DEVICE AND METHOD FOR PATTERNWISE PRINTING OF PIXELS CONSISTING OF ONE OR MORE DOTS

Abstract: Printing device for patternwise printing on a substrate of pixels which consist of one or more dots, at least comprising one or more delivery nozzles for delivering the dots, a support for the substrate, means for moving the nozzles with respect to the substrate, and a control assembly for controlling the printing device, which device comprises dot-position-disturbing means, by means of which the operation of the device can be influenced during printing in such a manner that the accuracy with which the separate dots are positioned on the substrate can be varied randomly. Furthermore, the invention describes a method for printing a substrate and a substrate which can be obtained in this way.

Description: DEVIC AND METHOD FOR PATTERNWISE PRINTING OF P IXELS CONSISTING OF ONE OR MORE J\J I o The invention relates firstly to a printing device for patternwise printing on a substrate of pixels which consist of one or more dots, at least comprising one or more delivery nozzles for delivering the dots, a support for the substrate, means for moving the nozzles with respect to the substrate, and a control assembly for controlling the printing device. Printing devices of this nature are generally known in the prior art and are also known as ink-jet printers. These printing devices are able to print images on the substrate without there being any contact between the printing unit, the nozzles and the substrate. There are two types of such printers : Drop-on-demand printers, in which the drops are delivered by gas bubbles formed by increasing the temperature, so as to correspond to the image which is to be printed; Continuous printers, in which the drops are formed with the aid of a pump and a piezo-electric vibrator, are charged with the aid of an electrode, and finally, with the aid of electrode plates, are deflected and collected or guided towards the substrate . Printing devices of this nature are able to produce images of photographic quality with a very high resolution. In other words, the accuracy with which the dots can be positioned on the substrate is extremely high. All this is a consequence of the accuracy with which the various mechanical components function, and of the advanced control used in the device and provided by the control assembly, which is usually connected to a data processing unit . In the present description, ink is intended to mean any substance which a printing device of the type referred to in the introduction can use for printing, i.e. ink, pigments, melt dyes, etc. - 2 - Images which are produced with the abovementioned printing devices consist of a pattern of pixels which are each built up from a number of dots. When a plurality of dots are applied at the position of a pixel using one or more nozzles, the position of these dots with respect to one another may vary between completely overlapping and completely separate. The human eye scarcely notices any difference between these two states at pixel level, since it is difficult to distinguish the separate dots. However, when a relatively large area is printed in one colour, which may be a mixture of dots of different colours, in practice a so-called colour hint has often been found. Colour hint refers to a certain prevailing colour impression which is undesirable. This often occurs in colour photographs, which then provide, for example, a somewhat green or blue overall impression. Colour hint has been found to be the consequence of a gradual change in the accuracy of positioning of the dots in each pixel during the course of the printing process. In other words, the correct number of drops is printed in each pixel, but the position of these dots relative to one another varies, often to a very slight extent. This is a very considerable problem to which hitherto no satisfactory solution has been found. The object of the present invention is to provide a solution to the abovementioned problem and, to this end, is characterized in that the device comprises dot-position- disturbing means, by means of which the operation of the device can be influenced during printing in such a manner that the accuracy with which the separate dots are positioned on the substrate can be varied randomly. In the prior art, it has been attempted to solve the problem of colour hint by increasing the accuracy still further. However, it has been found that this does not work, since this often leads to the devices becoming very slow, expensive and susceptible to faults. Surprisingly, the device according to the invention solves the problem by deliberately and randomly varying the accuracy of the printing process. The dot-position-disturbing means ensure - 3 - that over the course of time all possible positions of the dots relative to one another in each pixel occur equally frequently on the printed surface. In other words, the possible variation in accuracy of the drop positioning which occurs over the course of time is averaged out. In this context, reference is made to EP-A-O, 376, 596. This European patent application describes a method for positioning two drops of a colour at the same position, a so-called pixel position, on a substrate, using two nozzles. In this case, the first drop is delivered from one nozzle and the other drop is delivered from the other nozzle. The aim here is for the printing process as a whole to experience as little disruption as possible in the event of any blockage in one nozzle and to be able to maintain a certain printing quality even in the event of such a blockage. However, the nozzle alternation employed can in no way be said to be random, but rather it is a predetermined alternation. The range within which the random variation in the accuracy of the dots can be varied according to the invention is not critical, and will usually depend on the particular printing process to be carried out. Preferably, the dot-position-disturbing means are designed in such a manner that the random variation of the accuracy with which the separate dots are positioned on the substrate lies within the limits of the maximum accuracy variation which occurs over the course of time without the dot-position- disturbing means exerting any influence. An important feature of the device according to the invention is that the dot-position-disturbing means are used to influence a parameter of the printing process in a manner which is unnecessary for the printing process itself . With the aid of the dot-position-disturbing means, the printing process can be influenced in a very wide variety of ways in order to randomly vary the accuracy of the dot positioning. Advantageously, use is made of the inherent inaccuracy of the device. Preferably, the dot-position-disturbing means are _ 4 _ designed to make a nozzle undergo a relative displacement, which is unnecessary for the printing process, with respect to the substrate before a dot is delivered. Moving a nozzle with respect to the substrate, in other words bringing the nozzle out of and back into register, produces the desired random positioning inaccuracy of each subsequent drop with respect to a preceding drop. Advantageously, the dot-position-disturbing means comprise vibrator means for vibrating one or more of the delivery nozzles. The vibration is advantageously a so- called random vibration (arbitrary vibration) , which