ES2395183A1 - Method for the adjustment of pressures in a flexographic printing press and flexographic printing press for the implementation thereof - Google Patents

Abstract

The method comprises: in a first step, linearly moving a printing roll and an inking roll towards a central, counterpressure roll, causing said rolls to rotate, and monitoring the torque thereof, and, upon detection of a variation in torque, performing a first adjustment of pressure between said printing roll and one of said central, counterpressure or inking rolls and, in a second step, a) carrying out printing b) capturing an image from said printing c) examining the quality of said captured image and d) varying the pressure exerted between the printing roll and one of said central, counterpressure or inking rolls and repeating said sequence of actions a) to d) to achieve an optimum level of pressure between the rolls. The press is designed in order to implement the method.

Description

Pressure adjustment method in a flexographic printing machine and flexographic printing machine for its implementation

Technique sector

The present invention concerns in a first aspect a method for adjusting the printing pressure, using information captured from the printed image, applicable to a flexographic machine, conventionally equipped with various printing groups arranged around a back pressure cylinder central on which the material to be printed is supported, and integrating each of said printing groups at least one printing cylinder and an inking cylinder (also called an ink applicator) that receives ink supply, said adjustment being necessary to achieve a correct impression.

It is well known in this sector that in order to carry out printing tasks by means of a flexographic machine and achieve optimum ink transfer, it is necessary to provide, by means of an adjustment, a correct pressure both between the inking cylinder and the printing cylinder, and between said printing cylinder and the material to be printed, that is, the correct pressure between the said cylinders, inking cylinder and printing cylinder and between the latter and the central back pressure cylinder, which supports the material to be printed.

The correct pressure between the cylinders of the inking group (impression cylinder and applicator cylinder and between the impression cylinder and the central back pressure cylinder (material) is a critical factor for the correct impression, and depends among other factors on the type of printing plate to print, the adhesive with which the plate has been attached to the cylinder, the material to be printed on, the reason to be printed, etc. and therefore must be adjusted in each new print job.

In a second aspect, the invention also provides a printing machine to implement said method.

Background of the Invention Numerous methods are known for adjusting the printing pressure on a flexographic machine.

In patent ES 2308716 an automatic adjustment is described reading the printed surface and based on the comparison of the position of each of the monochrome impressions of the different groups registered at different times by, at least, one sensor station. The method requires a sensor system for each printing group, where during a registration process one printing cylinder after another is respectively in the printing position, and which comprises the following steps:

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a single sensor station records each monochrome print obtained

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the correction signals for the actuator or actuators of a printing cylinder are obtained from the monochrome image of each printing cylinder and from the information regarding the rotational movements of at least one printing cylinder.

Said patent ES 2308716 solves the problem of making the correct adjustment of the working pressures, but the described method implies that more than one color cannot be made at the same time (as they are monochrome images) and this entails a great waste of material and of time when adjusting.

To solve the problem of the material and time spent in making the adjustment, we can find in the state of the art various patents that try to make more or less precise adjustments with minimal waste.

Thus, in EP 1 916 102 an adjustment system is proposed based on a series of theoretical calculations outside the machine, where it is estimated what is the value of the diameter of the sleeves of the printing and inking cylinders. The final positions of the positioning drives are recalculated from these theoretical calculations. This procedure minimizes waste in time and material, but being a purely theoretical procedure, the reliability of the adjustment is very low, since in the printing process not only the diameter of the rollers is involved, but also and fundamentally the type of cliché to print, the ink, the substrate on which it is printed, ... all of them determining factors of the correct printing pressure. This procedure is valid to find the contact point of the printing and inking cylinders with each other and the printing cylinder with the material, but not to determine the correct pressure between them.

Patent application EP 1 018 426 discloses a flexographic printer in which an automatic procedure for adjusting a printing cylinder, an applicator cylinder and a back pressure cylinder to each other can be performed. Said procedure comprises the following stages:

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firstly, and with the help of the printing cylinder rotating drive motor, said printing cylinder is brought into slow rotation;

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the applicator cylinder, which has not yet rotated, then moves slowly in its radial direction against the impression cylinder;

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as soon as the printing cylinder and the applicator cylinder come into contact, a change in the torque of the printing cylinder's rotating drive motor takes place, which is registered by a rotary encoder (of said motor);

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the position that the applicator cylinder has reached at this time is stored by a printer control unit as the zero position of said applicator cylinder;

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the impression cylinder is then moved - which continues to rotate - against the back pressure cylinder until another modification of the torque of its rotating drive motor occurs;

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the position of the impression cylinder is stored in the control unit as zero position of the impression cylinder; and

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the path the impression cylinder has traveled until it comes into contact with the back pressure cylinder is added to the zero position of the applicator cylinder The adjustment process according to the approach sequence according to the EP

However, 1 018426 has drawbacks since the forces acting on the cylinder surface when the cylinders come into contact can be very high. Tests have shown that the printing plates mounted on the impression cylinders already show damage after some automatic adjustment cycles performed according to this approximation process.

