SOURCES TO A PRINTING MACHINE

FIELD OF THE INVENTION

The present invention concerns a system for the on-board ink management, mainly directed to flexographic or rotogravure printing machines. The system according to the invention achieves the feeding of the ink in an ink supplying module of the printing machine and also controls the adjustment of the printing parameters.

BACKGROUND OF THE INVENTION

As known, flexographic and rotogravure printing machines comprise a roller unit with two mutually tangent and counter-rotating rollers that make for the transfer of the ink to a printing support. In particular, there is at least one inking roller in direct contact with an ink feeding module (such as an ink storage tank with its doctor blade). The inking roller transfers the ink (directly, in the case of rotogravure printing, or indirectly via a further roller, in the case of flexographic printing) to the printing support, held by a counter- pressure roller.

In the currently known printing machines of the type as just described, the problem arises of replacing the ink of a certain colour or quality circulating in the machine, with a different one, as the printed image varies. Embedded systems are thus provided for taking care of a controlled replacement of the ink, at the same time cleaning the tank and the ducts.

These mentioned known systems are then directed to supply the printing machine with an ink having a predetermined desired quality (as far its nature or colour are concerned).

In flexographic and rotogravure printing machines a standard printing process has been recently developed making use of seven ink colours (besides white). With this standard a wide range of printed colours can be obtained, capable of covering about 90- 95% of the colours coded under the PMS (Pantone® Matching System). Such a standard overcomes the old concept of having a specific ink for each colour, and thus the necessity to replace the ink in the machine to change the printed colour. However, a seriously felt drawback, particularly in flexographic printing machines, is that when the printing speed increases (i.e. when the rotation speed of the printing rollers increases), the effectiveness of the transfer of the ink to the printing support is affected, resulting eventually in a drop of the print quality. In particular, as the quality of the ink transfer varies, there is a rampant and difficult to predict decay of the printing parameters, in terms e.g. of printing density and colorimetric parameters (Lab). These comprise Lightness (L) expressed as a percentage (0 for black and 100 for white) whereas with a and b tit is meant two colour ranges respectively from green to red and from blue to yellow with values from -120 to +120. STATE-OF-THE-ART

EP 2,985,150 discloses a system that corrects the ink composition in a press based on the result of a measurement of the printed support. The inking system uses two sets of ink and two mixing system. Each mixing system has a plurality of ink sources. The first set is used to produce the base ink, the second set (and second mixer) is used to correct the base ink based on the measurement of a printed patch. One drawback of the system is the complexity. Another drawback is the reaction time. The system lacks the elements to correct the printed output promptly: It can only add ink. Also, the corrected ink is not in direct contact with the printing roller (it is presented as an ink dispensing unit), and thus, it has to wait until some old ink is consumed before the corrected ink is used. In addition, to control the temperature the whole volume of ink is heated, which is slow, and the system cannot be used to cool the ink if the temperature happens to be temporarily too high.

US 2015/0210060 discloses an inking system with a recirculation circuit which compensates for the evaporation of the solvent in real time. The system is meant to be refilled with ink at the end of each printing job and is not suitable for an in-line adaptation of the printing parameters.

OBJECT AND SUMMARY OF THE INVENTION

An object of the present invention is therefore to provide a system for the on-board ink management, that is a system capable of effectively controlling the ink feeding in an ink supplying module of the printing machine and moreover of adjusting the printing parameters. A further object of the present invention is to provide a system that permits to vary the composition of the processed ink, for example the amount of pigment contained therein, and obtaining a variation of the printing parameters such as the Lab colorimetric parameters and the colour density that can be detected on the image transferred to the printing support.

These objects are accomplished by the device for supplying ink in an ink supplying module of a printing machine and for the adjustment of printing parameters according to the invention, the essential features of which are defined by the first of the appended claims. The objects are also attained by the related method for feeding ink in a ink supply module of a printing machine and for adjusting the printing parameters, the essential features of which are defined by appended claim 7.

LIST OF FIGU RES

Figure 1 shows schematically and in functional terms the system according to the invention. DETAILED DESCRIPTION OF THE INVENTION AND OF SOME OF ITS EMBODIMENTS

The characteristics and the advantages of the system according to the present invention shall become clearer from the following description of an embodiment thereof given as an example with reference to the attached drawing, in which figure 1 shows schematically and in functional terms the system according to the invention, installed on a printing unit of a flexographic or rotogravure printing machine.

