The present invention relates to continuous ink jet printing and, more particularly to such printing when applied to the printing of bar codes such as those used in retailing or by postal authorities.

Continuous ink jet printers are widely used for industrial coding and marking applications. A jet of ink is broken into a regular stream of uniform ink droplets which is then passed through a transverse electric field provided across a pair of deflector plates. In general droplets are not charged and are caught in a gutter to be recycled. If a droplet is charged, it is deflected from the recycling path by the electric field and is printed. Since the field across the deflector plates is constant, the deflection is proportional to the magnitude of the charge. The object or substrate being printed is generally moved at right angles to the deflection direction and this, together with deflection of droplets by varying amounts, provides the means for placing droplets at the desired position in a two dimensional array.

There are aerodynamic and electrostatic interactions between closely spaced droplets in flight and these lead to distortion of the printed image. At low speed printing, only occasional droplets will be charged (since the droplets are generated at the same speed regardless of the speed of printing and the characters or symbols being produced) and there is no problem. With high speed printing, however, as many droplets as possible must be printed. A common raster, or sequence of printable (but not necessarily printed) and non-printable (uncharged) droplets (known as guard droplets) , is used to print each stroke of a line of print, in order to provide an acceptable linear response. Since equal spacing is required between strokes of printed droplets on the substrate, each stroke of print is produced by the same number and sequence of printable and non-printable

droplets, but the actual printable droplets which are printed will vary from stroke to stroke. The linearity of the strokes is particularly important when bar codes are being printed. Various techniques are already known for optimising the placement of droplets by using carefully designed rasters in which a predefined sequence of charged and non-charged droplets is used for each stroke. See for example EP-A-0206614.

For printing the lines of droplets required in bar codes for instance, table 1 shows an interleaved raster in which sequential droplets numbered 1 to 9 are printed in interleaved droplet positions as indicated. Drops 1 and 2, for instance are printed at positions 1 and 5 on the substrata so that their flight paths are widely separated and there is little or no interaction. At high speed, however, this tends to produce a ragged line as shown in figure 1 and this is unacceptable for barcoding.

An alternative approach is to use a straight raster with guard drops as illustrated by the 26 drop raster shown in table 2.

Diff rent segments of a vertical stroke may be printed to form a character by using uncharged drops rather than charged drops at the appropriate point within the raster. Postal bar codes, in particular, demand very high s eed operation and a selection of bars and half bars in the case of USPS Postnet code (figure 5) or four different bars for the British Post Office BPO 4 State Code (figure 4).

The speed constraints are such that with current machines, it is not possible to design a raster with few enough guard drops to attain the required performance. In practice, the first printhead drop is decelerated most and the second drop catches up with it, so that, without a guard drop, the first and second drop are likely to coalesce and form a single large drop at the beginning of the stroke as shown in figure 2. If this does not occur, the delay in the initial drop will manifest itself as a

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misalignment or hook at the begining of the stroke as shown in figure 3.

The present invention is intended to provide a solution to this problem, by enabling high speed and uniform lines to be printed.

According to the present invention, there is provided a method of high speed printing of bar codes with a continuous ink jet printer, in which bars of varying length or in varied positions are printed by using a different raster of drops for each bar or at least for different bars and by switching rasters in real time to print the appropriate bar.

One example of a method according to the present invention will now be described with reference to the accompanying drawings and tables- in which:-

Figure 1 illustrates a conventionally printed line of droplets for a bar code;

Figure 2 illustrates another conventionally printed line of droplets for a bar code; Figure 3 illustrates a further line of droplets for a bar code in accordance with a prior art technique;

Figures 4 & 5 illustrate BPO 4 state bar code strokes and USPO Postnet bar code strokes respectively produced by the technique of the present invention; Tables 1 and 2 show conventional rasters for printing bar codes; and

Tables 3 to 6 show rasters used in accordance with the present invention.

As mentioned above, it is well known that the first printed drop of a stroke suffers greatest from aerodynamic deceleration forces. It is found that if a series of drops are printed in sequence, an acceptable 13 drop line (as shown in the first bar in figure 4) may be printed with the 17 drop raster shown in table 3. Only 4 guard droplets are necessary. Use of this raster in the conventional way to print the eight drop bars or lines of figure 4 produces unacceptable print quality. However, it is possible to

design excellent rasters for the remaining strokes, especially with the freedom of an additional 5 guard drops.

For instance, the rasters of tables 4 and 5 provide excellent print quality, well within the specifications of both the postal bar codes mentioned above. The three dot bar shown in figure 4 may be printed using the same raster as that of table 5, without the final 5 drops (drop positions 9-13) being printed, ie drop numbers 8 onwards in the raster all being uncharged drops and thus not being deflected for printing. This raster provides the important guard drop between the first and second drops of the raster.

Table 6 illustrates the rasters used for printing the section of bar code shown in figure 4, a different raster being used for each bar of print.

By switching the raster used in real time according to the type of stroke to be printed, it is possible to reduce the number of drops produced from 26 to 17 in the examples cited which provides an approximately 35% increase in printing speed.

This increase in printing speed is of critical importance in meeting the postal bar code specifications with current continuous ink jet printing equipment.

Using a machine according to the invention, a host computer can download bytes of data, relating to the bars to be printed, to an ink jet printer and a memory in the printer which contains a raster sequence of droplets can then be accessed according to the respective types of bar and in order to provide respective commands to the printhead for printing of the required drops. For example, a high level byte of data can be downloaded to the printer and converted to two 7-bit bytes which are stored in a printer buffer. Bits l to 12 may provide a code for the various bars (of 12 dots) to be printed and bits 13 and 14 used to select the related raster. The buffer may be sequentially fed with data until full.

For printing, the bits stored in the buffer are read sequentially, bits 13 and 14 being used to select the respective raster and bits l to 12 read sequentially to enable charge voltages from a look-up table to be applied to the droplets in order to cause the drops to be deflected to the corrct position.

TABLE 1

TABLE 2

TABLE 3

TABLE 4

TABLE 5

TABLE_6