Methods of Operating a Printhead
The present invention relates to methods of operating a printhead to print a linear pattern on a substrate, the pattern comprising printed and unprinted areas of width equal to an integer multiple of a unit width.
A typical example of such a linear pattern is a barcode, as used to store information in a machine-readable format in many diverse applications.
Such barcodes are of necessity standardised: EAN barcodes, for example, are made up of bars and spaces whose widths are one, two, three or four times a unit width (or "module"). In the case of the EAN 13, a small barcode for primary product, this unit width is 330μm at unity magnification with tolerances of 101//m on the widths of individual bars and spaces, 49 /m on bar and space pairs and 96μm on whole characters. As the magnification factor is reduced, these tolerances become disproportionately tighter (an EAN 13 barcode magnified by the factor 0.8, for example, has a tolerance of only 32μm on bar and space pairs).
There have been developed various techniques for printing barcodes to the required tolerances, the most common of which employ an array of heaters
either to form an image directly on a heat-sensitive substrate or to cause the transfer of a printing agent from a ribbon onto a substrate. Such techniques
are restricted, however, particularly in the kind of substrate onto which they
can print.
It has been recognised that inkjet printing techniques suffer fewer limitations regarding substrate than do the thermal techniques mentioned above. It has also been recognised, however, that care will be required in the placement of inkjet ink droplets on the substrate if the necessary tolerances are to be met. The present application describes methods for operating a printhead that allow such tolerances to be achieved.
Accordingly, the present invention comprises in a first aspect a method of operating a printhead to print a linear pattern on a substrate,
the printhead and substrate moving relative to one another at a velocity,
v; the pattern comprising printed and unprinted areas of width in the direction of relative motion equal to an integer multiple, i, of a unit width, w; the printed areas being made up of dots of diameter, d, printable by the printhead at a frequency, f; the method comprising the step of printing ni dots to form a printed area, n- being given by the formula:
(n, -1 ). rn^v/f) + d = i.w
where rrii is an integer.
In a second aspect, the invention consists in a method of operating a
printhead to print a linear pattern on a substrate, the printhead and substrate moving relative to one another at a velocity,
v;
the pattern comprising printed and unprinted areas of width in the direction of relative motion equal to an integer multiples, i, of first and second unit widths, w., w2.
the printed areas being made up of dots of diameter, d, printable by the printhead at a frequency, f; the method comprising the steps of printing n1 dots to form a first printed area, n., being given by the formula
(n,, -1 ). mM.(v/f) + d = i. w1 and printing n2 dots to form a second printed area, n2 being given by the formula
(ni2-1 ). mi2.(v/f) + d = i. w2 where m. and m2 are integers.
A method according to a third aspect of the invention comprises in a method of operating a printhead to print a linear pattern on a substrate, the printhead comprising a linear array of dot printing means arranged at a pitch, p; the printhead and substrate moving relative to one another at a velocity,
v; the pattern comprising printed and unprinted areas of width in a direction orthogonal to the direction of relative motion equal to an integer
multiple, i, of a unit width, w; the printed areas being made up of dots of diameter, d;
the method comprising the step of printing n dots to form a printed area, n being given by the formula:
(n, -1 ).p + d = i.w
Further advantageous embodiments of the invention are set out in the description and dependent claims.
The invention will now be described by way of example by reference to the following diagrams, of which:
Figure 1 is a schematic illustration of a printhead printing a barcode in "picket fence" mode;
Figures 2 and 3 are detail views of a linear barcode pattern printed by a single dot printing means;
Figure 4 is a schematic illustration of a printhead printing a barcode in "ladder" mode;
As mentioned above, figure 1 schematically illustrates the printing of a barcode 10 by a printhead 20 in "picket fence" mode, i.e. with the barcode bars 60 perpendicular to the direction 30 of relative motion of the printhead and substrate 40. Printhead 20 comprises an array of dot printing means such as ink jet nozzles 25, shown larger than scale for purposes of clarity.
It has been recognised by the present inventor that the major issue in this mode is the timing of the printing of a row of dots, as this determines the widths of the bars and spaces. If the speed of relative motion between
printhead and substrate is v and the frequency at which ink droplets can be
ejected from the printhead is f, the resolution of dot placement in the direction of relative motion is (v/f). Thus to achieve a typical resolution of 180 dots per inch (dpi) equal to a dot spacing of 141μm in a printhead operating at a typical ink droplet ejection frequency of 4kHz, a relative velocity, v, of 22 inches/second is required.
It is noted that barcodes are most commonly printed on packages such as bottles or boxes as they move on a conveyor mechanism past a stationary inkjet printhead. In such a case, 22 inches/second would be the velocity of the packages on the conveyor.
Referring to figure 2, let ^ dots 50 be used to print a bar 60 of width i.w, where w is the module width and i=1 , 2,3,4, and let the centre-to-centre separation of the dots be;
S| = m,(v/f) where ΠTI is an integer equal to the ratio of the actual separation of the dots on the substrate to the separation of the dots on the substrate were they to be ejected from the printhead at a - nominally maximum - frequency, f. Unless indicated otherwise, all the examples shown utilise a value of nr.p1. It will be appreciated that ^ must also be an integer. From the figure it will be seen that
(n, -1 ). m,.(v/f) + d = i.w.
