Staggered multi-phase firing of nozzle heads for a printer

Details Staggered multi-phase firing of nozzle heads for a printer Staggered multi-phase firing of nozzle heads for a printer

2003-04-10

2003-12-30

Nguyen, Thinh (Department: 2861)

Incremental printing of symbolic information

Ink jet

Ejector mechanism

Reexamination Certificate

active

06669330

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to apparatus and methods for printing and in particular to drop-on-demand (DOD) inkjet printing methods and apparatus.
BACKGROUND OF THE INVENTION
When DOD inkjet is considered, two main groups can be discerned: thermal inkjet and piezo inkjet.
With thermal inkjet technology, tiny resistors rapidly heat a thin layer of liquid ink. The heated ink causes a vapour bubble to be formed, expelling or ejecting drops of ink through nozzles and placing them precisely on a surface to form text or images. As the bubble collapses, it creates a vacuum that pulls in fresh ink. This process is repeated thousands of times per second. With thermal inkjet technology, water-based inks are used.
Piezoelectric printing technology—commonly called piezo—pumps ink through nozzles using pressure, like a squirt gun. A piezo crystal used as a very precise pump places ink onto the printing medium. A wide range of ink formulations (solvent, water, UV) may be used.
A number of different piezo concepts exist.
A typical concept, as described in U.S. Pat. No. 4,887,100, WO 96/10488, WO 97/04963 and WO 99/12738, uses so called shared walls. The pressure chambers containing the ink are next to each other, while their dividing walls are the actuators.
Because an actuator is always shared by two channels, it is not possible to jet a drop out of two neighbouring channels at the same time. In WO 96/10488 is described that the nozzles are divided in three interlaced groups (A, B, C). Neighbouring nozzles are fired in a sequence ABC. Two solutions are possible to print dots on a straight line.
A first solution uses a complete nozzle array under a certain angle. By doing this, the resolution is increased, and by using the right fast scan speed, dots fired in a sequence A, B, C are on a straight line.
A second solution uses a head perpendicular to the fast scan direction, in which the A, B, and C nozzles are staggered in the fast scan direction. Printing of a line of pixels is divided into three cycles. In the first cycle, the dividing walls to either side of the A channels are driven (if ink is to be ejected from them—depending on the image to be printed) with a pulsed signal. In the second cycle, the dividing walls to either side of the B channels are driven (if ink is to be ejected from them—depending on the image to be printed) with a pulsed signal. In the third cycle, the dividing walls to either side of the C channels are driven (if ink is to be ejected from them—depending on the image to be printed) with a pulsed signal. The pressure pulses developed in the channels that are not included in the current cycle are not larger than ½ of those in the channels that are intended to eject ink. The printing apparatus is arranged so that such pulses with ½ magnitude do not cause ink ejection.
A drawback of this concept is that, once the firing frequency is defined, only one fast scan speed can be used to print ABC dots on a straight line, as explained hereinafter. In the fast scan direction, the head will e.g. print each {fraction (1/360)}-inch.
FIG. 1
shows a piezo printhead
10
according to the prior art, having nozzles
12
which are divided into three sets, called a set of A nozzles, a set of B nozzles and a set of C nozzles, each set intended to be fired during different firing cycles. The different sets of nozzles are staggered with respect to each other over a stagger distance D
1
in the fast scan direction. If the nozzles are divided in groups G of three, every first nozzle is part of the set of A nozzles, every second nozzle is part of the set of B nozzles and every third nozzle is part of the set of C nozzles. All nozzles in one set A, B, C are positioned on a straight line in the slow scan direction S, which lines are located at the stagger distance D
1
with respect to each other in the fast scan direction F.
As an example, printhead 10 is considered to be a type
360
head. This means that the printhead 10 is provided for printing 360 dpi (=pixels per inch) in the fast scan direction F. In this type
360
printhead
10
, the distance D
1
between nozzles
12
in the fast scan direction F is {fraction (1/360)} inch/3=70.56 &mgr;m 3=23.52 &mgr;m.
If the firing frequency is 12.4 kHz, meaning that every set A, B, C of nozzles can be fired every 80.65 &mgr;, the speed of the printhead
10
in the fast scan direction F is {fraction (1/360)} inch*12.4 kHz=0.875 m/s. The nozzles
12
are fired in an ABC sequence, with the A nozzles at the leading edge of the printhead 10 in the fast scan direction F.
The cycle frequency is 12.4 kHz*3=37.2 kHz. Or formulated in another way: the set of B nozzles fires 26.88 &mgr;s after the set of A nozzles, and the set of C nozzles fires 53.76 &mgr;s after the set of A nozzles. After 80.65 &mgr;s, the set of A nozzles fires again.
One type of printing may be called “mutually interstitial printing”, also called shingling e.g. as in U.S. Pat. No. 4,967,203, in which adjacent pixels on a raster line in the fast scan direction are not printed by the same nozzle in the printhead. Printing dictionaries, however, refer to “shingling” as a method to compensate for creep in bookmaking. The inventors are not aware of any industrially accepted term for the printing method wherein no adjacent pixels on a raster line are printed by one and the same nozzle. Therefore, from here on and in what follows, the terms “mutually interstitial printing” or “interstitial mutually interspersed printing” are used. It is meant by these terms that an image to be printed is split up in a set of sub-images, each sub-image comprising printed parts and spaces, and wherein at least a part of the spaces in one printed sub-image form a location for the printed parts of another sub-image, and vice versa.
When it would be desired to keep the same firing frequency, but to print a 180*180 dpi image with the 360 type printhead of the example given above, the printhead speed should theoretically double to 1.750 m/s. In the above case of printing a 180*180 dpi image with a 360 type printhead, where the printhead speed must double to 1.750 m/s, the delays for firing B and C need to be shorter to make sure that dots are printed on the same line. Nozzle set B has to be fired 13.44 &mgr;s after nozzle set A, and nozzle set C 26.88 &mgr;s after nozzle set A. These firing frequencies are too close one to the other, and therefore a 360 type printhead cannot be used to print a 180*180 dpi image.
When it would be desired, on the other hand, to print a 720*720 dpi image with the 360 type printhead, the firing delay between the set of A nozzles, set of B nozzles and set of C nozzles increases to 53.76 &mgr;. As, however, after 80.65 &mgr;the set of A nozzles has to fire again, there is not enough time left to fire the set of C nozzles, and therefore a 360 type printhead cannot be used to print a 720*720 dpi image neither.
It is an object of the present invention to provide a method for printing, with one type of printhead, with a resolution which differs from the design resolution of the type of printhead used.
SUMMARY OF THE INVENTION
The above objective is accomplished by a method of driving a print head according to the present invention. A print head used has a longitudinal axis in a slow scan direction and has an array of marking elements comprising at least one group of marking elements. Marking elements of one group are staggered with respect to each other over a stagger distance in a fast scan direction, which is perpendicular to the slow scan direction. The print head is intended to be driven with a reference velocity Vref, which is equal to the stagger distance, multiplied by a reference firing frequency Fref. One marking element of a group is able to be fired at each reference firing frequency pulse (whether it fires depends upon the image to be printed). The marking elements of the print head are intended to be fired according to a reference firing order to print an image with a first resolution. The method of the present

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