heat the ink in the channel.
However, the present inventors have discovered that heating of ink in the chambers of a ‘variable volume chamber’ printhead can take place, particularly when it is operated at high frequency.
FIG. 1
of the accompanying drawings is a plot of droplet ejection velocity U against the amplitude V of the electrical signal applied to the piezoelectric side walls of a channel in a printhead of the kind shown in the aforementioned EP-A-0 364 136. Plot A corresponds to a droplet ejection rate of one drop every droplet ejection period, with each droplet ejection period lasting 0.25 milliseconds, whilst plot B corresponds to a droplet ejection rate of one drop every 66 droplet ejection periods. It will be seen that for a given amplitude V of the electrical signal, a significantly faster droplet will be ejected by the printhead when operating at the higher ejection rate than at the lower ejection rate. Such a velocity increase is attributable to a decrease in viscous losses during the droplet ejection process due to a reduction in the viscosity of the ink. This in turn is the result of an increase in the temperature of the ink between the two operating conditions A and B caused by the heating of ink in the channel which, it is believed, is due to inefficiencies in the printhead.
It will be appreciated that droplet ejection velocity has to be taken into account when synchronising droplet ejection from the printhead with the movement of the substrate relative to the printhead and that any variation in velocity will manifest itself as droplet placement errors in the final print For example, the drop placement tolerance is frequently specified as one quarter of a drop pitch. Thus for a print matrix density of 360 dots per inch, the drop placement tolerance will be &Dgr;X=18 &mgr;m. The variation in droplet ejection velocity, &Dgr;U is related to the dot placement tolerance by the formula
&Dgr;
U=U
d
2
.&Dgr;X/h.U
h
where h is the flight path length (typically 1.0 mm), Uh is the printhead velocity relative to the print substrate (typically 0.7 ms
−1
) and Ud is the mean droplet ejection velocity.
For mean droplet ejection velocities of 5,10 and 15 ms
−1
, the maximum acceptable variation in droplet ejection velocity is 0.65, 2.6 and 5.8 ms
−1
respectively. Thus there is a substantially greater allowable tolerance in the drop velocity when the mean droplet ejection velocity takes a value greater than 5 ms
−1
.
On the other hand, there is maximum droplet ejection velocity (‘threshold velocity’), U
thr
, which corresponds to the onset of capillary instability. In variable-volume (piezoelectric) printers, the inventors have found U
thr
to be usually in the range 12-15 ms
−1
when continuous high frequency droplet ejection is sustained, although higher droplet ejection velocities can be obtained during short bursts of drop formation.
It will also be appreciated that the rate at which a particular chamber in a printhead is actuated will depend on the incoming droplet ejection input data (which will be determined, by the image to be printed and generally vary from high to low). Thus in a printhead having a chamber operating in accordance with FIG.
1
and at a given amplitude—for example 35—of electrical signal V, droplet ejection input data causing the chamber to eject droplets frequently (equivalent to plot A) will result in a droplet velocity of 15 m/s whilst subsequent input data may only cause the chamber to eject droplets at a lower rate (equivalent to plot B) and consequently at a much reduced velocity of 2 m/s. Such a large (750%) variation in ejection velocity will clearly lead to inaccuracies in the placement of the droplets and a reduction in the quality of the printed image. Such an error may occur for every chamber in a multi-chamber printhead. The degree of difference between these two conditions increases with ink viscosity and also with operating frequency, making the control of this effect particularly important in high speed printers.
It will also be evident from
FIG. 1
that there is only a narrow range of magnitude V of actuation waveform—denoted W—over which droplet ejection at both high and low rates can be guaranteed. This in turn severely inhibits the operational flexibility of the printhead.
SUMMARY OF THE INVENTION
According to one aspect of the present invention, these problems are solved at least in preferred embodiments by a method of operation of droplet deposition apparatus comprising a chamber supplied with droplet fluid and communicating with a nozzle for ejection of droplets therefrom; and actuator means actuable by electrical signals to vary the volume of said chamber; volume variation sufficient to effect droplet ejection being effected in accordance with droplet ejection input data; the method comprising the steps of controlling said electrical signals such that the temperature of the droplet fluid in said chamber remains substantially independent of variations in the droplet ejection input data.
Such a method can avoid velocity variations between enabled channels due to variations in ink viscosity which in turn are attributable to temperature variants caused by differential actuation rates. Differential actuation rates are of course a result of differences in the droplet ejection input data between enabled channels.
This aspect of the present invention also comprises the method of operation of droplet deposition apparatus comprising first and second chambers each supplied with droplet fluid and communicating with a nozzle for ejection of droplets therefrom and having actuator means actuable by electrical signals to effect droplet ejection selectively from said chambers in accordance with droplet ejection input data; the method comprising operating said actuator means to effect droplet ejection from the first chamber but not from the second chamber, and selectively electrically heating the fluid in the second chamber to reduce the difference in temperature between fluid in the second chamber and fluid in the first chamber.
Again, by reducing variation in the temperature of the droplet fluid between first and second chamber, viscosity-related droplet ejection speed differences can be reduced.
Thus again according to the invention there is provided a method of operation of droplet deposition apparatus comprising a chamber supplied with droplet fluid and communicating with a nozzle for ejection of droplets therefrom; and actuator means actuable by electrical signals to effect droplet ejection from the chamber in accordance with droplet ejection input data; the method comprising controlling said electrical signals such that the maximum droplet ejection velocity lies just below a threshold velocity (U
thr
), as hereinbefore defined and the variation in the droplet ejection velocity due to variations in the temperature of the droplet fluid in said chamber lies within a range deter
Pulman Robert Mark
Temple Stephen
Webb Laura Ann
Dickens Charlene
Fuller Benjamin R.
Marshall Gerstein & Borun
XAAR Technology Limited
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