EP0842784B1 - Continuous multi-colour inkjet press, synchronization method for such press, and printed product obtained therewith - Google Patents

Description

The present invention relates to a press multi-color continuous inkjet, a process of implementation of such a press, and a product print obtained using such a press.

State of the art

Digital printing systems by inkjet have grown considerably these recent years, particularly in office image printing applications in color. The inkjet process has undeniable advantages with regard to technologies contact printing, such as the absence of operating noise, and lack of contact with the substrate.

Compared to other techniques digital color printing like electrophotography, inkjet offers a cost advantage and superior performance for the restitution of colors, as well as better printability substrates of various natures.

In industrial applications color printing, like textile printing, posters, wall or floor coverings, labels, plastic cards, or even printing of books / magazines or catalogs, printing systems used to date still put implement traditional technologies operating by contact, such as gravure, offset, or serigraphy. These technologies are heavy to implement work, because they require prior manufacturing mechanical printers that materialize the image to be printed, such as the engraved rolls for rotogravure, screen printing screens or offset plates. The cost and time of manufacturing these printer forms constitute a major obstacle to printing small series with short deadlines.

• La personnalisation de produits est effectuée hors ligne de production, ce qui est long et coûteux.
• La production en petites séries est découragée par les imprimeurs qui répercutent les coûts élevés causés par l'arrêt de la production lors des changements des formes imprimantes, les pertes d'encres lors de changements de couleurs et les pertes de produits engendrées par le calage des nouvelles formes lors de la remise en route de l'impression.
• La production est organisée en séries longues, regroupant un grand nombre de commandes identiques. La production « juste à temps », pour fournir aux circuits de distribution les produits correspondant à la demande immédiate des consommateurs, est impossible. Au contraire, ces systèmes de production traditionnels génèrent des inventaires volumineux et coûteux ; les invendus et produits démarqués sont fréquents, et les encours de production importants.

The design of industrial printed products has evolved under the constraint of traditional printing technologies:

• Product customization is done off the production line, which is time consuming and costly.
• The production in small series is discouraged by the printers who pass on the high costs caused by the stop of the production during the changes of the print forms, the losses of inks during changes of colors and the losses of products caused by the setting of the new forms when printing is restarted.
• Production is organized in long series, bringing together a large number of identical orders. Just-in-time production is impossible to supply the distribution channels with products corresponding to immediate consumer demand. On the contrary, these traditional production systems generate large and expensive inventories; unsold and unmarked products are frequent, and large production outstandings.

• avec l'avènement des systèmes de communications numériques, tels que les autoroutes de l'information, qui permettent de connaítre à chaque instant la demande de produits et de commander et fabriquer « juste à temps » ;
• sous la pression des consommateurs et utilisateurs dont les besoins, les goûts et les modes sont de plus en plus variés et changeants ;
• sous la contrainte des circuits de distribution qui veulent réduire leurs coûts, notamment ceux occasionnés par les stocks et les invendus.

However, traditional systems are in the process of being replaced by systems based on digital printing:

• with the advent of digital communications systems, such as the information superhighways, which make it possible to know demand for products at all times and to order and manufacture "just in time";
• under the pressure of consumers and users whose needs, tastes and fashions are more and more varied and changing;
• under the constraint of distribution channels which want to reduce their costs, in particular those caused by stocks and unsold products.

The printing industry will adopt digital production techniques which are more flexible, faster, provided they do not do not compromise print quality. The jet ink is one of the main candidate techniques considered.

• L'impression à vitesse variable doit être possible, car les séries sont courtes et engendrent des arrêts et redémarrages fréquents du défilement du substrat. Pour minimiser les en-cours de production, le processus d'impression est dorénavant effectué en ligne ou intégré avec d'autres étapes de production telles que la fabrication du substrat proprement dit, son contrecollage, son laminage, ou son emballage. Les variations de vitesse du substrat sont donc fréquentes, car liées aux aléas des autres processus de la ligne de production.
• Les exigences de qualité des produits imposent de travailler avec une résolution élevée et une précision de superposition et juxtaposition des couleurs accrues.
• Les séries ou métrages imprimés sont très courts, parfois même de longueur inférieure à la longueur du substrat présent dans la machine d'impression, ce qui conduit à l'impression simultanée de plusieurs motifs dans une même machine.
• Les impératifs économiques exigent de produire en continu, en minimisant les arrêts, avec des vitesses effectives de production de plus en plus élevées.
• La personnalisation en ligne des produits nécessite de pouvoir imprimer une image numérique variable sur un substrat comportant un premier motif de base préimprimé, ceci avec un excellent positionnement relatif des images.
• L'impression est de plus en plus fréquemment réalisée avec des encres aqueuses, donc sans solvants, afin de préserver l'environnement. Ceci nécessite d'interposer entre les unités d'impression de différentes couleurs des systèmes de réticulation et/ou de séchage, qui augmentent les longueurs de produits entre ces unités par leur encombrement et modifient la température du substrat. Ces deux facteurs, augmentation de la longueur de la ligne de production et environnement de températures variables, accentuent la déformation des substrats dans l'imprimante.

