PRINTING MACHINES

FIELD OF THE INVENTION

The present invention concerns a device and a method for controlling and adjusting printing parameters such as printing density and colorimetric parameters in printing machines, in particular, flexographic printing machines.

DESCRIPTION OF RELATED ART

As known, printing machines such as flexographic printing machines comprise a roller unit with mutually tangent and counter-rotating rollers that make for the transfer of the ink to a printing support. Generally speaking, in a flexographic printing machine these rollers comprise a roller with an engraved coating, the so-called anilox, a roller for supporting the printing plate, and a counter-pressure roller (or counter-roller for the sake of brevity).

The anilox is in direct contact with an ink feeding module (such as an ink storage tank or chamber with its doctor blade) of the machine and transfers the ink to the printing plate that in turn carries out the actual printing step, that is the transfer of the ink on the printing support, held by the counter-roller.

A seriously felt drawback in these type of machines is that as the printing speed increases (that is, as the rotation speed of the printing rollers increases), the effectiveness of the ink transfer to the printing support is affected so that the quality of the print is eventually worsened. More specifically, as the quality of the ink transfer varies, there is a rampant and unpredictable 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 color ranges respectively from green to red and from blue to yellow with values from -120 to +120.

It has been indeed understood that to fight this printing quality worsening, one has to ensure that the ink is effectively transferred in a correct manner, independently from the printing speed. A known method that aims to obtain this result makes for increasing or decreasing the mutual distance between the printing rollers, because in this way, as a result of this moving away/close of the rollers, the ink pressure with which the ink is transferred from one roller to the other varies, and this pressure variation, therefore, improves the eventual transfer of the ink to the printing support.

Among the known devices that make use of this method, the one disclosed in document EP2384892 can be mentioned. This device comprises detection means for detecting the printing parameters, actually consisting of video cameras and radiating means affecting a determined portion of the roller to assist the image capture of the cameras. Depending on the values detected by this detection means, the two rollers are mutually displaced. The ink transfer effectiveness remains thus constant as a result of the change of mutual pressure between the rollers.

However, this solution has drawbacks deriving mainly by the constructive complication that ensues from the provision of a control system that has to take care of the change of mutual distance between the rollers. Moreover, especially in the flexographic print in which the contact pressure between the roller and the support strongly affects the printing quality, acting on the pressure to fight the decay of the printing parameters can indeed easily result in a loss of quality, because the pressure, if excessive, can deform the printing plate (made of a plastic material), causing printing defects that are typical of an incorrect printing pressure.

Other known systems provide for a heating of the anilox, 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. BRIEF SUMMARY OF THE INVENTION

It is an object of the present invention to provide a device and a method for controlling and adjusting printing parameters (such as printing density and colorimetric parameters) namely in flexographic printing machines, that permit to solve the above- mentioned problems, and in particular to reduce, if not eliminate completely, the uncontrolled decay of the printing density and of the colorimetric parameters as the printing speed increases.

A particular object of the present invention is to overcome all the limitations of the known devices and provide an advantageous technical alternative with respect to such devices.

A further object of the present invention is to provide a device and a method for controlling and adjusting printing parameters in flexographic printing machines, that can also be implemented as a revamping of pre-existing printing units.

These objects are accomplished with the device and method for controlling and adjusting printing parameters such as printing density and colorimetric parameters in printing machine, in particular flexographic printing machines, according to the invention, having the essential features defined respectively by claim 1, 5 and 7.

The characteristics and the advantages of the device and method 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

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 shows schematically a printing unit in which a device according to the invention is mounted.

Figure 2 shows a top view and a side cut of an embodiment of the magnetic inductor. Figure 3 shows a side view of a magnetic inductor positioned to heat the anilox.

