WO2017182143A1

WO2017182143A1 - Device for cleaning a doctor blade in a printing unit of a rotogravure printing machines

Info

Publication number: WO2017182143A1
Application number: PCT/EP2017/025094
Authority: WO
Inventors: Renzo Melotti
Original assignee: Bobst Italia S.P.A.
Priority date: 2016-04-22
Filing date: 2017-04-21
Publication date: 2017-10-26
Also published as: EP3445587B1; ES2790742T3; EP3445587A1

Abstract

This invention discloses a system to automatically clean a doctor blade (1). The cleaning of the doctor blade (1) removes the dust particles or the dried ink aggregates. It is performed by a sliding blade (4), which slides over the doctor blade (1). By automating the cleaning process, we can place the safety equipment on the side of the print unit, thus allowing to position the blade (1, 4) closer to the printing roller (2) and improving the print quality. Also, the invention discloses a cleaning sequence where the dirt caused by the cleaning process is confined to a small portion of the print.

Description

DEVICE FOR CLEANING A DOCTOR BLADE IN A PRINTING UNIT OF A

ROTOGRAVURE PRINTING MACHINE

FIELD OF THE INVENTION

This invention relates to a device for the automated cleaning of a doctor blade of a printing unit in a rotogravure printing machine.

DESCRIPTION OF RELATED ART

A common rotogravure printing machine is made of a printing roll or printing cylinder in contact with a second roller, which is usually a rubber roller, called the impression roller. The printing cylinder exhibits little cells on its surface whose distribution defines the image to be printed. These cells are filled with ink which is then transferred onto a support by pressuring the support between the two rolls. To achieve this goal, the printing roller carrying the ink must be cleaned to transfer to the support the exact amount of ink needed to obtain a clean and sharp image. The cleaning is performed by the doctor blade, which is a blade positioned tangentially on the surface of the printing cylinder, parallel to the cylinder revolution axis. Thus, while the printing cylinder is rotating, the doctor blade removes the excess of 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 plagued by defects which are still unsolved today. In particular, the vibration of the doctor blade causes an accumulation of dried ink on its surface at its back edge. The back edge being situated downstream from the contact line between the blade and the printing cylinder considering the direction of rotation of the latter. The accumulated ink tends to be redistributed on the part of the cylinder that was already cleaned and causes printing defects known as "doctor blade stripes." When a doctor blade strip gets detected, the operator responsible for the quality control has first 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 an important waste of printed support given the important speed of today's printing machines.

In addition, a safety bar is placed between the doctor blade and the impression zone to prevent the operator to access this area particularly exposed to crushing or entanglement risks. The impression zone is located at the nip between the printing cylinder and the impression roller. Nevertheless, this implies that the doctor blade is placed further from the impression zone than required for an optimal print quality. The further the doctor blade from the impression zone, the more likely the drying of the ink. The drying of the ink causes some shifts in the tone reproduction. Consequently, improving the print quality would imply a closer positioning of the doctor blade from the impression zone and the removal of the security bar to accelerate the doctor blade manual cleaning operations. This would result in a higher risk for the operator, which is nowadays not acceptable.

BRIEF SUMMARY OF THE INVENTION An objective of the invention is to provide a solution to the problems mentioned above.

A further objective of the invention is to provide a device for the efficient cleaning of the doctor blade without any manual intervention of an operator.

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

Also, another objective of the invention is to provide an apparatus which improves the safety of the printing machine as a whole.