Inkjet printing, in particular the technology of the continuous deflected jet, lends itself well to the construction of large printheads, as described in the document referenced [1] at the end of the description. Continuous multi-color presses, in which several printheads are arranged in series to print a substrate in web running continuously under the printheads, can be manufactured. The cost of these electronic presses is higher than that of traditional mechanical presses, but their economic operating conditions are better because they allow just-in-time production, short series, customization of products in lines, and the elimination of related investments. the setting up of printing forms for new designs. The new operating conditions for digital presses, however, introduce new constraints, hitherto unknown:

• Printing at variable speed must be possible, because the series are short and cause frequent stops and restarts of the substrate scrolling. To minimize production in progress, the printing process is now carried out online or integrated with other production stages such as the manufacture of the substrate itself, its lamination, its lamination, or its packaging. Variations in substrate speed are therefore frequent, as they are linked to the vagaries of other production line processes.
• The quality requirements of the products make it necessary to work with a high resolution and a precision of superposition and juxtaposition of the increased colors.
• The printed series or measures are very short, sometimes even of length less than the length of the substrate present in the printing machine, which leads to the simultaneous printing of several patterns in the same machine.
• Economic imperatives require continuous production, minimizing downtime, with increasingly high effective production speeds.
• The online personalization of products requires the ability to print a variable digital image on a substrate comprising a first basic pre-printed pattern, this with excellent relative positioning of the images.
• Printing is more and more frequently carried out with aqueous inks, therefore without solvents, in order to preserve the environment. This requires interposing between the printing units of different colors of the crosslinking and / or drying systems, which increase the lengths of products between these units by their bulk and modify the temperature of the substrate. These two factors, an increase in the length of the production line and an environment of variable temperatures, accentuate the deformation of the substrates in the printer.

Traditional contact printers used to date, whether technology rotogravure, screen printing with rotary frame, or offset, operate at steady speed. Indeed, the phases substrate acceleration when starting the print are generally shorter than the time required to calibrate the printer forms (corresponding to the images of the different colors primary) relative to each other.

The problem of synchronization in transient speed phase (acceleration or deceleration) of the substrate is currently unknown. The shims are carried out at speed stabilized by relative mechanical displacements of the forms printers. When the frame rate is low, examining the quality of color matching is done visually on the printed substrate. When the substrate speed is higher, a help electronic adjustment is provided by printing repetitive calibration patterns at the edge of the width, and by viewing them on a control monitor, the targets being observed by a camera associated with a strobe lighting. A slow drift in the timing is always observed in practice, at due to variations in the environment, friction or even dimensional differences between different forms of printers, maintaining timing is provided by the printer operator who continuously monitors and adjusts the print timing.

The problem of synchronization between different colors has been addressed in printers desktop digital. Thus, the document referenced [2] at the end of the description describes the synchronization of a single pass color electrostatic printer in which print head of the first color prints at regular intervals and at the edge of the substrate synchronization targets. Scroll speed is kept constant thanks to the enslavement of the substrate drive motor. In phase of scrolling of the substrate this test pattern is reread by CCD cameras located downstream, each camera being associated with a print head. Each head then interprets the distance between the landmarks of the test pattern measured by his camera, so to print the dot lines in its own color of evenly distributed among the test marks on the substrate and thus obtain the superposition of different colours.

The distance between the test marks being smaller than the size of an image it is also necessary to determine the start of the image for each print head. This is done in determining, at nominal operating speed, the time difference between the different heads printing. This offset is determined by the operator which performs a sequence of impression tests of a other specific calibration target, combining different colours.

The document referenced [3] discloses another type of synchronization system applied to a electrophotographic printer. A difference with the electrostatic system mentioned above comes from fact that electrophotographic printing is not a direct printing technique. This requires indeed a transfer of the colored image, previously materialized on a transfer tape. This image is then transferred by mechanical contact between the transfer tape and the substrate to be printed. The disclosed synchronization system implements printing of different targets by each cylinder associated with each of the colors on the transfer tape. A single optical system located, in downstream of all printing cylinders (but upstream of the transfer location on the substrate), analyzes the positioning deviations of the test patterns materialized on the transfer tape in each of the colors. These deviations are used to generate corrections to apply to the engines driving the cylinders associated with each of the colors. In this case too, printing and synchronization are performed at constant speed of the substrate and the web of transfer. No solution is described elsewhere. to define the precise image start time.

The document referenced [4] discloses a synchronization system for a printer electrophotographic. The image start signal is materialized by a hole in the transfer tape. the detection of this hole by an optical system and the definition of delays for each printing cylinder allows you to synchronize the different colors. However, this solution does not allow overprinting or customize an already printed document.

Printing substrates at speed variable is, moreover, known in industrial marking applications, but in these cases, printing is done in one color, or several independent colors: positioning relative points of different colors is not required. Note however that even in printing monochrome, variable speed printing poses specific timing issues at the inkjet technology, due to response time intrinsic to the printheads. These project droplets at a distance ink, which will impact the substrate for the impression. The duration of the trajectory of the drops of the print head to the substrate being fixed by the ejection speed of the drops and the distance from the ejection nozzle to the substrate, it is understood that in case of substrate speed variation, compensation specific should be done to account for the duration of trajectory of the drops. Such systems of compensation of the trajectory time of the drops in theft are known in the state of the art and used to commercially, as in the jet printer IMAJE Series 4 ink.