Figure 4 shows a perspective view of the magnetic inductor in an idle position. A rotation around the axis according to the arrow is able to move the inductor in the position of Figure 3. DETAILED DESCRIPTION OF POSSIBLE EMBODIMENTS OF THE INVENTION

With reference to the figure, the printing unit of a printing machine, such as a flexographic printing machine, comprises an anilox roller 1 to which an ink-feeding module 2 is associated for transferring the ink over an engraved outer surface 10 of the anilox 1. The anilox is tangent with a plate-carrier roller 3, counter-rotating with respect to the first roller. The plate-carrier roller transfers the image to a printing support 4, pushed in abutment against the roller by a counter-roller or pressure roller 5. After passing through this printing process the support is referred to as printed support 4 (whereas printing support or support does not specify whether the support has been processed by the printing unit).

The device according to the invention comprises magnetic induction coil 6 arranged close to the anilox 1 so that a magnetic field generated by such inductor affects the roller on the whole width on a portion of the circumference.

The magnetic inductor 6, comprising for instance a coil inductor or solenoid, are such to generate, if a current (such as alternate current) runs through them, a magnetic field in their surrounding space. This surrounding space involves the whole width of the anilox, and at least its outer surface 10. By "the whole width", we mean the width W of the portion of the anilox configured to transport ink, which is the portion engraved with tiny cells that can contain ink. When the magnetic field hits the ferromagnetic material of which the anilox roller is made, parasitic currents are generated that cause a heating of the ferromagnetic material and thus of the outer surface of the anilox 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, in most cases becomes more fluid with increased temperature, consequently improving the capability of the ink to be transferred more effectively from a roller to the other. The improved transfer of the ink from the anilox roller 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. To keep the system simple and inexpensive, the preferred solution is an inductor 6, which consists of a single coil 12. By single coil, we mean a coil, which can be controlled by a single source of current 11. The single allows for a simple control of the magnetic fields, and thus saves costs. Preferably, the coil is spooled around a single E- shaped structure 13, as shown in Figure 2. The E-shaped structure is used to hold the wires 14 of the coil 12, and is also directing the magnetic field toward the anilox surface. The coil is positioned parallel to the rotation axis of the anilox. In operation, to coil is arranged to direct the magnetic field according to a direction along a radius of the anilox. The distance to the anilox can be adjusted according to the anilox radius and according to the magnetic field to be transmitted. The inductor is advantageously mounted on a pivoting structure 16 to be able to quickly set the induction into an idle position for exchanging the anilox with a different one.

The sensor which measures the printing parameters on the support can be a densitometer, a camera, a spectrophotometer, a colorimeter or even a high-resolution camera. The high-resolution camera being a camera whose resolution is high enough to distinguish the individual dots in a dot pattern and is able to measure the effective ink coverage. The sensor may deliver the density, or the LAB parameters, or the spectrum or the ink coverage or a combination of said parameters.

Activation means 7 are operationally connected to the source of current 11 that controls the magnetic inductor 6. The activation means can include an electronic controller that is in turn operationally linked with at least a first sensor 8 that measures a measured value of the printing parameters (such as in particular the density and the Lab colorimetric parameters) on the printed support.

The measured value obtained by the first sensor 8 is sent to the electronic controller that compares this measured value with a desired or target value. If the measured value does not match the desired value, the electronic controller 7 switches the magnetic inductor on, i.e. activates the current source (11), which 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 9 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 a desired value, the controller instructs the current source 11 to vary the frequency and power of the current circulating in the coil, until the measured temperature value matched the desired one. Indeed, by varying the frequency and the power of the supplied alternate current running in the coil, a magnetic field variation is obtained, and a consequent change of the induced currents, thereby tuning their penetration depth and their heat generating effect.

By heating the anilox by induction, and using an ink whose temperature is lower than the desired temperature allows the system to "cool" the ink: since the ink in the ink source is too cold, by reducing the heating applied on the anilox, it results in an overall drop in temperature of the ink transferred to the support. In this way, cooling is performed by heating less, and can, therefore, be as rapid as the heating itself.

The method, therefore, obtains the adjustment of the printing parameters, so that they can be unaffected from the printing speed, in a real-time manner, by acting on the transfer capability of the ink.

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