ES2790742T3 - Device and method for cleaning a tangent blade in a printing unit of a gravure printing machine - Google Patents

Abstract

Device for cleaning a tangent blade (1) in a printing unit such as a gravure printing machine, the printing unit comprising the tangent blade (1) and a rotating printing cylinder (2) with an outer surface, the printing unit extending blade (1) tangent along a longitudinal axis (X) and comprising a face (11, 12) delimited by an edge (10) adapted to come into contact with a generatrix of the outer surface of the printing cylinder (2), which defines a contact line on the tangent blade (1), the device being characterized in that it comprises a cleaning blade (4) and cleaning blade driving means (5), - the cleaning blade (4) being arranged superimposed and oblique with respect to said face (11, 12) of the tangent blade (1), - the cleaning blade (4) comprising a free edge (40) extending along a longitudinal axis (X ') of the cleaning blade cleaning parallel to the line contact line, - the cleaning blade driving means (5) being adapted to move the cleaning blade (4) between a separate resting position of the tangent blade (1) and a cleaning position in which the edge (40) free of the cleaning blade abuts against the face (11, 12) of the tangent blade (1) in proximity to the edge (10) of the tangent blade.

Description

DESCRIPTION

Device and method for cleaning a tangent blade in a printing unit of a gravure printing machine

Field of the invention

This invention relates to a device and a method for the automated cleaning of a tangent blade of a printing unit on a gravure printing machine.

Related Art Description

A typical gravure printing machine is composed of a printing roll or printing cylinder in contact with a second roller, which is usually a rubber roller, called a printing roller. The printing cylinder exhibits small cells on its surface whose distribution defines the image to be printed. These cells are filled with ink which is then transferred to a support by pressing the support between the two rolls. To achieve this, the ink-bearing printing roller must be cleaned to transfer the exact amount of ink needed to obtain a clean and sharp image to the media. Cleaning is done by the tangent blade, which is a blade placed tangentially on the surface of the impression cylinder, parallel to the axis of revolution of the cylinder. Therefore, while the printing cylinder is rotating, the tangent blade removes the excess ink present on the surface of the latter while leaving the ink inside the cells. This remaining ink is then transferred to the support during printing to obtain the desired image.

This common printing system is full of flaws that are currently still unsolved. In particular, the vibration of the tangent blade causes an accumulation of dry ink on its surface at its trailing edge. The trailing edge is located downstream of the contact line between the blade and the printing cylinder considering the direction of rotation of the latter. Accumulated ink tends to redistribute itself on the already clean part of the cylinder, causing print defects known as "tangent knife bands." When a tangent blade web is detected, the operator responsible for quality control first has to determine which printing unit causes the defect, and then has to use an ad hoc tool to clean the blade. This operation requires a significant amount of time, which translates into a significant expense of printed support given the significant speed of current printing machines.

Furthermore, a security bar is placed between the tangent blade and the printing area to prevent the operator from accessing this area particularly exposed to risks of crushing or entanglement. The printing zone is located on the tangential line between the printing cylinder and the printing roller. However, this implies that the tangent blade is positioned further from the print area than is required for optimum print quality. The further the knife is tangent from the printing area, the more likely the ink will dry. The drying of the ink causes some deviations in the tone reproduction.

Accordingly, improving print quality would involve closer positioning of the tangent blade to the print zone and removal of the security bar to speed up manual tangent blade cleaning operations. This would result in increased risk to the operator, which is not acceptable today.

The preamble of claim 1 of US patent 3,292,201 discloses a cleaning system for a tangent blade that continuously removes wet material (ink) that clogs the blade. This uses a series of blades that are positioned just above the tangent blade and that sweep the material to the blade side. The blades are driven by a chain that surrounds the tangent blade.

Brief summary of the invention

An object of the invention is to provide a solution to the problems mentioned above.

A further object of the invention is to provide a device for effective cleaning of the tangent blade without any manual intervention of an operator.

A further objective of the invention is to limit the amount of residue produced between the time when a printing defect is detected and the time when the defect is resolved.

Furthermore, another object of the invention is to provide an apparatus that improves the security of the printing machine as a whole.

Furthermore, the invention is intended to be easy to construct and easy to use.

These objectives are achieved by the device for the automated cleaning of a tangent blade in a printing unit of a gravure printing machine, the essential characteristics of which are defined in claim 1, and by the corresponding method according to claim 10.