• une excellente résolution pour effectuer les réglages fins de synchronisation. Ceci implique une horloge très rapide, et/ou une indexation spatiale très fine du déplacement du substrat ;
• une excellente représentativité de la position du substrat au niveau de chaque tête d'impression, afin que la position relative des points de couleurs différentes soit précise. L'horloge ne doit pas être entachée d'erreurs provenant de glissements ou de déformations du substrats entre les têtes d'impression, notamment lors d'accélérations ou décélérations ;
• un codage de la référence de la production (ou de l'image) en cours, plusieurs productions différentes pouvant être imprimées à un instant donné dans la machine d'impression.

The difficulty of synchronizing a multi-color printing system printing at variable speed stems from the need to have synchronization clocks and information presenting both:

• excellent resolution for fine-tuning synchronization. This implies a very fast clock, and / or a very fine spatial indexing of the displacement of the substrate;
• excellent representativeness of the position of the substrate at each print head, so that the relative position of the dots of different colors is precise. The clock must not be marred by errors resulting from slippage or deformation of the substrates between the print heads, in particular during accelerations or decelerations;
• a coding of the reference of the production (or of the image) in progress, several different productions being able to be printed at a given time in the printing machine.

• les vitesses de défilement élevées ;
• la structure, la couleur ou la texture des substrats qui ne permettent pas d'imprimer des marques d'indexation à haute résolution et qui soient lisibles en environnement industriel.

   la présente invention a pour objet une presse multicouleur à la continue par jet d'encre permettant de résoudre les problèmes énoncés ci-dessus.Such characteristics, new compared to traditional printing techniques, are also very difficult to obtain in an industrial environment, due to several factors, such as:

• high frame rates;
• the structure, color or texture of substrates which do not allow high resolution index marks to be printed and which are legible in an industrial environment.

the subject of the present invention is a continuous multicolor inkjet press enabling the problems set out above to be solved.

A patent application of known art, the Patent application EP 0 729 846, describes a device for control of the recording of images printed on a substrate in a jet printing device ink comprising several printheads, in which of the registration marks associated with images are printed on the substrate and then read by a captor.

Statement of the invention

• un moteur d'entraínement entraínant un substrat à passer successivement sous chacune de ces têtes,
• un codeur de position de résolution élevée, typiquement de 3000 points à 300000 points par tour de moteur, placé sur le moteur, qui délivre un signal à haute fréquence TACHY,
• plusieurs capteurs pour lire des marques, chaque capteur étant associé avec une tête d'impression,
• un circuit de synchronisation connecté aux têtes d'impression, aux capteurs et au codeur, ledit circuit de synchronisation étant contrôlé par un système informatique qui supervise l'impression de chacune des têtes d'impression,

caractérisée en ce que ces marques sont des premières marques régulièrement espacées, en ce que le circuit de synchronisation, qui reçoit le signal TACHY issu du codeur de position délivre une horloge trame HTRAMi associée à chaque tête d'impression Ti, dont la période est un multiple dudit signal TACHY, en ce que chaque capteur associé à une tête d'impression Ti, est un capteur optique permettant la lecture de ces première marques et la génération d'un signal impulsionnel DTOPi qui définit l'instant de passage d'une première marque sous ledit capteur, la durée entre deux signaux temporels impulsionnels DTOPi contenant en permanence un nombre entier et constant de périodes de l'horloge trame correspondante HTRAMi, et en ce que chaque capteur est placé en amont de la tête correspondante Ti à une distance légèrement supérieure à la distance séparant deux premières marques.The present invention relates to a continuous multicolour inkjet press, in which several Ti printheads are arranged in series to print at least one substrate traveling continuously beneath them, each head being supplied with an ink of color given by its own ink circuit, said press comprising:

• a drive motor driving a substrate to pass successively under each of these heads,
• a high resolution position encoder, typically from 3000 points to 300000 points per motor revolution, placed on the motor, which delivers a high frequency TACHY signal,
• several sensors for reading marks, each sensor being associated with a print head,
• a synchronization circuit connected to the print heads, to the sensors and to the encoder, said synchronization circuit being controlled by a computer system which supervises the printing of each of the print heads,

characterized in that these marks are first regularly spaced marks, in that the synchronization circuit, which receives the TACHY signal from the position encoder delivers a HTRAMi frame clock associated with each printhead Ti, whose period is one multiple of said TACHY signal, in that each sensor associated with a print head Ti, is an optical sensor allowing the reading of these first marks and the generation of a pulse signal DTOPi which defines the instant of passage of a first marks under said sensor, the duration between two impulse time signals DTOPi permanently containing a whole and constant number of periods of the corresponding frame clock HTRAMi, and in that each sensor is placed upstream of the corresponding head Ti at a distance slightly greater than the distance between the first two marks.

Advantageously, this press includes a system of the first brands, regularly printed on the substrate, located upstream of the print heads.

The use of an encoder placed for example on the axis of rotation of the motor, and preferably operating thanks to an optical device gives a very high signal resolution.

When using a tape conveyor, these first marks can be printed or even simply materialized by manufacturing on this same substrate conveyor belt. In the case of a substrate preprinted, the first marks will have been produced during the preprint.

These first brands have a geometry and a color allowing an unambiguous reading in environment industrial by an optical system such as a CCD camera and a lighting, or a sensor measuring the optical reflection of the substrate. A square block of one to a few millimeters in side and fluorescent color are possible choices particularly suitable. These marks can be indifferently printed on the front or back of the substrate or the conveyor belt if necessary, depending on the best lighting conditions and reading system. Reading marks on each printhead is done by a system optical. This reading allows the generation of a signal precise pulse time DTOPi, which defines the moment of passage of a first mark under the sensor associated with the Ti print head. The distance between two first marks is of the order of distance separating 100 to 5000 lines of printed dots.