In addition, the invention aims at being easy to build and easy to use. These objectives are met by the device for the automated cleaning of a doctor blade in a printing unit of a rotogravure printing machine, whose essential characteristics are defined in the first claim. BRIEF DESCRIPTION OF THE DRAWINGS

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

- Figure 2 shows the device represented in Figure 1 seen from the back; - Figure 3a and 3b shows the doctor blade on the printing cylinder, where figure

3a shows the cleaning blade in rest position and figure 3b shows the cleaning blade in cleaning position; and

- Figure 4a to 4c show different stages of the cleaning of the doctor blade by the cleaning blade DETAILED DESCRIPTION OF POSSIBLE EMBODIMENTS OF THE INVENTION

Figure 1 shows a doctor blade 1 in a printing unit such as a rotogravure printing machine, made in a traditional way for all the aspects not mentioned here. The doctor blade extends according to a longitudinal axis (X) and comprises a face delimited by an edge 10 adapted to contact a generatrix of the cylinder outer surface, as shown in Figures 4a to 4c. The line on the doctor blade in contact with the printing cylinder surface is called the "contact line," and is parallel to the revolution axis of the printing cylinder. The device comprises a cleaning blade 4 arranged in a superimposed and slanting fashion with respect to the face 10 of the doctor blade. The cleaning blade comprises a free edge 40 extending according to the cleaning blade longitudinal axis (Χ') which is parallel to the doctor blade longitudinal axis (X), and parallel to the contact line. The device further comprises cleaning blade driving means adapted to displace the cleaning blade between a rest position spaced from the doctor blade, shown in figure 4a, and a cleaning position in which the cleaning blade free edge abuts against the face of the doctor blade in proximity to the edge the doctor blade. In other words, in the cleaning position, the cleaning blade wipes away any residual dried ink that might have accumulated on the face of the doctor blade in proximity to the contact line. By proximity, we mean close enough to get rid of the excess of dried ink. In practice, it can touch the print cylinder. Thus the cleaning blade is made of a material that avoids damages to the print cylinder, for example, polyester or another type of suitable plastic material, but may result appropriate also a thin metallic blade

In the figures, the printing unit is schematically represented by only showing the printing cylinder with the doctor blade positioned tangentially along its edge 10. The doctor blade is attached to a frame 3 in a rigid way, represented by I the figures. The frame 3 being not specifically the object of this invention is not described in details.

Despite the fact that the doctor blade is static with respect to the printing cylinder, the rotation of the printing cylinder around its revolution axis Y determines the relative motion between the two elements. The doctor blade defines two planar faces: the front face 11 and the rear face 12. The front face 11 is the face of the doctor blade closer to the portion of the print cylinder surface located upstream from the contact line. The back face 12 being the face on the opposite side of the doctor blade, and parallel to the front face. Upstream and downstream being defined according to the rotation direction of the print cylinder. In other words, the ink in excess that must be removed by the doctor blade from the surface of the printing cylinder bumps against the front face of the doctor blade, as shown in figure 4a to 4c.

In general, an unwanted residue of ink responsible for printing defects accumulate on the rear face 12 of the doctor blade. Figure 4b shows an accumulation of ink S in excess on the rear 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 doctor blade. Please note that the same invention can be applied to clean the doctor blade on its front face as well.

As mentioned before, the device comprises a cleaning blade 4, which is substantially flat and is arranged in a superimposed and slanting fashion on the rear face 12 of the doctor blade. The cleaning blade is attached to drive means 5 adapted to displace the cleaning blade over a plane slanting with the doctor blade rear face 12. The angle of the between the plane and the doctor blade rear face may be comprised between 0° and The drive means 5 may comprise a linear actuator. The linear actuator may be made of a cylinder/piston group of hydraulic, pneumatic or electric type, depending on the situation and in particular on the electrostatic/electromagnetic requirements to be applied when using solvent based inks. Also, we will prefer a solution using two actuators 5, placed toward each end of the cleaning blade, to ensure a proper guiding of the blade. In other words, the actuators are placed on mutually opposite axial ends of the cleaning blade, as depicted in figure 1.