For its part, patent application W02010 / 142405 discloses a flexographic printer in which it tries to solve the drawbacks of the automatic cycle described in application EP 1 018 426, especially those related to damage that may be caused to the surface of the printing plate. Said patent application WO 2010/142405 proposes a device and a method for adjusting the cylinders of a printer that do not damage the printing cylinder, in which the rotation drive of the cylinders of the inking group is controlled by a control unit of the printer which is arranged in such a way that it operates in weakened flow mode (limited torque) at least one of the cylinders' rotary drive motors during their mutual adjustment.

According to patent application WO 2010/142405, in order not to damage the raised areas of the printing cylinder during the adjustment procedure, the control unit reduces the current that produces the torque that is applied to the cylinder drive, so that the drive undergoes a field weakening and in this way the magnetic flux in the motor working area is reduced below the nominal value, thereby achieving a reduction in torque at the same speed in the drive. When under such conditions a rotating cylinder contacts the rotating torque with another cylinder and with it the force acting during the contact of the cylinders on the cylinder surface, they are very low and thus the plate surfaces are not damaged.

Both in patent EP 1916102 and in the two patent applications EP 1018426 AND WO 2010/142405, however, the real problem of adjusting the correct working pressure is not solved. In this background, a zero point of contact of the printing cylinder with the material and of contact of the cylinders of the inking group (applicator cylinder and printing cylinder) with each other is detected, but then an approach movement of between 10 and 100 must be applied. microns (see page 5 lines 20 to 23 of application WO 2010/142405) to achieve the optimal working pressure. This approach of between 10 and 100 microns is left to the consideration of the machine operator, with which an optimal work adjustment is not automatically achieved.

In ES 2300209 a method of positional adjustment of printing bodies in flexographic printing machines is proposed, comprising the following steps:

a) move linearly in approach by guide provisions of a

first impression cylinder towards the support drum by means of a linear motor;

b) detecting that a contact point has been reached between said first cylinder

print, or a cliche fixed around it, and the sheet material

supported on the support drum;

c) rotate the support drum, and allow the first roller to rotate.

impression dragged by the support drum by friction, through the material

laminate, or rotate both synchronously: the support drum and the

first impression cylinder; and

d) detect the linear zoom-out positions on these

guide arrangements of the first printing roller.

This method comprises in step c) an action that involves friction that can in some cases result in damage to the material, or in errors, and the adjustment depends on the reliability of the detection in the final step d).

The objective of the present invention is to present a method of adjusting the working pressure applied to inking and printing cylinders, in which the optimum working pressure is actually automatically determined, with the help of a device for acquiring the printing image, such as a linear camera and allowing to carry out a part of the method simultaneously with several printing groups, avoiding the dead time that exists between the printing area and the viewing area, saving a significant volume of the printing material, automating the adjustment and avoiding the intervention of the operator and the possible errors that it can commit.

Brief exposition of the invention

To this end, the present method of adjusting pressures is carried out on a conventional flexographic printing machine equipped with several printing groups arranged around a central back-pressure cylinder on which the material to be printed is located, integrating each of said printing groups. at least one printing cylinder and one inking cylinder, capable of rotation by means of a rotating drive motor, said printing machine having associated image acquisition means, such as a linear camera or other system, capable of capturing an image of the printing of each printing group and of the set or of a selected group thereof. In addition, the flexographic printing machine has a control unit for detecting one or more parameters of the rotary movement of the cylinders, such as the torque, electric current consumed by said rotating drive motor or the speed of rotation.

The pressure adjustment of each of the printing groups in relation to the back pressure cylinder is carried out by means of positioning motors that apply linear displacement to the bearing axes of said printing and inking cylinders, in their radial direction.

The method, for a printing group, comprises two different stages.

In a first stage the following actions are carried out:

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linearly move said impression cylinder and said cylinder

inking towards said central back pressure cylinder, making them turn,

actively or passively, and monitoring its torque, and

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perform, from a detection of a torque variation, a first

pressure adjustment between at least said impression cylinder with said

back pressure cylinder or between said impression cylinders and

inker.

This ensures a contact position between the different cylinders.