has the desired result for the invention. The dot-position-disturbing means do not necessarily have to be an additional mechanical component. In particular, they form part of the control assembly. In this way, the dot-position-disturbing means are able to regulate the control of the printing process, with the result that colour hint problems can be resolved without the need for additional mechanical components . Preferably, the printing device is a continuous printer. In these devices, it is very simple, by influencing the printing process by means of the action of the dot-position-disturbing means on the control assembly, to vary the positional accuracy of the dots on the substrate in every pixel. In particular, in this case, the dot-position- disturbing means comprise means for varying the charge voltage of the dots formed. By suitably activating the charging electrode, the charge voltage of the dots can be varied randomly within defined limits, with the result that the positional accuracy of the dots is also varied randomly. Preferably, the charge voltage is varied for each drop. The extent of variation in the charge voltage will be such that it does not adversely affect the printing process but does have the desired result . Advantageously, the variation in the charge voltage is at most plus or minus 12%, more advantageously plus or minus 5%. - 5 - It will be clear that random variation in the charge voltage may also suitably be used for continuous printers in which a plurality of lines can be printed at the same time. In particular, the dot-position-disturbing means comprise means for randomly varying the time between the moment at which the drop breaks off a delivery nozzle and the time at which the drop is charged. In this way, the charge can be varied randomly, so that the charge deviates only slightly from the ideal charge. In other words, instead of varying the charge voltage itself, it is also possible to vary the time at which the drop breaks off the nozzle after it has been charged. A slight change in this phase will lead to a slight change in the charge on a drop. Since this takes place randomly and the variations in the phase lie within the range of the variations which are normally present, it is possible that the image to be built up will deteriorate slightly from the most ideal state, but tests have shown that the images are still adequate. The result is a more stable system (less possibility of external influence (temperature, air pressure, relative humidity )), a less complicated system (no need for a measurement and regulating system) , and a less expensive system (no electronics, air conditioning or the like required) . In continuous printers, a small group of drops is often charged, and a following group is not charged, then a following group of drops is charged, and so on. It is possible to randomly vary the time at which charging of a small group of drops is commenced. Preferably, however, the charge voltage is varied randomly for each drop. The device according to the invention is in no way limited with regard to the design of the delivery nozzles. For example, it is possible to use so-called print heads which contain a plurality of rows of nozzles, but a print head with a single nozzle is also possible. Nor is the actual printing process of critical importance, since the device according to the invention exhibits advantageous properties with regard to solving the problem of colour - 6 - hint for all printing processes which are known to date and can be carried out using this device. The invention furthermore provides a method for patternwise printing on a substrate of pixels consisting of one or more dots, using a printing device comprising one or more delivery nozzles for delivering the dots, a support for the substrate, means for providing a relative movement of the substrate with respect to the nozzles, and a control assembly for controlling the printing device, which method is characterized in that the operation of the device is influenced during printing in such a manner that the accuracy with which the separate dots are positioned on the substrate is varied randomly. Advantageously, the dot-position-disturbing means are designed in such a manner that the random variation of the accuracy with which the separate dots are positioned on the substrate lies within the limits of the accuracy variation which occurs without the dot -position-disturbing means exerting any influence. Preferably, each nozzle is subjected to a relative movement, which is unnecessary for the printing process, with respect to the substrate before delivering a dot. For this purpose, it is particularly preferable to make at least one nozzle vibrate. In a particular embodiment of the method according to the invention, the random variation of the accuracy is effected using the control assembly. Preferably, each successive dot is delivered using a different delivery nozzle, by means of regulating the control assembly. Furthermore, according to the invention, it is preferable to simultaneously build up various pixels randomly with the dots, by means of regulating the control assembly. In yet another preferred embodiment, the accuracy with which the drops leave a delivery nozzle is varied, by means of regulating the control assembly. In particular, the printing device used is a continuous printer. Preferably, in this case, the random _ 7 _ variation in the accuracy is effected by using the control assembly to activate the charging electrode in such a manner that the charge voltage is varied randomly during charging of the dots. All this may, for example, be effected by using a so-called random generator. Advantageously, the random variation in the charge voltage is at most plus or minus 12%, more advantageously plus or minus 5%. In particular, the random variation in the accuracy is effected by using the control assembly to drive the printing device in such a manner that the time between the instant at which the drop breaks off the nozzle and the instant at which the drop is charged is varied randomly. A slight change in this phase will lead to a slight change in the charge on a drop. By carrying this out randomly and ensuring that the variations in the phase fall within the range of the variations which are normally present, it will be ensured that the image to be built up is of sufficient quality. The invention furthermore provides a printed substrate which can be obtained using the method according to the invention. The invention will be explained below with reference to an example. A continuous printer (DUPONT, Cromalin AX4) was used to print a light grey area. A certain colour hint was clearly visible over the area. The area was not equally light grey. In a first test, the control assembly was reprogrammed and a random generator was used to vary the charge voltage of the charging electrode randomly between 55 and 65 V during printing, while the usual charge voltage was 60 V; a grey area was again printed. In a second test, using the same printing device, a vibrator and a random generator were used to impart a random vibration to the print head during printing. The same light grey area was again printed. The frequency of the vibrator was varied randomly between 600 and 650 Hz. In both tests, it was possible to print a light - 8 - grey area in which there was no perceptible colour hint whatsoever .

