Inkjets printing heads are most extensively used, available in a variety of types and capacities. Generally, a printing head as denoted H in Fig. 1 a comprises a series of tiny ink-emitting nozzles N, and a computer programmed mechanism for controlling the timing of each and every one of the nozzles according to the pattern to be printed. The printed pattern, for example the letter a as shown in Fig. 1b, is printed by the head H while traveling over a substrate, usually a paper sheet P, by a series of dots each formed by one of the vertically arranged nozzles. The accumulation of the dots completes the requested pattern. Once the head completes a line or row over the paper sheet, the latter is pulled upwards and the head commences its travel in the opposite direction (Fig. 1 c) to print the requested patterns such as s at the beginning (end) of the next raw and the letter o at the end of this reverse travel.

This bi-directionai method of printing head displacements is of course destined to increase the speed of the printing process, which is the general trend in the design of printers by the relevant industry at large. However, due to mechanical limitations governed on the one hand by the reaction time of the printing head nozzles and on the other hand by travel speed of the printing head carriage, the printing speed has not been significantly increased despite all directed efforts.

It is therefore an object of the present invention to substantially increase the printing speed by inkjets printing heads of computerized printers.

It is a further object of the present invention to improve the quality of the printing and to avoid vacant spaces generated due to clogged or otherwise incomplete injection of ink by malfunctioned heads.

It is a still further object of the present invention to make use of groups of two or more printing heads, operating in unison and thus reducing and/or selectively distributing the working load of each individual printing head of such group.

It is a still further object of the invention to increase the printing speed by providing a matrix of printing heads capable of printing more than one row at a time.

It is a still further object of the invention to provide matrixes of the printing heads operating in parallel and/or in series and thus allowing the distribution of the printing job in a selectively programmed form.

SUMMARY OF THE INVENTION According to one aspect of the invention there is provided a method of operating printing heads, particularly of the ink-jets type, comprising the steps of providing a series of at-least two printing heads; mounting the heads on a common carriage in a side-by-side, at least partly sidewise overlapping relative position, and activating the heads for printing on a substrate in a programmed periodical sequence.

According to another aspect of the invention there is provided a computerized printer comprising a matrix of printing heads operable in unison.

BRIEF DESCRIPTION OF THE DRAWINGS These and additional features and advantages of the invention will become more clearly understood in the light of the ensuring description of preferred embodiments thereof, given by way of example only with reference to the accompanying drawings, wherein- Fig. 1a schematicaity shows the face side of a typical ink-jets printing head; Fig. 1 b illustrates the conventional printing process of a first row ; Fig. le illustrates the conventional printing process of a second, following row; Fig. 2a shows an assembly of four printing heads, mounted onto a common carriage, illustrating a first arrangement according to the principles of the present invention; Fig. 2b illustrates the printing process according to a first mode of operation; Fig. 3 exemplifies the printing of certain symbols, assuming (for illustration purposes only) that the symbols are composed of a series of single dots only; Fig. 4 illustrates the use of the first mode of operation for printing the symbols of Fig. 3; Fig. 5 iIllustrates a second mode of operation; Fig. 6 illustrates a third mode of operation; Fig. 7 shows a second assembly arrangement of printing heads applicable to the method of the present invention; and Fig. 8 shows a third assembly arrangement of printing heads applicable to the method of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to Figs. 2a and 2b, there is shown that more than one head--4 in the present example, denoted A, B, C and D--have been combined and mounted onto the same carriage (not shown).

In operation, as most schematically and exaggerated illustrated in Fig. 2b, the heads are so programmed that ink-ejection cycles A-B-C-D-A and so forth are maintained, namely in exact succession (hereinafter"Mode 1").

It will be readily understood that the Mode 1 printing speed is multiplied by four (or other number equal to the number of heads) without diluting the (longitudinal) density of dots. The vertical density remains the same.

For illustrating the advantages and diversification capabilities, attainable by this mode of operation, reference shall be first made to Figs.

3 and 4.

In Fig. 3 there are shown the letters a, b and c. It is assumed, for the sake of explanation only, that the letters are composed of singular, discrete dots oniy-which of course is not the case in reality.

As most clearly seen in Fig. 4, the utilization of Mode 1 is such that the first column is produced by head A, the second by head, the third by C and the fourth by D, and so forth in the same sequence.

The main avantage derived is, as above said, the multiplication of the carriage speed, and hence the reduction of overall printing duration.

In printing processes, it sometimes occurs that the dots must be applied more than once on the same spot. This is required if the substrate is of an absorbent, fibrous or porous material such as cardboard or even cloth; or if certain parts of a text, or portions of colored prints are to be emphasized. Conventionally, in such cases, double or triple printing is applied, causing waste of time.

Mode 2 of the present invention method presents an effective solution, as exemplified in Fig., 5, Hence, since every portion of the printed area is passed-over by more than one head, it is most convenient to program the system so that two (or more) heads will eject the ink on one and the same spot.

Using the letters example of the previous example, it is shown that a pair of heads, say A+C and B+D are associated and used for the application of"double-printing", as explained.

Fig. 6 illustrates yet another mode of operation-Mode 3.

This mode contributes to the art of printing in the following manner. It may occur, during the printing process, that one or more of the nozzies become clogged or otherwise cease to function properly. In conventional systems, the complete head has to be replace, should the quality (density) of the print be retained.

In the usage mode exemplified in Fig. 6, pairs (or more) of the series of heads are linked in such a fashion that one can! enlace the other <BR> <BR> <BR> <BR> regarding any given point. Thus, per every column, a dot can be applied by either one of the pair of associated heads, in a selective, i. e. programmed, manner.

Mode 3 therefore allows for real time surveillance of the printing process, e. g. by using suitable scanners ; once a local quality deterioration is observed, say because one of head A nozzle stopped working, a command will be transmitted to head C to"take over"and appiy the ink to the same spot, previously fed by head A; and the same with regard to the pair of B and D heads.

The"multi-head"concept herein proposed is readily extendable in another direction schematically presented in Fig. 7.

Here, a matrix of 4x3 heads is assemble, driven by a common carriage (not shown).

All the attributes discussed above are applicable, in addition to, that 3 rows at a time are produced. The advancement speed of the paper sheet is trio) ed. besides the time gain of the printing process proper.

In the embodiment illustrated in Fig. 8, the heads A to D are not alligned one with respect to the other, resulting in a rhombus or rhomboid pattemed matrixes. Thus, a part of head is sidewis overlapped by head S; C overlaps A and B; and D overlaps t !, B and C. This arrangement attains most of the benefits of the Fig. 7 arrangement, but with a considerably smaller number of heads.

It should be emphasized that all heads of every group as heretofore described may be of the same color, of different colors, or of the multi-color type, at the designer's option.

The advantages of the method are most striking in printing colored patterns. Hence, the four heads group is the most efficient, each head containing ink of one of the four basic colors (cyano, magenta, yellow and black) used in the colors separation technique.

While the above description contains many specificities, these should not be construed as limitations on the scope of the invention, but rather as exemplification of the preferred embodiments, Those skilled in the art will envision other possible variations that are within its scope. Accordingly, the scope of the invention should be determined not by the embodiment illustrated, but by the appended claims and their legal equivalents.