The printing machine comprises an inking roller 1 and a counter-pressure roller 20. The inking roller 1 may be an anilox roller in a flexographic printing machine or the gravure cylinder, i.e. the cylinder comprising the pattern to print, in a rotogravure printing machine. The inking roller 1 is in direct contact with an ink supplying module 3 (such as a tank and doctor blade unit with a closed chamber or an open tray, or any other type known in the art) and transfers the ink to a printing support 4, held by the counter-pressure roller 20. The figure represents specifically a layout of a flexographic printing unit, and in this case, also a third roller 21 is shown, that in the flexographic machine has the function of supporting a printing plate. In rotogravure printing machines only the inking roller and the counter-pressure roller are instead present. Going now into further details as far as the aspects of the invention are concerned, the ink supplying module 3 is connected with a main feeding circuit 50 through which a recirculation of the ink is carried out between an outlet 30 and an inlet 31 of the ink supplying module 3. Let's call the assembly made of the main feeding circuit and the (connected) ink supplying module the "main circuit" 5. In fact, in the ink supplying modules that are most widely used in the field, an exceeding amount of ink over the surface of the inking roller is recovered by the doctor blade to be used again. Thus, advantageously, the main circuit 5, via the provision of circulation forcing means 51 , such as a pump, allows for a forced flow of the ink from the outlet 30 towards the inlet of the ink supplying module 3. Downstream of the circulation pump 51 , but upstream of the inlet 31 , the main circuit is connected with ink refill device 6 for providing an additional amount of ink to the main circuit 5. The refill device 6, described in further detail hereafter, is logically controlled by control means 7, adapted to detect a measured value of printing parameters on the printing support, and to compare said measured value with the desired value of such parameters; accordingly, if thus measured value does not match the desired value, the control means 7 activate the refill device 6 to provide the main circuit with the additional amount of ink.

Adding an additional amount of ink to the ink already present in the main circuit results in a new mixture of ink in the main circuit 5 whose characteristics are in-between the ink added, called the "corrective ink", and the one already present, called the "old ink"; the result of the mix being called the "adjusted ink". To perform the correction in a minimum amount of time, the main circuit 5 may preferably contain as little ink as possible, and thus an ink supplying module 3 with a closed chamber and two blades is preferred to a solution using an open tray. To further decrease the reaction time of the system, two options are proposed: the first uses an ink outlet 9, and the second varies the temperature of the inking roller 1 . Both options can be used independently or in combination.

A printed colour channel has typically 10% ink coverage, representing approximately 0.1 1 gramme per square meter of ink coverage. For a press printing on a 60 cm wide printing support at 150 meters per minute, it consumes approximately 0.6 litres of ink per hour. For an ink supplying module 3 that needs approximately 1 litre of ink per meter of printing support width, and ignoring the amount of ink present in the main feeding circuit 50, the total amount of ink in the main circuit is 0.6 litres. In other words, it takes one hour for the press to use 100% of the ink present in the main circuit 5. If the needed colour correction is large, performing a correction based solely on the addition of corrective ink to the main circuit will result in a system that is too slow and might not be able to correct the deviation of the print characteristics in a timely manner. Using a corrective ink with very different characteristics compared to the adjusted ink could be a solution for a fast correction using a small amount of ink. This solution is however not recommended because, there is a risk that, due to a temporary lack of proper mixing, the corrective ink might be transferred as is to the support, resulting in a large colour mismatch and an oscillation in the print characteristics. The lack of this possibility contributes to the slowness of the correction using only additive corrective ink.

To efficiently correct the ink characteristics, the refill device 6 may add an additional amount of ink to the main circuit 5, while the ink outlet 9 may remove a second amount of ink from the main circuit 5, resulting in a replacement of a given percentage of the ink. Preferably, the additional amount of ink added is equal to the second amount of ink removed (for the sake of simplicity of this disclosure, we disregard here the fact that in a normal running situation there is a given flow of additional supplementary ink that is added to the main circuit 5 by the refill device 6 to compensate for the consumption of the printing machine, said consumption being the flow of ink transferred to the printing support; thus the amount of ink removed might be slightly smaller than the amount of ink added). Preferably, the refill of the ink by the refill device 6 occurs simultaneously to the removal of the ink by the ink outlet 9. Thanks to this last characteristics, the main circuit does not need to have means to handle large volume variations of ink. The ink outlet is positioned on the main circuit 5 such that when corrective ink is added to the circuit, the ink removed with the outlet 9 is old ink (and not a mixture of old ink and corrective ink). In other words, following the ink flow in the main circuit 5, the ink outlet 9 is positioned upstream from the ink refill device 6, which is positioned upstream from the ink supplying module 3, which is positioned upstream from the ink outlet 9, etc. Preferably, the ink amount in the main circuit 5 between the ink outlet 9 and the ink refill device 6 (measured along the ink flow) is kept to a minimum compared to the total amount of ink in the main circuit, thus placing the ink outlet 9 just before the ink refill device 6 (according to the ink flow).

