Incremental printing of symbolic information – Ink jet – Ejector mechanism
Reexamination Certificate
1998-01-20
2001-08-28
Le, N. (Department: 2861)
Incremental printing of symbolic information
Ink jet
Ejector mechanism
Reexamination Certificate
active
06280023
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to so-called ‘continuous ink-jet’ (CIJ) printers, in which lines of printed droplets are printed on to a substrate after having been electrostatically charged and then deflected in accordance with the charge level.
BACKGROUND OF THE INVENTION
In continuous ink-jet printers,each character of print is made up of plural lines of droplets which extend in a direction transverse to the direction of relative movement between the printer and the substrate. Each line is printed from a so-called ‘raster’ of droplets in which each printable droplet has a defined print position. Non-printable ‘guard’ droplets separate the printable droplets and the printable droplets are either printed or not, depending on the character being formed.
Whilst such printers are capable of printing at very high speeds (droplets may be generated at 64kHz or even 128kHz), a particular problem often exists with the quality of printing produced by such printers, particularly where they are being used to print, from a single nozzle, a wide variety of very different characters or different character fonts. The characters cannot readily be printed to ‘near letter quality’ (NLQ), unlike so-called ‘drop-on-demand’ (DOD) ink-jet printers which use an array of closely spaced nozzles, with ink droplets from plural nozzles being used to make up each line of each character. Often, the lines of characters are slightly sloped, which itself may be undesirable, but the more significant problem is simply that the droplet resolution is not sufficiently high to allow characters to be well and fully shaped.
There has long been a desire to overcome this problem and techniques involving the use of multiple CIJ printheads have been proposed as one solution to the problem. Whilst reasonable resolution may be achieved, the increased cost and complexity of such printers rarely makes their use viable, particularly in marking and coding applications.
U.S. Pat. No. 4,670,761 discloses a continuous ink-jet character printing method which comprises providing a stream of droplets from a nozzle of a continuous ink-jet printhead; moving a print substrate past the printhead, said printhead having a pair of deflection electrodes for deflecting individual charged droplets from said stream of droplets to a required print position on the substrate; determining the speed of movement of the print substrate relative to the printhead; said deflection electrodes being disposed at a preselected angle relative to the path of movement of said print substrate, wherein the droplets are charged in dependence upon the speed of the substrate relative to the printhead to determine their print position.
SUMMARY OF THE INVENTION
According to the present invention, the method includes determining, in accordance with said deflection electrode angle, for each of a series of droplets to be printed on said substrate to form an image, the value of the charge to be applied to the droplet;
correcting the values of the charges to be applied to the droplets in said stream and adjusting the number of uncharged droplets between printable droplets in accordance with the determined speed of the substrate; and
applying the respective charges to each of the droplets in turn.
Effectively, therefore, the printer is operated in a non-raster or rasterless mode.
The term ‘substrate’ is used herein to refer to an article or plurality of articles on which characters or images are to be printed and the term ‘character’ herein refers to a discrete character, ideogram, image etc., and is not limited to simple alpha-numeric characters.
The invention also includes a continuous ink-jet printer comprising
means for providing a stream of droplets from a nozzle of a continuous ink-jet printhead;
means for applying a charge to individual ones of the droplets;
a pair of deflection electrodes for deflecting individual charged droplets from the stream of droplets to a required print position on a substrate moving past the printhead, said deflection electrodes being disposed at a preselected angle relative to the path of movement of said substrate; and
means for determining the speed of movement of the print substrate relative to the printhead; characterised by means for determining, in accordance with the deflection electrode angle, for each of a series of droplets to be printed on the substrate to form an image, the value of the charge to be applied to the droplet; and
means for correcting the values of the charges to be applied to the droplets in the stream and adjusting the number of uncharged droplets between printable droplets in accordance with the determined speed of the substrate.
A look-up table or other memory may contain a vector representation of the position of the droplets forming each printable character and the charge to be applied to the droplets may be calculated after reading the vector representation by means of a suitable algorithm. The algorithm may be hard- or soft-coded into the apparatus. Alternatively, the look-up table or other memory may contain sets of charge values for each droplet of each character that the printer is enabled to print or, when printing multiple lines of characters, a voltage offset may be added to the calculated charging voltages depending on the line in which the respective character drop is to be printed.
Multiple lines of characters may be printed using individual look-up tables or memories for each line of print or else a large look-up table containing values for the characters to be printed in each line (thus the values for the same character to be printed on different lines will be different).
Preferably, the angle a of the deflection electrodes to the path of movement of the substrate is chosen depending on the number of droplets N required or selected to print a line normal to the path, by the equation:
tan
α
=
n
+
1
N
where: n is the minimum number of non-printable droplets between adjacent printable droplets in a line of droplets normal to the path.
Furthermore, the determination of the speed of the substrate may be made by means of a suitable line speed sensor or else manually or the speed may be preset and the step of determining the speed may thus be achieved by setting the speed into the apparatus by means of a suitable manually adjustable input. In many applications, the articles or substrate pass beneath the printhead at a fixed speed determined by the packaging or other process with which the printing method is associated, but in other cases, a shaft encoder or similar means is used to determine the speed of movement of the substrate or articles in a process where their speed is variable according, say, to process conditions further upstream.
The correction of the values of the charges to be applied to the droplets in the stream may be carried out at one of a number of different stages in the process. For example, the charge values read from the look-up table or other memory may be corrected as the values are read therefrom or else corrections may be applied by a feedback method immediately prior to the charging signals being fed to the printhead. If multiple lines of characters are printed using individual look-up tables or memories for each line of print, then corrections may be applied after multiplexing of the look-up table values.
If the required print has no line that extends normal to the path of movement of the substrate, then a deflection electrode angle may be chosen to best suit the particular application.
An example of a CIJ printer operated according to the present invention, will now be described with reference to the accompany drawings, in which:
REFERENCES:
patent: 3588906 (1971-06-01), Brimer et al.
patent: 3916421 (1975-10-01), Hertz
patent: 4216480 (1980-08-01), Buehner et al.
patent: 4510503 (1985-04-01), Paranjpe et al.
patent: 4551731 (1985-11-01), Lewis et al.
patent: 4620198 (1986-10-01), Behun
patent: 4670761 (1987-06-01), Yoshino et al.
patent: 4809016 (1989-02-01), Padalino
patent: 4864323 (1989-09-01), Lecheheb et al.
patent: 5481
Conte Robert F. I.
Domino Printing Sciences PLC
Le N.
Lee Mann Smith McWilliams Sweeney & Ohlson
Nghiem Michael
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