These drive means 5 are thus arranged to be able to displace the cleaning blade 4 from the rest position where the cleaning blade 4 is retracted to a cleaning position where the free edge 40 of the cleaning blade 4 comes in contact with the doctor blade to remove the accumulation of ink S in excess. In particular, the cleaning blade enters into contact with the rear face 12 of the doctor blade up to the edge 10 and close to the contact line. The excess of ink S being dry, the contact with the cleaning blade free edge causes the shattering of the ink, and consequently its removal. The doctor blade comprises an additional stiffening plate 13 which ends before the edge 10. Thus, the plate defines an extremity 130 on which the cleaning blade 4 can lean during its motion. Also, the plate helps the cleaning blade to be correctly positioned in its motion toward the edge 10.

In some situation, dust particles may get stuck under the doctor blade, causing a local elevation of the blade and a wide stripe on the print where the density of the ink is too high and the contrast too low. By performing a cleaning as described in this invention, the cleaning blade presses over the doctor blade extremity. This augmented pressure can suffice to eject some of the dust from underneath the blade, thus removing the defect. The device, in particular the drive means 5, may 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 drive means and thus the cleaning of the doctor blade from a remote location. Indeed, every printing unit is provided with means to measure the printing quality. These means, which are usually made of 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 for cleaning the doctor blade without having to move toward the printing unit, and accelerating substantially the time needed to fix the printing defect. A printing unit is in most cases made of several printing units. Every printing unit is provided with the cleaning device. Thus, when the operator detects a defect, he might need to determine which printing unit causes the defect and trigger the cleaning device of that particular unit. As an alternative, the operator might trigger the cleaning of that particular unit followed by the cleaning of every printing unit situated downstream from it. As another alternative, the operator might trigger the cleaning of all the printing units, thus avoiding the task of identifying the problematic printing unit. When the printing unit is cleaned, and the ink in excess removed, this ink might affect the print quality by ending up on the support. To minimise the effect of the cleaning on the wasted support, the cleaning can be arranged in sequence to avoid that that the ink ejected by the cleaning of the printing units spreads across successive prints. The sequence can be performed in an automated manner by the control means.

A cleaning sequence of particular interest is one where once a starting unit is identified, the cleaning is performed using a sequence which causes the ink excess ejected by the cleaning to lands around a single x coordinate of the support. To achieve this goal, a cleaning of a printing unit situated downstream from the starting unit at a distance D measured along the path of the support must be triggered after a time equal to the distance D divided by the transport speed of the support. The sequence allows limiting the waste around the x coordinate of the support. The x coordinate is measured along the X axis of the support which is the one extending along the transport direction of the support, the Y axis being the one parallel to the rotation axis of the print roller. The precision needed in when setting the distance D is the order of a fraction of the length of a single print, for example, 20% of said length. For example, if the prints are 50 cm long, printed with a support speed of 2 meters per second, then we would want the cleaning to be synchronized such that it happens at a distance precision of about 10 cm, thus the precision needed in the sequencing of the cleaning is of the order of 10 [cm] / 2 [m/sec] = 50 milliseconds. The sequence of cleaning can either start from the first unit where the defect is identified or the first unit in the press, the latter being preferred because it generates the same amount of waste and might get rid of a defect which is already present but not visible yet. The units in the press are numbered according to the transport direction, in other words, the supports travels through the first unit, then through the second unit, etc.

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

Also, this invention renders unnecessary the cleaning interventions of an operator. In addition, thanks to the fact that the device can be run automatically from a distance, the printing area can be entirely isolated, for example by installing some protection means to limit the access to the printing area. Consequently, the risk of injury for the operator is drastically reduced. Also, thanks to these external protection means, the safety bar between the doctor blade and the impression zone can be eliminated. The invention also allows positioning the doctor blade very close to the impression zone which reduces to an important extent the drying phenomenon of the ink on the printing roller. Please note that coming closer with the doctor blade to the printing point, not only we increase the print quality but also made the printing press much more sensitive to the dust on the blade, so an automatic cleaning system became even more important. The possibility to trigger automatically the cleaning device reduces the intervention time from the moment the defect is detected. This implies an important reduction of the waste compared to today's printing machines and thus represents an economic advantage for the user of the machine.