In the synchronization circuit according to the invention, the duration between two signal pulses DTOPi permanently contains an integer and constant M of periods of a clock HTRAMi. The clock HTRAMi is the print command signal of a line of dots through the print head. this allows to have permanently the same number M of lines of dots printed on the substrate between two first marks, for each color. So, these brands being physically linked to the substrate, positioning relative of the different colors is noticeably assured, even if the substrate undergoes deformation between two printheads.

In practice, the optical generator sensor DTOPi signal, placed upstream of a head is more exactly arranged at a distance from the print head less than twice the distance between the first two marks.

According to a third characteristic of the invention, for substrates having a width, second marks are printed on the substrate, which can be distinguished unambiguously from first brands. These second marks can be printed on the edge of the substrate by the first head printing. A preferred embodiment consists of printing these marks at the edge of the substrate on a line parallel to the direction of scrolling, but located at a good distance from the first line brands. In the case of a preprinted substrate, the second marks will have been made during the preprint.

The function of these second marks is to report the change in the pattern to be printed. These second marks are read by an optical system, in order to generate a signal called MOTIF, more precise rough, indicating the change in the pattern to be printed. In a preferred embodiment, the signal MOTIF is detected by printing and detection a rapid succession of a few separate paving stones from a distance much less than that separating the first brands.

For substrates with sheet, preprinted or not, a second mark may be naturally generated by the appearance of the edge downstream of the sheet under the optical sensor, and the synchronization is performed in a similar way to width substrate case.

According to another characteristic of the invention, the synchronization circuit performs prediction, filtering, and windowing operations the reading operation of the DTOPi signal in order to confer a very high robustness to the system. Detection of first brands is first authorized in a limited time window, which is centered on the first brand likely to pass under the sensor. This solution makes it possible to limit the disruptive detections that could be related to the presence of parasites. In the absence of detection of a first mark in the reading window, a fictitious DTOPi signal is generated, from a prediction based on the interval between two previous impulses. This allows to continue printing, especially when changing the pattern, even when the first mark could not be detected. Simultaneously the reading window is expanded for the next detection time. Printing is stopped if the fault persists after four missing DTOPi signals.

In a preferred embodiment, the offsets between the print heads of the different colors making up the printing system are measured by intermittent test patterns of multi-color calibration printed by these same heads printing. The calibration patterns include geometric patterns that allow you to locate without ambiguity the dots printed by the different heads printing. The impression of the sights is interspersed in the sequenced process of producing printed products.

Target analysis can be done by output from the production line, if the time to product residence in the line is brief, allowing corrections and calibration within a short time. Yes however, the production line is long, which is the case for vinyl flooring which must spend several minutes in ovens placed in line immediately downstream of the place of printing, then an online analysis of the test patterns must be carried out work, before the release of the product at the end of the line production.

According to another characteristic of the invention, we have all the heads downstream a test pattern analysis system comprising a color camera (CCD type) equipped with optics adapted and mounted on a mechanical system displacement with micrometric position indexer arranged substantially perpendicular to the direction of substrate scrolling, as well as a system associated IT. The substrate conveyor line is stopped intermittently when the test pattern calibration is located significantly below the area swept by the movement of the camera. Detection the presence of the calibration target on the substrate can be made through the printing of a characteristic MOTIF mark at the edge of the substrate, signaling the presence of a calibration target and commanding the momentary cessation of the running of the substrate. The MOTIF mark is detected by a optical sensor associated with the target analysis system, similar to the sensors fitted to the print heads. When the substrate stops under the system action zone the camera is moved by the system mechanical, while analyzing the impacts of drops of different colors. The system IT simultaneously notes the characteristics of printed dots and the position of the camera thanks position information from the indexer of position on the axis of movement. By comparing the positions of the test pattern printed dots with their theoretical values, point position deviations prints of each color can be determined and compensated in the printing system during the next production. These compensations are automatically calculated by the computer system and uploaded to the process controller printing.

The present invention also relates to a width or sheet product (floor / wall covering, textile, poster) printed or overprinted using the synchronization method of the invention.

This (over) printed product, produced using the press of the invention is such that it includes a fixed background image and some parts of the decoration are variable, printed continuously by the object press of the invention, for example (address or photo of local distributor for an advertising poster of big brand in an international campaign or national, ...). The fixed and variable parts of the image are printed on the same substrate.

• lors des phases d'accélération ou décélération du substrat ;
• avec une résolution élevée et une précision de superposition et de juxtaposition des couleurs accrues ;
• permettant l'impression simultanée de plusieurs motifs dans la machine ;
• minimisant les arrêts, avec des vitesses effectives de production élevées ;
• autorisant la surimpression en ligne de produits qui comportent un premier motif de base préimprimé, ceci avec un excellent positionnement relatif des images ;
• autorisant l'impression avec des distances importantes entre têtes d'impression, notamment pour permettre d'interposer entre ces unités d'impression de différentes couleurs les systèmes de réticulation et/ou de séchage.

