EP3439883A1

EP3439883A1 - A system using an anilox with internal induction in printing machines

Info

Publication number: EP3439883A1
Application number: EP17718337.3A
Authority: EP
Inventors: Ludovico FRATI
Original assignee: Bobst Firenze SRL
Current assignee: Bobst Firenze SRL
Priority date: 2016-04-07
Filing date: 2017-04-06
Publication date: 2019-02-13
Also published as: ITUA20162391A1; WO2017174218A1

Abstract

The present invention concerns a printing unit provided with an engraved printing roller (1) and a relative method for controlling and adjusting printing parameters such as printing density and colorimetric parameters in printing machine, in particular flexographic printing machines.

Description

A SYSTEM USING AN ANILOX WITH INTERNAL INDUCTION IN PRINTING

MACHINES

DESCRIPTION

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

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.

The anilox is an engraved roller, that is with an outer surface in which cells or channels are formed, aimed at containing the ink transferred thereon by the ink-feeding module. This engraved outer surface can be the surface of a unitary roller body or, as in a presently more common solution, the anilox can be in the form of a tubular sleeve having the engraved outer surface, inserted by sliding over a sleeve-bearing core or mandrel. This is the so-called anilox sleeve.

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 (Z.) 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, in order 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.

It is an object of the present invention to provide an anilox roller 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 an anilox roller and a method for controlling and adjusting printing parameters in flexographic printing machines making use of such a roller, that can also be implemented as a revamping of pre-existing printing units.

These objects are accomplished with the anilox and the 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 and claim 12 attached.

The characteristics and the advantages of the anilox and the method according to the present invention shall become clearer from the following description of an embodiment thereof given as an example and not for limiting purposes with reference to the attached drawings, in which.

- figure 1 is a longitudinal sectional view of an anilox roller provided with magnetic induction means;

- figure 2 shows the magnetic induction means in isolation; and

- figure 3 shows schematically and in functional terms a printing unit including the anilox of figures 1 and 2. With reference to the above figures, and in particular to figures 1 and 2, an anilox roller 1 has an elongated development according to a longitudinal axis X and comprises an engraved outer surface 10 centered around the longitudinal axis.

The anilox 1 is adapted to be rotatably supported around the longitudinal axis X in a printing unit (generally indicated with the numeral 2) of a printing machine, namely a flexographic printing machine. The rotatable support means are represented only schematically in figure 1 , indicated by the reference numeral 20.

The anilox according to the invention comprises, underneath the engraved outer surface 10, an annulus tubular gap 1 1 , in turn centered around the longitudinal axis X and extending lengthwise substantially over the entire elongation of the anilox. The gap 1 1 is open to the outside through an access aperture 12 on a head face 13 of the roller.

Inside the tubular gap 1 1 there are arranged magnetic induction means 3 (shown in isolation in figure 2) such as, according to a preferred solution represented in the drawings, a coil inductor or solenoid 3. The coil 3 winds in a spiral developing, when arranged in the gap 1 1 , around the longitudinal axis X. The coil axially extends substantially for the same elongation of the annulus gap 1 1 .

Possibly, to house the coil steadily inside the annulus gap, a matrix of a non- ferromagnetic material can fill the gap, e.g. a polymeric material matrix 14.

The coil inductor is connected, via suitable electric wiring 40, to current generating means such as a current generator 4 (shown only schematically in figure 3 3) that supplies alternate current running in the coil and thus generating a magnetic field in its surrounding space This magnetic field induces, in turn, parasitic currents in the ferromagnetic material of which the anilox is made; the currents cause then a heating of the ferromagnetic material.

The connection between the current generator 4 and the coil 3 is obtained via a rotary electrical interface 5 with which the electric wiring 40 is linked; the rotary interface allows for the electrical connection to be maintained during the printing steps, i.e., as the anilox roller turns, being it arranged between the same roller and the structure 20 of the printing unit.

With particular reference now to figure 3, in the printing unit of a flexographic printing machine the anilox roller is associated with an ink-feeding module 21 and is tangent with a plate-carrier roller 22, counter-rotating with respect to the first roller. The plate-carrier roller transfers the image to a printing support 23, pushed in abutment against the roller by a counter-roller or pressure roller 24.

The printing unit further comprises an electronic controller 6 operationally connected to the current generator 4 and to at least a first sensor that measures a measured value of the printing parameters (such as in particular the density and the Lab colorimetric parameters) on the printing support 23. Possibly, a further or second sensor 8 can be provided to detect the temperature of the anilox roller 1 . The electronic controller 6, besides controlling the current generator to adjust the frequency and the power of the current circulating in the coil, carries out a comparison between the measured value of the printing parameters, as detected by the first sensor, with a desired target value.