A previously described, it is also possible to trigger the cleaning on several printing units simultaneously or in sequence. In this way, the doctor blades of every printing unit are constantly cleaned and prevent the error of one printing unit to propagate to the next, or even prevents any errors at all. In this invention, the term "printing" or "print" also refers to coating processes where the aim is not an image but a uniform colouring or varnishing scheme.

In another embodiment of the invention, the operator that triggers the cleaning can be replaced by an algorithm. The method preferably uses the comparison between the desired output - called here the master image - and a measured output. When the algorithm detects the presence of a doctor blade stripe, it triggers the cleaning sequence, to remove the stripe. The detection can be placed at the end of the press so that only one camera system can be used for the whole machine, or it can be placed after each printing unit if the response time is of particularly important. In the latter case, the cleaning of a single printing unit may suffice.

There exist many methods to detect a doctor blade stripe by comparison with a master image, and several methods to obtain a master image. For example, when the press is printing to the desired quality, the operator can trigger the recording of the master image. This master image can be made of a collection of images of selected areas to be verified, or of the complete cliche. In general, a master image does not need 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 here may then be applied to every image of the collection. The recording may be performed more than once, and the result averaged. If for some reasons the recording is not possible, the original i.e. the digital representation of the image to be printed, or a transformation of it, can be used as the master image.

The comparison with a master image can, for example, be made by a direct subtraction of the recorded image to be verified, called the test image(s), with the master image. This subtraction may be performed 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 support/camera or due to the timing of the recording. Also, to handle changes in illumination, the image values of the master and the test image may be converted to contrast values before the subtraction. The result of the subtraction (which is an image of the same size than the master image) is then used for the detection.

The detection of a particular pattern in an image, here a doctor blade stripe, is well known in the computer vision and machine learning field. It can be performed by supervised learning: in an off-line process, a system is trained to recognise 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 process, the system can detect the presence (and the position) of a doctor blade stripe in an image. The complexity of the detection is low because the doctor blade stripe 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 subsequent processing. For example, the image used as input might be the result of the subtraction of the test image and the master. It could be the test image itself, or the test image where the pixel values are converted to contrast values. The system must preferably be trained with the same type of data than the one used in detection. In this description, we present three different approached to apply the well- known computer vision algorithm to this specific task.

In a first example of the doctor blade strip detection, the test image is aligned to and subtracted from the master. The resulting image is fed to the detection algorithm which decides whether there is a doctor blade strip and triggers the cleaning sequence if the detection is positive. As mentioned earlier, the system must be trained in an offline phase with some examples of images, some of which containing the doctor blade stripes, some of which without doctor blade stripes, aligned to and subtracted from the corresponding master image.

In a second example, a system is trained to recognise the doctor blade stripe directly from the test images. From an image, the detector first outputs another image (of the same size or smaller), called the master stripe image, which carries information about the presence of a doctor blade stripe. For example, the image is made of ones where the detector detects the stripes and zeroes elsewhere. The areas of the master image that responded positively to the detection are classified as areas to ignore. Then the detector processes a test image; it outputs a test stripe image, which accounts for the presence or absence of doctor blade stripes. Then, it compares the test stripe image with the master stripe image, to remove the areas which responded positively to both stripe images. Once removed, the algorithm responds positively if there are remaining areas containing positive responses to the doctor blade stripes presence in the test stripe image. The method of the second example can be made much less sensitive to misalignments between the test and master image and is also more robust to illumination changes. On the downside, the detection of doctor blade stripes directly from an image, as performed in the method of the second example, requires one or two orders of magnitude more training data to reach a comparable performance for well aligned and well-exposed images. The master image may be computed from the digital representation of the image to be printed.