The press of the invention allows printing of quality color images:

• during the acceleration or deceleration phases of the substrate;
• with high resolution and increased color overlay and juxtaposition accuracy;
• allowing the simultaneous printing of several patterns in the machine;
• minimizing downtime, with high effective production speeds;
• allowing online overprinting of products which have a first basic preprinted pattern, with excellent relative positioning of the images;
• allowing printing with large distances between printheads, in particular to allow interposition between these printing units of different colors the crosslinking and / or drying systems.

Brief description of the drawings

• Figures 1A and 1B show schematically the mechanical architecture of a press conventional screen printing with rotating frame respectively in a side view and in a top view;
• Figures 2A and 2B show schematically the mechanical architecture of a press rotogravure, respectively in a side view and in a top view;
• Figures 3A, 4A; and 3B, 4B illustrate schematically two mechanical architectures of continuous inkjet printing machines, respectively in two side views and two side views above ;
• Figure 5 illustrates an architecture of an inkjet press according to the invention;
• Figure 6 illustrates synchronization the printing system illustrated in Figure 5;
• Figures 7 to 9 illustrate different characteristics of the press of the invention.

Detailed description of embodiments

Figures 1A and 1B show schematically the mechanical architecture of a press conventional screen printing printing on a substrate textile 10, continuously scrolling, fed from a roll 11. This substrate is stuck on a strip adhesive conveyor 12. The device 13 is a device for laminating and driving the substrate 10. The device 14 is a device gluing of the belt 12. This conveyor belt 12, less deformable than the textile substrate 10, is set in motion by a motor. Textile is therefore driven by the conveyor belt 12 and maintained in position as it scrolls under the units color printing of rolls engraved with serigraphy 15. Each roll 15 deposits a quantity ink on the substrate 10, the ink being circulated at inside the roller and forced through holes engraved in this roll, and corresponding to the image to print. Each roller or rotating frame 15 applies controlled pressure to the substrate 10, which conditions the amount of ink transferred. After his printing, the substrate 10 is peeled downstream of the conveyor belt 12 for the following operations of production, such as fixing or drying inks. A color is printed here while the previous color is still wet. The printing system includes a device for cleaning 16 of the conveyor belt 12, to remove the ink that has passed through the fabric and impregnated the latter.

Figures 2A and 2B show schematically the mechanical architecture of a press rotogravure printing on a substrate 20 passing by continuous, thanks to a drive motor 21. The roller 22 is the input roller of the substrate. This substrate 20, which can be, for example, a coating vinyl flooring, usually armed with a fabric core of fiberglass, is more resistant mechanically and less deformable than a textile. So it does require no conveyor belt and can be used mechanically by the conveyor system. Each printing cylinder 23 has etched cells intaglio corresponding to the image to be printed (process gravure). These cells are filled with ink by an inking device 24 (inkwell, inking roller and doctor blade) in contact with the cylinder. Considering the low porosity of substrate 20 and use classic water inks, a heating system 25 is interposed between each of the printing units 23, so that freshly printed ink is not transferred by contact to the rollers located downstream.

Figures 3A, 3B and 4A, 4B show schematically mechanical architectures of continuous inkjet printing machines. In these figures are represented heads inkjet printing 30.

The machine of Figures 3A and 3B uses a conveyor belt 31 and is particularly suitable printing porous and deformable substrates such as textiles presented in rolls, or as substrates in sheets or plates unstacked as input.

For mechanically resistant substrates such as vinyl siding, a machine such that shown in Figures 4A and 4B is sometimes better suited. In these figures 4A and 4B are represented first and second readers brands 32A and 32B, a device for marking first brands 33, a calibration target reader 34, a drive motor 35, as well as drying devices 36.

These machine architectures are directly adapted from printing machines traditional serigraphic or rotogravure, illustrated respectively in Figures 1 and 2, which operate by contact. A fundamental difference in their realization comes from the fact that the impression of ink jet drops should be synchronized with the displacement of the substrate, by a flexible process and robust that works in an industrial environment, even during transient speed phases; what is the subject of the invention.

Figure 5 shows the architecture of an inkjet press according to the invention.

In this figure are represented a printer 40 of first brands 51, sensors 41 and 49, a color camera 42, a drive motor 43, ink circuits 44 respectively connected to several print heads T1, T2, T3 and T4, as well a synchronization circuit 45, connected to the heads T1, T2, T3 and T4 and to sensors 41 (referenced 32 on Figures 3 and 4) and 49, and a circuit for reading calibration test patterns 47 linked to a computer system process controller 46.

The substrate 50 is driven directly as illustrated in Figure 4; or indirectly, glued, or simply worn, on a conveyor belt as illustrated in figure 3, to go under the successive print heads T1, T2, T3 and T4. he can be animated by one (or more) device (s) motorization. Each T1, T2, T3 or T4 printhead prints an ink associated with a primary color of the image to print. Printing is done using the simultaneous piloting of a multitude of jets arranged in parallel, as described in the document referenced [1]. Each print head is supplied with ink thanks to an ink circuit 44 which is specific to it. The computer system 46 called "process controller" supervise the printing of these different heads T1, T2, T3 or T4.