The printing unit according to the invention operates as follows. The first sensor 7, that can be e.g. (but this indication is not limitative) a spectrophotometer, detects the measured value of the printing parameters and sends this data to the electronic controller 6 that compares this measured value with the target value. If the measured value does not match the desired value, the electronic controller 7 switches the current generator on, to make the electric current run through the coil. The current circulating in the coil at a chosen frequency and power determines a controlled heating of the engraved surface of the anilox. The ink in the cells of the engraved surface becomes in turn heated, and this affects the viscosity of the ink, or more generally speaking its physical properties improving its capability to be effectively transferred from the anilox roller to the plate-carrier roller. The improved transfer of the ink from the anilox roller to the plate-carrier roller, and from the latter to the printing support. This more effective transfer has a direct positive outcome as far as the quality of the print is concerned, as measurable on the printing parameters.

The temperature sensor 8 measures the temperature on the engraved outer surface of the anilox and then, in case this measured temperature does not correspond to a desired value, the controller commands the current generator, varying 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.

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.

Moreover, the anilox roller according to the invention can be an anilox sleeve or a unitary body; in the case of a sleeve, like the one shown in figures 1 and 2, the anilox comprises an engraved hollow tubular sleeve 10', developing around the longitudinal axis X. The engraved tubular sleeve defines, with its outer surface, the engraved surface 10. The tubular sleeve is arranged by sliding in a coaxial fashion (according to the longitudinal axis X), over a sleeve-bearing core 15. In this type of sleeve roller, the tubular gap 1 1 is formed within the thickness of the sleeve 10', underneath the outer engraved surface. The anilox roller can even, advantageously, be mounted in already existing printing units, with no need of changing the structure of the unit.

The present invention has been described with reference to a preferred embodiment thereof. It should be understood that there can be other embodiments that belong to the same inventive concept, inasmuch as they fall within the scope of protection of the following claims.

Claims

1 . A printing unit of a printing machine, comprising an anilox roller (1 ) and a printing support (23),

The anilox roller (1 ) being elongated according to a longitudinal axis (X) and adapted to be rotatably supported by a structure (20) of the printing unit (2);

The anilox roller further comprising an outer surface (10) and a tubular annulus gap (1 1 ) formed around the longitudinal axis (X) underneath the outer surface (10); and magnetic induction means (3) housed in said gap;

The printing unit comprises electrical current generating means (4) connected to the magnetic induction means (3) of the anilox roller (1 ), an electronic controller unit (6) operationally connected to the electrical current generating means (4) and at least a first sensor (7) for the measurement of printing parameters on the printing support (23).

2. The printing unit according to claim 1 , wherein the magnetic induction means (3) comprise a coil inductor elongated along the longitudinal axis (X) when the inductor is mounted in the tubular gap.

3. The printing unit according to claim 1 or 2, wherein the coil inductor is adapted to be connected to electrical current suppling means (4) via a rotary interface between the anilox roller and the structure (20) of the printing unit.

4. The printing unit according to claim 2 or 3, wherein the coil inductor is supplied with alternate current.

5. The printing unit according to any of the previous claims, wherein the anilox roller (1 ) comprises a sleeve-bearing core (15) elongated along the longitudinal axis (X) adapted to be rotatably supported around the longitudinal axis in the printing unit (2), an anilox tubular sleeve (10') engaged in a coaxial arrangement, according to the longitudinal axis (X), with the core (15), an outer surface of the anilox tubular sleeve defining the outer surface (10).

6. The printing unit according to claim 5, wherein the tubular gap (1 1 ) is formed within the thickness of the anilox tubular sleeve (10'), underneath the outer surface (10).

7. The printing unit according to any preceding claim, further comprising a second sensor (8) for the measurement of the temperature of the outer surface (10) of the anilox roller (1 ), the second sensor being operationally linked with the electronic controller unit.

8. The printing unit according to any preceding claim, wherein the electronic controller unit (6) is adapted to receive a measured value of the printing parameters from the first sensor (7) and to compare the measured value with a target value, the controller unit activating the electrical current generating means (4) when the measured value differs from the target value.

9. The printing unit according to any preceding claim, wherein the controller unit (6) is also adapted to adjust at least the frequency and power of the current supplied by the electrical current generating means (4) to the magnetic induction means (3).

10. The printing unit according to any preceding claim, wherein the electrical current generating means (4) comprise a generator of alternate current.

1 1 . A method for the control and the adjustment of the printing parameters, such as density and colorimetric parameters, in a printing unit of a printing machine such as a flexographic printing machine, the printing unit comprising an anilox roller (1 ) and a printing support (23), the method comprising the steps of:

- measuring a value of said printing parameters on said printing support (23);

- comparing said measured value with a target value;

the method being characterised in that it further comprises the step of, when said measured value differs from the target value, activating electrical current generating means (4) of the printing unit to supply an electrical current to magnetic induction means (3) arranged inside the anilox roller in order to obtain a magnetic field that affects the anilox roller to generate a warming of an outer surface (10) of the anilox roller.

12. The method according to claim 1 1 , wherein the step of measuring the value of the printing parameters on the printing support (23) is implemented by a sensor (7).

13. The method according to claim 1 1 or 12, further comprising the step of measuring the temperature of the outer surface (10).

14. The method according to claim 13, further comprising the step of adjusting at least the frequency and the power the electrical current supplied by the electrical current generating means (4), depending on the temperature measured on the outer surface.