In a third example, the stripe detector of the second example is used, without comparison with the master image. The result of the detection is used directly to trigger the cleaning. If, after several attempts of cleaning the doctor blade the algorithm still detects the presence of doctor blade stripes, then three alternatives are proposed. The first alternative asks an operator whether there is a doctor blade stripe, and if not, classifies the region where the stripe is detected as a region to be ignored by the detection. The second alternative classifies the region where the stripe is detected as a region to be ignored by the detection without the confirmation of the operator. The third alternative uses the second alternative (i.e. the automatic classification) at the beginning of each printing job and switches to the first alternative (i.e. ask the operator) after a while. The rationale behind the last method is the following: it may happen that a well-printed region of the image is classified as a doctor blade stripe, but it is unlikely that a region first classified as background (i.e. not a stripe) suddenly becomes classified as doctor blade strip if it is printed correctly.

Claims

1. A device for cleaning a doctor blade in a printing unit such as a rotogravure printing machine, the printing unit comprising the doctor blade and a rotating printing cylinder with an outer surface, the doctor blade extending according to a longitudinal axis (X) and comprising a face delimited by an edge adapted to contact a generatrix of the cylinder outer surface, defining a contact line on the doctor blade, the device comprising

- a cleaning blade arranged in a superimposed and slanting fashion with respect to said face of the doctor blade,

- the cleaning blade comprising a free edge extending according to a cleaning blade longitudinal axis (Χ') parallel to the contact line,

- the device further comprising cleaning blade driving means adapted to displace the cleaning blade between a rest position spaced from the doctor blade and a cleaning position in which the cleaning blade free edge abuts against the face of the doctor blade in proximity to the doctor blade edge.

2. The device according to claim 1, wherein said doctor blade face is a rear face, namely a face downstream of the contact with the printing cylinder according to the direction of rotation of the same cylinder.

3. The device according to claim 1 or 2, wherein the drive means comprise a linear actuator adapted to displace the cleaning blade over a plane slanting with the doctor blade face.

4. The device according to claim 3, wherein said cleaning blade is slanting with said doctor blade face by an angle comprised between 0° and 45°.

5. The device according to claim 3 or 4, wherein the drive means comprise a pair of actuators at mutually opposite axial ends of the cleaning blade.

6. The device according to any of the previous claims, further comprising a frame (3) supporting said drive means and providing guide means for the movable support of the cleaning blade.

7. The device according to any of the previous claims, comprising or functionally associated with a programmable control system adapted to automatically control the activation of the drive means from the rest position to the cleaning position based upon pre-set instructions.

8. The device according to any preceding claim, for a printing unit printing on a support, comprising a camera connected to computation means, wherein the camera is arranged to take at least one picture of the support and is positioned downstream from the rotating printing cylinder; and wherein the computation means are configured to detect the presence of a doctor blade stripe from the picture; and wherein the computation means are functionally connected to the cleaning blade driving means for cleaning the doctor blade when a doctor blade stripe is detected.

9. A printing unit comprising a doctor blade cleaning device according to any of the previous claims.

10. A method for the cleaning of a doctor blade in a printing unit of a rotogravure printing machine printing on a support, the unit comprising a rotating printing cylinder, the method comprising the steps of:

- checking the print for printing defects caused by residues of dry ink on the doctor blade;

- if a defect is detected, commanding the displacement of a cleaning blade between a rest position in which the cleaning blade is spaced from said doctor blade and a cleaning position in which a free edge of said cleaning blade abuts against a face of said doctor blade in proximity to the contact line of the doctor blade; - and commanding the return of the cleaning blade to the rest position.

11. The method according to the claim 10, for a printing machine comprising a sequence of printing units each provided with a cleaning blade, wherein after commanding the displacement of said cleaning blade for the first printing unit, the cleaning blade of each unit following the first unit are commanded after a lapse of time equal to the time that the support needs to reach each unit starting from the first unit.

12. The method according to claim 10 or 11, comprising a camera configured to record a picture of the support, wherein the step of checking the print for printing defects is performed automatically from the picture.