Claims: - 9 - CLAIMS 1. Printing device for patternwise printing on a substrate of pixels which consist of one or more dots, at least comprising one or more delivery nozzles for delivering the dots, a support for the substrate, means for moving the nozzles with respect to the substrate, and a control assembly for controlling the printing device, characterized in that the device comprises dot-position- disturbing means, by means of which the operation of the device can be influenced during printing in such a manner that the accuracy with which the separate dots are positioned on the substrate can be varied randomly. 2. Printing device according to claim 1, characterized in that the dot-position-disturbing means are designed in such a manner that the random variation of the accuracy with which the separate dots are positioned on the substrate lies within the limits of the maximum accuracy variation which occurs over the course of time without the dot-position-disturbing means exerting any influence. 3. Printing device according to claim 1 or 2 , characterized in that the dot-position-disturbing means are designed to make each nozzle undergo a relative displacement, which is unnecessary for the printing process, with respect to the substrate before a dot is delivered. 4. Printing device according to one or more of claims 1-3, characterized in that the dot-position-disturbing means comprise vibrator means for vibrating one or more of the delivery nozzles. 5. Printing device according to one or more of claims 1-4, characterized in that the dot-position-disturbing means form part of the control assembly. 6. Printing device according to one or more of claims 1-5, characterized in that the printing device is a continuous printer. 7. Printing device according to claim 6, characterized in that the dot-position-disturbing means comprise means for varying the charge voltage of the dots formed. - 10 - 8. Printing device according to claim 7, characterized in that the variation in the charge voltage is at most plus or minus 12%. 9. Method for patternwise printing on a substrate of pixels consisting of one or more dots, using a printing device comprising one or more delivery nozzles for delivering the dots, a support for the substrate, means for providing a relative movement of the substrate with respect to the nozzles, and a control assembly for controlling the printing device, characterized in that the operation of the device is influenced during printing in such a manner that the accuracy with which the separate dots are positioned on the substrate is varied randomly. 10. Method according to claim 9, characterized in that the random variation of the accuracy with which the separate dots are positioned on the substrate lies within the limits of the accuracy variation which occurs without the dot-position-disturbing means exerting any influence. 11. Method according to claim 9 or 10, characterized in that each nozzle is subjected to a relative movement, which is unnecessary for the printing process, with respect to the substrate before delivering a dot. 12. Method according to claim 11, characterized in that at least one nozzle is made to vibrate. 13. Method according to one or more of claims 9-12, characterized in that the random variation in the accuracy is effected by means of the control assembly. 14. Method according to claim 13, characterized in that each dot is delivered by a randomly selected delivery nozzle, by means of regulating the control assembly. 15. Method according to claim 13, characterized in that various pixels are simultaneously built up randomly by the dots, by means of regulating the control assembly. 16. Method according to claim 13, characterized in that the accuracy with which the dots leave a delivery nozzle is varied randomly, by means of regulating the control assembly. 17. Method according to one or more of claims 9-16, characterized in that the printing device used is a - 11 - continuous printer. 18. Method according to claim 17, characterized in that the random variation in the accuracy is effected by using the control assembly to activate the charging electrode in such a manner that the charge voltage is varied randomly during charging of the dots . 19. Method according to claim 18, characterized in that the random variation in the charge voltage is at most plus or minus 12%. 20. Printed substrate which can be obtained using the method according to one or more of claims 9-19.

Inventors: BRAUN ROBERT EVERT ANTHONY, MANDERS MARTINUS GERARDUS JOZE, SMALLEGANGE MARTIN JAN, BRAUN, ROBERT, EVERT, ANTHONY, MANDERS, MARTINUS, GERARDUS, JOZEF, SMALLEGANGE, MARTIN, JAN
Applicants: BRAUN ROBERT EVERT ANTHONY, MANDERS MARTINUS GERARDUS JOZE, SMALLEGANGE MARTIN JAN, STORK DIGITAL IMAGING B V, BRAUN, ROBERT, EVERT, ANTHONY, MANDERS, MARTINUS, GERARDUS, JOZEF, SMALLEGANGE, MARTIN, JAN, STORK DIGITAL IMAGING B.V.
Application year: 1999
Main classification: B41J2
IPC codes: B41J2, H04N1