Incremental printing of symbolic information – Ink jet – Ejector mechanism
Reexamination Certificate
2000-08-11
2002-10-15
Nguyen, Lamson (Department: 2861)
Incremental printing of symbolic information
Ink jet
Ejector mechanism
C347S012000
Reexamination Certificate
active
06464331
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a method of printing a substrate with an inkjet printing device which comprises at least one print head provided with at least two rows of nozzles, wherein substantially fixed locations on the substrate, which locations form a regular field of pixel rows and pixel columns, are provided with ink drops image-wise, the resolution of the pixel columns being greater than the resolution of the rows of nozzles, so that p, where p is equal to the quotient of the resolution of the pixel columns and the resolution of the rows of nozzles, is an integer greater than or equal to 2. The method comprises a first printing stage in which a strip of pixel rows is provided with ink drops, whereafter the print head is displaced in a direction substantially parallel to the pixel columns, and a second printing stage in which the strip is provided with supplementary ink drops. The present invention also relates to a printing device suitable for the use of the method.
A printing method is known from U.S. Pat. 5,640,183. A known problem in inkjet printing devices is that deviations of individual nozzles may cause disturbing faults in the printed image. For example, a nozzle deviation may result in ink drops leaving the nozzle at the wrong angle (“skew jets”), so that they occupy a different place on the substrate with respect to the center (the normal position) of the fixed locations (pixels), or result in ink drops with a deviant volume, so that too much or too little ink reaches the substrate. This method is used to mask the faults. The print heads used for the use of this method are provided with two rows of nozzles each having a resolution (number of nozzles per unit of length) equal to half the required printing resolution (number of fixed locations per unit of length) in a direction parallel to the pixel columns, and which together, by occupying an interlaced position with respect to one another, form a print head with the required printing resolution. Each row of nozzles of a print head is provided with a number of extra nozzles. When a strip of pixel rows of the substrate is printed by the known method, a series of successive nozzles is selected in the first printing stage from the set of the rows of nozzles of a print head, the number of nozzles in this series being equal to the total number of nozzles of the print head less the number of extra nozzles. If a print head is provided with two rows of 50 nozzles and 3 extra nozzles per row (so that the total number of nozzles is equal to 106), a series of 100 successive nozzles is selected with which a strip in a width of 100 adjoining pixel rows of the substrate is printed. After this first printing stage, a new series of 100 successive nozzles is selected from the available 106 nozzles of the print head. There are thus 7 different options for selecting a second series, i.e. the same series as used in the first printing stage and one of the other 6 possible series of 100 successive nozzles. A choice from these 7 options is made at random. After the choice has been made, the print head is displaced with respect to the substrate in a direction substantially parallel to the pixel columns over a distance corresponding to the selected second series of successive nozzles. The relevant strip is then provided with supplementary ink drops in the second printing stage. By printing each strip of pixel rows of the substrate with a plurality of sub-images, each of said images being printed by a series of successive nozzles chosen at random, any printing faults due to deviations of nozzles are distributed at random over the substrate so that they are less visible to the human eye.
A significant disadvantage of the known method is that as a result of the random choice there is an appreciable risk that a pixel row may be printed entirely with ink drops having the same fault, for example because they occupy a different position with respect to the normal position. Consequently, linear faults may occur in the image. The human eye is very sensitive to such linear faults and these faults are thus found to be disturbing in the printed image. A linear fault forms in any case if the first and second (and any following) series of successive nozzles are identical in the printing of a strip of pixel rows so that all the ink drops printed in one pixel row originate from one specific nozzle. It has also been found that within one print head there are many nozzles which have substantially the same deviations, i.e. they result in ink drops printed with the same fault. Consequently there is a considerable risk of linear faults when the known method is used.
Another disadvantage of the known method is that prior to the second and any following printing stages the substrate must be displaced very accurately over a distance which, depending on the choice of the second series of successive nozzles, varies at random with the width of 0, 1 or a number of pixel rows (a maximum of 6 in the above-described example). A shift of this kind is obtained by moving the paper with respect to the print head by means of a motor. These small shifts chosen at random mean that the paper transport must meet very stringent requirements with respect to accuracy.
Finally, the printing device productivity is reduced with respect to the maximum obtainable productivity since a number of nozzles in each row must be reserved as extra nozzles to make it possible for a random choice to be made for the second and any subsequent series of nozzles.
SUMMARY OF THE INVENTION
The object of the present invention is to obviate these disadvantages. To this end, a method has been developed wherein the print head is displaced over a distance being equal to the width of a number of pixel rows selected from the set:
±(
i+kp
) (formula 1)
where i is the set of integers greater than or equal to 1 and less than or equal to (p−1), k is a natural number and p is equal to the quotient of the resolution of pixel columns and the resolution of rows of nozzles, wherein the resolution of the pixel columns is greater than the resolution of the rows of nozzles.
The present method is based on the realization that it is better to use the systematics of the deviations of the nozzles of the print head to mask printing faults than try to break through the same by a random choice as known from U.S. Pat. No. 5,640,183. The systematics governing the deviations of the nozzles may comprise a number of distinguishable forms of regularity.
First, it has been found that the deviation of the nozzle is substantially constant in time, irrespective of the intensity of the use of the nozzle. In other words, a nozzle will impart substantially the same fault to each drop ejected during the life of the print head. In addition, the deviations of the different nozzles within one row of a specific print head have been found to be not independent of one another in many types of print heads. It has been found that the deviation of an individual nozzle is substantially equal to the deviations of the adjoining nozzles within the same row. For example, if nozzle i in the first row of a print head has a deviation resulting in an ink drop originating from the same nozzle deviating from the normal position on the substrate by a distance of 20 &mgr;m, then the ink drops originating from the nozzles i−1 and i+1 will result in ink drops differing by about 20 &mgr;m from the normal position. It has also been found that the deviations of the individual nozzles within one row frequently have a slow progression, so that not only the directly adjoining nozzles within one row have substantially the same deviations, but also the more distant nozzles. The deviation progress may also be said to be periodic, so that even nozzles very far away from one another have practically the same deviation. As a result of these forms of regularity, there may be many nozzles within one row which exhibit substantially the same deviations. The reason for this regularity is not entirely clear.
De Grijs Eduard Theodorus Hendricus
van Doorn André
Weijkamp Clemens Theodorus
Westdijk Jacob Albert
Birch & Stewart Kolasch & Birch, LLP
Nguyen Lamson
Oce--Technologies B.V.
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