It has further been recognised that in the case of synchronous printhead
operation (i.e. the rows of dots are printed at a regular rate), the position of the edge of a bar - and therefore the width of the bar and the adjoining space -
can be controlled no more closely than v/f. Advantageous values of n, have been identified, however, at which tolerances on bar width, etc. referred to above are met. For example, at the value of (v/f) = 141μm referred to above and a printed dot diameter, d, of 220//m, bars of width equal to one, two, three
or four times 330/ m can be printed within tolerance by means of n. = 2,5,8 and 11 dots respectively.
From figure 2 it will also be evident that the widths of the spaces 70 between the bars are governed by integers q, according to the formula: q,.(v/f) - d = i.w. Advantageous values for the example above are q, = 5,8,11 and 14 giving tolerable space widths of one, two, three and four 330μm modules respectively. Figure 3 illustrates another scheme applicable to (v/f)=82.5μm (corresponding to one quarter of the module width 330μm) and d=165μm in which bars 60 and spaces 70 of width equal to one, two, three or four times 330μm can be printed within tolerance by means of n, = 3, 7, 11 and 15 dots and q, = 6,10, 14 and 18 spaces (effectively non-dot printing events) respectively. ITF is another barcode standard that utilises two widths of bar: narrow, having a module width, w of 1016μm at unity magnification and wide, having a module
width, w2, of 2540μm. For printing in "picket fence" mode, using similar terminology to that employed with regard to the EAN barcodes:
(n -1 ). mv(v/f) + d = i. w (nl2-1 ). m2.(v/f) + d = i. w2.
q (v/f) - d = i. w, ql2(v/f) - d = i. w2 A possible solution for (v/f) = 127μm (v=22 inches/second, f = 4403 Hz) and unity magnification is d=254μm, n = 7, nl2 = 17, m1 = m2 = 1 , q = 10, ql2 = 22. An alternative with smaller dots of d=170μm is achieved with n(1 = 11 , nl2 = 29, m1 = m2 = 1 , q = 14, ql2 = 32.
If the printed dots overlap too much, ink spread may be a problem. A desirable ratio of dot size to separation is around 1.4. One merit of the high degree of overlap of the drops in the direction of motion is that the overlap in the other direction, which is determined by the pitch of the ink droplet ejectors in the printhead - the "printhead resolution" - may not be critical.
As already mentioned, it may be desirable to print barcodes with magnification factors other than unity in which case other schemes of n, q and d can be determined from the above formulae by a simple process of trial and
error.
Figure 4 schematically illustrates the printing of an EAN barcode 10 by a printhead 20 in "ladder" mode, i.e. with the barcode bars 60 parallel to the direction 30 of relative motion of the printhead and substrate 40. With such configurations, a problem arises because the ink droplet ejectors 25 (shown enlarged for clarity in figure 4) of the printhead 20 may not align precisely with
the dot position required.
A similar analysis to that outlined above can be applied, with the pitch,
p, of the ink droplet ejectors in the printhead playing the role of (v/f). If t channels are fired to produce a bar of width i.w and q< ink droplet ejectors are left unfired in a space of width i.w, then:
(n, -1 ).p + d = i.w
(q; + 1 ).p - d = i.w for . =1 ,2,3,4.
Taking p=141μm and d=165μm as per the example shown in figure 3, an in-tolerance barcode having 0.855 magnification factor can be achieved with ni =2,4,6,8 and q; = 2,4,6,8. An in-tolerance barcode at a magnification factor of 1.28 can be achieved with n, =3,6,9,12 and q. = 3,6,9,12.
A barcode at a magnification factor of unity can be printed accurately using a printhead having ink droplet ejectors spaced at a pitch of 165μm (equivalent to 154 dpi) and a dot diameter of 210μm. n, and q- values are in both cases 2,4,6,8. Alternatively, ink ejectors mounted in a printhead at a pitch of 110μm (231 dpi) and printing dots of 150μm can provide tolerable barcodes when using n. and q. values both equal to 3,6,9,12.
The methods outlined above are particularly, but not exclusively, suited to inkjet printheads, particularly of the kind known from EP-A-0 277 703 (incorporated herein by reference) in which an ink chamber is bounded on at least one side by an actuator wall constructed from upper and lower portions of
piezoelectric material bonded together at their common surface and polarised
in the plane of the actuator wall in respective opposite senses. When subjected
to an electric field perpendicular to the direction of polarisation by electrodes located on opposite sides of the actuator wall, both upper and lower portions deform in shear towards the ink chamber, thereby causing the ejection of a droplet from the ink chamber via a nozzle. As is also commonly known - e.g. from EP-A-0 278 590, incorporated herein by reference - a number of actuator walls can be arranged in parallel to as to define ink channels between adjacent walls. Ink ejection is achieved by actuation of the walls on both sides of a channel, with each actuator wall being displaceable in opposite directions so as to effect droplet ejection from the channels located on either side of that actuator. Such "shared wall" operation is also known in the art and will not be discussed in further detail here.
It will be appreciated that the present invention has been described by way of examples only and that a wide variety of modifications can be made without departing from the scope of the invention.