Additional preferred embodiments of the invention are defined in the dependent claims.

Brief description of the drawings

- Figure 1 is a front perspective view of the device for cleaning the tangent blade of a printing machine, without showing the printing unit;

figure 2 shows the device represented in figure 1 seen from the rear;

Figures 3a and 3b show the tangent blade in the impression cylinder, where Figure 3a shows the cleaning blade in the rest position and Figure 3b shows the cleaning blade in the cleaning position; and - Figures 4a to 4c show different phases of cleaning the tangent blade by means of the cleaning blade.

Detailed description of possible embodiments of the invention

Figure 1 shows a tangent blade 1 in a printing unit such as a gravure printing machine, made in a traditional way for all aspects not mentioned in this case. The tangent blade extends along a longitudinal axis (X) and comprises a face delimited by an edge 10 adapted to come into contact with a generatrix of the outer surface of the printing cylinder 2, as shown in Figures 4a to 4c. The line on the knife tangent in contact with the surface of the impression cylinder is called the "line of contact," and is parallel to the axis of revolution of the impression cylinder. The device comprises a cleaning blade 4 arranged superimposed and obliquely relative to the face of the tangent blade. The cleaning blade comprises a free edge 40 extending along the longitudinal axis ( X) of the cleaning blade which is parallel to the longitudinal axis (X) of the tangent blade, and parallel to the line of contact. The device further comprises cleaning blade driving means adapted to move the cleaning blade between a separate rest position of the tangent blade, shown in Figure 4a, and a cleaning position in which the free edge of the cleaning blade cleaning abuts against the face of the tangent blade in proximity to the edge of the tangent blade. In other words, in the cleaning position, the cleaning blade removes any residual dry ink that may have accumulated on the face of the blade tangent in proximity to the nip. By "proximity" is meant close enough to get rid of excess dried ink. In practice, you can touch the impression cylinder. Thus, the cleaning blade is made of a material that prevents damage to the impression cylinder, for example polyester or other suitable plastic material, but a thin metal blade may also be appropriate.

In the figures, the printing unit is schematically represented showing only the printing cylinder with the tangent blade placed tangentially along its edge 10. The tangent blade is rigidly attached to a frame 3, represented by I in the figures . The frame 3 which is not specifically the object of this invention is not described in detail.

Despite the fact that the tangent blade is static with respect to the impression cylinder, the rotation of the impression cylinder about its axis of revolution Y determines the relative movement between the two elements. The tangent blade defines two flat faces: the front face 11 and the rear face 12. The front face 11 is the face of the blade tangent closest to the part of the surface of the impression cylinder located upstream of the nip. The rear face 12 is the face on the opposite side of the tangent blade, and parallel to the front face. Defined upstream and downstream according to the direction of rotation of the printing cylinder 2. In other words, excess ink to be removed by the tangent blade from the surface of the impression cylinder collides with the front face of the tangent blade, as shown in Figures 4a to 4c.

In general, an unwanted ink residue responsible for printing defects accumulates on the rear face 12 of the tangent knife. Figure 4b shows an excess ink accumulation S on the back face 12.

In the following description, and in the figures, the device is described as performing a cleaning operation on the rear face 12 of the tangent blade. Note that the same invention can be applied to also clean the front face of the tangent blade.

As mentioned above, the device comprises a cleaning blade 4, which is substantially flat and is arranged superimposed and obliquely on the rear face 12 of the tangent blade. The cleaning blade is attached to drive means 5 adapted to move the cleaning blade on an oblique plane with respect to the rear face 12 of the tangent blade. The angle between the plane and the rear face of the tangent blade can be between 0 ° and 45 °.

The cleaning blade driving means 5 may comprise a linear actuator. The linear actuator can be formed by a cylinder / piston group of hydraulic, pneumatic or electrical type, depending on the situation and in particular on the electrostatic / electromagnetic requirements to be applied when using solvent-based inks. Furthermore, a solution that uses two actuators 5, positioned towards each end of the cleaning blade, will be preferred to ensure proper guidance of the blade. In other words, the actuators are located at mutually opposite axial ends of the cleaning blade, as shown in Figure 1.