From the ink outlet 9, the ink is collected in a collection reservoir 63. The collected ink has a known characteristic and thus can be adjusted by adding some pigments or by adding some solvent in order to match the characteristics of the ink present in one of the tanks 62. Then, the collected ink can be reused in said tank. Optionally, the ink characteristics are measured offline in an ad hoc place, adjusted, and used to refill one of the tanks 62.

The ink outlet can either be an active device (preferably), comprising a valve and/or a pump which can be activated by the control means 7, or a passive device. It can also be a flux deviator. The outlet can be a device that regulates the amount of ink present in the main circuit 5, for example by keeping the volume constant. This constant volume function can be implemented as an overflow reservoir, which removes ink whenever the ink pressure exceeds a given threshold (the overflow reservoir can be a passive device, i.e. not connected to the control means 7). Naturally, whenever the ink refill device 6 inject an additional amount of ink in the circuit, the overflow reservoir 9 removes that same amount of ink from the main circuit, leading to a simple system.

The refill device 6 are connected with two or more auxiliary tanks 62 each containing ink with a higher or lower amount of pigment with respect to the ink in the ink supplying module, or ink with a different type of pigment. It is also possible to have an embodiment with one or several tanks 62 containing ink with a higher amount of pigment compared to the ink in the ink supplying module and have a tank 62 filled solely with solvent (the solvent may be water for water-soluble inks). The tanks 62 can be any receptacles suitable for storing liquids. In a first embodiment, the refill device 6 comprises a pump 61 connected to two or more tanks 62, as depicted in the figure. In this case, the pump 61 , in response to a determined input signal from the control means 7, selectively draws a certain selected amount of ink from either tank to which the same pump is associated, and directs such amount as an extra amount of ink to the main circuit. In this embodiment the pump 61 can be provided with a switching valve (not shown), to selectively clear or block the hydraulic connection of the pump with either tank.

In a second embodiment, the refill device 6 comprises more than one pump, and namely a pump for each tank 62. In this case, the control means 7 may activate only the pump of the selected tank, to draw a determined amount of ink and direct such amount as an additional ink amount to the main circuit. As an alternative, the control means may activate each pump simultaneously, by selecting the appropriate duration and/or flow for each pump to reach the desired ink mixture. The addition of such extra amount of ink to the main circuit can change the composition of the ink inside the main circuit and consequently permits to adjust the colour of the ink right inside the printing machine

The refill with the extra amount of ink can occur at a single junction point of the main circuit (as in the depicted example where the junction point is indicated by the reference character P), or at plurality of different junction points, e.g. a junction point for each pump when the refill device 6 comprises more than one pump.

A correct mixing of the extra amount of ink with the flow circulating in the main circuit is carried out through mixing means 8 arranged in the main circuit, downstream of the refill junction point P. The mixing means comprise e.g. a static mixer 8.

The ink in the main circuit is circulated at a higher speed than the flow of the ink determined by the ink consumption of the printing machine, thanks to the pump 51 . Thus, the ink passes on average several times through the mixer 8 before reaching the printed support. Preferably, the ink flow speed is set to at least 10 times said consumption speed. The pump 51 can be placed anywhere on the main circuit, but preferably just before the inlet P for the corrective ink.

Circulating the ink in the main circuit at high speed is important in some cases. It reduces the time needed for the ink mixture (after addition/replacement by the corrective ink) to reach the support. It also reduces the time needed for obtaining a well-mixed adjusted ink (even in the absence of the mixer 8) in the case when the corrective ink does not replace the old ink completely (but is mixed with it). For example, the addition of an extra amount of ink can be programmed to last the average time that the ink needs to circulate once through the main circuit. This last trick prevents the output characteristics to oscillate between the characteristics of the old ink present in the main circuit before the correction and the characteristics of the corrective ink. The oscillation might happen if there is a section of the main circuit filled with corrective ink and another section of the circuit filled with old ink provided that these sections have not been mixed yet. On the downside, adding ink in this way requires the knowledge of the total amount of ink in the circuit, which may be difficult to monitor over time, and requires an ad hoc sensor, for example, one or two flow sensors 72.