In a second stage, a fine pressure adjustment is carried out by means of at least one sequence that includes the following actions:

a) make an impression on a sheet material arranged on

said central back pressure cylinder,

b) capture at least an image of at least part of said

printing, for example using a linear camera,

c) analyze the quality of said captured image; and

d) vary the pressure exerted by the impression cylinder on the cylinder

central back pressure, or the pressure between these two cylinders,

by means of the aforementioned positioning motors.

and the aforementioned sequence of actions a) to d) will be repeated until reaching an optimum pressure point between the cylinders determined by the optimum quality of the captured image.

According to the proposed method, the first stage of pressure adjustment is carried out simultaneously in several printing groups and said second stage can be carried out group by group or simultaneously depending on the type of motif to be printed.

Furthermore, the second stage of fine adjustment is carried out with monitoring of the driving torque of at least one of said printing and inking cylinders.

During the aforementioned first stage of pressure adjustment, provision has also been made to monitor the torque of the positioning motors, and that said motors operate in at least said first stage in torque limitation (reduced torque) mode.

Other characteristics of the invention will appear more clearly on reading an embodiment which is described below with the aid of drawing sheets.

Brief Description of the Drawings Fig. 1 is a schematic view of a flexographic printing machine for implementing the method of this invention. Fig. 2 schematically illustrates the elements involved in the adjustment method of the present invention, according to an exemplary embodiment. Fig. 3 is a block diagram explaining the proposed method.

Detailed description of an execution example

In Fig. 2 a typical printing group of a flexographic printing machine is shown, associated with a central back pressure cylinder 1, integrating a printing cylinder 2 and an inking cylinder 3, which in a printing situation are opposite and susceptible to rotation with the aid of a respective rotating drive motor 5.4.

The printing machine that is to implement the method of this invention has means associated with a control unit (not shown) for detecting one or more parameters of the rotary movement of the cylinders 2, 3, in particular the torque, the electrical intensity consumed by said rotating drive motors 4, 5, or the rotating speed (obtained by means of detectors 6, 7, such as rotary transducers).

Figure 2 shows pressure adjusting means 8, 9, 10, 11, implemented in this embodiment by means of a set of electric spindle motor, hereinafter called positioning motors, which apply to the ends of the axes carrying said cylinders. of impression 2 and inker 3, in its radial direction, a linear displacement in approach or distance with respect to the central back pressure cylinder 1.

The printing machine also has image capture means e (see Fig. 1) for acquiring visual information on the impressions made by each group G1 to G8, by several groups or by the set of groups G1 to G8 of the machine .

The method proposed by this invention includes, for each printing group:

A first stage in which said printing cylinder 2 and said inking cylinder 3 are linearly displaced towards said central back pressure cylinder 1, making them rotate (by means of motors 4, 5), actively or passively, and monitoring their torque. rotation (by conventional means such as monitoring the evolution of the intensity of the current consumed by each motor 4, 5), and performing, from a detection during said monitoring, of a torque variation (increase above of a certain threshold) a first pressure adjustment between at least said printing cylinder 2 with said back pressure cylinder 1 or between said printing cylinders 2 and inker 3, thereby guaranteeing that a contact position has been reached between the different cylinders .

In a second stage, a fine adjustment of pressures is carried out by means of one or more sequences, said sequence including the following actions: a) making an impression on a sheet material L arranged on said central back pressure cylinder 1, b) capturing at least one image of at least part of said printing, for example by means of a linear camera e,

c) analyze the quality of said captured image; and

d) vary the pressure exerted by the impression cylinder 2 on the

central back pressure cylinder 1, or the pressure between these two

2.3 cylinders, using the aforementioned positioning motors 8,9, 10,

eleven.

One or more sequences will be used with the aforementioned actions until an optimum pressure point is reached between impression cylinder 2 and inking cylinder 3, and between impression cylinder 2 and back pressure cylinder 1.

Fig. 3 clearly shows the proposed method and it can be seen that the aforementioned first stage of pressure adjustment can be carried out simultaneously on several printing groups (GROUP01 to GROUP n), which represents a significant saving by avoiding printing successive of the different groups, also avoiding the dead time that exists between the printing groups and the viewing area, saving a significant number of meters of test sheet material.

On the other hand, the said second stage is carried out group by group and advantageously with a monitoring of the driving torque of at least one of said printing cylinders 2 and inking 3.

In accordance with other characteristics of the proposed method:

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In said first stage of adjustment, provision is also made for monitoring the torque of the positioning motors 8,9, 10, 11;

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in said first stage the positioning motors 8, 9, 10, 11 will operate in torque limiting mode (reduced torque);

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the linear movements of the cylinders 2, 3 are carried out alternatively by actuating the positioning motors 8, 11 on one end of a bearing axis of the cylinders 2, 3, or the motors 9, 10 on the other end or all the motors of positioned 8, 9, 10 and 11, on both ends of both cylinders 2, 3 at the same time.