According to a first characteristic of the invention, the motor 43 is equipped with an encoder 48 of high resolution position, typically 3000 at 300,000 points per engine revolution, which gives a high frequency pulse (typically 100-500 kHz) representative of a step of a few microns (3 to 30 microns) in advance of substrate 50. This resolution is on the order of ten to fifty times lower than addressability, i.e. nominal distance between adjacent printed dot lines, measured in the direction of travel of the substrate 50. This level of resolution allows, thanks to the system of synchronization, to precisely position the drops of different colors, with precision greater than about 1/10 of the addressability. This resolution would be inaccessible to a system operating with printed and reread marks on the substrate. The signal from the encoder 48, noted TACHY, is transmitted to the synchronization circuit 45. This signal, presented in Figures 6 and 9, gives an image approximate speed and position of the substrate 50. It is inaccurate in the sense that it does not does not take into account possible slippages or the deformation of the substrate. Using the encoder rotary 48 placed on the motor, and operating preferably through an optical device, gives a very high resolution signal.

The TACHY signal is the basis for the development of a frame clock, noted HTRAMi, associated with each print head Ti of color. This frame clock is the start signal each line of dots. Through construction, the HTRAMi signal period is a multiple of the TACHY signal (so it contains a number TACHY pulse integer), typically corresponding from 10 to 50 pulses, depending on the addressability. This number of TACHY pulses contained in the period HTRAMi signal, is variable over time, and moreover different for each Ti printhead, depending a second DTOPi signal, explained below.

According to a second characteristic of the invention, first marks 51 are regularly printed on the substrate 50, preferably using the printing system 40 located upstream of the Ti printheads. In the case the use of a conveyor belt, these first marks can be printed or even simply materialized by manufacturing on this same conveyor belt. The brands must already be present (therefore pre-printed) at the input of the system printing in the case of overprinting the product.

These marks 51 have a geometry and a color as they allow replay without ambiguity, in an industrial environment, by a system optics such as a CCD camera and lighting, or a sensor measuring the optical reflection of the substrate. A square paving stone of typical size 5 mm × 5 mm (or 1 cm × 1 cm) and a fluorescent color are choices particularly suitable. These brands can be indifferently printed on the front or back of the substrate, depending on the best conditions lighting and reading system.

Reading a first mark 51 at level of each printhead Ti is performed by the associated sensor 41 which is an optical system. This read allows generation of a time signal precise impulse, denoted DTOPi in FIG. 6. This DTOPi signal defines the moment of passage of a mark 51 under a sensor 41 associated with a print head Ti. Preferably, the DTOPi signal can be generated by appropriate processing of the read signal of optical sensor 41, using wired operators such as smoothing and time derivative, in order to translate the precise moment of passage of an edge of the printed mark 51. The distance between two marks 51 can be in the range of 100 to 5000 lines of dots printed. So the frequency of reading these marks 51 is about 100 to 5000 times lower than that of HTRAMi signal.

In the synchronization circuit according to the invention, the duration between two successive pulses of the DTOPi signal permanently contains an integer and constant of periods of the HTRAMi signal, noted M on the figures. This allows to have the same permanently number M of lines of dots printed on the substrate between two marks 51, for each color. So the marks 51 being physically linked to the substrate, the relative positioning of the different colors is substantially assured, even if the substrate is subjected to deformation between two printheads. In practice, the distance between the marks 51 is chosen such that so that for the extreme conditions of deformation substrate (maximum acceleration, deceleration maximum) the variation in length of the substrate 50 between two marks 51 consecutive is less than addressability (the distance between lines of points successive). This constraint is compatible with scrolling and deformation characteristics of substrates (or conveyor belt if applicable) commonly encountered (maximum deformations of around 1%).

The principle of clock correction HTRAMi, to take into account the deformation of the substrate 50 is described in more detail in FIG. 8. In practical, each optical sensor 41 generating a DTOPi signal is not placed at the head associated printing Ti, but placed upstream. It is more exactly arranged at a distance slightly greater than the distance between the first two marks and less than twice that distance. This offset allows synchronization circuit 45 to count the TACHY pulses in the interval between successive marks 51, before the same DTOP interval scrolls under the head print, and therefore calculate the corrected values HTRAMi clock parameters and transmit to the print head.

The number of TACHY pulses is redistributed in M substantially equal periods for constitute the HTRAMi clock which synchronizes printing the dots at the Ti print head.

At the running speed of the substrate established, its deformations are small to zero, and the successive periods of the HTRAM signal differ at most of a TACHY pulse. When there is deformation measurable substrate, the number of TACHY pulses counted between two successive marks 51 varies (this number increases when the substrate is stretched and decreases when the substrate is relaxed). The ΔTACHY gap between the TACHY pulse numbers measured for two intervals between first successive marks is used to modify the TACHY pulse numbers in HTRAMi clocks, to compensate for the deformation of the substrate 50. In a preferred mode of achievement, the ATACHY gap is redistributed approximately linearly in the interval between first brands considered, as shown in the figure 8. This compensation ensures a variation monotonic of the HTRAM clock period, and particular equality of the first HTRAM period of the interval between first marks considered with that of the last TRAM period of the interval previous. It also obviously ensures equality strict number of HTRAMi pulses in the interval between corresponding first marks, here equal to M.

According to a third characteristic of the invention, for substrates having a width, second marks are printed on the substrate 50 (not on the conveyor belt). These seconds brands can be distinguished unambiguously from first marks 51. These second marks can be printed on the edge of the substrate by the first head T1 printing. In the case of a preprinted substrate, the second marks will have been made during the preprint. A preferred embodiment consists of printing these second marks at the edge of the substrate on a line parallel to the direction of travel, but located a good distance from the first line marks 51.