A simple method to perform the correction consists in replacing the old ink by corrective ink (i.e. the adjusted ink is the corrective ink), by providing the corrective ink while fully opening the ink outlet 9, until the print parameters recorded by sensor 71 match the specifications. Once the specifications are met, the circuit is closed, and the printing continues as normal. In this strategy, the recirculation speed of the ink in the circuit 5 is less critical, since it does not involve mixing corrective ink with old ink (except for mixing the residue of the old ink). Getting back to the control means 7, these comprise an electronic controller 70 to which a sensor 71 is logically connected, for detecting the printing parameters such as the Lab colorimetric parameters and density, or the reflection spectrum, or the ink coverage, or any combination of these parameters, directly on the printing support. The sensor 71 can be for example a spectrophotometer, a colour camera, or a high-resolution camera. The spectrophotometer measures the spectrum (from which the Lab colorimetric parameters and density can be extracted), the colour camera measures density and Lab colorimetric parameters, and the high-resolution camera can measure, in addition to colour and density, the ink coverage, i.e. the percentage of the support area covered by the individual halftone dots. When, via the sensor 71 , a printing parameter is detected that does not match a predefined target value, the electronic controller 70 activates the refill device 6 so as to make it draw the desired extra amount of ink from a chosen tank 62 and have it mixed with the ink circulating in the main circuit. In the embodiment that uses heating means, the electronic controller 70 may activate the heating means so as to reach a target temperature that results in the desired ink viscosity. Also, if the mismatch in print parameters is out of specification, or close to be out of specification, The electronic controller 70 activates the ink outlet device to accelerate the replacement of the old ink by the adjusted ink. In extreme situations, the ink outlet device can be used to replace the old ink by the corrective ink completely. The mixing with an additional ink having a pigment of a different type affects the

(optical) density of the ink as well as its viscosity. On the one hand, using ink with an increased amount of pigments, thus a more viscous ink, reduces the quantity of ink that the inking system transfers to the support, potentially reducing the printed density. On the other hand, having more pigment in the ink increases the printed density. Also, in some inks, the viscosity might even vary according to the opposite trend (or not at all) with the pigment density. The overall result of these two phenomena depends on the situation, in particular on the ink characteristics, the solvent characteristics and the current viscosity. When heating the inking roller, the viscosity of the ink decreases (i.e. the ink gets more fluid), and the amount of liquid transferred to the support is increased. By increasing the amount of ink transferred, the ink coverage of a given halftone pattern is increased. Also, by increasing the speed of printing, the amount of ink that can be transferred by the inking roller decreases. All these phenomena put together result in a quite complex system. Nevertheless, since the system is built around a feedback loop (using sensor 71 ), the control system 7 does not necessarily need to know beforehand the exact behaviour of the inking system to perform the control (even if it might help). It may use any adaptive control technique known in the art to perform the correction. Adaptive control techniques estimate the dynamic behaviour of the printing system using measured data over time. Also, a solution to keep the effect of the corrective ink simpler is by controlling the temperature of the inking roller for keeping the viscosity of the ink constant, and independent of the pigment density. This is possible within the range of temperatures that are acceptable according to the ink and support specifications.

Possibly, a further sensor 72 can be provided in the main circuit upstream of the refill junction point P where the additional amount is received, and downstream from the ink supplying module 3. This is a flow rate sensor for detecting the flow rate circulating in the main circuit and transmitting the measured data to the controller 70 so that the control means can set the appropriate amount and rate of extra ink to be mixed with the flow that is already circulating.

From an operative standpoint, the system according to the invention can be thus further detailed as follows. At a zero time, the machine, and therefore the ink supplying module, contains a certain ink, with a defined amount and type of pigment. Once the printing step has started, the sensor 71 detects the printing parameters, for example, the Lab colorimetric parameters and the printing density on the printing support 4.

As long as these measured values are compliant with the target values, the refill device 6 remains mostly inactive (the refill device 6 may compensate for the normal ink consumption of the machine). When the values detected by the sensor 71 no longer match the target values, the controller 70 activates the refill device 6 selecting, upon consideration of the measured values detected by the sensor 71 , and upon computing the desired ink composition and the composition and amount of corrective ink, activates the refill device to draw the appropriate amount and percentages of ink from the different ink tanks 62. At this point, the extra amount of ink is pumped into the main circuit and then received by the ink supplying unit module in which it wets the inking roller and thus becomes used in the printing process. Depending on the embodiments, the control means can also act on the temperature of the inking roller to influence the ink viscosity, or decide to remove some ink from the main circuit 5 to accelerate the printing parameter correction.

The outlet device 9 can also be used for flushing the system with a cleaning fluid that circulating in the main circuit cleans it and empties it completely before processing an ink of a different nature.