These drive means 5 are therefore arranged to be able to move the cleaning blade 4 from the rest position in which the cleaning blade 4 retracts to a cleaning position in which the free edge 40 of the blade 4 The cleaning pad comes in contact with the tangent blade to remove excess ink build-up. Thus, the cleaning blade comes into contact with the rear face 12 of the tangent blade up to the edge 10 and close to the contact line. The excess of ink S that is drying, the contact with the free edge of the cleaning blade causes the cracking of the ink, and consequently its removal.

The tangent blade comprises a plate 13 for additional stiffness that ends before edge 10. Thus, the plate defines an end 130 on which the cleaning blade 4 can rest during its movement. In addition, the plate helps the cleaning blade to be positioned correctly in its movement towards edge 10.

In some situations, dust particles can get stuck under the tangent blade, causing local blade lift and a wide band in printing where the ink density is too high and the contrast is too low. By performing a cleaning as described in this invention, the cleaning blade presses on the end of the tangent blade. This increased pressure may be sufficient to expel some of the powder from under the blade thereby removing the defect.

The device, in particular the drive means 5, can be controlled by programmable control means arranged to execute a set of predefined instructions. For example, thanks to the control means, an operator can activate the actuation means and thus the cleaning of the tangent blade from a remote location. In fact, each printing unit is provided with means for measuring the printing quality. These means, which are typically digital cameras, can transmit an image to a control workstation monitored by the operator. When the operator detects a printing defect, he activates the device to clean the tangent blade without having to move towards the printing unit, and substantially accelerates the time required to repair the printing defect.

A printing unit is in most cases made up of several printing units. Each printing unit is equipped with the cleaning device. Therefore, when the operator detects a defect, he may have to determine which printing unit causes the defect and trigger the cleaning device of that particular unit. As an alternative, the operator can trigger cleaning of that particular unit followed by cleaning of each printing unit downstream of it. As another alternative, the operator can trigger the cleaning of all the printing units thereby avoiding the task of identifying the problem printing unit. When the printing unit is cleaned, and excess ink is removed, this ink can affect print quality by ending up on the media. To minimize the effect of cleaning on spent media, cleaning can be arranged in sequence to prevent ink expelled by cleaning the printing units from spreading through successive prints. The sequence can be performed in an automated manner by means of control.

A cleaning sequence of particular interest is one in which once a start unit is identified, cleaning is performed using a sequence that causes excess ink ejected by cleaning to settle around a single x-coordinate of the media. . To achieve this purpose, cleaning of a printing unit located downstream of the starting unit must be triggered at a distance D measured along the path of the medium after a time equal to the distance D divided by the transport speed of the support. The sequence allows limiting the remainder around the x coordinate of the support. The x coordinate is measured along the X axis of the support, which is the one that extends along the transport direction of the support, the Y axis being parallel to the axis of rotation of the printing roller. The precision required when setting distance D is on the order of a fraction of the length of a single print, for example 20% of that length. For example, if the prints are 50 cm long, printed with a media speed of 2 meters per second, then the cleaning is required to be timed so that it occurs at a distance accuracy of about 10 cm, therefore, the precision required in cleaning sequencing is of the order of 10 [cm] / 2 [m / sec] = 50 milliseconds. The cleaning sequence can start from either the first unit in which the defect is identified or from the first unit in the press, the latter being preferred because it generates the same amount of residue and could get rid of an existing defect. present but not yet visible. The units in the press are numbered according to the direction of transport, in other words, the supports travel through the first unit, then through the second unit, etc.

The cleaning device according to the invention has many advantages. For example, it can be installed on machines printing machines that are already in operation.

Furthermore, this invention makes cleaning interventions by an operator unnecessary. Furthermore, thanks to the fact that the device can be run automatically from a distance, the printing area can be completely isolated, for example by installing some means of protection to limit access to the printing area. Consequently, the risk of injury to the operator is drastically reduced. Furthermore, thanks to these external protection means, the security bar between the tangent blade and the printing area can be eliminated. The invention also allows the tangent blade to be positioned very close to the printing area, which reduces to a significant extent the phenomenon of ink drying on the printing roller. Note that by bringing the blade closer to the printing point, not only does the print quality increase but also makes the printing press much more sensitive to dust on the blade, so that an automatic cleaning system is even more efficient. important.

The possibility of automatically triggering the cleaning device reduces the intervention time from the moment the defect is detected. This implies a significant reduction in waste compared to current printing machines and therefore represents an economic advantage for the user of the machine.