The function of these second marks is to report the change in the pattern to be printed. These brands are read by an optical system (which can be same or the same type as the previous one), in order to generate a signal called MOTIF, of coarser precision, indicating the change of the pattern to be printed. In one preferred embodiment, the MOTIF signal is spotted by printing and detecting a rapid succession of paving stones 53 separated by a distance much less than the distance between first marks, as shown in figure 9. This redundancy of pavers allow unambiguous detection of change pattern. When the MOTIF signal is detected, the synchronization circuit 45 gives the order to the head printing to stop printing production in course and move on to the next production as soon as the next pulse of the DTOPi signal.

For substrates with sheet, pre-printed or not, mark 53 is naturally generated by the appearance of the downstream edge of the sheet under the optical sensor, and the synchronization is performed in a similar way to width substrate case.

According to another characteristic of the invention, the synchronization circuit 45 performs prediction, filtering, and windowing operations the reading operation of the DTOPi signal in order to confer a very high robustness to the system. Detection of a first mark 51 is first authorized in a limited time window, which is centered on the probable time of passage of this mark under the sensor. This solution makes it possible to limit the disruptive detections that could be related to the presence of parasites (printed defects or electrical disturbances). In the absence of detection of a first mark 51 in the reading window, a fictitious DTOPi signal is generated, from a prediction based on the interval between firsts previous brands. This allows to continue printing, especially when changing the pattern or between two sheets preprinted or not, even when the first mark 51 could not be detected. At the same time the reading window is widened to the next detection time. The impression is stopped if the fault persists after four pulses Missing DTOPi.

To synchronize correct, it is also necessary to take into account exact time offsets between each of the sensors and its associated print head as well as between different printheads. These shifts are expressed as whole numbers and fractions of HTRAMi. Of the same way, some lags may exist between the jets of the same printing unit. In one preferred embodiment, these shifts of printing system are measured by analysis intermittent multi-color calibration test patterns printed by the printing system all over the width of the substrate. Calibration targets have geometric patterns that allow unambiguously identify the dots printed by the different printing units. The impression of targets is interspersed in the sequenced process of the production of printed products. Analysis of targets can be made at the machine outlet, if the product residence in the line is brief, and allows corrections and calibration within a short time. Yes however, the production line is long, which is the case for vinyl flooring which must spend several minutes in ovens placed in line immediately downstream of the place of printing, then an online analysis of the test patterns must be carried out work, before the substrate leaves the line of production.

According to another characteristic of the invention, there are print heads downstream a test pattern analysis system including a camera color (CCD type) fitted with suitable optics, and mounted on a mechanical displacement system at micrometric position indexer arranged substantially perpendicular to the direction of travel of the substrate, and an associated processing system. The line of conveying of the substrate 50 is stopped so intermittent when the calibration target is placed substantially in the area scanned by the camera. The detection of the presence of the calibration target on the substrate can be made through the printing of a characteristic MOTIF mark at the edge of the substrate, signaling the presence of a calibration target. The MOTIF mark detection is carried out by a sensor optics 49 associated with the test pattern analysis system, similar to readers of second brands 41 associated to the Ti printheads: it triggers the shutdown momentary of the substrate. When the substrate stops under the analysis system, the camera 42 is moved by the mechanical system (transversely in the sense of scrolling of the substrate), at the same time as it analyzes the impacts of drops of different colors. The treatment system simultaneously records the characteristics of the printed dots and the position of camera 42 using position information from the position indexer on the axis of displacement. By comparing the positions of the points with their theoretical values, position differences can thus be determined and compensated for in the printing system during the next production. These compensations are automatically calculated by the processing system and remote transmissions to the controller of the printing process.

Even if the substrate is temporarily stopped, for reading the calibration target, penalizes the overall printer productivity, this solution appears the most robust to measure without ambiguity and with precision the dots printed in different colors on an industrial substrate whose texture can sometimes be complex. The impression being by elsewhere possible in acceleration phases and deceleration, this calibration phase only generates minor substrate losses, limited to the surface sights which are themselves very compact, limited at one, two or three DTOP intervals.

REFERENCES

[1] FR-A-91 11151

[2] "Design of a Paper Drive Mechanism of a Single-Pass Color Electrostatic Plotter for Accurate Image Registration ”by M. Dizechi, published in“ Journal of Imaging Technology ”, volume 15, number 16, December 1989

[3] US-A-5,452,073

[4] "A Strategy for Tandem Color Registration" by Caselli et al. in SPIE, volume 2658, pages 96-104, 1995.