Some advantages ensue from the system according to the invention. Not only the system obtains a continuous control of the printing quality thanks to a real-time adjustment of the printing parameters, but also to the possibility of modifying the type and amount of pigment in the ink without emptying the circuit. A further option is also to mix the ink circulating in the circuit with an ink characterised by a different ink, thus obtaining an even differently coloured ink. In this way, a number of different colours can be accomplished without having to replace the ink in the ink supplying module altogether.

Among the inks that can be used, one can mention UV inks, water-based inks and solvent-based inks, but this notwithstanding, the system according to the invention can be applicable also with inks of different nature.

To control the viscosity of the ink, heating the inking roller 1 is the preferred solution. Of course, one could heat the ink in the main circuit 5 and/or in the ink tanks 62, but this would imply heating the total mass of ink and thus be slow. In addition, if we need to cool the ink without having to setup expensive cooling means, it would imply waiting until the ink temperature drops naturally, and thus be very slow. Thus, heating the ink in the main circuit can advantageously be used to keep the ink at an intermediate temperature, which may be chosen as the minimal temperature expected to be ever used for printing, but not for bringing the ink to the desired temperature. Keeping the ink at a minimal temperature may also be a guarantee for a good mixing and flow of the ink in the main circuit. Nevertheless, by heating the inking roller, a tiny layer of ink is heated (i.e. the layer transported by the inking roller), which leads to a reactive system for heating as well as for "cooling". By cooling, we refer to the situation where the target temperature has to be decreased (over time) by the control system 7; this "decrease" in temperature is, in fact, heating to a lower temperature than before and is thus as fast as the heating process. Known systems can be used for a heating of the inking roller 1 , heating that is then transmitted by thermal conduction to the ink that is spread over the roller. For instance, the heating of the anilox can be obtained by a heat carrier fluid that is made to flow inside the anilox, underneath the engraved surface; the fluid is heated in a controlled manner by a heating surface inside or outside the anilox. Moreover, among known systems, some exploit the heating generated by incandescence of electrical resistances. However, these systems are structurally complex and costly. Thus, the preferred solution to heat the inking roller uses inductive heating.

The inking roller 1 according to the invention comprises a magnetic inductor 100 arranged close to the inking roller 1 so that a magnetic field generated by such inductor affects the roller at least partially. In rotogravure systems, the magnetic inductor may be used to heat an ad hoc inking roller, or may be used to heat the gravure cylinder directly (in the latter case, the gravure cylinder may act as an inking roller, thus the inking roller mentioned in the claims may refer to the gravure cylinder when the invention is applied to a rotogravure printing machine).

The magnetic inductor 100, comprising, for instance, a coil inductor or solenoid, are such to generate, when a current (such as alternate current) runs through it, a magnetic field in its surrounding space. This surrounding space involves at least part of the inking roller and at least its outer surface 10. It is configured to heat said part of the inking roller over the whole width of the inking roller. When the magnetic field hits the ferromagnetic material of which the inking roller is made, parasitic currents are generated that cause a heating of the ferromagnetic material and thus of the outer surface of the inking roller. The ink over the surface of the roller is also heated, as a result of the thermal conduction by the underlying surface, and as a result of the heating the physical properties, e.g. the viscosity, of the ink change, consequently improving the capability of the ink to be transferred more effectively from a roller to the other, or to the support. The improved transfer of the ink from the anilox inking to the plate-carrier roller, and from the latter to the printing support has a positive outcome as far as the quality of the print is concerned, an outcome that can be verified on the printing parameters. The control means 7 can be operationally associated with the magnetic inductor 100.

The control means can include an electronic controller that is in turn operationally linked with the sensor 71 that measures a measured value of the printing parameters on the printing support. To heat the inking roller, the electronic controller 70 switches the magnetic inductor on, i.e. makes the electric current run through the coil to generate the magnetic field causing the temperature to rise on the outer surface of the anilox.

Possibly, a temperature sensor 1 10 can also be linked to the electronic controller, to detect the temperature of the engraved outer surface 10; accordingly, in case this measured temperature does not correspond to the desired value, the controller 70 varies the frequency and/or the power of the current circulating in the coil, until the measured temperature value matched the desired one. Indeed, by varying the frequency and/or the power of the supplied alternate current running in the coil, a magnetic field variation is obtained, and a resulting change of the induced currents, thereby tuning their penetration depth and their heat generating effect.

The inductive heating device can be even applied to an already existing printing unit, and to any anilox roller, including the anilox sleeve (having an outer sleeve inserted over a sleeve-bearing core or mandrel).