As described above, it is also possible to trigger cleaning on several printing units simultaneously or in sequence. In this way, the tangent blades of each printing unit are constantly cleaned and error from one printing unit is prevented from spreading to the next, or even any error is prevented entirely.

In this invention, the term "printing" also refers to a coating process in which the purpose is not an image but a uniform color or varnish scheme. In another embodiment of the invention, the operator triggering the cleaning can be replaced by an algorithm. The method preferably uses the comparison between the desired result, called in this case the master image, and a measured result. When the algorithm detects the presence of a tangent blade band, it triggers the cleaning sequence, to remove the band. The detection can be located at the end of the press so that only one camera system can be used for the entire machine, or it can be placed after each printing unit if response time is particularly important. In the latter case, cleaning a single printing unit may be sufficient.

There are many methods for detecting a tangent blade band by comparing it to a master image, and various methods for obtaining a master image. For example, when the press is printing at the desired quality, the operator can trigger the registration of the master image. This master image can consist of a collection of images of selected areas to be verified, or of the entire cliche. In general, a master image does not have to be an image in its strict interpretation of a two-dimensional array of pixels, but can be a collection of such images; The methods described in this case can then be applied to each image in the collection. Registration can be done more than once, and the result can be averaged. If for some reason registration is not possible, the original representation, that is, the digital representation of the image to be printed, or a transformation of it can be used as the master image.

Comparison with a master image can be carried out, for example, by direct subtraction of the registered image to be checked, called the test image (s), with the master image. This subtraction can be done after an image alignment step to ensure that there is no global translation between the test image and the master image due to some tolerances in the control of the position of the holder / camera or due to registration timing. . Also, to handle changes in illumination, the image values of the master image and the test image can be converted to contrast values before subtraction. The result of the subtraction (which is an image the same size as the master image) is then used for detection.

Detecting a particular pattern in an image, in this case a tangent blade web, is well known in the field of computer vision and machine learning. It can be done through supervised learning: in an offline procedure, a system is trained to recognize the pattern by showing to the system, with the associated label, a set of images containing the pattern, and a set of images without the pattern. These two sets of images are called the "training data." A general description of well known techniques can be found in J. Schmidhuber. Deep Learning in Neural Networks: An Overview. Neural Networks, Volume 61, January 2015, Pages 85-117, published online in 2014. After the training procedure, the system can detect the presence (and position) of a tangent blade band in an image. The complexity of the detection is low because the tangent blade band appears as a small line, which is always oriented along the transport direction of the support. The image used as input can be the image output from the camera, with or without post-processing. For example, the image used as input can be the result of subtraction of the test image and the master. It could be the test image itself, or the test image in which pixel values are converted to contrast values. The system should preferably be trained with the same type of data as used in detection. In this description, three different approaches to apply the well-known computer vision algorithm are presented. for this specific task.

In a first example of tangent blade band detection, the test image is aligned with and subtracted from the master. The resulting image is fed to the detection algorithm that decides if there is a tangent blade band and triggers the cleaning sequence if the detection is positive. As mentioned above, the system must be trained in an offline phase with some sample images, some of which contain the tangent knife bands, some of which without tangent knife bands, aligned with and subtracted from the image. corresponding teacher.

In a second example, a system is trained to recognize the tangent blade band directly from the test images. From one image, the detector first outputs another image (the same size or smaller), called the master band image, which carries information about the presence of a tangent blade band. For example, the image consists of ones where the detector detects the bands and zeros in the rest. Areas of the master image that responded positively to detection are classified as areas to be ignored. The detector then processes a test image; outputs a test band image, which takes account of the presence or absence of tangent blade bands. Then it compares the test band image with the master band image, to eliminate the areas that responded positively to both band images. Once removed, the algorithm responds positively if there are remaining areas that contain positive responses to the presence of tangent knife bands in the test band image. The method of the second example can be made much less sensitive to misalignment between the test image and the master image and is also more robust to changes in lighting. On the other hand, the detection of tangent blade bands directly from an image, as performed in the method of the second example, requires an order of magnitude or two more training data to achieve comparable performance for well-aligned images and with good exposure. The master image can be calculated from the digital representation of the image to be printed.