Claims (28)

1. Continuous multicolour ink jet press, in which several printing heads (Ti) are arranged in series in order to print at least one substrate moving continuously beneath them, each head being supplied with ink of a given colour by means of its own individual ink circuit, said press comprising:

   a drive motor (43) driving a substrate (50), which passes successively under each of said heads,

   a high resolution position coder (48), typically of 3,000 to 300,000 dots per motor revolution, placed on said motor and which supplies a high frequency (TACHY) signal,

   several sensors (41) for reading marks, each sensor (41) being associated with a printing head,

   a synchronization circuit (45) connected to the printing heads, to the sensors (41) and to the coder (48), said synchronization circuit being controlled by a data processing system, which supervises the printing of each of the printing heads,

   characterized in that these marks (51) are first, regularly spaced marks, in that the synchronization circuit receiving the (TACHY) signal from the position coder (48) supplies a frame clock (HTRAMi) associated with each printing head (Ti), whose period is a multiple of said (TACHY) signal, in that each sensor (41) associated with a printing head (Ti) is an optical sensor making it possible to read said first marks (51) and the generation of a pulse signal (DTOPi), which defines the passage instant of a first mark (51) beneath said sensor the time between two time pulse signals (DTOPi) permanently containing an integral, constant number of periods of the corresponding frame clock (HTRAMi) and in that each sensor (41) is placed upstream of the corresponding head (Ti), at a distance slightly exceeding the distance separating two first marks (51).
2. Press according to claim 1, characterized in that it comprises a system (40) for printing first marks (51), which are regularly printed on the substrate upstream of the printing heads (Ti).
3. Press according to claim 1, characterized in that the coder (48) operates by means of an optical device.
4. Press according to claim 1, characterized in that it incorporates a conveyor belt, the first marks (51) being materialized by the fabrication on said conveyor belt.
5. Press according to claim 1, characterized in that it comprises a circuit processing the reading signal of the optical sensor (41) supplying said pulse signal (DTOPi), said circuit using hard, wired operators such as smoothing and time drift, in order to translate the precise instant at which one side of the first mark (51) passes.
6. Press according to claim 1, characterized in that a first mark (51) is formed by a square block with a side a few millimetres long.
7. Press according to claim 1, characterized in that a first mark (51) has a fluorescent colour.
8. Press according to either of the claims 1 or 4, characterized in that the distance between two first marks is approximately the same as the distance separating 100 to 500 lines of prited dots.
9. Press according to claim 1, characterized in that there is always the same number M of dots printed on the substrate between two first marks (51) for each colour.
10. Press according to claim 1, characterized in that the deviation between each optical sensor (41) and the associated printing head (Ti) is less than twice the distance between two first marks (51).
11. Press according to claim 1, characterized in that it incorporates a system of reading second marks.
12. Press according to claim 11, characterized in that the first printing head (Ti) prints said second marks.
13. Press according to claim 12, characterized in that said second marks are printed on the border of the substrate.
14. Press according to claim 13, characterized in that said second marks are located on the edge of the substrate on a line parallel to the advance direction at the correct distance from the line of first marks (51).
15. Press according to claim 12, characterized in that it comprises an optical system for reading said second marks, which generates a signal (MOTIF) indicating the change of pattern to be printed.
16. Press according to claim 12, characterized in that a second mark is formed by a succession of blocks (53) separated by a distance well below the distance between two first marks.
17. Press according to claim 4, characterized in that, for substrates in sheet form, second marks are generated by the appearance of the downstream edge of each sheet beneath a reading system for said second marks.
18. Press according to claim 1, characterized in that the synchronization circuit (45) performs prediction, filtering and windowing operations of the reading operation of the signal (DTOPi) corresponding to the passage of a first mark under a sensor (41).
19. Press according to claim 1, characterized in that, downstream of the printing heads (Ti), it comprises a test pattern analysis system (47) incorporating a colour camera (42) equipped with matched optics and mounted on a mechanical displacement system having a micrometric position indexer arranged substantially perpendicular to the advance direction of the substrate (50), as well as a processing system.
20. Press according to claim 19, characterized in that the calibration test patterns include geometric patterns making it possible to unambiguously identify dots printed by different printing heads (Ti) covering the width of the substrate.
21. Press according to claim 19, characterized in that the detection of the presence of a calibration test pattern on the substrate takes place by the printing of a characteristic mark at the edge of said substrate.
22. Press according to claim 21, characterized in that the detector (49) of the presence of a test pattern mark is similar to the sensors (41) associated with the printing heads (Ti).
23. Process for the synchronization of a press according to any one of the preceding claims, characterized in that the detection of a first mark (51) is firstly authorized in a limited time window centred on the probable passage instant of a first mark (51) beneath a sensor (41).
24. Process according to claim 23, characterized in that in the absence of detection of a first mark (51) in the reading window, a fictional signal (DTOPi) is generated starting from a first prediction based on the preceding interval (DTOP), and in that simultaneously the reading window is widened for the following detection instant, printing being stopped if the defect persists after four missing signals (DTOPi).
25. Process according to claim 23, characterized in that the offsets of the printing system are measured by intermittent analysis of multicolour calibration test patterns printed by the printing system, said calibration test patterns comprising geometric patterns making it possible to unambiguously identify dots printed by the different printing units (Ti).
26. Process according to claim 25, characterized in that the analysis of the test patterns takes place at the exit from the line, if the product residence time in the line is short.
27. Process according to claim 25, characterized in that the test patterns are analyzed in line by momentarily stopping the advance of the substrate when the production line is long.
28. Process according to claim 23, characterized in that on stopping the substrate beneath the analysis system, a camera (42) is displaced by the mechanical system transversely to the advance direction of the substrate (50), at the same time as it analyzes the impacts of droplets of different colours, in that a processing system simultaneously records the characteristics of the printed dots and the position of the camera (42) by means of position information coming from the position indexer on the displacement axis, in that by comparing the positions of the dots with their theoretical values, the position differences can consequently be determined and compensated in the printing system during the following production cycle and in that said compensations are automatically calculated by a processing system and remotely transmitted to a printing process controller.

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