As the invention has been indicated in a second aspect, it proposes a flexographic printing machine that has a conventional general structure, which is illustrated in Fig. 1, formed by a central counter-pressure cylinder 1, on which a sheet material L runs. displaced from a coil by an unwinder D and wound into a coil of printed material by means of a rewinder R.

The flexographic printing machine proposed by the second aspect of the invention has associated a linear camera C or another printing image acquisition system, and as a particularity it incorporates a control unit (not illustrated), in connection with said torque detection means. and with said system

image acquisition e, intended to execute said first and second stages of the explained method.

A person skilled in the art could introduce changes and modifications in the

Embodiment examples described without departing from the scope of the invention as is

defined in the appended claims.

Claims (7)

 Claims 1.-Method for pressure adjustment in a flexographic printing machine, equipped with several printing groups (G1 to G8) arranged around a central back-pressure cylinder (1) on which the material to be printed is located (L), integrating each of said printing groups (G1 to G8) at least one printing cylinder (2) and an inking cylinder (3), capable of rotation by means of a rotating drive motor (4, 5), the printer having means (6, 7) associated with a control unit for detecting one or more parameters of the rotary movement of the cylinders (2, 3), such as the torque, electrical intensity consumed by said rotating drive motor (4, 5) or the speed of rotation, and the printer having pressure adjustment means that apply to the ends of said printing cylinders (2) and inker (3), in their radial direction, a linear displacement, and integrate some positioning motors (8, 9, 10, 11), comprising e l method, for each printing group (G1 to G8):

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in a first step: -linearly moving said printing cylinder (2) and said inking cylinder (3) towards said central back-pressure cylinder (1), making them rotate, actively or passively, and monitoring their torque, and -performing, from a detection of a torque variation, a first pressure adjustment between at least said impression cylinder (2) with said central back pressure cylinder (1) or between said impression cylinders (2) and inker (3 ); and

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in a second stage, fine-tune the pressures by means of at least one sequence that includes the following actions: a) make an impression on a sheet material (L) arranged on

said central back pressure cylinder (1), b) capturing at least an image of at least part of said impression, c) analyzing the quality of said captured image; and d) vary the pressure exerted by the impression cylinder (2) on the central back pressure cylinder (1), or the pressure between said two printing cylinders (2) and inking (3), by means of said positioning motors (8, 9, 10, 11); And repeat said sequence of actions a) to d) until reaching an optimum pressure point between the cylinders. 2-Method according to claim 1, characterized in that said first stage of pressure adjustment is carried out simultaneously in several printing groups (G1 to G8) and said second stage is carried out group by group. 3-Method according to claim 1, characterized in that said first stage of pressure adjustment is carried out simultaneously on several printing groups (G1 to G8) and said second stage is also carried out simultaneously on several printing groups (G1 to G8). 4.- Method according to claim 1, characterized in that said second fine-tuning step is carried out with monitoring of the driving torque of at least one of said printing cylinders (2) and inking (3). 5. Method according to any one of the preceding claims, characterized in that said first stage further comprises also monitoring the torque of the positioning motors (8, 9, 10, 11). 6. Method according to claim 5, characterized in that said positioning motors (8, 9, 10, 11) operate in at least said first stage in torque limitation, or reduced torque mode. 7.- Method according to claim 1, characterized in that said linear displacements are carried out alternately by actuating the positioning motors (8, 9, 10, 11) on one end of a bearing axis of the cylinders (2, 3), on the other extreme or both at the same time. 8-Method according to claim 1, characterized in that said first stage is carried out by applying a variable tangential speed to the material supported in the central back-pressure cylinder (1) and to the printing (2) and inking cylinders (3). 9.-Method according to claim 8, characterized in that said first stage is carried out by applying a tangential speed difference between the material supported in the central back pressure cylinder (1) AND the printing cylinders (2) and inking (3). 10.- Method according to claim 9, characterized in that said first stage is carried out by applying a difference in tangential speed between the printing cylinders (2) and inking (3). 11. Flexographic printing machine, comprising: -a central back-pressure cylinder (1), operable in rotation, and at least one printing cylinder (2) and an inking cylinder (3), operable in rotation and linearly; -a drive means (4,5) to carry out said drives 10 in turn; -a drive means (8, 9, 10, 11) to carry out said linear drives; -at least one image acquisition unit (C) of the printed material; - means for the measurement of torque arranged to detect the torque of At least said printing cylinder (2); and

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a control unit, in connection with said torque detection means and with said image acquisition unit (C), and intended to implement the method according to any one of the preceding claims.

12. Flexographic printing machine, according to the preceding claim, comprising means for the measurement of the rotating torque arranged to detect the torque of both printing cylinders (2) and inking (3).