In a third example, the band detector of the second example is used, without comparison with the master image. The detection result is used directly to trigger cleaning. If, after several attempts to clean the tangent knife the algorithm still detects the presence of tangent knife bands, then three alternatives are proposed. The first alternative asks an operator if there is a tangent blade band, and if not, classifies the region in which the band is detected as a region to be ignored by detection. The second alternative classifies the region in which the band is detected as a region to be ignored by detection without operator confirmation. The third alternative uses the second alternative (that is, automatic sort) at the beginning of each print job and switches to the first alternative (that is, asks the operator) after a while. The reason behind the latter method is as follows: it may happen that a well-printed region of the image is classified as a tangent knife band, but a region classified first as a background is unlikely (i.e. not a band) suddenly classifies as a tangent knife band if printed correctly.

Claims (12)

i. Device for cleaning a tangent blade (1) in a printing unit such as a gravure printing machine, the printing unit comprising the tangent blade (1) and a rotating printing cylinder (2) with an outer surface, the printing unit extending blade (1) tangent along a longitudinal axis (X) and comprising a face (11, 12) delimited by an edge (10) adapted to come into contact with a generatrix of the outer surface of the printing cylinder (2), which defines a contact line on the tangent blade (1), the device being characterized in that it comprises a cleaning blade (4) and cleaning blade drive means (5), - the cleaning blade (4) being arranged superimposed and obliquely with respect to said face (11, 12) of the tangent blade (1), - the cleaning blade (4) comprising a free edge (40) extending along a longitudinal axis ( X) of the cleaning blade parallel to the contact line, - the cleaning blade driving means (5) being adapted to move the cleaning blade (4) between a rest position separated from the tangent blade (1) and a cleaning position in which the free edge (40) of the cleaning blade abuts against the face (11, 12) of the tangent blade (1) in proximity to the edge (10) of the tangent blade. Device according to claim 1, in which said tangent blade face is a rear face (12), specifically a face downstream of contact with the printing cylinder (2) according to the direction of rotation of said cylinder (2) of impression. Device according to claim 1 or 2, in which the cleaning blade driving means (5) comprise a linear actuator adapted to move the cleaning blade (4) on an oblique plane with respect to the face (11, 12) of tangent blade. Device according to claim 3, wherein said cleaning blade (4) is oblique with respect to said tangent blade face (11, 12) by an angle between 0 ° and 45 °. Device according to claim 3 or 4, wherein the cleaning blade driving means (5) comprise a pair of actuators at mutually opposite axial ends of the cleaning blade (4). Device according to any of the preceding claims, further comprising a frame (3) supporting said cleaning blade driving means (5) and providing guide means for movably supporting the cleaning blade (4). Device according to any one of the preceding claims, comprising or functionally associated with a programmable control system adapted to automatically control the activation of the cleaning blade drive means from the rest position to the cleaning position based on instructions preset. Device according to any preceding claim, for a printing unit that prints on a support, comprising a camera connected to calculation means, in which the camera is arranged to take at least one image of the support and is positioned downstream of the rotary impression cylinder (2); and wherein the computing means is configured to detect the presence of a tangent blade band from the image; and wherein the calculating means is operatively connected to the cleaning blade driving means (5) to clean the tangent blade (1) when a tangent blade band is detected. 9. Printing unit comprising a tangent blade cleaning device according to any of the preceding claims. 10. Method for cleaning a tangent blade (1) in a printing unit of a gravure printing machine providing a printing on a support, the unit comprising a rotating printing cylinder (2) and a cleaning device of tangent blade according to any of the preceding claims 1-8, the method comprising the steps of: - checking the printing to detect printing defects caused by residues of dry ink on the tangent blade (1); - if a defect is detected, order the movement of the cleaning blade (4) of the cleaning device tangent between a rest position in which the cleaning blade (4) is separated from said tangent blade (1) and a cleaning position in which the free edge (40) of said cleaning blade (4) abuts against said face (11, 12) of said tangent blade (1) in proximity to the line of contact of the blade (1) tangent; Y - order the return of the cleaning blade (4) to the rest position. Method according to claim 10, for a printing machine comprising a sequence of printing units each provided with a cleaning blade (4), wherein after commanding the movement of said cleaning blade (4) to the first printing unit, the cleaning blade (4) of each unit that follows the first unit is ordered after a period of time equal to the time that the media needs to reach each unit starting from the first unit. A method according to claim 10 or 11, comprising a camera configured to record an image of the medium, wherein the step of checking the print to detect print defects is performed automatically from the image.